Clinical biochemistry : metabolic and clinical aspects

EDITED BY

WILLIAM J. MARSHALL STEPHEN K. BANGERT

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Clinical Biochemistry

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Clinical Biochemistry Metabolic and Clinical Aspects EDITED BY

William J. Marshall

MA PhD MSc MBBS FRCP FRCPath

Senior lecturer in Chemical Pathology, King's College School of Medicine and Dentistry, University of I^ondon, I-ondon; Honorary Consultant Chemical Pathologist, King's Healthcare N H S Trust, London, UK and

Stephen K. Bangert

MA MSc MB BChir MRCPath

Consultant Chemical Pathologist, Eastbourne Hospitals N H S Trust, Eastbourne, UK

Churchill

Livingstone NEW YORK EDINBURGH LONDON MADRID MH1J30UKNK SAN FRANCISCO AND TOKYO 1995

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CHURCHILL LIVINGSTONE Medical Division of Pearson Professional Limited Disinbutcd in the United States of America by Churchill Livingstone Inc., 650 Avenue of Americas, New York, N Y . 10011, and by associated companies, branches and representatives throughout the world. С Pearson Professional Limited 1995 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without cither the prior permission of the pubhahcrs (Churchill tivingstone, Robert Stevenson House, 1-3 Baxter's Place, Leah Walk, Edinburgh EH1 3AF), or a licence permitting restrtcied copying in the United Kingdom issued by the Copyright licensing Agency I J d , 90 Tottenham Court Road, London, Wl P 9HE. First published 1995

ISBN 0 ЧЧЗ 0ЧЗЧ1Й British Library Cataloguing In Publication Data A catalogue record for mis book is available from the British Library. Library of Congress Cataloging In Publication Data A catalog record for this book is available from the Library of Congress.

The editors and authors have endeavoured to ensure that the information in this book is accurate and up to date. However, it is difficult entirely to eliminate human error, and in any case medicine is an ever changing science. Readers are particularly advised to check the product information sheets supplied with drugs, especially when planning to administer new or infrequently used formulanons.

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Contents

Preface

12. The assessment of hepatic function and investigation of jaundice 217 P. J. Johnson

vii

Contributors

ix

1. The uses of biochemical data in clinical medicine i William J. Marshall 2. The acquisition of biochemical data William J. Marshall

13. Acute and chronic liver disease P- 7- Johnson 14. Diabetes mellitus Simon Coppack

7

3. The interpretation of biochemical data William J. Marsha» 4. Sodium, water and potassium Michael Penney

15. Hypoglycaemia Mourad Labib

15

17. Adrenal disorders

5. Hydrogen ion homoeostasis, tissue oxygenation and their disorders 61 William J. Marshall 87

8. Protcinuria 143 Peter Gosling 9. Renal tubular disorders and renal calculi Stephen K. Bangert

11. Malabsorption 199 Stephen K. Bangert

281 295

315

P. L Drury 18. Thyroid dysfunction J. Butler and R. Pope

3 31

19. Hormonal aspects of non-endocrine disease Edward Hillhouse

7. The kidneys, renal function and renal failure William J. Marshall

10. The clinical biochemistry of nutrition Stephen K. Bangert

257

16. Hypothalamic and pituitary disorders Trevor A. Hoxoletl

25

6. Calcium, phosphate and magnesium T. Cundv and I. Reid

237

163

173

117

20. Abnormal sexual development leuan A. Hughes

355

375

21. Abnormalities of reproductive function 393 John Waterstone, John Parsons, John Miell and Richard Ross 22. Pregnancy, oral contraception and hormone replacement therapy 413 R. Bradley and D. Crook 23. Paediatric clinical biochemistry 423 Philip Mayne and Maggie Hancock

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CONTENTS

24. Biochemical aspects of haemolysis H. Hambley and A. Ixstas

453

25. Disorders of haem synthesis and the porphynas B. Adriaam

467

26. The haemoglobinopathies 483 Mark Layton, Roopen Arya and Alastair Beliingham 27. Paraproteinaemias P. Riches

493

28. Metabolic bone disease 7'. Cundy and I. Reid

36. Therapeutic drug monitoring M. J. Stewart

641

37. Poisoning 659 John Henry 38. Metabolic effects of tumours 679 E, W. Hillhousc and T. Loughlin

507

29. Biochemical aspects of articular disease Pauline Pitt

35. Lipid metabolism, hyper- and hypolipidaemias and atherosclerosis 621 С, B. Marenah

39. Tumour markers 705 Graham H. Beastail and И. Jean McAllister

533

40. Cellular aspects of clinical biochemistry A. Fleck and Af. Afym

30* Muscle disease 543 D. M. Tumbull and L. A. Bindoff

41. Molecular clinical biochemistry Af. Norman

31. Investigation of cercbrospinai fluid 557 R, Beetham 32. Biochemical aspects of psychiatric disorders Af. Murphy 33. Biochemical aspects of neurological disease J. Gareth Uexvdyn

569 587

34. Biochemical aspects of mental handicap 599 Elizabeth F. Aiarshall and Stephen P. Tyrer

717

739

42. Free radicals 765 Roberta J. Ward and Timothy J. Peters 43. Clinical biochemistry in the investigation of acute chest pain and the acute abdomen 779 James Hooper and Stephen K. Bangert Index

789

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Preface

Clinical biochemistry encompasses the use of biochemical techniques both in the study of fundamental disease proc­ esses and in the diagnosis and management of disease. It impinges on every medical and surgical speciality, and as research increasingly reveals a biochemical basis to disease, so there is an increasing requirement for the development of simple but reliable biochemical tests to diagnose, monitor, screen for and detect the complica­ tions of disease. Medical and surgical practice has become increasingly specialised. Most physicians are no longer general physi­ cians but chest physicians, neurologists, gastrocnterologists (even hcpatologists or 'hollow organ* gastrocnterologists), etc. Similarly, many surgeons specialise exclusively in, for example, urology, orthopaedics or neurosurgery» while even general surgeons tend to have their specialist inter­ ests. Despite this, it remains important that the specialist remains conversant with general medicine or surgery. Specialisation in pathology has resulted in the demise of the 'general pathologist', who was as adept at perform­ ing an autopsy as doing a Gram's stain or a blood glucose estimation. But although there is specialisation even within the pathology specialities, so that clinical bio­ chemists may have particular expertise in, for example paediatric or toxicological biochemistry, the practice of most clinical biochemists covers the whole range of medical and surgical specialities. A request for a bio­ chemical test is in effect a request for a clinical opinion on a patient, and clinical biochemists, be they medically or scientifically qualified, must be in a position to under­ stand the reason for the request and the use to which the result will be put as well as being able to ensure that the test itself is performed to a high standard.

There are several excellent textbooks devoted to the analytical aspects of clinical biochemistry. At a post­ graduate level, comprehensive coverage of the metabolic and clinical aspects of clinical biochemistry is more difficult to find. Specialist textbooks, reviews and original papers are all important sources of information, but we believe that there is a place for a single textbook covering the metabolic and clinical aspects of the whole of clinical biochemistry. For doctors and scientists, learning is a life-long process which does not stop after qualification or even after postgraduate examinations. We hope that this book will be of value both to people studying for examinations in clinical biochemistry (for example, MSc degrees and the Membership of the Royal College of Pathologists) and to clinical biochemists in career posts. The authors of individual chapters have contributed their specialist knowledge in their respective fields and as editors we have been fortunate in that they are also good writers. We have endeavoured to ensure comprehensive coverage of the individual topics to a similar level, a con­ sistency of style and minimum of overlap - all perennial headaches for the editors of multi-authors books - without destroying the character of individual contributions. We have opted to provide short, annotated lists of 'Further Reading* rather than for each chapter to be fully referenced. The bibliographies indicate the authors* 'best buys* for more detailed study. Information retrieval is now so simple that readers seeking more recent references should have no difficulty accessing them through their local libraries. We are grateful to our contributors for their chapters, their enthusiasm for the project, and their positive re-

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PREFACE

sponse to our editorial input. We have made friends along the way, and shared their vicissitudes. At the Eastbourne end of the editorial process, Gladys Stickler has provided unstinting secretarial support, whilst frequently lifting the spirits of the South lx>ndon editor with her good hu­ mour during many telephone conversations. At Churchill Livingstone, it was a pleasure to work with Dilys Jones, whose constant encouragement sustained us through many difficult periods, and whose editorial guidance has been exemplary. Graham Birnie took over the reins inside the final furlong, and saw us safely to the finish and into the unsaddling enclosure. Timothy Home, whose idea the book was, has been supportive throughout.

Wives and families seem always to be mentioned last, but if the last words that are read are the ones most likely to be remembered, that is appropriate here. Lorraine (Bangert) and Wendy (Marshall) have been nothing short of wonderful with their emotional and practical support throughout this project, and we dedicate this book to them. 1995

W.J. M S. K, B.

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Contributors

Beverley Adriaans MD FRCP Consultant Dermatologist, Brook General Hospital, London, UK

Simon W. Coppack BSc MD FRCP Senior Lecturer in Medicine, U C L Medical School, Whittington Hospital, London, UK

Stephen K. Bangert MA MSc MB BChir MRCPath Consultant Chemical Pathologist, Eastbourne Hospitals NHS Trust, Eastbourne, UK

David Crook PhD Laboratory Director, Wynn Institute for Metabolic Research, London; Honorary Lecturer, National Heart and Lung Institute, London, UK

G r a h a m H. В t us tall BSc PhD MRCPath Consultant Clinical Scientist, Institute of Biochemistry, Royal Infirmary, Glasgow, UK Robert Beetham PhD MCB FRCPaih Clinical Scientist, Frenchay Healthcare NHS Trust, Bristol, UK Alastair J. Bellingham FRCP PRCPath Professor of Haematoiogy, King's College School of Medicine and Dentistry, London, UK Laurence A, BindoffBSc MSc MBBS MRCP(UK) MD Senior Registrar, Middlesbrough General Hospital, Middlesbrough, UK Rob Bradley MD MRCOG MRCGP DCH Consultant Obstetrician and Gynaecologist, Royal Sussex County and Brighton General Hospitals,

Brighton, UK Joan Butler MA MSc Top Grade Biochemist, King's College Hospital, London; Honorary Lecturer, King's College School of Medicine and Dentistry, London, UK

Tim Cundy MA MD MRCP(UK) FRACP Senior Lecturer in Medicine, University of Auckland Medical School, Auckland, New Zealand Paul L. Drury MA FRCP Medical Director, Auckland Diabetic Centre; Physician, Auckland Hospital, New Zealand Adam Fleck BSc MBChB PhD FRCPath FRSC FRCP(G) FRS(Ed) Professor of Chemical Pathology, Charing Cross and Westminster Medical School (University of London), London, UK Peter Gosling PhD FRCPath Clinical Biochemist, Department of Clinical Biochemistry, Selly Oak Hospital, Birmingham, UK Henry Hambley BSc MBChB MRCPath Consultant Haematologist, Department of Haematological Medicine, King's College Hospital, London, UK Maggie Hancock BSc MSc PhD Biochemist, Department of Chemical Pathology, Charing

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CONTRIBITORS

Cross and Westminster Medical School, Chelsea and Westminster Hospital, London, UK J. A. Henry MB FRCP Consultant Physician, National Poisons Unit, Guy's and St Thomas' Hospital Trust, London, UK Edward W. Hillhouse BSe MBBS PhD FRCP Professor of Medicine, University of Warwick, Coventry, U K James Hooper BSc(Hons) MD MBBS MRCPath Consultant Chemical Pathologist and Honorary Senior 1-ecturer, University of London, Royal Brompton Hospital, London, UK Trevor A. Hewlett MD FRCP Consultant Physician and Endocnnologist, Leicester Royal Infirmary, Leicester, UK leuan A. Hughes MA MD FRCP FRCP(C) Professor of Paediatrics, University of Cambridge; Honorary Consultant Paediatrician, Addenbrooke's Hospital Trust, Cambridge, UK Philip J. Johnson MD FRCP Professor and Chairman, Department of Clinical Oncology, The Chinese University of Hong Kong, Prince of Wales Hospital, Shatin, Hong Kong; Honorary Consultant Physician, Institute of Liver Studies, King's College Hospital, London, UK Mourad H. Labib MBChB MRCPath Consultant Chemical Pathologist, Russells Hall Hospital» Dudley, UK D . Mark Layton MBBS MRCP(UK) Senior Lecturer and Honorary Consultant, Department of Haematological Medicine, King's College School of Medicine and Dentistry, London, UK A. N. Lestas BSc PhD CChem FRSC Top Grade Biochemist, Department of Haematological Medicine, King's College Hospital, London; Honorary Lecturer, King's College School of Medicine and Dentistry, London, UK J. Gareth Llewelyn BSc(Hons) MD MRCPCUK) Consultant Neurologist, Department of Neurological Science, Royal Free Hospital School of Medicine, London, UK Theresc Loughlin MBChB BAO Department of Oncology, Southampton General Hospital, Southampton, UK

E. Jean McAllister BSc DipCB Principal Biochemist, West Glasgow Hospitals University NHS Trust; Biochemistry Department, Gartnevel General Hospital, Glasgow, UK Christine B. Marenah BSc PhD MBBChir MRCPCUK) MRCPath Consultant Chemical Pathologist, Nottingham City Hospital NHS Trust, Nottingham, UK William J. Marshall MA PhD MSc MBBS FRCP FRCPath Senior I-ecturer in Chemical Pathology, King's College School of Medicine and Dentistry, University of London, London; Hon. Consultant Chemical Pathologist, King's Healthcare N H S Trust, London, UK Philip D . Maync MD MSc FRCPI FRCPath Consultant Chemical Pathologist, The Pathology Department, The Children's Hospital, Dublin, Ireland John P. Miell DM MRCP(UK) Lecturer in Endocrinology, King's College School of Medicine and Dentistry, University of I-ondon, I-ondon, UK Michael Murphy MBChB MRCP(UK) MRCPsych Consultant Psychiatrist, Queen Mary's University Hospital, London, UK M. A. Myers BSc PhD DipCB MRCPath Principal Clinical Biochemist, Department of Clinical Biochemistry, Northampton General Hospital N H S Trust, Northampton, UK Michael Norman BSc PhD MCB FRCPath Senior Lecturer in Medicine, University of Bristol,

Bristol, UK John H. Parson» MBChB FRCOG DA Senior Lecturer and Honorary Consultant, King's College Hospital, London, UK Michael D . Penney BSc MD FRCPath Consultant Chemical Pathologist, Royal Gwent Hospital, Newport, UK Timothy J. Peters PhD DSc FRCP FRCPath FRCP(Edm) Professor of Clinical Biochemistry, Head of Department and Sub-Dean, King's College School of Medicine and Dentistry, London; Director of Pathology and Honorary Consultant Physician and Chemical Pathologist, King's Healthcare (King's College Hospital), London, UK

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CONTRIBUTORS

Pauline I. Pitt MD MRCP(UK) Honorary lecturer, Department of Medicine, King's College Hospital, l.ondon; Consultant Rheumatologist, Farnborough Hospital, Oфinglon, UK Richard M. Pope MD MRCP(UK) Consultant Physician, Airedale General Hospital, Steeton, UK Ian R. Reid BSc MBChB MD FRACP Associate Professor of Medicine, University of Auckland Auckland, New Zealand Richard J. M. Ross MD MRCP(UK) Senior I-ccturcr in Endocrinology, Department of Medicine, Clinical Sciences Centre, Northern General Hospital, Sheffield, U K Pamela G. Riches BSc PhD FRCPath Director of the Protein Reference Unit, Royal Surrey County and St Luke's Hospital N H S Trust, Surrey, UK

xi

Michael J. Stewart BSc PhD MCB FRCPath Consultant Scientist, Drug Investigation Unit, Institute of Biochemistry, Glasgow Royal Infirmary, Glasgow, UK D . M. Turnbull MBBS MD PhD FRCP Professor of Neurology, Division of Clinical Ncuroscience, The Medical School, University of Ncwcasde upon Tyne, Newcastle upon Tyne, UK Stephen P. Tyrer MA FRCPsych LMCC DPM Consultant Psychiatrist, Royal Victoria Infirmary, Newcastle upon Tyne and Prudhoe Hospital, Northumberland, UK Roberta J- Ward MPhil PhD Senior Research Fellow, Unite de Biochimie, Catholique Universite de Louvain-Ia-Neuve, Louvain-la-Neuve, Belgium John J. Waterttone BSc MB MRCOG Senior Registrar, Greenwich District Hospital, London, UK

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C H A P T E R

The uses of biochemical data in clinical medicine William J. Marshall

INTRODUCTION The science of biochemistry is fundamental to the praaice of clinical medicine. Many diseases have long been known to have a biochemical basis and research in biochemistry is increasingly providing descriptions of pathological pro­ cesses and explanations for disease at a molecular level. As a result of the application of biochemical principles and techniques to the analysis of body fluids and tissues, clinicians have an extensive and still increasing range of biochemical tests that can be called upon to aid clinical decision making. Such tests can provide information vital to the diagnosis and management of many conditions, both those with an obvious metabolic basis (e.g. diabetes mellirus) and those in which metabolic disturbances occur as a consequence of the disease (e.g. renal failure). On the other hand, many conditions are successfully diagnosed and treated without recourse to any biochemical testing while there remain conditions in which it might be ex­ pected that biochemical tests would be of value but for which appropriate tests arc not yet available. For example, there are as yet no practical biochemical tests for the major affective disorders (see Ch. 32), although there is increasing evidence that biochemical disturbances arc involved in the pathogenesis of these conditions. The widespread availability of analysers, ranging from large automated instruments capable of performing multi­ ple biochemical tests on single serum samples to relatively simple instruments designed to measure only one or a few analytes, means that considerable quantities of clinical biochemical data can be made available quickly, reliably and economically. The ordering of a biochemical test is a simple procedure and there is no doubt that tests are often

requested automatically, without regard for their potential value in the specific clinical setting. Clinical biochemists decry this but do themselves no favour by their use of rhe term, widely employed by clinicians, 'routine tests' (referring to frequently performed tests) and even 'routine laboratories' (meaning the places where these tests are done). Ideally, tests should always be performed because there is a specific indication for them, that is, because it is anticipated that their results will provide information of benefit to the management of the patient. However, it cannot be denied that tests requested for no specific rea­ son can sometimes provide valuable information. Most clinicians are able to recall occasions when an unexpected result from a 'routine test' has provided the essential clue to the diagnosis in a difficult case. The potential range of investigations available to sup­ port the clinician is considerable, ranging, for example, from simple dip-stick tests on urine costing fractions of a pound to magnetic resonance imaging using equipment costing hundreds of thousands of pounds. There is an understandable tendency for clinical biochemists to think that biochemical tests are pre-eminent among special investigations. In some conditions they are, in others they have no role, while in many they are valuable when used in conjunction with the results of other investigations. The clinician should be aware of the whole range of investiga­ tions that are available, but needs also to be able to appre­ ciate their various advantages and limitations. The clinical biochemist, too, needs to be aware of the role of other investigations, so that he or she can view biochemical tests in context and advise on their suitability and the intcr-

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CUN1CAL ШОСШ-MIS'I'НУ

pretation of their results in specific clinical circumstances. It has been the editors" aim to ensure that this information is provided where relevant in this book. As will be discussed in this and the following two chap­ ters, the processes of acquiring, interpreting and using biochemical data are complex. A biochemical result on its own is of no value. It requires interpretation and proper interpretation requires that the reason for its being re­ quested is properly understood. It is to the potential uses of biochemical data in clinical practice that this chapter is devoted.

SPECIFIC USES OF BIOCHEMICAL TESTS Diagnosis It has been said that diagnosis in medicine is an art, not a science, yet the process of diagnosis is susceptible to scientific analysis. Making a diagnosis is the equivalent of propounding a hypothesis. A hypothesis should be tested by experiment, the results of which may confirm, lend support to or refute the hypothesis, which can then be extended, modified or discarded, as appropriate, in favour of an alternative. 1Ъс validity of a clinical diagnosis is tested by observation of the natural history or the re­ sponse to appropriate treatment, or by the results of definitive investigations; the diagnosis will be confirmed if these are as expected from knowledge of previous cases. If they are not, it must be reviewed. Clinical diagnosis is based on the patient's history and clinical examination. Taking general and hospital practice together, it has been estimated that in more than 80% of cases, a confident diagnosis can be made on the basis of the history or the history and clinical findings alone. Even when this cannot be done, it should be possible to formu­ late a differential diagnosis, that is, a list of diagnoses that could explain the clinical observations. The results of investigations may then lead to one of these being con­ sidered the most likely and providing a rational basis for treatment. Subsequent observation will indicate whether the diagnosis was correct. Although not necessarily required for the management of an individual patient, it may be possible to extend the clinical diagnosis by further investigation to determine the pathogenesis of the condition and ultimately its under­ lying cause. For example, measurement of plasma enzyme activities and an electrocardiogram may confirm a clinical diagnosis of myocardial infarction in a patient with chest pain; angiography could be used to demonstrate coronary atherosclerosis (although it is not necessary unless surgery or angioplasty is being considered); the finding of hypercholesterolaemia would indicate a causative factor for the atherosclerosis and a family history of premature heart disease would suggest that the hypereholesterolaemia was

familial. Familial hypereholesterolaemia is known to be due to a decrease in the normal number of low density lipoprotein receptors on cell surfaces and a number of mutations in DNA have been identified, any of which can be responsible. The ideal diagnostic test would be 100% sensitive (all cases of the condition in question would be correctly diag­ nosed using it) and 100% specific (no individual without the condition would be wrongly diagnosed as having it). The concepts of specificity and sensitivity arc examined fully in Chapter 3. In practice, the capacity of biochemical tests to provide precise diagnostic information is ex­ tremely variable. At one end of the spectrum, the tech­ niques of genetic analysis arc making it possible reliably to diagnose inherited metabolic diseases in urero; at the other, to take just one example, a decrease in plasma sodium concentration can occur in many different conditions and is on its own diagnostic of none of them. Molecular genetic analysis is a special case of the use of biochemical tests for diagnosis. It is used to detect the presence of a mutation responsible for a specific disease. Even when possession of a mutation does not inevitably result in the development of a disease» it can indicate in­ creased susceptibility to a condition. However, although molecular genetics is a rapidly developing field, many genetically determined conditions, including inherited metabolic diseases, are still diagnosed on the basis of their biochemical phenotype. With the exception of genetically determined diseases, the number of conditions in which biochemical tests alone provide a precise diagnosis is very small- There are several reasons for this. Biochemical changes are often a conse­ quence of a pathological process which can be common to many conditions. Thus although tissue destruction leads to the release of intracellular enzymes into the plasma, few such enzymes are specific to any one tissue; further, tissue destruction can occur for many reasons, for example, ischaemia, exposure to toxins, etc. It also frequently happens that a biochemical variable can be influenced by more than one type of process. T o cite a familiar example, the plasma albumin concentration can be influenced by changes in the rates of synthesis and degradation of the protein, and by changes in its volume of distribution, and the rate of synthesis in turn depends on substrate supply and hepatic function among other factors. Even when a biochemical change is specific to one condition, it may not indicate its cause and this may need to be established before the condition can be treated appropriately. For example, the demonstration of a high plasma concentra­ tion of the thyroid hormone, tri-iodothyroninc, is charac­ teristic of hyperthyroidism but this can be a result of several different thyroid diseases and treatment appropri­ ate for one of these may not be appropriate for another. When a biochemical test is used for diagnosis, the

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BIOCHEMICAL DATA IN CLINICAL MEDICINE

result obtained in the patient will usually have to be com­ pared with a reference range, that is, the range of values that can be expected in comparable (e.g. for age and sex) healthy individuals. T h e theory of reference ranges is dis­ cussed further in Chapter 3 but two points require particular emphasis here. First, the natural variation of biochemical parameters such as the concentrations of constituents of the plasma is likely to be less in an individual than in a group (even if well matched to the individual). Secondly, for many bio­ chemical variables, there is overlap, often considerable, between the range of values seen in healthy individuals and those characteristic of disease. Thus a test result in a patient with a disease may fall into the range typical of healthy people and vice versa. This overlap stems in part from the faci that some organs have considerable reserve capacity. The liver, kidney, pancreas and small intestine exemplify this. For example, in renal failure, renal func­ tion may still be sufficient to maintain normal homocostasis with respect to body fluid composition even when half the functional capacity of the kidneys has been lost. It should not therefore be surprising that simple measure­ ments of function can yield normal results in patients with renal disease. In chronic pancreatitis, biochemical evi­ dence of functional disturbance (e.g. of malabsorption) usually only becomes apparent when at least 80% of the functional capacity is lost, although the characteristic severe pain often occurs at an earlier stage. Similarly, disease of the small intestine by no means always results in malabsorption. When previous measurements are available in an indi­ vidual, test results can be compared with these values, rather than with a reference range. Indeed, biochemical tests are sometimes made to provide a 'baseline' against which to assess future results, particularly if there is risk of a particular complication developing or if a change can be anticipated from the natural history of the disease or the expected response to treatment. A change in a bio­ chemical variable in relation to a previous result may be of significance even if both results are within the reference range. The capacity of a biochemical test to provide diagnostic information can be quantified by the calculation of a mathematical function known as the predictive value. As will be discussed in Chapter 3, the predictive value of a diagnostic test depends on the prevalence of the condition in the group of people to whom the test is applied. If a diagnostic test is used indiscriminately, its predictive value will be low. The majority of biochemical tests made for diagnostic (and, indeed, for other) purposes involve analysis of plasma or serum, but changes in the concentration of analytcs in these fluids do not necessarily parallel changes in intracellular or whole-body content, either in quantity

3

or in their time course, and yet it may be these quantities that arc more relevant to the underlying pathology. Finally, whatever purpose biochemical data are used for, it is essential that they are reliable and are available in time to be of use. Under some circumstances, it may be permissible to sacrifice some quality in order to obtain a result rapidly but in general every attempt should be made to minimize the influence of both analytical and prcanalytical factors on the accuracy and precision of data. This topic is considered further in Chapter 2.

Management Assessment of disease severity Most biochemical tests are quantitative and the more ab­ normal a result is, the greater is likely to be the pathologi­ cal disturbance causing it. Often, the extent to which a test result is abnormal correlates well with the severity of a condition but this is not always the case. The diagnostic test may not reflect that aspect of the condition of greatest importance in terms of severity; thus two patients with hepatitis may have equally raised plasma aminotransferase activities (reflecting tissue damage) but the condition will be judged more severe if in one the prothrombin time is prolonged (reflecting impaired hepatic functional capac­ ity). Furthermore, overall disease severity (in relation to its effect on the patient) is likely to depend on many other factors, including the nature of the condition itself, the patient's age, previous state of health, the existence of other illness, etc.

Prognosis In general, the results of biochemical tests are poor indi­ cators of prognosis but there arc exceptions to this. For example, the plasma bilirubin concentration at the time of diagnosis in patients with primary biliary cirrhosis cor­ relates well with outcome; a high plasma concentration of alphafetoprotcin in a patient with tcsricular tcratoma is of prognostic significance but the concentration of paraprotein in a patient with myeloma is not. Other examples are discussed in the ensuing chapters.

Monitoring the progression of disease Although biochemical data alone are of limited use in diagnosis, serial measurements are of considerable value in monitoring the natural history of a disease or its response to treatment. The more closely the variable(s) being measured relates to the underlying pathological process or functional abnormality, the better it will be for this purpose. However, the reason for a change in a biochemical

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CLINICAL BUK:HEMISTRY

variable is not always the most obvious (or hoped for) so that even if an observed change is as expected or desired, the result should be interpreted with care. For example, a decrease in urine protein excretion in a patient with glomerular disease may indicate resolution of the under­ lying condition» but it can also be a result of deterioration leading to a decrease in the glomerular filtration rate. Biochemical data must always be interpreted in the light of clinical assessment and the results of other relevant investigations, not in isolation. Nevertheless, intervention may sometimes be appropriate on the basis of a bio­ chemical change alone, if this has been shown reliably to predict a significant clinical change. This is the principle on which biochemical screening tests for specific disorders are based. It is also true in other circumstances, for exam­ ple, in hyperkalaemia in a patient with renal failure. When serial biochemical measurements are used to follow the response to treatment, the failure of an ex­ pected change to occur may suggest that the treatment is inadequate or inappropriate. In therapeutic drug monitor­ ing (TDM, see Ch. 36), biochemical measurements may actually indicate a possible cause for non-response to treatment. Biochemical tests can also be used to detect the devel­ opment of complications of diseases or their treatment before these become obvious clinically and thus allow ap­ propriate action to be taken before there is any clinical deterioration. They may even be used to prevent compli­ cations: for certain drugs, T D M allows presymptomatic detection of potentially toxic concentrations of the drug. Screening Screening for disease implies attempting to detect disease before it becomes manifest through the development of a clinical disturbance. Inherent in the concept of screening is that appropriate management of subclinical disease is of potential benefit to the patient. Screening can involve clinical assessment, laboratory and other tests. For some conditions (particularly inherited metabolic diseases), screening may involve a single biochemical test. But the term is also used in relation to the performance of a battery of biochemical tests (often combined with other types of investigation) in healthy people in an attempt to detect any of a number of conditions, in the belief that a set of 'normal* results - that is, within the appropriate reference limits - excludes these conditions. As will be seem in Chapter 3, considerable care is required both in the devising and in the interpretation of the results of screening tests. While a set of'normal 1 results may appear reassuring and may indeed exclude the presence of certain diseases, they may convey a false impression since early disease will not always be identified; and the diagnosis of a condition when symptoms develop may even be delayed

if it was deemed to have been excluded by the earlier demonstration of 'normal' results. Because of the way in which reference ranges are defined, the more tests are performed, the more likely it is that an 'abnormal* result (i.e. outside the reference limits) will be generated which is not related to the presence of disease. A screening lest on its own should not usually be re­ garded as being diagnostic. When the prevalence of a condition in the population being screened is low, the predictive value of a positive result is often lower than is generally supposed. Diagnoses made on the basis of screening tests must always be confirmed by further inves­ tigation. The use of direct methods (e.g. oligonucleotide probes, see Ch. 41) to detect mutations in DNA is an exception to this. Properly conducted, they are definitive with regard to the detection of mutations, although not necessarily (for the defect may be recessive or penetrate incompletely) for the development of disease. Screening may be applied to a population, to groups within a population sharing a common characteristic or to individuals. According to the nature of the condition in question, screening may be carried out antenatally, shortly after birth, during childhood or during adult Life. The strategy adopted will depend on the risk of the condition, the probability of its presence, the availability of suitable screening tests and, inevitably, the cost - particularly the economic cost of the programme but also the personal cost to individuals, for example those who test 'false posi­ tive* (that is, who are detected by the programme but on further investigation are found not to have the condition in question) and, with inherited diseases, the relatives of individuals detected by the programme. Population screening Economic and logistic considerations preclude the screen­ ing of whole populations for disease, although it has been advocated, for example, that all adults (some suggest only males, others males and females) should be screened for hypercholesterolaemia. Although this would undoubtedly lead to the identification of a significant number of indi­ viduals at greatly increased risk of coronary heart disease because of severe but asymptomatic hypercholesterolae­ mia, such a programme, however desirable, would be very costly and it has been argued that resources would be better devoted to measures to improve the general health of the population, by encouraging a healthy diet and lifestyle. Selective screening Selective biochemical screening for disease is already practised extensively in developed countries. The neonatal screening programmes for phenylketonuria and congenital

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BIOCHEMICAL DATA IN CIJNIC-AL MEDICINE

hypothyroidism are the best known examples. These com­ plement the thorough clinical screening of the newborn for conditions such as congenital cataract, imperforate anus, etc. Although it would be possible to devise bio­ chemical tests to screen the newborn for many severe inherited metabolic diseases which are amenable to treat­ ment, the rarity of most of these conditions and hence the cost of the programme in relation to the benefit precludes this being done. However, where a condition is particularly common in a defined group, screening may be appropriate even though it would not be for the population at large. Ante­ natal screening for Tay-Sachs disease in Ashkenazi Jews is one example. For hypercholesterolacmia, selective screening is a more practicable procedure than population screening. It can be applied to people in whom there is a high probabil­ ity of hypercholesterolaemia being present, for example members of families in which there is a history of hyper­ cholesterolaemia or premature coronary heart disease. Such screening can also be directed towards people al­ ready at increased risk of coronary heart disease because, for example, they are smokers or have hypertension or diabetes, whose risk would be increased further by hyper­ cholesterolaemia. Other examples of selective screening are discussed in the relevant chapters of this book. Individual screening Examples of individual screening include antenatal screening of a fetus for an inherited disease when a pre­ vious child of ihe parents has been found to have the condition or when there is a strong family history of the condition. This has been practised for some time for certain inherited diseases, but the number for which it can be done will grow rapidly now that the mutations respon­ sible for inherited diseases are becoming known. Although undoubtedly it will become possible successfully to treat some of these conditions in utero, at the present time ante­ natal screening is mainly aimed at detecting conditions whose consequences are so severe that it is considered appropriate to terminate the pregnancy if the genetic abnormality is present. Given this possible outcome, it is clearly essential that if the diagnosis is to rest only on the result of the screening test, this should provide accurate and unequivocal results. Other uses of biochemical tests All the uses of biochemical tests that have been discussed thus far arc potentially of direct benefit to the patient. Other important uses include the provision of information for teaching and research. Usually, this will relate to one of the categories discussed. Although such data may not

5

be of immediate benefit to the patient, both these areas are of immense potential benefit, providing as they do information fundamental to the advance of knowledge. The use of biochemical tests to assess organ function in potential transplant donors is an example of the use of tests entirely for the benefit of other people. Extensive biochemical tests are usually carried out during trials of drugs; these may be required as pan of the assessment of their efficacy but are also essential for the detection of possible toxicity. Investigations may also be performed for the benefit of the doctor rather than the patient. Few doctors have not been guilty at some time of requesting biochemical tests for reassurance. The supposition is that if a battery of test results are within reference limits, then ihe condi­ tions in which abnormalities are known to occur cannot be present. As has been emphasized above, this sup­ position is erroneous and any reassurance may be un­ founded. Biochemical tests should be requested for one of the reasons discussed in the relevant chapter and not 'routinely 1 . Neither should junior medical stafT be put under pressure to request unnecessary tests to placate their seniors. It is regrettable that there is an increasingly perceived need for doctors to carry out a comprehensive range of investigations in case of subsequent litigation. Whilst this is understandable, it should not be necessary if investiga­ tions are requested and performed in response to the individual clinical circumstances. There will always be investigations which could have been done but no blame should attach to a doctor who failed to carry one out if it was not indicated clinically, cither on the basis of the known natural history of the disease or the predicted response to and known complications of treatment. CONCLUSIONS Biochemical data arc used extensively in medicine, both in the management of patients and in research. But before a test is requested, the rationale for doing this should always be considered. Automated analysers can perform many tests at a very low cost in relation to the total ex­ penditure on health care, but the cost is not negligible. There may also be a cost to the patient. Repeated venepunctures to obtain blood for 'routine* tests are at best a nuisance and at worse can, particularly in children, cause a significant fall in the haematoerit. The laboratory hand­ book at one hospital of the author's acquaintance con­ tained the following advice to junior medical staff: 'If you need advice or time to think, ask for it; do not ask for a full blood count and measurement of "urea and electrolytes" \ In common with other investigations, biochemical tests should be performed to answer specific questions; if there is no question, a test cannot provide an answer.

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6

CUKICA1. BKK-HI'MISI RV

FURTHER READING A&hcr R. In' Jone* H A (cd) Richard Ashcr talking seme. Ixmdon: Pitman Medkal, 1072 A coUeetwn of essay* by a clear thinking physician, his itbservations on the use of common sew* м medwsne, including the use of the laboratory.

Hraser С G. Interpretation of clinical chemistry laboratory data. Oxford: Blackwell Scientific Publications, 1986. A concise but comprehensive account of the user of laboratory (Lita, their acquisition and interpretation, relevant to this and the succeeding ttiw chapters, ivhieh should be required reading for clinical buxhewists.

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C H A P T E R

The acquisition of biochemical data William J. Marshall

INTRODUCTION It has been emphasized in the preceding chapter that all investigations in medicine should be performed to answer specific questions. Only if it is known why an investigation was performed can the value of the result be adequately assessed. \ \ Ъ с т с г data are to be used for diagnosis, management, screening, etc., it is essential that they arc appropriate, reliable, available in time to be of use, provided economically and interpreted correctly. Action should then follow, the outcome of which will in turn vali­ date the data and their use in that context. The achieve­ ment of this goal requires careful attention to every step in the process from the ordering of the test, through the analysis, to the delivery of a report to the clinician, appro­ priate action being taken and the effects of this action being assessed.

T H E T E S T REQUEST A test request is generated by the completion of a form, either manually or through a computer terminal, which prompts the collection of the appropriate sample and essentially instructs the laboratory on the test(s) to be performed. According to the reason for the request, the expertise of the clinician and the practice of the labora­ tory, the request may simply be for one or more specified analyses on a body fluid; for a more involved procedure such as a dynamic function test involving the collection of serial samples following a specific stimulus; or an open request to perform whatever tests are deemed appropriate by the laboratory staff to answer the question posed in the

request. The majority of biochemical test requests fall into the first category although with the trend towards the pro­ vision of medical care in the community, an increasing number are in the third. The information that must be given on a test request form is summarized in Table 2.1.

Table 2.1

Information required on a

etcd I

icst form

К

Information Patient's name unit number date of birth

Deification and (age. яех)

jntcrprwgtipo of m a i n

Return address (e.g ward, clinic, surgery; tclephone/pagB number if urgent

Delivery of report

Name of clinician tend telephone/page number)

liaison Audi! Hilling

meal treatmei

la 'including drug

Justification of request Audit Interpretation tion of appropriate tests oiceofan.i hod U> avoid drug interference)

T e n requested

In>

Sample(s) required

Instruction to phlcbotomret

Date (and time if арргоргЬ

Idcntincat*

hr

itm to analyst

ion (frith

timed/sequential requests)

^ M

8

CUNICAl. BIOCHEMISTRY

FACTORS AFFECTING T E S T RESULTS The generation of biochemical data is subject to potential error at every stage in the process. It is essential that the sources of error are identified and understood, so that their effects can be minimized. The sources of errors in biochemical tests arc conven­ tionally described in three categories: • prcanalytical, that is, either outside or within the laboratory but before the analysis is performed; • analytical, and • postanalytical, that is, during data processing or transmission, or in relation to the interpretation of the data. PREANALYTICAL FACTORS Prcanalytical factors may appear to be outwith the province of the clinical biochemist unless he or she is re­ sponsible for collecting the specimen, but it is the duty of laboratory staff to ensure that clinicians understand the problems that can arise, so that samples arc collected and transported appropriately. Prcanalytical factors fall into two categories: those which relate directly to the patient (biological factors) and those which relate to the specimen obtained for analysis (technical factors). Technical factors These include: • correct identification of the patient; • appropriate preparation of the patient where necessary; • collection of the sample required into a container with, where necessary, the correct preservative; • accurate labelling of the specimen container, and

details on the request form. In most laboratories, both specimen container and request form arc labelled with a unique number (the laboratory number) at this stage, after which they may be separated for further processing (usually analysis of the specimen and transfer of data from the request form to a worksheet or a computer database). When the specimen has to be divided before analysis, an aliquot taken or plasma/serum separated from whole blood, it is essential that the secondary container is posi­ tively identified, preferably by giving it the same number as the parent specimen. All these processes are facilitated if an integrated computer system allows the request form to be generated on the ward from information held in a central database, to which the laboratory has a direct link. Laboratories should have written protocols for the re­ ception and handling of all specimens to minimize the chance of error. Biological factors Numerous factors directly related to the patient can influ­ ence biochemical variables, in addition to pathological processes. They can conveniently be divided into endo­ genous factors, intrinsic to the patient, and exogenous fac­ tors, which arc imposed by the patient's circumstances. They are summarized in Table 2.2. In addition, all biochemical parameters show some intrinsic variation, tending to 'hunt' around the homocostatic set point for the individual. Endogenous factors Age The reference values for many biochemical vari­ ables do not vary with age, but for those for which ageTable 2.1 Some biologic»! factors effecting biochcnv Factor

mahlct

Example

• secure transport to the laboratory

f€nom

Care must be taken during specimen collection to avoid contamination (e.g. of blood with fluid being in­ fused intravenously or tissue fluid when capillary blood is being collected), haemolysis of blood or haemoconcentration (due to prolonged application of a tourniquet). Appropriate precautions are also required during the collection and transport of urine, spinal fluid, tissue (e.g. biopsy samples), etc. Biological material from patients is potentially infective and must be handled safely during collection, transport and analysis. Specimens known to be infective (e.g. from patients carrying the hepatitis В virus or HIV positive) arc usually handled specially, but it is good practice to handle all specimens as if they were potentially hazardous. On receipt in the laboratory, the patient's name and unit number on the sample must be checked against these

Ajtc Sex mu Time Strc«> Poeture

< taltftWOl Alkaline phosphatsse Urate Gotudotmphms Gonadal ttcrotds 'gtyceridei Cortiaol (daily) Gofiadotroptin women, catamcnul 2Vhydr i-imin I) (teaaonal) OProlactm Catecbolamine*

Rental Aldottcrone Protein»

Food intake

n

Increased synihcw binding proteins

Increased plasma Phenytotn. y-fdutamvltransferase phenobarbitone, al>

Increased enzvme synthesis ( о м у т е inductio

completeness), for example by inhibiting the generation of a signal or by crossreacting with the analytc in question and giving a spuriously high signal. This Field is well documented, bui the continuing introduction of new drugs means that new examples of this source of error arc continually being described. Other factors Exercise can cause an increase in plasma creatinc kinasc activity and may confound the di­ agnosis of myocardial infarction in a patient who develops chest pain during exercise. Even uncomplicated surgery may cause an increase in this enzyme, as a result of muscle damage, and tissue damage during surgery can also cause transient hypcrkalaemia. Major surgery and severe illness can elicit the 'metabolic response to trauma* (sec Chs 38 and 40) which can lead to changes in many biochemical variables. Intrinsic biological variation As mentioned above, the levels of analytes also show random variation around their homoeostatic set points. This variation contributes to the overall imprecision of measurements and must be taken into account in the interpretation of test results. It can be measured by col­ lecting a series of specimens from a small group of compa­ rable individuals over a period of time (typically several weeks) under conditions such that other sources of varia­ tion are minimized. The specimens arc handled identi­ cally and stored in such a way as to prevent degradation of the analyte. The specimens are then analysed in duplicate in a single batch. The analytical imprecision (see below) can be calculated from the differences in the duplicate analyses and is given by: ifferenccs)' SDA=

\

Jn

where n is the number of pairs of data. The standard deviation of a single set of data from each individual is then calculated. This will encompass both analytical variation and the intraindividual variation (SD,), such that:

защ1

чны.

-им пр

ACQUISITION OF HIO< НКМ1СЛ1. DATA

SD=

SD A 2 + SD*

Since SD A is known, the intraindividual variation can then be calculated. It is also possible to calculate the interindividual variation (SD (i , due to the difference in the individual homoeostatic set points for the analytc between each member of the group) by calculating the SD for all single sets of data for all subjects in the group, since this SD is given by: SD=

S D . 2 + SD. : + S D , \ A

I

It

These relationships arc illustrated in Figure 2.1. Some typical values for biological variation are given in Table 2.4. It should be noted that while in most instances the interindividual variation is greater than the intra­ individual variation, this is not always the case. When it is not, it means that the extent of natural variation around individuals' homoeostatic set points is more than the range of variation between these set points. The relative sizes of mtra- and interindividual variation have important conse-

.4 Г lnicnndmJual ■ia expressed a» cocfttcn

intraindrvulual and UCtll

m m o n |
0|EM1.STRY

represents an attempt to address these shortcomings. Essential to the concept of predictive values is that disease or freedom from that disease can be defined absolutely, i.e. that there is a test which can be regarded as the 'gold standard'. For some conditions, this will be histological examination of tissue obtained at surgery or post mortem, but it may be the eventual clinical outcome or some other more or less well defined endpoint. It is against the 'gold standard' that biochemical tests may be judged.

The sensitivity of a diagnostic test is defined in this context as the number of true positives in all individuals with disease. Thus: Sensitivity = TP/(TP + FN) T h e specificity of a diagnostic test is defined as the number of true negatives in all individuals without disease. Thus: Specificity ■ T N / ( T N + FP)

Definitions If all the member» of a population consisting of people with and without a particular disease are subjected to a particular test, it will be divided into four categories: • true positives (TP) - individuals with disease, who test positive; • false positives (FP) - individuals free of disease, who test positive; • true negatives (TN) - individuals free of disease, who test negative; and • false negatives (FN) - individuals with disease, who test negative. Clearly the number of individuals with disease equals (TP + FN) and the number without (TN + FP). The total number of positive tests is (TP + FP) and of negative (TN + FN). These data can conveniently be arranged in a matrix (Fig. 3.4). It is then easy to derive the other important parameters relating to test performance, that is, pre­ valence, sensitivity, specificity, predictive value and

efficiency. The prevalence of a disease is the number of indi­ viduals with the disease expressed as a fraction of the population. Thus: Prevalence ■ T P ♦ FN/(TP + FN + T N + FP) ■

It is often expressed as a percentage. Note that this is not the same as the incidence of a disease, which is the number of persons who develop the disease during a stated period of time. Test result Positive ) Negative

The predictive value of a test is a measure of its ability to allocate individuals correctly to cither the disease or non-disease category. It is given by the fraction of test results that are true results. Thus the predictive value of a positive test is given by: PV(+) = TP/(TP + FP) and the predictive value of a negative test by: PV(-) ■ T N / ( T N + FN) The efficiency of a test is a measure of its ability correctly to assign people to both the disease and the non-disease category. "I*hus: Efficiency: (TN + TF)/(TP + T F + F P + FN) These values are usually expressed as a percentage by multiplying by 100. It will be seen that while, for example, both sensitivity and the predictive value for a positive result are measures of positivity, with sensitivity, positivity is being related to the number of individuals with the condition whereas pre­ dictive value is related to the number of positive tests. Even if a test is very sensitive (the false negative rate is low), the predictive value of a positive test may not be as high as might be expected because it is decreased by false positive results. The predictive value model examines the performance of a test in defined circumstances. Clearly, it depends upon being able to determine the numbers of true and false positive and negative results, which depends upon there being an independent, definitive diagnostic test. The importance of the independence of the test under investi­ gation from the definitive test may appear self-evident but in practice, it is frequently overlooked.

Total

Example Disease status

FV)$Hive Negative

Tow

TP FP TP+FP

FN

TP+FN

TN

TN-fFP

TN+FN ' TP+TN +FP+FN

Fig. 3.4 A mainx for dusMfving test results: TP = tnie positive, TN = true negative, FP = false positive and FX = false negative.

These concepts, and some of the pitfalls inherent in their use, can be illustrated with reference to a hypothetical example. Suppose that it is desired to assess the value of the measurements of serum y-glutamyltransferase activity to diagnose alcohol abuse in patients attending a drug de­ pendency clinic. It is decided that the test will be regarded as positive if the y-glutamyltransferase activity exceeds the upper reference limit.

ч

ATION OF BIOCHEMICAL ПАТА

Over the course of a year, 200 patients are tested; enzyme activity is found to exceed the chosen limit in 73, but only 62 are judged to be abusing alcohol on the basis of a rigorous questionnaire. In 18 individuals judged to be abusing alcohol, enzyme activity is not elevated. "1Ъе results matrix can thus be completed: 1 Disease status

Test i «suit Positive Negative

Totals

Positive

TP = 62

FN-18

TP i FN = 80

Negative

FP - И

TN - 109

FP + T N - 1 2 0

TP + FP -73

Tote*

TN »FN j » 127

This example illustrates two important points about the predictive value model. The first is that sensitivity and specificity lend to vary inversely; second, appropriate selection of the criterion for positivity or negativity (here the level of enzyme activity) allows either one to be maxi­ mized. If the cut-off is set very high, positive results will only occur in individuals with disease; there will be no false positives and specificity* will be 100% although sensi­ tivity will be low. On the other hand, if the cut-off is set low, no cases will be missed but the false positive rate will be high; sensitivity will be 100% but specificity will be low. This is illustrated in Figure 3.5. Whether it is desirable to maximize sensitivity, speci­ ficity or efficiency* depends on the nature of the condition being investigated, as discussed below.

TP t FP ♦ I TN + FN - 200 ^

—

Calculation then shows that:

Prevalence a n d predictive value

Prevalence = ( 6 2 + 1 8 ) / 2 0 0 Sensitivity = 62/(62 + 18) Specificity = 1 0 9 / ( 1 0 9 + 11) = 6 2 / ( 6 2 + 11) PV (+) = 109/(109 + 18) PV(-) Efficiency = (62 + 109)/200

= = = = = =

0.40 0.775 0.908 0.849 0.858 0.855

= = = = = =

40% 78% 91% 85% 86% 86%

While sensitivity and specificity' arc dependent on the characteristics of the test and the nature of the condition

Thus the test correctly identifies 7 8 % of alcohol abusers; further, if a patient has an elevated enzyme activity, there is an 8 5 % chance that he is abusing alcohol. However, it might be considered that a significant number of patients are misclassified and that better performance might be achieved if the cut-off value for a positive test was set at a higher enzyme activity, say twice the upper reference limit. The results matrix would then appear as follows: Test result Positive 1 Negative Disease f t St US

Totals

Positive

TP-50

FN - 30

TP * FN - 80

Negative

FP = 2

TN = 118

FP ♦ TN = 120

TP*FP -52

TN*FN • 148

TP + FP ♦ TN ♦ FN - 200

Totals

Analyte concentration

1 Negative test

1

/

N

°

Positrve

£ = = = = = =

(50 + 30)/200 50/(50 + 30) 118/(118 + 2) 50/(50 + 2) 118 (118 + 30) ( 5 0 + 118)/200

= = = = = =

0.40 0.625 0.983 0.961 0.797 0.84

= = = = = =

40% 63% 98% 96% 80% 84%

The effect has been to decrease the sensitivity of the test, which now only correctly diagnoses 6 3 % of alcohol abusers. On the other hand, if a patient tests positive, the probability that he is an alcohol abuser is now 96%. The overall efficiency of the test is little changed.

/

TN

lest

cut

К

1 disease

from w h i c h : Prevalence Sensitivity Specificity PV(+) PV( ) Efficiency

21

\

jFP\ Anatyte concent ration

i •

^*-TP I

Fig. 3.5 The effect of moving the cut-off point that determines posiuvity/ncgativity of a test result. Hypothetical distributions for the concentration» of an analyte with and without disease are shown. Because these overlap, if the cut-off is selected to decrease the number of false positive results (and hence increase specificity) (panel A), there arc a significant number of false negative results (decreasing the sensitivity). If the cut-off is set lower (panel B), false negatives are eliminated (maximizing sensitivity) but at the expense of increasing the number of false positives (and decreasing specificity). The distribution for individuals with disease has been shown below the axis for clarity.

22

CIUNKIAI. BIOOIl-MIXTRY

being investigated, predictive values are dependent on the prevalence of the condition in the population under examination. Consider the application of the measure­ ment of /-glutamyltransferase activity in a population in which alcohol abuse might be expected to have a lower prevalence and in which there is not likely to be an in­ crease in the number of individuals with other conditions associated with an increase in enzvme activity. The test might reasonably be applied, for example, to screening for alcohol abuse during pregnancy. If 500 women were screened and the cut-off taken as the upper limit of the reference range, then the results matrix might show;

Test

Disease status

Totals

Positive

Negative

Poskrva

TP - ! i

FN-7

ГР

• LN

:n

Nagaiive

F- Г

TN-^^B

F-

. TN

^e?

7N .

TF*FP •

43 j

gerated claims tor tests have been made on the basis of their performance in a highly selected group. In this example, sensitivity and specificity were as­ sumed to be the same in the two groups examined. This was an assumption made for convenience, but it docs not have to be the case and indeed may well not be. In choos­ ing people in whom to examine the performance of a new test, it is natural to do this in healthy individuals and pa­ tients with well established disease. Under such circum­ stances, the proportion of false positive and false negative results are likely to be lower than when the test is applied to a more heterogeneous sample. This again emphasizes the need to compare like with like and not to make unwarranted extrapolations from experimental data col­ lected under carefully defined or controlled circumstances to practical applications. Practical applications of the predictive value model

1

TP• FF -67

Total*

FNH

1 N т F,\

J00

from which: Prevalence Sensitivity Specificity PV ( + ) PV (-) Efficiency

= ~ = = = =

(24 + ?}/500 24/(24 + 7) 426/(426 + 43) 24/(24 + 43) 426/(426 + 7} (426 + 24V500

= 0.062 = 0.774 =0.008 = 0.358 = 0.983 = 0.90

= 6% = 77% =91% = 36% = 98% = 90%

The sensitivity and specificity are unchanged but because of the much lower prevalence of alcohol abuse in this group, the predictive value of a positive test is low; only 3b% of those with a positive test are alcohol abusers. At the same time, the predictive value of a negative test is higher than in the patients attending the drug abuse clinic; this is again a consequence of the lower prevalence in the pregnant women. It is instructive to note that the efficiency of the test appears greater in this context (90% in com­ parison to 84%) in the drug abuse patients). Overall the proportion of tests which correctly assign patients to the alcohol abuse or non-abuse categories is higher, although its performance in diagnosing abuse alone is less good. The effect of disease prevalence on predictive values is an important one. It means that the performance of a test in one group of patients cannot be translated to a different group. Although the predictive value of a new test may appear high in the typical experimental setting where equal numbers with and without disease are tested (the prevalence is 50%) it is likely to be much lower *in the field*. Thus the performance of a new test must be ana­ lysed In groups of individuals comparable to those in whom it is to be used in practice. In the past, many exag­

High sensitivity in a diagnostic test is required when a test is being used to diagnose a serious and treatable condi­ tion. The false negative rate (missed diagnoses) must be as low as possible, even though this inevitably means that false positives will occur. Individuals testing positive will be subjected to further, definitive tc*ts and provided that these occasion no harm to those who test positive and turn out not to have the condition, this is an acceptable price to pay for making the diagnosis in all those who have the condition. The programmes for neonatal screening for phenylkctonuria and other harmful conditions in the newborn exemplify tests requiring maximum sensitivity. In this context, it is instructive to note that although the sensitivity of the screening test for phenylkctonuria is 100%, and the specificity approaches this value, because the condition has such a low prevalence (approximately one in 10 000), the predictive value of a positive test is only of the order of 10%. High specificity (no false positives) is desirable in a test to diagnose a condition which is severe but unbeatable, when knowledge of its absence is potentially beneficial. Multiple sclerosis is frequently cited as the classic example of such a condition. High specificity is also required when it is desired to select patients with a condition for a trial of some new form of treatment. If individuals without the condition are included in the treatment group, the results of the trial (assuming that the treatment is in some measure effective and does no harm to those without the disease) may give a false view of its efficacy. As alluded to above, diagnoses are rarely made on the basis of single tests. Ideally, a single test or combination of a small number of tests should be used to identity' indi­ viduals in whom the probability of a condition is signifi­ cantly higher (the prevalence is higher) who can then be further investigated. The initial tests can be simple, cheap

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r\TERPRETATION OF BIOCHEMICAL DATA

23

and unlikely to do harm* but the further tests may be more elaborate, expensive and possibly associated with some risk. The predictive value model can be applied to multiple testing and the interested reader is referred to Galen and Gambino's seminal text for further discussion of this topic. In essence, if the performance of individual tests is known and the objectives of testing can be pre­ cisely defined, the predictive value model can be used to determine the appropriate sequence or combination of tests to achieve the desired objectiveReceiver operating characteristic (ROC) curves Another use of the predictive value model is to compare the performance of two tests. This can be done by deter­ mining sensitivity and specificity for each test using a range of cut-off limits to define positivity and plotting one rate against the other. The resulting curves are known as receiver operating characteristic curves, a rerm which was coined during the early use of radar. The reader is warned that four variants of ROC curves may be encountered, according to the way the data are projected on the axes, but this does not affect their interpretation. Figure 3.6 shows hypothetical ROCs for two tests, A and B, each applied to the same group of individuals to make the same diagnosis. It is clear that test A gives the better discrimination, since for any level of sensitivity, its specificity is superior. This mav be useful information but these are bv no means the only factors that may need to be taken into account. The economics and practicalities of the tests will also be of relevance to which one is chosen. *1Ъс interpretation of the ROC- curves in Figure 3.6 is straightforward. In practice the curves for two tests may cross; the relative performance must then include assess­ ment of the areas under the curves. The greater the area, the greater the efficiency. Summary The predictive value model can thus be used to determine the true significance of a positive or negative result and to select the appropriate cut-off value that determines whether a result is positive or negative, according to the clinical requirements. However, it does nor give any indi­ cation of the degree of abnonnality of a result. The model can also be used to define a logical sequence or combina­ tion of tests and to compare the performance of tests designed to achieve the same object. But it must be appre­ ciated that its use depends on being able to distinguish between the presence and absence of disease using an in­ dependent technique, which may not always be possible particularly since the distinction between disease and non-

Fig. *.6 Receiver operaunj; characteristic curves for ww test* being ii?«evtcd for the diagiios» of the name condition in the same patients. Test A gives better specificity al any lcveJ of sensitivity.

disease may not be clear cut. Also, it is only valid to apply its conclusions in circumstances similar to those in which the data leading to those conclusions were gathered. T H E USE OF COMPUTERS IN T H E INTERPRETATION OF BIOCHEMICAL DATA Computers have long been used for data capture, process­ ing and storage in clinical biochemistry. They are now increasingly being used in [he interpretation of such data. They can be used to perform simple logistic procedures such as decision analysis but also, in expert systems and neural networks, they are being used to analyse data so that they can directly inform patient management and re­ spond interactively to changes as they occur. This topic is beyond the scope of this book, but some useful references are included in the farther reading list for the interested reader. CONCLUSION The interpretation of biochemical data requires adequate knowledge of all the factors which can affect the test result. These include the physiological, biochemical and pathological principles on which the test is based as well as the reliability of the analysis. It is also necessary to understand the statistical principles which relate to the distribution of data in healthy individuals and those with disease and how the test performs in the specific circum­ stances in which it is used. Such knowledge is also essen­ tial for the appropriate selection of laboratory tests.

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CUNICAl. BIOCHEMISTRY

F U R T H E R RKAD1NG Astion M L, Wilding P Application of neural networks lo the interpretation ot" laboratory d a u in cancer diagnosis. Clinical Chemistry 1992; *«: U-H. A lucid descriptum of ths new technique of mulhvanate analysis. It thoidd be read m conjunction icith the leading article in tlie same number of this journal (Cicchctti 1) V*. Neural networks and diagnosis in the clinical laboratory State of the An. Ibid 3R: 9-10}. Challand G S. Reference value* and the concept of normality. In* Williams D S, Marks V Teds) Scientific foundations of clinical biochemistry: volume 1. Principles of clinical biochemistry, 2nd edn. Oxford: Heincmann Medical Books. 19НЙ. А amcise inirvductum to this topic Fraser С G. Interpretation of clinical chemistry laboraton' data. Oxford: Blackwell Scientific Publication*, 19Й6 & r comments in further reading for Chapter I. Fraser С rtance of taking analytical and bu*logiial variation into account chen interpreting labortitory data. Fraser О G , Petersen P H, T h e importance of imprecision. Annals of Clinical Biochemistry 1991; 2ft: 207-11 A brief mnetv of the importance of imprecision in the interpretation of biochemical data.

Galen R S, Gambino 5 R. Beyond normality: the predictive value and efficiency of medical diagnoses. New York: John Wiley, 1975. The seminal text on the predictive value concept. Henderson A R. Assessing test accuracy and us clinical consequences: a primer for receiver operating characteristic curve апагумя. AnnaU of Clinical Biochemistry 1003; 30: 521 39. A detailed discumon of the theory and application of receiver operating characteristic curves and other mettu\is for assessing the clinical utility of tern. Morris H A. Strategic» for interpretation of clinical laboratory data. Clinical Biochemical Review* 1091; 12: M 7 A short, wetl~tvritten summary cf the theory of reference ranges, the influence of analytical and bs\»logical variation on the interpretation of biochemical data and introducing tht concept of critical difference. Mould R К Introductory medical statistics, 2nd edn. Bristol: Adam Hilget, I9$9. (hie of the more accessible basic texts on statistics as applied to medicine, feaiened by epigrams and amusing illustrations. Winkcl P. "l"hc application of expert systems in the clinical laboratory. Clinical Chemistry 1989; ^5: 1595-600. A reviete of the nature and application of expert *yacms> Kith an extensile bibliography iihieh includes references to other tech manes of computer» supported medical decision making.

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Sodium, water and potassium

C H A P T E R

Michael Penney

PHYSIOLOGY Introduction The fluid spaces of the adult male human body constitute approximately 60% of the total body weight. The physio­ logical control over the composition and distribution of these fluid spaces is a highly sensitive and complex homocostatic process necessary to maintain a constant milieu interieur. Two main fluid spaces exist - the intracellular fluid (ICF) and the extracellular (ECF) - the latter further separated into the intravascular space (plasma volume), the interstitial space (which includes lymph) and transcellular fluid such as pleural, pericardial, peritoneal, cercbrospinal and gastrointestinal fluids. Table 4.1 sum­ marizes the water content of the body and the distribution of fluid between the main body spaces: the proportion of body water to total body weight decreases with age and is also further reduced as the proportion of body fat increases. The electrolyte composition of the ECF and the ICF is different - in essence the extracellular space is a predomi­ nantly sodium-containing solution and the intracellular

is a potassium-containing solution (Table 4.2). This fundamental difference in electrolyte composition is maintained by cell membrane sodium pumps ( N a \ K'-ATPase). Body water moves between the main body spaces predominantly under the influence of osmotic pressure resultant from dissolved panicles in the ECF and ICF either side of the cell membrane. Under physiological conditions the osmotic pressure of the ICF equates with the plasma osmotic pressure. Osmolahty represents the molal concentration of solute within a litre of solvent (water) and is expressed as mmol/kg, as opposed to a molar (or calculated osmolar) solution, which is the concentration within the space of a litre of solution (which includes solute space) and is expressed as mmol/L. The apparent nicety in definition does have its uses in differen­ tiating certain real from apparent electrolyte disorders (see p. 43). The measurement of osmolahty in ECF, however, cannot always be equated with transcellular osmotic pressure. The cell membrane is selectively permeable to a variety of solutes and certain natural solutes such as urea, Tebl» m the N

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26

СЛ1X1 CAT BKXiHllMlSTRY

or exogenous solutes such as alcohol, are freely perme­ able. Thus an increase in plasma osmolality due to so­ dium implies an increase in osmotic pressure across the cell membrane, tending to withdraw water from the cell to equate osmolalities. However an increase in plasma osmolality due to urea does not have this effect because of the free permeability of urea within the ICF and ECF. This leads to the concept of effective osmolality or iomcitx\ which under physiological conditions is primarily de­ pendent on plasma sodium concentration, but under pathological conditions may be dependent on other solutes, e.g. the effect of glucose on somatic cells in untreated insulin-dependent diabetes mellitus.

between 5 mmol/24 h and 500 mmol/24 h have been re­ corded and physiological mechanisms balance renal excretion of sodium. Sodium delivery* to the renal tubules is a function of plasma sodium concentration and the glomerular filtration rate (GFR). Every 24 hours the kid­ neys of an average healthy adult male will filter in excess of 24 000 mmol of sodium, most of which is «absorbed in the tubules so that, in health, sodium balance is achieved. Thus urine sodium concentrations can vary from < 1 mmol/L to approximately 300 mmol/L.

Oncotic pressure (colloid osmotic pressure) is the osmotic pressure resulting from the difference within the E C F of the protein content of plasma and interstitial fluid. The major contribution to oncotic pressure under physio­ logical conditions is plasma albumin concentration and the difference arises as hydrostatic pressure causes ultrafiltration of the plasma across the capillary membrane: the combination of the oncotic and hydrostatic pressure gradients is sometimes described as the Starling forces. Because water is freely permeable across all except some highly specialized cell membranes and because the total solute content of cells under physiological conditions is essentially fixed, the volume of the ICF is determined by the body water content. ICF tonicity will in turn determine ECF tonicity» but ECF volume is essentially dependent on its sodium content. ECF and sodium

Intrinsic rettat control of tubular re absorption of sodium Under normal physiological conditions ap­ proximately 80% of the sodium in the glomerular filtrate is reabsorbed in the proximal tubule. '1Ъс protein concen­ tration of the blood within the postglomerular peritubular capillary bed is believed to exert a strong oncotic pressure on fluid within the proximal tubule and this in turn helps to regulate the volume of fluid reabsorbed. This process contributes to the autoregulation of filtration and reabsorption known as glomerulotubular balance. Despite a considerable physiological interest in the control of proximal tubular sodium reabsorption and other intrinsic renal control mechanisms, such as redistribution of filtering activity from superficial nephrons (relatively saltlosing) to juxtamedullary nephrons (relatively saltretaining), the major humoral influences on sodium reabsorption reside in the distal tubule and collecting duct. That is, fine tuning of sodium reabsorption appears to reside in these more distal sites.

The sodium content of a normal adult is 5*5 65 mmol/kg body weight. 1 Tic concentration of sodium in plasma is approximately 140 mmol/L (=- 152 mmol/kg). Plasma water moves freely into the interstiiium but the movement of protein is restricted. Sodium concentration in the inter­ stitial fluid water approximates to plasma molar concen­ tration as a result of the effect of the Gibbs-Donnan equilibrium to reduce the concentration of permeable cations. Under physiological conditions the control of the ECF volume is through the control of functioning or effective plasma volume (that part of the plasma volume actively perfusing tissues). There are a variety of afferent mecha­ nisms to monitor effective plasma volume (and thus E C F volume), which include intrathoracic volume receptors such as atrial stretch receptors, hepatic volume receptors, arterial baroreceptors, intrarenal baroreceptors and possi­ bly tissue receptors monitoring tissue perfusion. Whatever the actual or relative function of all these sensory systems, their major function is to influence the renal conservation of sodium and the appetite for oral sodium intake. Sodium intake varies considerably between different human cultural and ethnic groups. Variations in intake

Ren in—a ngiotensin-aldosterone The best under­ stood mechanism of tubular reabsorption of sodium is that controlled by aldosterone. Aldosterone is a steroid hormone released from the zona glomerulosa of the adre­ nal cortex. The major control of aldosterone secretion is through angiotensin II, an octapeptidc produced in the circulation as a final product of renin action. Renin is a proteolytic enzyme secreted by a small group of cells situated between the afferent and efferent glomerular anerioles and a part of the distal convoluted tubule - the macula densa. "1 "his organizational complex, known as the juxtaglomerular apparatus, ultimately controls aldoster­ one secretion and hence Tubular reabsorption of sodium. The renin angiotensin aldosterone system is summarized in Figure 4.1. The major physiological influence on aldosterone production is sodium deficiency, although hyperkalaemia can stimulate aldosterone release directly. Aldosterone acts on the cells lining the distal tubule and stimulates sodium reabsorption in exchange for potassium and/or hydrogen ion secretion. In addition, angiotensin II has direct vasoconstnetive actions, thus having an immediate influence on effective plasma volume. Satriuretic hormones Glomerular filtration and

Renal control of sodium output

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SODIUM, WATER ANP POTASSIUM

27

Sodium depletion Reduced arterial pressure ECFV contraction The Circulation Angrotenstnogen Renm Kidney AngKMensin Angiotensm converting enzyme Angotensm II

r

Action of aldosterone -Sodium retention

AWoeterone

t

Adrenal gland ANP Hyperkalaemia Secondary actions of angiotensin II • Enhanced myocardial contraction • vasoconsJhcbon • Salt and water reabsorplion in the intestine • Central effects: thirst, salt appetite Fig. 4.1 The renin-tngwtensin aldosterone %умст ♦. *iimulaior> signal, , inhibitory signal.

the action of aldosterone do not constitute the complete control over renal sodium excretion: a third factor (or factors) has been proposed. The first described was the cardiac hormone atrial natriurctic factor, now known as atrial natriuretic peptidc (ANP). 1Ъе major molecular form of circulating ANP is a 28 amino acid peptide with a 17 amino acid ring structure formed by a disulphide bridge between cysteincs at positions 7 and 23: this form of ANP is also described as «ANP. The ANP precursor molecule synthesized within the myocytes is a 151 amino acid peptide (prc-pro-ANP) which is cleaved to a 126 amino acid peptide (рго-ANP or yANP) - the main stor­ age form. On secretion into the circulation, рго-ANP is cleaved into the N-tcrminal 1-98 ANP and the biologi­ cally active 99-126 ANP (uANP). An antiparallel dimer form of ANP ((iANP) is also found in the heart and is convened to ocANP in the circulation. The major stimulus to the secretion of ANP is atrial stretch and the major sites of synthesis are the atria. Many of the early studies of ANP action in animal ex­ periments were made in vitro^ so possible interactions with other humoral systems controlling salt and water meta­ bolism could not be fully explored. However, more recent studies have suggested that physiological concentrations of ANP result in falls in plasma renin and aldosterone concentrations and thus provide evidence of a dual Tine-tuning' system of sodium control using afferent

information obtained from the heart and the kidneys si­ multaneously. Currently studies do not suggest an impor­ tant physiological role for direct interaction of ANP and argininc vasopressin (AVP), although ANP may modulate the renal response to AVP and blunt osmotically stimu­ lated thirst appreciation. ANP has both natriuretic and hypotensive actions. The renal action appears to be inhibition of sodium reabsorp­ lion by the inner medullary colleaing duct and is prob­ ably mediated through cyclic GMP. The hypotensive actions arc in part due to a direct effect on vascular smooth muscle. The physiology of ANP is summarized in Figure 4.2. Since the discovery of ANP two other natriuretic peptides have been described, each with a 17 amino acid ring structure - BNP and CNP. BNP has a pre-pro form (134 amino acids) and a pro form (108 amino acids), but the major storage form is the mature peptidc (32 amino acids). BNP was originally named brain natriuretic pep­ tidc because it was first isolated from porcine brain; it is, however, primarily synthesized in and secreted from the ventricles of the heart. The biological effects of ANP and BNP are similar. The alphabetical naming has continued with the most recently described natriurctic peptidc CNP, which again has a pre-pro form (126 amino acids) and a pro form (103 amino acids), with the mature hormone being 22 amino acids in length. CNP appears to be re-

28

CLINICAL BIOCHEMISTRY

Stimulus -Atria) stretch

Heart

ANP

Control of renal water output "••.

Delayed action

I

Adrenal / ~ \

Kidney

• Reduced tubular sodium reabsorpt юл • Inhesion о' renm secretion Fig. 4.2

except certain specialized membranes of epithelial sur­ faces, are freely permeable to water and thus the addition of water increases the volume of all body compartments. Cells behave as 'effective' osmotic meters - swelling when body water increases and contracting when body water decreases. Normally ECF and hence ICF osmolality arc maintained at about 285 mmol/kg (sodium concentration 140mmol/L).

Vascular smooth muscle

Inhibition o* aidosterone secretion

Systemic vasodiiatation

The atnal natriurctic pepride system

strictcd to the brain in humans and acts as a neuropeptide rather than a cardiac hormone. Sodium appetue The renal conservation of sodium is remarkably efficient, but when under physiological conditions sustained nonrenal losses occur, such as through sweating due to pro­ longed vigorous physical activity or as a result of prolonged exposure to high ambient temperature, then a mechanism for increasing sodium intake comes into play the salt appetite. That such a mechanism exists in humans can be observed in pathological states of impaired sodium con­ servation such as Addison's disease. However, accurately defining the sodium appetite in man under physiological conditions is a subjective and difficult task. Salt intakes are largely conditioned by traditions of food intake and cultural habits of seasoning food with salt. The impulse to add salt to prepared food appears to be automatic in many people, who do so even before tasting. Animal experiments have implicated an entirely sepa­ rate, brain-based, renin-angiotensin-aldosterone system as a controlling influence on active salt-seeking behaviour.

OsmoregulaU'on *1Ъсгс is a minimum obligatory loss of water by the kidney each day which is dependent upon the maximum achievable urine concentration and the osmotic load for excretion. The maximum renal loss of water occurs when, for a given osmotic load, the minimum urine concentration is achieved. The control of water output by the body is through secretion of antidiurctic hormone (argininc vasopressin, AVP) and its renal action. AVP is synthesized by two paired nuclei within the hypothalamus - the supraoptic and paraventncular nuclei. The hormone is transported by axonal flow to nerve terminals in the posterior pitui­ tary. Closely associated cells within the hypothalamus (the osmorcceptor cells), by virtue of their swelling or shrinking in response to changes in ECF osmolality, control the release of AVP from the posterior pituitary. The effect of changing plasma sodium (hence osmolality) upon plasma AVP concentration is shown in Figure 4.3. Other solutes confined to the ECF, e.g. exogenously ad­ ministered mannitol, have a similar effect. By contrast, urea produces no significant stimulation of AVT because Subjective thirst (visual analogue ile)

Plasma AVP (pmol/L)

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I C F a n d water Under physiological conditions the solute content of cells is constant and therefore cell volume is dependent on sol­ vent, not solute, content. I*he majority of cell membranes,

Plasma osmolality (mmoVkfl) Flf. 4.3

Osmorcgulstiao of AVP and thirst.

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it freely permeates cell membranes. The osmoreceptor re­ sponse is often characterized by its set point (variously denned by the osmotic pressure at which a measurable AVP response commences or by the basal state osmolality) and by its responsiveness (gain or sensitivity as judged by the slope of the response). Thus, in situations of water depletion extracellular osmolality will increase, osmo­ receptor cells will contract and AVP secretion will in­ crease. Hydration will reverse these events and suppress AVP. This system constitutes the osmoregulatory control of AVP. Non-osmotic control of AVP In addition to osmo­ regulation, certain non-osmotic controls over the secretion of AVP exist, including ECF hypovolacmia, hypotension and nausea. 'The AVP response to hypovolaemia and hypotension is relatively insensitive when changes are proportionally small (5-10% reductions), but increases exponentially as further reductions occur. Thus a reduc­ tion in ECF volume or blood pressure of 20% or more will result in a plasma AVP concentration far in excess of that observed during normal osmoregulation. The influ­ ence of baroreceptor or volume receptor afferent input appears to modulate the osmotic response but does not abolish it: modulation occurs by decreasing the threshold for AVP release and increasing the gain of the systems. Nausea is the single most powerful stimulus to AVP secretion. It overrides osmoregulatory control and plasma AVP concentrations may increase 100-fold or more. Thus the non-osmotic control of AVP may under certain pathological conditions significantly modulate or override normal osmoregulatory control. Renal responsiveness to A VP The luminal surface of the specialized cells lining the collecting ducts is impermeable to water except when AVP occupies specific receptors on the contraluminal surface. Under these conditions water is absorbed into the collecting duct cells under the influence of medullary hyperosmolality and is subsequently absorbed into the bloodstream; concen­ trated urine is formed. During states of overhydratiom when AVP secretion is suppressed, the luminal surfaces of the collecting duct cells remain impermeable to water. Luminal fluid rendered hypotonic in the diluting segment of the kidney is not exposed to the medullary hypcrtonicity and dilute urine is formed. The renal response to AVP is thus dependent on an intact receptor-effector mechanism within the collecting duct cell resulting in an alteration in luminal membrane permeability and upon the presence of a medullary osmotic gradient. Human urine osmolality varies between approximately 50 and 1400 mmol/kg.

Control of water intake Osmoregulation The physiological stimulus to water intake is thirst. However, the act of drinking in

29

human societies in temperate regions is predominantly a social or habitual act not dependent on thirst. The control of water balance under non-pathological conditions is thus in many individuals and for much of the time achieved by control of output. The osmoregulation of water output by the kidney can only modulate and restrict an established deficit, but to reverse such a deficit a specific water input mechanism thirst - is required. The osmoregulation of thirst is similar in principle to that of AVP and appears to be primarily governed by alteration in osmoreceptor cell size. T h e osmoreceptor cells controlling thirst arc considered to be closely linked but distinct from those controlling AVP secretion. The study of thirst in human subjects under conditions of continuous stimulation, as opposed to meas­ ures of satiation, is a highly subjective exercise with large intcrsubject variability. Controversy exists as to the exact relationship between the onset of effective renal water conservation and the onset of effective thirst - that is, a sensation of thirst sufficient to cause active seeking and intake of water. The response in Figure 4.3 of thirst dem­ onstrates a 'gap' between the onset of AVP secretion (and hence water conservation) and the onset of thirst. The magnitude of this difference in water output and input effector controls will determine whether an individual controls physiological water balance primarily by thirst and water intake or by renal conservation of water. Non-osmotic control of thirst Non-osmotic thirst occurs when extracellular fluid is lost without a corre­ sponding cellular dehydration, the osmotic pressure of the extracellular fluid remaining unchanged. In this respect the overall control of thirst parallels the control of AVP, with both osmotic and hypovolacmic stimuli. There is good evidence from animal experiments of both neural and hormonal mediators controlling non-osmotic thirst. Angiotensin II is the most potent human thirst stimulant and may act directly upon the brain, but even when the effects of angiotensin II are blocked significant hypovolaemia will still stimulate thirst. Thirst following haemorrhage is a commonly reported clinical observation but, like the AVP responses to extra­ cellular hypovolaemia, often a considerable degree of haemorrhage (15-20% of total blood volume) is necessary before the sensation becomes strong. Thus for day-to-day water balance the primary physiological control of thirst is osmotic. E C F , ICF and potassium In health, plasma potassium concentrations range between 3.1 and 4.6 mmol/L with serum levels approximately 0 . 3 0.4 mmol/L greater owing to release of potassium during clot formation (mean serum level 4.0 mmol/L; 4.3 mmol/ kg). The intracellular concentration of potassium is ap­ proximately 160 mmol/kg and 9 8 % of the total body

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*м правом

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CUMCAI. ию< .HI:MISTXY

potassium is present in the intracellular fluid. "ITicrc are two aspects to the physiological control of potassium» namely the total body content and its distribution be­ tween intra- and extracellular spaces. ECF and ICF distribution of potasmon The potassium content of cells is determined by the balance of activity between uptake of potassium due to membrane-bound Na', K'-ATPase and the passive loss or leakage of potassium out of the cell. Many factors can influence the distribution of potassium, e.g. acid-base status, hormones (insulin, catecholamines), osmolaliry and the cellular content of potassium. The influence of acid-base status is widely recognized as an important contributor to potassium distribution, with an association between hypokalacrnia and alkalosis and between hypcrkalaemia and acidosis. However, when the nature of acidbased disturbances is examined, marked variations in the magnitude of changes of ECF potassium are observed. Acidosis induced by mineral acid administration produces a far greater increase in plasma potassium than the equi­ valent acidosis induced by organic acid administration. Insulin promotes active uptake of potassium by cells, probably by direct stimulation of Na*, K'-ATPase, and this activity appears to be independent of the effect of insulin on glucose uptake. The importance of the effects of insu­ lin in controlling plasma potassium under physiological conditions is not understood, but its action has an impor­ tant therapeutic role in the treatment of hyperkalaemia. Catecholamines have an effect on potassium distri­ bution, with [J-adrenergic actions essentially promoting cellular uptake and «-adrenergic actions resulting in increases in plasma potassium, but again the significance of these effects under physiological conditions is not understood. The net effect of catecholamines on cellular uptake of potassium probably explains the transient hypokalaemia frequently observed in acutely ill patients. An acute increase in extracellular tonicity, such as occurs following hyperosmotic infusions of saline or mannitol, results in an increase in plasma potassium. 'I*his results from leakage of potassium from cells, a phenom­ enon which is not related to extracellular acidosis but mav be linked to cellular dehydration, altered cell membrane function or altered cell metabolism. An increase in extra­ cellular tonicity is also observed in patients with hyperglycacmia in the absence of insulin and has important therapeutic relevance in the provision of potassium re­ placement during the treatment of hyperglycaemia. The effects of tonicity under physiological conditions arc probably of no significance. Potassium depletion results in a greater loss of potas­ sium from the KC1 ; than the ICF and potassium excess results in a greater proportional rise of ECF potassium than ICF potassium. The controlling influences over these

changes arc not defined, but the result is a significant alteration in membrane potential: this is increased with potassium depletion and decreased with excess. The ef­ fects on neuromuscular function of either condition constitute the most important clinical complications of disorders of potassium metabolism. Renal control of potassium output Intrinsic tubular control '1Ъе traditional under­ standing of potassium handling by the kidney is that potassium is freely filtered by the glomerulus, but that up to ° 5 % has been reabsorbed before the tubular fluid reaches the distal convoluted tubule. The predominant control of potassium excretion appears to reside in the control of distal tubular reabsorption or secretion. Plasma potassium itself has a major effect on potassium secretion in the distal tubule, tending to correct any im­ balance. Acute changes in sodium delivery to the distal tubule may also influence potassium excretion - restricted sodium delivery impairs potassium excretion hut a ten­ dency to natriuresis is accompanied by a kaliuresis. How­ ever, chronic effects on potassium excretion as a result of changes in sodium intake are not seen because of the influence of the rcnin-angiotcnsin-aldosterone system. There is increasing evidence that medullary recycling of potassium occurs: a portion of the potassium within the luminal fluid of the medullary collecting duct is reabsorbed and a fraction of this is secreted into the descending limb of the loop of Henle. ЧЪс increase in potassium concentration inhibits sodium reabsorption in the ascending limb and in turn increases sodium delivery to the collecting duct, which then provides the necessary conditions for funher potassium secretion and hence repetition of the cycle. Evidence is accruing that, at the limes increased potassium excretion is required, cycling is active. Potassium excretion is enhanced by leakage from the cycle, but the cycle in turn provides a regulatory feedback svstem. Aldosterone Potassium directly influences aldosterone secretion from the adrenal cortex. A high plasma potassium will stimulate aldosterone secretion and a low concentration will suppress secretion. As stated previ­ ously, the effect of aldosterone is within the distal tubules and is primarily sited in the cortical collecting ducts. Aldosterone influences the sodium permeability of the apical membrane, which indirectly stimulates the sodiumpotassium pump on the basolateral membrane. In addi­ tion, the number of sodium-potassium pumps is enhanced by a direct influence on the synthesis of Na*, K*-ATPase. Acidosis is associated with reduced potassium secretion and alkalosis with an enhanced secretion. The entry of potassium into the luminal cells of the distal renal rubules is decreased in acidosis and enhanced in alkalosis. The effect of aldosterone is to stimulate exchange of potassium

Материал, защищенный авторским правом

SODIUM, WATER AND POTASSIUM

31

and hydrogen ions for sodium ions. Therefore, the relative proportions of potassium and hydrogen ions within the cells of the distal tubule, together with the ability to secrete hydrogen ions, will determine the effect of sys­ temic acidosis or alkalosis on potassium excretion. Acting alone an acidosis will promote potassium retention and an alkalosis will promote a kaliuresis. Urine potassium concentration can van,* between about 5 mmol/L and 150 mmol/L. Adaptation of urinary excretion to a variation in input tends to be slow, taking a few days to achieve a new balance. In this respect urinary control of potassium is less sensitive than the control of sodium.

Reduced intravascular volume, when mild, will result in postural hypotension and a compensatory increase in pulse rate; central venous pressure is reduced and this can be assessed clinically by observation of neck vein filling or directly measured following insertion of a cannula into a central vein. When the volume reduction is more severe, hypotension and eventually shock will result in oliguria; the central venous pressure is further reduced. Reduction in interstitial fluid volume results in reduced skin turgor and transccllular fluid reductions result in dry mouth and reduced intraocular pressure.

Extrarenal control of potassium

The causes of sodium deficiency can be broadly classified into extrarenal, primary renal (resulting from renal dis­ ease) and secondary renal (resulting from disturbed hor­ monal control of renal salt retention or diuretic abuse)- In addition, and somewhat difficult to classify*, is the sodium deficiency that can occur when isolated jejunal segments are incorporated into urinary diversion operations (jejunal conduit, jejunal continent diversion: sec p. 52). These procedures are rarely performed as alternative sources of donor intestine are preferred. When jejunum is used in such procedures there is a risk of postrcnal loss of sodium. Extrarenal sodium loss Extrarenal fluids have sodium concentrations which may approach the concen­ tration in plasma (Table 4.3). T h e major causes of extra­ renal sodium deficiency are summarized in Table 4.4.

There is evidence of additional potassium control via potassium secretion by the colon. In animal experiments a high potassium diet will result in an increase in faecal potassium and a potassium-deficient diet will result in a decrease. These adaptive processes are to some extent in­ dependent of changes in aldostcronc secretion, although for maximal colonic retention of potassium the presence of aldosterone secretion is required. IJKT1 RDERS

O F SODIUM MfcTABOiJSM

As sodium is predominantly an extracellular cation the control of sodium balance will control the volume of the BCF. T h e tonicity of body fluids is under osmoregulatory control, therefore sodium deficit or sodium excess presents clinically with primary changes in E C F volume rather than changes in sodium concentration within the IfCF. Hyponatracmic and hype mat гае mic states will therefore be discussed in the section on water metabolism. S o d i u m deficiency Clinical presentation Sodium is always lost from the body in association with water. As the sodium concentration of all body fluids is equal to or less than plasma (except on occasions of high sodium intake, when urine sodium concentration will exceed plasma concentration), then loss of any body fluid except plasma will generally result in an excess loss of water over sodium. Any loss of sodium, however, will result in a reduction of E C F volume, including a reduc­ tion in circulating plasma volume. Clinical presentation will depend on the severity of the decrease. When the changes are mild patients are often described clinically as being dehydrated, a description that should be confined to pure water deficiency but unfortunately in general usage is not. Except in rare instances, truly dehydrated (that is» water depleted) patients are extremely thirsty. Patients with all but the most severe salt and water deficiency arc not.

Causes of sodium deficiency

Teblt

>mpo> Sodium nmol/L)

i i-ittnc jufa nh p' cell secretion) Pancreatic iu:

Ввс

Deal fluid Stool water Sweat

l;ihlr-t.4

Potassium ■ 1

5-13

110 130 -M 11 0 '

oxtmtci

6-9 \2 6 75-<W VIS

< htorkk (mm i i

Bicarbonate :nmc4/I )

HO- 100

К10 N

0

И 15 ■

• 60 12-18 5-70

50 W-40

< Juse* of extrarenal «odium lost

Minting, asptrauon, futula, blood lose Jgut. e.g. ilemtomv. pancreatic and biliary fistulac Mindful, e.g.. diarrhoea, bleeding. tSCM mucus production

Sto Sweating, e.g. thermal Off increased Burns 1 \udativc skin disea«c

I urn content Ccyv

AtffNff m i Mi ft

GtmointmifiaJ, t % Bern, шмП bowel obftmction Other transccllular spaces, e.g. peritonitis, pleura! effusion

32

CUXICAL BIOCHEMISTRY

The commonest clinical presentations result from gastro­ intestinal disease. The clinical history may considerably underplay the degree of deficit, especially in chronic con­ ditions or conditions resulting in sequestration of fluid. Primary renal sodium loss 1Ъс major causes of primary renal sodium loss are summarized in Table 4.5. The recovery phase of acute renal failure is often associ­ ated with a polyuria, kaliuresis and natriuresis. Normally this stage is short-lived, lasting only a few days, but it may occasionally be prolonged. A natnuresis may occur fol­ lowing successful renal transplantation and this may be in part due to transient tubular dysfunction; the recovery phase usually lasts only a few days, but occasional patients show prolonged natriuresis. Relief of urinary tract obstruction, most commonly seen in patients with prostatic enlargement, is often fol­ lowed by a short period of diuresis and natriuresis. The exact mechanism of this is not fully understood, but is probably related to a urea-induced osmotic diuresis together with elimination of excess sodium retained during the obstruction phase. The natriuresis in these cases is corrective and is not strictly a primary renal condi­ tion: it is unlikely to lead to sodium depletion. Normally this phase of post-obstructive natriuresis and diuresis lasts between I and 7 days, but occasionally a more prolonged natriuresis occurs, leading to sodium deficiency. This rare event is secondary to tubular damage occurring during obstruction. Salt wasting has been described in associa­ tion with methicillin-induced acute interstitial nephritis. Full resolution of the condition may be delayed for several months, during which time minimum obligatory losses of sodium may exceed 100 mmol/24 h. Chronic renal failure is generally associated with an im­ paired ability to excrete sodium. Considerable adaptation occurs to increase the natriuretic capacity of the remain­ ing functioning nephrons, which paradoxically impairs the overall sodium-conserving function. Normally dietary' sodium intake exceeds minimum obligatory urine sodium loss, but if dietary salt is restricted or non-renal losses of sodium arc increased, the obligatory loss may induce sodium deficiency. Minimum obligatory losses may be as little as 40 mmol/24 h and even lower if enough time

ГмМс 4.Й

t ni л

i primary rv-nai lodium Iota

Acute Diuretic phase «if acute renal failure Poet renal transplanted •Hawing Ittid of urinary tract obstruction Acute interstitial nephritis Chronic renal failure with salt restriction Suit-losing ncptuDpaihv chronic pyelonephritis medullary cystic disease cphropathics, e.g. chronic analgetic abuse, cisplatm

is given for adaptation, but occasionally patients have minimum obligatory losses exceeding 150 mmol/24 h (3 mmol/kg body weight in children). The term salt-losing nephropathy is appropriate for this group of patients. Salt-losing nephropathy is a clinical state rather than a specific disease. It is normally associated with chronic renal failure due to tubulointcrstitial disease or to glomeruloncphriris with significant interstitial abnormalities. A common example of a toxic nephropathy inducing a saltlosing state is analgesic abuse, in which there is typically also reduced concentrating ability and renal tubular acidosis. Secondary renal sodium loss The disruption of extrinsic controls over renal sodium handling can give rise to a secondary renal sodium loss. Patients present with a variety of symptoms, some specifically related to a con­ tracted ECF volume, such as postural hypotension and an increased sodium appetite. Causes of secondary renal so­ dium loss, presented in Table 4.6, are essentially hormone or diuretic induced. In Addi son's disease the synthesis and secretion of aldosterone arc reduced because of adre­ nal gland destruction, leading to distal renal tubular sodium wasting. Congenital adrenal hyperplasia (САН) with associated sodium wasting is due to impaired mineralocorticoid synthesis and is associated with the most com­ mon form of САН (2l-hydroxyIase deficiency) and the less common 3(i-hydroxydchydrogcnase deficiency. In either condition, the degree of sodium wasting is variable; it is present in about two thirds of patients with 2 1 hydroxylasc deficiency (sec Ch. 17). Very rare forms also exist, including cholesterol desmolasc deficiency (lipoid adrenal hyperplasia) and corticosterone methyl oxidase deficiency - a deficicnev in the mixed-function oxidase catalysing the final steps of aldosterone synthesis. Corti­ costerone methyl oxidase deficiency is not strictly a type of САН as the synthesis of cortisol is unaffected and con­ sequently the adrenals are not hyperplastic. In all these conditions the associated hypovolacmia stimulates rcnin production and all may be associated with hypcrkalacmia. Treatment is predominantly by glucocorticoid and mineralocorticoid replacement. It might be predicted that deficient production of renin would also lead to renal sodium loss because of a second­ ary deficiency of aldosterone. The condition hyporcninaemic hypoaldosteronism is invariably associated with

Table 4,6

Causes of secondary renal salt Iota

Нура сшпаешк hvpoaldoatcronUm Addiaon'a disease congenital adrenal hyperplasia corticosterone methyl oxidase deficiency types I and II Нурогетпастк hvpoaldostcronism Pscudohypoaldostcrontsm (type Diuretics Banter's syndrome

SODIUM, WATER AND POTASSIUM

renal insufficiency', but the characteristic feature is hyper­ kalaemia rather than hypovolacmia. As in the majority of patients with renal insufficiency, inappropriate renal so­ dium loss will induce hypovolacmia if the patient is placed on a low-sodium intake, but it is unclear if this is more severe in hyporeninaemic hypoaldosteronism. Although this condition is a cause of secondary renal sodium loss, its main importance is in the differential diagnosis of hyperkalaemia and it is discussed in more detail later in this chapter (see Syndromes of hypoaldosteronism on p. 56). Pseudohypoaldosteronism (type 1) is a rare condition in which the mineralocorticoid receptor in the renal tu­ bule, as well as in the colon and salivary gland, is defec­ tive. The clinical features are similar to those observed in primary hypoaldosteronism but the aldostcrone levels in plasma are elevated - hence the application of the term p5i7* 1000 mmol/kg), although the capacity for urinary con­ centration is restricted in the neonate to about 500 mmol/ kg and in the adult urine concentrating capacity declines in the elderly and may fall below 700 mmol/kg over the age of 60. In patients whose water deficiency is due to complete DI (cranial or nephrogenic) the urine is not concentrated. However, if the water deficiency is sufficient to reduce ECF volume and thus renal function significantly, the urine may not be maximally dilute but will approximate to the osmolality of plasma. In addition, in partial forms of DI, severe water deficiency may result in near maximally concentrated urine. In order to avoid diagnostic confu­ sion, it is therefore advisable to correct any water defi­ ciency prior to establishing the diagnosis with a water deprivation test. Water deficiency without thirst Patients may rarely present with severe hypernatraemia but no thirst or polyuria. The cause is a group of condi­ tions known collectively as adipsic or hypodipsic hyper-

Plasma osmolality (mmovVg)

Flf. 4.5 Relationship between plauna AVP and plasma osmolality in patients with hypodipsic hypernatraemia; 1, normal regulation of AVP release; 2, partial defect in AVP release. 3, total defect; 4, reset osmostat. The dotted Ime represents the limit of detection of current

AVP вами Adapted ton RUIMISUU G I . Agpctocm r. Zeta R I Ncurogcnic disorders of osmoregulation. American Journal of Medicine 1Q82;72: m - 5 3 .

natraemia or essential hypernatraemia. The hypothalamic disorder causing hypodipsic hypernatraemia may be due to trauma, a primary or secondary tumour, a granuloma (e.g. sarcoid or histiocytosis) or vascular impairment, or the condition may be idiopathic. The actiological similar­ ity to those conditions causing CDI underlines the close proximity of osmorcccptors for AVP and thirst control within the hypothalamus. Four subtypes of hypodipsic hypernatraemia have been recognized (Fig. 4.5). In type 1 normal osmotic control over AVP release is retained but thirst appreciation is absent. This dis­ order is very rare but demonstrates the separate osmoregulation of thirst and AVP. An example of this defect is shown in Figure 4.6. Thirst was entirely absent in this patient when he was hvpernatraemic (and hence hyperosmolal), but he retained normal osmotic control over plasma AVP release and normal renal responsiveness to AVP. He had undergone hypothalamic surgery for severe behavioural disturbance and his plasma sodium was found to be consistently above 150 mmol/L without any experience of thirst. In type 2 hypodipsic hypernatraemia the plasma AVP response is similar to that seen in patients with severe CDI, with a substantial loss of incremental gain of plasma AVP as plasma osmolality rises. Unlike the situation in CDI, however, the osmotic thirst response is severely blunted. This group of patients is of interest as the renal responsiveness to AVP is often considerably enhanced and patients are thus not polyuric. Urine concentration

SODIUM, WATER AND POTASSIUM

hypernatraemia, but without expressed thirst, is by defini­ tion suffering from one or other form of hypodipsic hypernatraemic syndrome. Although it is possible to differentiate pure adipsia (type I) from other types of the syndrome by the use of tests of water deprivation and water loading (sec Appendix 2), in practice the diagnosis can only be confirmed by the study of AVP secretion and thirst in response to hypcrtonic saline infusion.

В Plasma AVP ■.

Although acute and chronic hyponatraemia may share certain common causes and the definition is somewhat arbitrary (acute hyponatracmia is of less than 48 hours' duration) these conditions present and are managed in quite separate ways. This is because of the adaptation of cells, in particular brain cells, to chronic hypotonicity. The nature of this adaptation is summarized in Figure 4.7.

ampUruj G

rScudohyponatraemu icmic hyponatracmu renal «odium lot» diuretic excess adrenal failure sodium-loune; ncphropathy non-rcnal sodium Iota

Acute dilutional hyponatraemia The main causes of acute dilutional hyponatraemia are shown in Table 4.13. Self-inflicted acute hyponatraemia has been described but is extremely difficult to induce due to the enormous human capacity for renal water excretion - urine flow rates of 27.5 m l / m i n (equivalent to 39.6 litres of urine in 24 hours) have been described. One unusual cause of hyponatraemia requiring special mention is beer potomania. This condition is caused by the consumption of large volumes of salt-poor beer (such as certain conti­ nental beers) together with a very poor dietary intake of protein and other minerals, although caloric intake is

burns 1 Ivpervolacmic hyponatnicmui with .'edema rrho.

CCF ncpti lyndroa u it oedema acute or chronic renal failure ЫаааоггЫаав ponatracnua acute dilutional hyponatraemia (see Table 4.1 iaJ hyponatraemia (see Table 4.14)

exogenous hypertriglyceridaemia and in patients with high protein concentrations (usually due to paraprotcinacmia). The concentration of sodium within plasma water is normal, but each litre of plasma now contains less water. Plasma osmolality (molal concentration) is normal. Patients with hypovolaemic or hypervolaemic hyponat­ raemia present with signs of ECF volume depletion or excess. These conditions are discussed under disorders of sodium metabolism; they are usually differentiated clinically from dilutional hyponatraemias and also by the presence of biochemical evidence of the underlying cause, e.g. renal or hepatic dysfunction.

if-indue»

Psycbogemc I-reshwatcr drowning latrofccnii. mappropnati tluid regimen absorption of trrigam tranvurcthral prostatectomy traiuccrvical endometnectomy traoscutaneotu ureterolithot. vesfcal ultrasonic tithotripvy Syntocinon mdu if labour with isotomc dextrose infusion.

Plasma

Brain cell

Sodium 140mmol/l_

Volume 100%

OsmolaWy 285mmoVkg

Osmolality 285 mmoUkg

Acute hyponatraemia

Chronic hyponatraemia

Plasma

Brain call

Plasma

Brain call

Sodium IWmmoVL Osmolality 225 mmoVkg

Volume 127%

Sochum 110mmol/L Osmolality mmofrkg

volume 104%

225 mmobVg

Osmolality mmot^g

Fig. 4.7 Schematic representation of the maior differences in brain cell volume in acute hyponatraemia and chronic hyponatraemia In chronic hyponatraemia cell osmotic content has been reduced.

1Л, защи

SODIUM, WATKR AND POTASSIUM

more than adequate. The daily osmolal load for excretion may thus be very low indeed (»!i. paralysis IMHC

s-

M

56

CUNKAl- HIOCHKMISTRY

occur locally in an ischaemic limb, such as may happen during prolonged venous stasis prior to venesection. Hyperkalacmia has been described in patients undergoing chemotherapy for malignancy, which causes massive lysis of neoplastic cells. This phenomenon has been described in the treatment of chronic lymphocytic leukaemia, acute lymphoblastic leukaemia and lymphosarcoma. Its occur­ rence emphasizes the need for maintenance of hydration and for careful electrolyte monitoring during aggressive chemotherapy. Acute hacmolytic disorders can give rise to hyperkalaemia by a similar mechanism. Hyperkalaemic periodic paralysis Hyperkalaemic perio­ dic paralysis (HYPP) is a rare autosomal dominant condi­ tion which presents with attacks of muscular weakness, paralysis (usually sparing the respiratory muscles) and an associated acute rise in plasma potassium (up to 8.0 mmol/L). The periodicity of attacks is variable, rang­ ing from daily to only a few attacks a year. This type of periodic paralysis is provoked by a high potassium intake, glucocorticoids, hypothermia and during the recoveryphase after vigorous exercise. During attacks ECG moni­ toring shows tall T waves but cardiac arrhythmias are rare. The management of HYPP is administration of the |i -agonist saibutamol, which can easily be taken by inha­ lation. Salbutamol has also been used prophylactically. Recently the mutation causing HYPP has been identified in the skeletal muscle sodium channel gene. Potassium retention The major causes of potassium retention are listed in Table 4.23. An increased potassium intake alone, without associated renal impairment, in prac­ tice only occurs as an tatrogenic complication of inappro­ priate intravenous loading. Excessively high oral loads of potassium are counteracted by a combination of reduced gastrointestinal absorption, vomiting and diarrhoea. A reduction in GFR of whatever cause increases the risk of development of hyperkalaemia. However, on a nor­ mal diet containing approximately 100 mmol of potas­ sium per day, GFR may fall to less than 10 ml/min before a significant risk of hyperkalaemia arises and increased coIonic secretion of potassium may further protect the pa­ tient when GFR is reduced beyond this. It is important, however, that patients with significantly reduced GFR avoid high intakes of potassium or any condition which results in endogenous shifts of potassium.

Decreased tubular secretion of potassium may occur in response to treatment with the potassium-sparing diu­ retics: spironolactone, triamtercne or amiloride. Hyper­ kalaemia is a particular risk when such a drug is used in a patient with impaired renal function or in a patient with a high potassium intake. Urinary diversions which involve jejunal segments (see p. 52) result in a significant inci­ dence of hyperkalaemia due to rcabsorption of potassium from the urine while contained within the segment. Syttdnmtcs of hypt>aidosteronism This broad group of conditions is summarized in Table 4.24. Primary hyperreninaemic hypoaldosteronism is found in Addison's disease (see Ch. 17) and the rare condition corticosterone methyl oxidase deficiency, in which aldosterone synthesis is impaired. Heparin given in continuous high doses can also inhibit aldosterone synthesis through inhibition of 18-hydroxyIasc and possibly by inducing atrophy in the zona glomerulosa. Two forms of congenital adrenal hyperplasia result in impaired mincralocorticoid synthesis. 21-Hydroxylase deficiency is the most common form of САН. Virilization is characteristic, but not all patients exhibit features of hypoaldosteronism; up to two thirds develop renal sodium wasting with hyperkalacmia. 3(J-Hydroxydehydrogenase deficiency is a rarer condition, in which the majority of patients have renal sodium wasting and hyperkalaemia. Hypoaldosteronism may occur in patients receiving angiotensin-converting enzyme inhibitors such as captopril. Patients at particular risk of developing hyper­ kalaemia include those with associated renal impairment or with high rcnin levels, for example resulting from congestive heart failure. Hyporcninacmic hypoaldosteronism is an increasingly recognized syndrome in which patients present with hyperkalaemia out of proportion to the reduction in CiFR. The majority of such patients develop a hyperchloraemic metabolic acidosis and fall, therefore, into the classifica­ tion of type IV renal tubular acidosis (sec Ch. 5). The syndrome is particularly prevalent in elderly patients with

Table 4.M and aldosterone concentration* norm..

asm with rrlativc plauna сю*. . N Ren m

Table 4.2Л l*crtaud

t '.*.■.

*bc:

ponmiur

muikt

Deemwd output Decrated GFR Dccreatcd tubular accretion pouisium-fpurtntt diuretic* SYDUfUIDCV Ов BVOOVMOV

Increased POM-tubular reab*orption following surgical urinary diversion involving lenmal aegmenti

I'nman lv. poaldoMcronism AddlSOn'* isolated aldoMCfonc deficiency ncpaxin treatment Congenital adrenal hyperpUwa Angtotcnun-convcrting enzyme inhibition Hyporcnmaemic hvpoaidoiterorusm ■ •ndjrv tubular disorder* PscudohypoaJdosteroniun rapt I type II

ian, защи1

Aldosterone

* *

N N .

N gh

\toi

SODIUM, VATUR AN13 POTASSIUM

non-insulin-dependent diabetes but is also found in many other diseases in association with interstitial nephritis, in­ cluding systemic lupus erythematosus, multiple myeloma, chronic obstructive uropathy, gout, sickle-cell disease, lead nephropathy, following renal transplantation and in association with treatment with prostaglandin synthetase inhibitors and treatment with cyclosporin. Plasma renin activity is reduced, as are aldostcronc levels. Renin re­ sponse to upright posture or to salt depletion is also reduced. The pathogenesis is not fully understood but includes structural damage to the kidney» including the juxtaglomerular apparatus, and there is evidence in diabe­ tes of impaired conversion of renin precursor to active renin. One intriguing aspect of hyporeninaemic hypoaldosteronism is why hyperkalaemia occurs, as aldosterone secretion is known to be stimulated by hyperkalaemia. However, it is likely that this direct stimulation of aldos­ tcronc can only occur in the presence of angiotensin II. An important aspect of this syndrome is that, because it occurs more frequently in the elderly, drugs that suppress renin and aldosterone, such as (i-bloekers and calcium channel bluckers as well as prostaglandin synthetase in­ hibitors, should generally be avoided in this age group. Interstitial nephritis may give rise to hyperkalaemia in which the aldosterone is not suppressed. The patho­ genesis in these conditions is presumed to be a direct impairment of tubular secretion of potassium. Many of the diseases associated with tubular dvsfunction are those which are also associated with the development of hypo­ reninaemic hypoaldostcronism and include obstructive uropathy, sickle-cell disease and systemic lupus erythematosus. The main clinical difference between direct tubular dysfunction and hyporeninaemic hypoaldosteronism is the failure of the former condition to respond adequately to mineralocorricoid replacement and alternative treatments such as thiazide diuretics must be used, Pseudohypoaldostcronism is an exceedingly rare condi­ tion existing in two forms. Type 1 pseudohypoaldosteronism is an autosomal recessive condition usually presenting within the first 2 years of life with failure to thrive, hyperkalaemic acidosis, renal sodium wasting, hyponatraemia and hypotension. There is evidence that the colon, salivary glands and sweat glands, as well as the renal tubules, are unresponsive to endogenous or exo­ genous mineralocorticoids. Plasma levels of renin and aldosterone are elevated. Patients are primarily managed by sodium replacement and symptoms often ameliorate in childhood but may recur if dietary sodium is restricted. Type 2 pscudohypoaldosteronism (sometimes called Gordon's syndrome) presents in late childhood with hyperkalaemic acidosis but, in marked contrast to type 1. with hypertension. The primary defect docs not appear to be at the aldosterone receptor but is believed to involve increased chloride reahsorption in the thick ascending limb of the loop of Henle, resulting in enhanced sodium

57

reahsorption, ECF volume expansion and hypertension. Plasma renin and plasma aldosterone are low. The condition responds to thiazide diuretics. laboratory investigation of hyperkalaemia The first stage in any laboratory investigation of hyper­ kalaemia is to ensure that the plasma potassium is a true reflection of in vivo concentration. The storage of wholeblood specimens in a refrigerator at 4°C is widely prac­ tised by clinicians in the belief that this will aid the preservation of a specimen, but this practice can greatly increase plasma potassium without any evidence of haemolysis. This effect is particularly noticeable in famil­ ial pseudohyperkalaemia, but can affect any blood sample if storage is prolonged for 8-12 hours. As previously mentioned, hyperkalaemia in patients with high WBC counts or platelet counts should be confirmed in freshly separated plasma, rather than serum. Having confirmed a true hyperkalaemia, the clinical history is required with particular emphasis on drug and dietary regimens, with information sought regarding in vivo redistribution. Assessment of blood acid-base status and glucose con­ centration may be valuable. Plasma potassium is usually measured with other electrolytes and markers of renal function, including urea and creatinine. When GFR is reduced below lOmlVmin then hyperkalaemia is likely to develop unless dietary potassium is restricted. The measurement of urine potassium output is of marginal value except possibly in steady-state conditions when a 24 hour urine potassium will provide evidence of excessive ingestion. If GFR is not reduced sufficiently to explain the hyperkalaemia then a syndrome of hypoaldosteronism should be considered. Normally, clinical presentation of Addison's disease is sufficiently characteristic to require only studies of cortisol response to exogenous ACTH pre­ parations, but other forms of hypoaldostcronism require measurement of renin and aldosterone. Unfortunately, the various combinations of findings do not fit snugly with clinical classification of disorders. Hyper-reninacmic hypoaldosteronism is found in primary hypoaldosteron­ ism, the exceedingly rare condition of corticosterone mcthyloxidase deficiency, some forms of САН and with ACE inhibitor treatment. Hyporeninaemic hypoaldos­ teronism is found in the conditions grouped under the syndrome of the same name and is also found in the rare type II pseudohypoaldosteronism. In contrast, high renin and high aldosterone are found in type I pseudohypo­ aldostcronism. However, normal levels of renin and aldosterone may be found in those conditions causing interstitial nephritis with direct tubular inhibition of po­ tassium excretion. Renin and aldosterone levels arc not always easy to interpret as high plasma potassium directly

Материал, за

ленный asTOf

*м правом

58

CIJNK:AI. BIOCHEMISTRY

stimulates aldosterone and suppresses renin activity. In addition, aldosterone is increased as the GFR falls without corresponding changes in renin activity. In situations in which diagnostic difficultly may exist, for example in hyporcninacmic hypoaldosteronism, it may be necessary to reassess aldosterone levels when plasma potassium has been reduced to within the reference range or alternatively и trial of mineralocorticoid replacement will differentiate hyporcninacmic hypoaldosteronism from a direct tubular dysfunction. Management of hyperkalaemia Hyperkalaemia is a serious condition requiring immediate treatment because of the risk of sudden death. Emergency treatment is intravenous 10% calcium gluconate - 10 m L injected over 60-120 seconds and repeated every 15 min­ utes or so until the EGG changes improve (maximum dose 50 ml.). This does not correct the hyperkalaemia, but is directly cardioprotective. In patients receiving dig» oxin, calcium gluconate should be infused more slowly 10 mL over 30 minutes - to avoid digoxin toxicity induced by hypercalcaemia. Two therapeutic regimens are available to lower plasma potassium rapidly. Glucose (50 ml- of 50% glucose) can be infused over 15 minutes together with 10 units of solu­ ble insulin. This regimen may be repeated at hourly inter­ vals and should be accompanied by plasma potassium and glucose monitoring. Alternatively* plasma potassium may be reduced by the infusion of 50-100 mL of 4.2% sodium bicarbonate {500 mmol/L) over a 15-30 minute period. For haemodialysis patients with hyperkalaemia a regi­ men which has been shown to be of equal benefit to glucose and insulin in reducing plasma potassium is treat­ ment with the P r adrenergic agonist salbutamol. Inhala-

tion of nebulized salbutamol (10 20 mg) will reduce plasma potassium by approximately 1 mmol/L within 30 minutes. If hyperkalaemia is the result of increased body stores of potassium then this excess must be removed from the body. The relationship of plasma potassium to excess body potassium is highly variable and is not strictly pre­ dictable. As an approximation, elevation of plasma potas­ sium by 1 mmol/L above normal without evidence of redistribution will roughly equate with a 200 mmol total excess. Polystyrene sulphonare resins (sodium or calcium salts) may be given by mouth (15 g> 3—4 times daily in water) or as an enema (30 g in mcthylccllulosc, retained for 9 hours). As an approximation each gram of resin re­ moves 1 mmol of potassium so that up to 60 mmol may be removed in each 24-hour period. For more dramatic reductions in body potassium dialysis is required. Perito­ neal dialysis is capable of removing 10 15 mmol of potas­ sium each hour» whereas the efficiency of haemodialysis will allow up to 30 mmol to be removed each hour. SUMMARY The physiological control over sodium, water and potas­ sium within the human body is a complex interrelated series of systems of extreme precision and sensitivity. These systems regulate the extracellular fluid volume, the extra- and intraccllular solute content, the imracellular volume and neuromuscular functions and therefore in­ directly influence myriad functional and metabolic proc­ esses essential for life. The pathological causes and consequences of recognized abnormalities in the control of sodium, water and potassium have been explored in this chapter, together with the details of diagnosis and treatment.

FURTHER READING Anonymous. Hyperkalaemia silent and deadly. Lancet I**8**; 1: 1240. A lucctnet editorial providing information on the current management of hyperkalaemia. Burner F C, Schwartz W R. The syndrome of inappropriate secretion of an( uii иге tic hormone. American Journal of Medicine l°b7j 42: 700-ЯОо.

A detailed review by the author* и/ю originally coined the term SIADIL The diagnostic criteria for thu syndrome are clearly defined. Baylts P H Regulation of vasopressin secretion. In: Baylis P H (cd) Water and salt homeosmsis in health and disease. Clinical Endocrinology and Metabolism 1 W ; 3: 313-30. A revietv of the complex physiological control of argmine vawpressw secretionBcrl T . Treating hyponatraemia: damned it we do and damned it" we don't. Kidney- International 1990; V7: 1006-1Й. Ditcwsvms around a case conference on hyptmatracmia; the ratwnalc of acute correction and the atwuianee of the det-ebiptHcnt of central peinrine myehnolysii are covered in detail. Flcar C T G , Singh С M. Hyponatraemia and sick cells. British Journal of Anaesthesia 1973; 45: 976 94. '1Ъе anginal description of sich-cclt syndrome. The complexities of the syndrome are exposed.

Lindcman R D. Mypukalaemia: causes consequence* and correction. American journal of the Medical Sciences 1976; 272" 5 17. Part of tht proceedings of a s\ntp*is»um on fluid and electrolyte disorders. The aetiologies. ctmseaitencet and therapies of hypi*kataemta are discussed in detail Morgan D B, Thomas T H. Water balance ami hvrxmatracmui. Clinical Science IV?1.»; 56: 517 22. An ехрояпоп of acute and chronic hyponatraemia, and the fundamental differences between tlie patkophysiology of each. Pcnncy M D, Walters Ci. Arc oitmolaluy measurement* clinically useful?1 Annals of Clinical Biochemistry 1987; 24: 506-71. A critical n m n r of the clinical usefulness of btxiv fluid osnaUahty meanamients including measurtmentj applied to fluid balance dtwrdery. Robertson G L, Aycincna P, Zerbe R L. Neurogcnic disorder» of osmoregulation American Journal of Medicine 1*^2; 72 )V> 53 The osmoregulatory control of A V'Pm hypodtpstc hypernatraemu conditions i\ discussed in detail. Zerbe R I., Stropc* !•? Robertson Ci l_ Vasoprewm function in the syndrome of inappropriate antidiurcsiv Annual Review of Medicine 1480; 31: 315 27 Patterns P/plasma AI'P responses to hvpertonie saline infusion in patunts with dilutivrud hvponatruemta are presented.

Материал, защищенный авторским правом

SODIUM, WATER AND POTASSIUM

APPENDIX 1: FORMU1-AE Formulae (a)-(d) are approximations only and are sup­ plied as illustrative guidelines. Any corrective procedure based upon any of these formulae should be accompanied by detailed clinical and laboratory monitoring. (a) Estimate of ECF volume reduction from change in haemacocrit (hct) ECF volume

body weight

reduction (litres!

(кжЭ

weighed accurately. Subsequently, urine is collected each hour and the volume and osmolality are measured. At 3, 5 and 8 hours blood (5 mL) is collected and the patient weighed. Osmolality measurements should be made as the test proceeds. If after 8 hours the urine osmolality remains below 600 mmol/kg then proceed to the vasopressin test. Vasopressin test

normal hct measured hct i

Give 2 jig of dDAVP i.v. The patient is allowed to drink, but total fluid intake should be restricted to 1000 m L until 09.00 hours next morning, unless the patient's weight loss continues above 3%, when free fluids should be allowed. A further urine collection is made 2 hours after administration of desmoprcssin and, if necessary, after 5, 14 and 16 hours.

(b) Estimate of sodium deficit in patients with hypovolaemic h y p o n a t r a e m i a Sodium deficit body weight ■ O.O N x MO - [Na*] in mmol/I. plasma (mmoll (kgi

59

'

(c) Estimate of water deficit in hypcrnatracmia 140

Interpretation

Hypcrtonic saline (5%) = 855 mmol/L.

A normal subject will concentrate urine to above 600 mmol/kg during the period of water deprivation and the plasma osmolality will remain within the physiological range (or, more strictly, the urine - plasma osmolality ra­ tio will be greater than 2:1). If urine fails to increase above 600 mmol/kg but increases following dDAVP by greater than 20% then CDI is confirmed. The weight loss re­ corded should always equate with total urine volume passed.

(e) Calculation of plasma osmolality (osmolarity)

Reference

Calculated osmolality = 1.89 (Na*) + 1.38 [K'J + 1.03 [urea] + 1.08 [glucose] + 7.45

This protocol is modified from the original description of the 'short' water deprivation test by Dashe A M, Cramm R E, Crist A C, Habener J F, Solomon D H. A water deprivation test for the differential diagnosis of polyuria. Journal of the American Medical Association 1963; 185:699-703.

VTatcr defiat i.lnres)

,.

bodv weight (kg>

I

1

j *a

Jm mmo!/l. plasma

(d) Estimate of sodium required in acute water intoxication (to increase plasma sodium to 125 mmol/L) Sodium required ,._ ... , .... „, bodv weight l25 Na n /««Jr. " l "l » mmol/I.plasma « 0.6 * ,.f'ktf, M

From Behagat С I, Garcia-Webb P, Fletcher E, Beilby J P. Calculated vs measured osmolalities revisited. Clinical Chemistry 1084; 30: 1703 1705. ( 0 Calculation of osmolal gap Osmolal gap = measured osmolality — calculated osmolality APPENDIX 2: DYNAMIC F U N C T I O N TESTS (a) Water deprivation test The patient is denied fluid and sloppy food from 08.30 a.m. onwards. All patients should be observed closely; urine should be collected at the bedside to prevent covert access to fluid. If the patient develops symptoms of water loss or loses more than 3 % of initial weight, urine and plasma osmolality should be measured and water deprivation discontinued. The vasopressin test is then performed (see below). At the commencement of the test blood and urine are collected for measurement of osmolality and the patient is

(b) Hypcrtonic saline infusion The patient is food fasted overnight (12 hours) but is al­ lowed free access to water. Smoking is not allowed during this 12-hour period or during any part of the test. No fluid of any kind should be consumed during the test, including sips, mouthwashes or ice cubes: all of these oropharyngeal stimuli can suppress the release of AVP from the posterior pituitary. Э.СЮЬоигъ

9.00-10.00 hours 10.00 hour*

Weigh Supine position Indwelling cannulae with both amccubital veins kept patent with heparinized saline Rest hour Basal sample 2S m l .

{

One for eventual infusion of hypcrtonic saline, one for blood sampling 20 ml- (hepann) AVP 4 ml. - urea and electrolytes, osmolality

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60

CUN1CAL BIOCHEMISTRY

10.50 hour* 1 I .00 hour* 11.30 noun

12.00 hour,

Commence 5% saline (0 06 mUkg/min) mfuuon into one cannula following blood sampling j 20 ml- Cbepihn) - A \ T s Sample 2 * ml. I 5 m l . - urea and I electrolytes, o»inolahtv | 20 mL (hepann) AVP Sample 25 mL \ Ь m L urea and \ electrolytes, osmolahty - AVP " !0mL(hcparin; 20 m l . (hep: Sample 2*> ml д and 5 mL - urea ! electroK'TM, osmolality ( 20 m L (hepann) AVP Sample 25 mL I 5 mL - urea and I electrolytes, cKmolahrv

For interpretation of results, see Figure 4.4 and text. Note 1. If information is required on subjective thirst rating then the patient should be shown, at each time blood is sampled, a unitlcss 100-mm scale with a maximum limit entitled 'Extreme thirst' and a minimum limit 'No thirst'. The patient is asked to indicate their where­ abouts on the scale. Two scales should be completed on each occasion to obtain a measure of precision. 2. Information concerning the handling of blood samples for AVP should be obtained from the relevant labora­ tory. In general, samples should be collected into prechillcd heparin tubes, transported on ice to the labo­ ratory immediately, centrifuged rapidly at 4°C and the plasma stored at a maximum temperature of -20 U C (preferably -70 U C). The time from collection to storage should not usually exceed 20 minutes. 3. Patients with a history of congestive cardiac failure should be closely monitored and if necessary the test curtailed and frusemide (40 mg i.v.) administered.

(c) Water load test 1 he patient is allowed free access to fluid 12 hours prior to the test to ensure adequate patient-determined hydraxion at commencement. No smoking is allowed during the test period or for the 12 hours prior to this. Adequate glucocorticoid replacement is required for patients with hypoadrenocorticism. At 09.00 hours the bladder is emptied and an aliquot of urine (10-15 mL) saved for osmolality measurement. Blood (for urea and electrolytes, osmolality) is collected and the patient weighed. An oral water load (20 mL/kg) is then consumed within 20 minutes. Further samples and weighings are obtained according to the following sched­ ule. Urine volume output is accurately measured each hour.

Sample

Time of urine collection (h)

Time of blood collection (h)

Time of weighing (h)

1 2 1 1 5

09.00 09.00-10.00 10.00-11.00 11.00 12.00 12.00-13.00

09.00

09.00

13.00

13.00

Interpretation In a normally hydrated subject over 8 0 % of a standard water load is excreted within 4 hours of ingestion. Urine osmolality will typically fall below 100 mmol/kg. Weight changes should confirm the observed difference between the water volume ingested and the urine volume loss. Difficulties in micturition may make the test difficult or impossible to interpret.

REFERENCE

REFERENCE Adapted from Robertson G L, Athar S. The interaction of blood osmolality and blood volume in regulating plasma vaaoprmin in т а л . Journal of Clinical Endocrinology and Metabolism 1976; 42:613-20.

Penney M D, Murphy D, Waller* Ci. Resetting of the osmoreceptor response а* л cause of hyponatraemia in acute idtopaOiic potyncuriitv British Medical Journal 1979; 2: 1474 Ъ.

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C H A P T E R

Hydrogen ion homoeostasis, tissue oxygenation and their disorders William J. Marshall

INTRODUCTION Disorders of both hydrogen ion homocostasis and tissue oxygenation will be discussed in this chapter. Abnormali­ ties of hydrogen ion homoeostasis occur frequently in respiratory disorders, as a result of changes in the rate of excretion of carbon dioxide. Such disorders can also affect oxygenation, and impaired tissue oxygenation is an impor­ tant potential cause of acidosis. Furthermore, hydrogen ion concentration, carbon dioxide and oxygen are meas­ ured using related technologies, usually with the same instrument. The first pan of this chapter deals with hydrogen ion homoeostasis and its disorders (colloquially often referred to as 'acid- base balance* and 'acid-base disorders' respec­ tively), while the second deals with the mechanism whereby oxygen is made available to the tissues and dis­ orders in which tissue oxygenation is impaired. T H E PHYSIOLOGICAL ROI-E OF HYDROGEN IONS Hydrogen ions are ubiquitous in the body and mainte­ nance of appropriate concentrations is critical to normal function. The gradient of hydrogen ion concentration between the inner and outer mitochondria! membrane drives oxidativc phosphorvlation; changes in hydrogen ion concentration can affect the surface charge and physical conformation of proteins and thus their function; and hydrogen ion concentration determines the degree of ionization of weak acids and bases and can thus affect the disposition of such substances, amongst which are many having important physiological and pharmacologi­ cal functions.

The hydrogen ion concentration of the blood is nor­ mally controlled within narrow limits, in health rarely exceeding 46 nmol/L or falling below 35 nmol/L. This regulation is achieved in spite of the continuous produc­ tion of hydrogen ions as a result of the normal processes of metabolism. Intracetlular hydrogen ion concentration is higher, that in the cytosol being slightly so while in some organelles, notably lysosomes, it is several orders of magnitude higher. Definitions An increase in the hydrogen ion concentration of the blood is termed acidaemia and a decrease alkalaemia. 1Ъс term acidosis strictly describes a pathological disturbance which can result in acidaemia but does not necessarily do so because of the simultaneous existence of another disturbance (possibly the result of a physiological com­ pensatory process) having an opposing effect. Similarly, alkalaemia is not always present in alkalosis. These dis­ tinctions, although made much of by some authors, often only introduce confusion and will not be pursued in this chapter. Strictly speaking, it is the actiznty of hydrogen ions and not their concentration that is relevant, and devices for measuring hydrogen ions respond to their activity. Activ­ ity and concentration are only the same in ideal solutions, which biological fluids are not, but the distinction can be ignored for practical purposes. Hydrogen ion concentration can also be expressed in terms of pH. The pH of a solution is the logarithm (base 10) of the reciprocal of hydrogen ion activity. Thus a solu­ tion with hydrogen ion concentration of 100 nmol/L (100 ol

Мг

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62

OUKIOAL BKHHl-MISTRY

x 10*mol/L) has pH 7.00 (log,,, 1/100 x 10")- pH does not have units. It varies in a reciprocal and non-linear fashion with hydrogen ion concentration. The reference range for the pH of blood, corresponding to the range for hydrogen ion concentration given above, is 7,36-7.42. It is beyond the scope of this chapter to discuss the reasons why the pH nomenclature was developed. It is clearly nec­ essary in discussing hydrogen ion homoeostasis and its disorders to consider the production and disposal of hy­ drogen ions; it is therefore logical to discuss the effects of changes in these processes on hydrogen ion concentration rather than on a derived unit. Furthermore, as will be seen, the analysis of disorders of hydrogen ion homo­ eostasis is considerably facilitated if direct measurements are used. Disturbances of hydrogen ion homoeostasis occur frequently and in a wide variety of disease states. They are traditionally classified as respiratory or non-respiratory in origin, according to whether the primary abnormality is due to an excess or deficiency of carbon dioxide (respira­ tory) or of bicarbonate (non-respiratory). Non-respiratory disturbances are often referred to as metabolic. As will be seen, however, an underlying respiratory disorder which produces hypoxia may produce an acid-base abnormality whose characteristics are non-respiratory or even combine respiratory and non-respiratory features. Nevertheless, the distinction is a useful one and is of considerable value in analysing disorders of hydrogen ion homoeostasis.

HYDROGEN ION HOMOEOSTASIS

What follows is a brief, simplified account of the essential features of buffering, sufficient for an appreciation of the physiology of hydrogen ion homoeostasis. In essence, buffer systems limit the extent to which the hydrogen ion concentration changes in the face of any tendency to change. A buffer system (or buffer pair) con­ sists of a weak (that is, only partly dissociated) acid and its conjugate base (that is, the anion which combines with a hydrogen ion to form the acid). If the acid is HB and the conjugate base is В , the relevant reactions are: HB

(1)

and HB + OH H : 0 + В

(2)

Bicarbonate An example of central importance to physiology is the carbonic acid-bicarbonate system; carbonic acid is a weak acid which partly dissociates to bicarbonate and hydrogen ion: H : C O , 1 rcagencvia

кт л Ш

elimination of acid in a it I 2-l b

-000 1300

;

Lull mmol'24 h

.,;

Excretion bj lung» Gluconcogcncv» 1 '\idation

■

1 UK)

Recsceriflcaaoa Oxidation

600 400 П40

Oxidation Охи' of unino at id* Renal excretion •i buffered

400 1100 40

acid)

balance, although any discrepancy can have a major effect on hydrogen ion concentration. But even this turnover of hydrogen ion appears insignificant in comparison with that which is associated with the turnover of adenine nucleotides, the movement of hydrogen ions which takes place across the mitochondrial membrane during oxidative phosphorylation and the synthesis and hydrolysis of ATP. This has been estimated at 500 mol/24 h. The po­ tential for a disturbance in these processes to cause an acidosis is clearly colossal, although in health the rates of adenine nuclcotidc reduction and oxidation and of ATP formation and utilization are equal, so that these processes have no net effect on hydrogen ion homoeostasis. This may not, however, be true in disease. As a result of oxidative metabolism carbon dioxide is produced and excreted by the lungs. The rates of forma­ tion and excretion are normally equal, but carbon dioxide can combine with water to form carbonic acid and the daily production of carbon dioxide in a resting adult is a potential source of approximately 20 mol of hydrogen ion. As has been alluded to, disorders of the excretion of carbon dioxide are an important cause of abnormalities of hydrogen ion homoeostasis. Tendencies for hydrogen ion concentration to change can be limited to some extent by buffering, but this proc­ ess can only offer a temporary solution to an imbalance between the rates of hydrogen ion production and dis­ posal because the body's buffers have a limited capacity. Physiological processes often bring about a partial reversal of a change in hydrogen ion concentration (compensation, sec below) but ultimate correction of any disturbance re­ quires equalization of the rates of production and disposal of hydrogen ion. Hydrogen ion production The processes involved in hydrogen ion production and utilization are summarized in Table 5.1. They comprise: processes involving carbon dioxide formation; reactions of intermediary metabolism; and processes involving 'fixed' acids. In addition, and responsible for the bulk of

hydrogen ion turnover, arc the reactions involved in the complete oxidation of energy substrates. These processes are interlinked, but it is instructive to consider them separately. Carbon dioxide The role of carbon dioxide in rela­ tion to the formation of hydrogen ion has been mentioned above. Carbon dioxide, produced by oxidative metabo­ lism, can become hydrated to carbonic acid, a weak acid which partly dissociates to hydrogen ion and bicarbonate: C O , + R O *-> H , C O , *-> H 4 + HCO,

(9)

The equilibrium for this reaction strongly favours carbon dioxide and water, but in tissues which contain carbonate dchydratase the rate of formation of carbonic acid is increased and it can become an important source of bicarbonate and hydrogen ion. Incomplete metabolism of glucose: glycolysis and lactate metabolism The most familiar process of inter­ mediary metabolism that results in the formation of hydro­ gen ion is anaerobic glycolysis, the metabolism of glucose to lactate. The overall equation for this reaction is: C.H.,0. Glucose

2CH,CHOHCOO Lactate

+2H

(10)

This process, which takes place particularly in skeletal muscle and erythrocytes, results in the formation of ap­ proximately 1.3 mol of hydrogen ion per 24 h in a 70 kg man at rest. The major route of disposal of lactate is glu­ cose synthesis by gluconcogenesis in liver and kidney. The overall equation for this process is the reverse of that for glycolysis; although most enzymes involved arc common to both pathways, some are unique. Gluconeogenesis thus consumes hydrogen ions. In health, lactate production and disposal are equal and so, too, are the production and disposal of hydrogen ion by these pathways. At rest, most of the lactate produced is converted back to glucose; during exercise, when lactate production is increased, 50% or more is instead completely oxidized. This process also consumes the hydrogen ions that are generated in its production and results in the formation of carbon dioxide and water:

ким np

• 4

HYDROGEN ION HOMOEOSTASIS AND TISSUH OXYGIiNA IION

С Н . С Н О Н С О О + Н ' + 3 0 ; - * З С 0 2 + З Н . О (11) I-actate Thus, when lactate production and utilization are in balance, there is no net production of hydrogen ion. However, in disease states, an imbalance between these processes can be responsible for the development of acidosis. '1Ъс hydrogen ion generated in lactate production will be buffered principally by bicarbonate so that Equation 10 could be written: CJHnQh + 2 HCO> -» 2 C H X H O H C O O + 2 CO, + 2 R O (12) Clearly the reverse reaction, gluconeogenesis, will regen­ erate the bicarbonate and Equation 11 can be rewritten to show that this also occurs with complete oxidation of lactate:

сн,снонсоо- + 30,-»2 co2 + HCO, + 2 Н , Ь

(П)

Incomplete metabolism of triglycerides: ketogenesis The liberation of free fatty acids from triglyceride (triacylglycerol) in adipose tissue results in the generation of hydrogen ions. The process is exemplified by the equation: (CH, (CH 2 ) 4 COO),R + 3 r l O - > Triglyceride 3 СН,(СН ; ) я СОО + R(OH), + 3 H Free fatty acid Glycerol

(14)

In the liver and adipose tissue, free fatty acids can be re-esterified to triacylglycerol, a process which consumes three hydrogen ions for each molecule of triacylglycerol synthesized. The further metabolism of free fatty acids to kctones in the liver (ketogenesis) also results in hydrogen ion production. An example of the reaction, starting from palmitate, is: CH» (CH : ) M COO + 6 tV -> 2 CH.COCH^COO + Palmitate Acetoacetate 2 CH,CHOHCH,COO + 2 H : 0 + 3H* (15) 2-Hvdroxvbutvrate In health, this process may account for up to 0.4 mol hydrogen ion per 24 h, although in pathological states its contribution may be much greater. However, ketoacids arc utilized as energy sources by skeletal muscle and other tissues. Their oxidation consumes hydrogen ions so that, in health, the rates of hydrogen ion production and disposal are equal: C H , C O C H , C O O + H + 4 02 -+ 3 H 2 0 + 4 CO.(16) Acetoacetate 2 CHiCHOHCH.COO + 2 H * + 9 0 2 - > 2-Hydroxybutyratc 8 H.O + S C O , (17)

65

In disease, notably in diabetic ketoacidosis, excessive ketogenesis is an important cause of acidosis. Complete oxidation of glucose and triglycerides Glucose can also be completely oxidized to carbon diox­ ide and water; indeed, this is the major route of glucose metabolism in the body. СьН 1 2 0, + 6 Ok -> 6 C O , + 6 H.O

(18)

This is a complex process; oxidation is indirect, involving the transfer of hydrogen ions to adenine nucleotides, which arc mainly reoxidized by the mitochondria! electron transport system. Electrons are transferred to oxygen and combination with hydrogen ions produces water, while the energy released is transferred to ATP. But as Equation 8 indicates, the complete oxidation of glucose to carbon dioxide docs not give rise to net formation of hydrogen ion; the products of its oxidation are carbon dioxide and water. The same is true for the complete oxidation of triacylglycerols: [ C H l ( C H : ) N C O O ] 1 C , H , + 72", O, ^ 51 C O , + 49 H , 0 Tripalmitoylglycerol (19) As with glucose oxidation, the process is far more com­ plicated than appears from Equation 19, again involv­ ing the reduction of nucleotides, transfer of electrons to molecular oxygen, formation of water and trapping of energy in ATP, but the net production of hydrogen ion is zero. Amino acid tnetaboKsm Amino acid metabolism both produces and consumes hydrogen ions, according to the type of amino acid concerned. *l~hc metabolism of neutral amino acids eventually results in the formation of urea and carbon dioxide, for example: 2 C H , C H N H / C O O H O : - » CO(NH,) Alanine Urea 5 CO, + 5 К О

2

+ (20)

It is instructive, however, to look at this process in more detail. Most amino acids arc metabolized by transamination in the liver to yield the corresponding oxoacid, the amino group being transferred to 2-oxogluiarate to form glutamatc. Glutamalc undergoes oxidativc dcamination, the amino group being convened to ammonium. The car­ bon skeletons of amino acids are in general glucogenic, although some are ketogenic. Ultimately they will be completely metabolized. Omitting the transamination step, the intermediate stages are: 2 C H , C H N H / C O O + O, -» 2 C H . C O C O O + 2 NH, Alanine Pyruvate (21) 2 C H , C O C O O + 50ь, + 2 Н * - » 6 C O , + 4 r l O

(22)

C O , + 2 N r V -» CO(NH>) , + R O + 2 H*

(23)

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Thus, although urea synthesis generates hydrogen ions, these are utilized during the metabolism of the carbon skeleton so that the metabolism of neutral amino acids does not result in net generation of hydrogen ions provided that the nitrogen is converted into urea. If this does not occur, the metabolism of these amino acids consumes hydrogen ions. The relevance of this is discussed in a later section of this chapter. The complete oxidation of a dibasic amino acid results in the generation of hydrogen ion, e.g. for lysine: ЫН|'СН,(СН 3 ),СН ( N r V ) C O O * 7 0 , ч Lysine CO(NH : ) t + 5 CO. + 5 H , 0 + H ' (24} The complete oxidation of a dicarboxylic acid consumes hydrogen ions, e.g. for aspartatc: 2COOCH(NH,')CH.CtX) + 2 H - » Aspartate C O ( N H : ) 2 ± 6 C O , + 5 H.O

(25)

The complete oxidation of sulphur-containing amino acids (cysteine, methionine) generates hydrogen ions, e.g. for methionine: 2 CH,S(CH,) : CH (NH, + )COO + 15 Ог -> Methionine C:O(NH.0, + 9 CO, + 7 R O + 4 И" + 2 S O /

(26)

In each of these last three examples it has been assumed that the end-product of the amino nitrogen is urea. If it is not, the effect on hydrogen ion metabolism may be different. Overall, the amino acid composition of dietary protein and the manner of amino acid metabolism is such that in health there is a small net production of hydrogen ion. This is disposed of primarily by renal excretion. Hydrogen ion excretion Carbon dioxide Carbon dioxide is excreted via the lungs. The respiratory control mechanisms are exquisitely sensitive to carbon di­ oxide so that in health (provided that no conscious effort results in under- or overvenrilation) the rate of carbon dioxide elimination is made equal to the rate of produc­ tion and blood carbon dioxide concentration remains constant. Hydrogen ion Excess hydrogen ions arc excreted in the urine and, because overall the body is a net producer of acid, the urine is usually acidic. But before the urine can be acidi­ fied, there must be complete reabsorption of filtered bicarbonate.

Bicarbonate reabsorption The glomcrular filtrate contains bicarbonate at the same concentration as the plasma. Normally, the urine is virtually bicarbonate-free and clearly, were the filtered bicarbonate not to be reabsorbed, the body's bicarbonate pool, and thus buffer­ ing capacity, would rapidly be depleted. Generation of bi­ carbonate from carbon dioxide and water necessarily also involves the production of hydrogen ion. The luminal membrane of renal tubular cells is rela­ tively impermeable to bicarbonate, so that filtered bicar­ bonate cannot be reabsorbed directly. Instead, carbonic acid is generated from carbon dioxide and water in renal tubular cells (catalysed by carbonate dchydraiase) and dissociates into hydrogen ion and bicarbonate; the former crosses the luminal cell membrane in exchange for actively reabsorbed sodium and reacts with filtered bicarbonate, generating carbon dioxide. This can diffuse into tubular cells and, together with the carbon dioxide produced by aerobic metabolism, provides the substrate for the contin­ ued formation of carbonic acid. Although the equilibrium for the reaction favours carbon dioxide and water, the presence of the catalyst, and the continual removal of hy­ drogen ion, drives the reaction in the direction of hydro­ gen ion and bicarbonate formation. The bicarbonate formed accompanies sodium as it is pumped across the basal membrane of tubular cells into the interstitial fluid. The net effect is the reabsorption of filtered bicarbonate, with an equivalent amount of sodium. In health, bicar­ bonate 'reabsorption* is complete in the proximal renal tubule. This process is summarized in Figure 5.2. Hydrogen ion excretion When bicarbonate reab­ sorption is complete, continued production of hydrogen ion bv renal tubular cells and its movement into the tubular fluid will constitute net hydrogen ion excretion. There is, however, a limit to the acidity of urine that can be achieved. This is a pH of approximately 4.5, equivalent to a hydrogen ion concentration of 38 fjmol/L. This repre­ sents a thousandfold concentration gradient with respect to the extracellular fluid, but clearly excretion of hydrogen ion itself is insufficient to remove the burden of acid produced by metabolic processes, which is measured in mmolcs. Significant acid excretion is achieved by hydro­ gen ion being buffered by phosphate, titrating the monohydrogen to the dihydrogen ion. It should be noted that, since the formation of hydrogen ion in renal tubular cells is accompanied by the production of an equal amount of bicarbonate, the process of hydrogen ion excretion regen­ erates bicarbonate that is consumed by buffering. This process is summarized in Figure 5.3. The role of urinary ammonium excretion Although the urine contains ammonium, and indeed the amount excreted increases in states of chronic acidosis, this cannot, for the reasons indicated above, constitute net hydrogen ion excretion. Ammonium is produced in renal tubular cells by the action of the enzyme glutami-

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HYDROGEN ION HOMOEOSTASIS AND TISSUF. OXYGEKATION

Glomerulu»

67

HOC

Proximal tubular cell

Carbonate lehydratase Plasma

*н2СОз

HCO

Tubular lumen

Fig. 5.2 Reabsorption of filtered bicarbonate. Bicarbonate ions cannot be direvtly rcahsorbed. Instead, hydrogen ions and bicarbonate are generated from carbon dioxide in tubular cell*; the hydrogen ions are secreted into the tubular lumen in exchange for sodium and titrate the filtered bicarbonate. Seme of the carbon dioxide thus formed diffuse* intu renal tubular cells. "I ne bicarbonate formed in the cells accompanies sodium as it is pumped into the interstitial fluid and thence reaches the plasma.

Gtometuius

Monoriyojogen obosphate in gtomerular filtrate

Tubular call

Plasma

HC л tanner increase in plasma bicarbonate concentration (in addition to that generated directly by the erythrocytc mechanism). Although this is usually considered to be a consequence of increased renal hydrogen ion excretion (since bicarbonate is generated pan passu with hydrogen ions in renal tubular cells), it is probably mainly a result of diversion of ammo­ nium from ureagenesis, requiring decreased buffering by bicarbonate of hydrogen ions produced during this process. Practically, it is important to appreciate that this compensation evolves over several days of carbon dioxide retention. If an attempt is made to reduce PCO: rapidly, for example by artificial ventilation, temporary persistence of the compensatory process may result in the develop­ ment of an alkalosis. '1Ъеге is a limit to which changes in renal acid excre­ tion and in ammonium metabolism can compensate for an increase in /*C0 2 ; if this rises above 8 kPa, arterial hydrogen ion concentration will always be increased. Biochemical characteristics of respiratory acidosis The cardinal features of respiratory acidosis are an in­ creased blood P C O . and a high, or high-normal, hydro­ gen ion concentration; bicarbonate concentration is increased. The hydrogen ion and bicarbonate concentra­ tions for any P C 0 2 depend on the extent of the compen­ satory increase in renal hydrogen ion and ammonium excretion (sec above). In an acute disturbance, the in­ crease in bicarbonate is only of the order of 2-A rnmol/L, even with massive increases in PCO,, but in compensated disturbances the increase is much greater.

Systemic effects in respiratory acidosis In patients with respiratory acidosis, the manifestations of the underlying disorder and of hypoxaemia, if present, usually dominate the clinical findings, but effects due to acidosis and to hypercapnia may also be present. Hypo­ xaemia causes breathlessness, cyanosis and drowsiness. The consequences of acidosis have been discussed above. The effects of hypercapnia are seen predominantly in the central nervous and cardiovascular systems. The neurological effects of hypercapnia cover a spec­ trum from anxiety and confusion through to impaired consciousness and coma. Particularly in chronic carbon dioxide retention, headache, papilloedema, extensor plantar responses and myoclonus may occur. Most of these effects arc due to the increased cerebral blood flow that is a consequence of the vasodilatory action of carbon dioxide. Systemic vasodilatation also occurs, but the cardiac output is increased so that blood pressure is usually main­ tained. The skin is warm and arterial pulses are bounding. '1Ъе acidosis may cause venous constriction and chronic hypoxaemia may cause pulmonary hypertension and cor pulmonale, rendering the patient very susceptible to pulmonary oedema should intravenous fluids be given injudiciously.

Managemettt 1Ъс logical management of respiratory acidosis is to treat the underlying cause and thus restore IK10£ to normal. This may not be possible and, in chronic carbon dioxide retention» if compensatory processes have restored the blood hydrogen ion concentration to normal or near nor­ mal, it may not be necessary. In practice, the management of respiratory disorders is usually dictated by the necessity to maintain an adequate arterial 1Ю:. As mentioned above, rapid correction of an elevated ЛССХ in a patient with chronic carbon dioxide retention is potentially dan­ gerous. The compensatory changes can persist for several hours, or even a few days, and may cause the patient to become alkalotic. This demonstrates the fact that the compensatory process in a respiratory acidosis can be regarded as the physiological generation of a non-respiratory alkalosis, although the patient's hydrogen ion concentration does not fall below normal, i.e. the patient docs not become frankly alkalotic, as long as the PCO, remains elevated. Non-respiratory alkalosis This disorder can develop because of excessive loss of hydrogen ion, decreased generation of hydrogen ion or exogenous alkali administration. Some of the causes are

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HYDROGEN ION HOMOEOSTASIS AND TISSUE OXYGENATKW ТаМс S. 7

rcspi/

Sal: tpomnm Gastrointestinal cause* ■mmng gastric drainage congenital chloride-losing durTtioea u* alkali admini%tr> sodium ЬвШгЫаШе, U a j i e . acetate espcuallv if GFR reduced l • ■ causes poorly reabMirhjhle action therapy 'e.g. carbenwilltn) ром [>nia diureticv especuillv in congestive cardiac failure. ncphronc syndrome, rirrhosi» Satimt-mnrftpi-nitvt Associated with hypertension primary aklostcrofusm aahanf*! syndrome secondary aldosteronism bqu ori carbenoxolonc Not usually associated with hypertension

Banter

ndmne

n >ig after starvation severe potassium depletion magnesium deficient-

indicated in Table 5.7 and a selection of these arc discussed later in this section. Non-respiratory alkalosis is characterized by an in­ crease in plasma bicarbonate concentration. Since this bicarbonate is filtered at the glomcrulus and therefore available for urinary excretion, the persistence of a nonrespiratory alkalosis implies that it is perpetuated by in­ appropriate reabsorption of filtered bicarbonate, Indeed, in healthy subjects, it is very difficult to produce a sus­ tained alkalosis by the administration of, for example, sodium bicarbonate, whether orally or intravenously, because the excess is excreted in the urine. In considering the body's responses to non-respiratory alkalosis, it is therefore necessary to take into account not only the underlying cause, but also the factors responsible for its perpetuation. Three such factors appear to be important: • extracellular volume contraction; • potassium deficiency; • mineralocorticoid excess. These are discussed further below. Compensation for non-respiratory alkalosis Buffering A fall in blood hydrogen ion concentration results in the release of buffered hydrogen ion and in consequence the blood bicarbonate concentration in­ creases. The excess bicarbonate could be excreted by the kidney but, as alluded to above, this process is impeded in sustained alkalosis. Hypoventilation In a systemic alkalosis, decreased

77

stimulation of chcmorcccptors would be expected to de­ crease the respiratory drive, leading to compensatory re­ tention of carbon dioxide. However, any increase in /°C0 2 will tend to stimulate respiration and lessen the extent of the acute compensatory response. With the passage of time, this response may increase, because the central respiratory centre becomes less sensitive to an increase in PCO,. However, hypoventilation will tend to decrease arterial P02 and a hypoxaemic stimulus will override any inhibitory effect of a low hydrogen ion concentration. For these reasons, the respiratory compensation for a non-respiratory alkalosis is usually incomplete. Renal bicarbonate excretion As indicated above, persistence of a non-respiratory alkalosis implies contin­ ued and inappropriate renal bicarbonate reabsorption. This could be achieved by the combination of a fall in the glomerular filtration rate with maintenance of normal rates of tubular bicarbonate reabsorption or by enhanced tubular reabsorption with a normal glomerular filtration rate. In many patients with non-respiratory alkalosis, there is both a decrease in the glomerular filtration rate and increased bicarbonate reabsorption. A decrease in extracellular fluid volume may lead to a decrease in the glomerular filtration rate. If this is associated with chloride deficiency, the requirement to maximize tubular sodium reabsorption may cause an obligatory increase in the reabsorption of filtered bicarbo­ nate to maintain electrochemical neutrality. In the major­ ity of patients with non-respiratory alkalosis, correction of the alkalosis follows repletion of extracellular fluid volume by the infusion of an isotonic sodium chloride solution. Non-respiratory alkalosis is frequently associated with potassium deficiency, but the fact that in these cases the alkalosis can usually be corrected by volume expansion without replacement of potassium casts doubt on the pre­ cise role of potassium. However, potassium depletion can contribute to the maintenance of a non-respiratory alkalo­ sis through an effect on bicarbonate reabsorption. Severe potassium depletion enhances proximal bicarbonate reab­ sorption. Distal tubular sodium reabsorption takes place in exchange for potassium and hydrogen ions; particularly when there is enhanced distal sodium reabsorption, potas­ sium depletion may result in an increased secretion of hydrogen ions into the tubular fluid. The third factor which can maintain a non-respiratory alkalosis is an increase in mineralocorticoid activity. This promotes distal tubular sodium reabsorption and results in increased excretion of both potassium and hydrogen ion. The effect is potentiated by potassium depletion, in­ creased distal sodium delivery (as with diuretic treatment) and by the presence of non-rcsorbablc anions, which accentuate the negativity of the luminal aspect of tubular cells. Increased mineralocorticoid secretion can occur secondarily to extracellular fluid volume contraction or be

78

CUNICAt. BUHHKMIS! RY

primary, as in Conn's and Cunning's syndromes. In these two conditions, extracellular fluid volume is expanded and the alkalosis, unusually, is not corrected bv saline infusion. Biochemical characteristics oj non-respiratory alkalous *1Ъе blood hydrogen ion concentration is low and the bicarbonate concentration increased; respiratory compen­ sation may increase PC02y but not to more than about 8 kPa. Hypokalaemia is almost always present. Systemic effects of alkalosts In general, the effects of alkaiosis are the opposite of those of acidosis The effects on the cardiovascular system are, however, less in alkalosis and infrequently of clinical con­ sequence. The effects of alkalosis on oxygen delivery to tissues are unfavourable in that oxygen delivery is im­ paired. Alkalosis is rarely sustained over a long period and there is no evidence of any adverse effects on bone. Chronic non-respiratory alkalosis is frequently associ­ ated with potassium depiction and hypokalaemia. This is related to increased distal tubular secretion of potassium as a consequence of decreased hydrogen ion secretion. Thus alkalosis can cause potassium depletion and potas­ sium depletion sustains alkalosis. If sodium depletion is also present, the stimulation of maximal distal tubular sodium reabsorption will also contribute to potassium depletion. Neuromuscular hyperexcitability is frequently present in patients with acute respiratory alkalosis, manifest as paraesthesia, muscle cramps and tetany. It is unusual in non-respiratory alkalosis, except when the hydrogen ion concentration falls rapidly, as has been reported in pa­ tients with chronic respiratory acidosis treated with me­ chanical ventilation. Grand mal convulsions have been reported in such patients and alkalosis can precipitate a fit in patients with epilepsy. The usual explanation for these observations is that buffering of hydrogen ion by plasma proteins, particularly albumin, decreases in alkalosis and that calcium binding to protein increases as a result, thus lowering the plasma ionized calcium concentration. Whether this is a complete explanation is, however, open to doubt. Management of non-respiratory alkalosts Management should be directed towards treatment of the underlying cause of the alkalosis, when possible. As al­ luded to above, treatment can also be directed towards the correction of any factors tending to sustain the alkalosis. Most patients will respond to expansion of the extracellu­ lar fluid volume with isotonic saline. The demonstration of a low urinary chloride concentration reliably predicts

those patients who will respond to this treatment. This is frequently combined with potassium replacement, although in many instances the alkalosis can be corrected by the administration of saline alone, even if there is potassium depletion, although this latter may require correction in it own right. Administration of saline is inappropriate (and poten­ tially dangerous) in patients with saline-unresponsive causes of non-respiratory alkalosis. Management must be directed towards the underlying cause, e.g. removing the source of excessive mineralocorticoid secretion (or blockade of mineralocorticoid action) or replacement of potassium or magnesium, as appropriate. Specific causes of non-respiratory alkalosis Loss of gastric acid The most severe non-respira­ tory alkalosis is seen in patients losing unbuffered acid from the stomach because of either gastric drainage or prolonged vomiting, particularly in association with pyloric stenosis, which prevents the concomitant loss of alkaline secretions from the proximal small intestine. The acid is hydrochloric acid so that patients become chloride depleted. The hydrogen ions are derived from carbonic acid, pan passu with bicarbonate. Initially, renal excretion of the excess bicarbonate may prevent the development of alkalosis, but with extracellular fluid volume contraction the requirement to maximize renal sodium reabsorption in the face of hypochloraemia necessitates increased reab­ sorption of sodium with bicarbonate. Indeed, in severe cases, renal bicarbonate reabsorption may be complete in spite of the high plasma concentration, resulting in the excretion of an acid urine. Potassium is lost in gastric fluid, but increased aldosterone secretion may result in significant loss of potassium in the urine as well, exacer­ bating the potassium depletion and the alkalosis. Thus all three of the factors mentioned above can be involved in the maintenance of the alkalosis. The alkalosis usually re­ sponds to re-expansion of the extracellular fluid volume with 'normal' saline. Mineralocorticoid excess Syndromes of mineralo­ corticoid excess arc almost invariably associated with nonrespiratory alkalosis, for reasons that have been outlined above. When the excess is due to primary adrenal disease (Cushing's syndrome, Conn's syndrome), patients are ex­ tracellular fluid volume expanded. This should oppose the effect of potassium depletion on proximal bicarbonate reabsorption and it is likely that the alkalosis is maintained largely by increased distal reabsorption. Chronic hypercapnia It has been pointed out that the rapid lowering of a chronically elevated PCO: can result in the compensatory processes causing a frank non-respiratory alkalosis. If P C O . falls to normal over a few days, bicarbonate is excreted and an alkalosis does not develop, but if this occurs in patients with contraction

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HYDROGEN ION HOMOEOSTASIS AND TISSUE OXYC.I-NATION

of the extracellular fluid volume, for example as a result of diuretic treatment, alkalosis may become apparent and persist. Respiratory' alkalosis Respiratory alkalosis is a consequence of the rate of excre­ tion of carbon dioxide exceeding the rate of production, leading to a decrease in PCO : . This is usually due to stimulation of the respiratory' centre; the stimulus may be toxic, reflex, psychogenic or related to the presence of an intracranial lesion. The exception is mechanical venti­ lation, when normal respiratory control is overridden. Some of the causes of respiratory alkalosis arc indicated in Table 5.8. Compensatory транш

in respiratory alkalosis

Buffering In acute respiratory alkalosis, the fall in PCX), causes a decrease in hydrogen ion concentration and a slight fall in bicarbonate. Other buffers release hydrogen ions, so tending to counter the effect of the fall in /^CO : ; some of these hydrogen ions will combine with bicarbonate, causing its concentration in the blood to fall further. A new steady state can be attained rapidly and can persist for approximately 6 hours, after which the ef­ fect of changes in renal hydrogen ion metabolism become detectable. Hypoventilation Correction of a respiratory alkalo­ sis is only possible if the rate of excretion of carbon diox­ ide can be restored to normal. The fact that an alkalosis develops (other than with mechanical ventilation) indi­ cates that the inhibitory effect of the decrease in PCQ2 on

Voluntary hvrH-rvcnuUoan Mechanical усппЫню Reflex hypcrvcniiUtion decreased pulmonary compliance disease affecting I wall irritative lesions of the air passage* Other stimuli to respiratory centre meal influence* pain fever anxiety ei local disease trauma tum>w) and PCO, (also low) in such patients may suggest a partially compensated acute respiratory disturbance, but the clinical features will be inconsistent with this as the sole diagnosis.

TISSUE OXYGENATION Introduction The process whereby atmospheric oxygen is made avail­ able to mitochondria is a complex one, depending on adequate alveolar ventilation and function, pulmonary and tissue blood flow and the ability of the blood itself to take up oxygen in the alveoli and release it to tissues. Tissue oxygenation can be compromised by disease affect­ ing any of these functions. Until relatively recently, the only readily available index of tissue oxygen supply was the arterial partial pressure of oxygen, / Ъ 0 2 . This meas­ urement is still regarded as essential, but it has a number of limitations. It requires access to arterial blood, either by direct puncture or through an indwelling catheter; blood samples must be collected with considerable care

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CUSUIAI. HJCXIHKMISTRY

and analysis performed without delay; but, perhaps most importantly, measurement of P J O . provides in­ complete information on oxygen transport. It is a measure of partial pressure, not of the oxygen content of blood or the delivery of oxygen to tissues. However, although tissue oxygenation depends upon factors other than ftO,, main­ tenance of an adequate ftO, is a prerequisite for normal tissue oxygenation. This is illustrated in the following sec­ tions, which discuss the transport of oxygen from the inspired gas to the tissues, where it is used for oxidative metabolism. P u l m o n a r y function Alveolar ventilation The partial pressure of oxygen in arterial blood, PaO-., depends on the alveolar oxygen tension ( P A O : ) , which is in turn dependent on the fraction of the inspired gas which is oxygen (FiO_0* the alveolar carbon dioxide ten­ sion (РлСО : ), the respiratory quotient (RQ), atmospheric barometric pressure (PB) and the partial pressure of water vapour (РНЛЭ) such that: PAO:

= FiO, :< (PB - P H : 0 )

- PACO/RQ

(29)

The respiratory quotient depends on the relative propor­ tions of free fatty acids, carbohydrate and protein being used as energy substrates by the tissues, but it varies through only a small range even in disease. Alveolar air is always saturated with water so that fil:0 is constant. It follows that P A O : can increase significantly as a result of either an increase in PB (requiring a hyperbaric chamber) or FiO;, (requiring the administration of oxygen) or a decrease in Р л С О ; (requiring an increase in ventilation). Only the last two are commonly available to treat patients with a low РлО : . Inspection of Equation 29 will indicate that the effect of a change in PACO: on P A 0 3 will be greater if FiO, is normal than if it is raised; an increase in PACO_. (usually accepted as being equal to ACO_,) can significantly decrease PAOZ at normal values of FiO., but it will have a lesser effect if the FiO : is increased.

alveoli is such that any impairment of diffusion must be considerable before it affects PaO, at rest. The maintenance of a normal 1лаО: also requires a nor­ mal relationship between the perfusion of alveoli and their ventilation; the effects of a disturbance in this relationship (venttlation-perfusion imbalance) are considered further below. In health, a small proportion of the blood reaching the lungs from the tissues bypasses the alveoli and does not take part in gas exchange (a right-uHcft shunt). This is because the bronchial veins drain directly into the pulmo­ nary veins, while some blood that perfuses the myo­ cardium drains directly into the cavity of the left ventricle. Any increase in shunting due to a pathological process will tend to decrease PaO : . The final factor which can affect P&02 is the oxygen tension in the blood reaching the lungs, that is mixed venous P0 2 > or /V0 2 . If this is low, increased alveolar oxygen transport will be necessary to allow maximal /\iO ; . In health, increased tissue oxygen requirements (as for example during exercise) result in a fall in PvO, and the increased oxygen requirement is met by hypcrvcntilation. Tissue oxygen requirements arc often increased in dis­ ease, e.g. as a result of sepsis or the metabolic response to trauma, but at the same time the physiological responses leading to hypcrvcntilation may be attenuated. Other fac­ tors which can contribute to a low PvOz include decreased oxygen saturation, anaemia and a decrease in cardiac out­ put, all of which, as will be discussed below, can decrease the delivery of oxygen to tissues.

■ ■

In summary, a low Pa02t that is, hypoxaemia, can be caused by any of: • a decrease in PAO: (due to a decrease in the proportion of oxygen in the inspired gas, an increase in P A C O : or a decreased barometric pressure); • hypovcntilation; • decreased diffusion; • imbalance of ventilation and perfusion; • an increase in shunting; • a decrease in /VO..

Oxygen uptake into blood The continuous delivery of mixed venous blood of low oxygen content to the alveolar capillaries and diffusion of oxygen down the concentration gradient from the alveolar space to the blood results in a constant tendency for РлО, to fall which is prevented by the delivery of oxygen to the alveoli by ventilation. In healthy young individuals, oxy­ gen diffuses readily from alveoli into the plasma and PaO, is usually onlv about 1 kPa less than P A O , ; the difference ((л-а)/Ю,) increases with age and in healthy 60-year-olds may reach nearly 4 kPa. Pulmonary disease which impairs diffusion is a potential cause of an increase in (A-a)JX). and hence of a decrease in /ЪО„ but the nature of the

The role of haemoglobin in oxygen transport Although the amount of oxygen present in physical solu­ tion in the blood is directly related to ftO; (0.23 xxiUU kPa), only a small amount of oxygen is carried in this way (at normal PaO : , approximately 3 m L per Hire of plasma). Oxygen is principally transported in the blood bound to haemoglobin. One gram of haemoglobin can bind 1.34 m L of oxygen when fully saturated. The normal P&0: of arterial blood is approximately 13.3kPa, at which haemoglobin is approximately ° 7 % saturated. Total arterial oxygen content (Ca0 2 ) is given by sum of the dissolved and haemoglobin-bound fractions:

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HYDROGEN ION HOMOEOS'I AStS ANI> TlSSLT OX YGK NATION

CaO. = ([Hb] x А С У 100) x 1Л4 + (0.23 x /ЪО>) (30) where [Hb] is the haemoglobin concentration (g/L) and SaO, is the percentage saturation of haemoglobin with oxygen. At sea level, CaO_, is normally about 200 m l / L . SaO, can be measured using a co-oximeter; this instru­ ment also measures haemoglobin concentration and thus allows calculation of arterial oxygen content, which pro­ vides far more information than ftiO, alone. Modern blood gas analysers often incorporate a co-oximeter and such instruments are widely used in intensive therapy units. Arterial oxygen content can be measured noninvasively (transcutaneously) using a pulse oximcter. It is beyond the scope of this book to discuss the analytical principles utilized in these instruments. The familiar sigmoid relationship between Hb satura­ tion and oxygen tension (Fig. 5.7) has a number of impor­ tant clinical consequences. First, a considerable reduction in ftO, below the normal has little effect on the amount of oxygen carried in the blood. Saturation only falls below 90% when ftO, falls below 8 kpa. If ЛаО, falls further, however, oxygen saturation (and thus the amount of oxy­ gen carried) decreases rapidly. A further consequence is that, because haemoglobin is saturable, increasing PaO, above that necessary to provide complete saturation has relatively little effect on arterial oxygen content^ since only the small fraction present in solution is significantly increased. The effects of pulmonary disease on oxygen uptake into blood The effects of alveolar hypoventilation and impaired diffu­ sion on pulmonary function have been alluded to above. Two other functional defects, shunting and ventilationSaturation (%}

!

j

-*

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5

:

,

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TO2 (kPa) Fig. 5.7 The oxygen-haemoglobin dissociation curve. Ai a normal arterial PO} (point V ) , haemoglobin h aboui 47?*© saturated (SaOi = 97'Уо). Point у represents the normal venous № , , at which haemoglobin is 75?'n saturated uVvO. = 7St'»)- A decrease in arterial Ю . to 8 kPa (point '%') is associated with a decrease in Л*аО? to only 90%, but a further decrease will have a proportionately greater effect on S»Q. and thus on arterial oxygen content.

83

perfusion imbalance, can also have profound effects on the oxygenation of blood. Shunting Haemoglobin is only about 97% saturated in normal arte­ rial blood. The is in part because of physiological shunt­ ing. In some conditions (for example lobar pneumonia, pulmonary oedema, adult respiratory distress syndrome) some alveoli become filled with fluid and do not take part in gas exchange, although they arc still perfused with blood. Shunting is thereby increased and this leads to arterial hypoxacmia. Atelectasis (collapse of a lung or part of a lung so that it is not aerated) has the same effect. Under such circumstances, increasing P\0? by increasing F i 0 2 has little effect on overall ftO:. This is because no gas exchange will be taking place in the alveoli which are acting as shunts, while haemoglobin in blood leaving normally functioning alveoli will already be fully saturated with oxygen. Increasing FiO_, will increase only the small proportion of oxygen held in physical solution in this blood and this will have little efFect on total arterial oxygen content.

Vemilation—perfusion imbalance The fact that haemoglobin is not fully saturated in normal arterial blood is also related to an imbalance between alveolar ventilation and perfusion. At rest, ventilation is about 4.2 LVmin and pulmonary blood flow 5.5 L/min, so that the overall ventilation-perfusion ratio (V/Q ratio) is approximately 0.8. However, this ratio is not uniform throughout the lungs, ranging between the approximate limits 0.5 and 3.0. Some alveoli arc better ventilated than they are perfused ( Р / ф > I), so that a proportion of venti­ lation is 'wasted 1 and the effect is an increase in 'dead space'; in others, ir and £) arc in balance. In both cases, effective oxygenation of the blood takes place. However, in those alveoli in which perfusion exceeds ventilation (VfQ < 1), complete oxygenation of the blood is impossi­ ble. Because haemoglobin is saturable, oxygen transport in the well-ventilated and normally ventilated alveoli cannot compensate for decreased oxygenation of blood in poorly ventilated alveoli. Many pulmonary diseases, nota­ bly chronic obstructive airways disease and interstitial pul­ monary disease, give rise ю an increase over the normal imbalance of ventilation and perfusion. However, in con­ trast to disease in which significant shunting occurs, an increase in FiO : , because it will increase oxygen transport in poorly ventilated alveoli, can increase overall /ЪО : .

Differential effects of pulmonary disease on PaCG2 and PO, It will be instructive at this point to compare the effects of respiratory disease on /ЪСО, and PaO:. Carbon dioxide

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C U M C A I . BKH:HKMISTKY

is transported in blood effectively in physical solution. In contrast to the oxyhaemoglobin dissociation curve, the curve relating the carbon dioxide content of blood to par­ tial pressure is almost linear over the physiological range. As a result, a significant change in ЛаССУ. will always cause a significant change in the carbon dioxide content of blood. In contrast, as has been seen, considerable changes in ftO, can occur with little effect on oxygen content. ftCO, is determined by alveolar ventilation; in health, any increase in carbon dioxide formation can be matched by an increase in excretion. In pulmonary disease, if A C O : is increased, then Рг02 will always be decreased. Thus, if hypoventilation causes hypercapnia, it will also cause hypoxacmia. However, hypercapnia is not always present in patients who arc hypoxaemic. Carbon dioxide can diffuse between the blood and the alveolar air more readily than oxygen; defects in diffusion rarely cause hy­ percapnia, although they can cause hypoxaemia. Shunting causes a decrease in PzO„ but this will stimulate hyperventilation, increasing the excretion of carbon dioxide from ventilated alveoli and preventing a rise in ftiCO: or even causing hypocapnia. Only with extensive shunting will the PzC02 be increased. In pulmonary disease causing an increase in Г/ф imbalance, there is a tendency for А О , to fall, for the reasons discussed above; this will stimulate respiration but, while the increased ventilation of well-perfused alveoli cannot compensate for the impaired oxygenation, it can increase carbon dioxide excretion and A G O , may fall. With more severe disease, however, hypoxaemia will be accompanied by hypercapnia.

Oxygen transport to tissues Oxygen delivery The transport of oxygen to tissues depends not only on adequate transfer of oxygen from the inspired gas to the alveolar capillaries, but also on an adequate cardiac out­ put. '1Ъс total amount of oxygen delivered by the cardiopulmonary apparatus (D0 2 ) is given by the product of oxygen content (CaOO and cardiac output (CO):

/ХУ=Са0.хСО

(31)

However, the amount of oxygen available to tissues will depend upon local perfusion and the affinity of haemo­ globin for oxygen, which determines how readily oxygen can be released. Oxygen uptake Oxygen is taken up into tissues because their P O : is lower than that of the blood. But examination of the oxyhaemo­ globin dissociation curve will indicate that the amount of oxygen taken up will depend on the haemoglobin saturaU"ii SO nt л fciivcn / W Mixed Vi/MOU:. oxygen 4itUr.i tion (AVO,) at rest is approximately 75%, corresponding to a / V 0 2 of 5.3 kPa and permitting the uptake of 46 m L of oxygen from each litre of blood. Total tissue uptake of oxygen is about 250 mL/min at rest. However, if the oxy­ haemoglobin curve were to shift to the left, less oxygen would be released from haemoglobin for the same fall in ГО:, whereas a right shift would increase the availability of oxygen. An increase in ЯСО. or [H*) (both of which

% saturation

arterial oxygen delivered to tissue %

1 2 , 3 - OPG i[H«] IPCOO

P 0 2 (kPa) P» 50

Normal P*0.

Fig. S. a *I "he oxygen-haemoglobin dissociation curve. An increase ш ЛСО>, [H\[ от red cell 2,5-<Jir»ho*phoglyccraie (24*-DKG) causes a right shift in the curve, decreasing the affinity "f haemoglobin for oxygen (PK„ increased) and increasing the amount of oxygen available to the tissues- A decrease in /XJO>, [H*) or red cell 2,*-Dt*G causes a left shift, which has the opposite effect.

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HVMOGBN ION HO.MOBOSTASIS AND TISSUE OXYCINATION

occur as blood traverses the capillary beds) causes a right shift, as docs an increase in temperature. The position of the curve is also determined by the concentration of 2,3-diphosphoglyccratc (2,3-DPG) in crythrocytcs. An increase in 2,3-DPG concentration causes a right shift. This occurs in chronic hypoxia and thus facilitates oxygen uptake by tissues. These effects are illustrated in Figure 5.8. The position of the oxyhaemoglobin saturation curve can be defined by /*w» the partial pressure of oxygen at which haemoglobin is 50% saturated. It is normally about

3.7 kPa. Hypoxia Tissue hypoxia can be due to a disturbance occurring at any stage in the delivery of oxygen to the cells where it is utilized (Table 5.10), to increased demand or to defi­ cient oxygen uptake. The rational treatment of hypoxia obviously depends on knowing the cause. Measurement of oxygen delivery to tissues This requires determination of arterial oxygen content, preferably determined from measurements of haemo­ globin and .S'aO . and of cardiac output. \a< > can be cal­ culated from AiO,, using an assumed value for P w but direct measurement, by oximetry, is more reliable. It may be possible to infer the adequacy of cardiac output from clinical observation, but it can be measured directly by the thermodilution method using a pulmonary artery (Swan-Ganz) catheter incorporating a thermistor. Detection of tissue hypoxia Although the accumulation of lactic acid is a consequence of severe tissue hypoxia, the development of hyper-

ТиЫс 5Л0 hypoxia

1 ICUX1aflevtme tissue . ixvgenaf и■n 1

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'oxia

lactataemia is a relatively late phenomenon and by the time it is detectable hypoxic tissue damage may already have occurred. Furthermore, it can occur for other rea­ sons, e.g. impaired hepatic function, strenuous muscle contraction (not only in exercise but due to rigors or con­ vulsions) and as a result of improved perfusion of pre­ viously poorly perfused tissue (a *washout* phenomenon). The measurement of mixed venous oxygen saturation (5v0 2 ) is now commonly used in the assessment of the critically ill. SvO* can be measured in a blood sample taken through a pulmonary artery catheter or in vivo using a catheter containing fibreoptic bundles, which transmit light of appropriate wavelength to the blood and transmit reflected light back to a measuring device. It reflects the difference between arterial oxygen supply and tissue con­ sumption. .SvO, is normally of the order of 75%, but frequently falls to 50% or less when there is anaerobic metabolism, although such a fall can also be due to appro­ priately increased tissue uptake of oxygen or to a decrease in cardiac output. Because the measured SvO, is effec­ tively a mean value of venous blood from all tissues, it can be affected by changes in the relative distribution of blood between different tissues. Measurements of SvO, can be misleading; in severe sepsis, reduced tissue oxygenation is sometimes associated with a high SvO,, as a result of im­ paired oxygen extraction and local arteriovenous shunting. Oxygen delivery to tissues normally exceeds demand, so that if deliver, falls demand may still be met and con­ sumption is independent of supply. Eventually, however, there will come a point when oxygen consumption be­ comes supply dependent (Fig. 5.9). Determining whether oxygen consumption increases in response to an increase in delivery (usually effected by giving a vasodilating drug) is a theoretically attractive way of detecting impaired tis­ sue oxygenation, but it has not proved useful in clinical practice.

Oxygen consumption

type of hypoxia Critical point

Inspired itx\y content 1 к >,> AlvcoUr oxygen ic;

Aftcnal oxygen tension M»i* and some of its disorders. In: Cohen R I)» t<ewis B, Alberti К Ci M M , Dcnman A M (eds) l*he metabolic and molecular basis of acquired disease. London: Bailbtrrc TmdaJI, >°Q0, p p ° o 2 - 1 0 0 l . A detailed account of the topic, clearly written, bv a leading authority m this field. Kurtzman N A, BattUe D С (cds>. Acid-base disorders. Medical Climes of North America 1483; 6 7 : 7 5 1 - 9 3 2 . Contains nrt'Utri on many aspects of acid-base disorders, а-ЛкЛ, although now somewhat doled, are still tvortfty of study Rilcy 1-J Jr. Hson В E, Narins R G. Acute metabolic acid base disorders. Critical Care Clinics 1487; 5: 699-724.

A cttmeaily based account, including treatment. Other papers in this issue (October J9H?) entitled 'Renal failure and associated metabolic disturbance' are also of reLi ыпее. Schadc D S ted). Metabolic acido*i*. Clinic* in Endocrinology and Metabolism 1983; 1 2 : 2 6 5 - 4 * 5 . A scries of wicKs, the first triv of which, particularly, provide valuable background information; others discuss most of t/ie causes of increased hydntgen ion generation. Tobin M J. hvscnti.ils of cnnc.il care medicine. New York: Churchill Livingstone* 1*)89. Of the many textbooks dealing zcith intensive care, respiratory failure and disorders of oxygenation, thii is one of the clearest Walmslcy R N , White О H. Mixed acid-base disorders Clinical Chemistry 1**5; 31: 321 5. A short revietc, emphasizing the clinical approach through the use of clinical case studies.

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C H A P T E R

Calcium, phosphate and magnesium T. Cundy and L Reid

PART 1 CALCIUM METABOLISM Biological role of calcium Calcium is a divalent cation with multiple roles in verte­ brate physiology which can be grouped as either structural or metabolic. Its structural role is in the skeleton where calcium deficiency leads to mechanical failure, either in the form of osteoporosis or osteomalacia. Its metabolic roles are more numerous. The extracellular concentration of calcium ions influ­ ences the threshold for nerve action potentials, a high calcium raising the threshold and a low calcium having the opposite effect. The extracellular calcium concentra­ tion appears to alter the gating of sodium and potassium channels. It has been proposed that this occurs as a result of calcium ions being attracted to and 'screening' the negative charge on the cell surface in the region of these channels. Calcium has an important role as an intraccllular messenger. The depolarization of nerve or muscle cells, or the binding of a hormone or cyrokine to its receptor on the surface of other types of cell, results in an increase in cytosolic calcium concentration via either one or both of two mechanisms. These are the influx of calcium through plasma membrane channels and the release of calcium from intracellular stores (for example, the sarcoplasmic or cndoplasmic reticulum). The latter pathway depends on receptor-activated hydrolysis of inositol phospholipids in the plasma membrane, resulting in the release of inositol

trisphosphate into the cytosol. This compound hinds to a receptor on the cndoplasmic reticulum resulting in the release of calcium into the cytosol. The regulation of a variety of cell functions can then follow, some enzymes being directly affected by the cytosolic calcium concentra­ tion (e.g. protein kinasc C) and others indirectly by the calcium receptor protein, calmodulin. These pathways are integral to muscle contraction, neuroendocrine secretion, cell metabolism and growth.

Distribution of calcium The total body calcium in a human adult is approximately 1 kg, 99% of which is contained in the skeleton. Approxi­ mately 1% of skeletal calcium is freely exchangeable with calcium in the extracellular fluid. Calcium ions diffuse freely throughout the extracellular space where their con­ centration is approximately 1.2 mmol/L. The plasma ion­ ized calcium concentration is the same, but the plasma total calcium is approximately twofold higher because of protein-binding of calcium and the formation of calcium complexes with phosphate, citrate and bicarbonate in plasma (Table 6.1). It is the ionized calcium concentration which is physiologically important and closely regulated. Calcium is principally an extracellular ion and cytosolic concentrations are of the order of 10 4 to 10 * mmol/L. This low intracellular calcium concentration is necessary in order that calcium can function as an intracellular messenger and it is maintained by calcium pumps and exchangers on cell membranes. The endoplasmic reti­ culum and the mitochondria also have the capacity to remove calcium from the cytosol. ST

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plasma t %toml

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лЫе

53 47 6

nuecd Coraplcxed l*i -und Albumin Globulin Total (2.;

47 10 OKSttl

100

Calcium fluxes There arc three principal organs involved in the body's handling of calcium: the gastrointestinal tract, bone and the kidneys (Fig. 6.1). Gastrointestinal tract Intestinal absorption of calcium is mediated by two mechanisms. One is an active process, regulated by calcitnol (1,25-dihydroxyvitamin D, l,25(OH),D). This appears to involve a calcium binding protein whose syn­ thesis is increased by l,25(OH)*D. Active calcium ab­ sorption occurs principally in the duodenum and upper jejunum. Calcium is also absorbed passively throughout the small intestine and possibly in the colon. Because the duodenum is only a relatively small part of the small bowel, it may in fact account for less than half of the

calcium absorbed at normal dietary intakes, despite its greater absorptive capacity per unit length. At low calcium intakes active absorption predominates but this is saturablc and at higher intakes the change in net absorbed cal­ cium in relation to any change in intake is relatively small and mediated by passive absorption. The gut not only absorbs calcium but also secretes it in the digestive juices. The unabsorbed portion of this secreted calcium is called the endogenous faecal calcium and is in the order of 2-3 mmol per 24 h. Thus, at very low intakes, the absorbed dietary calcium will be less than that lost from digestive juice secretion and the net calcium absorption will be negative. Net calcium absorp­ tion is positive in healthy adults when their daily intake of calcium is greater than 5 mmol. The absorption of calcium is influenced by other dietary constituents. The presence of anions such as phos­ phate, oxalate (found in some green vegetables) and phytatc (found in some unrefined cereals) diminishes cal­ cium solubility and thus net absorption. Gastrointestinal absorption of calcium tends to decline with age but is increased during pregnancy and lactation. The principal regulator of intestinal absorption is l,25(OH) 2 D and so either deficiency or excess of this hormone is associated with parallel changes in calcium absorption. The average calcium intake of healthy adults consum­ ing a Western diet is about 20 mmol per day, of which 20-40% is absorbed. The principal source of calcium in the Western diet is dairy products. There is a significant

Resorption

FoiiMiion

• 5 Digestive Calcium pool

Intestine 24 aosorpiiof 8 3

Filtered

Reabsorbed 246

ec 15.7

Fig. 6.1 Representative calcium (luxes (mmol/24 h) in a healthy adult (70 kg body weight) in zero calcium balance. The rapid exchange of calcium on bone surfaces and the exchange in soft tissues are not illustrated in this diagram. The major sites of action of parathyroid hormone (*> and l,25(OH)>D (•*) are indicated. (Modified with permission from Wilkinson R. Absorption of calcium, phosphorus and magnesium. In: Nordin В Е С (ed) Calcium, phosphate and magnesium metabolism. Edinburgh: Churchill Livingstone, 1976.)

ким пр.

i l l

CALCIUM, PHOSPHATE AND MAGNESIUM

amount of calcium in some green vegetables such as spinach» though the bioavailabiliiy of this appears to be lower. Ktdncv The ionized and complexed fractions of plasma calcium are filtered at the glomerulus» amounting to approxi­ mately 250 mmol per 24 h. Of this, 9 8 % is reabsorbed, 6 5 % in the proximal tubule and the remaining fraction progressively along the ncphron. Proximal tubular cal­ cium reabsorption is an active process which is closely linked with that of sodium and does not appear to be hormonally regulated. Only the 10% of calcium reabsorp­ tion which takes place in the distal nephron has been shown to be subject to hormonal regulation, principally by parathyroid hormone (PTH) but possibly also by 1 , 2 5 ( O H ) J D . Other factors which increase renal tubular calcium reabsorption are hypovolaemia, alkalosis and thiazide diuretics, whereas volume expansion, acidosis and loop diuretics (e.g. fruscmidc) have the opposite effect. Urine calcium excretion is increased in subjects con­ suming a high protein diet, because of acid production in the metabolism of sulphur-containing amino acids and complexation of calcium by sulphate in the urine, result­ ing in inhibition of calcium reabsorption. Probably the largest dietary influence on calcium reabsorption is that of sodium intake, which influences proximal tubular sodium and calcium reabsorption.

8^

Control of calcium metabolism Plasma calcium concentration is principally controlled by P T H and l,25(OH)X>. Calcitonin may also be regarded as a calcitropic hormone although it has no clearly defined physiological function. It is also apparent that many other hormones, growth factors and cytokines can influence bone metabolism. Parathyroid hormone (PTH) Parathyroid hormone is an 84-amino acid, single-chain polypcptidc secreted by the parathyroid glands. The gene is on chromosome 11 and consists of three exons coding for a peptide of 115 amino acids (pre-pro-PTH) which is cleaved to produce pro-PTH (90 amino acids) and sub­ sequently the mature peptide (84 amino acids), prior to secretion. The biological activity of Р Г Н appears to reside in the first 34 amino acids at the amino terminal end of the peptide. The principal regulator of PTH secretion is the extra­ cellular fluid ionized calcium concentration, low levels stimulating secretion and high levels inhibiting it. The relationship between P T H secretion and ionized calcium concentrations is not linear. Marked hysteresis is evident from experiments where ionized calcium levels are raised or lowered- The P T H concentration at any given level of plasma ionized calcium is lower when the ionized calcium concentration is rising than it is when ionized calcium is falling (Fig. 6.2). Plasma РГН concentrations exhibit a intact PTH

Bone

(pmol/l) I D - f — — —

In the mature adult, the movement of calcium into bone equals its rate of efflux and bone mass remains constant. The situation is different in both growth and senescence. In spite of this constancy of adult bone mass, there is an active exchange of calcium between bone and the extra­ cellular fluid. This can take place as a result of bone remodelling (see Ch. 28) or it can be accomplished by a process of mineral exchange between bone and the extra­ cellular fluid, without local changes in bone matrix. Bone remodelling probably accounts for changes in bone den­ sity that take place with ageing or disease. In the normal adult about 5%. of the entire skeleton is remodelled in one year. In contrast, radioisotope studies have indicated that 1-2% of total body calcium can be exchanged between the bone and extracellular fluid over a period of several days. The precise mechanisms of this exchange and the factors influencing it are not known, but the quantities of calcium involved suggest that it may be an important part of normal calcium homoeostatis. Calcium losses in sweat are in the order of 0.3 mmol/ 24 h. Breast milk has a high calcium content (around 7.5 mmol/L) and a breastfeeding woman loses 4-8 mmol/ 24 h in the milk,

i

e

105 1.10 1.15 1.20 125 1.30 1.36 140 145 Ionized calcium (mmoVL) Fig. b.2 Mean concentrations of plasma uiiati J'TH in relation ю plasma ionized calcium concentration in response to infusions of citrate (M) and calcium gluconatc ( • ) and during recovery from hypocalcacmia (П) and hypcrcukaemm (O*. For any given ionized calcium concentration the I T U concentration is lower when the ionized calcium is rising than when it is tailing. (I ; rom Conlin P R ei al. Journal of Clinical Endocrinology and Mctaboli&m 1989; 6°; 5*>3 9, reproduced with permission of the Endocrine Society.;

Материал, защищенный авторским правом

90

CLINICAL BIOCHEMISTRY

diurnal variation. They are stable during the afternoon and evening but rise by about 50% around 2 a.m. and sub­ sequently fall to values approximately 50% below after­ noon values around 9 a.m. The zenith to nadir difference (in normal men) is around 2.3 pmol/L, using an intact PTH assay. Many other factors have been shown to influence PTH secretion including l,25(OH),D, adrenergic agonists, prostaglandins and magnesium, but whether any of these are physiologically important is debatable. Severe mag­ nesium deficiency can produce u state of reversible hypoparathyroidism. PTH binds to cell surface receptors in its target tissues. The PTH receptor is similar in structure to the calcitonin and sceretin receptors with seven transmembranc domains, an extensive extracellular domain involved in hormone binding and an iniracellular domain regulating intraccltular signalling. In its two principal target tissues, bone and kidney, this results in activation of both adenylate cyclase (with production of cyclic adenosine monophosphate) and phospholipase С (with production of inositol trisphosphate) and subsequent mobilization of iniracellular calcium. In bone, PTH receptors are present on cells of osteoblastic lineage. The osteoblasts in turn regulate osteoclast function. PTH has three principal actions on the kidneys. It reduces proximal tubular reabsorption of phosphate, increases distal tubular reabsorption of calcium and in­ creases the activity of the 25-hydroxyvitamin D la-hydroxylase enzyme in proximal tubular cells. It also decreases proximal reabsorption of bicarbonate, leading to a mild hypcrchloracmic acidosis in states of FI*H excess. Measurement of circulating parathyroid hormone The development of assays for P T H has greatly simplified the investigation of patients with abnormal plasma calcium levels but has brought with it the danger of misdiagnosis if the limitations of these assays arc not recog­ nized. The parathyroid gland secretes both the intact 84 amino acid peptide and inactive PTH fragments. Hepatic and renal metabolism of intact PTH result in the appear­ ance of mid-region and carboxy terminal peptides in the circulation. In vivo4 the principal biologically active pep­ tide is intact parathyroid hormone, which is present at a low concentration (1-5 pmol/L) and has a short half-life (2—4 min). However, mid-molecule and carboxy terminal fragments have significantly longer half-lives and accumu­ late in concentrations 5-20 times greater than that of the intact peptide. These inactive fragments are cleared by the kidney and their levels increase substantially in the pres­ ence of renal insufficiency. Although these fragments are not biologically active they may be immunoreactive, if they crossreact with the particular antibodies used in the radioimmunoassay. Assays which detect inactive frag­

ments report higher PTH concentrations than those which detect the intact molecule. There are a number of antigenic sites in the PTH molecule. Antibodies are classified as being directed at the amino terminal, mid-molecule (the 46-48 region) or carboxy terminal (68-84 region). Many current radioimmunoassays have midregion specificity. Two-site immunoradiometric and chemiluminescence assays of intact PTH have been developed, which measure only the bio­ logically active intact peptide. Inactive fragments are not detected and neither are parathyroid hormone-like molecules which can cross-react in one-site assays. Both mid-molecule and intact PTH assays show an increase in normal values with age, though this is more marked for the mid-molecule assays because of increased fragment accumulation secondary to the age-related de­ cline in renal function. Because of the dependency of mid-molecule and carboxy terminal P T H assays on renal function, their interpretation must always take into account the patient's plasma creatinine concentration. Because of the heterogeneity of PTH immunoassays cur­ rently in use, it is very important that laboratories deter­ mine their own normal ranges and the range of values to be expected in hyperparathyroidism, hypercalcaemia of malignancy, hypoparathyroidism and renal disease. The action of PTH on the kidney causes cyclic adenos­ ine monophosphate (cAMP) to appear in the urine, so 'nephrogenous' с AMP has been used as a surrogate meas­ ure of PTH. The excretion rate is usually expressed as nmol/L glomcrular filtrate (CJF) and is calculated from: urine cAMP (nmol) x plasma creatinine (mmol/L) urine creatinine (mmol/L) The normal range is 5 25 nmol/L GF. Classification of hyperparathyroidism Increases in plasma concentrations of P T H are seen in a variety of circumstances. It is useful clinically to distinguish condi­ tions in which the increased secretion of P T H is a normal physiological response to hypocalcaemia (secondary hyperparathyroidism) from those in which hypersecretion of PTH is the principal abnormality' (primary hyper­ parathyroidism). If secondary hyperparathyroidism per­ sists over long periods then the parathyroids can become hypcrplasuc. A gradual rise in plasma calcium accom­ panies this change and overt hypercalcaemia can develop, which is known as tertiary hyperparathyroidism. This arises chiefly in long-term dialysis patients, but it has also been described in patients with malabsorption syndromes. This classification is unsatisfactory in some respects. For example, the kidney transplant recipient with hypercalcae­ mia due to hyperparathyroidism which developed whilst on dialysis is difficult to categorize. An alternative classification, which has the attraction of being more closely related to treatment stratagems, is to

Материал, защищенный авторским правом

CALCIUM, PHOSPHATE AND MAGNESIUM

describe hyperparathyroidisrn in relation to the prevailing plasma calcium concentration. Thus hyperparathyroidism can be described as either hypocalcacmic, normocalcacmic or hypercalcaemic. Vitamin D

Synthesis and metabolism (Fig. 6.3) The term vi­ tamin D is a misnomer since it implies that this substance is an essential component of the diet. In fact, the majority of vitamin D is synthesized in the skin by the action of ultraviolet light on 7-dehydrocholesterol. This produces cholecalciferol or vitamin D v Vitamin D is present in a variety of foodstuffs. It can be added to food in the form 7- denydroc+raiesteroi Ultraviolet * light

Skin Dietary sources

1 1

Calciferol Uvtr [ Live ) 23,25 grafHiomatoue ?lymphoma)

Bone scan (? metaslases)

1

PTH high

PTH suppressed

25i < leOfft syndrome pharyngeal pouch dwmbryogenesi

• Idiopathic sporadic familial type I potyglanrfular autoimmune syndrome autosomal dominant autosomal recessive X-hnked recessive • Foraurgicjl. pOM neck irra n • Seven hypomagncsaemia • Infiltrauve P thalassaemia Qrofl V ■

.лрег)

malignani • О -ni/cd rare association Kcnm ( uMc syndrome Kceraa-Sayrc syndrome /\jrjrАлтай/ ЛОГИКИ* mutanct Pieudohypoparathyroidism (Types la, lb, k , II Abnarmaima imtn I) meiaboiism • Pnv.uion.at vitamin D d ticicncy • i lent lo-favdiiuylttfcM renal failure acidosis

Causes of hypocetcattma Causes of hypocalcaemia are set out in Table b.b. Because hypoalbuminaemia is observed in patients with a wide variety of other diagnoses, it is mandatory to make allow­ ance for the plasma albumin level or to measure ionized calcium when entertaining the diagnosis of hypocalcae­ mia. The common causes of chronic hypocalcaemia relate to abnormalities in the synthesis, secretion or action of PTH or l,25(OH),D or both these hormones. Hypoparathyroidism The most common type of hypoparathyroidism is that occurring after surgery to the thyroid or other structures in the neck. It varies widely in severity and in some patients is transient, with plasma cal­ cium returning to normal in the weeks after surgery. Overt hypocalcaemia can be precipitated in previously asympto­ matic patients with partial hypoparathyroidism by therapy with exogenous oestrogens. Idiopathic hypoparathyroidism is a very rare disorder. It may present in early childhood associated with congeni­ tal absence of the thymus - the Di George syndrome. These patients have a deficiency in cell-mediated immu­ nity and the condition has a substantial mortality. Idio­ pathic hypoparathyroidism presenting between the ages of 5 and 10 years is more likely to have an autoimmune basis and may be associated with other deficiencies such as ad­ renal insufficiency, gonadal failure, diabetes mcllitus and pernicious anaemia. The appearance of these abnormali­ ties may be preceded by the development of chronic moniliasis. This group of abnormalities is referred to as the type I poly-glandular autoimmune syndrome, which may be sporadic or familial with autosomal recessive inheritance. Other forms of idiopathic hypoparathyroidism are also

vitamin D dependent rickets, type 1 imm l) тиЫЯСЛ Vitamin D dependent rickets. Type II Other causa Acute pancreatitis fh|4-rpbo4phatacni Acute rhabdomyob Multiple transfusion! ol titrated blood Severe acute illness (e.g. thttck syndrome. Gram ncgar sepsis) Osteoblastic mctasuues (C.R prostate, brca

ntad bypocalcaemia Inhibitor, о 1 bone resorption

likely to have a genetic basis in view of their frequent familial occurrence. Т Ы variable pattern of inheritance suggests that a variety of mutations can produce hypo­ parathyroidism. Mutations in or near the PTH gene have been described in kindreds with autosomal recessive and autosomal dominant inheritance respectively. However, the occurrence of X-linked recessive inheritance, and the finding of associated abnormalities (such as renal dysplasia and sensorineural deafness) and normal PTH genes in some kindreds with autosomal dominant inheritance, suggest that hypoparathyroidism can also result from mutations at loci distant to the PTH gene, which affect embryological development, cellular composition or homoeostatic regulation of the parathyroids. Hypoparathyroidism is characterized by hypocalcae­ mia, with plasma calcium levels as low as I.25mmol/L being found. Urine calcium excretion is subnormal and the TmP/GFR is increased, leading to hyperphosphatacmia. Bone turnover is reduced, as are intestinal calcium absorption and circulating levels of both 1,25 (OH).D and intact P T H . Some P T H radioimmunoassays are not

s-

M

100

C:UNK:AI

вюснкмтто'

able to distinguish low levels from those within the normal range. Pseudohypoparathyroidism Pscudohypoparathyroidism refers to a heterogeneous group of rare condi­ tions in which there is resistance to parathyroid hormone action. These patients present with much the same clini­ cal and biochemical findings as in hypoparathyroidism but their levels of tmmunoreactive PTH are high rather than low. The resistance to P T H seems primarily to be in the kidney, so some patients show evidence of parathyroid bone disease. '1Ъс1г hypocalcacmia may be less severe than in hypoparathyroidism and may fluctuate. The current classification of the disorder is shown in Table 6.7. The most common form of the disorder, type la, appears to be attributable to reduced levels of the protein G,a in cell plasma membranes. G,a is part of the guanine nucleotide-binding protein complex that links the PTH receptor to the adcnylatc cyclase catalytic unit. The diagnosis of pseudohypoparathyroidism and the classification of patients as type I or type II is dependent upon the modified Ellsworth Howard test (see Appendix Vitamin D disorders Hypocalcaemia is frequently encountered where abnormalities of vitamin D metabo­ lism mean there is a failure to synthesize adequate quanti­ ties of l,25(OH) 2 D. This can arise through there being insufficient supplies of the precursors (privational vitamin D deficiency) or defects in renal la-hydroxylation (either inherited or acquired). Finally, there can be end-organ resistance to the actions of l,25(OH)^D. In these dis­ orders, in addition to hypocalcaemia there is usually sec­ ondary hyperparathyroidism with hypophosphataemia and, in bone, osteomalacia or rickets. These disorders arc discussed more fully in Chapter 28.

TabU

Other causes of hypocalcaemia Acute hypocalcae­ mia can result from the release into the circulation of any factor capable of binding substantial amounts of calcium. In acute pancreatitis, free fatty acids fill this role, whereas in acute rhabdomyolysis and in the t u m o u r lysis s y n d r o m e , large quantities of intraccllular p h o s ­ p h a t e are released. Incautious infusion or ingestion of phosphate can also result in hypocalcaemia by the same mechanism and the infusion of large quantities of citrate as a result of blood transfusions has a similar effect. Some r a d i o g r a p h i c contrast dyes contain cither citrate or the calcium chelator EDTAand can also produce hypo­ calcaemia. These mechanisms, together with m a g n e s i u m deficiency which impairs both PTH secretion and action, contribute to the hypocalcaemia observed in a quarter to one half of patients admitted to intensive care units. In patients with widespread osteoblastic met «states, modest hypocalcaemia may occur secondarily to increased incorporation of calcium into bone around the secondary deposits. Hypocalcacmia is sometimes seen in the n e o ­ natal period, being more common in premature infants and in the offspring of diabetic and hypcrparathyroid mothers. Its aetiology is varied (see Ch. 23). Investigation of hypocalcaemia Figure 6.6 sets out an approach to the differential diagno­ sis of hypocalcaemia. Clearly, the first step is to confirm that hypocalcacmia is not just a reflection of low concen­ trations of plasma albumin. An ionized calcium measure­ ment may be necessary, particularly in acutely ill patients in whom calcium binding to albumin or other compo­ nents of plasma may be abnormal. A number of the causes of hypocalcacmia shown in Table 6.6 are easily distin­ guished clinically. Thus, the acutely ill patient is clearly

iastitication and feature* of pscudohypoparath

PC

Re Phosf

l,-

i П

IHcujfobyproanthynsidiam*

N

PTH ic сАМГ

G acti\

rmone resistance*

AHO T

Postulated abnormality

+

1 1 1

N

N

N

Г4ТС

1 receptor } Receptor adcnylaic cyclase complex ? cAMP-dependent protein кийке } Caicium-inoetd 2nd messenger system

N

fteudohypoparamyroidiun » a related disorder, hut ha* no associated abnormalitiea in calcium metabolism N ~ normal •Other than IM 11 e.g. thyrotrophin ISHi. gonadotrophfas. glucagon LHO Albnghr's hereditary ostcodystror und fa*. hort suture- obesity, short phalanges, short mctacarpals) I fpe la and pwudohypoparathyroid. Tnmonljr occur within the same kindred G,a - a protean forming рал of die guarunc nuclcondc-bindine proum complex that links the 1' 1 M receptor to the adcnylatc cyclase catunit

1Л, защи

CAIXIUM, PHOSPHATE AND MAGNESHJM

101

Low total plasma caksum

I I I I

intnaemia lormaJ ionized/ 'corroctec calcium

Иал^н rflburr.ri ± ionized calcium

No acton required

True hypocalcaemta

CWc^cai assessment

mvesl»gate appropriately

Increased

Plasma croatrnme

RecaJ lai^te

>

No<mal LOW

Plav^a 'пазгечшт

Lowmormai f

Hypomagnesaemu

| Increased

Hypopa/athyroKlism I r,F4iK cause)

Plasma Df-osphato

I

}

TLOW 25 and a breastfeeding woman will lose 1.4 mmol/24 h in the milk. There arc two specific mechanisms which react to vari­ ations in phosphate status in order to maintain phosphate homoeostasis, despite fluctuations either in supply (diet, intestinal absorption) or demand (growth, mineralization, cellular metabolism). These two mechanisms comprise (a) 1,25(ОН)Л) synthesis, through the 1 a-hydroxylation of 25(OH)D, by the cells of the proximal tubules of the kidney, and (b) the response of the renal tubular phos­ phate transport system (in both proximal and distal tu­ bules). l,25(OH),D regulates plasma phosphate primarily by its action on intestinal phosphate absorption, whereas the renal tubular transport system regulates urinary phos­ phate excretion. Both mechanisms are rapidly stimulated in response to restrictions in phosphate supply. 1 Ъ е м re­ sponses occur independently of PTH and are independent of each other, i.e. the increase in l,25(OH)»D production is not responsible for the change in renal tubular reabsorptive capacity. However, both mechanisms operate from the same anatomic site (the renal tubule), and in many circumstances there is a striking parallelism in the response of the 1 a-hydroxylase system and the renal tubu­ lar phosphate transport system, which suggests that these two controlling elements might be regulated by a single

Phosphorus homoeostasis The main sources of phosphate transfer to and from ihc plasma pool are the intestine, bone, soft tissues and the kidneys. Under steady-state conditions the net intestinal

Formation T

П ,.- •■ ■■ . iuice 7

I Resorpton

11

Intestine 52 Int..-.'

;■: .i 33

Faeces 19

Fig. 6.7 Representative fluxes (mmol/24 h) in a healthy adult (70 kg body weight) for phosphate. The exchanges with soft tissues are not illustrated in this diagram. For phosphate metabolism the major sites of action of parathyroid hormone ( # ) and l,25(OH).D (**) arc indicated. PTH reduces renal tubular reabsorption of phosphate and increases unnc phosphate excretion. (Modified with permission from Wilkinson R. Absorption of calcium, phosphorus and magnesium. In: Nordin В К С (ed) Calcium, phosphate and magnesium metabolism. Edinburgh: Churchill UvingMone, l°76.)

защищенны

ким пр

104

CUK1CAI. BUM I1EM1STRY

signal. This signal is probably not the plasma phosphate level itself, but rather an intracellular phosphate pool by which supply and demand are matched. Thus during lactation, growth or recovery from privational ostcomalacia, when demand for phosphate is high, then both l,25(OH) 3 D production and the renal tubular absorptive capacity for phosphate are increased and in consequence so is plasma phosphate. In early renal failure the satura­ tion of this intracellular pool is responsible for a fall in l,25(OH) : D production and tubular absorptive capacity long before plasma phosphate levels become elevated. Dietary phosphate and intestinal absorption Phosphate is present in a wide range of foodstuffs, so that if the diet is adequate in other nutrients it is usually also adequate in phosphate. A typical Western diet contains around 6.5 mmol/kg body weight/day. There is a con­ siderable power of adaptation to low phosphate intake, so it is difficult to establish a minimum dietary require­ ment. Hypophosphataemia secondary solely to inadequate intake is extremely rare. A measurable increment in 1,25(OH) 2 D production, which is one of the adaptive re­ sponses, occurs when dietary phosphate is reduced to 3.2 mmol/kg/day. Phosphate absorption takes place most efficiently in the duodenum and jejunum, though the ileum contributes most in absolute quantity because of its greater length. Approximately 67% of dietary phosphate is absorbed from the jejunum and ileum even under conditions of phos­ phate repletion, by a passive non-saturable mechanism. Thus the net absorption increases in parallel with dietary phosphate content. The active, l,25(OH)>D-depcndent, saturablc component to phosphate absorption mainly in­ creases jcjunal uptake. The bioavailability of intestinal phosphate is reduced by a high calcium intake, which forms insoluble complexes with phosphate in the intesti­ nal lumen. Aluminium salts react similarly. Phosphate is secreted in the digestive juices at a rate of approximately 0.1 mmol/kg/24 h, about 67% of which is reabsorbed from the intestine. Phosphate fluxes are summarized in Figure 6.7.

increment in urine phosphate; that is, the relationship between the phosphate excretion rate and the plasma phosphate is linear, with a slope that equals glomcrular filtration rate at high (saturating) filtered loads. Extrapo­ lating this line back to where the urine phosphate excre­ tion rate is zero defines the threshold of glomerular filtrate (and hence plasma) phosphate concentration (TmP) which the renal tubular homocostatic mechanism is seek­ ing to maintain, in relation to GFR. In vivo this can be determined by calculating the clearance of phosphate rela­ tive to creatinine clearance on a fasting urine sample and using the nomogram of Bijvoet and Walton, which relates this value to the prevailing plasma phosphate level (TmP/ GFR). This is performed fasting to avoid changes in plasma phosphate due to food intake and requires a urine sample and a plasma sample (sec Appendix 5 for method). Factors regulating TmP/GFR A number of endo­ crine and non-endocrine factors are involved in the regu­ lation of TmP/GFR and hence plasma phosphate (Table 6.9). Amongst the most important is parathyroid hor­ mone (PTH). Changes in plasma phosphate do not di­ rectly affect PTH secretion, but PTH acts on the renal tubule. An elevation in PTH (for example, in primary hyperparathyroidism or secondary hyperparathyroidism due to vitamin D deficiency) reduces TmP/GFR. When PTH levels arc low or there is insensitivity to the hormone (hypoparathyroidism or pseudohvpoparathyroidism) then TmP/GFR is increased. Parathyroid hormone-related peptide has similar effects to PTH on renal phosphate handling. The phosphaturic effects of PTH and PTHrP result from action at multiple sites along the nephron and are mediated through cAMP production. The high TmP/GFR seen in childhood and in acromegaly is mediated through insulin-like growth factor I

ГаЫс л.9

~

Endocnne

Increased TmP/GFR

Decrease

HypttpgrathvruKltsni P*eudobypoperath>Toidi«m

1 Ivpcrparathyroidum Pin-related peptide Cortscuateroid» Ocatrogen replacement therapy

owth hormone/} Recovery from privwjnnal vitamin D doScfcfl Lactation

Tfte renal tubular reabsorphon of phosphate A high proportion of the phosphate in plasma is ultrafilterable and about 7 5 % of filtered phosphate is reabsorbed by the proximal tubule. A further 5-20% is reabsorbed in the distal tubule and/or cortical collecting duct and the remainder (15-40 mmol/24 h in an adult) appears in the urine. If phosphate is infused intravenously and the urine ex­ cretion rate of phosphate measured, then above a certain plasma concentration of phosphate all additional incre­ ments in the filtered load will be paralleled by the same

Fact o n affecting

Drug»

Hiaphoaphonatck (particularly etwlronatc)

1 PA rFR

(ilflin

Uuretic* (partKuLirly ucctazolarmde) lume loading Single kidney, diuretics Poorly controlled diabetca

Netrhm

Metabolic

Aeftdo*

Alkalotb

Crcnclic or unknown cause*

Tuznoral cttlcinom

И \r* T*>oapbetacmic condition* with pen.alciuna and ostcomalacia/nckcta

МГ;

ЗТОрСКИМ n p s

CALCIUM, PHOSPHATE AND MAGNESIUM

(somatomcdin С). TmP/GFR is increased in thyrotoxicosis and decreased by corticosteroids. The increase in plasma phosphate seen after the meno­ pause is abolished by oestrogen replacement therapy. '1Ъсге is little evidence that l,25(OH) 2 D greatly affects renal phosphate handling. Calcitonin given in pharmaco­ logical doses acutely decreases the TmP/GFR. In general, acidosis (whether respiratory or metabolic) reduces phos­ phate rcabsorption, whereas alkalosis has the opposite effect. Calcium itself can affect phosphate transport in the kidney. In normal subjects this is difficult to demonstrate because of concurrent changes in PTH, but calcium infusion in hypoparathyroid subjects ultimately increases urine phosphate losses and reduces the plasma phosphate by reducing TmP/GFR. Various drugs may induce changes in TmP/GFR. In general these are small, the exceptions being acetazolamidc which reduces TmP/GFR, and the bisphosphonate, etidronate disodium. >ХЪеп given at doses >10mg/kg etidronatc causes a dose-dependent increase in TmP/GFR and plasma phosphate (which can rise by up to 1 mmol/L above pretreatment values). Renal phosphate handling may also be affected by haemodynamic changes. Patients with a single kidney (for example, transplant recipients or live kidney donors) have a high filtered load per nephron and lower TmP/GFR as an adaptation. Phosphaturia also accompanies the natriuresis which follows volume loading, poorly controlled dia­ betes or diuretic use, because renal phosphate transport is related to sodium transport. PTH is needed for the full expression of this phenomenon. There are a number of conditions which share as a central feature a loss of the PTH-scnsitive component of phosphate transport in the proximal tubule, a low TmP/ GFR and hypophosphataemia. This defect may be inher­ ited or acquired. The clinical manifestations depend on the severity of the phosphate leak, the degree of response to the la-hydroxylase enzyme to the ensuing hypophosphatuemia, the age at onset and associated defects. In some instances (for example, in some cases of idiopathic hypercalciuria) the primary defect appears to be a low TmP/GFR with modest hypophosphataemia and in response to this there is an appropriate increase in l,25(OH) 3 D production. Intestinal absorption of calcium and phosphate is enhanced and the extra calcium that is absorbed appears in the urine, predisposing to renal stones, but there is no bone disease. In the various forms of hypophosphatacmic rickets and ostcomalacia there is an inadequate I,25(OH) : D response and the combination of lower plasma phosphate and low plasma l,25(OH)>D causes a failure of mineralization. An intermediate dis­ order with both bone disease and hypercalciuria has also been described. The precise physiological and molecular basis of these disorders is unknown. The commonest form of hypo-

105

phosphatacmic rickets in humans is X-linkcd and the gene has been mapped to the short arm of the X chromosome. In mice, two distinct genetic hypophosphataemic dis­ orders are recognized. These also map to loci on the X chromosome. As yet the gene products and their mecha­ nisms of action are unknown. The proximal tubular cells each have 6-7000 microvilli on their luminal surface ('the brush border') and it is transport of phosphate across the brush border luminal membrane that is affected. Phos­ phate transport occurs against a large electrochemical gradient and appears to be coupled secondarily to active sodium transport, but the chemical nature of the putative brush border membrane phosphate transporter is un­ known. It is probable that a variety of defects exist accounting for the clinical heterogeneity of the hypo­ phosphataemic conditions. The bone diseases associated with hypophosphataemia are discussed in Chapter 28.

Plasma phosphate concentratiotts There are marked age-related changes in plasma phos­ phate, with the highest levels seen in early infancy when growth velocity is highest. Throughout childhood and adolescence, plasma phosphate concentrations remain higher than in adults. In adulthood a decline in fasting plasma phosphate concentrations is seen in men after the age of 40. In women there is little change, but a small increment (approximately 0.07 mmol/L) occurs after the menopause. Plasma phosphate does not change during pregnancy but is significantly elevated during lactation. The age-related and lactation-related changes in plasma phosphate arc mediated through changes in TmP/GFR (Table 6.10). There is marked diurnal variation in plasma phosphate. During the day levels are substantially higher in the midafternoon than in the early morning and there is a second

Table 6.10 *ex

Plasma |

Aft

oncentratiot» according to age and

Sen phosphate (mmol/I

Last trimester i« utero \ day I day - 1 month 1 12 months 1 l l v.-an 11 IHyean i 8 -»5 years > 45 vcars >45 years lactation

Mi i M. 1 M. I ■1 M. i M. F* M F M

ь

0.8-1 4 \A 1 \ 21 i 1-1.7 0А-М

o.a-i.4 II

i 14

' J: г,, in plasma phosphate levels from adolescent to aduli value rage 2 years earlier in girii than boy». Adult level* are reached around age 15 m girls and age 17 in boyv I"hia parallels the earlier start an. t the adolescent gr spun m gtris.

106

CLINICAL ВКХ:НКМ!.Ч~ГКУ

peak in the early hours of the morning (Fig. 6.4). This early morning peak is attenuated with age. Plasma phos­ phate nses postprandially and subsequently falls as phos­ phate either enters cells or is excreted. 1Ъс fall in plasma phosphate after a meal is partly attributable to insulindependent stimulation of glycolysis, which increases intracellular phosphate utilization. Hyperphosphataemia The major causes of hyperphosphataemia arc listed in Table 6.11. Hyperphosphataemia may arise through an increase of phosphate input into the blood or decreased excretion. The latter may result from a reduction in GFR or an increase in TmP/GFR. Conditions in which the TmP/GFR is raised generally cause modest hyperphos­ phataemia (plasma phosphate 1.4-2.4 mmol/L). Tumoral calcinosis in non-uracmic subjects is a rare, often dominantly inherited disorder, associated with a high TmP/ GFR and hyperphosphataemia. Periarticular calcific masses accumulate which arc subject to breakdown and chronic inflammation. Renal failure, whether acute or chronic, can cause hyperphosphataemia but it docs not usually occur until the GFR falls below 30 mlVmin. If renal function deterio­ rates further, plasma phosphate rises exponentially and may reach 6 mmol/L at end stage. Other abnormalities of phosphate metabolism which arc present early in the course of renal failure are important in the aetiology of

Table f-.ll

' ,

".-rph"*phAUcmia

P%€ndokyp<rphosphatatmia • 11 acmolyscd specimen • \Ucli»ma IncrrauJ pKnphaU трш • Intravenous phosphate • Rectal phosphate • I ■■;. death tumour lysis syndrome rhabdomyolysis malignant hyperpyrcxia heat m o t e DimimituJp/nnphaU exatboH • Reduced GFR acute renal failure chronic renal failure • Increased 1шГ ' К Physiological normal childhood lactation recovery from vitamin I) deficiency Pathology i reduced P T H 01 m i

vitamin D toxicity thyrotoxicosis acromegajy ctidronatc tumoral

resistance)

hynerparathyroidism and are discussed turther in Chapter 26. The diet rarely contains sufficient phosphate to cause significant postprandial hyperphosphataemia, but exces­ sive input of phosphate can occur with the use of intrave­ nous phosphate salts or phosphate enemas. The latter is particularly liable to occur with children unless the dose of phosphate in the enema is reduced. Massive cell death with release of intracellular phosphate can cause hyper­ phosphataemia. Examples of the latter include rhabdomyolysis and the tumour lysis syndrome, seen after the initiation of chemotherapy in childhood leukaemia. It should be emphasized that, as with hypercalcacmia, an increased input of phosphate into the circulation is much more likely to cause severe hyperphosphataemia if there is coexistent renal impairment and thus a reduced capacity to excrete phosphate. This is often the case in critically ill patients. Plasma phosphate may be artefactually raised (by about 30%) in severely haemolysed samples. Pseudohyperphosphatacmia has also been described with certain chromogenic assay methods of automated analysis in patients with paraproteinaemia or hypertriglyceridaemia. Consequences of hyperphosphataemia No specific symptoms are directly attributable to hyper­ phosphataemia. However, when the calcium x phosphate product in blood exceeds the solubility product (approxi­ mately 4.85 x 10* molar units) soft tissue (metastatic) calcification occurs. This is most commonly seen in chronic renal failure where calcification occurs in blood vessels, the skin, heart, lungs, kidneys, conjunctivae and around joints. Interstitial calcification of the kidneys is of particular importance since it impairs renal function fur­ ther and reduces the ability to excrete phosphate loads. The deposition of calcium phosphate salts into the skin is thought to contribute to the pruritus of chronic uraemia, a symptom which may improve with better control of hyperphosphataemia. The syndrome of progressive ischacmic skin ulceration (calciphylaxis) in renal failure patients has also been attributed to calcium phosphate deposition. In this syndrome ulcers spread rapidly with infection of the necrotic tissues. Histologically, there is extensive arterial and artcriolar calcification and thrombo­ sis. These lesions do not respond to local therapy, but do heal after parathyroidectomy (which acutely reduces the calcium x phosphate product). Urgent parathyroidectomy is indicated in this condition. If plasma phosphate is raised acutely then plasma calcium levels fall by a mass action effect and this is seen, for example, with the tumour lysis syndrome and rectal or intravenous phosphate administration. This is the basis of the use of intravenous phosphate in the management of malignant hypercalcacmia. However, the perils of soft

CALCIUM, PHOSPHATE AND MAGNESIUM

tissue calcification induced by this treatment outweigh the potential benefits.

disease, both to limit damaging mctastatic and vascular calcification and to control the development of hyperparathyroidism. There arc various salts of aluminium, magnesium and calcium which, when administered orally, bind phosphorus in the intestine and limit its absorption. Aluminium hydroxide is the most effective of these agents, but significant quantities of aluminium may be absorbed which have toxic effects on bone. Other phosphatebinders such as calcium acetate, calcium carbonate and magnesium carbonate are increasingly used instead of alu­ minium salts. In dialysis patients inadequate phosphate control may be due to a number of factors (see Table 28.8).

Diagnostic approach to hyperphosphatacmia The history should demonstrate excessive exogenous phosphate supply (intravenous or rectal). Massive cell death in, for example, rhabdomyolysis or the tumour lysis syndrome should also be evident from the clinical setting. Rhabdomyolysis can be confirmed by measurement of plasma creatine kinase activity. Diminished excretion of phosphate due to severe renal failure is evident from measurement of the plasma crcatinine. The most convenient method of detecting changes in TmP/GFR is to use the nomogram of Walton and Bijvoet (see Appendix 5). In interpreting the result it is important to bear in mind the normal age-related changes in this index. There is no diagnostic value in measuring the 24 h urine phosphate excretion, which in steady-state conditions reflects the product of dietary phosphate con­ tent and fractional phosphate absorption. Other measures of urinary phosphate clearance have theoretical failings compared to the TmP/GFR.

Acute renal failure can complicate the tumour lysis syn­ drome and is attributable to phosphate and urate released from cells. Maintaining a brisk saline diuresis at the time chemotherapy is begun reduces the risk of developing acute renal failure. Hypophosphataemia

Mechanisms Hypophosphataemia can arise by three mechanisms: (1) inadequate phosphate absorption from the intestine; (2) shifts of phosphate from extracellular fluid into cells; (3) abnormal urinary phosphate losses (Table 6.12). (1) As indicated earlier it is very rare that the diet is so inadequate in phosphate that on its own it can cause

Therapeutic approach to hyperphosphatacmia Hyperphosphatacmia due to a raised TmP/GFR is rarely high enough to require specific action. Phosphate control is, however, important in patients with endstage renal

I

Tabled, и

Mccharusras under.

Aetiology of hvpophosphatacmiii Respiratory ilkPrimary hvperpurathyroidism tonal vitamin D deficiency with secondj hvpcrparathynndism Post renal transplant ilipophosphataenui ticket*, or ostcomalacia Antacid consumption Sepsis, burns Alcoholism npethomniu-tiet i5- (i-agoruv Sahcylaa- р-им-шпц Early asthmatic crisis

107

Cellular uptake

i Gastrointestinal input

Rtffctfdinjt

Inadequate intake

Muscle

Urinary lost

Poor absorption

I

Г (.IK

•

(•)•

Alcoholic withdrawal Diabetic kctoacidotis rccovi Hyperalinv n Hepatic failure li tancc running

у •

-

ill v

'ConrriV KHphatc majabsorption depends on 1,25 о Л I) response 10 hypitas\ium depletion Diuretic* (non-potassium-spanng) Osmotic diuresis IruraeeBular shift Post myocardial infarction Postptrathyroidcct Recovery from diabetic kctoacidosis Rccovci m starvation Acute pancreatitis

11 1

nerve, muscle and cardiac cells appear not to be direct, since as far as is known magnesium is not involved in the normal action potential, in conduction or contraction. The effects appear to be mediated by inhibition of the N a \ K*-ATPase enzyme and in practice it is difficult to distinguish direct neuromuscular and cardiac effects of severe hypomagnesaemia from those secondary to altera­ tions in potassium and calcium metabolism. Cardiac effects. Hypomagnesaemia of a more modest degree (0.5-0.8 mmol/L) is not uncommon and whilst it is not associated with obvious symptoms, it is not neces­ sarily benign. In patients admitted to coronary care units there is a striking relationship between hypomagnesaemia and the incidence of serious dysrhythmias (including multifocal atrial tachycardia, ventricular tachycardia, ventricular fibrillation and torsades de pointes). Hypo­ magnesaemia is strongly associated with hypokalaemia in this circumstance and hypokalaemia is another potent cause of dysrhythmia. The association between low plasma levels of magnesium and potassium is again expli­ cable by reduced membrane ATPase activity, a loss of intracellular potassium and subsequent urinary losses. The ratio of intra- to extracellular potassium, which largely determines the resting membrane electrical poten­ tial, is reduced with a consequent increase in electrical excitability. Hypokalaemia can be refractory to replace­ ment therapy whilst magnesium deficiency persists. Some cardiac dysrhythmias are less responsive to antidysrhythmic therapy in the presence of hypomagnesaemia. In addition to the dysrhythmias, magnesium depletion is as­ sociated with other ECG abnormalities, including PR and Q T interval prolongation and T-wavc flattening. Follow­ ing acute myocardial infarction plasma magnesium levels fall and are at their lowest 12-24 h after admission to hos­ pital. Drug therapy, particularly non-potassium-sparing diuretics, may exacerbate this hypomagnesaemia by in­ creasing urinary losses.

Diagnostic approach The plasma magnesium concentration remains the only widely available measure of magnesium status. Renal magnesium loss may be assessed by measuring the urinary excretion rate. Where hypomagnesaemia is due to inad­ equate intake or malabsorption, urine magnesium falls rapidly to 15 mmol/24h). This measure is thus of diagnostic value in the diagnosis of magnesium-induced diarrhoea. Therapeutic approach Conditions which may benefit from magnesium repletion are listed in Table 6.16. The amount and rate of magne­ sium administration depends on the cause and severity of the magnesium depletion and on renal and intestinal function. Symptomatic deficiency is best treated by the intra­ venous route. Adults with good renal function should be given 12 mmol magnesium (in the form of MgSO* in sa­ line or 5% dextrose) over 2-3 h and a further 12 mmol infused slowly over the following 24 h. This regimen may need to be repeated over several days as the tissue magne­ sium deficit takes longer to correct than the plasma level. If the plasma levels are raised too high then unnary mag­ nesium losses increase. In infants replacement is given in a similar manner but the dosage should be reduced to

Table 6.1ft plcmcnt

luioaa thai

Acute myocardial intention wMi recurrent ventricular fibrillation

recurrent ventricular i complex щ

cardia

uUr с

sopMvcntneuter tachycardia Digitals IrtfrntkatJoo with arrhythmu Л:г;.-.. tibnllanon with rapM wniruular rc»pomc requiring immediate digKakut; I r&ado Jc pointe* Hypokalaemia unresponsis mentation Neurological manifestation* of hvpomagnesacmu Hvpornugnenaemia without clinical manifestation* Dbtfi iiempy Hvpocalcaernia unresfHtnsive to calcium supplementation 1-rom Rcinhart R Л 1981 MagncMum metabolism. Archives of Internal MeJuine 14S 2 115-20.

0.2-0.3 mmol/kg. Plasma magnesium levels need to be monitored during and after replacement therapy. The in­ tramuscular route may also be used for replacement but the injections are painful. V№ere feasible magnesium supplements may also be given orally (5-10 mmol/day, in divided doses). Suitable preparations include MgO, MgS(3 4 and MgCO, which arc available as antacids or purgatives. Higher doses may cause diarrhoea. Hypermagnesaemia The kidneys normally excrete excess magnesium very effi­ ciently and can handle loads up to 400 mmol/24 h, so hypermagnesaemia as a clinical problem is restricted to patients with acute or chronic renal failure. In these subjects potential sources of excess magnesium load arc the dialysate, the use of magnesium containing antacids (as phosphate binding agents), magnesium containing cathartics and, in acute renal failure, release of magne­ sium from tissues. Hypermagnesaemia in patients with renal impairment can also occur with the use of citrate-gluconic acid solu­ tions in urological practice. These solutions, which con­ tain substantial quantities of magnesium salts, are used either for bladder irrigation or are infused into the renal pelvis through a nephrostomy tube for the dissolution of renal stones. Hypermagnesaemia in the order of 1.5-2.5 mmol/L may be associated with hypotension but is commonly asymptomatic. In the range 2.5-5.0 mmol/L areflexia may be present and ECG changes occur (prolonged PR and QRS interval, peaked T waves). At higher levels respiratory paralysis and cardiac arrest ensue. Ai these very high plasma concentrations magnesium has direct effects on neuromuscular cells by inhibiting acetylcholine release and causing peripheral blockade. Pharmacologi­ cally, use is made of this in the management of seizures associated with eclampsia of pregnancy, where an effective remedy is the parenteral administration of magnesium sulphate to maintain plasma concentrations in the range of 3-4 mmol/L. The treatment of hypermagnesaemia consists initially of cessation of magnesium administration. In an emer­ gency intravenous calcium gluconate (10 mL, 10% solu­ tion; 2.2 mmol calcium) will reverse the effects of hypermagnesaemia. Dialysis is also effective. SUMMARY The physiology and pathology of calcium, phosphate and magnesium are closely related. The principal hormones controlling calcium and phosphate homoeostasis arc 1,25dihydroxyvitamin D and parathyroid hormone. The great majority of calcium and phosphate are present in the skcl-

( V» L i 1 tw

и

\IM

wi

CAU:U;M,

eton, as is over half the magnesium, but all three ions have many other vital functions. Pathologically, increased and decreased concentrations of calcium, phosphate and magnesium can occur and all can have serious consequences. For each disorder, a rela­ tively small number of conditions account for the majority of presentations: for hypercalcacmia, primary hypcrparathyroidism and malignancy; for hypocalcaemia, lack or impaired metabolism of vitamin D; for hyperphosphataemia, renal failure; and for severe hypophosphaiaemia, recovery from diabetic ketoacidosis, intravenous feeding and withdrawal from alcohol. Hypermagnesaemia is usu­ ally due to renal failure; hypomagnesaemia is probably common, but often undiagnoscd because the possibility of its occurrence is not considered.

FURTHER READING Favus M J (cd). Primer on the metabolic bone diseases and disorders of mineral metabolism. Kclscyvillc, С A: American Society of Bone and Mineral Research, 1ЧЧ0. A succinct and aiahtmtative account of the current state of knouledge in the fields of banc biology. calcium metabolism and metabolic bone disease, tentten by the leaders in the rcspectnv fields. Mundy G R. Calcium homctmasis: hjpcrcalcacmia and hypocalcaemia» 2nd edn. I-ondon: Martin Dunit?, |Q90. This is a detailed consideration of ail aspects of abnormal plasma calcium homoeostasis.

APPENDIX 1; S T R O N T I U M ABSORPTION T E S T The intestinal absorption of stable strontium has been shown to be an adequate predictor of radio-calcium absorption.'^Its advantages are freedom from radioactivity, low cost and independence from the constraints of using short half-life radioisotopes. T h e study is performed after an overnight fast. T h e subject takes 2.5 mmol of strontium chloride in 200 m L distilled water, followed by a light, low-calcium breakfast (e.g. stewed fruit). Scrum strontium is measured 4 hours later, by atomic absorption spectrometry. The result is expressed as the fraction of the administered dose in the extracellular fluid: _ WOQSr] x 0.15 x body weight)

2.5

0/ i 0

where (Sr] is the measured serum strontium concentra­ tion. The reference range is 6-18%. 1. Rcid I R, Pybua J, IJm T M T> Hunnon S, Ibbertson H K. The assessment of intestinal calcium absorption using stable strontium Calcified Tissue International 1986; 38: MH-5, 2 Milftom S» Ibbcrtson H K, Hannon S, Shaw 1>, Pybu* J. Simple ie*t of intestinal calcium absorption measured by stable strontium, Bnnsh Medical Journal I*>87; 205: 231-4.

A P P E N D I X 2: C A L C I U M A B S O R P T I O N T E S T

This test was originally described by Broadus et all for

mospHATM AN"» MAC.NKSIIM

113

use in the investigation of hypercalciuria, but it can also be used to estimate intestinal calcium absorption in other contexts. The patients takes a 10 mmol (400mg)/24h calcium diet for 1 week and is studied after an overnight fast (distilled water only). A water intake of 250 mlTh is maintained from 1 hour before the test until its comple­ tion. The test consists of three consecutive 2 h urine collections for measurement of calcium and creatinine with a plasma calcium and creatinine measurement at the midpoint of the first and third of these collections. At the beginning of the second urine collection a 1 g (25 mmol) oral calcium load is taken. Interpretation Normal values: rise in urine calcium output = 0.180.73 mmol/2 h rise in plasma calcium = 0.070.30mmol/L I. Broadus A E, Dorninguez M, Banter F C. Paihophysiological studies in idiopathic hypercalciuria: use of an oral calcium tolerance test to characterize distinctive hypcrcalciunc subgroups. Journal of Clinical Endocrinology and Metabolism 1978; 47: 7V-60.

A P P E N D I X 3: ANALYSIS O F T U B U L A R H A N D U N G OF CALCIUM

The normal relationship between urine calcium excretion per litre of glomerular filtrate (Ca E ) and plasma calcium has been established from calcium infusion experiments in normal subjects. The renal tubular handling of calcium can be assessed by reference to this normal relationship. CaK is estimated from: Urine calcium x plasma creatinine urine creatinine obtained from the second void sample after rising in the morning, whilst still fasting. All these concentrations are expressed as mmol/L. The units of Cafc arc mmol/L glomerular filtrate. Interpretation Hypcrcalcaemic patients falling to the right of the normal range are excreting less calcium than expected for their prevailing plasma calcium level. Provided that this is documented at a time when the patient is not volume depleted, it can then be inferred that abnormally high renal tubular calcium reabsorption is contributing to the patient's hypercalcaemia. Typically this is seen in primary hyperparathyroidism, familial hypocalciuric hypcrcalcaemia and malignant hypercalcacmia due to PTHrP. Similar considerations apply in the investigation of hypocalcae­ mia. Patients in whom CaK falls to the left of the normal range are excreting more calcium than would be predicted from the low plasma calcium concentration and it can be

Материал, защищенный авторским правом

1 14

CLINICAL BIOCHEMISTRY CaE( m nx>l/LGF)

Analysis

0.5

urine specimens - cAMP, creatinine, phosphate concentrations blood specimens - plasma creatinine, phosphate concentrations cAMP excretion expressed as nmol/L G F urine cAMP(nmol/L x plasma creatinine (mmol/L) urine creatinine (mmol/L) Phosphate excretion expressed as TmP/GFR (mmol/L G F - see Appendix 5). Interpretation 20

25

30

35

Plasma catewm (mmol/L) 6.9 Reproduced with permission from Nordin В Е С Diagnostic procedures. In: Nordin В Е С (cd) Calcium, phosphate and magnesium metabolism. Edinburgh: Churchill Livingstone, 1976.

inferred that abnormally low renal tubular calcium reabsorption is contributory to the hypocalcacmia (typically seen in hypoparathyroidism) (Fig. 6.9). APPENDIX 4: CLASSIFICATION OF PSEUDOHYPOPARATHYROIDISM This protocol is that described by Malletie et al1 and is a modification of the Ellsworth-Howard and ChaseAurbach tests. It measures the response of urine cAMP and TmP/GFR to an injection of synthetic human PTH (1-34). The test is performed in a fasting patient with a fluid intake of 400 mlTh being maintained for 2 h before the test until its conclusion. Six consecutive 30 min urine collections arc made, with blood samples taken at the midpoints of these. At the beginning of the fourth period P T H 1-34, 3 units/kg (maximum dose 200 units) is in­ fused intravenously over 10 min (sec Fig. 6.10).

1

4

2

r

T

I 4

T

2 | 3) 4 | 5 | 6

t

5 1

hours

]

fluids 0.4 Lyhr

Normal response: 10-12-fold increase in cAMP excretion, fall in TmP/GFR of 20% Pscudohypoparathyroidism: Type I < 5-fold increase in cAMP Type II normal increase in cAMP Both types < 10% fall in TmP/GFR I. Malletie L E, KirWand J L, Gagel R P, Law W M, Heath H. Synthetic human parathyroid hormone (1-34) for ihc study of pscudohypoparathyroidism. Journal of Clinical Endocrinology and Metabolism 1988; 67: 964-72.

APPENDIX 5: ESTIMATION O F TmP/GFR A fasting, second voided urine sample is obtained in the morning. *1Ъс convention is that this should be a 2-h col­ lection, but the timing is in fact immaterial since time does not enter into the calculation. A plasma sample is obtained at the same time. Measurements: plasma phosphate and creatinine; urine phosphate and creatinine (all expressed in the same units).

30 min urine collections blood samples PTH (1-34) infusion

Fig. 6.10 Test protocol for the classification of pscudohypoparathyroidism.

Fig. 6.11 Reproduced with permission from Walton К J. Bnvoct O L M . Lancei 1975; 2: 509-10.

CALCIUM, I'HOSPHAIT- AND MAC. N IS 1С M

The ratio of the clearance of phosphate (С(Ап.Ч1Лж|с) to the clearance of creatinine (C LfnMMt ) is calculated: . C ^ ^ C , ^ ^ = u n n c ^ H ^ x P' a s m a .r» I g »,. P*asmartio*Phj(r x unnecrf.Qii.fK The tubular ^absorption of phosphate (TRP) = 1 (C 1С л TmP/GFR is read from the nomogram which relates thc prevailing plasma phosphate concentration [POJ to TRP (Fig. 6.11).

I15

A straight line through the appropriate values of plasma phosphate and T R P (or Crbut^ltJCi:n„intnt.) passes through the corresponding value of TmP/GFR. It should be noted ±ul T m p / G F R a n d p i a s m a phosphate concentrations are expressed in the same units. Two scales have been given: thc 0 ' ° - 2 - 0 «= aie i s suitable for estimating values of TmP/GFR close to the normal range expressed in SI units (0.80-1.35 mmol/L GF) and the 0.0-5.0 scale for values close to the normal range expressed in mass units ( 2 . 5 4 2 mg/КЮ ™L GF).

Материал, защищенный авторским правом

п С Z

с >

2

I APPENDIX 6: MAGNESIUM 1OLERANCE TESTING Suggested schema for clinical use of magnesium tolerance test in normomagncsaemic patients 1. 2. 3« 4.

Collect baseline urine (spot or timed) for magncsium/creatininc molar ratio Infuse 0.1 mmol MgSC>4/kg lean body weight in 50 m L dextrose saline over 4 h Collect urine for 24 h (starting with infusion) for measurement of excretion of magnesium and creatinine (Cr) (both in mmol) Calculate % Mg retained using following formula:

Mg retained

Postinfusion urine Mg - (preinfusion urine Mg/Cr x postinfusion urine Cr)

> 100

total elemental Mg infused

5. Criteria for Mg deficiency: > 50% retention at 24 h From Rvzen E et al. Magnesium 1985; 4: 137-47. With permission of the publishers Kargcr, Basel.

со -

в о

—
-aminohippuric acid, which is almost completely cleared from the blood in a single passage through the kidneys by a combination of glomerular filtration and secretion. Inutin clearance Inulin is a plant polysaccharide which satisfies all the physiological criteria mentioned in the previous section. The measurement of inulin clear­ ance remains the 'gold standard' for the estimation of

Материал, защищенный авторским правом

1 26

1Л JN1CAL BIOCHEMISTRY

1 able

■ !"горсГОс» of substances uv to assess glomcrular

Property*

Urea

Ocatininc

Inut

•^Tc-DTPA

Not pin ml» bound FVcciv B i o c d by glomerulu» Neither tecrcicd oor absorbed in ncphron l*r« Muted endogenou at constant rate Easily measured

Yea Ye»

Уса Ye»

Ye» Yes

Yes Ye»

Flow relaicd reabsorpuon

Some secretion

Ye»

Ye»

No Ye»

Ye» Yet

N..

No No

-

* The ideal substance would have all these properties DTPA = dicthylenetmmincpentaaccuc acid

GFR. However, it is a relatively complex procedure and although test kits are commercially available, it is infre­ quently performed in clinical practice. In essence, the test involves the injection of a bolus dose of inulin, followed by a maintenance infusion designed to produce a constant plasma concentration. Once this has been achieved, a scries of timed urine samples arc collected and blood is drawn for the measurement of inulin at the midpoints of the collection periods. The GFR is taken as the mean of the inulin clearances for each period. Creatinine clearance Creatinine is an endogenous substance, a normal product of muscle metabolism. Its rate of production is fairly constant from day to day, being determined by muscle bulk rather than by activity. Creati­ nine is removed from the body mainly by glomerular filtration and creatinine clearance is frequently measured as an index of GFR. Creatinine is actively secreted into the urine, so that its clearance tends to overestimate the GFR. This effect is of little significance at normal filtration rates, the ratio of (creatinine clearancc/inulin clearance) being between 1.1 and 1.2, but in advanced renal failure the contribution of active secretion to the total amount of creatinine excreted in the urine becomes high in relation to the amount filtered and creatinine clearance may significantly over­ estimate the true GFR. This is despite the fact that in severe renal failure, bacterial degradation of creatinine secreted into the gut may contribute approximately 2 ml/ min to the total clearance. Inaccuracies arising from methodological problems with creatinine measurement have largely been overcome, but the major cause of inaccuracy in the determination of GFR by creatinine clearance is the accurate measurement of urine volume. Traditionally} patients have been re­ quired to collect urine over a 24 h period. This is a con­ venient time, but requires the patient to void their urine completely at the beginning of the 24 h period and collect all the urine passed over the ensuing 24 h, making a final collection at the end of this period. The possibilities for mistakes resulting in an incomplete collection are consid­ erable and even under ideal conditions, for example in a

metabolic unit with highly motivated patients, the coeffi­ cient of variation for repeated measurements in the same individual may only be as low as 1 1 % and is frequently higher. Even at this level, the critical difference (the amount by which two estimates must differ to give a 9 5 % probability that there has been a true change in GFR) is 33%. The accuracy of estimates of the GFR by measure­ ment of creatinine clearance may be improved by making two or more consecutive 24 h urine collections, but this is often not practicable even if it is acceptable to the patient. There is, however, nothing magical about the 24 h period. The use of shorter periods, although more con­ venient for the patient, may reduce accuracy through in­ adequate bladder emptying, but a good compromise is to make a collection overnight. As long as the bladder was emptied at the beginning of the test (before retiring) and when the final collection was made (on rising), the urine production rate can be calculated and substituted in the (U x V)/P formula. It is essential that a reliable method is employed to measure plasma and urine creatinine concentrations. Colorimetric assays tend to overestimate creatinine since they detect non-creatinine chromogens. But even when reliable methods are used, it should be appreciated that the creatinine clearance is based on four measurements: plasma and urine creatinine concentrations, urine volume and time. Each has an inherent inaccuracy and the overall analytical variance will be the sum of the individual variances. Plasma creatinine concentration 1Ъс way in which creatinine is handled by the kidney, coupled with the fact that in any one individual the rate of production is relatively constant from day to day, means that the plasma creatinine concentration alone can be used as an index of renal function. But although it is widely used, it has a number of disadvantages and these must be borne in mind when interpreting plasma creatinine concentrations. Perhaps the most important becomes apparent from con­ sidering the familiar (U x V)/P formula for deriving the GFR. Plasma creatinine concentration is inversely related to GFR, not linearly related. This means that a plot of

ким np

l l i

KIDNEYS, RliNAl. FUNCTION AND KKNAl. rAH.UKK

plasma creatinine concentration against GFR has the form of a hyperbola (Fig. 7.4). At any concentration of creatinine, halving the GFR will double the plasma con­ centration. But given the wide reference range for plasma creatinine concentration in the normal population, this means that in an individual, the plasma creatinine concen­ tration could be within the reference limits, yet the GFR be only one half of normal. This is of critical importance to clinical practice, since in early renal disease (when it might be supposed that therapeutic intervention would be most beneficial), the plasma creatinine concentration may be 'normal' - that is, within the reference range - in spite of the GFR being reduced. Plasma creatinine concentration is thus a relatively insensitive index of mild renal functional impairment. At low values of GFR, however, it becomes extremely sensitive, although its accuracy declines be­ cause of the increased contribution of tubular secretion to overall renal excretion. Other factors which must be taken into account in assessing the significance of plasma creatinine concentra­ tions include muscle mass, diet (a recent meal including meat - particularly if stewed - may cause a transient in­ crease in plasma creatinine concentration) and the pres­ ence in the plasma of substances which interfere with the assay. Ketones, bilirubin, cephalosporins and spironolactone have all been implicated, depending on the method used. The importance of muscle mass is exemplified by the changes that occur with age. In healthy individuals, glomerular filtration rates, and thus creatinine clearances, decline with increasing age from about the end of the fourth decade, at a rate of approximately 1 mL/min/year. However, plasma creatinine concentration docs not nor­ mally rise with age; this is because of a decrease in pro­ duction rate, thought to reflect the tendency for muscle mass to fall with age.

Although comparison of a measured plasma creatinine concentration with a reference range can be misleading, the fact that intraindivtdual variation is less than interindixndual variation means that the detection of a change in creatinine concentration in an individual provides a more sensitive indication of a change in renal function. Never­ theless, the biological and analytical variance (even using the best assays) results in a critical difference of about 17% (14pmol/L at a plasma creatinine concentration of 120jimol/L). In patients with advancing renal failure, the progressive loss of renal function with time can be expressed as a graph of reciprocal plasma creatinine concentration against time (since GFR is proportional to l/[plasma creatinine]). A steady loss of renal function will cause this plot to be rectilinear (Fig. 7.5). Such plots are useful in patients with irreversibly declining renal function to help predict when renal replacement will become necessary, so that, for example, appropriate means for vascular access for hacmodialysis can be provided. Given the problems associated with the measurement of plasma creatinine concentration and creatinine clear­ ance, the practical application of these measurements to patients with or suspected of having renal disease is not straightforward. Since the rate of creatinine production is relatively constant in most individuals (the essential

GFR (mL/min)

1600 P.asma creatinine (umol/L)

600

127

1 2 0

°800 400-

Plasma creatinine <jimol/L)

500 H 0 4 0 0

"Г 2

т 3

4

1 0.05 0.04-

300-

Ptasma 0.03 creatirwne o.02 -

200

(pmol/L}'1 O.01 -

100

0 Years Creatinine clearance (mUmm) Fig. 7.4 Relationship between creatinine clearance and plasma creatinine- concentration. As the clearance is related to the reciprocal of the plasma concentration, considerable lo« of renal function can occur without the plasma creatinine concentration rising above the reference range (shaded)

Fig, 7.5 The progression of chronic renal failure. Hypothetical curves lo show (a) the decline in glomerular filtration rate (GFR), 40mmol/L

>10

videi a go**d imnhiucruyn to renal medicine.

лтериал. з а щ т

*ый авторским правом

C H A P T E R

Proteinuria Peter Gosling

INTRODUCTION For over 150 years proteinuria has been recognized as a sign of renal disease and it remains the most common finding in patients with ncphropathy. The application of modern analytical techniques to urinary protein measure­ ment, together with increasing understanding of renal pathophysiology, has allowed non-invasive monitoring of the kidney to an extent which is not possible for any other organ in the body. In the future, assessment of proteinuria is likely to extend beyond the role of diagnosis and man­ agement of renal disease. The unfavourable prognosis for cardiovascular disease of trace proteinuria in population studies, the correlation of microalbuminuria with diabetic microangiopathy, as well as with diabetic ncphropathy, and the association of microproteinuria with a variety of acute inflammatory stimuli such as injury, ischaemia or pancreatitis suggest that urinary protein excretion reflects not only renal Ьш systemic disease processes. T H E MECHANISM OF PROTEIN CONSERVATION BY T H E KIDNEY Under normal circumstances the kidneys receive approxi­ mately 2 5 % of the resting cardiac output, which repre­ sents approximately 1.2 Umin of blood or 650 mL/min of plasma. The kidneys' capacity to conserve protein can be judged from a simple calculation. Every 24 hours approxi­ mately 930 litres of plasma containing about 70g/I- of protein pass through the kidneys, representing 65 kilo­ grams of protein, of which less than lOOmg (0.00015%) appear in the urine. The filtration process is dependent on adequate renal blood flow which is preserved in spite of variations in

blood pressure by an autoregulatory system. This mecha­ nism allows vasodilatation as perfusion pressure falls and vasoconstriction as pressure rises. The mediators of this process include prostaglandins, kinins and atrial peptides (vasodilators) and angiotensin II, ot-adrenergic hormones, rhromboxane A : , noradrenaline and vasoprcssin (vaso­ constrictors). In addition renal arterioles respond within seconds to changes in vessel wall tension; thus when renal perfusion pressure rises, vessel wall tone increases and conversely when renal perfusion pressure falls abruptly there is a compensatory decrease in vessel wall tone. This phenomenon is called the myogenic reflex and helps to maintain a constant renal blood flow across a range of perfusion pressures. A minimum intraglomcrular pressure, derived from the pumping action of the heart, is required to overcome the two main opposing forces to filtration: the colloidal oncotic pressure and the hydrostatic pressure of Bowman's space. When the renal perfusion pressure falls below 50 60 mmHg then further vasodilatation does not occur and renal blood flow declines in proportion to the reduction in renal perfusion pressure. These mechanisms maintain renal blood flow and hence glomerular filtration independently of the normal fluctuations of blood pressure; whether they become overwhelmed when renal perfusion pressure rises above normal, such as in hypertension, is not clear. The glomerular capillary wall The glomerular membrane consists of a modified capillary wall comprising endothelium, a cell-less basement membrane and an outer specialized epithelial cell layer 143

Материал, защищенный авторским правом

144

CUMGAL в к х : ш MISTRY Nucleus

Epithelial cell Foot processes Filtration sirt Slit diaphragm (55nm)

Lamna rara externa

I

300 nm

Lamina densa Lamina rara miema

EncMhalal овй cytoplasm Enctothelial •enestration (50-t00nm)

1;|ц. 8.1 Glomcrular membrane structure. Reproduced with permission from Swcny c! |L The kidney and its disorders. Oxford: Blackwcll Scientific Publications» 1989.

(Fig. 8.1). T h e endothelial cells are thin and fenestraied with 50-100 nm pores, the basement membrane is around 300-350 nm thick and appears to be a gel-like structure containing 3-5 nm long fibrils but no detectable pores. Moving from the endothelium across the basement mem­ brane, the latter can be subdivided respectively into the lamina rara interna, lamina densa and lamina rara ex­ terna, each clearly discernible by electron microscopy. T h e epithelial cell forms numerous foot processes that interdigitatc and envelop the outer surface of the glomcru­ lar membrane. The foot processes arc separated by slit diaphragms about 55 nm in width. T h e whole of the glomcrular membrane carries a fixed net negative charge which is partly due to a glycosialoprotein coat covering both endothelium and epithelium. T h e charge increases in density from the lamina interna towards the lamina rara externa with the greatest density at the slit diaphragm of the epithelium. T h e glomcrular capillary acts as a high pressure filter, allowing solvent and low molecular weight solutes to pass through freely and excluding macromolecules; the almost complete retention of protein by the glomerular mem­ brane produces the colloidal oncotic pressure which must be overcome for filtration to take place. T h e selectiv­ ity of the glomerular membrane for protein filtration depends on the molecular size, shape and electrical charge of individual proteins.

certain molecules. Classic experiments using uncharged dcxtrans revealed their greater glomerular permeability compared with anionic proteins of similar molecular weight and atomic radii. "1Ъс pore size of the fenestra of endothelial cells is too great to provide a major restriction to the passage of most proteins. T h e lamina rara interna of the basement membrane is probably the first impediment to confront charged molecules, with the lamina densa pro­ viding the most effective barrier to macromolecules. Some macromolecules accumulate at the slit diaphragms of the epithelium where the net negative charge is greatest and there is evidence that some molecules are pinocytosed by the podocytcs. Different proteins appear to penetrate the glomerular membrane to a variable extent related to their molecular radius and charge; for example, albumin (radius 3.6 nm and isoelectric point 4.7) is restricted at the lamina rara interna, whilst lactopcroxidasc (radius 3.8 nm and isoelec­ tric point 8.0) may reach the slit pores of the epithelial cell. Despite the protein-retaining properties of the glomer­ ular membrane, some protein does pass into the proximal tubular fluid. In normal man the albumin content of glomerular filtrate is probably about 5 mg/L which, with a glomcrular filtration rate of 180 IVday, represents ap­ proximately I g of albumin presented ю the renal tubules each day, most of which is reabsorbed.

T h e theory of molecular sieving

Tubular rcabsorption of proteins

T h e size selectivity of the glomerular membrane is only partly consistent with the theory that the membrane con­ tains pores of definite size which allow selective passage of

Only a very small proportion of plasma proteins filtered at the glomerulus reaches the unne. Proteins filtered by the glomcrulus are generally less than 69 000 Da (Table 8 -1 )«

ЩИЩ1

I'RCvn-INUWA

Table К. t

Relative lar wct^hi

Protein

M (Da)

IgG Albumin Amyiasc

150 000 1000 48 000 17800 14900

ЬАуофЫ

«ozyme

Clearance (%CFR' i

75 80

There is some evidence that of the filtered pi which reach the proximal tubule, those with lower mo­ lecular weights are reabsorbed by different mechanisms from those with higher molecular weights. For example, although increased amounts of low molecular weight pro­ teins arc excreted in tubular disease, they are absent from the urine of patients with massive glomerular proteinuria, when the reabsorptive mechanisms for large molecular weight proteins such as albumin must be saturated. Measurement of the urinary content of low molecular weight proteins (,5> I

Role

I 'rtnarv content (mg/24 h)

Cast formation nn removal Bactericidal

1I

146

CUNICIAI. HUH MI MISTRY

physical and catalytic properties. Urokinase may have a role in removing fibrin from the renal microvasculaturc and possibly in removing urinary casts. Secretory IgA is thought to be produced by lympho­ cytes as a 7S-protein IgA which is transported to tubular epithelial cells where an additional 'transport protein' is added before it is secreted into the tubular lumen. Secre­ tory IgA may provide a defence against infection. URINE PROTEIN C O N T E N T IN HEALTH Following the application of sensitive immunoassays to the measurement of urinary proteins, the concept of normal urine being protein-free is no longer tenable. A normal adult excretes about 300 mg/24 h of non-dialysable material, of which protein represents approximately 140 mg/24 h, the remainder being largely aminoglycan polymers. Published reference ranges for total urinary protein excretion vary considerably with the type of ana­ lytical method employed. The protein content of normal urine is the net result of glomerular filtration, tubular rcabsorption and tubular sccreuon. Plasma proteins only represent some 25 mg/24 h of total urinary* protein, the remainder being made up of proteins of renal origin of which Tamm-Horsfall glycoprotein is the major contribu­ tor (Table 8.3). D e t e r m i n a n t s of u r i n a r y protein excretion Age and sex In neonates albumin excretion tends to be higher than in older children and adults; this has been attributed to greater permeability of the neonatal glomerulus. Total urine protein tends to fail after birth and rises with in­ creasing age reaching adult excretion rates by puberty. However, if urinary protein excretion is expressed in pro-

ТиЫ.

1ЯП1Д prolemi

:aJ in normjl annc Men

I'rcalbumm Albumin й oproton -Antiirvpvn 1 cruloputunin Haptoglobtn i r a n u c m 11 1 i icrnopcxin I*A IgG IgM 1 jght d u o » Lambda i Kappa P; Mi roglobuhn

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• 'П m t . 4 h 0.05) 21.9)

12.7

portion to urinar>' crcatinine the protein/creatinine con­ centration ratio remains constant from 3-15 years of age, since urinary crcatinine excretion also rises with increas­ ing body mass. Of the plasma proteins normally present in urine, albumin is in the greatest concentration. In children between the ages of 4 and 16 years albumin excretion rate, corrected for body surface area, rises with age and is slightly higher in girls. Daytime excretion rates are higher than during the night and the sex difference disappears in overnight collections. There appears to be no sex differ­ ence in urinarv albumin excretion in adults. Posture In both adults and children ambulatory urine protein excretion is higher than overnight or during recumbency, with two- to ten-fold differences being reported for uri­ nary albumin. Onhostatic protcinuria in otherwise healthy subjects has been the subject of controversy for some time. The discussion has been complicated by the variety and differing sensitivities of protein assays used. Isolated protcinuria of less than 1000 mg/24 h has been described in between 0.6 and ° % of healthy young adults in the absence of urinary red cells, white cells or casts and can be divided into 'constant' and 'postural' based on its persistence after recumbency. Light microscopy of renal biopsies of patients with postural protcinuria revealed 8% having unequivocal evidence of well-defined disease and 4 5 % with subtle alterations in glomerular structure. Since urinary protein excretion increases in the upright posture, it has been suggested that onhostatic protcinuria merely represents the crossing of the limit of detection of the urinary protein assay in use in patients with low-level proteinuria. However, other explanations have been proposed including increased venous pressure due to prolapse of the kidney or to increased inferior vena cava compression during extreme lordosis on standing. The medical management of isolated or postural pro­ tcinuria in an otherwise healthy patient tends to be con­ servative with annual assessment of protcinuria and renal function; biopsy is reserved for the rare patient who has evidence of progressive renal impairment.

0 . 4 1

0 И 0.06 0.18 n22 0.20 51

[0.19 (0.05-0.06) (up tn о 12) (0.1 V O W ) «J ! » 29) (IM (1.97-3.01)

0 M 1 40 2.30

a

Adapted with pcrau>won 1 n Реке Л I л I Proianuna: an integrated I n edn N ri \i*rtel Ockkcr. 1979.

Exercise and diurnal variation Exercise-induced proteinuria was discovered over a century ago in soldiers after marches or drills. Five- to 100-fold increases in the excretion of proteins such as albumin, transfernn and immunoglobulins have been observed following 26-milc marathons, with smaller in­ creases after less strenuous activities. The pattern of exer­ cise-induced protcinuria is generally glomerular although mixed glomerular and tubular protcinuria has also been described which persists for over 3 hours after exercise. The reason for exercise-induced proteinuria is unclear but

4

I'ROIT.IM'RIA

some degree of renal ischaemia due to redistribution of blood during exercise has been suggested as a possible mechanism. A large protein meal is associated with an increased urinary albumin excretion which appears to be due to an associated increase in GFR. The lowest and most repro­ ducible estimates of urine protein excretion are obtained from an early morning urine specimen after overnight recumbency. I*regnancy During normal pregnancy the urinary albumin excretion rate generally remains within the non-pregnant range, although there is some evidence for a small increase in albumin excretion during the third trimester which may be related to increased glomerular permeability. PROTEINURIA IN RENAL DISEASE Richard Bright, in 1836, is generally credited with noting the association between protcinuria and renal disease. Proteinuria remains the most common clinical finding in renal disease and quantitation of proteinuria is still a valuable test in monitoring renal function. For adults total protein excretion is normally < 1 5 0 m g / 2 4 h and < 140 mg/m : /24 h in children. Application of sensitive immunoassay to urinary pro­ teins has identified a pathological range of proteinuria < 500 mg/24 h which is undetectable by semiquantitative reagent stick testing, but which is pathologically signifi­ cant. Assessment of 'microprotcinuria' has found a place in clinical practice in the early diagnosis of diabetic nephropathy and appears to predict those patients who are likely to develop oven nephropathy. Sensitive urine pro­ tein assays arc also being routinely applied in paediatric ncphrology. Proteinuria can now be regarded as a con­ tinuum which extends from the measurable amount of protein normally excreted up to 1000-fold increases found in ncphrotic syndrome. Conventionally, proteinuria has been classified into glomerular proteinuria, tubular proteinuria, nephrogenic proteinuria (e.g. due to Tamm-Horsfall glycoprotein, basement membrane and tubular proteins), proteinuria of prcrcnal origin (e.g. overflow proteinurias such as light chain disease, myoglobinuria, haemaglobinuria, lysozymc in leukaemia or amylasc in pancreatitis) and postrenal proteinuria due to inflammation of the urinary tract such as in urinary tract infection. Whilst this may be an over­ simplification because, for example, glomerular disease leading to proteinuria may well have consequences for tubular function, it serves as a useful framework. Glomerular proteinuria - nephrotic syndrome In the normal adult the renal tubule probably reabsorbs

Table K.4

< JU

147

neph'

Гпшагу renal disease minimal change (steroid response 'mcruloncphu focal segmcninl giomcnilo>deracii membranous IKphropathj prolifcraiivc p ■ uUmcphnri* lomerular disease ш systemic con.!:' amvloidosis ncphropaiE lystemk lupui crythematosus Henoch Schonlcin purpura

(.hildren •/.of all ncphrouo

Adults % of all ncphrotici

80 7

25 9

I

10

-

2

7 1 8 3.5 g/ 24 h. However, it should be remembered that the amount of proteinuria may lessen due to decreased plasma protein concentration or a fall in the glomerular filtration rate. Causes of ncphrotic syndrome arc listed in Table 8.4. In addition to the nephrotic syndrome, glomerular proteinuria is a feature of several other syndromes of ncphron injury and the severity of proteinuria taken together with other clinical findings can allow useful diagnostic classification (Table 8.5). Mechanisms underlying glomerular proteinuria Glomerular injury can occur as a result of both primary and secondary renal disease and there is no single pathogenetic pathway which will embrace all the possible mechanisms. However, since many glomerular diseases are immunologically mediated, the immune responses leading to glomerular injury* and increased glomerular permeability can be considered. Glomerulonephritis can be subdivided immunologi­ cally into conditions mediated by antibodies against agents which arc cither extrinsic or intrinsic to the kidney. Glomerular basement membrane represents an example of an intrinsic structural component to which antibodies can bind, leading to immune complex formation. Extrin­ sic agents include bacteria (e.g. subacutc bacterial endoirduis or sireptococcal infections) and DNA (e.g.

148

CLINICAL BIIK:HI MIS TRY

1 able К.-в.5 ГяЫс

Summary

.!

Degree proteinuna I). zic? of .-I pffOttfcrarii

( limcii! c ocrttaca rrttan

Massive protemuria

'

> VS щ,': ally no Ьааплипд. plasma albumin concentration Vcrj plasma lipid*. very high, gross о i. BP normal or low , < i! rmal or increased, urine volume normal or mcreaaed Moderate pro г i irmria

Ncp' syiMlWiBM (acute ncphnnv-. acute renal failure чктасе to marked Ьлеташпа. plasma чип n plasma lipids normal or slight iv increased, oedema absent lunv oad sometimes with im reavrd iugular venous pr \\V raised. (il-'R reduced, urine volume reduced renal uulurc As above but over mun -an. Only moderate proieinuna may reflci d rcn.il d i m of functaooang nephron* rather than cxtcn disease

Asymptomatic. vanablc/transici.

Perwuani mtcroprotcinuria

ftutainwii < l &2Л h no other abnormalities, probably benign fonhostatu protein una una >2g/2\ h po». glocnerulonephr Associated with inflammatory con ncbacmia and artcriopat

(

*,

systemic lupus erythematosus). Extrinsic antigens or their antigen-antibody complexes can become trapped in the glomerulus. Irrespective of the source of the antigen, immune complexes become embedded in the glomerular basement membrane. Immune complexes activate both the classical and alternative complement pathways leading to the re­ lease of anaphy la toxic components such as C3a and C5a. Anaphylatoxins, together with locally released kinins, prostaglandins and leukotrienes, attract polymorphonuclear neutrophils to the basement membrane where the latter release lysosomal enzymes leading to membrane viisruprion and glomerular proteinuria (Fig. 8.2). Light and electron microscopy reveal a gradation of morphological changes with increasing glomerular injury. Loss of anionic charge associated with fusion of the epi­ thelial foot processes can produce a massive but selective proteinuria, while changes in the basement membrane ranging from its exposure to frank discontinuities tends to be associated with increasingly unselective proieinuna. Hisrological classification of glomertdonephritis The classification of glomerulonephritis has traditionally been based on light microscopy of renal biopsy material

Fig. 8.2(1)

ЩИЩ1

PROTON-UNA

149

Activated С

Fig. 8.2 r i g . 8.2 Mechanisms of glomcrular inrury induced by circulating immune complexes (GIC) or anti-CiBM antibodies depicted schematically. Hither CIC or anti-GBM antibodies arc deposited in (IBM and tubule basement membrane* (TBMj, leading to activation of complement (C) through classical or alternative pathways (panels I, 2a, and 2b). As the complement sequence proceeds, the generated anaphylatoxic components СЭа and C5a result in inflammation and membrane permeability (panel 3). Together with other» non-complement mediators of the inflammatory response (e.g. kimns, prostaglandins, leukotnencs), complement chemotaxins serve to attract ncutrnphiU to the site of injury (panel 4). Concurrently, monocytes infiltrate and surround the GBM and enter the mesangium. C3b proximity to the GBM surface facilitates immune attachment of neutrophils (panel 5), which release lytosomal enzymes and disrupt membrane integrity. From WiNon С B, Oixon F J. Immunological mechanisms in nephritogenesis. Hospital Practice April 1979, with permission.

and the severity of the disease assessed in terms of the extent and nature of changes in glomerular architecture. Histological classification of glomerulonephritis supported by immunofluorcscencc and electron microscopy data remains an essential diagnostic and sometimes prognostic tool. A detailed description of the histological patterns of glomerulonephritis is beyond the scope of this chapter and only a brief overview of the five major histological classifications which are found in association with nephrotic syndrome can be given. Minimal change glomerulonephritis Light micro­ scopy shows little or no abnormality (Fig. 8.3) and there are no immunoglobulin or complement components seen on immunofluorescence. Electron microscopy shows fusion of epithelial cell foot processes. The mechanisms of protcinuna probably involve loss of fixed negative charge on glomcrular membrane. In the Western world minimal change glomerulo­ nephritis, frequently of unknown aetiology, accounts for 90% of nephrotic syndrome in 2-6 year olds and about 20% in adults. In children there is massive proteinuria which is predominantly albumin with little or no immunoglobulins present. (Such proteinuria is termed 'selective', while a generalized leak of plasma proteins, including proteins larger than albumin, is termed 'unselective* proteinuria.)

Because in paediatric practice this form of nephrotic syndrome usually responds to a short course of prednisolone (steroid-responsive nephrotic syndrome) a renal biopsy is unnecessary to confirm the diagnosis. The absence of serological evidence for complement activation gives additional confirmation. In adults the diagnosis is more difficult and a renal biopsy is generally needed before treatment is started. Membranous glomerulonephritis This is the most common morphology underlying nephrotic syndrome in adults. Light microscopy shows thickening of the glomcrular basement membrane which is more marked in later stages of the disease (Fig. 8.3). Immunofluoresence microscopy always shows IgG deposits, and when other immunoglobulin classes (e.g. IgA, IgjVl) arc present the condition is often secondary to systemic lupus crythematosus. Electron microscopy shows a progressive thickening of the basement membrane. Over 80% of patients with nephrotic syndrome present with massive proteinuria which is only moderately or poorly selective, while the remaining 20% have asympto­ matic proteinuria or microscopic hacmaturia. The pathogenesis of the glomerular disease is not understood, but theories centre on the role of extrinsic antigen deposition in the glomerulus with subsequent immune complex formation, glomerular trapping of circulating immune

PRO I-KINX'RIA

GBM

Urinary space

Epithelial cell (parietal layer)

1 5 1

Eostnophi lie deposits (PAS positive)

Epithelial cell (visceral layer)

\ Mesangial matrix

WesangiaJ celt Endothelial cell

Small capsular adhesion

Basement membrane of Bowman's capsule

Minimal change glomerulonephrltls

Collapse of capillary loop Expansion of mesangial matrix-like material (some increase in mesangial cells te seen occasionally) Focal and segmental glomerulonephrltls

Thickening of the GBM seen with PAS stain Normal mesangial area Bubbry or foamy GBM

Spikes on the GBM are seen with stiver stains

Membranous glomerulonephrltts

Mesangial overceflulahty Polymorphonuclear leucocyte infiltration

Endothelial cell proliferation

Epithelial eel: proliferation

Eprthelloid cells forming a crescent (fibrosis wrfh increasing age) Fibrin

Encroachment or occlusion of capillary lumen

P r o l i f e r a t e glomerulonephrltls

Underlying gkxnerulertuft: Focal necrosis is common but proNferation or even a normal glomerular tuft may be found

Crescent may become circumferential

Crescentlc glomerulonephrltls

F i g . 8.3 Major hiMological classifications of glomerulonephruis associated with ncphrotic syndrome. Reproduced wich регшдаюп from Swcny P ex al. The kidney and its disorders. Oxford: Biackwell Scientific Publications, 1989.

Материал, защищенный авторским правом

152

CLINICAL

шосшгмшиу

complexes or antibodies binding to intrinsic glomerular antigens. Proliferative glomerulonephritis The term 'proliferative' indicates that the glomcruli appear hypcrccllular, due for example to invading macrophages or mesangial cells (Fig. 8.3). Further subdivision is based on the site(s) of proliferation which may include mesangial cells (mesangial prolifcrative glomerulonephritis), cndothclial cells (cndocapiltary proliferate glomerulonephritis) or basement membrane (membranoproliferative glomeru­ lonephritis). Many different diseases, e.g. bacterial endo­ carditis, group A streptococca! infections and multisystem vasculitides, produce a similar histological pattern. Immunofluorescence microscopy generally shows IgG and C3 deposits, and circulaung complement concentrations arc frequently reduced with high concentrations of immune complexes present in some conditions. About 10% of patients with nephrotic syndrome show histological features of membranoproliferative glomeru­ lonephritis on biopsy (Fig. 8.3). All patients with mem­ branoproliferative glomerulonephritis have proteinuria and microscopic haematuria; approximately 50% of patients with nephrotic syndrome present with an unsclccrivc proteinuria. Focal and segment at glomerulonephritis Hyaline material is deposited in the subendothelial spaces of affected capillary loops of some but not all glomeruli, hence the terms focal and segmental. Patients present with mild proteinuria or recurrent haematuria and only 10-20% of adults and children have nephrotic syndrome. The aetiology is not known although some patients have evidence of systemic lupus crythcmatosus. In most cases the condition goes into spontaneous remission, but may recur in association with respiratory infections. Crescentic glomerulonephritis Bowman's capsule is infiltrated by a crescent of proliferating cells, probably macrophages, which can encircle the whole glomerulus. The crescents become fibrotic and immunofluorescence reveals IgG and C3 deposits when the condition is due to antibodies against basement membrane, whilst a variety* of immunoglobulins may be found if the cause is immune complex deposition (Fig. 8.3). The relationship between proteinuria and histological classification Following the introduction of immunochemical tech­ niques which allowed separate measurement of plasma proteins appearing in urine, attempts were made to corre­ late the pattern of proteinuria with renal histology* in patients with glomerular proteinuria. The concept of * se­ lectivity1 was based on the assumption that under normal circumstances the glomerular barrier had a sharp molecu­ lar size cut-off above which protein molecules were excluded, but that in patients with glomerular disease

there was an increasing tendency ю allow large molecular weight proteins to pass into the tubule. Protein selectivity is based on a comparison of the relative concentrations of proteins of differing molecular weight in plasma and urine. Two proteins frequently used for selectivity studies are IgG and transferrin. '№cir relative clearances can be calculated as follows: Clearance of IgG Clearance of transferrin

_ (IgG] u x (Trans) p IlgG] p x [Trans| u

where [IgG]^ = urine IgG concentration [Trans] p = plasma transferrin concentration [IgG) r = plasma IgG concentration (Trans] u = urine transferrin concentration. VPhere the selectivity index is >0.5 the proteinuria is said to be 'non-selective', 0.3-0.5 'moderately selective' and 600mQ/24h

Reassure patient Follow up annually

Вюрзу

L

T Diagnosis

Fig. 8.5

Diagnosis

Stcpwisc invesugation of patient* with proteinuna.

F U R T H E R Kl A P I N G Brenner В M, ( д к HI-, Rector F С Clinical ncphrology, Phil»» W B S a u n d e r s , 19Я7. Brenner В M, О м : I. C , Rector F С Renal phvMnlogy in health and disease, 1st edn. Philadelphia: W В Saunderv 1087. These ttvu гЫите$ provide an up-to-date treatment of mechanisms of rrnal disease ami methods of investigation Cameron J S, Glassock R J. T h e nephrotk syndrome. New York: Marvel Dckker, 1988

In addition to being a valuable reference text. Chapter 6 provides a clear desenptum of current theories explaining mechanisms of tubular rtabsorptton of filtered albumin. Gosling P. Hughes E A, Reynolds T M ci at. Micxoalbuminuha is an early response following acute myocardial infarction. European Heart Journal 1991; 12:90$. This paper provides an example of lot? level protetnuna in non-renal disease and gives references to other examples of рпнапипа m a variety of acute inflammatory eondstums.

ЩИЩ|

PRO 11-INt RIA Pesce A J, First M J. Protcinuna, an integrated review, h t edn. New York: Marcel Detter, 1979. Although /A years old at the time of unting, this text remains one of the finest reviews of original rt*>nV on promnuria. Poortmans J R et al. Renal protein excretion in man. European Journal of Applied Physiology 1489; 58: 47b. A current and auiisontam* reviete of protemuna.

161

Swcny P, Farhngton K, Moorhcad J F. The kidney and its disorders. Oxford: BlackwclJ Scientific Publications, 1989. The clear description of the histoiogical elastifkaricm of ghmerular disease it a major feature for the clmkal biochemist. Vfallcr К V, Ward К M, Mahan J D ct al. Current concepts in proteinuria. Clinical Chemistry 1989; 35: 755. 1Ъп reviete provides a bibliography of 117 references covering the majority of recent advances in the understanding of proteinuria oxvr the last decade.

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C H A P T E R

Renal tubular disorders and renal calculi Stephen K. Bangert

INTRODUCTION Most patients with renal disease have some element of renal tubular involvement, but the other manifestations of the disease tend to be clinically more obvious and impor­ tant. However, in a small number of patients, the clinical picture results primarily from a disorder of renal tubular function. These disorders maybe inherited or acquired and may affect tubular handling of a limited number of speci­ fic substances or may encompass more generalized defects. These renal tubular defects are conveniently consid­ ered with renal stone formation, since calculi do at least sometimes form as a result of one of these tubular defects. RENAL TUBULAR DISORDERS Introduction renal tubular disease includes certain develop­

mental disorders of the tubules, for example polycystic renal disease and medullary cystic disease. While these can result in disorders of renal function, including renal tubular function, they will not be considered in detail here. Renal tubular physiology will be briefly discussed, followed by a discussion of some wcil-rccognizcd disorders of renal tubular function. Physiology Renal function has already been outlined in Chapter 7, but in essence the process involves filtration at the glomerulus, followed by modification of this glomerular filtrate by both tubular reabsorption and tubular secretion. Since 170 L of filtrate are formed each 24 h but onlv about one hundredth this amount of urine is produced, reabsorption is quantitatively the more significant (Table 9.1). This is largely an active, energy-requiring process and explains

table 9 . Д Renal tubular handttni m* plavna coi П iigure* given are ustrai rily and anun. i l70LGln i and reduced la innc in an adult «HI л normal diet. thai 1c rcabwrpi quantitat^ more the tubule

Пнин constituent Sodium

Potassium г. цЬошц Urea cahnme Unac jcose

Typical plasm.i i ooKcntration (mraoi/L)

Filtered

iЮ 4.0

23 800 >.Ы)

40в0 1 i'

0 4.5

MO 19 41 765

h t mmol) Amount* per Keabeortv Secreted l;XCrCtcd the urine 23 700 650 4 080 3.'

100 30

0 360 1.0 5

90 765

60

-

1

0 163

^ M

164

CUMCM. BIOCHEMISTRY

why the kidneys account for some 6-8% of the resting oxygen consumption of the body, while representing less than 1% of bodv mass. Some of the mechanisms by which active transport in the renal tubule occurs are shown in Figure 9.1. The control of renal tubular handling of certain substances is covered in detail in other chapters, for example, sodium and water in Chapter 4. Only the renal tubular handling of substances that arc important in disorders of renal tubular function will be considered further here. Glucose is absorbed in the early part of the proximal tubule with sodium ions, in a secondary active transport process. Glucose and sodium bind to a common carrier protein in the luminal membrane and sodium moves down its electrochemical gradient, carrying glucose into the cell. N a \ K'-ATPasc in the non-luminal (basolateral) membrane of the tubular cell pumps the sodium ions out into the interstitial fluid, while glucose moves out by passive diffusion. Amino acids arc also reabsorbed in the early part of the proximal renal tubule, again by a secondary active trans­ port system linked to sodium reabsorption. There appear to be separate cotransporter proteins for certain groups of amino acids, although some of these probably have over­ lapping specificities. The process is driven by the Na%

K*-ATPase in the basolateral membrane pumping sodium out of the cell, with amino acids leaving by passive or facilitated diffusion. Phosphate reabsorption in the renal tubule is influenced by the dietary intake of phosphate, certain hormones and a variety of other factors, and these are described in Chapter 6. However, in summary, about 90% of the in­ organic phosphate in the plasma is freely filtered at the glomerulus (although this decreases as the plasma calcium concentration rises) and then about 7 5 % is reabsorbed in the proximal tubule. A small, variable amount is also absorbed in the distal tubule, but overall reabsorption is incomplete and up to 40 mmol/24 h appears in normal adult urine. The rate-limiting step in reabsorption appears to be a secondary active transport system linked to sodium reabsorption, with a phosphate/sodium cotransporter located in the luminal membrane of the tubular cell. As described in Chapter 5, the renal tubular secretion of hydrogen ions is linked to the 'reabsorption' of bicarbo­ nate. Around 4000 mmol of bicarbonate is filtered every 24 hours, but normal urine contains virtually no bicarbo­ nate so the tubules must secrete 4000 mmol of hydrogen ions to achieve this. They must also secrete the hydrogen ions produced each day in normal metabolism (see Ch. 5), a further 40-80 mmol/24 h.

LUMEN

INTERSTITIAL FLUID

Tight junction PRIMARY ACTIVE TRANSPORT

A DP + P.

ATP~-

ATP

ADP^ +

Ы»

Tubular cell

SECONDARY ACTIVE TRANSPORT

ATP ADP

R g . 9.1

NaК

Active transport mcvhAnisrm in the renal tubule.

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правом

RJ-NAL TUHl'MR DISORDERS AND ККХЛ1- CALCL'JJ

There are two distinct mechanisms by which hydrogen ions arc secreted into the tubular lumen. A secondary active transport system linked to sodium operates in the epithelial cells of the early tubular segments, so that the Na'j K'-ATPasc on the basolatcral membrane produces an electrochemical gradient for sodium to enter the cell from the luminal surface, but in contrast to glucose and amino acids, a hydrogen ion is simultaneously secreted into the lumen. Although a very high hydrogen ion gradi­ ent cannot be achieved, this mechanism is responsible for the bulk of hydrogen ion secretion, so that most bicarbonate '«absorption' occurs in the proximal tubule. There may be other H* secretory mechanisms in the pro­ ximal tubule, but they do not appear to be quantitatively important. In the late tubular segments (the late distal tubule and the collecting ducts), a completely different mechanism for hydrogen ion secretion exists. This is relatively inde­ pendent of tubular sodium content and occurs through primary* active transport. The 'intercalated' cells in this part of the nephron have a hydrogen ion-transporting ATPase on their luminal surfaces and although this accounts for less than 5% of the total hydrogen ions se­ creted, it is important because it can generate a hydrogen ion gradient of almost 1000 to 1. It is this that is respons­ ible for the final acidification of urine and dictates the minimum achievable urinary* pH of about 4.5. Specific abnormalities of t u b u l a r function Renal glycosuria Glucose is freely filtered at the glomerulus, but is nor­ mally then reabsorbed in the proximal tubule so that ii is undetectable in urine. If plasma glucose concentrations rise or if the glomcrular filtration rate increases (as in pregnancy)? then the capacity of the proximal tubule to reabsorb filtered glucose is exceeded and glycosuria occurs. Generalized defects in renal tubular function mav also result in glycosuria (see later), but a small group of people appear to have an isolated defect of tubular glucose rcabsorption. They consequently excrete a vari­ able amount of glucose in their urine at normal plasma glucose concentrations. 'ITie defect seems to be in the N a \ i>g!ucosc cotransport system, which is present in both the proximal renal tubule and the small intestine and which is respons­ ible for both glucose and galactose absorption. This sys­ tem is probably heterogeneous, since when the defect affects the gut, severe, watery, acidic diarrhoea due to glucose and galactose malabsorption occurs together with glycosuria, but in the renal form there is glycosuria with no effect apparent clinically in the gut. Renal glycosuria is less rare than small intestinal glu­ cose and galactose malabsorption, but is not common. It

I 65

is inherited in an autosomal recessive manner, although there appears to be more than one mutation capable of causing the condition. It is generally recognized to be a benign condition, with no clinical sequelae. Amino acidurias Amino acids are normally freely filtered at the glomerulus and then almost entirely reabsorbed in the proximal convoluted tubule. There is a maximal capacity to each reabsorptive mechanism and in most cases of amino aciduria some extrarenal disorder leads to accumulation of amino acid(s) in the plasma, which are then filtered in amounts higher than the reabsorptive capacity of the tubule, with consequent •overflow* amino aciduria. Cystinuria Cystinuria is a classic example of an amino aciduria due to a true defect in renal tubular func­ tion, in that the amino aciduria occurs at normal or even low plasma concentrations of the amino acids involved. In most cases of cystinuria there is renal loss not only of cystine but also of the dibasic amino acids ornithine, arginine and lysine. There is also an associated failure of intestinal absorption of the same amino acids. Inspection of their molecular structures (Fig. 9.2) shows that each has two amino groups separated by 4-Ъ bonds, which suggests that malfunction of a single membrane carrier protein might explain the disorder. However, the true ex­ planation is probably not this simple, since the clearance of cystine may exceed the creatinine clearance, suggesting secretion of cystine into the tubule, and furthermore since dibasic amino aciduria (e.g. lysinuric protein intolerance) or cystinuria may each occasionally occur alone. The main clinical manifestation of cystinuria is recur­ rent urinary tract stone formation, the name 'cystine' coming from the original (erroneous) assumption that the source of these stones was the bladder. The disease occurs with equal frequency in both sexes, although males tend to be more severely affected. It may present at any time from the first year of life up to the ninth decade, with a peak incidence in the second and third decades. Cystine stones form readily in acidic urine. They are yellow-brown in colour and are radio-opaque because of their sulphur content, although there may also be some calcium deposition if tficrc is infection secondary to the cystine calculi. They tend to occur as staghorn or multiple recurrent stones and often require some form of surgical intervention. The prevalence of cystinuria varies between racial groups and according to whether the figures are taken from neonatal amino acid screening programmes or from known cystinuric stoneformers, but the overall prevalence from the former may be as high as one in 7000, making it one of the commonest inherited disorders. The mode of inheritance is autosomal recessive, although in some families it appears to be incompletely recessive, with

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166

HUHHhMIMKY

СООН

г

H—

I

сн,

t

сн.

I н,

I н— с—ми,

Fig. 9.2

P

I

Н—C-NH,

I

СООН Cystinc

t NH

Н— С— NH.

СИ,

н—

?

H—C —NH,

СООН

COOH

Lytine

Ornlthlne

Argirnne

Chemical structures of the dibasic amino acids involved in cystinuna.

?СИ, H— C-NH,

H—

s I

f

г

H—C —NH

I

COOH

C(CHj, H —C - N H ,

C-NH,

COOH

COOH

COOH Cysteme Fig. 9.3

Cyslinc

Cysteine-penictllaminc

Chemical strucrurc* of cystinc. cystcinc and cyitcmc-pcnicillaminc.

heterozygotes excreting more urinary cystinc, ornithinc, arginine and lysine than normal, although less than in the homozygous state. The intestinal defect appears to cause no clinical problems, since cystinuric patients can still absorb dibasic amino acids as dipeptides, but in fact it has proved useful as a sensitive genetic marker, allowing the classification of cystinuna into three subtypes. Medical treatment of cystinuna begins with main­ tenance of a high fluid intake throughout the 24 h and alkalinization of the urine, both aimed at decreasing the chance of precipitation of cystinc in the renal tract. If these measures fail, then it may be possible to convert the cystinc to a more soluble compound, most commonly by the use of ivpcnicillamine. This can form the mixed disulphide cystcinc-pcnicillaminc (Fig. 9.3), which is significantly more soluble than cystinc. Unfortunately, r> pcnicillaminc commonly causes an allergic reaction and can also cause nephrotic syndrome and pancytopaenia, so careful monitoring is essential. Occasionally, in spite of both medical and surgical treatment of stone formation, cystinuria causes sufficient renal damage to result in chronic renal failure. In this case, renal transplantation may be effective, since the donor kidney should not be affected by the amino acid transport defect and should therefore remain disease-free. Hartnup disorder A second example of an amino aciduria due to a true defect in renal tubular function, rather than an 'overflow* effect, is found in Hartnup dis­

order. This is named after the family in which it was first described and is again a defect of both renal and intestinal amino acid transport. The constant feature is a failure to rcabsorb the neutral amino acids (Table 9.2) in the renal tubule, with their consequent appearance in the urine. The failure of reabsorption is not absolute, as renal clear­ ances of the affected amino acids arc generally lower than the creatinine clearance. Most affected individuals also have increased amounts of indoles (e.g. indican) in the urine, which originate from the bacterial breakdown in the gut of unabsorbed tryptophan. Rarely, the renal and intestinal lesions may each occur alone.

Table *.2 exces* in Ha< C'

Jtrai ammo acids ■ inch appear in the unnc m ftiaoittci i f the 1 mcthioiune ma be mcrea-

\ lonoanunomonocarboxv IIL amino acids !

Alamnc Sennc Thrc i mine Valinc I cuv. me IsoJeuctnc Phenylalaninc 1 :c

«mauiiuodtcarboxyUc lltii.!.

Ляралцрпс Cilutann

i ryptopfaan H

nc

защ1

ннык

ким пр

RENAL

The original description of Hartnup disease included a pellagra-like skin rash, temporary' cerebellar ataxia, conslant renal amino aciduria and certain other biochemical features. Some affected individuals have also shown psy­ chotic Ьспагаоиг, while others have been mentally handi­ capped (see Ch. 34). The pcllagra-likc rash and its response to nicotinamide suggest that the clinical features of the disease may be due to failure to absorb tryptophan in the intestine and reabsorb it in the renal tubule, leading to a deficiency of nicotinamide. However, investigation of siblings of individuals with Hartnup disease and the results of neonatal urine amino acid screening pro­ grammes both suggest that the typical amino aciduria (Hartnup disorder) may exist without the features of Hartnup disease. It would appear that Hartnup disorder is an autosomal recessive inherited disorder, but that this is not expressed as the 'disease' without the addition of certain other environmental or genetic influences. The 4 disorder' alone is a benign condition, both to the indi­ vidual who inherits it and to normal children of affected parents. Familial renal iminoglycinuria A further example of a specific abnormality in renal tubular reabsorption causing a distinct pattern of amino/imino aciduria occurs in familial renal iminoglycinuria. Here a common mem­ brane carrier system for L-proline, hydroxy-L-proline and glycine appears to be at fault, with the consequent appear­ ance of these substances in the urine. There is probably more than one type of transport system for these amino/ imino acids, since significant tubular reabsorption still occurs. The condition, which is benign, is autosomal recessive although some heterozygotes are 'incomplete' and have hyperglycinuria. In some but not all homozygotes, an im­ paired intestinal transport of proline can be demonstrated. Neonatal screening for amino acidurias suggests that familial renal iminoglycinuria occurs in one in 15 000 live births in a Caucasian population. However, it should be appreciated that in normal neonates the renal tubular reabsorption of proline, hydroxyproline and glycine is less efficient than in adults, although imino aciduria normally disappears by Ъ months and hyperglycinuria by 6 months of age. Phosphate transport defects Specific disorders of renal tubular phosphate handling may be inherited or acquired. There is reduced proximal tubular reabsorption of phosphate (in particular, the TmP/GFR is reduced, as explained in Chapter 6) and hypophosphaiaemia. The normal response to this would be an increase in the ltt-hydroxylaiion of vitamin D and if this is adequate then the end result is hypercalciuria (with an increased risk of renal stones), but no bone disease. If the vitamin D response is inadequate, then the result

TUBU1J4R

DISORDERS AND RliNAI. CAU'.UU

1 67

may be bone disease with hypercalciuria (e.g. as in heredi­ tary hypophosphataemic rickets with hypercalciuria) or bone disease alone (e.g. X-linked hypophosphataemic rickets and oncogenic hypophosphataemic osteomalacia). Inherited and acquired forms of hypophosphataemic bone disease are discussed further in Chapter 28. The phosphate loss and other changes in more general­ ized forms of renal tubular disorder may also be severe enough to cause bone disease (see below). Renal tubular acidosis The renal tubular acidoses are a group of hyperchloracmic metabolic acidoses which occur secondarily to an abnor­ mality in urinary acidification. The plasma bicarbonate concentration is low, the plasma chloride raised and the anion gap normal, in contrast to the acidosis seen with reduction in the glomcrular filtration rate in which the plasma bicarbonate is low, chloride normal and anion gap increased. Renal tubular acidoses (RTAs) are classified according to the presumed site of the disorder in hydrogen ion secre­ tion. In proximal (type 2) RTA, there is a failure in bicar­ bonate reabsorption, while in distal RTA there is a failure in net acid excretion. If hydrogen ion secretion is the sole defect in the distal nephron it is known as classic distal (type 1) RTA, whereas if the defect is more generalized, causing hyperkalaemia as well as acidosis, it is known as generalized distal (type 4) RTA. These conditions are described in Chapter 5 but the underlying mechanisms arc briefly summarized again here, together with details of diagnostic tests. Type 3 RTA is a mixture of types 1 and 2 and is very rare. Proximal (type 2) RTA The exact defect which prevents adequate hydrogen ion secretion into the proxi­ mal tubule in type 2 RTA remains to be elucidated, but the net effect is that bicarbonate reabsorption is impaired. This leads to metabolic acidosis and bicarbonaturia, with a decrease in the plasma bicarbonate concentration until the filtered load of bicarbonate falls to a point at which hydrogen ion secretion is sufficient to 'reabsorb* all fil­ tered bicarbonate. Thus a new steady state is established, in which there is a metabolic acidosis with a low plasma bicarbonate but no bicarbonaturia. Administration of bicarbonate to such patients will establish a steady state closer to normal, but quite large doses of bicarbonate are required (e.g. up to 20 mmol/kg body weight/24 h or more), since as the plasma bicarbo­ nate rises, so does the magnitude of renal bicarbonate loss. Hydrogen ion secretion in the distal tubule seems to be normal and the urine is acidified in response to an acid load. Proximal RTA may occur as an isolated defect, but is more usually part of a generalized defect of proximal tubular function, the Fanconi syndrome (see below).

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168

CUN1CAL BIOCHEMISTRY

Distal (type 1) RTA As with proximal RTA, the exact defect causing inadequate hydrogen ion secretion into the late tubular segments has not been defined. However, while other distal tubular functions remain un­ impaired, hydrogen ion secretion is reduced so that urinary pH cannot be lowered below 5.5. A resultant increase in the exchange of potassium for sodium ions in the distal tubule can increase renal potassium losses significantly and the condition is often accompanied by hypokalaemia. Diagnosis depends on the demonstration of an inap­ propriately high urinary pH (>5.5) during either sponta­ neous acidosis or an ammonium chloride loading test (sec Appendix, p. 172). Treatment involves potassium supple­ mentation together with bicarbonate. Unlike proximal RTA, the dose of bicarbonate required is small (e.g. a little over 1 mmol/kg body wcight/24 h in adults), since there is only a minor degree of bicarbonaturia and this docs not increase as the plasma bicarbonate concentration rises. Type 1 RTA may be inherited as an isolated defect or may be acquired, often as part of a systemic disease (e.g. in certain hyperglobulinaemic conditions, in hypcrcalciuria with nephrocalcinosis and following treatment with drugs such as amphotericin B, lithium or analgesics). Distal RTA with hyperkalaemia (type 4) In type IV RTA there is a generalized impairment of distal tubular function, with decreased tubular secretion of both hydro­ gen and potassium ions. The hypcrchloracmic metabolic acidosis of the RTA is accompanied by a relatively severe hyperkalaemia. The condition may arise due to a lack of mineralocorticoid activity (e.g. in adrenal failure), disease of the kidney resulting in impaired produciion of renin (hyporcninacmic hypoaldosteronism, e.g. in diabetic nephropathy) or resistance to the action of mineralocorticoids on the distal tubule (e.g. in patients treated with spironolactone). Treat­ ment of the condition depends on the underlying cause.

in a distinct syndrome known as the renal Fanconi syndrome (which must be distinguished from Fanconi's anaemia, a form of congenital aplastic anaemia). There is a failure in net proximal tubular reabsorption of glucose, ammo acids, phosphate and bicarbonate, with consequent glycosuria, amino aciduria, phosphaturia and acidosis, together with the development of vitamin D-rcsistant metabolic bone disease. There may also be increased uri­ nary losses of water and other substances, for example, sodium, potassium, calcium, magnesium, urate and low molecular weight proteins. The exact mechanism by which the Fanconi svndrome occurs is not clear and there may be distal, as well as proximal, tubular dysfunction. Clauses of the renal Fanconi syndrome may be broadly divided into inherited and acquired and arc listed in Table 9.3. The most common cause of inherited Fanconi syndrome in children is cystinosis, an autosomal recessive disorder of cystine transport across lysosomal membranes. There is accumulation of cystine in the lysosomes of most tissues, including the kidney, and in the infantile form of the disease the Fanconi syndrome progresses to glomerular damage and renal failure during childhood. There is no specific treatment, but renal transplantation is particu­ larly useful when required for endstage renal failure, since the transplanted kidney does not have the genetic defect and so docs not develop cystinosis. There is also an adult-onset form of cystinosis in which there is no Fanconi syndrome or glomcrular impairment and an intermediate form which presents during adoles­ cence and does eventually lead to renal failure. Other inherited metabolic diseases which are associated with the Fanconi syndrome are listed in Table 9.3. There is also an idiopathic inherited form, but this is a diagnosis of exclusion. It is interesting that in inherited metabolic diseases for which there is a specific treatment (e.g. avoid­ ance of lactose and galactose in galactosaemia), the Fanconi syndrome resolves on treatment, presumably as the concentration of a toxic metabolite goes down.

Hereditary renal hypouricaemia This is a rare disorder of the renal tubular handling of urate, in which there is a net decrease in reabsorption of unite, resulting in hypouricaemia and an increased renal urate clearance. Plasma urate concentrations are less than 150|imol/L in men and 126umol/L in women, when measured by a colorimctric method. The condition is generally harmless, although some patients also have hypercalciuria and about 2 5 % have a tendency to stone formation. It is transmitted in an autosomal recessive manner and heterozygotes tend to have intermediate plasma urate concentrations.

ГшЫ

ic c i u u

Inherited opathic oculoccrcbroicnal (Lowe) syndrome associated with inherited metabolic disease i»

fructose inr..i,TJiuc

galactosaemu glycogen storage disease type I rosinacmia Wilson's disease Acquib exogenous toxins heavy metals

Generalized t u b u l a r defects Generalized renal tubular delects tend to occur together

i cnal Fanconi syndrome

certain drugs paraprouiruemia

■myjoidoih

RENAL TLBUtARTOSORDKR.SAND RKNAI. (AlXX'U

White the inherited diseases associated with the Fanconi syndrome tend to present in childhood, the acquired forms tend to occur in adults. These also seem to be reversible if exposure to the causative agent can be stopped. The clinical features of the Fanconi syndrome tend to be rather non-specific and include polyuria, polydipsia, dehydration, hypokaiacmia and acidosis, with impaired growth and rickets in children and osteomalacia in adults. Treatment is primarily directed at the underlying cause, but inappropriate renal losses need to be replaced (for example with fluids, bicarbonate and potassium) and the bone disease treated with phosphate replacement and vitamin D.

169

logical factors in the formation of each are dealt with in detail below. The classic clinical manifestation of nephrolithiasis is renal or ureteric colic. The pain is characteristically severe, requiring opiates for analgesia, and is usually accompanied by haematuria. However, some stones may be discovered incidentally when abdominal X-rays are taken for some unrelated reason, and passage of tiny stones may only cause minimal discomfort. Investigation and treatment of stoncformers is undertaken to prevent recurrent stone formation, but in general permanent intrinsic renal damage does not occur unless there is superadded infection in an obstructed kidney. Pathogenesis of renal stones

RENAL CALCULI Introduction Stone formation in the urinary tract (urolithiasis) has been described since ancient times but in the past, lower uri­ nary tract stones, particularly those arising in the bladder, appear to have been more frequent, engendering a surgi­ cal enthusiasm for 'cutting for stone'. In industrialized, relatively affluent populations, renal stone formation (ncphrolithiasis) has increased in frequency, while bladder stone formation has almost disappeared as the prevalence of malnutrition and infection have decreased. However, while the frequency of nephrolithiasis is ac­ knowledged ю be increasing in the Western world, precise incidence and prevalence figures have not been well estab­ lished. Nevertheless, it would appear that as many as 5% of the population (possibly even more) will have a clinical stone event at some time during their lives. Stone forma­ tion occurs in men about four times as frequently as in women and in both sexes tends to be both recurrent and unpredictable, The clinical effects of stone formation tend to be simi­ lar whatever the type of stone, but it is apparent that there are a number of distinct metabolic derangements which each give rise to stones of a characteristic composition. The main types of renal stones are listed in Table 9.4 in approximate order of frequency of occurrence, and aetio-

'I'ublc 9.4 frcqui

rial «tone, in approximate order -ccun-cnce

IM: of atone Calcium oxalate and phosphate Calcium oxabu um ammonium phosphate and c* phosphate (struvit

:qucncy 45 I

Uric add

5%

Calcium phosphate Cystrnc

I I I -2%

The role of the kidney in water conservation means that it is often necessary to excrete concentrated urine. Since some of the other constituents of urine are relatively in­ soluble in water, to the extent that supersaturated solu­ tions form, it is not surprising that these constituents sometimes crystallize out to form renal calculi. Certain factors predispose to this process and chronic dehydration, as may occur in hot climates, is one. In­ creased urinary excretion of certain constituents (e.g. hypercalciuria, hyperuricosuria or hyperoxaluria) in­ creases the likelihood of supcrsaturation occurring and alteration of urinary pH may adversely affect the solubility of certain solutes. For example, uratc stones arc more likely to form in an acid urine, whereas alkaline condi­ tions, as may occur with a urinary tract infection, make calcium precipitation more likely. These factors arc opposed by the presence in urine of inhibitors of crystallization (e.g. magnesium, pyrophosphate, citrate and certain glycoproteins), so that crystal formation in urine is usually slower than in simple salt solutions. Each of the types of stone listed in Table 9.4 will now be considered in more detail. Calcium stones Calcium-containing stones form the majority of renal calculi and may consist of calcium oxalate, calcium phosphate or a mixture of the two. Hyperoxaluria is likely to be involved in the formation of pure calcium oxalate stones, whereas pure calcium phosphate stones suggest either hypercalciuria or a urine which has a higher pH than normal for some reason. Hypercalciuria Hypercalciuria is present in around 30% of patients with calcium-containing stones, although the exact prevalence depends on the population studied and the upper limit accepted for the reference range (of the order of 7.5 mmol/24 h for men and 6.5 mmol/24 h for women).

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ctJNtcAL BIOCHEMISTRY

A minority of hypercalciuric stoneformcrs also have hypercalcacmia and it is then important to establish a cause for this in this group of patients it is most likely to be due to primary hyperparathyroidism. In patients who are normocalcacmic with hypercalciuria, there may be a cause apparent (e.g. slightly in­ creased vitamin D activity» renal tubular acidosis, high salt intake, excessive glucocorticoid activity, Paget's disease, prolonged immobilization or chronic frusemide therapy) but in most patients with nephrolithiasis, normocalcacmic hypcrcalciuria is idiopathic in origin. Detailed study of the condition suggests it may be due to a vitamin Dindependent increase in intestinal calcium absorption or to impaired renal tubular rcabsorption of calcium, but the precise interaction of the various factors remains to be elucidated. Hyperoxaluria Until relatively recently it has been difficult to measure oxalate reliably, but it would now appear that hyperoxaluria is more important in calcium oxalate stone formation than is hypercalciuria. In most instances, hyperoxaluria results from excessive dietary in­ take or increased intestinal absorption of oxalate, although primary hyperoxalurias have been described. The upper reference limit for urinary oxalate excretion is usually said to be somewhere between 400 and 500umol/24h. A dietary excess of oxalatc-containing foods (e.g. spinach, rhubarb, chocolate) can increase this to 700 цто1/24 h or even higher, particularly if the dietarycalcium content is low. Increased intestinal absorption of oxalate may occur in malabsorption, whatever the cause. Increased amounts of unabsorbed fatty acids in the gut lumen combine with calcium, making larger than usual amounts of oxalate available for absorption. In addition, exposure of the colonic mucosa to substances such as fatty acids and bile salts, which have detergent properties, may increase its permeability to oxalate and further increase the amount absorbed. Primary oxaluria is a rare inherited metabolic disease, but must be considered if nephrolithiasts occurs during childhood (although a few affected individuals do not present until adult life). There is increased synthesis of oxalate, with a consequent increase in its urinary excre­ tion, together with that of other organic acids. The pattern of accompanying organic aciduria has allowed classifica­ tion into two types. In type 1 primary hyperoxaluria, excessive amounts of glyoxylic and glycolic acids arc excreted, whereas in the rarer type 2, excretion of these acids is normal but that of i-glyceric acid is increased. The biochemical defect in type 1 is a deficiency of the enzyme alanine:glyoxylate aminotransferase, but in type 2 the defect has yet to be fully worked out. The inheritance of both types appears to be autosomal recessive and in both the 24 hour excretion of oxalate is of the order of 1.5 3 mmol. Besides renal stone formation, there is also a

tubulointerstitial nephropathy which progresses to chronic renal failure. This raises plasma oxalate concentrations even higher and oxalate deposition in other tissues (e.g. myocardium, synovial membranes) may occur. However, the condition appears to exhibit marked clinical and biochemical heterogeneity. Other causes of hyperoxaluria include acute ingestion of ethylene glycol, which is rapidly converted to glycolate and can lead to calcium oxalate crystals forming in the renal tubules, and excessive ingestion of vitamin C, which seems to lead to increased oxalate formation in some indi­ viduals. A syndrome of'mild metabolic hyperoxaluria* has also been described in adults and pyndoxine deficiency causes hyperoxaluria in rats. Other /actors in calcium stone formation Although uric acid can itself form renal stones (see below), hyperuricosuria may also contribute to calcium stone formation. This may be through heterogeneous nucleation following uric acid crystallization or through other mechanisms and remains a controversial area. Citrate is a recognized inhibitor of calcium stone formation and some stoneformers have low urinary citrate concentrations. This is particularly so in patients with distal renal tubular acidosis, who are already at in­ creased risk of calcium stone formation because of failure adequately to acidify the urine. Normal urine also contains a variety of protein-based inhibitors of stone formation and it may well be that defi­ ciencies or defects in these will be found to be the cause of some types of stone. Triple phosphate stones Triple phosphate stones are composed predominandy of magnesium ammonium phosphate (struvite), with vari­ able amounts of calcium phosphate as carbonate-apatite. They form in the presence of high urinary concentrations of ammonia, bicarbonate and carbonate, which essentially means that they only form when the urine is infected with urea-splitting bacteria (e.g. Proteus* Klebsiella and Pseudomonas spp.). For this reason, and in contrast to other types of stones, they occur more frequently in women (and in other people with a predisposition to urinary tract infection). They may also occasionally occur in other stoneformers. Uric acid stones Hyperuricaemia and gout are discussed in Chapter 29. In normal people, the amount of uric acid excreted in the urine depends on, amongst other things, the purine con­ tent of the diet. This makes the definition of a reference range rather difficult, particularly since even at 'normal' excretion rates of 3.6-4.8 mmol/24 h, the urine is super­ saturated with uric acid and yet most people do not form

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RENAL TUBULAR DISORDERS AND RENAL CALCUU

uric acid stones. Stone formation seems to depend on the presence of hyperuricosuria (more than 6-7 mmol/24 h) together with a tendency towards urine with a mean pH that is lower than normal (uric acid is less soluble in acid conditions). This offers a useful therapeutic intervention, in that if alkali is used to maintain the urine pH at around 6.5, the formation of stones will be prevented (or even reversed) in mild hyperuricosuria. More pronounced alkalinization of the urine should be avoided, as it increases the risk of calcium deposition. Patients with severe hyper­ uricosuria may need allopurinol to prevent stone formation. Cystine stones These have already been discussed in the section on cystinuria (p. 165).

171

Investigation of stoncformcrs The most important investigation in renal stone disease is analysis of the stone, if it is available. l*his is generally accepted by most writers in the field, even though the results of external quality assurance surveys in the United Kingdom demonstrate that stone analysis is not generally well done. However, assuming an accurate result, knowl­ edge of the stone constituent(s) obviously directs further biochemical investigation and monitoring. It is also im­ portant to continue to analyse the stones if there is recur­ rence - there may be changes in the constituents which would necessitate a change in management. Where the stone is not available for analysis, a more generalized screening approach has to be taken. This remains a controversial area, but a suggested selection of tests is shown in Table 9.5.

Miscellaneous ramies

CONCLUSION

Renal stones submitted for analysis may occasionally not tall into one of the categories discussed above. Some may be entirely unrelated to the renal tract (a manifestation of the Munchauscn syndrome), whereas others may be from the renal tract but not be true stones (e.g. blood clots, sloughed papillae, encrusted sutures). However, there arc a few other inherited metabolic diseases which do result in the formation of stones of unusual composition. In hcreditarv xanthinuria there is a dcficicncv in xanthine oxidase, with the consequent replacement of uricacid in the urine by xanthine and hypoxanthinc. In about two thirds of cases this remains an asymptomatic meta­ bolic abnormality, usually detected because of very low plasma urate concentrations, but in the remaining one third xanthine stones form in the renal tract. There may also be associated myopathy or arthritis. An even rarer subtype of xanthinuria has been de­ scribed, in which there is deficiency of xanthine oxidase together with sulphite oxidase, but here the main clinical concern is the neurological symptoms Xanthine stones have also been described in patients with normal plasma urate concentrations, who clearly do not have xanthine oxidase deficiency. The cause of these is unknown. Another inherited disorder of purine metabolism asso­ ciated with renal stone formation is adeninc phosphoribosyl transferasc (APRT) deficiency. This enzyme is involved in the salvage pathway for the purine base ad­ eninc and deficiency results in increased urinary excretion of 2,8-dihydroxyadenine. This leads to stone formation in most homozygotcs, although up to 15% remain clinically stone-free. It is worth noting that many of the chemical tests used for uric acid also give a positive reaction with 2,8-dihydroxyadenine, so uric acid stones discovered in childhood are worth reanalysing by a more specific technique.

Primary disorders of the renal tubules are not common, but biochemical investigations are important in both diag­ nosing and monitoring them. The pathogenesis of renal stone formation is still not fully understood, but enough is known for a logical approach to be taken in the biochemical investigation of stoneformers. This is something which has probably not been uniformly well done in the past. APPENDIX Diagnosis of RTA A variety of provocative tests of urinary acidification have been used in the investigation of RTAs, but whether these can firmly diagnose the type of defect remains to be established. In general, hyperchloraemic metabolic acidosis that is not explained by bicarbonate loss from the intestinal tract should raise the suspicion of a urinary acidification defect. The plasma potassium may give a clue as to the type (high

Table 9.5 Fini line b when a u-nc w not available !or analysis

i n ММПСИ1ГЛ1С|Л

Plasma calcium phosphate urate Spot unnc V\\ microbiolnipr it infection *u*peetcd'

greening tcM KM cystine 24h unnc urine volume calcium m l iu urate

ГОыСКИМ Dp

172

CUKICAI. BIOCHKMISTRY

in type 4, low in types 1 and 2). The presence of other features of the renal Fanconi syndrome suggests type 2 RTA. The amount of bicarbonate required to correct the acidosis also gives an indication of the type of RTA (type 1 responding most readily and type 2 least readily). How­ ever, it may be necessary to confirm the diagnosis by using one of the following two tests. Urinary acidification test This can be used to confirm the diagnosis of distal RTA. The test is not necessary if the pH of a urine specimen collected after an overnight fast is 40

fh« 'dcurablc' and other

rw eight able Overweight

0 I II Ш

ии

elude muscle, fat and bone, or attempts can be made to refine the measurement to reflect muscle mass alone. The first stage is to correct for fat by including the triceps skin fold measurement (TSF; see below) and using the formula (MAC - к x TSF) to derive the mid-arm muscle circumference. This can be further manipulated to give uncorrccted muscle area and finally an adjustment made for the contribution of the humcrus» to give corrected muscle area. The latter can then be used to estimate total body muscle mass, although given the difficulties in making the two original measurements accurately, the margin of error by this stage must be quite high. In the research environment, dual-energy X-ray absorptiometry (DEXA) is being used to make similar measurements of body composition. While these calculated values may be useful in epidemiological surveys, in nutritional support the initial MAC measurement itself may help to indicate whether support is required, and serial measurements help to assess the success of treatment, although short-term changes are more likely to be due to imprecision in measurement than changes in tissue bulk. Another 'circumference* parameter which may be use­ ful in assessing the health risks of moderate obesity is to measure waist and hip circumferences and calculate the waist:hip ratio. This reflects the distribution of fat rather than the degree of obesity, and the risk of ischaemic heart disease and stroke rises sharply with waist:hip ratios of more than 1.0 for men and 0.8 for women. Skin/old thickness Measurement of skinfold thick­ ness is useful in assessing and monitoring nutritional sta­ tus in patients who cannot be weighed and also has a place in epidcmiological surveys. Cheap plastic calipers are in­ accurate and the more precisely calibrated metal ones are to be preferred. However, even then the technique is prone to large variations, both within and between observers. The imprecision arises in identification of the exact loca­ tion for measurement; the way the skinfold is picked up; the way the calipers are placed on the fold; the compres­ sion of the fold by the calipers; and the exact timing of the reading. Some improvement in performance may be achieved by taking the mean of three readings, usually on the left (or non-dominant) side. The presence of oedema at the measurement site may be a further confounding factor. A variety of sites have been used for skinfold meas­ urement, but the most common are triceps, biceps, subscapular and suprailiac. Equations are available for calculation of total body fat from these measurements (usually for research purposes), but this assumes that sub­ cutaneous fat reflects total body fat, which is not always the case: obese men tend to lay down more intra-abdominal fat than women. In clinical practice, body weight is more useful than skinfold thickness in the management of obesity, but in undernourished patients the latter may be useful. Age and sex-related reference standards arc

C1JN1CAL BIOCHHMISTRY OV NtmUTtON

published (for example, for triceps skinfold thickness) and so measurement at presentation may identify severe malnourishment: a triceps skinfold of less than 5 mm almost always reflects low body fat stores. Serial measurements can help in monitoring nutritional support but again short-term changes are more likely to reflect imprecision in measurement than sudden changes in fat stores. When total fat stores reach very low levels, physical condition can deteriorate rapidly. Functional The effects of nutritional status on certain aspects of bodilv function can be used in the assessment of undernutrition, although with variable degrees of success. The best example is probably the effect of haematinic defi­ ciency on red blood cell morphology where, for example, a hypochromic, microcytic picture may be the first indica­ tion of iron deficiency and macrocytosis of B,, or folate deficiency (see Ch. 11). Functional tests of muscle mass have also been used, e -8- grip strength, isometric knee extension and response to electrical stimulation. However, while muscle strength correlates with muscle mass in normal subjects, there are many non-nutritional factors which can cause weakness in sick patients and malnutrition alone has to be quite severe before strength diminishes. There is also the possibility that repeated measurements may have a training effect on the muscles involved. Visceral protein is sometimes disproportionately de­ creased in protein deficiency and various measures of vis­ ceral function have been used in nutritional assessment. Two of these are discussed below. Hepatic secretory proteins The liver synthesizes most of the circulating plasma proteins (apart from immunoglobulins) and in epidemiological studies there is a clear correlation between plasma concentrations of these proteins and other markers of malnutrition. For example, in adults who are otherwise well, a plasma albumin of less than 35 g/L and a plasma transfemn below 1.5 g/L usually indicate protein malnutrition. Since most of these proteins can be measured quite easily, there is a temptation to use them as nutritional markers in individual patients, but there are pitfalls for the unwary. Albumin is probably one of the most frequently meas­ ured plasma proteins and low concentrations may reflect a deficiency in dietary protein intake. However, it must be remembered that decreased synthesis may be due to other factors, for example liver disease, and that the plasma con­ centration is also afFccted by fluid balance, loss of protein from the body, tissue catabolism and distribution of albu­ min across the various body compartments. It should also be remembered that while the intravascular concentration of albumin is relatively high, more than half of the total mass of albumin is actuallv extravascular.

187

Even in circumstances where the plasma albumin con­ centration does reflect nutritional status, the relatively long half-life (20 days) means that it does not respond to rapid changes and so other plasma proteins with shorter half-lives have been used. These include transfemn (9 days), prealbumin (1-2 days) and retinol-binding protein (10 h), but unfortunately these all have similar drawbacks to albumin and arc also affected by factors such as the acute phase response, oestrogen concentrations and fac­ tors relating to their own specific function (e.g. iron defi­ ciency in the case of transfemn). Serial determinations may be of more use than single measurements. Insulin-like growth factor 1 (IGF 1) shows some promise as a nutritional indicator, as long as allowance is made for the age of the patient and the possibility of any abnormality in growth hormone secretion. The immune response Malnourished people are more susceptible to infections. Plasma immunoglobulin concentrations arc generally maintained, but cell-medi­ ated immunity may be impaired. The circulating absolute lymphocyte count is often low (less than 2.0 x 107L) in malnutrition, although this is a very non-specific finding. Delayed cutaneous hypersensitivity testing against common allergens has been used to assess malnutrition, but many non-nutritional factors (e.g. infection, malig­ nancy, radiation, surgery, drugs) affect the response and in any case, the results may not be reproducible. In vitro tests of T cell function may be an alternative, but in clini­ cal practice neither of these approaches is in common use. Laboratory-based assessment of individual nutrients Energy Laboratory-based techniques are not in general use for the assessment of energy stores, whether of fat or carbo­ hydrate. However, there are techniques for the measure­ ment of energy expenditure which may be of use in assessing how much energy to provide as part of nutri­ tional support. Direct calorimetry measures heat loss from the body, which can be used to derive the metabolic rate assuming that body temperature remains constant and no external work is performed. The technique involves the subject re­ maining in a special insulated room equipped for heat ex­ change, so is not applicable in general clinical practice. Indirect calorimetry measures oxygen consumption and carbon dioxide production and from these the respiratory quotient can be calculated, together with the energy ex­ penditure (Fig. 10.7). This is the technique used by the 'metabolic measurement carts* which are popular in some intensive care units. The use of doubly-labelled water (containing the stable isotopes : H and twO) has produced new data on total energy expenditure in free-living individuals, but the

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In disease free individuals, the following formulae give an estimate ot 6MR in kcatf24h:

RESPIRATORY QUOTIENT (RQ)

1 RO =

ADULT MALES

VOLUME CO, PRODUCED per unit time.

BMR = 66.5 + 13.75W + 5H - 6-77A

RQs when metabolising individual energy sources: ADULT FEMALES Carbohydrate Fat Protein (average) Excess energy used tot net fat synthesis

1 0 0.7 0.8

BMR = 665.1 ♦ 9.56W + 1.85H - 4.67A W = body weight in kg

> 1.01

H - height in cm A m age m years ENERGY EXPENDITURE

Flf. 10.8 Harm - Benedict ecjuauon* tor the calculation ot basal metabolic rate (BMR).

At steaOy state

ENERGY RELEASED FROM FOODS

is proportional to

OXYGEN CONSUMED

The amount of eoergy released per litre of oxygen vanes with different foods, but on average is aboul 4 82 kcal per litre of oxygen (at standard temperature and pressure). Thus: ENERGY PRODUCTION (kcal)

LITRES OF OXYGEN CONSUMED

X

4,82

A more accurate estimation can be obtained by using a measurement of nitrogen excretk>n to assess protein metabolism, and then the RO to assess the relative proportions of carbohydrate and fat being used. Fig. 10.7

Calculation of respiratory quotient and energy expenditure.

technique is not suitable for the assessment of individual patients. Basal metabolic rate may be calculated for healthy indi­ viduals from knowledge of age, sex, height and weight using the Harris-Benedict (Fig. 10.8), Schofield or similar equations and for sick patients suitable adjustment may then be made for disease state, pyrexia, mobility, etc. However, the more severely ill the patient, the less likely it is that the calculated figure truly reflects total encrgy expenditure. l*rotcm Laboratory-based tests for visceral protein have already been discussed under functional tests of nutritional status (p. 187). However, there are a number of other rests which assess total body protein (predominantly muscle).

For research purposes, total body nitrogen can be measured by neutron activation (an estimate of total body protein is simply obtained by multiplying this figure by 6.25) and this can demonstrate changes over a period of only a few weeks. Almost all body potassium is intracellular, there is virtually none in adipocytes and potassium in the skeleton and red blood cells is not readily exchange­ able, so exchangeable potassium measured using dilution of 4 i K is a measure of lean tissue mass. Neutron activation can also be used to measure total body potassium and by monitoring both nitrogen and potassium, it can be shown that nitrogen losses in malnutrition arc relatively higher from lean tissue (high potassium content) than from fibrous tissue (low potassium content). Measurements of bioelectrical impedance have also been used to predict lean body mass. However, the technique actually measures total body water and the extension to lean bodv mass assumes a constant level of hydration m such tissue, which may not always be the case, particularly in sick individuals. Outside the research environment, a direct measure of muscle mass mav be obtained from the 24 h excretion of creatinine, a metabolite of muscle creatinc. This is some­ times corrected for height (the creatinine:height ratio) and standard values are available. However, urinary creatinine excretion does fluctuate (e.g. with exercise, pyrexia, meat intake) and the technique depends on complete urine collections and normal renal function, so that in practice it is not widely used. It may sometimes be useful to assess nitrogen balance. Nitrogen intake may require dietary analysis or, for pa­ tients on entcral or parcntcral formulations, it will already be known. Nitrogen output for research purposes involves analysing stool and urine for total nitrogen content, but for clinical purposes urinary urea nitrogen may be meas­ ured and standard adjustments made for urinary non-urea

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C:UNK:AI. RIOCHKMISTKY OP NXTRITION NITROGEN EXCRETION

URINARY NITROGEN

FAECAL NITROGEN

,\N\\\\W

£*3*3*й

URINARY NITROGEN

URINARY UREA NITROGEN

URINARY UREA NITROGEN (o/24h)

URINARY UREA (mmol/24h)

■

-^ -^

FAECAL NITROGEN

189

(20% added for non-urea nitrogen)

60 • 000

__ -^ - . ^ ^ r - I

1 - 2 0/24h

Thus'

NITROGEN EXCRETION (g/24h) Fl». 10.V

(

URINARY UREA (mmol^h)

0.028

x

1.2 )

Formula for the calculation of nitrogen output.

nitrogen and faecal nitrogen (Fig. 10.9). This approach is valid for reasonably well patients (assuming the urine is not infected with a urea-splitting organism), but diverges further from the true nitrogen output the more severely ill the patient. Vitamins It is sometimes useful to assess body stores of fat-soluble vitamins, as oudincd below, but only rarely necessary to measure the water-soluble vitamins (with the exception of B I2 and folate - see Ch. 11). This is because they are rela­ tively non-toxic and if deficiency is suspected a trial of therapy is safe and simpler than trying to measure them. Vitamin A Vitamin A is stored in the liver so meas­ urement of the liver vitamin A content would probably be the best indicator of vitamin A status, but this is not feasible in human subjects. Plasma retinol concentration can be measured, but tends to be maintained over a wide range of liver reserves, making it an insensitive detector of deficiency. Retinol is carried bound to retinol-binding protein (RBP) in the plasma, so conditions reducing RBP concentration (e.g. liver disease) may lower plasma retinol concentration without reflecting decreased stores. Very low concentrations (below 0.7 pmol/L) confirm deficiency, but by this stage the clinical effects are almost always manifest and in general the diagnosis of vitamin A deficiency is made on clinical grounds. Very high retinol intakes may increase the plasma concentration.

Plasma carotenes can also be measured and in general reflect dietary intake. Dietary assessment is usually all that is necessary for this, but measurement is occasionally helpful to establish whether an odd skin colouration is due to hypercarotenaemia or not. Vitamin D Suspicion of a nutritional deficiency of vitamin D may be raised by an increase in plasma alkaline phosphatasc activity. Since deficiency is likely to be part of more generalized malnourishment, the plasma calcium may be difficult to interpret in view of the reduced circu­ lating albumin concentration, even when the calcium is Corrected' using one of the standard formulae (see Ch. 6) and in any case, the secondary hyperparathyroidism sti­ mulated in these circumstances tends to raise the ionized calcium concentration towards normal. The main storage form of vitamin D in the body is 25hydroxycholecalciferol and this is also the main form in plasma, where it is bound to a specific binding protein. Measurement of plasma 25-hydroxycholecalciferol pro­ vides a useful indicator of vitamin D stores, as long as the result is interpreted with regard to a seasonally adjusted reference range (e.g. 15-35 pg/L in summer, 8-18 jig/L in winter). It is also possible to measure the active metabolite of vitamin D (1,25-dihydroxycholecalcifcrol), although this is more difficult as the plasma concentrations are approximately 1000-fold lower than those of 25-hydroxy­ cholecalciferol. It is normally closely regulated and does not reflect dietary vitamin D deficiency.

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Vitamin D toxicity can occur fairly easily if supple­ m e n t are given, as the difference between appropriate and toxic intakes is fairly small. This is generally detected in the laboratory as hypercalcaemia which gets better when the supplements are reduced or stopped, but it is occasionally necessary to confirm the diagnosis with plasma vitamin D assay. The appropriate metabolite to measure will depend on the type of supplement given. Vitamin E Vitamin E status has been assessed by measuring the in vitro haemolysis of red blood cells follow­ ing exposure to hydrogen peroxide, although this occurs well before the vitamin E deficiency is severe enough to cause clinical problems. Measurement of plasma tocopherol concentration is an acceptable alternative, although increases in plasma lipoprotein concentrations tend to result in increases in plasma tocopherol. It is thus better to express the results in relation to plasma lipids, for example, as the plasma tocopherol:cholesterol ratio. Vitamin К Although it is possible to measure vitamin К in plasma, the usual laboratory approach is to assess the plasma concentrations of the vitamin In­ dependent clotting factors. In vitamin К deficiency these are present in normal quantities but in a non-functional state (PIVKA - proteins formed in vitamin К absence) and so a functional assay, conventionally the prothrombin time, is used (although the activated partial thromboplastin time is also abnormal in vitamin К deficiency). There is no generally available test of vitamin К excess, largely because its lack of toxicity makes it unnecessary. Thiamin The single most important factor in detect­ ing thiamin deficiency is a high index of clinical suspicion and the safest confirmatory test a trial of thiamin supple­ ments, since delay may be harmful in a deficient patient. However, assuming appropriate samples can be obtained without delaying treatment, there are laboratory tests which may be used to assess thiamin status. Measurement of plasma thiamin concentration has not been widely adopted, probably because of the low con­ centration and consequent analytical problems. Urinary thiamin measurement has been used, with low outputs being found in clinical deficiency states. Discrimination is said to be improved if the percentage excretion of a test dose is measured, with retention of a higher proportion in deficiency*. Functional tests of thiamin deficiency depend upon the role of thiamin pyrophosphate (TPP) as a cofactor for certain enzymes. Thus the activity of the pyruvate dehydrogenase complex may be decreased, with a rise in the circulating pyruvate concentration. ЧЪсге are other causes of a raised pyruvate and measurement of blood pyruvate following a glucose load has been suggested as a way of making the test more specific, although this has not been widely adopted. The idea is that there is a higher peak and slower decline in pyruvate in thiamin deficiency than otherwise occurs.

Probably the most commonly used laboratory check on thiamin status is the measurement of erythrocyte transkctolase activity* with and without the addition of TPP. In severe deficiency the T P P enhancement of transketolase activty is greater than 25%, while it is only 0 - 1 5 % in normal subjects, with marginal deficency in between. Riboflavin "1Ъе laboratory approach to the assess­ ment of riboilavin status is similar to that taken with thia­ min. Urinary excretion can be measured and an output of 0.5—0.8 mg/24 h probably indicates an adequate intake, but docs not necessarily reflect total body content- Factors such as age, physical activity and illness can also influence urinary excretion. Discrimination may be improved by measuring the percentage excretion of a loading dose of ribotlavin, an increased proportion being retained in deficiency states. In the blood, about 9 0 % of riboflavin is present in the red cells. This can be measured directly, but measurement of red cell glutathione reductase activity is probably more useful. The enzyme is FAD dependent* so that activity falls in riboflavin deficiency, but is restored on addition of FAD. An increase in activity of 30% or more confirms riboflavin deficiency. The test assesses long-term tissue riboflavin status and is very sensitive. It cannot be used if there is coexistent red cell glucose 6-phosphatase dcficicncv. Xicotinamide Laboratory tests of nicotinamidc sta­ tus tend not to be entirely satisfactory, possibly because no specific functional test has yet been developed. Approaches used to date include measurement of whole blood NAD concentration and of urinary excretion of nicotinamidc metabolites, in particular 1-methyl nicotinamidc Vitamin B6 A variety of biochemical techniques have been described for the assessment of vitamin B„ status. Urinary excretion of 4-pyridoxic acid reflects immediate dietary intake and plasma pyridoxal 5-phosphatc can also be measured, but indirect approaches are less technically demanding and have been more widely adopted. In B,, deficient subjects, the urinary excretion of xanthurcnic acid shows a marked increase following an l.-tryptophan load and this is the basis of the tryptophan load test. This is relatively easy to perform and has been widely used, but interpretation of the results requires cau­ tion as there arc a number of other factors which can affect tryptophan metabolism. T h e most useful test cur­ rently available is probably the measurement of red cell aminotransfcrase (e.g. aspartate aminotransferase) activ­ ity, with and without the addition of pyridoxal 5-phosphate, in a technique analogous to that used for the functional assessment of thiamin (red cell transketolase) and riboflavin (red cell glutathione reductase). Plasma aminotransferase activities are also affected by B«, defi­ ciency, but the very wide fluctuations seen in disease states make them less useful than erythrocyte activities in ihis context. Pantothenic

acid

Tests of pantothenic acid status

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( I JNICAt. BIOCHEMISTRY OF NUTRITION

arc rarely necessary and no biochemical test has become generally accepted. Pantothemc acid itself can be meas­ ured in urine and plasma, the urinary excretion being more immediately affected by dietary intake. Assessment of the red cell content of coenzymc A may be a functional test of pantothenic acid status. Biotin Techniques have been described for the meas­ urement of biotin in both urine and blood. However, these arc not particularly simple and in practice biotin status is rarely assessed biochemically. Vitamin С Vitamin С can be measured in plasma, white blood cells and urine. Unfortunately, none of these is entirely satisfactory in assessing vitamin С status. Plasma concentration tends to reflect recent intake and so will go down with decreased intake before body stores are depleted. Leukocyte (or buffy coat) vitamin С content probably reflects body stores better than does the plasma concentration, but the measurement is technically diffi­ cult, requires large quantities of blood and if the buffy coat content is measured, any abnormality in the relative proportions of white cells and platelets will introduce further error. Urinary ascorbate can be measured relatively easily in a fresh sample and output decreases quite markedly in defi­ ciency. However, the technique is not very specific and various ascorbic acid saturation tests have been devised to improve on this. These all assume that if body stores are depleted, most of a standard daily dose of ascorbic acid will be retained until stores are repleted, after which rela­ tively large amounts of ascorbate appear in the urine. These tests are not perfect, but are simple to carry out and appear to be satisfactory if a clinical suspicion of vitamin С deficiency needs to be confirmed. An added advantage is that any deficiency is actually treated by the lest. Minerals There is a general problem in assessing body stores of trace elements in that plasma concentrations are not necessarily related to tissue content, as is apparent in the following discussion. Zinc Assessment of body zinc status is not easy. Measurement of plasma zinc concentration is probably the simplest approach, but the result is influenced by many things other than total body zinc. There is a diurnal variation with a peak at about 10 a.m., fluctuation with recent dietary intake and a fall with hypoalbuminaemia. It also tends to fall in disease, for example, in acute inflammation intcrleukin-I stimulates zinc uptake by the liver, although in tissue destruction the plasma zinc may be maintained as it is released from the tissues, even though net balance is actually negative as losses in urine increase. Urinary zinc excretion is also difficult to interpret. Low levels may reflect deficiency, but even if environmental contamination can be avoided, normal or raised excretion

191

does not exclude deficiency - urinary losses may even be the cause of the deficiency. Since plasma and urine zinc measurements are gen­ erally unsatisfactory, a variety of other approaches have been tried. Hair zinc is liable to environmental contami­ nation (e.g. from certain shampoos), turns over relatively slowly and depends on the rate of hair growth. Erythrocytc zinc content responds only slowly to changes in zinc status, which can be rapid. (However, the high zinc con­ tent of red cells, in the enzyme carbonate dehydratasc, is important since it means that in vitro haemolysis increases the measured plasma zinc concentration.) Changes in the plasma and tissue activities of zinc-dependent enzymes like carbonate dehydratasc and alkaline phosphatase have given varying results. Leukocyte zinc content correlates well with muscle content, but it is not an easy technique and requires large quantities of blood. Plasma zinc remains a common, albeit unsatisfactory measure of zinc status, although when true deficiency is suspected a therapeutic trial of zinc supplementation with careful clinical monitoring is probably the best approach. Copper The diagnosis of copper overload is dis­ cussed in Chapters 13 and 23. In the detection of copper deficiency, plasma copper concentration may be low, but deficiency may be masked because caeruloplasmin synthe­ sis is increased in the acute phase response. Caerulo­ plasmin is also increased in pregnancy and by exogenous oestrogens. Measurement of caeruloplasmin before and after moderate copper supplementation may be a useful technique, an increase confirming deficiency. Interpreta­ tion of copper concentrations in other body fluids and tissues is difficult when looking for deficiency, although changes in the activities of copper-containing enzymes like superoxide dismutase may prove to be useful functional tests of copper deficiency in the future. Selenium Plasma selenium concentration is an indi­ cator of recent selenium intake, although whole blood selenium concentration has been used. The latter may obviously give a false picture if there has been recent blood transfusion, whereas plasma selenium is bound to Hpoproteuis, which may decrease in concentration in mal­ nutrition, causing a secondary decrease in plasma sele­ nium not necessarily related to body content. The activity of the selenium-containing enzyme glutathionc peroxidasc tends to decrease in selenium deficiency and may be a better functional indicator, but it is not clear whether the measurement is best made in erythrocytes, platelets or even plasma. Excess selenium is excreted in the urine, so urinary measurements arc useful in suspected toxicity, but are not helpful in deficiency. Molybdenum Plasma and urine concentrations of molybdenum are normally very low and measurements in these fluids are not useful in detecting deficiency. Increased urinary excretion of xanthine and sulphite which decreases with molybdate supplementation may be a useful approach in confirming suspected deficiency.

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CUNICAI. HUK:HFMISTRY

Manganese Measurement of plasma and urine manganese is useful when investigating suspected toxicity, but since most manganese is intracellular, whole blood measurements are used in the detection of deficiency. Chrotniutn Plasma and urine chromium can be measured by atomic absorptiometry, but the limit of de­ tection is such that they are only really useful in detecting toxicity. Thus in suspected deficiency, measurements are only helpful in avoiding oversupplementation. NUTRITION AND DISEASE The relationship between nutrition and disease is a com­ plex one, with several different aspects to it. In the first place, inadequate supplies of individual nutrients can cause the specific deficiency syndromes already discussed, which are generally curable by supplying the appropriate nutrient(s). Secondly, dietary factors may be important in the aetiology of specific diseases. In some cases, this may imply scope for dietary modification to alter the course of the disease (e.g. in hyperlipidaemia), but this is not always so (e.g. where carcinogenesis has already been stimulated by a dietary component). In other conditions where diet has no role in the aetiology, specific therapeutic diets may still be important in improving clinical outcome (e.g. in phenylketonuna). Finally, in patients with primarily non-nutritional diseases, general nutritional support may improve outcome, especially if they are, or are likely to become, malnourished either as a secondary effect of the disease or coincidentally.

Diet in the aetiology of disease This is a huge topic and no attempt will be made to cover it comprehensively here. In trying to establish associations between specific dietary factors and diseases, problems almost inevitably arise because a difference in the intake of one foodstuff is confounded by compensatory changes in the intake of others. A few of the better established associations are described briefly below. Protein-energy malnutrition Deficiency of protein and energy (protein-energy mal­ nutrition; РЕМ) gives rise to a number of clinical syn­ dromes. In the developing world, malnutrition affects whole populations, but the effects of РЕМ are particularly prominent in children, presumably because of their high requirements for growth. In contrast, in the UK, severe РЕМ is more commonly seen in adults, usually in association with illness (e.g. gastrointestinal disease). Although some of the features of РЕМ arc common to both, the circumstances of each are different and they are considered separately here. РЕМ in Third World children Worldwide, PExM

remains the most common disorder of infancy and childhood: as many as 100 million children may be signifi­ cantly affected and it accounts for over half the deaths in the under-5 age group in developing countries. Two clinical syndromes of РЕМ arc recognized in these children: marasmus and kwashiorkor. Marasmus is the commonest severe form of РЕМ and tends to occur earlier in life than kwashiorkor, usually within the first year. It is classically associated with a diet that is very low in both energy and protein, as may occur if the infant is weaned early and fed with dilute breast milk substitute. Affected infants are grossly underweight for their age, with no fat reserves (and a consequent wrinkled appearance to the skin) and severe muscle wasting. They are alert with a good or even voracious appetite and typically do not have oedema. Kwashiorkor was first described in the early 1930s by Cicely Williams. It tends to present between the ages of 1 and 4 years and is most common in poor rural children who are displaced from breastfeeding by the next sibling and who arc then fed on very low protein, starchy foods (e.g. based on maize, cassava or plantains). The presenta­ tion is often more acute than that of marasmus, with oedema, irritability and a characteristic desquamation and scaly cracking of patches of the skin. Hepatomegaly is often present, due to fatty infiltration of the liver. The oedema is associated with hypoalbuminaemia. The classic explanation for the two different syndromes rests on the difference in response to a diet poor in protein and energy and one poor in protein but with relatively more carbohydrate. The latter leads to insulin secretion being maintained and thus a relative sparing of muscle protein. This limits even futhcr the amino acids available for the synthesis of liver proteins, in particular albumin (leading to hypoalbuminaemia and oedema) and lipoproteins (leading to the accumulation of lipids in the liver). However, it has been observed that children from the same community, on apparently similar diets, may develop either marasmus or kwashiorkor, so it would seem that differences in the ratio of protein to energy arc not the whole explanation. Other suggested factors in­ clude those intrinsic to the child (there is some evidence that there are differences in cortisol secretion between the two groups) and extrinsic effects such as deficiency of other nutrients (such as zinc and possibly essential fatty acids) and ingestion of toxic factors (e.g. aflatoxins). There also seems to be an increase in free radical injury in kwashiorkor, due both to increased free radical activity and deficiency of antioxidants. In practice, marasmus and kwashiorkor can be re­ garded as the two ends of a spectrum of severe РЕМ. A clinical classification of malnutrition based on this idea is shown in Table 10.4, although it should be remembered that oedema may sometimes be due to coexistent disease rather than malnutrition. This classification does not dis-

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CLINICAL BIOCHEMISTRY OF NUTRITION

Table 10.4

< .lirucml cUaufic.

n in children I

infantile nuilnutr Percentage of normal

With «>cdema

Without t»cdcm*

Kwathiorb M.ir.iMiiK kwmhmrkor

Undernourishment

TttgC

'•" 4i - 60

tinguish the chronically malnourished child who adapts by decreasing linear growth so that height and weight arc in proportion but arc both low for age - the nutritional dwarf. РЕМ in Western adults Studies of both medical and surgical inpaticms in Western countries concur in their detection of a significant proportion of patients with РЕМ, although variation in the criteria used to define this means that the exact prevalence is uncertain. Even in severe cases, caution is required in applying the marasmus/kwashiorkor distinction to such patients, since the presence of hypoalbuminaemia or oedema is more likely to be due to the underlying illness than malnutrition. Nonetheless, severe РЕМ in such patients has important effects. Wasting of respiratory' muscles increases the risk of chest infection and may delay weaning from a ventila­ tor. Myocardial function may be impaired and skeletal muscle wasting delays mobilization, with a consequent in­ creased risk of thromboembolism or bed sores. РЕМ also results in impaired resistance to infection and gut perme­ ability may be increased, allowing entry of bacteria and toxins through the gut wall. Apathy and depression impair active efforts at recovery and may also impair appetite, worsening the situation further.

Obesity Obesity is a condition in which body stores of fat are increased. It appears to occur almost exclusively in mam­ mals. The most useful definition of obesity in man is a body mass index (BMI) of 30 or above, since the BMI is easy to measure and the adverse effects on mortality become manifest above this figure. However, people with lesser degrees of overweight (BMI in the range 25-30) may still wish to lose weight for social or aesthetic reasons and from a medical point of view it is important to en­ courage them to do so, as gross obesity is much harder to treat. In the UK, approximately 10% of the adult popula­ tion (excluding the elderly) has a BMI of 30 or above. Obesity is important because it is associated with an increase in mortality and morbidity. Obese people arc more likely to develop non-insulin-dcpendcnt diabetes mellitus, hypertension, cerebrovascular disease and coro­ nary artery disease. More cholesterol than usual is se­ creted into the bile, increasing gallstone formation.

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Arthritis, hernias, varicose veins and accidents are more likely to occur, presumably from the mechanical effects of increased fat stores. Obesity is associated with polycystic ovary disease in women. There is an increased mortality from cancers of the colon, rectum and prostate in obese men and of the gallbladder, uterus, ovary and breast in women. Sometimes the cause of weight gain is obvious, for example, due to a sudden decrease in energy expenditure (e.g. change to a less active job) or an increase in energyintake (e.g. overeating associated with stress). More often, the cause is not obvious. Endocrine disturbances (e.g. myxoedema, Cushing's syndrome) are uncommon causes of overweight. Research into the causes of obesity is ham­ pered by the fact that it is not possible to identify and study obese people before they start to gain weight and because overfeeding normal humans for extended periods is unethical. However, it docs seem that there is a genetic tendency to obesity and that on average obese people have higher (not lower) metabolic rates than slim people. Treatment of obesity is worthwhile, since although carcinogenesis or arterial damage cannot be reversed, other features such as hypertension and glucose intolerance do respond to weight loss. The basis of treatment is reduction in dietary' energy intake, but this has to be prolonged to be successful and in practice is often not sustained. Exercise is helpful, not particularly because of the energy used, but because it promotes a sense of well-being and has benefi­ cial effects on general health. Appetite suppressant drugs arc not generally considered to be useful and jaw wiring and gastric surgery are reserved only for extreme cases. Ischaemk heart disease T h e three main risk factors for ischaemic heart disease arc a raised plasma L D L (or total) cholesterol concentration, arterial hypertension and cigarette smoking. Dietary fac­ tors have a role in the aetiology of the first two. LDL cholesterol is increased by diets rich in saturated fatty acids, particularly palmitic (C16:0) and myristic (C14:0) acids, while it is reduced by ш-polyunsaturated fatty acids and, to a lesser extent, monounsarurated fatty acids. The cholesterol content of the diet appears to be less im­ portant. Non-starch polysaccharidcs, particularly soluble NSPs, may have a cholesterol-lowering effect, but this is difficult to separate from the way NSP-rich foods arc likely to displace foods containing fat from the diet. Л moderate alcohol intake may be beneficial in raising the plasma H D L concentration. Sustained high salt intake may be a factor in the aetiol­ ogy of essential hypertension and reduction of salt intake can certainly help in the management of mild to moderate hypertension. Obese people are more likely to have hyper­ tension than lean people and weight loss can lead to a fall in blood pressure (and, indeed, to improvement in

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i. HI

ISTKY

hyperlipidaemia). A high alcohol intake also causes a rise in blood pressure, which improves if the intake is reduced.

Table K>

миле example» of association* between dietary factor*

and cancer Site of cancer

■OCaB)

try factor*

Dental caries

Non-milk extrinsic sugars are the main dietary compo­ nents associated with dental caries. Streptococcus mutatis metabolizes these to lactic acid, which dissolves the dental enamel and also polymerizes some of the sugars to form an insoluble polysaccharide coating, under which the bacteria are shielded. Other carbohydrates are potentially cariogenic, but it seems to be the non-milk extrinsic sugars which arc important in practice. Dietary fluoride, for example in fluoridated drinking water, is also important in preventing the development of dental caries. A concentration of I mg/I. increases the re­ sistance of the enamel, particularly if the exposure occurs while it is being laid down prior to the tooth erupting, and reduces tooth decay in children by about 50%.

Mouth, pharynx and larynx

Alcohol

Oesophagus

Alcohol (cspcojllv that derived from apples) Nitroaammes (in preserve»! foods)

Stoeaach

Smoked fish, pickled foods, cured meats, valt Nitrosazrunes formed in th m nitrates in the diet

IJirge inieMine

High fat, high protein diet Low 'fibre' mtakc »mc types of beer

Liver

Aflatoxin contaminated food* (ail a toxins are a product of the fungus Азрег&Вш flovu

Breast

High fat diet 4*-derate alcohol intake

T h e r a p e u t i c diets Cancer Many associations have been noted between specific di­ etary components and various cancers (Table 10.5). The overall risk of cancer appears to be highest in populations consuming diets high in fat and energy, where people arc obese and alcohol intake is high. Diets that are high in 'fibre', particularly those with a high fruit and vegetable content, appear to offer some protection from certain can­ cers. Other diet components may be important, for exam­ ple, the various antioxidants (e.g. vitamins А, С and E, selenium) and food additives and contaminants, but the precise relationship with cancer risk is not well defined.

Table 10

"I"hc mam featur-

Details of therapeutic diets can be found in textbooks of dietetics, but the main features of some of the commoner ones are summarized in Table 10.6. Provision of nutritional s u p p o r t Deficiencies of specific vitamins or minerals are treated by appropriate supplementation of individual nutrients, which could be thought of as a form of nutritional sup­ port. However, the term is usually used to mean the provi­ sion of balanced mixtures of nutrients to replace all or part of normal food intake. This can varv between encouraging

-ome thcrapcun

Disease

Main tenures of therapeutic d

Dtabctes mcUitus

Mote than half of energy intake from unrefined, starchy car1 Pal intake limited to 35% of total energy, with saturated fat restricted the most Restr il energy intake if overweight

HvpercholcMcrolaemia

ilergy intak Increase proportion of unsarv raced fats (so that saturates = monounsatu rates = yunssturati Re ,il energy intake if overweight total cholesterol mtakc probably not verv important

ironic rcnai failure

R
undiruj an atom of cobalt which form* the core of cobalamin. The other two ligand* to cobalt (above and below the plane of the corrin ring), and subsrirucnTs on the pyrrole units, are not shown.

Vilamtn В]? Homocysteme Methyl THF Fig. 11.2 The link between folate and vitamin B,. deficiency in mcgaloblastic anaemia 5,HVmcthylenc T H F polyg)uiamatc н required for a rate-limiting step in DNA synthesis, the conversion of dcoscyundmc monophosphate to deoxythymidinc monophosphate. Vitamin Bl2 is required in one of the reactions convening the main circulating form of folate, 5-methyI T H F , to 5,10-mcthylcne T H F .

rcctcd, but any neurological manifestations (e.g. subacutc combined degeneration of the cord) are likely to get worse.

of cobalt in humans. This is at the centre of a corrin ring, made up of four pyrrole units in a similar manner to porphyrins (Fig. 11.3), with two other substitutions above and below the ring, variations here giving rise to the various forms of cobalamin. Naturally occurring forms include methyl-, hydroxo- and adenosylcobalamins. The only reactions known to be dependent on cobalamin in mammals are the conversion of L-methvlmalonyl CoA into succinyl CoA and the formation of methionine by mcthvlation of homoevstcine. B,, absorption

Assessment of/olate status Folate deficiency Both plasma and red cell folate can be measured, with plasma concentrations indicating recent dietary intake and red cell values generally reflect­ ing tissue reserves. In folate deficiency both plasma and red cell folate are low, but in B i 2 deficiency' the plasma folate tends to rise while the red cell folate falls (due to a failure of folate polyglutamatc synthesis). Results in combined deficiencies may thus present some difficulty in interpretation. Tests of folate status involving measurement of the urinary excretion of formiminoglutamic acid (FIGLU) following a histidine load (increased in folate deficiency) are now obselete. Folate absorption Folic acid absorption tests have been described, but are of little use in normal clinical practice. Their only role is in the diagnosis of congenital folate malabsorption, a rare condition presenting with a severe megaloblastic anaemia in the first 3 months of life. Vitamin B l 2 The molecular structure of cobalamin (vitamin B,,) is based around an atom of cobalt, the only known function

liver is the richest food source of B u , but it is present in almost all animal products including fish and milk and is also synthesized by certain micro-organisms. No vegetable food source consistently contains significant amounts of vitamin B, 2 , unless contaminated with bacteria or algae. The mechanisms underlying B u absorption arc more complex than for iron or folate and require important con­ tributions from the stomach, pancreas and terminal ileum. Luminal events in Bt> absorption Dietary cobala­ min is bound to food proteins and must be freed by gastric acid, although there is no firm evidence that malabsorp­ tion occurs in man from diseases or drugs that interfere with gastric acid secretion. Free cobalamin becomes bound to R binding proteins which are secreted in saliva. The parietal cells of the stomach secrete the glycoprotein intrinsic factor (IF), which enters the proximal small intestine with the cobalamin-R protein complex. The complex is hydrolysed by pancreatic proteases and free cobalamin now complexes with intrinsic factor. The B i ; IF complex resists proteolytic digestion and allows cobala­ min to reach its site of active transport in the terminal

ileum. Enterocyte events in Bu absorption Although the issue is unresolved, it appears that the intact B,,-IF com-

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MAIABSORPTION

plex is taken up by receptor-mediated endocytosis in terminal ileal enterocytes. B i : is liberated in the epithelial cell and transported in blood after binding to the carrier protein transcohalamin II. Vitamin B w absorption is slow: cobaiamin does not appear in the blood for some 3 hours after it arrives at the terminal ileum and peak plasma concentrations are not reached for some 8 hours after ingestion. BJ: deficiency Since adult body stores of cobaiamin are high in relation to need and since most diets contain adequate amounts, dietary deficiency of B t : is rare. It is occasionally seen in strict vegetarians or vegans. However, B 1? deficiency occurs more commonly as a result of impaired absorption, cither because of a deficiency of intrinsic factor (as in pernicious anaemia or following total gastrectomy) or because of disease of the terminal ileum. Body stores arc such, though, that even following gastrectomy, B,> deficiency may not occur for several years. Although pancreatic insufficiency might be expected to reduce B, : absorption as the B, : -R protein proteases would be deficient, in practice clinically important B t . deficiency is most unusual in pancreatic disease alone, no matter how severe. Vitamin B I : is involved in the processing of folate coenzymes (see Fig. 11.2) and so deficiency results in a megaloblastic anaemia in a similar manner to that seen in folate deficiency. Deficiency can also present with neuro­ logical symptoms, which may be irreversible if the defi­ ciency is prolonged. The cause of the neuropathy is uncertain, but it may be linked to decreased availability of methionine, from which some choline (necessary for the formation of sphingomyelin and other phospholipids used in myelin synthesis) is formed, or to accumulation of the toxic metabolite S-adenosyl homocysteine.

205

neurological defects. Once deficiency is established, further tests are required to establish the cause. Bu absorption B, 2 malabsorption is assessed by the Schilling test. This is performed in two parts. In sum­ mary, the 24 h urinary recovery of an oral dose of radiolabelled B )2 is measured in part one. In order to saturate potential binding sites for the oral B | : , a 'flushing' dose of non-radioactive Bl2 is given intramuscularly at the beginning of the test. In part two, the test is repeated with the addition of oral intrinsic factor to see whether a low urinary recovery of labelled B, 2 is corrected. A low urinary recovery in part one, becoming normal in part two, suggests that malabsorption is due to gastric disease, such as pernicious anaemia. Low recovery in both parts implies defective intestinal absorption as, for example, after resection of the distal ileum. CARBOHYDRATE ABSORPTION Dietary carbohydrates In most human diets, carbohydrates are the principal source of energy. The major form in the diet is starch, which accounts for about two thirds of dietary carbo­ hydrate, the remaining third being made up of sucrose and lactose. Starch is a polymer of glucose consisting of two forms: amylose and amylopectin. Amylosc consists of long, unbranched chains of glucose molecules joined by a-1,4 links. Amylopectin consists of chains of glucose molecules with branching points for side chains every 1225 glucose molecules. At the beginning of the branch, there is an a-1,6 link. Sucrose and lactose are disaccharides, sucrose consisting of one glucose molecule bound to one of fructose, while lactose is a dimer of glucose and galactose. Digestion of carbohydrates Luminal events in carbohydrate digestion

Bi: excess Supplements of B , : may occasionally precipitate hypersensitivity reactions, but otherwise there are no known toxic effects, even at daily doses of orders of magnitude greater than the physiological requirement. Assessment of Bu status BIZ deficiency Suspicion of a deficiency of vitamin B,i or of folate may be raised by the finding of anaemia and macrocytic red cells in the peripheral blood, but there are other causes of macrocytosis (e.g. alcohol, liver dis­ ease, hypothyroidism) and so biochemical tests are useful in assessing Eti and folate status. The plasma B 12 concentration is usually very low when body stores are low enough to cause haematological or

T h e initial step in starch absorption is enzymatic digestion of a-1,4 links by salivary amylase to release maltose (dimer of two glucose molecules) and maltotriose (trimer of three glucose molecules). The salivary enzyme is rap­ idly inactivated by gastric acid, but digestion continues with pancreatic amylase which cleaves a-1,4 bonds but not a-1,6 bonds. The end products are maltose, malto­ triose, short branched oligosaccharides and a-limit dcxtrins, the latter two of which are residual branched segments resulting from incomplete digestion of amylo­ pectin. Both dietary oligosaccharides and those resulting from the action of amylase on starch are further digested by the cnterocytc. Enterocyu events in carbohydrate digestion Carbohydrate digestion is mediated by enzymes in the

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206

CLINICAL BKKIHEMISTKY

apical membrane or brush border of the enterocyte which convert sugars into monosaccharides. Three oligosaccharidases or disaccharidascs have been identified: maltase, sucrase-isomaltase and lactase (Table 11.3). They are synthesized on the endoplasmic reticulum, transported to the Golgi apparatus and thence to the brush border. They arc distributed throughout the length of the small intes­ tine but sucrase and lactase are in highest concentration in the jejunum. They are normally present in considerable excess so that absorption is the rate-limiting step. This is not the case for lactose, which is hydrolysed much more slowly than the other sugars. The three main monosaccharides derived from the diet (glucose, galactose and fructose) are absorbed by saturable carrier-mediated transport systems in the luminal membrane of cntcrocytes. Secondary active transport of glucose and galactose occurs through a sodium cotransport system, driven by a sodium gradient dependent on Na*, K*-ATPase on the basolateral surface of the cell. Fructose absorption is not active, but occurs through carrier-mediated (facilitated) diffusion, lixit from the enterocyte for each of the monosaccharides is also through facilitated diffusion. Both the digestion and absorption of carbohydrates are highly efficient, but the process is incomplete, particularly for starch, whereby up to one fifth of dietary starch is not absorbed. The efficiency of starch absorption varies depending on the foodstuff from which it is derived. For example, rice starch is highly absorbed, but between 10 and 20% of starch in wheat and certain beans enters the colon. Clinical aspects of c a r b o h y d r a t e absorption Carbohydrate in the large bowel is metabolized by colonic bacteria to produce short chain fatty acids (which arc an

I1J

Brush border c n ^ m c

n carbohyUra•maltu* ;ed u • ge«tKin «ml single mokcta Um-mg in the brush bonier membran. r o b vet the liisatich. гсЬд1о*с, 1 onh trehftU minor i v ! ■iming mainly from mu&hroom» ■

En/ymc

Макам

tcDOQ Splits glucote monomers from O-1.-1 linked .'Itgowcchariiics up rcttdi. tig

in* lactose into constituent nxmowecharidet

Lactase dtficieticy Activity of lactase is highest in the earliest months of life, but lc%'cls of the enzyme decline after weaning. The prevalence of lactase deficiency is highly variable between races so, for example, between 5 and 15% of northern Europeans have demonstrable hypolactasia or alactasia, but over 7 0 % of Africans and Asians show deficiency of the enzyme. These figures have been determined by tests of lactose absorption and the vast maiority of affected individuals show no clinical features. However, sympto­ matic persons will experience some combination of nau­ sea, bloating, abdominal discomfort and diarrhoea after ingestion of milk. T o some extent, symptoms will depend on whether the enzyme activity is merely reduced or whether it is absent. In some individuals, colonic bacteria may reduce the osmotic load on the colon by metabolizing the undigested sugar. In practice, the diagnosis is often made by a therapeutic trial of milk withdrawal but meta­ bolic tests are available (see below) and the diagnosis can be confirmed by assaying lactase activity in small bowel biopsy samples. Congenital lactase deficiency (i.e. lactase deficiency which is present at birth) has been described, but is extremely rare. The form of enzyme deficiency discussed above is usually known as primary lactase deficiency. However, a lactase deficiency may also complicate mucosal diseases of the small intestine. This secondary lactase deficiency may occur in coeliac disease, tropical sprue, inflammatory bowel disease, radiation enteritis, chronic alcoholism with malnutrition and the cnteropathy associated with acquired immune deficiency syndrome (AIDS). It may also accom­ pany infections such as acute gastroenteritis or giardiasis.

i гооякав ' .luctfie

SucraAC-iBomakast _ ^ ^ ^ ^ H ^ ^ ^ ^ H L ^ H ^ I Spin» tucrote into con icni SucoMfl (VluciMC And nMTKnaccharktai fructoHc Емвмкде Splits ii-ltmit d c x t n m at 0 :UCOK Jink* Both In gJucoftc monomers from (iluCOAC n-rcJucing a i limit dextrin»

LactaM

important energy source for eolonocytes), methane and hydrogen. Malabsorption of carbohydrates tends to pro­ duce osmotic diarrhoea as well as excessive Gatus and associated abdominal discomfort. Inherited disorders of sucrase-isomaltase have been reported but are very rare. Congenital glucose-galactose malabsorption may be even rarer. However, lactase deficiency is common.

ueosc And gaUctotc

Investigation of c a r b o h y d r a t e absorption The glucose tolerance test is of no value. Even in severe mucosal disease, the ability to absorb glucose is usually unimpaired. Furthermore, blood levels of glucose arc dependent on gastric emptying and metabolic factors unrelated to absorption of the sugar. Xylose absorption test A more useful test involves measuring the excretion of the pentose sugar, n-xylose. This sugar is only partly

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absorbed by the proximal small intestine, so its absorption is affected by smaller changes in gut function than sugars (e.g. glucose) for which there is a large absorptive capac­ ity. About half of the xylose absorbed is excreted in the urine; the remainder is metabolized or excreted in the bile. A protocol for the test is given in the appendix (p. 214) but briefly, the urinary excretion is measured for 5 hours after an oral xylose load. An alternative measure of absorption is to measure the xylose in blood taken at 1 hour after ingestion. The urine test is affected by renal function, but may also be difficult to interpret if there is ascites (in which xylose accumulates), rapid intestinal transit, blind loop syndrome (bacteria may metabolize the xylose) and in the elderly. The test is sufficiently un­ reliable for manv laboratories to have abandoned it. Its reliability can be greatly enhanced by giving n-xylose and 3-O-methyl-D-glucose and using the ratio of plasma concentrations at 1 h this test is said to be capable of excluding coeliac disease in children. luxctose tolerance test This is analogous to the glucose tolerance test, but is used in suspected lactase deficiency (see appendix, p. 214). In essence, serial blood samples for glucose estimation are taken after oral ingestion of 50 g of lactose. A rise in blood sugar of less than 1.1 mmol/L is indicative of lactose malabsorption. The occurrence of the symptoms discussed above can give additional diagnostic information. Hydrogen breath test T h e production of hydrogen by bacterial fermentation of unabsorbed carbohydrate has been utilized in a number of different breath tests which have in common the detection of a rise in breath hydrogen after administration of an oral sugar. The apparatus is simple and the tests are easy to perform. In the detection of lactase deficiency, an oral test dose of lactose is given, which should cause an increase in breath hydrogen if it passes unabsorbed into the colon. However, there are inherent limitations in the detection of breath hydrogen, for example with smoking and hydro­ gen production by oral flora. Carbohydrate may also be retained within the gut from an earlier meal. Abnormali­ ties in gut transit and concomitant antibiotic therapy can influence the result. Finally, some individuals fail to pro­ duce hydrogen in the breath even when challenged with a non-absorbed carbohvdratc such as lactulose. PROTEIN ABSORPTION The average Western diet contains some 80-100 g of pro­ tein, which contains not only the amino acids necessary for protein synthesis but also provides some 10-15% of

207

the energy content of the diet. In addition to this dietary protein, the gut digests and absorbs a further substantial amount (perhaps 60 g) of endogenous protein derived from intestinal secretions, mucus and the shedding of mucosal cells. Digestion of proteins Luminal digestion This begins in the stomach with the grinding and mixing action of the stomach on food particles exposed to gastric acid and pepsin. Pepsin is an endopeptidase which cleaves internal peptide bonds adjacent to hydrophobic amino acids, but which is rapidly inactivated above acid pH as food enters the duodenum. In the proximal small intestine, digestion proceeds further through the action of the pancreatic proteases, trypsin, chymotrypsin, elastase and carboxypeptidases A and B. These are secreted from the pancreas as inactive proenzymes. Trypsin is released from trypsinogen by the brush border-derived enterokinase and activates the other proenzymes. Trypsin, chymotrypsin and elastase arc endopeptidases, hydrolysing peptide bonds adjacent to certain specific amino acids, while carboxypeptidases A and В are exopeptidases, removing amino acids one at a time from the C-terminal ends of the peptides. The proteases reduce chyme to a mixture of free amino acids and oligopeptides (of between two and six amino acid residues). These are then further digested by brush border peptidascs. Enterocyte digestion There are at least eight mu­ cosal peptidases which are synthesized and transported in a similar manner to the disacchahdases. However, unlike carbohydrates, the peptides are not completely digested to their constituent amino acids, brush border enzyme action leaving a mixture of amino acids and di- and tripeptides. Peptide absorption There are specific transport systems which effectively absorb di- and tripeptides and indeed, do so at a faster rate than for amino acids. (This discovery has had implications for the development of enteral feeding regimes.) Peptide uptake is by active transport. Once inside the mucosa, peptidases within the enterocyte cleave small peptides to their constituent amino acids. A number of transport systems exist for the uptake of luminal amino acids. Protein digestion takes place throughout the length of the small intestine and is efficient, only 5% of protein entering the gut lumen being lost in the stools each day. Clinical aspects of protein absorption There are a number of rare inherited defects in protein absorption, including Hartnup disease and cystinuria (sec Ch. 9). There are no specific syndromes of global protein malabsorption although protein deficiency can contribute to the wasting and nutritional disturbance of severe malabsorption syndromes. In several diseases,

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e.g. Crohn's disease, ulcerative colitis, Whipple's disease, excessive protein loss into the gut produces the syndrome of protein-losing cntcropathy. This process is not one of malabsorption, although there may be associated malabsorption. Investigation of protein absorption There is no satisfactory method of assessing protein ab­ sorption since amino acids undergo rapid assimilation into protein synthesis or are catabolized as an energy source. FAT ABSORPTION Although the average fat content of the diet has been falling in the Western world over the last 20 years, the average fat intake is still of the order of 90-100 g per day. There has also been a relative decline in the proportion of saturated fat in the diet with a concomitant increase of polyunsaturated fatty acids. The principal long chain polyunsaturated fatty acids are linoleic (C18:2) and linolenic (C18:3) acids (see Ch. 10 for an explanation of nomenclature). 1Ъс other dominant fatty acids are oleic (C18:I) and palmitic (Cl6:0) acids. The polyunsaturatcs are called essential fatty acids as they cannot be synthe­ sized in man. Additional dietary fat consists of phospholipid and cholesterol, and the fat-soluble vitamins arc also important. Because fat is insoluble in water, the mecha­ nisms for its digestion and absorption are complex, but an understanding of the mechanism of fat digestion and absorption is crucial to an understanding of how a wide variety of diseases can result in malabsorption of fat and consequently result in steatorrhoea.

Digestion of triacylglycerols

Pancreatic lipasc acts in conjunction with a cofactor, pancreatic colipase, which is secreted in an inactive form (procolipase) and is activated by tryptic digestion. Pancre­ atic lipase is most active at near neutral pH and thus its activity depends on effective bicarbonate secretion from the pancreas (under the influence of secretin). The combined action of lipase and colipase hydrolyses triacyl­ glycerols to release fatty acids from the 1 and 3 positions (Fig. 11.4) so that one triacylglyccrol molecule is hydrolysed to a monoacylglycerol with a fatty acid attached at the 2 position, and two fatty acids. Bile salts are amphipathic, that is, they contain both water-soluble and lipidsoluble components. This property allows them to aggregate in micelles such that the hydrophobic compo­ nents line up adjacent to one another on the inside of the micelle with the hydrophilic aspect pointing outwards into the aqueous phase. The insoluble fatty acids, monoglycerides and cholesterol, therefore, are held 'inside* the micelle to form a highly stable paniculate emulsion. While it has been shown that triacylglycerol absorption remains fairly efficient in the absence of bile salts, by contrast cholesterol and fat-soluble vitamins undergo severe

C H 3 — О — С — Х — СН 3

Free fatty acids О НО

II

7

о

.

Fig. 11.4 Digestion of a triacylgrycerol molecule. The fatty acids in the example arc aJl shown as saturated molecules with different chain lengths, but this is not always the case,

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malabsorpiion unless the bile salt concentration in the duodenum is above a critical micellar concentration. Absorption of triacylgiycerols It has been believed hitherto that transport of Hpids across the cnterocyte surface is by die process of passive diffu­ sion. The microclimate at the cnterocyte surface is slightly more acidic than in the jejunal lumen, which encourages the liberation of fatty acids from the micelle, which per­ mits diffusion across the mucosa. There is some sugges­ tion, however, that a component of fat absorption for certain fatty acids, particularly Hnolcic and oleic acids, may occur by facilitated diffusion. In the endoplasmic rcticulum of the mucosal cell, tri­ acylgiycerols are resynthesized into chylomicrons and very low density lipoproteins. The carrier protein apolipoprotein В is essential for chylomicron synthesis and, in the absence of this protein, as in the condition congenital abctalipoprotcinacmia, fat malabsorpiion is severe. While medium chain triacylgiycerols are absorbed directly into the portal venous blood, the longer chain triacylgiycerols (above С10) pass into the lymphatics. Medium chain triacylgiycerols (C6-C10) can be partly absorbed intact, presumably by passive diffusion. Medium chain fatty acids do not need to be rc-estcrified to triacylgiycerols and so exit the cell directly and travel to the liver via the portal circulation.

209

duodenum, pancreatic exocrine deficiency, failure to achieve near-neutral duodenal pH either from gastric acid hypersccretion or pancreatic bicarbonate hyposecretion, and mucosal disease either in the form of villous atrophy or, as mentioned above, abetalipoproteinaemia, can ell result in steatorrhoea. Not surprisingly, because of the complexity of the process, tests of fat absorption have been used as screening methods to detect malabsorption. Investigation of fat absorption Quantitative measurement of faecal fat excretion is the most sensitive method of detecting fat malabsorption. Faecal fat excretion This test can be unpleasant both for the patient and for laboratory staff. It is essential that it is done correctly and, where possible, it is often best that the stool collection is performed in hospital. Normal stool fat excretion is less than l&mmol per 24 h. Faecal fat excretion above this level is a highly sensitive indicator of malabsorption but does not, of course, allow the cause of the steatorrhoea to be defined. As a general rule, fat malabsorption is more severe when it is a result of pancreatic disease. Because of its unpleasant nature, there have been a number of efforts to find alternative measures of fat ab­ sorption. The MC-triolein breath test is probably the best of these.

Digestion and absorption of other fats

**C-triotein breath test

Phospholipids are hydrolysed by pancreatic phosphoiipase A ; to fatty acids and lysophosphatidyl choline, while cholesterol esters arc hydrolysed by a pancreatic cstcrasc. The fatty acids form micelles with bile acids and thus both lipids are ultimately transported to the enterocyte in a similar way to the digestion products of triacylgiycerols. Phospholipids are resynthesized within the mucosal cell in the endoplasmic rcticulum and cholesterol is cstcrified by acyl-CoA cholesterol acyltransferase before being incorporated into chylomicrons. The efficiency of fat absorption is considerable so that, in health, the process is virtually 9 8 % complete. The vast majority of fat absorption occurs within the proximal jejunum.

T h e principle of this test is that if l4 C labelled triolein is absorbed, U C 0 2 will appear in the expired breath. A pro­ tocol is described in the appendix (p. 215). The test is, however, less reliable than faecal fat measurement because patients with lung disease or altered tnacylglyccrol meta­ bolism (e.g. obesity, hyperlipidaemia, diabetes, chronic liver disease) may give erroneous values. The measure­ ment also changes with age and has limited value when the degree of malabsorption is mild. The exposure of the patient to radiation can be avoided by the use of n C , a stable isotope of carbon, but a mass spectrometer is required to measure it. O T H E R DIETARY CONSTITUENTS

Clinical aspects of fat malabsorption It can be seen that fat absorption is a complex process, requiring the delivery' of bile salts and pancreatic enzymes to the duodenum, together with efficient enterocyte func­ tion and normal mixing. Therefore, disease at several levels in the alimentary system can result in fat malabsorp­ tion, which is characterized by the presence of steator­ rhoea. Interference with delivery of bile salts into the

Vitamins, apart from B,. and folate, are dealt with in Chapter 10, together with most of the minerals of dietary importance. However, iron is discussed above and cal­ cium and magnesium are considered here. Calcium Milk and dairy products contribute about 70% of dietary calcium, other less important sources being cereals and

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grains, where calcium is complexcd with organic anions derived from phytic and oxalic acids which reduce its po­ tential for absorption. The mechanism of absorption is complex and incompletely understood. There is an active mucosal transport system in the duodenum and jejunum but, in addition, there is an associated passive uptake of calcium through the epithelial tight junctions. Calcium is absorbed in ionized form and so other luminal nutrients, as well as intraluminal pH, arc important. Divalent cati­ ons tend to remain ionized at acid pH, but will precipitate with increasing concentrations of luminal phosphate. Luminal pH and phosphate concentrations are lowest in the duodenum and proximal jejunum, explaining why cal­ cium absorption is most favoured at that site. It is worth noting here that malabsorbcd long chain fatty acids also tend to form insoluble soaps with calcium. In addition to daily intake of calcium of about 25 mmol, a further 6 mmol of calcium enters the intesti­ nal lumen via digestive secretions each day. About 12 mmol of calcium is absorbed daily. There arc a number of other factors influencing calcium absorption. Calcitriol seems to increase passive uptake of calcium through tight junc­ tions. There is also a key calcium-binding protein, calbindin, which transports absorbed calcium through the mucosal cell against an electrochemical gradient. The intracellular concentration of calbindin is also regulated by vitamin D. Magnesium Magnesium is widely distributed in foods, probably the major source in the Western diet being vegetables. '1Ъс average daily intake of magnesium is 12 mmol. like cal­ cium, magnesium is absorbed in an ionized form. Unlike calcium, however, absorption is greater in the ileum than jejunum and occurs through both passive diffusion and a facilitated process. MECHANISMS O F M A I ^ B S O R P T I O N It must be said that there is no entirely satisfactory way of classifying malabsorption. There are a number of reasons for this difficulty, but even the two processes of digestion and absorption cannot be entirely distinguished when it is appreciated that the enterocytc brush border may have a key role in both events. Clinicians look for methods of classification which they can associate with particular diseases, but a classification based on pathology is far too crude for these complex metabolic processes. The biochemistry of malabsorption is not so clearly and comprehensively understood as to allow it to be classified in a way which would not result in a number of question marks. The most satisfactory defini­ tions arc those based on physiology. The classification suggested here basically resolves the processes of digestion

Table 11,4 BumpJ rer *ug« 1

ulabsorption ai luminal, mucosal and Isavri in the t
xylose are measured and

overnight. 3-O-methyl-n(5.0 g) are dissolved in and a blood sample taken of 3-0-methyl-i>-glucose the ratio calculated.

Interpretation of results If the two sugars are measured in mg/dL, the ratio r> xylosc/3-O-mcthyl-D-glucosc is normally 0.83-1.17. A ratio below this is found in virtually 100% of children with untreated coeliac disease. Increased ratios may be found in some cases of anorexia nervosa.

FURTHER READING Lawson L, Chcsner I Tests of exocrinc pancreatic function. Annals of Clinical Biochemistry 1994; 31: 305-14. A recent critical r r a m . including dctittis of some tests not mentioned here. Romano R J, Dobbins J W. Evaluation of the patient with suspected malabsorption. In: Fisher R I- (ed) («stroenicrolojn/ clinics of North America 18(5]. Philadelphia: W В Saundcrs, 1*»8г measurement is only of any significance when applied to liver pathology. T o this extent understanding of the conventional liver function tests relies on a broad grasp of the principles of liver disease and the present chapter should therefore be read in conjunction with Chapter 13. This is not to sav that the function of the liver is not well understood. While it is conventional to list the func­ tions that the liver can perform, this detracts from gaining a global or broad conceptual picture of what the liver 'docs*. The liver is a regulatory barrier between the systemic circulation and the organism's environment as experienced via the gut. More succinctly put, the job of the acinus, the functional unit of the Hver, is to regulate the concentration of solutes in the terminal hepatic venules and bile. It is the principal organ of metabolic homoeostasis, i.e. maintenance of blood composition within physio­ logically acceptable limits by the conversion, synthesis and release of components required by other organs. This chapter reviews briefly the anatomy, physiology

and biochemistry of the normal liver as a basis for under­ standing the tests currently applied in clinical practice and those that may be developed in the future. ANATOMY O F T H E LIVER T h e macroscopic and microscopic anatomy of the liver is difficult to understand, partly because of its inherently complicated three-dimensional structure and partly be­ cause of the recent trend to replace simple (but mislead­ ing) morphological descriptions with more accurate, but less obvious, functional descriptions. Macroscopic structure The liver is a wedge-shaped organ in the right upper quadrant of the abdomen. Its mass varies with that of the individual, being in the order of 22 g/kg body weight. In a typical 70 kg subject the liver weighs about 1.5 kg. It has a large right lobe, a smaller left lobe anteriorly and two further small lobes, the quadrate and the caudate lobes (Fig. 12.1). These lobes relate to the venous drainage, not to the portal distribution (see below). Thus the left hepatic vein drains the left hepatic lobe and the right and middle hepatic veins drain the right hepatic lobe. In terms of the portal structures, there are two functional lobes defined by the right and left portal veins. The division is marked by a line joining the inferior vena cava and the gallbladder bed (Fig. 12.1). Microscopic structure As viewed with the microscope, the unit of the liver ap­ pears to be the lobule and it is in terms of this structure 217

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CLINICAL BIOCHEMISTRY Falciform ligament

from a functional point of view, the basic unit of the liver is the acinus (see below). This, too, is important to under­ stand because of the idea of liver cell heterogeneity; differ­ ent zones of the acinus have different physiological and biochemical functions. The lobule The concept of the lobule is based on a central efferent hepatic arteriolc surrounded by radically oriented plates of hepatocytes and sinusoids. Three to five portal tracts (or 'triads'), comprising portal vein, hepatic artery and bile duct, are situated around the periphery of the lobule (Fig. 12.2A).

Uver

The acinus

Fig. 12.1 The anterior surface of the liver. The labelling in the upper part of the figure represent* the conventional description into a righi and left lobe separated by the falciform ligament. The lower pan of the labelling refers to the right and left lobes defined by distribution of the portal structures. The right and left portal structures (portal vein, hepatic artery and bile ducts) enter the functional right and left lobes, respectively.

that pathological changes arc described. Since liver biopsy has, under many circumstances, become the de facto "gold standard* of liver disease diagnosis, particularly for evaluating the diagnostic usefulness of liver function tests, it is important to understand the terminology. However,

В Acinus

A uobuie Brancn of hepatic artery Branch of portal vein ducts

This is the true structural and functional unit of hepatic parenchyma which represents a group of liver cells fed by a single terminal portal venule and hepatic arteriole, the blood from which passes via the sinusoid into a terminal hepatic vein (Fig. 12.2B). Flow is unidirectional and the 20 or so hcpatocytcs that separate the portal tracts from the terminal hepatic vein have been arbitrarily separated into three zones, through which the portal blood passes sequentially. Hepatocytes nearest the feeding artery (zone one) receive blood rich in substrates, hormones and oxy­ gen and have a higher level of metabolic activity*. It should

Hepatic cells Biliary canabculi

TPV THV HA BD

Terminal portal venule Terminal hepatic venule Hepatic arteriole BUe dud

Fig• 12-2 (A) The conventional hepatic lobule based on the central vein with surrounding portal tracts. (B) The hepatic acinus and its three functional zones. The axis is formed by the terminal portal venule, hepatic arteriole and bile duct. Blood flows from the periphery of the acinus to the terminal hepatic venule.

HEPATIC FUN'CTION AND JAUNDICE

be emphasized that there are no anatomical boundaries between these three zones, but hepatocytes in zone three ('ccntrilobular' or 'perivenous*) are particularly rich in smooth cndoplasmic reticulum whereas rough endoplasmic reticulum is more plentiful in those in zone one ( 4 pcriportaP). Clearly, as blood perfuses each zone sequentially its composition will change and the func­ tional heterogeneity of hepatocytes reflects their changing microenvironment. Space does not allow a detailed discussion of the functional heterogeneity of the three zones but we may mention three examples which impinge on areas discussed later. Glucose production (from glycogen) occurs mainly in zone one, hepatocytes and glucose uptake in zone three. Urea synthesis occurs mainly in zone one and glutamine synthesis in zone three. Phase 1 (biotransformation, e.g. hydroxylation) activity and alcohol dehydrogenase activity is sited in zones two and three, whereas phase II activity (conjugation, e.g. glucuronidation) is in zone one. It is not difficult to appreciate that different pathological insults may differentially damage the various zones and to envis­ age that eventually tests investigating the integrity of the different zones will be developed. Ultra&tructurc

2 1(»

filter fluid exchanged between the sinusoidal lumen and the space of Disse, the sinusoids also contain KupfTer cells (a major part of the rcticuloendothelial system), fatstoring cells (which contain vitamin A and produce sev­ eral connective tissue components) and Pit cells (which share similarities with natural killer cells). These nonparenchymal cells arc an area of intensive research and current evidence suggests that they play a major role in synthesis of growth factors responsible for control of liver regeneration (see below). Bile, bile ducts and biliary drainage As noted above, the biliary canaliculi are formed from the plasma membrane of the hepatocyte but these eventually drain into ducts lined by specific biliary cells and ulti­ mately into the major bile ducts, thence to the common bile duct and the gut (Ch. 13). Interruption of the flow of bile, the exocrine secretion of the liver, is responsible for many signs of hepatobiliary disease. Bile pigment and bile acid metabolism are described below and the pathological anatomy of the liver and biliary tract is described in the next chapter. T h e hepatic circulation

The hepatocytes are arranged in single cell plates, sepa­ rated from the sinusoids by endothelial cells. The biliary canaliculi are formed by the basolateral parts of opposed hepatocytes but have no direct connection with the space of Disse that separates the hepatocyte membranes from the sinusoids (Fig. 12.3). Besides the endothelial cells that

Erxtothelial cell Pensmusoidal space of Dtsse PenceUular space

&\1ЯЛлИпг1л^ ■

1

Hepatocyte

'.

Parac*i;ui.v flow

Nucleus

,

Bile canalculus Junctionel complex Qap junctions

Flf* 12.3 I'ltraKiructure of the liver. Solutes may pass out of the мпияок) winch doc* not have A conventional basement membrane and across the spate of Disse. Mere they may be taken up across the hepatocyte membrane and subsequently across the canahcular membrane or enter the canaliculus through the "paracellular pathway via the intercellular junctions

■■ >

A major cause of abnormal function in chronic liver disease is disturbance of blood flow through the liver consequent on the fibrosis that follows chronic liver cell damage. The liver has a dual blood supply. Arterial blood, direct from the aorta, comes via the hepatic artery from the coeliac axis. T h e second source is the portal vein that is formed by the joining of the superior mesenteric and splenic veins and which collects blood from the intraabdominal part of the gut, together with the spleen, pan­ creas and gallbladder. After passage through the sinusoids (sec below) blood drains from the liver via the hepatic veins into the inferior vena cava and thence to the right side of the heart. The portal system is responsible for the greater part of the blood supply but if either the vein or artery is blocked the other can maintain relatively normal liver function.

HEPATIC REGENERATION The fact that the liver has a great capacity for regeneration is the rationale for much of the practice of clinical hepatology, particularly for the prolonged periods of support during acute liver failure and the feasibility of extensive hepatic resection. Hepatic regeneration has been studied mainly in animals after partial hepatic resection. Twenty- four hours after liver resection there is extensive division of the re­ maining cells accompanied by a surge in DNA synthesis; this is followed 24 hours later by increased replication of

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the non-parenchymai cells. The initiation of these events is by growth factors that are now being identified. Epider­ mal growth factor (EGF), transforming growth factor a (TGF a ) and hepatocyte growth factor (HGF) are all in­ volved in the switching on of regeneration, and transform­ ing growth factor (J in it switching off. There can be little doubt thai measurement of these substances will soon form part of the investigation of liver disease.

PHYSIOLOGICAL F U N C T I O N S Because the various functions of the liver may fail at different time*, it is necessary to consider each function separately. The physiology and biochemistry of the liver subsume most of intermediary metabolism and, as such, are clearly beyond the scope of this chapter. Other than giving the briefest outline, emphasis is placed on those functions, the measurement or disturbance of which is important in liver tests or pathology respectively.

Carbohydrate metabolism As has been known for many years, removal of the liver in an animal leads to death from hypoglycaemia, A major metabolic function of the liver is to store sugar and damp down the oscillations in blood sugar consequent upon man's habit of eating intermittently. Thus, during a meal the liver stores glucose as glvcogen and then releases it (glycogenolysis) slowly when food is not being taken. This is particularly important for those organs such as brain and red blood cells that have an obligatory requirement for glucose. Specific glucose transporter molecules located in the sinusoidal membrane which mediate facilitated dif­ fusion are involved in both processes. Between meals, as the supply of glvcogen decreases (only about 75 g can be stored), the liver starts to make glucose (gluconcogcncsis) from other sources, particularly lactatc, but also pyruvatc, glycerol and alanine, although only from alanine is there a net production of glucose. During more prolonged periods of starvation the total body requirement for glu­ cose falls and demand is increasingly met by production of ketone bodies, derived from acetyl CoA. Apart from being stored, glucose can be used by the liver as an energy substrate via the glycolytic and citric acid cycles or for the synthesis of fatty acids and triglycerides. Insulin is secreted in response to the rise in blood glu­ cose concentration after a meal and leads to an increase in peripheral glucose uptake and a decrease in gluconeogenesis. In acute liver failure, the liver may not be able to maintain an adequate level of blood glucose and hypoglycacmia may become a life-threatening complication; in chrome liver failure /ryperglycacmia is more common, most likely because of a failure of the liver to store glvcogen and failure of peripheral tissues to take up glucose adequately.

The liver also metabolizes other dietary sugars including fructose and galactose, converting them to glucose phos­ phates (Ch. 13). Lipid metabolism After a meal, dietary iriglycerides are hydrolysed to free fatty acids and monoglycerides by pancreatic lipase and taken up into an aqueous medium through the action of bile salts. The liver meets its own metabolic energy* requirements, and those of the body as a whole, by mito­ chondria! P-oxidation of short chain fatty acids. The re­ sultant acetyl CoA either enters the citric acid cycle or reacts with another molecule of acetyl CoA to form ketone bodies. Although the role of the liver is central to the oxidation of fatty acids, most tissues have the enzymes required to undertake complete oxidation. The liver also synthesizes fatty acids, triglycerides, cholesterol, phospholipids and lipoprotcins. Disturbances of fatty acid meta­ bolism, including decreased oxidation (as in excessive alcohol consumption), increased hepatic fatty acid synthe­ sis and decreased triglyceride conversion, may all be involved in the development of fatty liver ('stcatosis'), a problem common to several liver disorders.

Protein metabolism Hepatic protein metabolism is central to the assessment of liver function and its disturbance important in the pathogenesis of many liver diseases.

Synthesis Other than the immunoglobulins, most circulating pro­ teins are synthesized wholly or in part by the liver and the levels of several are used as a measure of synthetic hepatic function. Except for albumin, transcobalamin II and C-reactive protein, all are glycoproteins. There is no ap­ parent general biological reason why proteins are glycated; in some (for example, fibronectin) it serves to make the protein resistant to proteolysis, in others it affects function (for example, follicle stimulating hormone) and in yet others (for example, caeruloplasmin) it affects the pro­ tein's half-life in the circulation. Disturbance in glycation produces some specific defects in protein structure that may be clinically useful in the diagnosis of alcoholic (see p. 244) and malignant liver disease (sec Ch. 13).

Metabolism of ammo acids and disposal of urta A 70 kg man on a normal diet needs to excrete between 10 and 20 g of nitrogen per 24 h. This derives from amino acids which arc surplus to requirements (and cannot be stored) and those that are not rcutilizcd after normal

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turnover. 'I"hc nitrogen is converted into urea in the liver and excreted by the kidneys. The liver processes incoming dietary amino acids from the portal vein and from muscle both for its own requirements and for peripheral tissues. Aromatic amino acids (AAA: phenylalaninc, tyrosinc and tryptophan) arc metabolized by the liver but hepatic ex­ traction of branched chain amino acids (BCAA; leucine, isolcucinc and valinc) is small and these arc taken up largely by muscle. The ratio of BCAA/AAA is decreased in acute liver failure and this alteration forms the basis of one theory of the pathogencsis of hepatic enccphalopathy. The major pathways of ammonia production and clear­ ance are shown in Fig. 12.4. Amino acids first undergo transamination to glutamate followed by oxidative deamination with the formation of ammonia. The resultant ammonia is fed into the Krebs-Henseleit (urea) cycle and excreted as urea or stored transiendy as glutamine (through the action of glutamine synthasc). Additional ammonia is produced by intestinal bacteria from dietary protein and urea present in gut fluids. These pathways are important as ammonia is toxic and is one of the compounds impli­ cated in the pathogenesis of hepatic encephalopathy. Its plasma concentration is still measured in many laborato­ ries and a raised level taken as evidence that an cnccphalopathic state is due to hepatic pathology. Measurement of the various enzymes involved in the urea cycle is increas­ ingly undertaken for diagnosis of inherited abnormalities of urea synthesis.

a-amino acid

a*oxoQiutarate

Oxo actd

Glutamate dehydr

Carbamoyl phosphate

И * . 12.4

Production and disposal o f ammonia.

The lung and kidney are effective in excreting volatile and water-soluble substances but many exogenous com­ pounds (such as drugs) and endogenous compounds, including end products of metabolism, arc lipid-soluble and non-volatile. It is one of the liver's functions to render such substances more water-soluble so that they can be excreted in urine or bile. Two stages are recognized in biotransformation. In phase I, a suitable polar group is made available which is conjugated in phase II. Phase I reactions occur in the smooth endoplasmic reticulum and are mediated mainly by the mixed function oxidase system (P450 isoenzymes) that utilize atmospheric oxygen, typically generating hydroxylatcd or carboxylatcd compounds. These are subse­ quently conjugated with glucuronic acid, acctyl or methyl radicals or, in the case of bile acids, with glycinc, taurinc or sulphate (see below). There is considerable functional heterogeneity of the glucuronyl transferases with several isoenzymes that have variable substrate specificities, particularly for exogenous compounds. A second, nonoxidative pathway of biotransformation is via glutathione. Increasingly, tests purporting to be true measures of liver function are being based on assessment of the integ­ rity of such systems by measuring the clearance of an exo­ genous compound (see p. 229). It should be noted that while biotransformation usually results in detoxification, it can sometimes be responsible for generating toxic or carcinogenic metabolites.

Bile, the exocrine secretion of the liver, consists of bile pigments, bile acids, cholesterol, phospholipids (mainly lecithin), inorganic ions and small amounts of protein. The Nucleic acids liver is a major site of cholesterol biosynthesis and the sole site of conversion of cholesterol into bile acids, which are the major organic anions excreted by the liver. The bile acids undergo conjugation which enhances their solubility at the pH of body fluids and this facilitates their main function of solubilizing biliary cholesterol and the prod­ ucts of lipid hydrolysis. Bile acids circulate from the hepatocytc, through the biliary tract and the gut and then, after reabsorption in the terminal ileum, are returned via the Glutamine portal blood to the liver (Fig. 12.5).

L-glutamate

Argmme

Biotransformation and excretion

Bile secretion

Amines

Ornithic*

22 1

CtruHme

Arginosucanate

The rate-limiting step in bile acid synthesis is hepatic microsomal 7ot-hydroxylation which is under negative feedback control, being inhibited by reabsorbed bile salts. Before secretion, bile acids arc conjugated with either taurinc or glycine and, after passage through the small intestine, are reabsorbed from the terminal ileum. Those reaching the large bowel arc partially deconjugated and converted to secondary bile acids by bacterial 7adchydroxylase to deoxycholic and lithocholic acids. A third metabolite, ursodeoxycholate (a stcrcoisomcr of

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222

ISTRY

Cholesterol

i

СпоИсас*

Cnenodoovycfic ac»d (CDCA) CDC taurme

Glycocholic acid

Fig. 12.5

Synthesis and cntcrohepatic circulation of bile acids.

chcnodcoxycholic acid), is found in trace amounts and has been classified as a tertiary bile acid. The measure­ ment of scrum bile acids has been extensively investigated as a test of liver function (see p. 228), LIVER F U N C T I O N TESTS Liver function application:

tests

CDCglycme

have

four

potential

areas

of

1. As an aid to establishing whether a particular patient has liver disease. In effect, they arc being used to answer the question 'Is there any evidence of liver dysfunction?* 2. As an aid to making a specific diagnosis. While function tests are clearly distinct from diagnostic tests it is still reasonable to assume that certain patterns of dys­ function may be characteristic of specific diseases. 3. T o establish the severity of li\fer dysfunction once a specific diagnosis has been established. This is important from a prognostic point of view. 4. T o monitor the progression of the disease and any response of it to therapeutic intervention. Within this framework three classes of tests will be con­

sidered. The first arc the standard liver function tests - a battery of tests often applied irrespective of the suspected diagnosis and to which all the caveats about lack of true functional assessment apply. The second are a group of tests that arc used in an attempt to measure function in the true sense of the word - these arc more complicated to perform and are not used routinely; an example is the measurement of galactose elimination capacity (see p. 230). The third group arc those biochemical tests used for assessment of liver disease in specific situations, for example a,-antitrypsin in suspected deficiency of this protein and alphafetoprotein in suspected primary hepatic malignancy. The latter are described briefly here for the sake of completeness and in more detail in the next chapter in the context of their diagnostic application. The standard liver function tests arc usually considered to include the plasma bilirubin concentration, the activity in plasma of certain enzymes, particularly aminotransfcrases, alkaline phosphatase and y-glutamyl transferasc, total plasma protein and albumin concentration. It has been estimated that this group of tests will correctly allo­ cate patients to a liver discasc/non-liver disease category in about 7 5 % of cases.

защ1

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ким пр

ННРА'ПС FUN* HON AND JAUNDICE

Bilinibin and bile pigment metabolism Although bilinibin has been characterized as a non-toxic metabolic product of a relatively minor metabolic path­ way, elevation in its plasma concentration leads to the alarming sign of jaundice and usually suggests the pres­ ence of underlying liver or biliary tract disease that may range from trivial to life-threatening. Accurate interpreta­ tion of the laboratory tests associated with bile pigment metabolism in the jaundiced individual requires a clear understanding of the physiology and biochemistry of the bile pigments. Most bilirubin is derived from the breakdown of haem, itself derived from senescent red blood cells (Fig. 12.6). A much smaller proportion comes from other hacmoproteins such as catalase, myoglobin and the cytochromes. An even smaller fraction comes from 'ineffective crythropoiesis* although the latter may represent a significant source of bilirubin in haematological conditions such as thalassacmia and pernicious anaemia. The initial and ratelimiting step is the oxidation of haem to biliverdin by haem oxygenase; this is followed by reduction (catalysed

Hepatic haem proteins (15%)

Haem

(Fe-protoporpnynn IX)

Effete red blood cells (80%)

^'

"y

v

N'

^

223

by biliverdin reductase) to bilirubin with the production of an equimolar amount of carbon monoxide and ferric iron. These reactions take place in the reticulocndothelial system, predominantly the liver, spleen and bone marrow. The resultant 'unconjugated' bilirubin is tightly bound to albumin in a molar ratio 1:1, but additional binding sites of lower affinity are recruited in hyperbilirubinaemic states. This binding limits extrahepatje uptake of the po­ tentially toxic unconjugated bilirubin and permits trans­ port to the liver. Other molecules such as thyroxine and certain drugs can compete for albumin binding sites and thereby displace bilirubin. Tightly bound to albumin, bilirubin is transported to the liver where it is actively transported across the sinu­ soidal membrane and binds to ligandin (glutathione transferasc B). In the hepatocyte, the bilirubin is conjugated mainly with glucuronic acid to form mono- and diglucuronides and thereby rendered water-soluble (Fig. 12.7). Secretion, which is probably rate-limiting in the overall transport of bilirubin from plasma to bile, occurs by active transport into the biliary system. In the gut some bilirubin is deconjugated by bacterial glucuronidascs and (being

Ineffective erythropoiesis (5%)

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Bilirubin (IXu) (275mg/24h)

*c'

M«me 2, in a patient who appears on clinical grounds to have a hepatitic illness, strongly suggests that alcohol is involved. The mitochondria! isoenzyme of AST (mAST) mAST accounts for about 80% of total AST activity within the liver cells. With the recent development of immunochemical methods for measurement of this en­ zyme in serum there has been considerable interest in the use of the mAST/total AST ratio as a marker of chronic alcohol consumption. Preliminary studies suggest that the test adequately distinguishes between alcoholics and normal subjects, irrespective of the presence or absence of liver disease, and is only elevated in association with chronic abuse. It may also be useful in distinguishing alcoholic from non-alcoholic liver disease. y-Glutamyl transferase (yOT) This is a microsomal enzyme responsible for transfer of glutamyl groups from Y-glutamyl pepudes to other

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HFPATIC FUNCTION AND JAUNDICE

peptides or amino acids. Although widely distributed throughout most body organs except muscle, the plasma activity is mainly attributable to the liver isoenzyme. It may be involved in the transport of peptides across cell membranes as y-glutamyl peptides. 7GT has poor spe­ cificity for liver disease and even in a patient with jaundice it adds little to the information gained from measurement of AST and ALP. However, its measurement can be use­ ful in two particular circumstances. First, when the origin of an elevated serum ALP is uncertain, a concomitant elevation in 7GT suggests that the ALP is of hepatic origin. The second relates to the controversial area of the relation of 7GT to chronic alcohol consumption. The laboratory tests for chronic alcoholism are dis­ cussed in the next chapter. Here, we attempt to synthesize what is accepted about 7GT and chronic alcohol con­ sumption. Levels of 7GT activity may be raised in patients with chronic alcoholism because of enzyme in­ duction by the alcohol and because of the liver damage. The absolute level of yGT is higher in those alcoholics with liver disease and there is a tendency for levels to re­ main high after abstinence. In contrast, among alcoholic patients without liver disease only about half will have a raised 7OT and this will usually fall to normal after 8 weeks of abstinence. The height of the 7GT activity is not direcdy related to either the amount of alcohol consumed or the duration of its consumption. It is evident from the above that the efficiency of using -^GT for screening populations for excessive alcohol consumption will be poor. False positive results occur in those taking enzymeinducing drugs and false negative results in those who do not have liver disease. Nonetheless, the finding of a markedly raised "yGT (greater than five times the upper limit of the reference range) provides good reason to enquire diligently about possible alcohol abuse. yCJ/T re­ mains the best of the simple laboratory screening tests and, depending on the population studied, the sensitivity is in the order of 50% and specificity about 85%.

5'-Nucleotidasc (5'~NT) This enzyme catalyses the hydrolysis of nucleoside 5'phosphates such as adenosine 5'-monophosphate (AMP), releasing inorganic phosphate. The enzyme is located mainly on the biliary canalicular membrane and its plasma activity is increased in cholestasis. The main advantage of its use in this context is that there is no increased activity in subjects with any of the non-hepatic conditions in which ALP activity1 is increased, including bone diseases and Hodgkin's disease or during pregnancy or periods of rapid bone growth. However, the assay is complex as account must be taken of non-specific alkaline phosphatases that catalyse similar reactions. For this reason this enzyme is now seldom measured in the investigation of liver disease.

227

Glutathione S-transferase (GST) Isoenzymes of G S T are involved in the detoxification and conjugation of several clcctrophilic compounds with glu­ tathione. As noted above» bilirubin and bile acids bind strongly to GST which is also involved in the metabolism of endogenous compounds such as some of the prostaglandins. Immunological tests are now available for assay of the various isoenzymes and type B l , in particular, seems to be a sensitive test of acute hepatocellular dam­ age. However, the assay is time-consuming and unlikely to enter clinical practice in the near future.

P l a s m a proteins The concentrations of proteins in plasma reflect the bal­ ance between availability of their precursors and rate of synthesis, release and clearance as well as their volumes of distribution, and it is therefore not surprising that in patients with liver disease the levels are very variable. Apart from albumin, prealbumin and some coagulation proteins, specific plasma proteins have little utility' in as­ sessment of liver function in general; more often they are measured in the diagnosis of specific conditions such as a,-antitrypsin deficiency and Wilson's disease.

Albumin The liver synthesizes about 12 g of albumin each day and, of the total body pool of 300 g, about 6 0 % is in the extravascular pool and 40% in the intravascular pool. The plasma half-life is about 21 days. Albumin is responsible for maintaining plasma oncotic pressure and binds several hormones, anions, drugs and fatty acids. "1Ъсгс is no doubt that as chronic liver disease progresses the plasma albumin concentration tends to fall and it is a major prog­ nostic faaor (see p. 235). Nonetheless, it must not be assumed that plasma albumin is simply an indicator of hepatic synthetic function. The rate of hepatic albumin synthesis falls in the face of inadequate protein intake. This is a frequent occurrence in patients with advanced liver disease and particularly those in whom excessive alcohol consumption is impli­ cated, riven when the rate of synthesis falls, plasma levels may remain within the reference range because of a com­ pensatory reduction in the rate of degradation. Further­ more, hypoalbuminaemia may occur in the face of normal or even increased rates of synthesis when protein 'leaks' into lymph, ascitcs or otherwise into the extravascular compartment.

frothrombin time and coagulation factors The prothrombin time (PT) is a test that has become widely used in hepatology. Quick's one-step prothrombin

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228

CUNK.AI, BIUCHEMISTKY

time measures the rate at which prothrombin is converted to thrombin in the presence of thromboplastin, calcium, fibrinogen and other coagulation factors (V, VII and X). In turn, the thrombin leads to the conversion of fibrino­ gen to fibrin. Prothrombin and factors VII, IX and X all require vitamin К to become active. Thus, there are two reasons why patients with liver dis­ ease may have a prolonged P T , each with different diag­ nostic implications. Firstly, the liver may be so damaged that it cannot adequately synthesize the clotting factors which require vitamin К for their activation. Secondly, since vitamin К is a fat-soluble vitamin, it may be defi­ cient because of impaired fat absorption when there is obstructive jaundice. The latter situation is remediable by parenteral administration of vitamin К (10 mg). Thus return of the prothrombin rime to the normal range within 18 hours may be taken as evidence of obstructive jaundice, whereas failure to respond implies severe parenchymal disease. "l*he concentration of factor V is becoming a widely used test in assessing the severity of fulminant hepatic failure particularly in continental Europe; low factor V concentration is associated with a poor prognosis.

Caertdopiasmin This protein? the principal copper-containing protein in the plasma, has oxidase activity including the fcrroxidase activity essential for transformation of Fe(II) to Fe(III). It consists of a single polypeptide chain containing six copper atoms but there is minimal turnover of the copper suggesting that it does not act in respect to copper as transferrin does to iron. In the context of liver disease it plays an important role in the diagnosis of Wilson's disease since in this condition it is virtually absent from plasma. Acuu phase reactants In response to tissue damage there is an increase in hepatic synthesis of se%'eral plasma proteins, notably Creactivc protein» antichymotrypsin, fibrinogen and caeruloplasmin. It has been suggested that this reaction helps phagocytic cells during the inflammatory response, with interleukins acting as the messenger between macrophages and hepatocytes. This topic is discussed in greater detail in Chapter 40. Bile acids

A Iphafeioprotein Under normal circumstances, the synthesis of this pro­ tein, the fetal equivalent of albumin, virtually ceases shortly after birth. The protein is synthesized in great amounts in about 70% of patients with hepatocellular carcinoma and to lesser degrees in certain other tumours and benign liver diseases (see Ch. 39).

a^Antitrypsin This protein is the major a,-globulin and responsible for 90% of plasma tryptic inhibitory capacity. a,-Antitrypsin deficiency is a major cause of chronic liver disease in children (see p. 252). Transfemn Transferrin is the major circulating iron-binding protein and its concentration correlates with the total iron-bind­ ing capacity of plasma. In states of iron overload, such as idiopathic haemochromatosis (see Ch. 13) the concentra­ tion is normal, but the saturation is between 55 and 90% compared to the reference range of 30-40%. Alcohol appears to inhibit the glycation of several glycoproteins and measurement of desialylated transferrin has been proposed as a marker of excessive alcohol consumption. Plasma transferrin has been suggested as a more appro­ priate indicator of protein synthesis in view of its shorter half-life.

The physiology of bile acids has been described in brief above. In normal subjects, the level of plasma bile acids (РВА) is determined by the difference between the amount absorbed from the gut and that taken up by the liver. The latter is fairly constant and normally» it is the amount absorbed from the gut which determines plasma concentrations. Under pathological conditions, hepatic blood flow and clearance become the determining factors so ihat measurement of РВА may be considered as an endogenous clearance test. However, РВА estimations have not been adopted in routine clinical practice. Partly this is because, until re­ cently, the tests were difficult to perform, but even with the development of rapid commercially available assays it has been difficult to show clear advantages in terms of sensitivity and specificity over conventional liver tests as described above. The results in terms of abnormal tests seem to be the same whether different techniques are used for example, radioimmunoassay for conjugated primary bile acids (glycocholic and chenodeoxycholic) or enzymic assays for 3a bile acids. It is helpful to consider the merits of РВА estimations along the same lines as those proposed for the standard liver function tests: 1. Does the test help answer the question, 4s there any evidence of liver ays/unction ? Here the answer is yes but the test is no better than measurement of one of the aminotransferases. False positive results may occur when there is bacterial overgrowth in the small intestine and when there is portosystcmic shunting. False negative results may

Материал, защиш
!02 >170

44 25 17

32 74 19 13

rvrvel is related to the depth of jaundice m patient» with cirrhosis. The q f bihrubm level* arc required to have been exceeded 1 at least 6 months i Shapiro ct aL Gut !?*>; 20: 137

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HEPATIC FVNCTION AND JACNDICE

235

Estimated probability of surviving x years 10 PI = 2.52 x log serum bilirubin + 0.0069 *exp(age-20710) ~0 05 к serum albumin (o/l) +0 88 (if arrhosis * present) ♦0.52 (if not treated with ага(пюрппе)

2 3 4 5 6 Prognostic index (Pt) Fig. 12.10 Prognosis for survival in primary biliary cirrhosis. The prognostic index is calculated from the above equation and the estimated survival read off from the accompanying graph. (Ghristcmen К ct al., Ciasirocnterology 198$; 89; 1084 91 j

median survival periods in relation to the plasma bilirubin level or changes therein (Table 12.2). Since then» more sophisticated equations have been derived from multivariate analysis of large patient databases. In general, the statistical analysis generates a formula from which a 'prog­ nostic index' (PI) can be calculated. The PI may then be translated into a figure representing the probability of sur­ viving a certain number of years. One example of a prog­ nostic index used for primary biliary cirrhosis is illustrated (Table 12.2 and Fig. 12.10). The standard liver function tests feature prominently in all indices derived from patients with chronic liver disease to date. In general, plasma albumin and prothrombin time are the best indicators of long-term survival. Plasma bilirubin elevations are, in most forms of chronic liver disease, a late occurrence and also of poor prognostic significance. Acute liver failure Liver transplantation for acute liver failure is a more re­ cent development but again laboratory investigations are important in assessing the likelihood of survival without transplantation. For example, in patients with paraceta­ mol-induced acute liver failure an arterial pH of < 7 . 3 or, if this feature is absent, the combination of grade 3 enccphalopathy, a prothrombin time > 100 sees and plasma

IURTHKRRKADING

Branch R A. Drug* as indicator* of hepativ function. Hepalology 1(>И2; 2: 97 105. A refietv of the measurement of the clearance of endogenousty administered suhnjriL*\ as Г0Г.1 of hver funelitm. Chri&tensrn L, Neuberger J M, Crowe J» Doniach D, Popper H» WiJJjams R. Beneficial effect of azathiopnne and prediction of

crcatinine of > 100 |imol/L indicate a very slight chance of spontaneous recovery and represent a strong indication to proceed to transplantation. SUMMARY There are three categories of liver function tests. These are the standard tests, e.g. plasma bilirubin and albumin concentrations and the activities of various enzymes; clearance tests, and thirdly, tests used in the management of specific liver diseases. In general, the standard liver function tests are poor indicators of hepatic function and seldom provide a spe­ cific diagnosis; it is important not to ovcrintcrpret their results. They may, however, provide a guide to further investigations which may give a specific diagnosis, for example, imaging by isotopes, ultrasound or radiography or histological examination of biopsy material. The stand­ ard liver function tests can be of value prognosticaliy and in screening for liver disease and are vital in the monitor­ ing of liver disease and its response to treatment. Many clearance tests, designed to be true tests of func­ tion, have been devised. All are cumbersome to perform and none has been accepted into routine practice. Tests for specific liver disease, e.g. plasma alphafetoprotcin, plasma copper and cacruloplasmin, can be diag­ nostic and are also used in management.

prognosis in primary biliary cirrhosis Gastrocntcrology 19R5; 89: 1084-91. Describes the use of hver function tests m a model of survtvai for patients with pnmary biliary cirrhosis. Clcrmom R J, Chalmers T C. 'I"hc transaminase tests in liver disease. Medicine 1967; 4o: 197 207. The ciassie paper on the subject of ammotrans}erases as a test of hver function.

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шенный авторски*

236

UiNICAl. HIOOIKMISTKY

Ferraris R, Colombatri G, Fiorentini M T, Carosso R, Arossa W, dc la Pierre M. Diagnostic value of scrum bile acids and routine liver function tests in hcpatobiliary diseases. Sensitivity» specificity and predictive value. Digestive Disease and Sciences 1983; 28: 129-46. Gamucto J J, Chianalc J. liver cell heterogeneity and Jiver function. In Ала I M, Jacoby W B, Popper H r Schach'ter D, Shafritz D A (eds.j The liver: biology and pathology-, 2nd edn. New York: Raven Press, 1968. The title of this referenet is stlf-explanatory. Greenfield S M. Soloway R D, Ganthcrs R I- el al. Evaluation of postprandial serum bile acid response as a test of hepatic function. Digestive Disease and Sciences 1986; 31: 785 91. Hayes P C, Houchier I A D , Beckett G ]. Glutathione S-transferase in humans in health and disease. Gut I 9Q I; 32 8 14 18 Huffman A. The aminopyrine dcmethyUnon breath test and the serum bile acid level: nominated but not yet elected to join the common liver tests. Hepatology 1982; 4: 512-17. Hulcranu R, Glaumann II, lindbcrg G, Nilsson S. liver investigation in 149 asymptomatic patients with moderately elevated activities of scrum aminotransferases. Scandinavian Journal of Gastroenterology 1986; 21: 109-14. An important stiuh discussing th* Mgmfitante of I.FT abnormalities detected in asymptomatic imimJuali. Kaplan M M. Scrum alkaline phosphatase another piece is added to the puzzle. Hepatology 19Я6; 6: 526-8. Describes the history of th< use of serum alkaline pitosphatase anJ updates Keidmg S. Galactose clearance measurements and liver blood flow. Gastroenteroiogy 1988; 94: 177-81. Lumeng I.. New diagnostic markers of alcohol abuse. Hepatologv 1986; 4-742 V Reviews the uie of mttochowhral AST Marks V. Clinical pathology of akohol. Journal of Clinical Pathology 1984; 36: 365 78.

A comprehensive nrtinr of the pathological effects of alcohol consumption. Moussavian S M, Becker R -4>cclusive disease

f -прврщюис

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Pyogenu liver аЫссм

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Aummmum Autoimmune chronic active \ persistent hepatitis Pnmarv Nliar

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Мйи&теош Potycysoc liver disease Congenital hepatic fihros» Amyloid

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238

t IJNH'AI. nilKMI-MlSTRY

compensated' implies that the liver is failing in some vital function. The terms are not well defined but the develop­ ment of ascites, jaundice or hepatic encephalopathy, in a patient known to have liver disease, would all be regarded as signs of decompensation. Finally, the presumed evolu­ tion of the disease in time is given by the terms of fulmi­ nant, acute, subacute or chronic. ACUTE HEPATITIS AND ITS SEQUELAE '1Ъе term 'hepatitis' is not synonymous with viral hepati­ tis. It simply means that there is hepatic inflammation for which there are several causes: viral infections, drugs (including alcohol) and toxins. 1Ъс standard 'liver func­ tion tests' (LFTs) arc involved in determining that acute hepatitis is present, how severe it is, documenting the natural progression of the disease and assessing response to therapy. By contrast, their role in assessing the aetiol­ ogy of the hepatitis is very limited. When the patient presents with an acute hepatitis, the liver tests characteristically show the so-called 'hepatitic' picture: • a pronounced rise in aspartatc or alanine aminotransferase (AST, ALT) activity, often to over 1000 i.u./I^ • a modest (less than twice the upper limit of the refer­ ence range) increase in plasma alkaline phosphatase (ALP) activity; • bilirubinuria and, in the more severe cases, hyperbilirubinaemia which is often manifest as jaundice. As a very broad generalization the plasma activity of the aminotransferases reflect the severity of the disease as assessed histologically in patients with viral hepatitis. Cenainly, patients who progress to fulminant hepatitis usually have exceptionally high levels. The features of acute alcoholic hepatitis and drug-induced hepatitis are described under their respective headings and acute viral hepatitis immediately below. Acute viral hepatitis After a variable incubation period (the duration of which depends on the nature of the virus and the viral load) there is usually a rise in the activity of the plasma amino­ transferases, although it is said that some infants can be infected without any disturbance of LFTs. In many cases, particularly among young children, this is the only indica­ tion of the hepatitic process and the patient remains asymptomatic. When symptoms do develop they are coin­ cident with the maximal aminotransferase levels. The pa­ tient begins to feel unwell, tired, depressed and anorexic, and becomes pyrcxial. The urine may become dark (due to bilirubinuria), the stools pale and in more severe cases, jaundice becomes evident a few days later. The liver is enlarged and tender but symptoms tend to resolve with

the onset of jaundice. The jaundice usually subsides over a few days but in type A hepatitis the patient may, despite being asymptomatic, enter a cholestatic phase. This is heralded by rising plasma activities of alkaline phos­ phatase and Y-glutamyl transferase which may persist for many weeks, causing considerable diagnostic confusion. Particularly in the older patient, it is an indication for ultrasound examination to rule out any obstruction to the biliary tract. The jaundice usually abates before amino­ transferase activity returns to the reference range but it has been a frequent observation that bilirubin may dis­ appear from the urine whilst the patient remains clinically jaundiced. "I"he explanation for this situation lies in the development of'bili-albumin* (sec p. 225). Differential diagnosis 1Ъегс arc two differential diagnoses to be made. The first is to distinguish between viral hepatitis and other, nonviral causes of a hepatitic picture and then, within the former group, to identify the virus. Generally speaking, the standard LFTs are not helpful in separating viral from non-viral causes and histological examination or imaging is required for a definitive diagnosis. Plasma aminotransferase levels are not usually grossly raised in alcoholic hepatitis; in this condition a ratio of AST: ALT of greater than 2 is characteristic while in hepa­ titis due to other agents the ratio is usually less than 2. In fulminant hepatitis, aminotransferase levels may be close to the reference range by the time patients reach hospital, emphasizing that results of LFTs can change rapidly with rime and depend on the particular clinical course that ensues. It is said that hepatitis В infection is more severe than hepatitis A and that biochemical abnormalities are more protracted. Raised plasma IgM concentration and atypical lymphocytes are more characteristic of type A, but the standard LFTs do not permit distinction between the different types of viral hepatitis. The specific diagnosis is established by serological tests for the hepatitis viruses (Table 1 3.2). The extent to which individual cases require biochemical and serological in­ vestigation depends on the clinical situation. For example, in children, a clinical diagnosis can be established with reasonable certainty if the clinical features are compa­ tible with infectious hepatitis, particularly if the physician knows of local occurrence. In adults, particularly where there has been no contact with infectious hepatitis, LFTs arc usually performed together with appropriate serology. Outcome of actiU vtraJ hepatitis There are three main possible outcomes of acute viral hepatitis, each with its own characteristic progression of clinical features and results of LFTs. Complete resolution As noted above, the plasma

Материал, защищенный авторским правом

ACUTE AND CHRONIC LIVER DISEASE Table 1 3 J

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ТУрС of virus

\l\\

Picornaviru*

IIBV

1 lenadnaviru* CDNA

HIT

i

rai hepatitis caused

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iiise* NANB

Incubat: period

Spread

Progression 10 chronic liver •liseasc

Other features

1 О WC4

Facco-oral

No

Raised IgM. Uypb lymphocyte

6 weeks - 6 months

Parent era!

Yes

2 2tt weeks

Parent с ral

Vcs

Mainly parcnteral Facco-oral

Yes

A)

: Iff .iruWRNA)

HDV D 0

RNA, Ml:

I nccnain

HEV

depend fold 1 tffchrtnu (RNA)

5 Id weeks

aminotransferase activities start to rise before the onset of jaundice and often before hepatomegaly is detectable. Levels begin to fall coincidental!}* with the onsci of jaun­ dice and symptoms and aminotransferase activity returns to normal (AST before ALT) at the same time as, or shortly before, the plasma bilirubin. This is the progres­ sion in the vast majority of cases and many patients will be unaware that they have had acute hepatitis. Progression to chronic liver disease Classically, this mode of progression is limited to viral hepatitis types В and С but there are occasional reports of type A trigger­ ing autoimmune chronic active hepatitis and a relapsing course for hepatitis A is also well described. Persistence of symptoms, signs and/or abnormal liver tests, particularly aminotransferase activity in the range of 2-10 times the upper reference limit, for more than 6 months constitutes» by definition, chronic hepatitis (see p. 240). The accom­ panying changes in viral serology are complex and beyond the scope of the present discussion. Progression to acute liver failure Very rarely (in less than 1% of cases) any type of viral hepatitis may pur­ sue a fulminant course. This course of events is character­ ized by grossly elevated plasma aminotransferase activity, an increasingly prolonged prothrombin time and the de­ velopment of hepatic encephalopathy which may occa­ sionally develop before jaundice becomes a prominent feature. ЧЪс prognosis is poor. The syndrome of acute liver failure, one cause of which is acute viral hepatitis, is described in more detail below. ACUTE LIVER FAILURE (ALF) Acute liver failure implies the development of severe hepatic dysfunction within 6 months of the first onset of liver disease and in the absence of any pre-existing liver disease. Hepatic encephalopathy and a prolonged and persistent increase in prothrombin time arc the character­ istic features and where these occur within 8 weeks of the first symptom the condition is known as fulminant hepatic

No

Rjiwd !«t»it

IgM class anu-HAV JIHsAg* I g M c l a » anti I Ilk Anti-IKA' antibodies Detection of HCV-RNAby P C R IgM class anti-Ill >V

Ann Ml л (ши vet routinely available)

failure (FHF). In those who survive there are no longterm hepatic sequelae. The commonest cause in the UK is paracetamol overdose (Ch. 37) followed by viral hepa­ titis, types А, В and non-A, non-B (NANB). Rarer causes include drug reactions, other viruses, Amanita mushroom poisoning, Wilson's disease and secondary hepatic malignancy. Laboratory features *1Ъс laboratory investigation involves determination of the cause of the ALF and the characterization of the conse­ quences and complications of ALF which are very similar irrespective of the aetiology. Measurement of blood con­ centration of paracetamol is of crucial importance, par­ ticularly in view of the current evidence that treatment by acetylcysteine infusion may be useful up to 36 hours after ingestion. 1Ъс serology of viral cases is complicated be­ cause the hepatitis В surface antigen may disappear before the patient reaches hospital. Generally speaking, even when a condition such as Wilson's disease is detected it is too late for specific treatment (i.e. penicillamine) to be effective and management is by general supportive measures and, if feasible, liver transplantation. The standard LFTs show a grossly hepatitic picture at the time of onset of the encephalopathy. Jaundice is deep and progressive and AST levels of several thousand are usually found, although these may have fallen consider­ ably by the time the patient is sent to a referral centre. A rare cause of F H F is massive infiltration with secondary malignancy. This is characterized by hepatomegaly (whereas in most other causes the liver is small) and a much more cholcstatic picture so that the ASTrALP ratio is less than 4 compared to greater than 4 in most other cases. Coagulation defects arc a consistent finding. Levels of fibrinogen are decreased as are those of factors II, V, VII, IX and X. These are reflected in a prolonged prothrombin time which, in the UK, is used as the main measure of

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240

< i INK \i им

severity of disease and is the most widely used parameter for following progress. Hypoglycaemia, due to a combina­ tion of impaired gluconeogenesis and glycogen breakdown and synthesis, is such a consistent finding that glucose infusion is a routine part of management particularly during transfer to a referral centre. Where paracetamol is involved the paracetamol may interfere with blood glucose estimation giving erroneously high values. Acute renal failure is a frequent and ominous complication which oc­ curs in at least 50% of cases, particularly with paracetamol poisoning. Hypcrbilirubinacmia interferes with crcatininc measurement, producing erroneously low results, and precipitation of bilirubin and plasma protein is required for an accurate estimation. In about half the cases the renal lesion is acute tubular necrosis and in the remainder 'functional' renal failure (see p. 246). Laboratory criteria for liver transplantation Overall, with the best supportive care, 20-30% of patients with fulminant hepatic failure will survive; the figures are rather better for those with paracetamol overdose and rather poorer for those with viral hepatitis, particularly those classified as NANB or drug-related. This bad prog­ nosis has led to the introduction of liver transplantation as a therapeutic option. It is clearly important to reserve this option for those with the worst prognosis and who are most unlike!v to recover with other treatment. With this in mind a number of laboratory-based criteria have been identified to ascertain the likelihood of death. The major adverse factors are an arterial hydrogen ion concentration > 50 nmol/L (pH < 7.3) and progression to grade III coma (see p. 244) with a prolongation of the prothrombin time to more than 100 s and plasma crcatininc concentration > 300 цто1/Ь. Recently, there has been considerable

iblc 13.3

i! J J J ; luclciir a n

Type

I .abora

Viral type В rypcC

MBsAg Am, H C \

Alcoholic

Drop -Antitrypttsn deficiency WtUon'* ditcstc

Amoimmu rypc 1 rypc2

interest in the measurement of plasma factor V concentra­ tion. If the ratio of factor V:factor VIII is very low the chance of survival without transplantation is very small. C H R O N I C HEPATITIS Chronic hepatitis is diagnosed when clinical or bio­ chemical features of liver disease persist for more than 6 months. On histological grounds the condition is split into two categories, chronic persistent hepatitis (CPH) and chronic active hepatitis (САН). In the former the inflam­ mation is confined to the portal tracts whereas in the latter, the inflammatory cells spill out into the hepatic parenchyma leading to nibbling away of the penportal hepatocytes, so-called piecemeal necrosis, the hallmark of chronic active hepatitis. CPH is of little clinical signifi­ cance, being detected in several conditions and in patients with САН who arc in remission. On the other hand САН carries a graver prognosis and may progress to cirrhosis. The inflammatory activity may continue even when cir­ rhosis has developed ('active cirrhosis'). The term chronic active hepatitis is best considered as a histological diagno­ sis for which there are several quite distinct causes and laboratory investigation plays a crucial role in the differen­ tial diagnosis and management of these conditions (Table 13.3). Following acute hepatitis, the presence of abnormal LFTs, particularly aminotransferascs, defines the pro­ gression to chronicity. Patients with untreated САН, re­ lated to any of the conditions described below, have a characteristic pattern of standard LFT results. Typically the aminotransfcrase activities are markedly raised, in the range 200-1000 i.u./L, with deep jaundice when the in­ flammation is severe. The activity of plasma alkaline phosphatase is usually only minimally increased, but changes

of chronic active her» г-kidney imcrosomaj an ostt

H< V K N A b v l ' C R

Blood.. •!, >GT. M< v dcsialylatcd irantferrin

Comment»

Ma* characti

hiuological features

I ahoratory test» are supplementary

ю distal hi*tory

May develop nuiounubodics

Oxyphcnitann. mcthvldtipa, noniazid. dantmlenc, с

Low u , - A T . typical ptllQOtypC P i / /

Character on histology

Low caerulopUftmin, high tissue copper and urinary copper oocentra

Low ALP ace low AST activity for degree of inflammation

Ann SWA and/or ANA Anti-I KM aotabodirt

Markedly rawed f-gJobulm and IgG

-оыпоргиЬс globules

ким np

ACUTE AND CHRONIC UVF.R DISEASE

may become more pronounced as cirrhosis develops with its associated architectural distortion. The prothrombin time is often mildly prolonged and plasma albumin con­ centration is in the low normal range. Particularly in the presence of hypoalbuminaemia, plasma globulins are raised, most notably when there is an autoimmune aetiol­ ogy. It should be noted that a typical phase of acute hepa­ titis is not always prominent in many of the conditions described below.

the procedures for handling potentially dangerous and in­ fectious samples such as those earning hepatitis viruses is not the concern of this chapter, it is worth noting that in inner city areas as many as 1-2% of the population may be chronic carriers of either HBV or HCV. The majority of these will have no symptoms of liver disease and their blood may be examined in the laboratory for illnesses unrelated to liver disease.) Antiviral treatment of chronic viral hepatitis is a rapidly growing area of hcpatology in which firm guidelines about monitoring response are not yet available. In the case of HBV infection the aim has been to inhibit viral replica­ tion, usually with interferon, and those with high initial levels of aminotransferase are most likely to respond. Suc­ cessful treatment is heralded by a change from hepatitis *e' antigen to V antibody positivity and this event is accompanied by an acute hepatitic illness, the so-called 'hepatitic flare* (Fig. 13.1). In the case of hepatitis С treatment with interferon, a fall in plasma aminotransferase levels is being used as an index of remission. A side-effect of interferon treatment is hypothyroidism and thyroid function tests should be monitored before treatment and at 3-monthly intervals.

Differential diagnosis of chronic active hepatitis Viral hepatitis types В and С The specific diagnosis is based on virological tests: in the case of hepatitis В virus (HBV), by the detection of hepa­ titis В surface antigen (HBsAg); in the case of hepatitis С virus (HCV), by detection of antibodies to the virus. The serological diagnosis of HCV infection is much more diffi­ cult than HBV and is an area of intense current research activity. At the time of writing the gold standard for a diagnosis of chronic HCV infection is the detection of HCV-RNA using the polymerasc chain reaction but this is available in only a few specialized laboratories. (Although

Interferon 50 mm x 2

I - 5 0

i

1 5

1

i

1

1

1

Г

10

15

20

25

30

35

Weeks Fig. 13.1 Changes in ALT activity in a patient with chronic hepanm H virus infection successfully treated with interferon. Note the hepatitis response heralding clearance of HBsAg and appearance of the antibody. Clearance of the virus is also indicated by a decrease in viral DNA polymerasc activity.

i
1:40) of organ non-specific autoantibodies (smooth muscle anti­ body (SMA), antinuclcar antibody (ANA) and anti-liverkidney microsomal antibodies). Plasma y-globulin levels (mainly IgG) are grossly raised, sometimes approaching lOOg/L. Monitoring response to therapy Once the diag­ nosis of AI-CAH has been established, the standard liver tests are used to monitor progress. With appropriate immunosuppressivc therapy the LFTs, particularly aminotransferase activity should return to within the reference range and should then be kept in this range for at least 2 years before any attempt is made to withdraw therapy. Nonetheless, it should not be assumed that a normal aminotransfcrasc activity excludes continuing inflammatory activity and it is as well to confirm this histologically. On the other hand, particularly after withdrawal of treatment» a rapid rise to more than the upper limit of the aminotransferase reference range is a good indicator of relapse.

This is a chronic cholcstatic condition of unknown cause in which there is destruction of septal and interlobular bile ducts. Most patients present with pruritus, jaundice or non-specific symptoms including tiredness and hepatic pain. However, an increasing number are detected inci­ dentally when abnormal liver tests, particularly an in­ creased activity of alkaline phosphatasc, arc detected during opportunistic screening. The LFTs reveal a characteristic cholestatic picture with, as the disease progresses, increasing plasma alkaline phosphatase (of biliary origin), increasing bilirubin con­ centration and falling albumin. The plasma concentrations of total IgM and monomeric IgM are frequently raised. The most specific serological tests relate to antimitochondrial antibodies (AMA) which are detectable in about 95% of cases. Recently it has been recognized that AMAs recognize several distinct autoantigens. The most characteristic for PBC are the so-called M2 which react with epitopes on three mitochondrial enzymes. These are the E2 subunit of the pyruvate dehydrogenase complex (PDH-E2), the E2 subunit of the branched chain oxoaciddehydrogenase (BCOADH-E2) and the E2-oxoglutarate dehydrogenase complex, laboratory tests, particularly the plasma bilirubin concentration, are used in assessing prognosis (see p. 235).

! |

PRIMARY SCLEROSING CHOLANGITIS (PSC) This is a progressive disease characterized by diffuse in­ flammation and fibrosis of the biliary system which leads to obliteration of the intrahepatic ducts and, eventually, biliary cirrhosis (see below). With the advent of endoscopic retrograde pancrcatography (ERCP) the condition is being increasingly diagnosed. It is diagnosed on the basis of characteristic cholangiographic appearances together with compatible clinical, biochemical and histo­ logical features and exclusion of several other conditions known to cause secondary sclerosing cholangitis such as biliary calculi and previous biliary tract surgery. Primary sclerosing cholangitis is also being diagnosed earlier in its natural history, particularly by the finding of persistently abnormal LFTs in patients with inflammatory bowel disease which coexists in more than 50% of cases. Consequently, in the early stages there is usually a minor elevation of aminotransferase and moderate elevation of

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ACITE ANO CHRONIC UVF.R DISEASE

ALP which progresses, over several years, to a very severe cholcstatic condition with deep jaundice, ALP activity often over 10 times the upper limit of the reference range and ultimately death from liver failure. Rapid clinical de­ terioration with a progressive cholcstatic picture may indi­ cate the development of cholangiocarcinoma, a frequent complication of this condition. Copper retention appears to be a general feature of chronic cholestatic conditions and in PSC most patients will have raised hepatic copper levels (of a similar order to that seen in Wilson's disease and primary biliary cirrhosis) and urinary copper concen­ trations will also be raised. However, in contrast to Wilson's disease, the caeruloplasmin and serum copper concentrations will be within the reference range (see p. 251).

ALCOHOLIC LIVER DISEASE Ethanolic liver disease is by far the most common cause of liver disease in the Western world although it often exists as part of a wider spectrum of social, psychological and pathological effects of alcohol-related damage. Most current evidence suggests that alcohol itself is the primary pathogenetic factor although the associated malnutrition may be a contributory factor. Considering that there is a vast variation in the actual amount of alcohol drunk by different individuals, the time course over which it is drunk and the individual susceptibility, it is not surprising that the consequences of excessive alcohol consumption vary widely.

Ethanol metabolism Alcohol is absorbed from the stomach and small bowel. Absorption is most efficient when there is no other food (particularly carbohydrate) in rhe gut and when its con­ centration in the ingested fluid is of the order of 20%. The liver is exposed to the highest concentration of alcohol in blood and is responsible for more than 9 5 % of its metabolism. The concentrations attained after ingestion of a standard amount of ethanol depend, amongst other things, on sex, weight, previous exposure to alcohol, the type of alcoholic be%'erage and rate of gastric empty­ ing. Ethanol is metabolized (oxidized) to acetaldehydc mainly by the cytosolic enzyme alcohol dehydrogenase but also, particularly at high levels, by the cytochrome P450 system. Acetaldehydc is particularly toxic. It is meta­ bolized by aldehyde dehydrogenase in the mitochondria to acetate which is, in rum, oxidized by peripheral tissues to carbon dioxide and water. Both enzyme systems can be induced by alcohol. The intoxicating and metabolic effects of alcohol are mediated directly by ethanol; it is likely that acetaldehydc is an important factor in causing tissue damage.

243

Types of liver pathologyThree patterns of histological change in the liver tissue have been described in association with ethanol ingestion. Fatty liver (steatosis) appears to be a response to excessive alcohol consumption in all individuals. It seems seldom to progress to chronic liver disease and resolves with abstinence. In alcoholic hepatitis the appearances are of steatosis, with Mallory bodies, megamitochondria and creeping pericellular fibrosis, particularly involving zone three. This is a serious condition with a marked propen­ sity to progress to fibrosis and cirrhosis, the third and final stage of chronic alcoholic liver disease. The occasional association with the histological picture of chronic active hepatitis has already been mentioned. A significant per­ centage of subjects with genetic haemochromatosis are alcoholic (see below), as are most subjects with porphyria cutanca tarda. The metabolism of ethanol also predisposes to various metabolic problems including hypoglycaemia and nonrespiratory acidosis; these are discussed elsewhere in this book.

Biochemical and histological associations Alcoholic steatosis Biochemical changes arc mini­ mal and consist of subclinical hyperbilirubinaemia and a very mild elevation of plasma aminotransferase activities. Very occasionally, a cholestatic syndrome develops but associated alcoholic pancreatitis leading to biliary ob­ struction should be considered if jaundice is marked. Episodes of delirium tremens and alcoholic myopathy may give markedly raised aminotransferase levels. Plasma y-glutamyl transferase activity is elevated in the majority of cases, but usually reflects enzyme induction rather than hepatic injury. Alcoholic hepatitis There is a very wide spectrum of severity associated with this histological diagnosis, ranging from the classic syndrome of deep prolonged jaundice, hepatic failure, fever and leukocytosis through to a com­ plete absence of symptoms and physical signs. Alcoholic hepatitis generally occurs after very heavy bouts of drink­ ing and when subjects have been drinking heavily for sev­ eral years. The laboratory tests reveal an anaemia, usually with a leukocytosis and, very consistently, raised levels of aminotransferases. However, the values of AST arc sel­ dom above 10 times the upper limit of the reference range and ALT values are usually lower. This results in an AST:ALT ratio of > 2 and a value below this in subjects with hepatitis strongly suggests that alcohol is not a major factor. Alcoholism and haemochromatosis (see p. 250) Iron overload is common in alcoholic liver disease, occurring in perhaps 50% of cases. Occasionally this may progress to a similar degree to that seen in genetic haemo-

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ленный asTOf

*м правом

244

CUNKIAI. BHK:III-:MISTKY

chromatosis and the patient may have similar metabolic and endocrine manifestations. More commonly, it ap­ pears that excessive alcohol consumption leads to the un­ masking of hacmochromatosis in patients with the genetic disease. Porphyria cutanea tarda As with hacmochromato­ sis, this genetically determined condition, comprising bullous skin lesions on exposure to sunlight, liver disease and iron overload, is often brought to medical attention by excessive alcohol consumption. The underlying biochemi­ cal lesion is a deficiency of uroporphyrinogen decarboxylase and this results in high hepatic porphyrins, a high level of uroporphyrinogen in the liver and increased coproporphyrin in the faeces.

U*e of laboratory tests in clinical practice There arc three questions of fundamental interest to the physician: 1. In a person who is known to drink excessively, is liver disease present and, if so, of what nature and sever­ ity? The detection of raised activity of aminotransferases ■iir 7GT does not correlate well with the degree of liver damage as assessed histologically. Over 50% of patients with only minimal liver disease on biopsy have raised plasma aminotransfcrasc activity. Consequently, histological examination of liver tissue is essential to determine the degree of liver damage. It is also noteworthy that pa­ tients with advanced hepatic cirrhosis who subsequently abstain from alcohol may have normal LFTs. 2. In a patient who is known to have liver disease, yet also denies excessive alcohol consumption, is it possible that covert alcoholism is responsible? Although failure to detect alcohol in the blood is not particularly helpful, detection in patients who deny drinking is. A fall in 7GT activity during hospital admission is also suggestive, as is an AST:ALT ratio of > 2 . It should be noted that other abnormalities frequently seen in alcoholic liver disease, such as macrocytosis, may also occur in other liver dis­ eases or during treatment with drugs such as azathioprine. It has been claimed that estimation of the ratio of desialylatcd transferrin to total transferrin is highly specific (81%) and sensitive (97%) for chronic alcohol abuse. At present the assays are not routinely available as clcctrophoretic separation of the transferrin isoproteins is re­ quired» but if these problems can be overcome the test may prove valuable. 3. Among a group of apparently healthy members of a population, are there some who are drinking to the extent that their work performance will be affected and who will shortlv be at risk of alcohol-related medical and social problems? 7GT and mean corpuscular volume (MCV) are perhaps the most reliable tests.

T H E C O N C E P T OF CIRRHOSIS Cirrhosis is not primarily a clinical diagnosis but a patho­ logical description of the liver in which there is: • diffuse hepatic fibrosis; • nodular regeneration; and • a disturbance of the normal hepatic architecture, i.e. a distortion of the normal relationship of the portal tracts to the central veins (Ch. 12). It is the end result of several chronic liver diseases which are usually, but not always, associated with recur­ rent episodes of cell death and attempts by the liver at regeneration. Liver function is disturbed, not usually because of the cirrhosis per sc, which has little effect on standard liver function tests, but because of the initiat­ ing agent such as alcohol., an ongoing immunologically mediated liver cell damage and the capilliarization of the sinusoids due to the fibrosis (see Ch. 12). Efforts to make this diagnosis on any grounds other than histological are unreliable. A great part of clinical hepatology is taken up with the diagnosis and management of the complications of cirrho­ sis: hepatic encephalopathy, ascites and the hepatorenal syndrome, infections, primary hepatic malignancy and endocrine dysfunction Hepatic encephalopathy This is seen in patients with advanced cirrhosis and a precipitating factor such as a large protein meal, infection or electrolyte imbalance can usually be identified. It is particularly frequent in subjects who have undergone a portocaval shunt for treatment of portal hypertension. As with the encephalopathy associated with acute liver failure, the severity of the condition is graded from I-IV: Alert but with astcrixis ('hepatic flap1), inversion of sleep rhythm II Confusion, disorientation, inappropriate behaviour III Restless, sleepy, uncommunicative IV Coma I

The diagnosis is essentially a clinical one and although characteristic EEG appearances are described, these arc not used diagnostically. Similarly, the blood ammonia concentration is usually raised but correlates only poorly with the degree of encephalopathy. Occasionally, estima­ tion of blood ammonia may be useful when the source of coma is clinically uncertain. High levels of ammonia are also seen in inherited disorders in which there are deficiencies of urea cycle enzymes. Ascites

Ascites, the excessive accumulation of extracellular fluid

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ACUTE AND CHRONIC LIVER DISEASE

in the peritoneal cavity (over 30 L in some cases), is usually a late complication of cirrhosis. The precise mechanism is still controversial, but the primary event is probably sodium retention (due in part to secondary hyperaldostcronism), compounded by hypoalbuminaemia and localized to the peritoneal cavity by portal hyperten­ sion. Thus the urine may become virtually sodium free and, so long as sodium intake exceeds output, fluid will continue to accumulate. Ascites usually develops in pa­ tients with advanced, decompensated disease, but the tendency to retain salt is present before ascites actually develops and a sudden increase in dietary salt intake may lead to the generation of ascites which clears spontane­ ously when the salt intake is restricted. Excess water re­ tention leading to hyponatraemia is frequent and probably attributable to increased concentrations of antidiuretic hormone. ЧЪегс arc, however, several liver and non-liver diseases other than cirrhosis (Table 13.4) which can lead to the formation of ascites and the establishment of a pre­ cise diagnosis is important for its correct management. If the standard LFTs are abnormal in a patient with ascites, then it is likely that one of the liver diseases listed in Table 13.4 is responsible and it is often already known that a patient who develops ascites has cirrhosis. How­ ever, patients with liver disease can develop ascites for reasons other than cirrhosis. For example, patients with alcoholic cirrhosis may develop tuberculous ascites, pan­ creatic ascites and also 'malignant' ascites if hepatocellular carcinoma supervenes. Cardiac causes can usually be

Table n.4 H and non-1 dJ ' romc i* thi

Non-hcpanc d a m

CMxM ■ ivpc) Hu.l.t < "hiaл syndrome Poml von thrombosis Acute and subacutc liver failure

Abdominal malignancy Cardiac failure instrictrve pericardium itoml pecially tuberculous) Pancreatic disease

T a b l e П.5 reprcsci

P cape-

diagnosed confidently on clinical grounds, but the other causes may be difficult to distinguish and laboratory investigation can be useful in such cases. Using a hollow needle and syringe, it is simple to aspirate some ascitic fluid (a so-called 'diagnostic tap') for analysis. The inves­ tigations and their interpretation arc shown in Table 13.5. It should be recognized that sodium retention is not a diagnostic feature of cirrhotic ascites; equally intense sodium retention is seen in 'malignant* ascites. The figures for ascitic protein given in Table 13.5 arc given only as guidelines. Exceptions are so frequent that many would consider the test useless. However, as the changes become more extreme, so their diagnostic speci­ ficity increases. Thus a protein concentration below 10 g/ L is not infrequent in uncomplicated cirrhosis, but virtu­ ally rules out malignant disease. Conversely, protein con­ centrations above 35 g/L arc the rule in malignant ascites and are unusual in uncomplicated cirrhosis. Several tests have been proposed to increase the specifi­ city of total ascitic protein measurement in differentiating between cirrhotic and 'malignant* ascites. These include calculation of the plasma to ascites albumin gradient and measurement of ascitic lactate dehydrogenase activity or ascitic cholesterol concentration (Fig. 13.2). Adcnosinc dcaminasc activity in ascites of more than 60 i.u./L is par­ ticularly sensitive and specific for tuberculous ascites. Monitoring treatment of ascites Ascites due to cirrhosis is usually managed by a combina­ tion of dietary salt restriction and diuretic therapy, though paracentesis with albumin infusion is now being used more widely. The aim should be to achieve a net fluid loss of 500 mlV24 h until the ascites clears. Plasma urea, crcatininc and sodium and potassium concentrations should be checked at least once a week at the Stan of treatment. A rising plasma urea or crcatininc and falling plasma sodium concentration (irect

(pection

Protein content (g/L)

Cell*

Bacteria

< 5007mm * (moiiotnu'icjr . Malignant cell* nd red Ыо- : White b ССШ (mainly lympho nc

None

Uncomplicated Canted

Clear

25

| ibcrculous pern ;

Turbid

>25"

Abdominal lymphoma

Chylou*

let below

None Acid-faat bacilli None

• T h e charactcnstic Laboratory feature i 50%, and usually > 60%, saturated. Serum ferri-

hicmochrumau»u» and accondary tmn ■ > tin reference rang-

гфсоажк get. tu hacmov h n»m u t osia

Asymptomatic gcnci hacmochromatos»

Secondary iron overload

Non-spccifical! v abnormal ►30

N

Variable

Г>рка0у > 30

>30

N

but may be N N

N

> 5 0 % , often >6Q

>50%

> 60% hut olten N

» 5 0 0 . o f t e n >5000

90O-200Q

< 500, often N

г

ACUTE AND CHRONIC 1JVER DISEASE

tin concentration» which is proportional to the iron excess, is usually over 1000 pg/L (upper limit of reference range, 200 Jig/L) and starts to rise when hepatic iron stores exceed twice the reference range. If these tests suggest iron overload a liver biopsy is un­ dertaken ю assess the degree of liver damage. Histological staining for iron (using Perls* stain) gives a semiquantitative assessment of the degree of iron overload, which in­ volves both hepatocytes and Kupffer cells, but the amount of iron in a biopsy can also be quantitated directly. ITie reference range is up to about 20 pmol/g dry liver weight but is over 40 nmol/g dry liver weight in patients with haemochromatosis. Treatment involves weekly venesection until the pa­ tient starts to develop iron deficiency anaemia and the ferritin concentration falls to within the reference range. Since 500 rnL of blood contain about 250 mg of iron, the total body iron at the time of diagnosis can be calculated (after correcting for 2 mg/24 h loss from the gut). It is usually in the order of 20-50 g compared to less than 5 g in the normal individual. The same figures can be obtained from a knowledge of the plasma ferritin which may also be used to monitor the progress of treatment (1 Mg/L of ferritin is equivalent to approximately 8 mg of iron). Unless decompensation has already occurred phlebotomy is successful in preventing progressive liver disease, although the high risk of development of hepatocellular carcinoma remains. It is also important to screen the patient's relatives for haemochromatosis so that treat­ ment can be instituted before liver damage occurs. Plasma ferritin and transferrin saturation are the most sensitive tests for presymptomatic detection. A major practical problem in diagnosing GH is that there tends to be overlap between patients with alcoholic liver disease and those with G H . In particular, there is overlap in terms of the tissue concentration of iron as­ sessed either by histological staging or biochemically. One solution to this problem has been calculation of the hepatic tissue iron index which is the hepatic iron tissue conccntrauon divided by age (Fig. 13.3). This appears to give a complete distinction. The problem of coexisting alcoholism and G H паь been discussed above.

W i l s o n ' s disease

Wilson's disease is a disorder of copper metabolism which is inherited in a recessive manner. Like haemochromato­ sis, it is treatable and thus has an importance in liver disease out of proportion to its frequency. Liver disease is the most prominent aspect in about 40% of cases, the remainder exhibiting neurological, psychiatric or haernatological complications. The hepatic presentation may be with signs of chronic liver disease or with fulminant hepa­ titis but there may be no clinical abnormalities in affected siblings detected by screening. In the absence of Kayser-

25I

Tissue iron index 100

Ю-

0.1-

001 « *

у

s

S V

Fig. 13.3 Tissue iron index in patients with alcoholic liver disease (ALD), genetic haemochromatosis (GH) and other forms of chronic liver ebsease (CLD). (Reproduced from SalUe R W, Reed W D, Shilkin К К Out I W ; 52:207 JO.)

Fleischer rings*, the diagnosis is often entirely based on laboratory investigations. Among those presenting with chronic liver disease the elevation of AST is said to be inappropriately low for the degree of hepatic inflammation and ALP is often in the reference range. There may be a low grade haemolytic anaemia leading to mild hypcrbilirubinacmia and an in­ creased risk of pigment gallstones. Copper deposition may lead to renal tubular dysfunction with loss of potassium, glucose, amino acids and phosphate. Diagnosis The diagnosis or exclusion of Wilson's disease is of crucial importance to patients and their relations. It is seldom as straightforward as may appear in standard texts; if there is any doubt about the diagnosis, advice should be sought from a specialist centre. The characteristic laboratory features (sec Table 13.11) arc: 1. A low plasma caeruloplasmin concentration. More than 90% of patients will have a value below the reference range of 200—400 mg/L. The only exception is when there is very 'active' liver disease where values may fall within the lower part of the reference range. About 10% of carriers will also exhibit a low plasma caeruloplasmin concentration. 2. A high tissue copper concentration. It is obviously important that the diagnosis of Wilson's disease is enter»

*A brown ring around the periphery of the cornea caused by copper deposition in Desccmet's membrane.

Ma

;:иал. за

нный авторским правом

252

< IJNICAI. BKM:I

Table 13.11

a l b b a m a FJ

T c « (reference range)

г Ibnrtion te*tt Plasma copper Toul< ' МШ1Л.1 Free u mol/L) Plasma cacruloplasmir. KM .. ■1 тагу copper h a u l (0.25-0,7SumoI/:M pcnicillamine >iunuUtcd i IcpatK copper A y weighn

i umoL'g

\s vuth м primary Ы1 Any icvcrc hepa' Incs*

B H f l M f l LX1 ^ A»v* 1П

Cbrom dbaaac

г

MilJlv abnormal

.tease and other с Wihon'idiMMc Fulminant hcp.i failure »ros*ly deranged > M

Variable

in which V il OQ*I disease may enter the d

1

Asympiomui

Acute hepatic necrosis3

Chronic cholcMasis1

Indian childhood cirrhosis

Normal

(Grossly deranged

Cholesta

MiMlv abnormal

Variable >2

>I5 2S 2S 10

>to

10-fold HtTftffPl1

>1

1

>0.75

20-Ю

1-4

9. A revtetv of the current and potential role oflti#r transplantation in the management of ln<er disease. Sallic К W\ Reed VC 1), Shilkin К В. Confirmation of the efficacy cif hepatic i m u c iron index in differentiating genetic haemochromaiosts from alcoholic liver disease complicated by alcoholic hacmowderosis. Gut 1901;12:207-1». l*ftwitLi the validatum for the u\e of thi tissue inm index, as illustrated in Fig /.?..?,

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C H A P T E R

14

Diabetes mellitus Simon Coppack

T H E PHYSIOLOGY O F GLUCOSE HOMOEOSTASIS M a i n t e n a n c e of n o r m o g l y c a e m i a Blood glucose concentrations arc maintained within very close limits in healthy people. Any given individual has a very strictly maintained posiabsorptivc (e.g. fasted over­ night) blood glucose concentration of 4.5-5.2 mmol/L, with intraindividual coefficients of variation (CV) of 1-2%. Interindividual coefficients of variation (assuming similar times since previous meal, levels of activity, com­ position of previous meal, etc.) arc < 5%. Glucose con­ centrations in healthy people increase after meals but typical meals will not raise blood glucose above 10 mmol/L and normoglycaemia is usually restored within 2-4 hours. Reductions in glycacmia arc produced by severe sudden unaccustomed exercise or prolonged fasting (or both), by the pathological conditions discussed in Chapter 15 and by pharmacological means, but are not typically encoun­ tered in a healthy Western adult on a daily basis. The strictness of glucorcgulation is most remarkable when compared to the relative laxity of regulation of other circulating metabolic fuels such as ketone bodies and non-csterified fatty acids (NEFA, also known as free fatty acids, FFA). The reason for strict avoidance of lowblood sugars is readily apparent in terms of the avoidance of hypoglycaemia (sec Ch. 15). The threshold for the onset of detectable ncuroglycopacnia is of the order of 3.0-3.5 mmol/L and it is thus appropriate that counterregulatory mechanisms are set to respond to maintain glycaemia comfortably above this level. The reason for the strict avoidance of hyperglycaemia is less immediately ap­ parent. Symptoms of hyperglycaemia are florid (in sub­

jects used to relative normoglycaemia) at blood glucose concentrations of 12-13 mmol/L and mav commence at concentrations below 10 mmol/L. The severe metabolic consequences of hyperglycaemia at levels above 20 mmol/L are discussed in the section on diabetic emergencies (p. 275). In contrast, mild hyperglycaemia is usually asymptomatic. The ideological value of the strict avoid­ ance of mild hyperglycaemia is not apparent except in terms of avoiding those problems we recognize as conse­ quences of prolonged hyperglycaemia and usually refer to as "long-term diabetic complications' or 'diabetic tissue damage* (p. 271). The mechanisms for regulation of normoglycaemia are outlined in Table 14.1, which lists counter-regulatory Table 14.1

\U Jianiwns prcventing/rcvcrbine hypogiycacmu

Adrtntrpc/tympathtnc m a w » Promotes glycogcn.viyas, gluconcofcncsts and increased glucose outpii Brer, reduces glucose clearance by skeletal muscle and adipose rime Promote* lipolysis to provide alternative fuel source(s) Inhibits insulin secretion XUnmitr-rzguLitoiy H*wmoiu ucagon .jcnolysit, gSuconeogcnesis and thus increased glucose output by the IIM-I May increase hepu ketone body production Cortisol рсОЮОШ tfycogenesis, gluconeogencMs and increased glucose output by the liver, reduces glucose clearance by skeletal muscle and adipose tissue Growth hormone: promotes hepatic gtycogenofysis and increased hepatic glucose output, promote* 1 n Other mixhiimsmi Insulin secretion inhibited .lings of hunger proni :ig 11 \poglvcaemia per *e stimulate* hepatic glucose output

257

258

CLINICAL BIOCHEMISTRY Adipose tissue

Liver

Muscle Fig. 14.1 Insulin's mam antihypcrglycaemic actions. Insulin (T) reduces the production of gluconcogcmc precursors such as give erol, alanine and lactate, (2) reduces activity of hepatic gluconcogcmc enzymes, and (3) reduces hepatic glycogenolysis to glucose. These effects contribute to (3) reduced hepatic glucose output Insulin ® reduces cellular glucose uptake mediated by insulinsensitive glucose transporters (GLUT4) and © r e d u c e s competition for glucose oxidation by alternative fuels ("Handle effect*)- The reduction of competing fuels involves ©inhibition of NEFA release from adipose tissue and(§) reduced hepatic ketogenesis. ® Insulm promotes glucose storage as glycogen.

mechanisms responsible for preventing hypoglycaemia, and Figure 14.1 which outlines the main sites of insulin action relevant to prevention of hyperglycaemia. N o r m a l glucose m e t a b o l i s m Glucose enters the body directly from the gut, as the re­ sult of hydrolysis of a variety of carbohydrates. Other sug­ ars absorbed are converted to glucose in the liver. Glucose moves around the body in the bloodstream, being released by the liver and taken up by almost all other cells. Glucose is made in the liver (with smaller amounts being synthe­ sized in other organs capable of gluconcogcncsis such as the kidney) from glucogcnic amino acids and from glycerol, lactate and pyruvate. Hepatic glucose output is ■ 2.0 mg/kg body weight/min in the resting postabsorptive state or 200-300 g during the average day (varying with the availability of glucose from food and with the body's requirements due to exercise). The balance of glucose in­ flux into the circulation (principally from hepatic glucose production) and peripheral clearance of glucose deter­ mines the level of glycaemia. Glucose moieties are stored as glycogen, a 70 kg man typically having a total of 700-1000 g (hydratcd) glyco­

gen. Most of this is stored in the liver (60-125 g) and skeletal muscle (400-600 g) with lesser amounts in other tissues. Glycogen is synthesized from both glucose and the gluconcogenic substrates such as lactate, pyruvate, glycerol and some amino acids. Glycogen in skeletal muscle can provide fuel for muscle but does not provide a source of glucose for release into the circulation. Glucose provides approximately 40-60% (on a typical Western diet) of the total fuel expenditure of the body during a 24 h period. It provides almost all the energy- of the central nervous system (especially in well-nourished subjects, although sustained fasting allows cerebral ketone body utilization). During high intensity exercise and dur­ ing the 4-6 h postprandially glucose is the predominant fuel of the whole body. Glucose is the most efficient fuel for oxidation in terms of the liberation of energy (112.2kcal or 6 mole ATP) per mole of oxygen con­ sumed. Many tissues can use either ketone bodies, fatty acids or glucose for their basal metabolism, switching between these different fuels depending upon their avail­ ability in the circulation. Glucose is fully oxidized to carbon dioxide and water in skeletal muscle, brain and liver. The brain accounts for most of the glucose oxidized in the fasting state (100-

DIABETES MELLITUS

low because of its active metabolism by phosphorylating enzymes (glucokinases, hexokinases). In resting, postabsorptive subjects, approximately 70% of the body's glu­ cose metabolism occurs in tissues such as the CNS and liver, independently of the action of insulin. However, these insulin-independent mechanisms cannot maintain normoglycacmia for very long without insulin orchestrat­ ing the response to food and integrating the balance between fatty acid/kctonc body and glucose metabolism.

125 g per 24 h). Skeletal muscle may take up 10-20% of hepatic glucose output in the resting state. Usually 1020% of hepatic glucose output is not oxidized but con­ vened to lactate, pyruvate, glycerol or amino acids and subsequently returns to the liver and is resynthesized to glucose. Fatty acids (or their partial oxidation products, ketone bodies) are the prime fuel of resting muscle, heart and liver. Other tissues such as red blood cells, skin, adi­ pose tissue and the renal medulla derive energy- from glycolysis to lactate and pyruvate, even in the normal state. Glycolysis to lactate is an anaerobic process to which many cells may resort when faced with hypoxia, for exam­ ple, skeletal muscle during high intensity exercise.

The ability to move glucose against a concentration gradient is conferred by Na'-glucose cotransporters which are possessed by only a few tissues such as renal tubules and intestine. In some models of diabetes mellitus, glucose trans­ porter number is decreased. Increased expression of low K^ G L U T l or G L U T 3 transporters has been reported in insulinoma cells. The role of genetic variants of glucose transporters in diabetes (especially non-insulin-dependent diabetes mellitus, NIDDM) is currently being examined.

Glucose t r a n s p o r t e r s Glucose uptake into cells is a process of facilitated diffu­ sion mediated by glucose transporter proteins. Five such membrane-spanning transporters (GLUT1-5) are recog­ nized to date (Table 14.2), and their protein and DNA sequences have been established. These transporters allow uptake of glucose into cells from the interstitial fluid into which glucose diffuses from the bloodstream. Different tissues possess different mixtures of glucose transporters which confer upon each tissue different characteristics of glucose uptake. G L U T l and G L U T 3 transporters are present on the cell surface at all times. In contrast, G L U T 4 transporters are stored in the cell cytoplasm when insulin is not present. G L U T 4 responds to insulin by moving from intracellular stores to the cell membrane, thereby increasing transporter number (6-10-fold). Erythrocytcs are insulin non-responsive because they possess only G L U T l . The central nervous system is relatively protected from neuroglycopaenia by the low K^ of its G L U T 3 transporters. Many cells can express a variety of different G L U T transporters and the expression of G L U T receptors changes with circumstances, e.g. liver cells ex­ press more G L U T l and G L U T 3 during starvation.

Insulin Biosynthesis Insulin is a peptidc hormone (51 amino acids, molecular weight 5807 Da) which is secreted physiologically only by the (i cells of the islets of Langerhans in the pancreas. In the synthesis of insulin, translation of mRNA yields prcproinsulin, a prohormone which undergoes post-translational modification prior to release of the mature insulin molecule. Removal of 24 amino acids from preproinsulin yields proinsulin with 86 amino acids. Having been syn­ thesized, proinsulin is stored in secretory granules prior to release from the p cell by exocytosis. In healthy subjects over 90% of proinsulin is converted to mature insulin by the removal of metabolically inert C-pcptide component, prior to secretion of the granule contents. The other products of the post-translational modification are either released when exocytosis occurs or are degraded within the secretory granules prior to release. C-pcptide is cosecretcd in cquimolar amounts with mature insulin. In

Tissue glucose uptake via G L U T transporters involves facilitated diffusion occurring down a concentration gradi­ ent, the intracellular concentration of glucose being very

Table 1-4.-

Character

I i.if

GLUTl •LUT2 GLUT3 GLUT4 GLUTS Na'-gtacose cotramponcr(s)

I tsSUCS

Ubiquitous, erythrocytc. placenta, v kidn> Liver* *mall intestine, kidney. p Cttft Ubiquir hram. plaixnta, kidney Skeletal muvclc, adi; e» heart Jcrunum Intestine, kidney tubules

259

[crs Kinetic*

Transport type

Low K*. (1-2 mmol/L

Facilitated diffusion

High K,. ( M O m m o U highV^ Low K, 1 JmmoLI w V P « 7 nameW K,„ 2 lOmmoVL

Facilitated diffusion Bidfan rial Facilitated diffusion Facilitated diffusion Insulin respond I.uii

Morct itfuLotc ак I concentration gradient

( dlffuM.'Tl

Av :гал»роП, lymport using Na* gradient

260

CLINICAL BIOCHEMISTRY

HUMAN PRQJNSUl IN

f, Chain NH.

a Cham

t

s

s—s

>;

1

Г

СПпт;с1|у>1^01у>о^^у\;,7'г,|Л«^!ЛДГ^^ ^>'

j8 A-IiJ

Л 1 i ПGin

'

40

C-Petfioe

Fig. 14.2 Structure of human proinsulin» showing site» of cleavage to insulin and C-pcptidc (Luft R (cdl Insulin, i&Jei paihology» islei function, insulin treatment. Gcntofte, Denmark: Nordisk Insulinfaboratonum, 1Q76).

healthy subjects only small amounts (< 10% of mature insulin output) of proinsulin and partially split proinsulin are released. Substances stimulating the synthesis and storage of insulin include glucose, mannose, leucine and a variety of mctabolizable sugars or sugar derivatives. Most of these also promote secretion. Secretion and kinetics The mechanisms whereby insulin release is triggered are the focus of much research. It is apparent that there is an ATP-dependent, sulphonylurea sensitive K* channel whose closure is a late event in the intracellular signalling mechanism within the (i cell. K' channel closure triggers calcium influx and exocytosis. It is not clear how this K* channel is activated although a wide range of secretogogucs will stimulate activation of this final common pathway. The most important of these is hyperglycaemia although mannose, 1 act ate, arginine, leucine and other ammo acids, glucagon, gastric inhibitory polypepride (GIP), cholecystokinin, vasoactive intestinal peptide (VIP), sulphonylureas and parasympathetic cholinergic (muscarinic) nerve activity also stimulate insulin release. Many of these secretogogues have synergistic effects. Because of cephalic and gastric influences, oral glucose is a more potent stimulus to insulin secretion than intra­ venous glucose. Glucose has complex time-dependent ef­ fects such that previous exposure to hyperglycaemia will augment subsequent insulinaemic responses. Sympathetic tone or catecholamincs inhibit insulin secretion.

In healthy adults, insulin is secreted in pulses with a pulse periodicity of 11-15 min. Stimuli of insulin secre­ tion increase the frequency and amplitude of these pulses. Approximately 30-40 U (240 pmol) of insulin are se­ creted per 24 h in healthy subjects. Insulin secretion is basal (0.25-1.0 U/h) until glycaemia exceeds a threshold level of about 5 mmol/L and insulin output is maximal at glycaemia of 15-20 mmol/L. Insulin is secreted into the portal venous system and thus must traverse the liver prior to reaching the systemic circulation. Approximately half of the insulin is cleared in the 'first pass' through the liver. Portal venous insulin concentrations are known to reach high levels in animals and hence the liver is exposed to insulin concentrations higher than other tissues when insulin is secreted endogenously. Autocrine and paracrine regulation of insulin secretion by pancreatic and gut hormones (which may reach very high concentrations within the islet) is not well understood. Increased secretion of insulin involves recruitment of more P cells to the secreting mode. Fasting peripheral insulin concentrations vary between 20 and 100 pmol/L (= 3-15 mU/L) as measured by radioimmunoassays in healthy subjects, the higher values being associated with increasing age and obesity. After a typical mixed meal (700-800 kcal) the peak plasma insu­ lin concentration will be 350-580 pmol/L (= 50-80 mU/ L) in young lean adults. The half-life of insulin injected into a peripheral vein is 2-6 min with the liver clearing most of this insulin and smaller amounts being cleared in other tissues having insulin receptors, such as skeletal muscle.

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MELLITUS 261 Table 14.i

Some рспсисаИ1

rmincd abnormal insulin*

Condition

Abnormality

Consequences

initial hyperproimulinaemte Insulin Chicago

1 allure ю i-tcavc C-peptidc >m pr Iin I-eU lor Phc ПвЪа&ЧШЗОП on position 25 of [i chain Sir ti>i Pbc substitution on position 24 of [1 %-ham I лги foe Val substitution on ромшп i of « chain II IN foe Arg substitution on position 65 Arg substitution on position 65 Asp for His subsnturion on г

True insulin concentrations and glucose tolerance normal

Insulin I хк Angeles Insulin Wakayama Promsulin Boston Promsulin l*rovidcnce Proinsulin Tokyo

Inability to cleave proinsulin to insulin Inabihtv to cleave jroiusulin in Inabihtv to cleave promsulin ы insulin

Abnormalities of insulin synthesis and secretion There arc a number of recognized genetic abnormalities of insulin structure involving mutations of the DNA code for insulin and hence altered amino acid sequences. Some of these are listed in Table 14.3 along with their consequences. More common abnormalities of insulin secretion in­ volve loss of the normal pulsatility of insulin release, an early feature in obesity and NIDDM. The progressive loss of insulin secretory capacity in diabetes is discussed below. Actions of insulin Insulin has widespread actions, some of which are listed in Table 14.4. Insulin is the dominant hormone regulating blood glucose concentration. It should be noted that whilst the mechanisms of its glucorcgulatory action have been the subject of extensive research, much less is known about its other actions. At present only two receptors have been identified that are responsive to insulin: 'the' insulin receptor and the IGF (insulin-like growth factor) receptor. It is apparent that there arc individual dose-response curves for the dif­ ferent actions of insulin in different tissues. For example, the ED V) for insulin's antilipolytic action on adipose tissue is below 140 pmol/L (= 20 mU/L) (and may be below 70 pmol/L), whilst the ED*, for the inhibition of hepatic glucose output is between 210 and 350 pmol/L ( = 3 0 50 mU/L) and the ED*, for stimulation of glucose uptake into skeletal muscle is between 350 and 490 pmol/L (= 50-70 mU/L). 1Ъе major target organs for the regulation of blood glu­ cose concentration by insulin are liver and skeletal muscle, the relative contribution of the two changing with the in­ sulin concentration. In fasting states it is hepatic glucose output (regulated by the ratio of insulin to glucagon) that determines the level of glycaemia. In postprandial states,

Table 14.4

Some action

чай lin

Actions /

I

Mi rhamsm

/■

Inhibition of hepatic glucose output Stimulation of hepatic glycogen storage Stimulation gtycotysis for intermediary metabolism Stimulation i-t hepatic Iipogc. Stimulation of hepatic glucose oxidation Sktktoiimud* Stimulation of glucose transport Stimulation of muscle glycogen synthesis Stimulation of muscle glycotys» AJifs*w uttm Inhibition of hpolvsi* (stored lipid) Promotion of re-oterification

1 i m ir.ition of substrate *upply Inhibition nt •genorysi Inhibitor «ncogencsis imulation of glycogen synthasc Stimulation ofl phc*spfc»rnjctokmase imulation of pyruvate dehydrogenase Activation of glucose transporter (GLU Stimulation of glycogen ■yntnasc Simulation of phosph 'olunasc Inhibition of hormone sensitive

Item Increased supply of glycerol

pboaphati Stimulation of lipolysis (circulating lipid) Increased glucose uptake

Several (? as for muscle/liver)

4 ( . nmaj navout tytum Satiei\ Changes in sympathetic tone Postprandial thcrmogrnesii

Uncertain 1 ricertam Uncertain

5 Other Promote* I >\'.\ synthesis Promotes RNA synthesis Stimulation of ammo acid uptake N a \ K'-ATPasc stimulation SaVH" antipon activation Na* retention

Stimulation Caffipopl

lipase

Uncertain nous ' .ertain } Increase in inrracetlular energy availability icertain «bably several mechanisms

262

CIJN'IGM- HICXIIKMISTRY

hyperinsuhnaemia increases peripheral glucose clearance. During hyperinsulinaemia (of > 350 pmol/L (> 50 mU/IJ) over 7 5 % of peripheral glucose disposal occurs in skeletal muscle where the majority is converted to glycogen. The different actions of insulin have different time courses, glucoregulatory and antilipolytic occurring within a few minutes, whilst growth regulation and actions de­ pendent upon synthesis of new proteins occur over periods of hours or days. Intravenous injection of insulin typically has little effect on blood glucose for 5-10 minutes, the maximal hypoglycaemic action occurring after 5-15 minutes. Insulin stimulation of skeletal muscle glucose uptake declines with a half-life of 10-25 minutes after the insulinaemic stimulus is removed. Proinsulin and partially split proinsulins have meta­ bolic activity generally similar to insulin although plasma half-life is 3-5 times longer and biological potency is only 8-15% that of insulin. It is suggested that proinsulin may be relatively potent in terms of hepatic activity and rela­ tively less potent in terms of peripheral glucose uptake. The insulin receptor Insulin's main glucoregulatory effects are mediated by a transmembrane receptor found on insulin-sensitive cells.

Phe-86.89 CYS-ПСЛ ys-ncn

i 1 56-312)

Cys-435 Су»-4ва Cys-524

This receptor is a glycoprotein, total molecular weight of 350 000 Da, comprising four peptide subchains, two a and two |i subunits, linked by disulphide bridges and re­ ferred to as 'the* insulin receptor (Fig. 14.3). The DNA and amino acid structure of the insulin receptor have been characterized and show homology with the IGFI receptor (see p. 264). Within the iniracellular domain of the [J subunit is a tyrosine kinase capability which is activated when insulin binds with the extracellular domain of the a subunits. 1Ъе tyrosine kinase promotes autophosphorylation of the receptor followed by activation of thrconinc and serine kinases. Rare DNA mutations of the insulin receptor have been described and result in severe glucose intolerance with resistance to exogenous insulin (p. 270). Second messengers mediating the effects of insulin Insulin can have multiple actions even on a single respon­ sive cell and hence there arc probably several different inrraccllular pathways mediating these actions. Gluco­ regulatory and antilipolytic responses are rapid and prob­ ably mediated via serine and threonine kinases and cAMP. Stimulation of lipid and protein synthesis, inhibition of proteolysis, the nuclear transcription of RNA and the

Lys 15

(Impaired transport to eel surface; decreased binding afflni»

Stop

(Decreased mRNA li

Arg20*

(Impaired transport to cell surface)

Pro233

(Decreased binding)

Vaf382

(impaired transport to eel surface)

Glu4*

(Increased bsvAng *) and of a relatively low insulin secretory reserve predicts the later onset of N I D D M . The differentiation of idiopathic N I D D M and 'secondary' diabetes can be difficult.

Genetic susceptibility

Genetic factors in NIDDM

Much work has demonstrated that certain genetic markers are associated with a high prevalence of IDDM. Most of these genetic markers are found on chromosome 6, in genes related to histocompatibility linked antigens (HLA). Some of the associations arc listed in Table 14.7. Most IDDM associated with HLA-DR4 presents in childhood whilst that associated with HLA-DR3 has a more variable age of onset.

Family studies suggest that N I D D M is strongly inherit­ able. Concordance rates for identical twins arc reported to exceed 90%. Some racial groups have very high incidences of NIDDM. Notable examples of this include the Pima Indians of Arizona and South Sea Islanders with prevalence rates of up to 50%. In the UK the prevalence of N I D D M amongst those of Asian extraction is perhaps two or three times that amongst people of Europid origin. Afro-Caribbeans show an intermediate prevalence. The natural history of N I D D M and its propensity to give rise to long-term complications varies between races.

Non-insulin-dependent diabetes mellitus (NIDDM) N I D D M is the commonest form of diabetes mellitus in Western societies. It is often considered a diagnosis of exclusion. Heterogeneity of NIDDM N I D D M is probably not a single condition. Some late onset diabetes initially presumed to be N I D D M will turn

Table 14. T

1

In the majority of patients with N I D D M the pattern of inheritance suggests a polygenic disorder, with an impor­ tant role for environmental factors such as obesity and a low level of exercise. Molecular biological techniques have not yet shown N I D D M to be consistently associated with any abnor­ malities of the DNA coding of insulin» the insulin receptor or glucose transporter peptides (except in a small percent­ age (R» HE Л DIM

6

Relative risk = n Relative mk ■ ■ linkage luihbnum with DR3 & DR4 Relative riifcs up to ж 5 year) exces­ sive alcohol consumption with recurrent episodes of mild abdominal pain. Hospitalization due to these episodes of pancreatitis is unusual. There may or may not be pancre­ atic calcification, but sclerosis of islets occurs. Insulin secretion is reduced, causing diabetes, but patients arc highly sensitive to exogenous insulin therapy (perhaps because glucagon secretion is also reduced). Alcohol may also induce severe exocrine pancreatic failure and conse­ quent steatorrhoea. Acute pancreatitis is associated with glucose intoler­ ance consequent upon reduced insulin secretion and the insulin resistance of systemic illness. Acute haemorrhagic pancreatitis occasionally produces such damage to the pancreas that permanent diabetes mcllitus results.

DIABETES MEI1JTUS

Mild (subclinical) abnormalities of exocrine pancreatic function and reductions in pancreatic lipase and immunoreactive trypsin are seen in 20-70% of diabetic patients. The cause for these abnormalities is not established although the pancreas is smaller in diabetes (especially IDDM). Changes in insulin secretion, glucagon secretion and autonomic neural function may also play a role in exocrine pancreatic dysfunction. Hacnwchromatosis Iron storage disease, whether due to familial defects or excessive iron intake, results in liver damage which may progress to cirrhosis (see Ch. 13) and also to P cell damage, as well as damage to other endocrine and nonendocrinc tissues (e.g. cardiac muscle, gonads). Diabetes is a common result of such problems. Glucose tolerance improves with treatment of the iron overload but the more severe the damage, the less the prospect of recovery with treatment. Screening diabetic clinic populations typi­ cally reveals 0.1-0.5% of N I D D M to be associated with undiagnosed iron storage disorders. Coeliac- disease Diabetes is statistically associated with coeliac disease and both are related to HLA-DR4. The combined dietetic requirements of the two conditions may be onerous for patients. The diarrhoea of unrecognized coeliac disease may be mistaken for diabetic diarrhoea. Endocrine disorders In many of the conditions listed below, insulin resistance occurs but, in most subjects, |J cell reserve is sufficient to produce compensatory hyperinsulinaemia so that, although there is mild glucose intolerance, full-blown dia­ betes mellitm is unusual. The unrecognized development of an endocrine condition in a known diabetic patient may present as worsening glycaemic control or recurrent kctoacidosis. It is more common for concurrent endocrine disease to exacerbate or unmask pre-existing glucose in­ tolerance rather than for it to be the sole cause of diabetes. Polycystic ovary syndrome is a common condition associated with obesity, insulin resistance and glucose in­ tolerance or N I D D M (see below and Ch. 21). Although the aetiology of polycystic ovary syndrome is uncertain, obesity and insulin resistance are usual features and it is a frequent cause of secondary diabetes. Active thyrotoxicosis is associated with glucose intoler­ ance in 30-50% of patients, but this rarely extends to frank diabetes. The mechanism causing this may vary be­ tween different patients. Hepatic glucose production will be increased. Increased gastrointestinal motility may exag­ gerate postprandial hyperglycaemia. Insulin clearance is

269

increased. P-Adrenoreceptor hvpersensitivity may mediate the elevated non-esterified fatty acid levels of hyperthyroidism. 30% of subjects with successfully treated thyrotoxicosis will be found to have established glucose intolerance 12 years after diagnosis. The increased incidence of impaired glucose tolerance in patients with hypothyroidism is well established al­ though the mechanism for this is uncertain. Hypciprolactinaemia causes insulin resistance and glu­ cose intolerance which is reversed by treating the primary condition and symptomatic diabetes is rare. The hypercorrisolaemia of Cushing's syndrome, what­ ever the cause, results in increased hepatic gluconeogencsis and hepatic glucose output whilst skeletal muscle becomes insulin-resistant. Glucocorticoids increase lipolysis and protein catabolism, increasing the circulating concentrations of alternative fuels and reducing glucose clearance (see p. 257). Glucose intolerance is found in 80-90% of patients with Cushing's and frank diabetes in 15-20%. Treatment with glucocorticoids is a common diabctogenic event. Conn's syndrome was originally described as including glucose intolerance. However, this is usually mild and only a small minority of patients have even moderate glucose intolerance. The prime defect is thought to be impaired insulin secretion consequent upon severe potassium deplerion. Growth hormone is a counter-regulatory hormone. Acromcgaly commonly causes glucose intolerance (6070% of cases) and even symptomatic diabetes (6-25%). Successful treatment of acromcgaly usually improves glucose tolerance. Isolated growth hormone deficiencyinduced dwarfism is also associated with glucose intoler­ ance and insulin deficiency. Diabetic microvascular disease is rare in this group. Phaeochromocytoma is associated with multiple abnor­ malities of glucorcgulation, adrenaline having a greater hyperglycaemic effect than noradrenaline. Catecholamines stimulate hepatic glycogenolysis, hepatic glucose output and inhibit insulin secretion via tt-rcccpiors. (JReceptor effects include promotion of adipose tissue lipolysis, increased skeletal muscle glycogenolysis and reduced skeletal muscle glucose uptake. Both a- and [J-receptors augment glucagon secretion. Improvement of glucose tolerance occurs within a few weeks of successful surgical resection. Hypercalcaemia and hypophosphataemia, as seen in primary Ь у р е ф а г а Л у п ж ^ т , reduce peripheral insulin sensitivity and are associated with hyperinsulinaemia although glucose tolerance is rarely impaired. Gastrointestinal endocrine tumours of several sorts are associated with glucose intolerance. Glucose intoler­ ance is a cardinal feature in patients with glucagonomas. Zollinger-Ellison (gastrinoma) syndrome, VernerMorrison or WDHA (taatery diarrhoea, Aypokalacmia,

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270

CLINICAL BIOCHEMISTRY

achlorhydria, or VIPoma) syndrome, carcinoid syndrome, I'OEMS (polyneuropathy, organomegaly, iTidocrinopathy, monoclonal gammopathy, 5kin changes) syndrome and somatostatinoma all have glucose intolerance as a fre­ quent, and often florid, feature. Iatrogenic diabetes Treatment with steroids is the most common form of iatrogenic diabetes. Thiazides frequently worsen glucose tolerance, their effects being most important in diabetic patients. Sympathomimctic drugs and carbcnoxolonc worsen glucose tolerance but the effect is usually mild. Pancreatic surgery is a rare cause of diabetes. Rare conditions associated with glucose intolerance Several congenital conditions with varying degrees of im­ pairment of glucose intolerance are listed in Table 14.9. In most of these conditions, the diabetes is not the most pressing clinical problem. Conditions associated with severe insulin resistance Most of the endocrine causes of glucose intolerance are the result of circulating insulin antagonists causing insulin resistance. A variety of other medical conditions, although rare, may be associated with severe resistance to insulin. Acanthosis nigricans is characterized by the presence of velvety brown hyperkeratotic lesions in the axillae and groins. There is a well-recognized association with malig­ nancy. Acanthosis nigricans not associated with malig­ nancy may be classified into two groups, both associated

lai

CooftrmuU condition»

associated ajncou

Condltl

Main feature*

Klctnfclicr\ %vndmroe I 11 ■ .- •■ syndrome Down4 svndrome r-'ncdreich** ataxia Ataxia telangiecta Dy%tro|
• or continuous subcutaneous infusion of insulin using a syringe driver, have the potential to achieve better control than twice-daily injections of a mixture of a short and longer acting insulin, but are used by only a minority of patients with IDDM. There arc also some specific circumstances which may interfere with efforts to achieve optimal glycaemic control.
  
  Obstacles to achieving glycaemic control Dawn phenomenon Non-diabetic subjects show arcadian changes in their plasma glucose concentration and glucose tolerance based upon changes in counter-regulatory hormones, insulin secretion and availability of alternative fuels. The most marked such arcadian effect is the 'dawn phenomenon' which typically occurs between 4.00 and 7.00 a.m. and is an increase in plasma glucose concentration and decreasein insulin sensitivity consequent upon the increased secre­ tion of counter-regulatory hormones at that time. During this period diabetics usually have modest rises (1-2 mmol/ L) in plasma glucose concentrations without recent ingestion of food. In some patients who have more serious in­ sulin deficiency, the rise in plasma glucose concentration may be much more marked. Intercurrcnt illness and stress Intercurrent illnesses such as viral infections usually cause hyperglycaemia but may give rise to hypoglycaemia. The effect on glycaemic control may persist for up to 2 weeks and insulin requirements, even during simple infections (e.g. colds), may increase by 100%. As most patients can­ not safely adapt to such changes in insulin requirement without risk of hypoglycaemia, it is often simpler to accept less strict glycaemic control during and shortly after the infection. In patients with severe intercurrent illness such as septicaemia, pancreatitis or myocardial infarction (all more common in diabetes), intravenous doses of insulin equivalent to those used in ketoacidosis are often re­ quired, even in subjects not usually requiring exogenous insulin treatment. Emotional stress of many forms will result in changes in secretion of counter-regulatory hormones and autonomic tone which will alter glucose tolerance. Since emo­ tional stress is often associated with alterations in sleep, eating and exercise patterns it may be practically impossi­ ble for diabetic patients under emotional stress to main­ tain usual glycaemic control, even if their morale remains high enough for them to bother trying. Previous hypoglycaemia and Somogyi effect The response of counter-regulatory hormones to hypogly­ caemia is multiphasic, with glucagon, circulating catecholamincs, neuronal catccholamines, growth hormone and steroids acting sequentially to reduce peripheral glucose clearance (increased insulin resistance) and increasing he­ patic glucose output by both glycogenolysis and increased gluconeogenesis. After a counter-regulatory response to a hypoglycaemic episode and the administration of glucose to reverse the symptoms, hyperglycaemia is common. The
  
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  DIABETES MEUJTUS
  
  patient's usual glycaemic control, glucose tolerance and insulin resistance may take several hours to be restored. A special variant of the rebound from hypoglycaemia is the Somogyi phenomenon in which nocturnal hypoglycaemia occurs. 1Ъе patient may not be aware of the hypoglycae­ mia, even in retrospect, although awakening with malaise, headache and bedclothes damp from sweating are sugges­ tive. The rebound from the nocturnal hypoglycaemia results in the patient waking with a blood glucose concen­ tration higher than desirable, causing the temptation to take at least as much insulin the next night. Exercise Exercise imposes a perturbation of the patient's usual balance between food intake and insulin requirement. The metabolic effects of exercise vary with the intensity and duration of that exercise. For example, jogging typi­ cally elicits an increase in both glucose and lipid oxidation and plasma glucose and NEFA turnover increase. In more strenuous exercise, muscle stores of glycogen and triglyceride will be depleted. Plasma glucose concentrations may increase or decline during exercise, depending on counter-regulatory hormone drive to hyperglycaemia and whether glycogen reserves are adequate, but in healthy subjects plasma insulin concentrations decrease. This de­ cline in insulin permits increased hepatic glucose output to limit the fall in blood glucose. In subjects on exogenous insulin treatment, insulin concentrations may not fall (indeed, they may increase if increased skin blood flow during exercise increases insulin delivery from the subcu­ taneous injections site) and hence hepatic glucose output may not be able to prevent hypoglycaemia. After exercise there is a phase of glycogen repletion in liver and muscle and in diabetic subjects, delayed hypoglycaemia may be a problem at any time up to several hours afterwards.
  
  Elhanol The effects of ethanol on diabetic subjects include all its effects on non-diabetic subjects. In addition, the effect of ethanol to inhibit gluconeogenesis may reduce hepatic glucose output so as to provoke hypoglycaemia. Typically this docs not occur immediately whilst the patient is con­ suming ethanol since most beverages contain adequate carbohydrate to prevent this (or the patient cats as well as drinks) and hyperglycaemia is usual within 120min of drinking. However, the inhibition of gluconeogenesis persists for several hours and a rebound hypoglycaemic episode may occur a few hours (typically 2-4 h) after drinking ethanol. If the clinical features of hypoglycaemia are dismissed as the consequences of inebriation by the patient's attendants (or police) then the failure to treat the hypoglycaemia may be serious. Even non-diabetics may
  
  275
  
  suffer symptomatic cthanol-induccd hypoglycaemia, but the problem is most severe in patients taking exogenous insulin therapy. Diabetic complications Diabetic autonomic neuropathy may affect gastrointes­ tinal motility which may be occult or manifest as constipa­ tion, diarrhoea, unexplained (or easily triggered) vomiting or as malabsorption. The features of malabsorption are dealt with in Chapter 11. Altered gastrointestinal motility will change the rate of absorption of nutrients from the food and cause rapid and unpredictable delivery of glu­ cose into the circulation. Such unpredictable absorption is particularly unwelcome in diabetics and may undermine completely the attempts of even a well-motivated patient to maintain tight glycaemic control. If gastrointestinal motility can be improved, this renders glycaemic control somewhat easier; however, such amelioration is usually spontaneous rather than related to medical intervention.
  
  Brittle diabetes Brittle diabetes is a term with no universally accepted definition. In 1977, Tattcrsall used a clinical definition, describing 'the patient whose life is constantly being dis­ rupted by episodes of hypo- or hyperglycaemia, whatever their cause'. This definition has been modified by various workers. Causes of brittle diabetes include psychological abnor­ malities such as eating disorders (e.g. bulimia), personal­ ity disorders, communication disorders or manipulative behaviour. Such factors are often inferred if good glycae­ mic control can be achieved in hospital by the use of intra­ venous or nurse-administered subcutaneous insulin. If intravenous insulin therapy fails to achieve good glycae­ mic control then a systemic cause for insulin resistance should be sought. Other causes of brittle diabetes include inappropriate education, an inappropriate insulin regimen, intercurrent illness such as thyroid disease, Addison's disease, systemic lupus erythematosus (via antibodies to insulin or its receptor), disorders of intestinal motility (erratic absorption of food) and interactions with other drugs (prescribed or non-prescribed). Problems related to insulin injections, such as faulty technique, abnormali­ ties of local anatomy or subcutaneous tissue blood flow, are well-recognized causes. The suggestion that subcuta­ neous proteases may prevent insulin absorption remains unproven.
  
  DIABETIC EMERGENCIES General medical emergencies in diabetic patients are usually treated in the same way as in non-diabetic sub-
  
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  UJNICAl.BUXJil MIMRY
  
  jects, although frequent monitoring of blood glucose is necessary. Specific diabetic emergencies arc classified below although mixed forms may occur rarely. Appro­ priate management requires accurate diagnosis of the condition. Any patient being admitted as an emergency with cerebral impairment, acidosis (or hypcrvcntilation) or dehydration requires an urgent laboratory measure­ ment of blood glucose concentration; stick testing of blood glucose, although useful as a rapid screening proceedure, should not be relied upon for such patients.
  
  Declining GFR
  
  Aados'S
  
  t
  
  Polyune
  
  !
  
  Glycosuna Unrestrained ketogenesis
  
  Diabetic ketoacidosis imatcly 30% of patients with IDDM present with ketoacidosis. Fortunately the onset in newly presenting patients tends to be slower than in known diabetic sub­ jects. An infection is found in approximately 35-55% of cases of ketoacidosis. Inappropriate self-treatment (or lack of it) accounts for another 30% cases. The clinical features include dehydration, acidosis and cerebral im­ pairment. Biochemical features include hyperglycaemia, ketosis (kctonaemia and kctonuria) and metabolic acido­ sis. The characteristic ketosis is a consequence of in­ creased lipolysis and decreased fat synthesis. Excess acetyl CoA derived from (J-oxidation of fatty acids is converted to the kctonc bodies, acetoacctate and f}-hydroxybutyrate. There are four mechanisms which predispose to keto­ acidosis: insulin deficiency, counter-regulatory hormone excess, fasting and dehydration. Of these the most impor­ tant is insulin deficiency. Mechanisms of the development of diabetic ketoacidosis are shown in Figure 14.4. In a vicious circle, hyperglycaemia and excess lipolysis cause dehydration and high circulating concentrations of NEFAs. These promote acidosis and dehydration and increase the secretion of counter-regulatory hormones. The acido­ sis, dehydration, counter-regulatory hormones (especially catecholamines and glucagon, which some consider essen­ tial for the development of ketosis) and excess lipolysis induce insulin resistance, thereby increasing hypergly­ caemia and lipolysis. Protein caiabolism and hypertriglyccridacmia also occur and compound the situation. Rarely patients may present with normoglycaemic keto­ acidosis; typically this occurs in situations where exercise, starvation or infection is the major precipitant of ketosis. Biochemical features Bedside tests may demonstrate high blood glucose levels, glycosuria and kctonuria. Significant ketonaemia can be detected by putting a drop of blood on a reagent stick designed for detecting ketonuria. The nitroprussidc reac­ tion (Acetest) detects acetoacetate. Plasma [i-hydroxybutyratc concentrations are typically three times higher than those of acetoacctate and the ratio may be even greater if there is also an element of lactic acidosis, so that
  
  Dehydration
  
  Hyperglycaemia
  
  t^^t
  
  Inadequate msulai secretion (or injection)
  
  Increased counter-regulatory hormones
  
  Fig. 14.4 VICIOUS cycles in ketoacidosis and hypcrosmolar states. In both conditions the trigger evcnt(s) cause an initial inadequate insulin supply and hypcrgki-ueniui. In ketoacidosis both acidosis and dehydration cycles occur In hypcrosmolar states only the dehydration cycle occurs. Features in italics arc restricted to ketoacidosis.
  
  the kctonaemia may be underestimated or not recognized. An ECG may point to hypcrkalaemia and is an important baseline in older patients. Typical biochemical changes are shown in Table 14.10. The key to successful management is early institution of therapy and repeated clinical and biochemical assess-
  
  Tablc 14.10
  
  initial laboratory
  
  I'l.tsina (glucose! - *7rnmol/L Plasma \K | - 5, Inmolfl wbok body depletion typical "> nmol/kg body weight PUsma [NV| ■ I И tnmol/K whole body depletion typically 8.0 mmol/kg body weight Plasma jurca) > 15 mmol/L Plasma (crcatininej > I5 umol'I Plasma |ketoncs] a > 15 mmol/L Plasma |M 1 7.30> PaCO- a normal P.» I normal [HCO, ] = lemmd/LOow normal) Aniongap iabctes melhius Kndocnnology and metabolism clinics of North America №92, 2 1 . 1Ф9-1Я2. l''rirtiwww authantaiii •■ »VT f4t*4 ■■►« the pal)u*XK*twti\, management and нкыЬмИх ecmplicuiiont ofdiabetes mtUiru*. Pickup J C , Williams G ieds- T e x t b o o k o f D i a b c t o . Oxford: Blackwell Scientific, 1 W 1 . A cotttprehensn^ anount of all aspects i)j diabetes. Heaven Ci R. Role of insulin resistance in human disease (syndrome X ' : an expanded definition. Annual Review of Medicine lrd ciinj. London: British Medical Journal, l*W3 A gihtd general mtroduetum to diabetes.
  
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  CHAPTER
  
  15
  
  Hypoglycaemia Mourad Labib
  
  GLUCOSE HOMOEOSTASIS IN T H E FED AND T H E POSTABSORFriVE STATES In healthy subjects, the blood glucose concentration is maintained within relatively narrow limits through a tightly controlled balance between glucose production and glucose utilization. Fundamentally, glucose is derived either from dietary intake (in the fed state) or from glyco­ genolysis and gluconeogenesis (in the fasting or postabsorptive state). It is metabolized by oxidation or stored in the form of glycogen or fat (Fig. 15.1). Blood glucose concentrations in the fed and postabsorptive states are regulated by the interaction between insulin and glucagon. Insulin, which is primarily an anabolic hormone, enhances storage of nutrients by pro­ moting glycogencsis, lipogenesis and protein synthesis. Glucagon, on the other hand, functions primarily to prevent hypoglycaemia by stimulating glycogenolysis and gluconeogenesis. After the ingestion of a mixed meal, the increase in glucose and insulin levels results in stimulation of glyco­ gen synthase and inhibition of glycogen phosphorylase, causing a net increase in hepatic glycogen. Three to four hours following a meal, glucose and insulin levels decline and the liver reverts from net glucose uptake to net glu­ cose release. Hepatic glucose release continues to increase over the next several hours until it equals glucose utiliza­ tion. Initially, about 7 5 % of glucose production is attrib­ utable to glycogenolysis and 2 5 % to gluconeogenesis but as hepatic glycogen stores decline, the proportion from gluconeogenesis increases steadily. During the early postabsorptivc phase, glucose utiliza­ tion continues at the rate of 0.11 mmol/kg/min, of which 40-50% is due to obligatory uptake by the brain and other
  
  non-insulin-dependent tissues. With prolonged fasting, the decline in plasma insulin concentration is accompa­ nied by an increase in free fatty acid and ketone body concentrations. These can be used as alternative fuels, thereby decreasing the need for glucose.
  
  Fed «tat*
  
  .
  
  1
  
  4 Peripheral uptake and oxidation of glucose
  
  * Hepatic giycogenesis
  
  Dietary carbohydrate and nutrients
  
  * » \ Insulri _ l 1
  
  t
  
  1 G»ycogeno«y5ts and
  
  *
  
  gluconeogenesis
  
  V
  
  Lipotys'S and ketogenesis
  
  Feeling state Peripheral uptake crfgiuooee
  
  .
  
  *
  
  Dietary carbohydrate and nutrients
  
  l ; i g . 15.1
  
  1
  
  I
  
  Glycogenolysis
  
  T
  
  Gluconeogenesis
  
  t
  
  LipO*yStS
  
  J
  
  3
  
  Glucose hotnueu&tasb in the fed and fasting states.
  
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  CLINICAL HUH Ml MISTRY
  
  HYPOGLYCAEMIA
  
  The hormonal response to hypoglycaemia
  
  The definition of hypoglycaemia is somewhat arbitrary since the glycaemic threshold at which symptoms occur differs between individuals and depends on the age of the patient and the prevailing plasma glucose level before the hypoglycacmic episode. A venous plasma glucose concen­ tration below 2.5 mmol/l-, regardless of the presence or absence of symptoms, has been commonly used. How­ ever, many apparently healthy people, especially women, may have plasma glucose concentrations below 2.5 mmol/ L during a 72 h fast without experiencing any symptoms. Therefore, the diagnosis of pathological hypoglycaemia necessitates the demonstration of Whipple's triad. This consists of: (1) symptoms of hypoglycaemia in the pres­ ence of (2) a low plasma glucose concentration and that (3) symptoms are relieved by administration of glucose.
  
  Acute insulin-induced hypoglycaemia causes prompt inhibition of hepatic glucose release and stimulation of glucose utilization resulting in a fall in blood glucose con­ centration. The continued decline in blood glucose is, however, prevented by a compensatory increase in hepatic glucose release which follows the increase in plasma glucagon and adrenaline concentrations. Plasma growth hormone and cortisol concentrations increase shortly afterwards. It has been estimated that the glycaemic threshold for glucagon, adrenaline and growth hormone secretion (3.8 mmol/L) is higher than that for cortisol secretion (3.2 mmol/L) or symptoms of hypoglycaemia (2.9 mmol/ L). Several experimental studies indicate that glucagon is the most important counterregulatory hormone during acute hypoglycaemia. If glucagon secretion is deficient or absent, adrenaline becomes the principal counterregula­ tory hormone. Cortisol and growth hormone arc of sec­ ondary importance and appear to have a permissive role during recovery from acute hypoglycaemia. When both glucagon and adrenaline are absent, as in long-standing insulin-dependent diabetes, cortisol and growth hormone compensate poorly, if at all, for hypoglycaemia.
  
  The ncurogenic response to hypoglycaemia Under normal metabolic conditions, the central nervous system is wholly dependent on adequate glucose supply. In hypoglycaemia, the shortage of glucose in the neurons activates cells in the hypothalamus and an autonomic re­ sponse is triggered to restore and maintain glucose supply to the brain. The activation of the autonomic nervous sys­ tem, mainly the sympathetic, acts directly by mobilizing storage depots of glycogen and fat and also by stimulating catecholamines which potentiate the adrcnergic mobiliza­ tion of energy stores (Table 15.1). Hepatic glycogen stores provide glucose, particularly for the brain, whereas the mobilization of fatty acids from fat depots provides energy for tissues which can utilize fatty acids as their basic fuel, e.g. skeletal and cardiac muscles, renal cortex and liver, thus sparing glucose for use by the central nervous system.
  
  ' mpathc; out tysutm (a) a-adrenrrgftc effect» л inhibition ,it endogenous insulin release Increase rcbral blood flow (by causing peripheral vasoconstneaon) (b) p - a d r v n e r y u i-llccta -nutation of glycugcnolyvu tffjocafjl mutation of liporyv
  
  lease
  
  Inhibition of muscle glucose uptake Increase m cerebral blood flow b\ camtag increase in output) (c) Catecholamine release from the adrenal mcdulln Potential с the above a- and IV-adrencrgic cfTccts mpalhitu пепчпи rvtttr* muiation Ы the vagus n c n Vsmulation of gastric acid secretion nulanon of parotid salivary secretion
  
  S y m p t o m s of hypoglycaemia The symptoms of hypoglycaemia are non-specific and vary depending on the degree of hypoglycaemia, the age of the patient and the rapidity of the decline in blood glucose. They arc conveniently subdivided into adrcnergic and neuroglycopenic symptoms but they all reflect the effects of hypoglycaemia on the central nervous system. Adrcnergic symptoms The adrenergic symptoms result mainly from activation of the sympathetic nervous system and characteristically occur when there has been a rapid decline in blood glu­ cose concentration. They consist of rather sudden onset of hunger, tremors, palpitation, sweating, weakness and anxiety (Table 15.2), A rapid fall in blood glucose may occur in diabetics due to excessive absorption of exogenous insulin either from overtreatment or from rapid mobilization from the injection site during exercise. In non-diabetics, reactive hypcrsccretion of insulin may be responsible, e.g. in postgastrectomy patients. The adrcnergic symptoms of hypoglycaemia may mimic those of anxiety attacks or hypcrvcntilation. Neuroglycopenic symptoms The neuroglycopenic symptoms result from a more gradual decline in blood glucose concentration and range
  
  HYPOGLYCAEMIA
  
  Tub
  
  ■
  
  Adrcncrgic
  
  Anxi. Restlessness gcr Nausea I (cmulousncaa ■ eating Palpitation*
  
  283
  
  ■
  
  Fasting hypogfacacin
  
  Ncurogtycopi Suhacme Reduction "i •'poruan wm Bcbu\ KoarH change redness Somnolen Otzzincss Confusion Incoordin > Headache Blurred vision Amnesia Paracsthetia лдкпеа* Transient hemiplejpa Seizures Coma
  
  ' hrmx Personality change» .uvc memory PsychcHi4 Dementia
  
  from a generalized decrease in spontaneous activity, poor concentration, confusion, incoordination, slurred speech and behavioural changes to seizures, focal neurological signs and coma (Table 15.2). It is unusual for neuroglycopenic symptoms to appear in patients with fasting hypoglycaemia unless the blood glucose concentration falls below 2.2 mmol/U although in elderly patients the neuroglycopcnic threshold may be as high as 3.0-3.5 mmol/L. Patients with neuroglycopenic symptoms of hypoglycacmia have been misdiagnoscd as having epilepsy, transient ischaemic attacks, psychosis, hysteria, chronic nervous exhaustion, personality disorder or inebriation. Chronic neuroglycopenia is very rare and may occur in patients with insulin-secreting tumours unsuspected for years. Symptoms include personality changes and mental deterioration. CLASSIFICATION OF HYPOGLYCAEMIC DISORDERS Disorders causing hypoglycaemia can be classified, according to the timing of symptoms in relationship to meals, into fasting and reactive hypoglycaemia (Table 15.3). While patients with fasting hypoglycaemia, e.g. insulinoma, may also have reactive hypoglycaemia, pa­ tients with reactive hypoglycaemia never have symptoms with fasting.
  
  With hypcrinsuUnaemU ulin auminiMTJiHin
  
  Sulpbooyhansa а^шШипйоа Islet cell tumours Autoimmune hypoglycaemia
  
  Drug qumi pcnumidmc Septicaemia Without hypcrinsulinacmia Chrome renal impairment Severe liver isc Deficient calorie intake Exercise-induced h>^>grycacmta Non-islet cell tumours
  
  Endocrine insufficiency: hypopituitansm AddisonS disease isolated GH « AC ГН ' Drugs. salicvUics p-blockcrs i ipyramidc Rtactwe hypogtycacm* Alimentary hypoglycaemia
  
  Diabetes meffiras Alcohol-induced hvpoglycaemia
  
  Idiopathk
  
  1. Those associated with increased or inappropriate plasma insulin levels, such as insulinoma, overdose or surreptitious administration of insulin or a sulphonylurea, autoimmune hypoglycaemia and certain drugs such as quinine and pentamidine; 2. Those associated with suppressed insulin levels, such as severe liver disease, chronic renal failure, severe malnutrition, hypopituitarism, adrenal insufficiency, non-pancreatic tumours and drugs, e.g. salicylatcs and (i-blockers. Non-fasting, p o s t p r a n d i a l o r reactive hypoglycaemia Symptoms typically occur within 2 -5 h after a meal and are predominantly adrcnergic. Reactive hypoglycaemia is rare and may occur in postgastrcctomy patients and mild diabetes mellitus or it may be idiopathic. The majority of patients with postprandial symptoms do not in fact have hypoglycaemia (sec later).
  
  hypoglycaemia Symptoms typically occur after a period of fast, usually after the longest regular fast of the day (e.g. before break­ fast) or after missing a meal. They are predominantly neuroglycopenic and may be precipitated by exercise. Causes of fasting hypoglycaemia may be further sub­ divided into two categories:
  
  PRACTICAL APPROACH AND INVESTIGATION O F HYPOGLYCAEMIA History and clinical picture The direction of investigation will depend essentially on whether the history is suggestive of 'fasting' or 'reactive'
  
  284
  
  CUNICAI BIOCHEMISTRY
  
  HISTORY
  
  Suggestive of feeling hypoglycaemia
  
  Suggestive erf reactive by pog I у caem la
  
  I
  
  T
  
  n
  
  Documentation hypoglycaemia
  
  During spontaneous symptoms or During a prolonged
  
  Documentation of hypogrycaemia
  
  ]
  
  Durmg spontaneous symptoms Mixed meal tolerance test
  
  test
  
  т
  
  .
  
  т
  
  Identrfl cation
  
  of the cause
  
  i den tin cation of the cause
  
  !
  
  History of gastric
  
  1 1
  
  surgery Exclude mild diabetes meliitus Alcohol-induced: g r and tonic test idiopaihic
  
  * Insulin and C-peptlde
  
  k^
  
  Islet-cell tumours Suipnonyiurea-mduced Autoimmune insulm syndrome Dn*g-»nduced
  
  Insulin and C-peptide
  
  Chronic renal failure Liver disease Non-islet cell tumours Endocrine deficiencies Drug-induced Alcohol-induced Anorexia nervose
  
  * Insulin * C-peptide
  
  U
  
  Insulin administration Anti-receptor antibodies
  
  T Further investigations Plasma proinsulin SuJphonyiuree screen Localization studies Arrti-msulm antibodies
  
  Depend on dmical picture
  
  Anti-insulin antibodies Artti- receptor antibodies
  
  Fta. 15.2 A practical approach to rhc investigation and diajtno»i*. of hypoglycaemia
  
  hypoglycaemia (Fig. 15.2). A thorough and detailed his­ tory of the symptoms (frequency, type, relationship to meals and exercise) is therefore essential. If symptoms are relieved by food ingestion, it is important to enquire about the type of food and speed of recovery. All medications should be inspected to exclude a prescribing or dispensing error and the possibility of surreptitiously induced hypo­ glycaemia should always be considered in health care pro­ fessionals and relatives of diabetic patients. Assessment of alcohol intake and pattern of drinking is important since alcohol can cause both fasting and reactive hypoglycae­
  
  mia- A family history of multiple endocrine neoplasia (MEN1) should be sought. Clinical assessment should include assessment of nutritional status with measurement of weight and body mass index. Underlying illnesses that can produce fasting hypoglycaemia such as severe renal or hepatic disease, congestive cardiac failure and anorexia nervosa are usually obvious clinically. Endocrine deficiencies such as hypopituitarism and adrcnocortical insufficiency should be sought and excluded by appropriate investigations if necessary.
  
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  HYPOGLYC AF-MIA
  
  Investigation offasting hypoglycaemia The aims of the investigation are firstly, to demonstrate that hypoglycaemia is the cause of the symptoms and, secondly, to identify the cause of the hypoglycaemia. Demonstration of hypoglycaemia Measurement of blood glucose during spontane­ ous neuroglycopenia The measurement of the blood glucose concentration (and collection of a suitable blood specimen for subsequent measurement of plasma insulin and C-peptide concentrations,) during a spontaneous epi­ sode of neuroglycopenia, and before glucose is given, is without doubt the best test for spontaneous hypoglycae­ mia. Unfortunately, failure to collect blood for proper laboratory measurement of glucose and insulin during a spontaneous episode of hypoglycaemia is common and provocation (prolonged fast) is often employed to repro­ duce the hypoglycaemia (see below). Improper blood collection, delayed separation, severe hypertriglyccridaemia and increased number of blood cells such as in leukaemia and polycythaemia may all lead to artificially low blood glucose values. Dry reagent strips for glucose measurement should not be used for the diag­ nosis of hypoglycaemia as they may give falsely low results due, for example, to insufficient blood being placed on the strip or incorrect riming. Prolonged fast A prolonged fast (up to 72 h) is the single most useful test employed in the evaluation of fast­ ing hypoglycaemia. The aim of the test is to demonstrate spontaneous hypoglycaemia in the presence of neuroglycopenic symptoms and that the symptoms resolve on administration of glucose. During a 72 h fast, normal sub­ jects rarely develop plasma glucose concentrations below 2.2 mmol/I. and almost never develop neuroglycopenic symptoms. The fast must be conducted in hospital and under strict medical supervision. Before initiation of the fast, a simple assessment of coordination, recent memory and calcula­ tions (e.g. counting up in sevens) should be performed as a baseline. The time of initiating the fast is determined depending on a reasonable estimate of the patient's likely tolerance for fasting. If fasting starts after the evening meal or at midnight, the majority of patients with insulinoma will develop hypoglycaemia by midday when adequate staff and laboratory facilities are available. Patients should have an intravenous indwelling catheter and an ampoule of 50% glucose solution should be readily available. During the test, the patient is allowed water and non-caloric beverages and should be exercised under su­ pervision. Blood for glucose and insulin measurements is collected at the beginning and every 4 hours thereafter. Plasma glucose should be determined in the laboratory as soon as possible and plasma or serum should be stored at -20°C for subsequent analysis of insulin and C-pcptidc.
  
  285
  
  Bedside glucose meters can be used to monitor the pa­ tient's blood glucose frequendy but low values, especially in the absence of symptoms of hypoglycaemia, should always be confirmed in the laboratory before any action, such as glucose administration, is taken. When the plasma glucose has fallen below 3.0 or 2.5 mmol/L, blood should be collected more frequently and the patient should be assessed for neuroglycopenia by repeating the same tests performed at the initiation of the fast. If neuroglycopenia develops and blood glucose is below 2.5 mmol/L, several specimens should be obtained for glucose and insulin/ C-peptide measurements before terminating the test by administering intravenous glucose. The patient can then be allowed to eat. If hypoglycaemia docs not occur after a 72 h fast, the test should be terminated. The measurement of 3-hydroxyburyraie concentrations during the fast is quite useful in that rising plasma 3hydroxybutyrate levels indicate indirectly that circulating insulin levels have been suppressed and the fast could be terminated. Blood should always be collected in this situa­ tion to confirm suppressed plasma insulin and C-peptide levels. Identification of the cause of hypogiycaania Plastna insulin and C-peptide Although insulin and C-peptide are secreted in equimolar amounts by the (J-cells of the pancreas, the metabolic clearance of insulin is much higher than that of C-peptide. Therefore, Cpeptide has a longer half-life and is present in peripheral blood in higher molar concentrations than insulin, making it less prone to marked fluctuations. Consequently, the measurement of plasma C-peptide levels may be more reliable as an indication of endogenous insulin produc­ tion. However, C-peptide is cleared by the kidneys and raised levels may occur in renal impairment. The measurement of plasma insulin and C-peptide concentrations, in the prcsercc of hypoglycaemia, is the most useful test in identifying the cause of hypogiycaemia (see Fig. 15.2). Hyperinsulinaemia or inappropriate plas­ ma insulin levels (i.e. not suppressed) in the presence of hypoglycaemia usually indicate the presence of insuli­ noma, or self-administration of insulin or sulphonylurea. Suppressed plasma C-peptide levels in the presence of hyperinsulinaemia will identify patients with self-adminis­ tration of insulin. If both plasma insulin and C-peptide levels are inappropriately raised, a sulphonylurea screen is important in identifying patients with surreptitious or inadvertent intake of sulphonylurea. The presence of circulating antibodies to insulin, either due to previous exposure to exogenous insulin or sponta­ neously, may give falsely high plasma insulin concentra­ tions. Because human C-peptide does not cross-react with insulin antibodies, the measurement of C-peptide in these situations can be used as an index of fi-cell function.
  
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  286
  
  CLINICAL BIOCHEMISTRY
  
  Plasma 3-hydroxybutyrate Lipolysis is very sensi­ tive to circulating insulin levels. During fasting, normal individuals will show a gradual decline in insulin and a progressive increase in lipolysis and hence kctone bodies, e.g. 3-hydroxybutyratc (Fig. 15.3). In patients with hypoglycaemia due to hyperinsulinaemia, lipolysis is suppressed and 3-hydroxybutyratc levels arc low (600|imol/ L), the only exception being in severe inanition when fat stores are depleted. Therefore, the measurement of plasma 3-hydroxybutyrate provides an indirect measure of the prevailing insulin level during the hypoglycaemia. Its main advantage, however, is that it is easy to measure and the result can be made available long before those of insulin or C-peptidc measurements. Plasma proinsulin Proinsulin is normally converted to insulin and C-pcptidc but a small amount, less than 20% of the total fasting immunoreactive insulin, is secTtted into the circulation. In islet cell tumours, the amount of circulating proinsulin is increased and may
  
  account for the majority of the insulin immunorcactivity. Occasionally, some rumours may secrete mainly or ex­ clusively proinsulin and therefore it is important that the insulin antiscra employed in insulin assays cross-react with proinsulin, otherwise those tumours would be missed. Investigation of reactive hypoglycaemia The aim of the investigation is to demonstrate that hypo­ glycaemia is the cause of the patient's symptoms. Measurement of blood glucose during symptoms If the patient is seen at the time of spontaneous symp­ toms, blood should be collected for the measurement of glucose. Otherwise, the patient is instructed to collect blood from a finger prick onto a filter paper for later analysis of blood glucose. The patient should collect blood whilst symptomatic and should document the na­ ture and timing of symptoms in relation to meals. It is also advisable to ask the patient to collect blood about 2-3 h after meals, in the absence of symptoms. The diagnosis is
  
  A. In normal subjects after prolonged fasting or in hypoglycaemia with hypoinsulinaemia
  
  Uvw
  
  Adipose tissue
  
  Vlneulin t etacaeon
  
  | ketones (plasma з-hyoVoxybutyrate > 600 umol/L)
  
  В In hypoglycaemia due to hyperinsulinaemia Adpose tissue
  
  I ,vo-
  
  Intulin
  
  Key
  
  + Stimulation — Inhibrtton
  
  {kef ones (plasma 3-hydroxybutyrate < 800 umoVL)
  
  FFA ж Free fatty acids TCA « Tncarboxyiic acid cycte
  
  Fif. 15.3 Change* in the concentrations of hormone* and metabolites in normal subjects after prolonged fasting and in hypoglycaemia with hypoin&ulinaemia \.A) and in hypoglycaemia due to hypermsulinaemia (B).
  
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  HYPOGLYCAEMIA
  
  made if the patient's blood glucose concentrations are consistently less than 2.8mmol/L during symptoms, on more than one occasion, provided that symptoms did not occur at times when blood glucose concentrations were normal. These patients should be further investigated by means of mixed meal tolerance tests. Self-monitoring by patients at home using a glucose meter has been suggested as an alternative method to filter paper blood collec­ tion but this is not advisable since it may lead to false self-diagnosis. Mixed meal tolerance test Patients suspected of having postprandial hypoglycacmia should have a mixed meal tolerance test, in which a stand­ ard meal of normal composition is ingested and then plasma glucose concentration is monitored every 30 min for 5 h and also at any time while the patient is sympto­ matic. Capillary or arteriali7.ed blood (from a vein on the back of a warmed hand) should be used since venous plasma glucose levels arc about 10% lower than arterial concentrations after a meal due to extraction of glucose by muscle. Patients should be observed for symptoms and the timing of these symptoms in relation to the meal and to the blood glucose concentrations should be docu­ mented. Patients who develop symptoms during hypo­ glycacmia, but not at other times during the test, may be considered to have postprandial hypoglycaemia. The diagnosis of postprandial or reactive hypoglycae­ mia should not be made on the basis of an oral glucose tolerance test (OGTT). At least 10% of'normal' subjects may have plasma glucose nadirs of less than 2.6 mmol/L during an O G T T and 2.5% may have values less than 2.2 mmol/L. In a study involving 192 patients with sus­ pected reactive hypoglycaemia, 129 patients had symp­ toms during the O G T T but these were unrelated to the level of plasma glucose nadir or to the rate of decline of the glucose level.
  
  287
  
  Diabetics who are treated with insulin arc particularly prone to exercise-induced hypoglycaemia. In normal sub­ jects, glucose uptake in skeletal muscles is increased by 20-30 times during exercise. This is compensated for by an increase in hepatic glucose release mediated by a fall in circulating insulin levels. In insulin-treated diabetics, the continuous release of insulin from subcutaneous depots inhibits glucose output from the liver. In addition» in­ creased absorption of insulin from the injection site may occur if the site is near the muscles being exercised. Severe hypoglycaemia, sometimes fatal, can occur in diabetic patients treated with sulphonylurea drugs. Elderly diabetic patients, especially those with renal or liver impairment, arc particularly susceptible to hypogly­ caemia and a reduction in sulphonylurea dosages is often required. The hypoglycaemia is usually prolonged and may last for 36 h to 7 days despite continuous infusion of glucose. Additional treatment with glucagon and/or diazoxidc may be required to prevent a fatal outcome. Several drugs such as P-blockers, sajicylates and alcohol (see below) may predispose to hypoglycaemia in diabetics. Inadvertent insulin overdose in elderly diabetic patients with poor vision or deliberate overdose to seek attention or to attempt suicide should always be considered. Occasionally, increased insulin sensitivity in a diabetic patient may be due to concurrent hypopituitarism, hypothyroidism or hypoadrenocorticalism (Addison's disease). Surreptitious administration of hypoglycaemic agents in non-diabetic patient» (factitious hypoglycaemia)
  
  Insulin- or sulphonylurea-induced hypoglycacmia in diabetics
  
  Surreptitious or self-induced hypoglycaemia should always be considered in patients with hypoglycaemia. The diag­ nosis should be suspected in individuals with access to insulin or sulphonylureas, such as health care profession­ als and relations or friends of diabetic patients. Of these patients, 80% are women between the ages of 20 and 40 years who vehemently deny self-administration and rarely show signs of psychiatric disturbances. Many have the characteristics of Munchausen's syndrome with a history of frequent hospitalizations, previous surgical procedures and extensive travel.
  
  This is by far the commonest cause of spontaneous fasting hypoglycaemia. Inadequate food intake or a missed meal is usually the precipitating factor. Variable absorption of mixtures of short- and intermediate-acting insulin from subcutaneous depots, which may vary by 50% from one day to another, is also an important factor. Many patients with long-standing diabetes have impaired countcrrcgulatory responses (glucagon and adrenaline) to hypogly­ caemia. These patients, and also those with autonomic neuropathy, may have 'hypoglycaemia unawareness' and are susceptible to frequent, severe and prolonged episodes of hypoglycacmia.
  
  Once suspected, the patient should be admitted to hospital for detailed observation and blood should be collected during hypoglycaemia for the measurement of plasma insulin, C-pepiide, insulin antibodies and sulpho­ nylureas. A raised plasma insulin level and a suppressed C-peptide concentration will confirm the diagnosis of selfadministration of insulin. In the past, the presence of cir­ culating insulin antibodies, a common consequence of the use of bovine or porcine insulin preparations, would sug­ gest previous exogenous insulin administration. However, now that most insulin preparations are of human types, the likelihood of insulin antibodies being produced is
  
  FASTING HYPOGLYCAEMIA
  
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  lower and if found, they may indicate autoimmune hypoglycacmia (see below). If plasma insulin and C-peptide levels are both inappropriately normal or raised, a sulpho­ nylurea screen (plasma and urine) should be performed if a false positive diagnosis of insulinoma is to be avoided. Once the diagnosis of factitious hypoglycaemia is made, the patient should be referred for counselling and psychi­ atric therapy. Inadvertent intake of sulphonylurea drugs Accidental intake of sulphonylurea drugs is rare but should be considered in all patients presenting with hypo­ glycaemia. 'l*hc finding that a spouse or a relation is taking a sulphonylurea drug should alert the physician to the pos­ sibility of accidental intake. The possibility of a prescrip­ tion or dispensing error should also be considered. Islet cell tumours (insulinoma) Insulinomas are rare, with an annual incidence of about 1 or 2 cases per million persons. *1Ъсу can occur at any age with approximately 8 0 % occurring between the ages of 20 and 60 years and with, probably, a slight preponderance in women. In approximately 8 0 % of patients, there is a solitary benign adenoma. Benign adenomas are generally small (0.5-2 cm in diameter) and are distributed equally among the head, body and tail of the pancreas. Multiple tumours occur in about 10% of cases and at least 10% of all cases of insulinomas are malignant, with regional or distant metastascs. About 4 % of insulinomas arc associ­ ated with multiple endocrine neoplasia type 1 (MEN1). Nesidioblastosis and diffuse islet cell hyperplasia have both been reported in adults. Many insulinomas contain and secrete other hormones in addition to insulin (glucagon, gastrin, somatostutin and pancreatic polypeptide) bui hypcrsccrction of these hormones is not usually asso­ ciated with clinical symptoms.
  
  Clinical features Hypoglycaemia is the clinical hallmark of insulinomas. Neuroglycopenic symptoms, especially confusion, slurred speech and irrational behaviour, typically occur before breakfast though 50% of patients may also have symptoms late in the afternoon. More than 50% of patients have amnesia and therefore the history should alwavs be corroboratcd. Nocturnal hypoglycaemia is not uncommon and can manifest by nightmares and morning headaches. Symptoms can be provoked by exercise, missing a meal or by following a restricted calorie diet. Transient hemiplegia, focal neurological signs and coma can occur but permanent brain damage is rare and is only seen with pro­ found hypoglycaemia, unrecognized and uncorrected for several hours.
  
  Diagnosis Insulinoma is a rare cause of fasting hypoglycaemia. More common causes such as surreptitious or inadvertent ad­ ministration of insulin or oral hypoglycaemic agents, alco­ hol abuse and severe liver disease (see Table 15.3) should be excluded first. The diagnosis of insulinoma requires the demonstration of hypoglycaemia, unsuppressed or elevated plasma insulin levels during hypoglycaemia and a pancreatic tumour. Patients with suspected insulinoma should have a prolonged fast with monitoring of plasma glucose, 3-hydroxybutyrate and insulin levels. In patients with insulinoma, plasma insulin levels fail to suppress de­ spite the presence of hypoglycaemia and plasma 3-hydro­ xybutyrate concentrations are usually below 600 nmol/L. Plasma C-peptide levels are raised and the insulin/Cpeptide ratio is 1:5 compared to 1:10 in normal subjects due to decreased hepatic extraction of insulin. Plasma proinsulin levels are significantly higher in patients with insulinoma than in normal subjects and the molar ratio of insulin/proinsulin is 1:1 compared to 6 :1 in normal sub­ jects . Occasionally, all circulating immunorcactivc insulin is in fact proinsulin and highly specific insulin assays may therefore give misleading results. A variety of suppression tests (insulin, diazoxide and somatostatin tests) and stimulation tests (tolbutamide, glucagon, leucine and calcium tests) have been used in the past to detect insulinoma. These tests, however, are po­ tentially dangerous, unreliable and suffer from a high rate of false negative and false positive results. This is mainlydue to the variable degrees of differentiation of insulinsecreting rumours. These tests are rarely used nowadays and should be reserved for difficult cases. Localization Once hypoglycaemia and inappropriate hyperinsulinism have been established, attempts to localize an insulinsecreting tumour should be undertaken. Unfortunately, non-invasive tests such as C T scanning and ultrasonography can only identify less than half of the cases. Selective coeliac axis angiography may identify about 6 0 - 8 5 % of cases but the procedure is invasive, time-consuming and requires a lot of experience. In addition, false localization is not uncommon, that is, the location suggested by angio­ graphy does not correspond to the tumour found at opera­ tion. Percutaneous transhepatic portal venography with selective blood sampling for insulin measurement has a sensitivity as high as 97% but the procedure is difficult, tedious, expensive and not without risks. Although preoperative localization is desirable, it is not essential since experienced surgeons can palpate the tumour at laparotomy in 80-90% of cases. Recently, intraoperative locali­ zation by ultrasound scanning has been shown to be highly sensitive, identifying between 8b and 100% of cases.
  
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  Treatment Surgical resection is the treatment of choice for insulinsecreting tumours and is curative in over 80% of cases. In 10% of cases, no tumour can be found at laparoiomy. In these cases, a blind distal two thirds pancrcatectomy can be performed but the success rate is only 25%. There­ fore, this procedure should only be attempted in patients who have not responded or arc intoleram to a preoperative trial of diazoxide. The latter, therefore, should be attempted on patients in whom preopcrative localization is unsuccessful. Hypcrglycaemia is common following the successful removal of insulinoma and is due to a combination of factors, namely, chronic downregulation of insulin recep­ tors by the previously high circulating insulin levels, high concentrations of counter-regulatory hormones, oedema of the pancreas and probably suppression of normal pan­ creatic islet cells caused by long-standing hypoglycaemia. It is usually mild and temporary and may last from a few days to up to 2 weeks. II surgery is contraindicated, medical treatment using diazoxide or, occasionally, a somatostatin analogue such as octreotide can reduce insulin release. Diazoxide sup­ presses insulin release in about 50% of cases and can be useful, in responsive cases, in preopcrative management. It directly inhibits insulin release via a-adrenergic stimula­ tion and enhances glycogcnolysis by inhibiting cyclic AMP phosphodiesterase. Its main side-effect is sodium and water retention which can be prevented by giving a diuretic. Strepiozotocin and 5-fluorouracil, either singly or in combination, can be used in the treatment of malig­ nant insulinoma. Hypoglycaemia associated with renal failure Renal impairment is a common predisposing factor for hypoglycaemia and is probably the second most common cause of hypoglycaemia, after insulin therapy, in hospital­ ized patients. The most important predisposing factor for hypoglycaemia in renal failure is caloric restriction whether acute, due to anorexia and vomiting, or chronic. Other predisposing factors include liver dysfunction, con­ gestive cardiac failure, septicaemia and drug therapy such as insulin, oral hypoglycaemics, salieylates and [Miockers. The pathogencsis of hypoglycaemia in renal failure is complex and several mechanisms have been proposed. In normal subjects, the kidneys play a major role in gluconeogenesis and may supply as much as 4 5 % of new glucose during prolonged starvation. In uracmic patients, who are often malnourished, renal gluconeogenesis may not be able to maintain an adequate glucose supply even if hepatic gluconeogenesis is normal. Other mechanisms include impaired hepatic glycogenolysis and gluconeo­ genesis; increased insulin half-life due to decreased renal degradation; diminished availability of alaninc, and im-
  
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  paired counter-regulatory mechanisms. Intercurrent con­ ditions such as hepatic dysfunction, congestive heart failure, septicaemia and drugs may also predispose to hypoglycaemia in uraemic patients. Therapy for diabetes mellitus, with insulin or sulphonylurea drugs, is by far the most common cause of hypo­ glycaemia in patients with renal impairment. Decreased renal degradation, frequently necessitating a reduction in insulin or sulphonylurea requirements, is the main reason, latrogenic hypoglycaemia may be a consequence of haemodialysis or peritoneal dialysis. This may be due, in part, to exaggerated glucose-mediated insulin secretion to high glucose content of the dialysate and impaired insulin degradation by the kidneys. The symptoms of uraemic hypoglycaemia are mainly neuroglycopenic (drowsiness, headache, lethargy, confu­ sion, convulsions and coma) and may be confused with the symptoms of the dialysis dysequilibrium syndrome. Spontaneous hypoglycaemia in renal failure, not associ­ ated with hypoglycaemic agents or dialysis, carries a grave prognosis with over half of the patients dying within months of onset* Hypoglycaemia associated with liver disease Although the liver plays a central role in normal glucose homoeostasis, hypoglycaemia is rare in patients with liver disease. Glucose homoeostasis can be maintained with the mass of functioning hepatocytcs reduced to less than 20% of normal and hypoglycaemia does not occur until the liver is extensively damaged. Hypoglycaemia is not usually a feature of chronic hepatic failure nor is it implicated in the production of hepatic coma. Conversely, it may occa­ sionally be associated with mild liver disease and has been reported in a wide variety of liver diseases such as fatty infiltration, portal cirrhosis, infective hepatitis and hepatocellular carcinoma. Frank hypoglycaemia is also uncommon in acute (fulminant) hepatic failure, whether due to viral infection or hepatotoxic drugs. When it does occur, it can be severe and persistent. The diagnosis of hepatogenous hypo­ glycaemia may be difficult since the extent of liver disease, as assessed by standard liver function tests, does not cor­ relate with the degree of hypoglycaemia. ЧЪегеГогс, other causes of hypoglycaemia should always be sought in a patient with hypoglycaemia and abnormal 'liver function tests'. The hypoglycaemia associated with congestive heart failure, septicaemia and Rcye's syndrome is thought to be due to hepatic mechanisms. Hypoglycaemia associated with non-islet cell tumours A wide variety of tumours have been associated with hypo­ glycaemia (Table 15.4). Mescnchymal tumours account
  
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  ■ ю-nlet cell tumour» associated with hyj
  
  icmia
  
  Inmoun of tkt соп/исм* tiiitus (яшвшеядя Mcsotbettora rnat ncurofibroma and fibrosarcom* Rhabdomyosarcoma 1 потуоздгеота Hacmangiopci na Ijposarcoma Tumoun of epithelial ortf.n patera* Adrenocorucal carcinoma WiunV tumour and rnpemephroma wjrcinoma I'hucochi" чти Breast carcinoma ctnoma of the cervix Tu*hwn of кьшнорошк origin Leu каст I.ymphoma Myeloma mid tumou
  
  for 45-64% of the reported cases and hepatomas, the most common epithelial tumours to cause hypoglycacmia, account for 22% of cases. Most patients are elderly and present with predominantly neuroglycopenic symptoms which may antedate the recognition of the existence of the tumour by many years. In most cases, especially those due to large mesenchymal tumours, the tumour is readily detectable by physical or radiological examination but in others, the tumour may be small and difficult to detect. Plasma insulin and C-peptide arc almost invariably sup­ pressed during hypoglycacmic episodes. The treatment of the hypoglycaemia is symptomatic, with administration of glucose orally or intravenously. Definitive treatment should be aimed at the primary tumour and successful treatment with surgery, radiation or chemotherapy usually results in amelioration of the hypoglycacmia. Several mechanisms by which these non-pancreatic rumours produce hypoglycaemia have been proposed. Increased glucose uptake by tumours coupled with a re­ duction in hepatic production of glucose has been sug­ gested. Weight loss and cachexia may greatly reduce the flow of gluconeogenic substrates and contribute to the hypoglycaemia. In recent years, the possibility that these tumours produce peptides with insulin-like activity has been exten­ sively investigated. Non-supprcssible insulin-like activity (NSILA) in plasma can be attributed to two components: one is a group of substances which are soluble in acid cthanol (NSlLAs) and the other group of substances precipitate in acid cthanol (NSILAp). Increased levels of NSII-Ap have been reported in some patients with nonislet cell tumours and hypoglycacmia but other patients with tumours and similarly elevated levels of NSILAp arc not hypoglycacmic.
  
  The NSILAs component can be largely accounted for by the two insulin-like growth factors 1 and 2 (IGFl and IGF2), also known as somatomedins. IGFl is mainly un­ der the control of integrated growth hormone output and is modulated by insulin and nutritional status. In hypo­ glycacmia associated with high circulating insulin levels, plasma IGFl concentrations are higher than those in con­ trols. Conversely, low circulating plasma IGFl concentra­ tions have been consistently reported in patients with non-islet cell tumour-induced hypoglycaemia and the ratio of IGFl to IGF2 is low. Elevated levels of IGF2 have been observed in some, but not all, patients with tumourinduced hypoglycaemia. The conflicting data may be due to methodological differences or storage conditions. High levels of IGF2 mRNA have been reported in Wilms* tu­ mours and more recently, high concentrations of IGF2 were found in phaeochromocytomas and a leiomyosarcoma. Although these findings suggest that IGF2 may be a factor in the hypoglycaemia which occasionally occurs in these tumours, the serum levels of IGF2 are only mod­ estly elevated and may not have been sufficient to produce hypoglycaemia. Recently, tumour necrosis factor ( T N F a ) has been shown to increase in vivo glucose utilization by macrophagc-rich tissues, increase the rate of glucose transport and increase insulin receptor number. However, the ability of T N F to induce sustained hypoglycacmia in experimental animals has not been established. Whether cytokines, released by tumours or macrophages activated by tumours, play a role in tumour-associated hypoglycac­ mia remains to be seen. Hypoglycaemia due to endocrine deficiencies Endocrine deficiencies, especially those involving the pi­ tuitary and the adrenal glands, are well-recognized causes of hypoglycaemia. Spontaneous hypoglycaemia due to pituitary insufficiency is more commonly seen in neonates and children than in adults and may occasionally be the presenting symptom, especially in the elderly. '1Ъе hypo­ glycaemia is mainly due to growth hormone deficiency but the associated deficiency of ACTH accentuates the tendency to hypoglycaemia. The diagnosis is usually suspected on clinical examination and is confirmed by ap­ propriate combined pituitary function tests (see Ch. 16). Plasma insulin and C-peptide levels are both appropri­ ately suppressed but 3-hydroxybutyratc levels are not sup­ pressed, indicating that ketogenesis may not be impaired in the absence of growth hormone. The hypoglycacmia in this setting should be treated with both glucose and hydrocortisone. Spontaneous hypoglycacmia is also well recognized in isolated ACTH deficiency and growth hormone defi­ ciency and has been observed in adults with isolated growth hormone deficiency after prolonged fasting.
  
  HYPOC.l VCAEMIA
  
  Hypoglycaemia is uncommon in primary adrenal insuf­ ficiency {Addison's disease); when it occurs, it is mainly due to impaired gluconeogenesis. Patients with Addison's disease are sensitive to fasting and hypoglycaemia can be precipitated by missing a meal and by exercise. Alcohol, by further impeding gluconeogenesis, may also provoke hypoglycaemia in these patients. Plasma insulin and Cpeptide levels are appropriately low and 3-hydroxybutyrate levels are high. Hypoglycaemia occurring in acute adrenal insufficiency (Addisonian crisis) is a medical emergency and requires immediate correction by intra­ venous infusion of glucose in addition to hydrocortisonc and other measures. Congenital adrenal hyperplasia (adrenogenital syn­ drome) is a rare cause of neonatal hypoglycaemia. In the majority of cases, cortisol deficiency is not often severe due to a compensatory increase in ACTH secretion. Although untreated hypothyroidism is associated with some lowering of fasting blood glucose levels, sympto­ matic hypoglycaemia has only been reported in a few cases and its existence has been questioned. Deficiency of other hormones, such as adrenaline and glucagon, does not seem to cause hypoglycaemia. Drug-induced hypoglycaemia Sulphonylurea drugs, either singularly or in combination with other drugs, account for the majority of druginduced hypoglycaemia. They are by far the most com­ mon cause of hypoglycaemia in diabetic patients over 60. Additional important precipitating factors are restricted carbohydrate intake, liver and renal impairment. Sulpho­ nylurea drugs can cross the placcntal barrier and stimulate insulin secretion in the fetus. Life-threatening hypoglycae­ mia has been reported in newborn infants of diabetic mothers who were treated with chlorpropamide during the third trimester. Sulphonylurea-induced hypoglycae­ mia may be prolonged, especially in those with renal impairment, lasting for up to 7 days and necessitating continuous treatment. Mortality is high, ranging from 7.5%-8,4%, and in attempted suicide with sulphonylurea drugs, the mortality is even higher with seven deaths out of 20 reported cases. Salicylates overdose has been associated with hypo­ glycaemia in children. In adults, therapeutic doses of salicylates have been shown to lower the blood glucose concentration in both diabetic and non-diabetic patients. The mechanisms by which salicylates produce hypogly­ caemia are unknown. Enhancing insulin secretion and in­ hibiting hepatic gluconeogenesis are possible mechanisms. Non-selective (J-blockers, e.g. propranolol, in thera­ peutic doses may induce hypoglycaemia, particularly in the presence of other precipitating factors such as liver disease, fasting or strenuous exercise. Hypoglycaemia has also been reported in newborn infants of mothers who
  
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  were treated by propranolol until hours before delivery. The mechanism is thought to be through the prevention of the normal glucagon-mediated glycogenolytic and gluconeogenetic responses by the liver. In diabetic patients, propranolol may mask the adrenergic symptoms of hypoglycaemia (except sweating) but may also delay recovery from hypoglycaemia because this requires mobi­ lization of glucose from liver. Selective (Ji-blockers, e.g. atenolol and metoprolol, have a lesser effect because the hepatic p-adrenoreceptor is of the P^-subtype. The advan­ tage of a selective (i-blocker is limited to a faster recovery phase from hypoglycaemia. Quinine stimulates insulin secretion and its intravenous use in the treatment of falciparum malaria has been asso­ ciated with profound hypoglycaemia. Plasma insulin and C-peptide levels are both elevated, indicating increased endogenous secretion. Falciparum malaria, especially in children, can itself cause hypoglycaemia. This, however, is associated with suppressed plasma insulin levels and is probably due to high glucose uptake by the parasitized crythrocytcs. Renal failure, hepatic dysfunction and star­ vation are common in malaria and mav be additional factors in precipitating the hypoglycaemia. Symptoms of hypoglycaemia may be mistaken for those of cerebral ma­ laria and blood glucose should always be monitored in these patients. Pentamidine, a biguanide derivative, has a direct toxic effect on jj-cclls causing release of preformed insulin and hypoglycaemia. This can be followed by (i-cell destruction with ultimate insulin deficiency and diabetes. Pentamidine is used for the treatment o( Pneumocystis carinii infection and there are at least 30 case reports of severe pentamidine-induced hypoglycaemia during treatment of pneumocystis pneumonia in undernourished patients with AIDS. Disopyramide, an antiarrhythmic drug, has also been associated with severe hypoglycaemia, particularly in elderly patients with renal or hepatic impairment. Other drugs which have occasionally caused hypo­ glycaemia include lidocainc, sulphonamides, dcxtropropoxyphene and />-aminobenzoic acid. Alcohol-induced fasting hypoglycaemia (fasting) Alcohol can be associated with both fasting and reactive hypoglycaemia (see later). Several mechanisms may be operational in alcohol-induced fasting hypoglycaemia but the most important is direct inhibition of gluconeogenesis. This is mainly due to accumulation of NADH and in­ creased NADU/NAD ratio resulting from the oxidation of ethanol. Hypoglycaemia characteristically occurs 6-36 hours after ingestion of moderate to large amounts of alcohol, mosdy in malnourished chronic alcoholics. Occasionally, it can occur in healthy binge drinkers or in children after
  
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  a relatively small amount of alcohol. Patients present with neuroglycopenic symptoms including stupor and coma. The patient's breath may smell of alcohol and the symp­ toms mav be mistaken for acute alcoholic intoxication. Therefore, delayed recovery from a presumed alcoholic intoxication should alert the doctor to the possibility of hypoglycaemia. The blood glucose level is usually below 2.0mmol/L and alcohol is nearly always detectable in the blood, though the concentration is not necessarily very highSevere metabolic aeidosis with high blood lactate concen­ tration is a characteristic feature. Hyperketonacmia and ketonuria are almost invariably present but predominantly in the form of 3-hydroxybutyrate, since the accumulation of NADH suppresses the conversion of 3-hydroxybutyrate to acetoacetate. Ketosis may, therefore, go unrecognized if methods which only detect aeetoacetate are employed (e.g. Rothcra's reaction or Kctostix). Plasma insulin and C-peptide levels during the hypoglycaemia are usually ap­ propriately suppressed. Prompt diagnosis and treatment, with intravenous glucose, is essential since mortality is relatively high. Glucagon is not effective because hepatic glycogen stores are depleted by the time hypoglycaemia ensues. Alcohol potentiates the hypoglycaemic effect of insulin and sulphonylurea drugs. In insulin-treated diabetics, ingestion of alcohol may produce profound hypoglycaemia which can be fatal. Hypoglycaemia, resulting from the combined effect of alcohol and sulphonylurea drugs, tends to be less profound possibly because diabetic pa­ tients receiving these drugs tend to be obese and, there­ fore, are in part protected from the hypoglycaemic effects of alcohol. Hypoglycaemia due to deficient energy intake Symptomatic hypoglycaemia is well recognized in starva­ tion and has been observed in patients with protein-calorie malnutrition and anorexia nervosa. The hypoglycftemia is usually due to reduced hepatic glycogen reserves as well as gluconeogenic substrates. In addition, the lack of ketosis due to markedly depleted fat stores deprives the central nervous system of an alternative source of energy. '1Ъс major risk factors arc low body weight and intercurrent infection. Compulsive excessive exercise in patients with anorexia nervosa may also be a factor. Plasma insulin and C-peptide levels are appropriately suppressed. Despite low circulating insulin levels, 3-hydroxybutyraTe levels are low due to depleted fat stores. Hypoglycaemia in anorexia nervosa has a poor prognosis. Autoimmune hypoglycaemia Two autoimmune syndromes which can cause hypogly­ caemia have been described. In one syndrome, antibodies
  
  bind insulin receptors mimicking the action of insulin and producing fasting hypoglycaemia. Most patients with this syndrome also have evidence, either in the laboratory or clinically, of other autoimmune diseases such as sys­ temic lupus erythematosus, primary' biliary cirrhosis or Hashimoto's thyroiditis. Laboratory investigations show high plasma insulin levels but suppressed C-pcptidc levels, similar to the findings with surreptitious adminis­ tration of insulin. Therefore, the demonstration of the presence of antibodies directed against the insulin re­ ceptor is essential to make the diagnosis. The hypoglycae­ mia seems to respond rapidly to high doses of steroids but not to immunosuppression or plasmapheresis. Prognosis is poor but in those who survive, the antireceptor anti­ bodies disappear and remission occurs over several months or vears. w
  
  The autoimmune insulin syndrome, in which autoantibodies are directed towards insulin, has been reported mainly in Japan and is extremely rare. This syndrome is classically associated with reactive hypoglycaemia but may rarely cause fasting hypoglycaemia. Most patients are middle-aged with no history of previous administration of insulin and may have evidence of other autoimmune diseases such as Graves* disease, rheumatoid arthritis or systemic lupus erythematosus. An association with admin­ istration of certain drugs such as hydralazine, procainamide, penicillaminc, glutathione and carbimazolc has also been suggested. Several possible mechanisms for the hypoglycaemia have been suggested. The sudden dissocia­ tion of insulin from the antibodies is the most plausible one. Laboratory investigations reveal high plasma insulin levels but because the insulin is produced endogenously, C-peptide levels arc not suppressed. The majority of cir­ culating insulin is bound to antibodies and the demonstra­ tion of the presence of anti-insulin antibodies is essential to make the diagnosis.
  
  Septicaemia Bacterial septicaemia, especially Gram-negative, can oc­ casionally cause hypoglycaemia. It has been postulated that cytokines produced by macrophages in response to endotoxin stimulation may induce hypoglycaemia by in­ creasing insulin secretion. '1Ъе endotoxins released may also have a direct hypoglycaemic effect, probably by inhib­ iting gluconeogenesis. Renal failure, which is often associ­ ated with septicaemia, may also be an important factor in the pathogenesis of the hypoglycaemia. Exercise-related hypoglycaemia Hxercise is associated with a marked increase in glucose uptake by muscles. During the first 5-10 min of severe exercise, glucose is supplied by the breakdown of muscle glycogen but by 40 min 75- 90% of the glucose is supplied
  
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  by the blood, mainly from increased hepatic glucose production. Initially, 7 5 % of increased heparic glucose output is derived from glycogenolysis and 25% from gluconcogcncsis. With prolonged exercise, gluconeogenesis becomes more important and contributes up to 4 5 % of the total hepatic glucose output. Decreased plasma insulin and increased catecholamines, glucagon, cortisol and growth hormone concentrations all contribute to the in­ creased hepatic glucose output. If hepatic glucose produc­ tion is inadequate, blood glucose cannot be maintained during exercise and hypoglycaemia ensues. Hypoglycaemia following excessive exercise is well recognized and may be severe enough to cause seizures. Although exercising to exhaustion can produce hypo­ glycaemia, the symptoms of exhaustion are not related to hypoglycaemia and glucose administration does not modify the time of exercise to exhaustion. POSTPRANDIAL OR REACTIVE HYPOGLYCAEMIA Alimentary hypoglycaemia Hypoglycaemia may occur in patients who have under­ gone gastric operations such as subtotal gastrectomy and pyloroplasty. The incidence of hypoglycaemia in these patients ranges from 5 % to 37%. Following a high carbo­ hydrate meal, the plasma glucose may rise to higher than normal due to rapid gastric emptying and rapid absorp­ tion of nutrients. This early hypcrglycacmia stimulates excessive insulin release which results in a rapid fall in plasma glucose and secondary hypoglycaemia. Symptoms, which are usually adrenergic, occur 2-3 h after ingestion of food and last for about 10-20 min. These symptoms are different from those of the dumping syndrome (flush­ ing, sweating, abdominal cramps and hypotension) which occur within half an hour of eating. Patients with alimen­ tary hypoglycaemia should be advised to eat frequent, small, high protein meals with a low content of refined carbohydrate.
  
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  cause reactive hypoglycaemia. This effect is not observed, however, when a complex carbohydrate is ingested with alcohol. Alcohol has been shown to potentiate the insulinstimulating effect of glucose. Idiopathic reactive hypoglycaemia and the postprandial syndrome The diagnosis of idiopathic reactive hypoglycaemia is con­ troversial. True idiopathic postprandial hypoglycaemia probably does occur in a very small minority of patients. The majority of patients, however, with postprandial non­ specific symptoms do not have hypoglycaemia and should be referred to as having the postprandial syndrome. This syndrome consists of non-specific symptoms, such as feelings of vague ill health, lighiheadedness, dizziness, anxiety, palpitations, fatigue, hunger and 'inner trem­ bling', which may resemble the adrenergic symptoms of hypoglycaemia. Many patients, especially those who are well informed about hypoglycaemia from the lay litera­ ture, state that symptoms are relieved by eating, thereby suggesting hypoglycaemia. However in the majority of cases, detailed questioning reveals that symptoms are relieved either immediately or within 2-3 min of eating foods such as cheese or bread, which are unlikely to raise blood glucose levels in such a short time. Many of the patients complain of generalized weakness and inability to concentrate in between attacks. The symptoms are not progressive but usually last for many years.
  
  Some patients with mild diabetes may have symptomatic hypoglycaemia 4-5 h after a meal. This is probably due to a delayed insulin release from defective ($-cells in response to the rising blood glucose level. Although patients with early diabetes mellitus may have blood glucose levels less than 2.8 mmol/L 4-5 h following an oral glucose load, a minority of them experience symptoms of hypoglycaemia after meab.
  
  Reliance on the use of oral glucose tolerance tests in the investigation of this condition in the past has led to the misdiagnosis of reactive hypoglycaemia in many of these patients. The oral glucose tolerance test ( O G T T ) is inad­ equate for detecting true postprandial hypoglycaemia since 10% of healthy individuals may have low blood glu­ cose levels ( bioassay for oestrogen and aim simply to achieve regular withdrawal bleeds. It is possible to measure plasma oestradiol in patients receiving transdermal oestradiol replacement, but other oral preparations of modified synthetic or equine oestrogens cannot logically be moni­ tored biochemically. Desmopressin replacement therapy can be monitored by measurement of plasma osmolality, aiming w maintain a normal osmolality and avoid dehydration or fluid over­ load. Patients should be instructed to allow the diuresis to resume prior to each treatment dose. CONCLUSIONS In the past two decades, clinical ncuroendocrinology has been revolutionized by the ready availability of assays for the relevant pituitary hormones. The relationship between cndocrinologist and clinical biochemist must be a close one, since the biochemical tests to be performed depend closely on the clinical assessment of the patient, whilst the treatment which the patient will receive is equally depend­ ent on the biochemical results obtained. In this chapter I hope that I have laid down a logical framework for such a harmonious relationship. APPENDIX: T E S T PROTOCOLS Assessment of basal pituitary function A basal 9.00 a.m. blood sample for measurement of: • cortisol • total (or free) T 4 • TSH • testosterone (male) or oestradiol (female) • LH, FSH • prolactin • osmolality.* Simultaneous random urine for osmolality.*
  
  •Assessment of posterior pituitary function is only required if indicated by the clinical context
  
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  Insulin stress test Indications: Assessment reserve.
  
  of ACTH/cortisol
  
  and
  
  GH
  
  Contraindications: Ischaemic heart disease* epilepsy or un­ explained blackouts, severe long-standing hypoadrenalism. Precautions: ECG must be normal, basal 9.00 a.m. cortisol must be above lOOnmol/L. Total or tree T 4 must be normal.
  
  Note: only this shortened protocol has actually been correlated with cortisol responses to an 1ST. Alternative regimes including i.m. Synacthen and cortisol responses at 60 min might be assumed to give the same information, but this has not been demonstrated in the literature and I believe that such equivalence cannot be assumed without validation.
  
  KEFEKKKCKS
  
  Procedure: Fasting from midnight, i.v. cannulu, soluble insulin, 0.15 LTkg i.v. at 9.00 a.m. Observe and record signs of hypoglycaemia. Hypoglycaemia only needs to be reversed with i.v. dextrose if severe and prolonged or with impending or actual loss of consciousness or fits.
  
  Lindholm J, Kchlet H. Re-evaluation of the clinical value of the 30 minute ACT1 i test in assessing hypothalamo-pituitary-adrenocortical function. Clinical Endocrinology 1987; 26; 5^-** Stewart P M, C o m e J, Seckl J R, Edwards С R W, Padfield ? L A rational approach for assessing the hypothalamo-pituitary-adrenal axis. Lancet ! Noradr > Isoprcn Adr > Noradr > Isoprcn Kopren > Adr = Noradr Isoprcn > Adr > Noradr
  
  Praz > P b - Yob Phcntol > Yoh > Pra/ Metop > But Butox > Mctop
  
  Abbreviation .-vii ■ jdrciialinc. Ndndi
  
  : "ыЛггплЬп.
  
  Isoprcn a isoprcnahnc, Pra/ = prazoun, Phcntol ■ phcntolarmm Yoh ■ yohimbinc. Mctop = mctoprolol, Ншох = hutoxammc
  
  327
  
  the multiple endocrine ncoplasia syndromes (MEN) types 2a and 2b (see Ch. 38). Clinical syndromes and their presentation The classic presentation is of intermittent hypertension with additional intermittent symptoms of catecholamine excess. Symptoms can include headaches, palpitation, anxiety and intermittent panic attacks, nausea and vomiting, weight loss, abdominal pain, constipation and sweating; signs include blood pressure instability, hypertension, bradycardia and tachycardia, tremor, pyrcxia, hypcrglycaemia and heart-failure. The attacks can be precipitated by pressure on the tumour and/or many drugs such as histamine, metoclopramide, droperidol, tricyclic antidepressants, phenothiazincs and A C T H . The differential diagnosis is clearly wide. Diagnosis and investigation Screening is often carried out using 24 h urinary metanephrines which have a 90% specificity and sensitivity or, less accurately, 4-hydroxy-3-methoxymandclic acid (HMMA), though patients need to be on a vanilla-free diet. Plasma catechol­ amines (under resting conditions) which exceed 10 nmol/L for noradrenaline or 1.5nmol/L for adrenaline arc also around 90% specific and sensitive. Small tumours remain difficult to diagnose. Urinary* free catecholamine measurements are also of value though small intermittently-secreting tumours may be missed. Noradrenaline levels are more often (80-90%) raised than adrenaline (50-70%) and overall sensitivity is over 90%. Urinary measurements arc obviously well suited for screening purposes. Many suppression and stimulation tests are employed in specialist units, but disagreement about their respective merits continues. The glucagon stimulation test is prob­ ably the safest and most accurate test in provoking a rise in plasma catecholamines and blood pressure. Despite all these advances it remains occasionally necessary to use a therapeutic trial of the a-blocker phenoxybenzaminc. C T and MR scanning are widely used and radionuclidc imaging using metaiodobenzylguanidine (mlBG); this is specifically taken up by chromaffin cells, though is not widely available. Treatment The choice of treatment lies between surgery, desirable wherever possible as it is curative, and drug therapy including a- and fi-blockade. Surgery on un­ prepared patients is hazardous as there is frequently great instability of blood pressure and cardiovascular status. Patients with phaeochromocytoma therefore need careful preparation, and preparation and surgery should only be carried out by surgeons, physicians and anaesthetists with experience in this field. Patients should be commenced on an a-blockcr, usu­ ally phenoxybenzamine, before use of a P-Ыоскег which unopposed may cause hypertension. Once a-blockade is
  
  328
  
  CUXlCAt, BIOCHEMISTRY
  
  established, (J-blockade should be started with propranolo]. Completion of blockade usually takes about 3 days and may be accompanied by a significant drop in haemoglobin concentration as vascular volume, which is reduced by about 20 30% by excess catecholamines, is re-expanded: transfusion is often prudent before surgery takes place. Some authorities recommend infusion of phenoxybenzamine intravenously for 2-3 days before operation. During surgery a solution of sodium nitroprussidc should be available as blood pressure may still prove unstable, especially when the tumour is first handled. Patients require long-term follow-up to check for the occurrence of further tumours. Where operation is not possible, long-term treatment with phenoxybenzamine and propranolol is effective. The combined a- and p-blocker, labetolol, is less effective for this indication. Neuroblastoma/xanglioneuroma These uncommon rumours of prenatal life, infancy and childhood, commonest in boys under 3 years of age, arise from the adrenal medulla or the sympathetic chain and may be accompanied by symptoms and signs of catecholamine excess, although their usual presentation is as an abdominal mass. They may be unilateral or bilateral and vary between relatively benign (ganghoncuroma) and very malignant behaviour including widespread metastatic spread (neuroblastoma). Approximately 75% of cases produce excess catechol­ amines and arc detected as for phaeochromocytoma, though dopamine secretion is often prominent. Treatment depends on the extent of the tumour: where localized, surgical cure may be expected in 6080%; where the disease is extensive chemotherapy is used, though with poor results. APPENDIX: PROTOCOLS FOR C O M M O N T E S T S O F ADRENAL F U N C T I O N Protocols and reference ranges for endocrine tests are often difficult to give exactly, as minor modifications of sample timing, dose, collection conditions, chemical and other analytical methods may make significant differences. Additionally many laboratories have their own minor modifications (and sometimes their own reference ranges); the local laboratory should always be consulted before performing a complex, time-consuming, expensive or invasive test. The protocols and reference values given should there­ fore be taken as a guide rather than as absolute; clearly normal and clearly abnormal responses can be seen as such, but borderline values require further thought and advice.
  
  Short Synacthcn test In the short test, 0.25 mg of Synacthen is given intra­ muscularly at time 0 after an initial sample has been taken for plasma cortisol. The test is normally performed at 9.00 a.m., to which time the reference range relates. If adrenal function is intact, there is an increase of plasma cortisol in samples taken exactly 30 and 60 min later, with a peak that should exceed 550 nmol/L and with an incre­ ment of at least 200 nmol/L. Long Synacthen test A 1 mg dose of Depo-Synacthen (a long-acting form) is given intramuscularly and plasma cortisol is measured before the injection and at + 1 , 2, 4, 6, 24 and 48 h there­ after. The normal response is an immediate and substan­ tial rise in plasma cortisol while, in hypopituitarism or adrenal suppression, the rise is delayed and attenuated. 9 5 % confidence limits for cortisol (in nmol/L) are: 60 min 2h 4h 8h 24h
  
  605-1265 750-1520 960-1650 1025-1600 610-1500
  
  Overnight suppression tests A dose of oral dexamethasone (usually 1 mg but up to 2 mg have been used by some authorities) is given at midnight with a single measurement of plasma cortisol at 9.00 a.m. the following morning. Suppression of plasma cortisol to 100 nmol/L or below excludes the diagnosis of Cushing's syndrome. Failure to 4uppress requires further investigation. Low dose dexamethasone suppression The low dose test works on the same principle as the overnight one but requires the administration of dexam­ ethasone 0.5 mg every 6 h for 48 h with cortisol being measured at 9.00 a.m. on day 0 before the first dose and again at 9.00 a.m. on day 2 (+ 48h from day 0). It is essential that dexamethasone is taken 6 hourly, which means the patient must wake at 3.00 a.m. each night. Suppression is defined as a fall in plasma cortisol below 50 nmol/L at 48 h. High dose dexamethasone suppression The protocol is identical to that for the low dose test, except that the individual doses are 2 mg every 6 h, giving a total dose of 8 mg per day. It is used only in the differen­ tial diagnosis of Cushing's syndrome. Suppression is defined as a fall of 50% or more in the 9.00 a.m. cortisol value after 48 h.
  
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  ADRENAL DISORDERS
  
  329
  
  FURTHER READING Drury P I,. Endocrinology. In: Kumar P J, Clark M (eds) Clinival medicine, 3rd edn, London: Hailhcrc Tindalc, 1994. A undent and postgraduate tdxtbt**k ofclinical medicine. Greenspan F S (cd) Basic and clinical endocrinology, 3rd edn. East Norwalk: Appleion and Lunge, 1991. Derailed endocrine text including much baste science. Grovcman A. Clinical endocrinology, part» 6 and 7. Oxford: Blackwell Scientific Publications, 1992. Large\ detailed text, predominantly for chmcians.
  
  James V H T. The human adrenal cortex, 2nd edn. New York: Raven Press, 1992.
  
  Very well referenced definitive work on the cortex. Re^nck R, Armstrong P. Imaging in endocrinology. The adrenal gland. Clinical Endocrinology, 1994; 40: 961 576. Up-to-date detailed r m w of radiological techniques applied to the adrenal.
  
  Мат
  
  й авторским пр.
  
  Материал, защищенный авторским правом
  
  C H A P T E R
  
  Thyroid dysfunction J. Butler and R. Pope
  
  Interfoiiicuiar PART 1 NORMAL T H Y R O I D P H Y S I O L O G Y T H E THYROID GLAND The thyroid gland lies in front of the trachea just below the larynx. Embryologically it is derived from the floor of the pharyngcal cavity and migrates from the base of the tongue to its final position in the neck via the thyroglossaJ duct. Abnormalities of this migration give rise to ectopic glands which may not function normally. Occasionally the gland is completely absent. The gland is bilobed with a central isthmus and weighs 10^20g in the adult. The blood flow is about 5 mLAnin/g of tissue, nearly twice that of the kidney. The gland consists of thousands of follicles, each a spheroidal sac of epithelial cells surrounding a lu­ men containing colloid, largely thyroglobulin, the pre­ cursor of thyroid hormones (Fig. 18.1). T h e thyroid also contains C-cells which secrete calcitonin; calcitonin is discussed in Chapter 6. KV-ICJ яг ce I
  
  i O ^ l Is
  
  {pyocyie;
  
  easement membrar>e Synthesis, storage a n d release of thyroid h o r m o n e s Synthesis of the thyroid hormones thyroxinc (T4) and tri-iodothyronine (T3) (Fig. 18.2) occurs in the follicles, beginning with the uptake of iodide from the bloodstream, against a steep concentration gradient, by the iodide pump of the basal membrane of the follicular cell (Fig. 18.3). The iodide pump is competitively inhibited by anions of similar size to iodide. Pertechnetate may be used for radioactive imaging of the gland and perchlorate is used
  
  Flj. 18.1 Structure of thyroid follicle. to block the uptake of iodide, for example after accidental exposure to radioactive iodide. Thiocyanate competitively inhibits the iodide pump but is not taken up into the gland. Both thyroglobulin, a glycoprotein of molecular weight 660 000 Da containing many tyrosyl residues, and the peroxidase enzyme system responsible for iodination of 331
  
  Материал, за
  
  чный авторски»-
  
  332
  
  CUNICAL BIO
  
  1
  
  McchauavB
  
  Result
  
  Exam pi c
  
  Comp'-unds occu; 11 binding I n serum pro
  
  Total Г I reduced
  
  Sabcylai Other NSAin* Fhenytoin
  
  Hcpann m two
  
  iipoprotcm | ase, increasing free fatty acids
  
  Free fatty acids
  
  rltgfa fee 1 l a n d f r e I \b
  
  •mpounds increasing
  
  H
  
  11 .ind 'I
  
  Oestrogena
  
  TBG Interference in immunoasaaya for
  
  Spin i high I * and free T 3
  
  N S.MI"» = non-ttcroidal ami-inflammatory drug
  
  Soffit NSAIDl
  
  И 2
  
  CLINICAL BIOCHEMISTRY
  
  Thyroid crisis PART 3 HYPERTHYROIDISM
  
  Florid hyperthyroidism may result in a life-threatening illness. In these fortunately uncommon cases (thyroid 'crisis' or 'storm') cardiovascular symptoms and signs pre­ dominate. Circulatory collapse is characteristic and results from both the above mechanisms and from a reduction in intravascular volume due to fluid loss (excessive sweating, gastrointestinal losses» etc.). Pharmacological attempts to correct abnormal cardiac rhythms are usually unsuccess­ ful in the face of continuing hyperthyroidism. Manage­ ment aims to control the hyperthyroidism rapidly (using large doses of antithyroid drugs, including iodine) whilst treating the patient symptomarically. (i-Blocking drugs such as propranolol are used in these patients (perhaps the only indication for their use in the presence of heart fail­ ure) and measures to cool the patient are often needed. Use of steroids under these circumstances may be of addi­ tional value.
  
  Clinical features The clinical symptoms and signs which may result from the hyperthyroid state are summarized in Table 18.9. The clinical picture seen may be modified by the presence of coexisting systemic diseases and can depend on the age of the patient. Certain of the thyroid diseases themselves also produce characteristic physical signs; for example, the orbital and cutaneous manifestations of Graves' disease. The changes seen in the major organ systems are de­ scribed below. Cardiovascular system Many of the manifestations of hyperthyroidism relate to the increased demands placed upon the cardiovascular system. High circulating levels of thyroid hormones have a direct stimulatory effect on cardiac muscle. Heart rate and stroke volume arc both increased at rest (including during sleep) and peripheral vascular resistance is reduced, lead­ ing to a marked rise in cardiac output in patients who have no pre-existing cardiac disease. Arrhythmias, usually supraventricular, occur frequently. Atrial flutter/fibrillation, producing a pulse which is irregular in both rate and vol­ ume, is seen in over 10% of patients with hyperthyroidism and may be the presenting feature in some cases. Patients who suffer from ischaemic chest pain typically find that angina worsens as a result of the increased metabolic demands placed on the myocardium.
  
  Table IH.9 ТЫ |ЩфЦЩЦ and signCommon S^mptmu Increased imubll; Increased sweating Heat intolerance
  
  Nptatfao
  
  Lethargy Lots of wciuht BrndfttaMDCM Increased bowel movement Signs Tachycardia mopmg of upper с
  
  Diagnosis '1Ъс diagnosis of Graves' disease is not difficult when the typical triad of hyperthyroidism, goitre and extrathyroidal involvement is present. The diagnosis may be made in other circumstances by demonstrating the presence of thyroid-stimulating antibody and is supported by the finding of diffuse, increased uptake of radioisotope on thyroid scintiscanning (Fig. 18.8). Biochemical evi­ dence of hypenhyroidism is usually but not always present at presentation. Natural history The natural history of this disease is one of relapses and infrequent (perhaps 10-15%) remis­ sions of the hypenhyroidism with time. Approximately 40% of patients relapse shortly after 12 months antithyroid drug therapy; of these, 50% will achieve lasting remission after a second course of these agents. The extra­ thyroidal manifestations of the disease can develop with
  
  заьт
  
  THVKOCD DYSFUNCTION
  
  34 5
  
  fetus hypothyroid. 'Block and replace' treatment should not be used as T 4 crosses the placenta only poorly. Iodinc-131 can be used therapeutically to treat hyperthyroidism due to Graves' and other thyroid disorders. Radioiodinc works initially by interfering with organification of iodine but later predominantly inhibits replication of thyrocytcs by inducing radiation damage in the gland and hence controlling thyroid ovcractivity, after a delay of weeks or months. The major side-effect of this form of therapy is long-term hypothyroidism, with up to 80% of patients becoming hypothyroid within the first 10 years post-treatment. There is no evidence of an increase in the risk of malignancy following therapeutic doses of radioiodine and the dose can be repeated if necessary in the future.
  
  l-"if. 18.Я Thyroid scmtiscan showing diffuse homogeneous uptake of isotope throughout the jtfand, y^nb vcr>' low background activity. This jppearance is characteristic of Grave*' disease.
  
  or without thyroid dysfunction, occasionally many years after the initial onset of thyroid disease. Treatment Three forms of antithyroid treatment arc available for treatment of patients with hyperthyroid Graves 1 disease: antithyroid drugs, radioactive iodine and subtotal thyroidectomy. Choice of treatment modality is dependent on factors such as the age of the patient, size of goitre and patient preference. The antithyroid drugs mcthimazole (carbimazole) and propylthiouracil exert their action in two ways. They in­ hibit the action of thyroid peroxidase and are directly immunosupprcssivc. Treatment should ideally be continued until TSH receptor antibody levels return to normal, but in the absence of routinely available assays, therapy is often continued empirically for 12 or 18 months. The antithyroid drugs can be given either in titrated doses or as part of a 'block and replace' regimen using both carbimazole and thyroxine. Lasting remission is only achieved in 6 0 % of cases; adverse prognostic indicators include high pretreatment levels of T 3 , the presence of a large goitre and persistence of T S H receptor antibody at the end of the planned treatment period. As the drugs do not affect release of thyroid hormone from the gland, there is typically a delay of 4-8 weeks before euthyroidism is restored. Symptomatic relief can be obtained in nearly all pa­ tients by also using P-blocking drugs for these first few weeks of therapy in order to reduce their tremor and tachycardia. In pregnant thyrotoxic patients careful moni­ toring is required in order to keep the dose of antithyroid drugs to a minimum, especially during the last trimester, as these compounds cross the placenta and render the
  
  Subtotal thyroidectomy is a highly effective treatment for this condition. Perioperative prohlcms include haem­ orrhage and the risk of damage to the recurrent laryngeal nerve and postoperative difficulties may arise due to haem­ orrhage leading to trachea! compression or to (usually transient) hypocalcaemia. The risk of late and permanent hypothyroidism is lower than with radioiodinc treatment but is still significant. Patients should be rendered euthyroid prior to operation using carbimazolc or propylthiour­ acil wherever possible or large doses of oral iodine. Toxic multmodular goitre Hyperthyroidism arising in a previously multinodular goitrc occurs in an older population than that affected by Graves' disease; typically over the age of 50, with females being affected more than males. Clinical features The cardiovascular features of hyperthyroidism tend to be prominent in this often elderly group of patients, though all the features of hyperthy­ roidism mentioned earlier may occur. The goitre itself is classically nodular and may be large, often having been present for many years prior to the onset of thyroid dysfunction. Diagnosis The biochemical diagnosis of hyper­ thyroidism in this situation is fairly straightforward, with suppression of T S H , though thyroid hormone levels may not be grossly elevated, in some cases lying at the upper limit of normal. Thyroid autoantibodies are not usually present. Thyroid scimiscanning shows a patchy uptake of isotope, with multiple hot and cold areas being seen throughout the gland. Radioiodinc uptake values may lie at or above the upper limit of normal, but are not usually grossly elevated and thyroid ultrasound can be used to confirm the multinodular nature of the gland. Treatment In general, radioiodinc is the treatment of choice for the patient with toxic multinodular goitre. Antithyroid drugs can be used until radioiodine becomes effective. Where comprcssivc symptoms result from gland enlargement, surgery should be used in place of radioiodine.
  
  346
  
  cusncAi. BtOOfliMISTRY
  
  Toxic adenoma Isolated, autonomously functioning adenomas within the thyroid are a less common cause of hyperthyroidism and are typically seen in the third and fourth decades. Patients present with characteristic features of hyperthyroidism and are usually found to have a well-defined, solitary nod­ ule within the gland. Nodules felt to be solitary on clinical examination may be found to represent a larger nodule within a multinodular gland if thyroid imaging is under­ taken; this may form an important diagnostic procedure. Diagnosis Scimiscans of the gland in cases of toxic adenoma show an area of high uptake in the region of the nodule» with suppression of uptake in the remaining, nor­ mally responsive, areas of the gland. Early in the course of the adenoma's development thyroid function tests may be normal, but later they show typically hyperthyroid results. T3-toxicosis may be more commonly found in this disease than with other forms of hyperthyroidism. Thyroid anti­ bodies are typically not found. Treatment Patients with proven toxic adenoma may be successfully treated either by radioiodine, which will of course be concentrated by the hyperfimctioning adenoma, or by surgery. TSH-see red figpttmtary lumour Very rarely, adenomas of the pituitary gland secreting T S H may produce hyperthyroidism. Diagnosis The biochemical key to making this diag­ nosis is the persistence of T S H secretion despite definite overproduction of thyroid hormones. High circulating levels of the a subunit of T S H may be found in the serum. T S H secretion is not increased by the administration of T R H and a diffuse goitre is found in the absence of immunological evidence of thyroid autoimmunity. If large, the pituitary lesion may produce local damage, with loss of other anterior pituitary hormone secretion and impair­ ment of the visual fields. Computerized tomography or magnetic resonance imaging of the region will usually confirm the presence of a mass lesion. Treatment Treatment consists of surgical removal of the tumour or attempts to reduce T S H secretion pharma­ cologically. The long-acting somatostatin analogue octreotide has been reported to reduce T S H secretion and tumour size in individual cases and the dopamincrgic agonist bromocriptine has also been used. Ablative antithyroid therapy will lead to control of hyperthyroidism, but will not deal with the primary problem and is there­ fore not appropriate. Other causes of hyperthyroidism In some of the areas discussed below, the term 'hyper­ thyroidism' is strictly not applicable, as the elevation of thyroid hormones results from cxtrathyroidal sources.
  
  However, for continuity, this term will be used to repre­ sent states of clinical and biochemical thyroid hormone excess. Iodine In individuals with goitre due to previous iodine deficiency, chronic administration of excess iodine in the diet can induce a hyperthyroid state. This phenom­ enon (sometimes called the Jod Basedow phenomenon) is particularly likely to occur in patients who already have a degree of pre-existing thyroid autonomy, expression of which may have been masked by the lack of iodine. In iodine-replete areas a similar phenomenon may be seen and in some cases the hyperthyroidism reverses completely when the source of additional iodine is withdrawn from the diet. The precise relationship between iodine dose and thyroid response is clearly complex, since it is dependent not only on the individual's prior exposure to iodine and the degree of gland autonomy, but also on the time course of iodine administration; use of large doses of iodine as a therapeutic means of rapidly reducing gland reactivity has been practised for many years, quite the opposite of the effect seen with chronic administration of more moderate doses. In iodine-induced hyperthyroidism, the radioiodine uptake is characteristically reduced and urinary iodine ex­ cretion increased, with coincident T S H suppression and evidence of elevated T 4 and elevated or high normal T 3 levels. Amiodarone Use of this iodine-containing antiarrhythmic drug can result in the production of confusing changes in thyroid function test results. Amiodarone has a structure similar to that of thyroid hormones and inter­ feres with the peripheral conversion of T 4 to T 3 . Result­ ing levels of T 4 may therefore be high and T 3 low, with an increase in levels of rT3. Basal and TRH-stimulated TSH levels may both be high. In addition, the high iodine content of the drug can itself cause a hyperthyroid state, though this is less common than the abnormalities described above. It is advisable to check basal thyroid function tests before commencing amiodarone therapy. Assessment of thyroid function during treatment should, however, be primarily based on careful clinical evaluation of the patient, as the biochemical changes are difficult to interpret. The long half-life of the drug means that changes in thyroid test results may persist for some time after ceasing therapy. Thyrotoxicosis factitia Cases of hyperthyroidism occur due to self-administration of thyroid hormone, taken either as T 3 or T4, Radioiodine uptake is low and goitre is absent. Unless there is a prior history of thyroid autoimmunity, thyroid-directed antibodies will be absent. T S H will be undetectable; total and free T 4 concentra­ tions will be high if the patient is taking T 4 and suppressed if the patient is taking T 3 ; total and free T 3 concentra­ tions will be high in both cases. Careful supervision of patients with serial measurement of thyroid function in hospital may be necessary and specific psychological
  
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  THYROID DYSFUNCTION
  
  assessment and counselling may be required after the diagnosis has been made. Ectopic thyroid tissue Metastatic thyroid follicular carcinoma may rarely produce sufficient thyroid hormone to result in hyperthyroidism. Other tumours, such as ovarian teratomata, may contain functional thyroid tissue in sufficient quantity to produce symptoms and signs of thyrotoxicosis (struma ovarii); endogenous thyroid radioisotope uptake will be suppressed and functioning tissue demonstrable in the tumour under these most uncommon circumstances. Other thyroid stimulators Trophoblastic tumours such as choriocarcinoma, hydatidiform mole and meta­ static embryonal testicular carcinoma may secrete an unu­ sual thyroid stimulator and produce biochemical (though rarely clinical) evidence of severe hyperthyroidism.
  
  347
  
  blood flow, which contributes to the intolerance to cold that is so often present. In the thorax, cardiac dilatation on chest X-ray may be seen and occurs in association with pericardia] effusion, which occasionally leads to com­ promised myocardial function. Plasma activities of muscle enzymes such as crcatine kinasc may be raised, but ischacmic chest pain is uncommon, more frequently being seen in hypothyroid patients with coincidental ischaemic heart disease when they first receive thyroid hormone replace­ ment therapy. The cardiac changes reverse with appropri­ ate thyroid replacement therapy. Gastrointestinal system
  
  PART 4 HYPOTHYROIDISM
  
  Gross weight gain is rarely due to hypothyroidism, though most individuals show a moderate gain in weight despite reduced appetite, due primarily to fluid retention. Intesti­ nal absorption of nutrients is affected both by the reduc­ tion in rates of absorption seen with hypothyroidism and the increased intestinal transit time, such that total ab­ sorption may actually increase. Results of biochemical liver function tests are usually normal.
  
  Clinical features
  
  Central and peripheral nervous system
  
  The clinical features seen as a consequence of low circu­ lating levels of thyroid hormone are summarized in Table 18.12.
  
  Deficiency of thyroid hormones in fetal or early neonatal life, if not promptly treated, results in irreversible damage to the central nervous system, with structural abnorma­ lities evident on histological examination. In adult life neurological defects resulting from hypothyroidism are usually reversible. The classic picture of cretinism is fortunately only rarely seen now, due largely to the intro­ duction of neonatal screening for hypothyroidism and the efficacy of therapy, if introduced sufficiently early in life.
  
  Cardiovascular system A reduction in resting cardiac output occurs. Cool periph­ eries are characteristic, due to a reduction in cutaneous
  
  Table IS.12 The symptom* and «jpu i Common .•mptoms Lethargy Skin dry and coarse Slow speech and mentation Facial puffinesa (oedema) Cold intolerance
  
  Ра&м
  
  n
  
  i ncommon
  
  Апогсхгл Irritability Menorrhacia Angina Dcaniess Poof co-ordination
  
  Ноагче voice mtipation Weight gain Hair dry and lallmx out Weakness/stimiesft Breathiesaness
  
  Sfem Pcnorhual/facui oedema I*ale. drv skin Goitre Obtundcd menial Mate Cool peripheries Diffuse alopecia Rradycaniia Median nerve compression Delayed relaxation phase of reflexes
  
  PaycbOssI Pleural/pcncardial effusions Ccrcbcllar ataxia alactorrhoea Mydrocoelc (males)
  
  In adult life, the characteristic features are of general­ ized slowing in intellectual function, with inanition, slow mentation, somnolence and, occasionally, a frankly psy­ chotic state. Speech becomes slow and the voice coarse and gruff in nature, the latter in part due to oedema within the vocal apparatus. Ccrcbcllar ataxia may be seen with prolonged hypothyroidism and may become irrevers­ ible with delay in treatment. Seizures may also occur in severe cases. Peripheral nervous system manifestations are also common, with compression of the median nerve at the wrist being perhaps the best known (carpal tunnel syn­ drome). Relaxation of the tendon jerks is characteristically delayed. luKomotor system Muscular stiffness is a particularly common complaint in hypothyroidism and relates to reduced relaxation rate. The muscles show abnormal structure on microscopy, with loss of striations, oedema, swelling of fibres and rela­ tive deficiency of type II fibres. Muscular weakness is often evident clinically.
  
  348
  
  CUVICAI. SKXTHEMISTRY
  
  Respiratory system Similar changes to those described above may occur in the respiratory muscles, with abnormal muscle function leading» in patients with pre-existing lung disease» to exac­ erbation of carbon dioxide retention and sleep apnoea. Chest X-ray may show pleura! effusions, though these are seldom large. Skin and hair Increased water binding due to deposition of mucopolysaccharides is seen in the skin, in common with the other tissues. The indurated oedema that results gives rise to the 'myxoedematous' appearance of the typical hypothyroid patient. Associated anaemia and hypercarotenaemia may also render the skin pale or yellow, respectively. Body hair tends to be easily lost, though the classic description of loss of the outer third of the eyebrows is, in our experi­ ence, rarely seen today.
  
  The skeleton Thvroid hormone deficiency in eariv life leads to abnormalities of the epiphyscs with marked reduction in linear growth and stunted final height. In common with all sys­ temic dlesses, prolonged hypothyroidism in childhood leads to retardation of hone compared with chronological age. Rates of bone turnover are reduced, leading to a reduction in the pool of exchangeable calcium. Plasma concentrations of calcium and phosphate remain normal. Alkaline phosphatase activity tends to be low in children with hypothyroidism.
  
  77/e kidneys: mineral and water balance Renal blood flow and glomerular filtration are both de­ creased, but total body water has been shown to increase with hypothyroidism, due to impaired renal excretion of water which itself results from a reduction in delivery to the distal tubule and abnormal osmoregulatory function in the hypothalamus and posterior pituitary. Although ex­ changeable body sodium is increased, the dilutional effect typically leads to a mild hyponairaemia. Plasma creatinine and urea concentrations remain normal. Reproductive system In adults of both sexes, hypothyroidism leads to a reduc­ tion in libido and subfcrtiliry. Menorrhagia due to failed progesterone secretion with anovulatory cycles is common in the female, as is oligospermia in the male. These changes may be related to impaired LH secretion, particularly in long-standing cases. Basal gonadotrophin levels are, how­ ever, typically within the normal range, unless pituitary
  
  disease is responsible for the hypothyroid state. Reduc­ tions in levels of sex hormone-binding globulin lead to an elevation in free sex hormone levels, with a reduction in total levels of both oestrogen and testosterone following from both this and alterations in sex steroid svnthesis. Other systems Cortisol turnover is often reduced and in some patients the cortisol response to hypoglycaemia is blunted, though response to exogenously administered ACTH is normal. I-nng-standing primary hypothyroidism can, however, lead to secondary suppression of pituitary and adrenal function and introduction of thyroid hormone therapy should be made with caution in these circumstances, as frank adrenal insufficiency may be precipitated. Hyperprolactinaemia is a common finding, resulting from in­ creased T R H release. The hyperprolactinaemia correlates with the degree of elevation of T S H , though frank galactorrhoea is only relatively infrequently found. A normochromic anaemia, which may be either normo- or maerocytie in nature is often seen and is likely to reflect diminished production of erythropoietin, with low red cell mass. Macrocytosis is frequently ascribed to coincidental occurrence of vitamin B, ; deficiency due to autoimmune pernicious anaemia, but a deficiency in the response to available B i r and concurrent malabsorption of folate from the gut may contribute. If menorrhagia has been prolonged or severe, micro- rather than macrocytosis may be present as part of an iron deficiency state. Plasma levels of clotting factors VIII and IX may be reduced in hypothyroidism and platelet adhesiveness reduced, caus­ ing a mild bleeding tendency that may exacerbate the existing risk of anaemia. The effects of thyroid hormones on lipids have been outlined earlier in this chapter. Glu­ cose absorption from the gut and uptake by the tissues from plasma are both delayed but in established diabetics a characteristic increase in sensitivity to exogenous insulin is seen, probably due to decreased clearance of insulin.
  
  Causes of hypothyroidism Table 18.13 lists the causes of hypothyroidism. These are described in detail below, following a brief outline of monitoring thyroid function at diagnosis and during treat­ ment. Where the use of specific immunological and imaging tests arc of value in the diagnosis or management of particular causes of hypothyroidism, this is discussed. Measurement of plasma T S H levels provides the cor­ nerstone of the biochemical evaluation of hypothyroidism. As circulating concentrations of thyroid hormone fall, T S H secretion increases and is used to monitor thyroid status. Secretion of T 3 is preferentially maintained in the presence of the high T S H concentrations that accompany declining thyroid function. T3/T4 ratios therefore rise and
  
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  THYROID DYSHNCTION
  
  Table 18.П
  
  Post-surgery or post-radioiodinc
  
  Ста Hypothvroidism due to
  
  i >i
  
  349
  
  /Viman (dm to thuase of the thynnd gland) Thyroiditis НЫит.'юЧ Autoimmune destructi
  
  ihe gland
  
  Loss of a critical mass of functioning thyroid tissue, from whatever cause, may lead to a permanent hypothyroid state, although transient restoration of a euthyroid state may occur after radioiodine-induced early hypothyroidism.
  
  PoMfwrtnm Dc Qucrvain'v mi Riedcl's (ran 1
  
  mged mlUmmnt^n damage * «land become» пЪгоясИ
  
  P n m u y myxocdema
  
  ind atrophy; r*posi-sutounmunc m many case»
  
  Ahlati"
  
  Post-radiowdine or surgery; gland bulk reduced
  
  Athyreos» Dytbonsooofnc Drags
  
  Abnormal or absent thyroid ti*»ue
  
  Iodine deficiency
  
  Lowered gland iodine, unpaired hormone synthev
  
  Асиг
  
  1 Г4ПМС-ПГ inhibition »f hormone svnthe \y become permanent in the presence of ciiexuting th'. ' Jcstructi amvimmune activity
  
  me excess
  
  Abnormal synthesis ot thyroid hormone» Inhibit l thyroid hormone synthesa induction of ihyroid auioimmun
  
  ScctrnJiir\- (Jut- to extrathyrouial dueas Antcnor ! failure tumours granulomatous deposit* Sheehan'* svndromc Loss of'1 M ! stimulation of Post pituitary destructive therapy thvroid Developmental abnormalities Post head injury Hypotfutefnlc dysfunction
  
  TRH Simulation anterior pituitary
  
  plasma T 4 concentrations correlate better with thyroid activity than do plasma T 3 concentrations in these pa­ tients. Replacement therapy for primary hypothyroidism can be monitored using plasma TSH concentrations, aim­ ing to restore these to the normal range. The particular problems resulting from pituitary failure are described below. Hashimoto's disease and other forms of thyroiditis arc described in the following section on thyroiditis. Primary myxocdema In a large proportion of patients with primary hypo­ thyroidism, no goitre, or history of goitre, will be found. These individuals will have high plasma T S H concentra­ tions with low total and free T 4 concentrations and the majority will have one or many types of serum thyroid autoantibodies, including antiperoxidasc, antithyroglobulin and T S H receptor-blocking antibodies. Thyroid growthblocking antibodies have also been described in these patients. Whilst the majority of patients may represent endstagc Hashimoto's thyroiditis with lack of clinical rec­ ognition of the early phases of the disease, clearly other immunological processes may be responsible for some of them.
  
  Congenital hypothyroidism Causes of congenital hypothyroidism arc listed in Table 18.14. Clinical detection of hypothyroidism in the neonate may be difficult. In many developed countries screening programmes measuring T S H or T 4 on dried bloodspots from hcclprick blood samples (taken at 6 days when phenylketonuria screening is performed) have been operating for some years with considerable success. Abnormal ihyroid development is responsible for most congenital hypothyroidism. Scintiscanmng is occasionally used in order to help to clarify* the diagnosis. Outlines of the methods used to clarify- individual dyshormonogenic diagnoses arc given below, though in routine practice it is often not necessary to make a precise diagnosis of the type of enzyme defect involved. Hypothyroidism can be con­ firmed biochemically using T S H and T 4 measurement on scrum samples prior to starting treatment. The diagnosis can be confirmed at a later date by transferring the patient onto T 3 therapy before stopping this hormone in order to follow the rise in T S H that should result. T 3 , with a shorter half-life, is used rather than T 4 under these circumstances in order to promote a more rapid T S H response. Inability to organify trapped thyroidal iodide can be recognized by an abnormal perchloratc discharge test and is usually associated with goitre. Patients with mild forms of this type of defect may also have deafness (Pendred's syndrome) but arc in this case not often hypothyroid. Ineffective transport of iodide into the thyroid gland can produce both goitre and hypothyroidism and will be found in individuals with abnormally low radioiodine uptake. The block to transport can in some cases be overcome by treating patients with high doses of oral iodide, reducing goitre and restoring normal thyroid function.
  
  Table I X.I 4
  
  Cause
  
  .foidiam
  
  Structural abnormalities absent gland ectoptc site fcnzyme defects (dyshormemogenesis) iodide trail tide organisation iotyrotine dchalogenasc Defev
  
  em *ci
  
  m
  
  Maternal antibodies (transient)
  
  350
  
  CLINICAL BJOCHK.MI5TRY
  
  Deficiency of iodotyrosine dehalogenase produces impaired deiodination of iodotyrosine, such that large amounts of mono- and di-iodotyrosines appear in the blood and iodine deficiency can result from the urinary loss of large quantities of dcaminatcd metabolites of these molecules. Again, large doses of iodine may reduce the severity of the hypothyroidism and resulting goitre. Biochemically, parcntcral administration of mono- and di-iodoryrosines will be followed by their rapid urinary excretion and this can be used as a diagnostic test for the condition. Diversion of thyroidal iodine into metabolically inactive or less active iodoproteins rather than thyroid hormones can lead to hypothyroidism. Radioactive iodine uptake will be high, due to high TSH drive to synthetic activity within the gland, and the abnormal quantities of iodinated products of the gland can be measured in serum. Thyroid hormone replacement therapy is required. Maternal T S H receptor-blocking antibodies can cross the placenta to produce transient hypothyroidism in the fetus, which resolves within a few weeks after birth. In these cases bloodspot T S H values are usually only mod­ estly raised. Testing for T S H receptor antibodies would distinguish these infants from those requiring T4 replace­ ment therapy. Lithium treatment Lithium therapy may lead to reduced thyroid hormone synthesis in a similar manner to the acute effects of iodine, but more commonly leads to the induction of a destruc­ tive thyroid-directed autoimmune process. Female pa­ tients are most often affected and thyroid autoantibodics are often present. Many other drugs can produce hypothyroidism, either directly, as with the amithyroid drugs themselves, or as a side-effect of therapy for another disease state. Examples in this latter group include ethionamide and, in the past, iodine-containing preparations used in the treatment of asthma. Iodine Both excess and deficiency of dietary iodine may result in hypothyroidism. Although oral use of large doses of iodine would nor­ mally lead to transient suppression of thyroid hormone synthesis, iodine can rarely produce more lasting hypothy­ roidism, particularly in individuals who have a pre-exist­ ing thyroid abnormality, including Hashimoto's disease, Graves' disease or mild dyshormonogenesis resulting from enzymatic defects in organisation. Thyroid scintiscan and/or immunological abnormalities of the underlying dis­ order may therefore be found, goitre is usually present and radioiodine uptake studies will show high early up­
  
  takes (due to the TSH drive to produce thyroid hor­ mone). The failure to organify iodine will, however, lead to the 24 h uptake being low, with high urinary iodine excretion and plasma iodine concentrations. A reduction in dietary iodine intake will often correct the goitre; thy­ roid hormone therapy may be required. Environmental iodine deficiency remains a major cause, worldwide, of goitre and hypothyroidism. In many cases other factors, such as dietary intake of goitrogens or coexistence of autoimmune or dyshormonogenic thyroid disease also contribute to the clinical state seen. In the absence of these factors, clinical euthyToidism is often maintained by preferential production of T 3 and com­ pensator}-- thyroid hyperfunction, leading to large goitre formation. Iodine deficiency leads to increased thyroidal radioiodine uptake with a proportional reduction in urinary iodine excretion.
  
  Secondary hypothyroidism I-oss of T S H drive to thyroid function may result from any of the causes of pituitary gland hypofiinction. Recog­ nition of the aetiology of this type of hypothyroidism is critical and is usually made by observing an inappropri­ ately low serum T S H level in the presence of low or low normal total and free T4. Increased T3:T4 ratios are not seen in this type of hypothyroidism as T S H drive is absent. T S H response to T R H stimulation may be absent or delayed.
  
  Treatment of hypothyroidism Myxoedema coma Profound hypothyroidism may produce the state known as myxoedema coma. '\Ъ{ъ medical emergency comprises coma in association with marked bradycardia, hypo­ thermia and circulatory collapse. Patients should be moni­ tored on an intensive care unit where possible. Therapy is supportive with thyroid hormone replacement using nasogastric T 4 or parenteral T 3 therapy. Replacement doses of glucocorticoids should be used until coincidental adrenal insufficiency has been excluded, infections should be treated aggressively and further heat loss prevented to allow progressive rewarming to occur. Despite appropri­ ate therapy, the mortality from this condition remains high.
  
  Oral replacement therapy Where hypothyroidism is likely to be permanent, replace­ ment therapy should be instituted. Therapy in 'compen­ sated hypothyroidism*, with elevation of T S H but normal T 4 levels, should be expectant until cither symptoms or further elevation of T S H and falling T 4 supervene. Where
  
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  THYROin DYSFtJNCmON
  
  this state is due to autoimmune thyroid disease, it has been estimated that there is an approximately 5% per annum risk of progression to frank hypothyroidism. «Joth T 4 and T ^ are available for therapeutic use, T 3 producing a more rapid resolution of symptoms and signs, but T 4 , with its longer half-life, being used far more frequently in clinical practice. T 3 , with its shorter duration of action, is also useful therapy prior to treating patients with thy­ roid carcinoma using ablative doses of radioiodine, as is described in pan 6 of this chapter. Care is required when commencing therapy in patients with known or potential ischaemic heart disease and in individuals who have been hypothyroid for prolonged periods, when associated secondary hypofunction of pitui­ tary and adrenals may be present. It is vital that thyroxine replacement is not commenced in the presence of un­ treated adrenal insufficiency, as an adrenal crisis may very well result. Subnormal responses of cortisol to hypoglycaemia are often found under these circumstances. The biochemical goal of therapy should be to restore normal circulating levels of T 4 and T S H . Considerable controversy has existed regarding this matter but it now seems clear that, in primary hypothyroidism, use of T 4 in doses sufficient to suppress T S H secretion, even in the face of normal T 3 levels, may have significant and adverse effects on bone turnover, with increased bone loss and effects on hepatic and cardiac function similar to those seen in frank hyperthyroidism. Biochemical monitoring of therapy with thyroid hor­ mone should initially include regular measurement of T S H and free thyroxine levels. When euthyroidism has been restored the frequency of monitoring can be reduced to annual or biannual T S H measurements, as thyroid re­ placement doses usually remain constant, with a need for slight reduction in old age. In pregnant hypothyroid pa­ tients the dose of T 4 should be increased to maintain nor­ mal T S H levels. The development of new, non-thyroidal illness with time will limit the interpretation of thyroid function test results, as has been described above.
  
  PART 5 THYROmiTIS Inflammation of the thyroid gland (thyroiditis) can result in both over- and underactivity of the gland. Thyroiditis producing hyperthyroidism Autoimmune thyroiditis (typified by Hashimoto's disease) is a common cause of hypothyroidism and as such will be reviewed in detail below. However, in the early phase of production of autoimmune damage to the gland, re-
  
  35 1
  
  lease of stored thyroid hormone may in some patients lead to transient hypenhyroidism. This usually lasts for less than 6 months before ongoing gland damage leads to res­ toration of a euthyroid state, often progressing through this point to produce permanent hypothyroidism. A simi­ lar phenomenon may be observed in cases of postpartum thyroiditis. Diagnosis and treatment The diagnosis of autoimmune thyroiditis is described below. The temporary nature and frequently mild degree of hyperthyroidism means that treatment should essen­ tially be supportive. Use of non-selective j)-blockade will abrogate many of the symptoms during the period of hyperthyroidism. Hypothyroidism resulting from Hashimoto's thyroiditis In this, the commonest form of autoimmune hypo­ thyroidism, destructive thyroiditis results from both a cell-mediated and humoral attack on the thyroid tissue. Females are affected with greater frequency than males. The gland is typically enlarged but small and firm in early cases, with a palpable pyramidal lobe. At this stage, hyper­ thyroidism may transiently occur, but further gland dam­ age rapidly leads to permanent hypothyroidism, at which stage goitre is lost and the gland remnant is composed of fibrous tissue. Diagnosis Thyroid-directed autoantibodies (described earlier) are almost invariably present in the early phase of the disease, though they may ultimately be lost over a period of years. After an initial period of hyperthyroidism, classic hypo­ thyroid features are found, with high TSH and low total and free T 4 . Radioiodine isotope scans give a picture of zero or very low uptake, even in patients who have previously had a low dietary iodine intake, Immunological and biochemical evidence of other organ-specific autoimmune diseases such as diabetes, pernicious anaemia, Addison's disease and hypoparathyroidism may also be found. Other forms of thyroiditis De Qucrvain's thyroiditis is an unusual condition which often follows a viral illness. Patients typically present with pain in the thyroid and neck region with fever and ma­ laise. Thyroid function tests may transiently become hypcrthyroid, then hypothyroid before returning to nor­ mal. Rarely, permanent hypothyroidism may follow re­ peated episodes. Riedel's thyroiditis is a condition of
  
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  352
  
  CLINICAL BIOCHEMISTRY
  
  unknown aetiology. Extensive replacement of the thyroid with fibrous tissue causes the gland to become stony hard. Hypothyroidism is a rare complication. Hypothyroidism a n d the p o s t p a r t u m period Postpartum thyroiditis, due to autoimmune thyroid dis­ ease developing during the early postpartum period and usually histologically and biochemically identical with Hashimoto's disease, may produce h\pothyroidism, occa­ sionally following transient hyperthyroidism. Published series suggest that the overall incidence of the syndrome is between 5-9% and that 40% of patients have failed to recover euthyroid function by 1 year post-delivery. A sig­ nificant proportion of patients do recover normal thyroid function, but of these a further 2 0 - 3 0 % will have become hypothyroid within 3-4 years time. After the initial epi­ sode, the future risk of developing the syndrome increases in successive pregnancies. Although the presence of antithyroid antibodies during pregnancy is a risk factor for postpartum thyroid disease, at most only half of these women will go on to develop frank thyroid dysfunction. Swings in thyroid function probably contribute to the aetiology of postnatal depres­ sion and thyroid function should be tested in all such patients.
  
  disease. Clinical suspicion of the diagnosis is further raised by the finding of a non-functioning solitary nodule on thyroid scintiscanning (Fig. 18.9) that is shown on ultrasound examination to be solid. The diagnosis of thy­ roid carcinoma ultimately depends on histological exami­ nation of the gland. Biochemical assessment (other than establishing the usually euthyroid status of the patient and excluding evidence of autoimmune processes) plays little part in making the diagnosis. Measurement of plasma thyroglobulin is of no diagnostic value. Treatment Therapy centres on the removal of the lesion together with a variable portion of surrounding thyroid gland, followed by whole-body scintiscanning and possible irradiation, using И | 1 , of remaining functioning thyroid tissue. Main­ tenance therapy of supprcssive doses of thyroid hormone is typically used, with repeat whole-body scanning at in­ tervals and further irradiation of local or metastatic recur­ rences as necessary. Prior to these scans, it is important to withdraw thyroid replacement therapy and allow the T S H level to rise (typically to > 50 mU/L) in order to maximize the chance of detecting neoplastic tissue. I'umouf markers Measurement of scrum thyroglobulin is principally of value in the management of thyroid cancer when follow­ ing patients whom it is hoped have no residual functional thyroid tissue, because of either total thyroid ablation or administration of effective suppressive doses of thyroid hormone. Under these circumstances thyroglobulin levels
  
  PART 6 NEOPLASIA Thyroid neoplasia forms a small but clearly important proportion of all thyroid disease. The types of primary and secondary thyroid neoplasm arc summarized in Table 18.15. Diagnosis Neoplasms (usually carcinomas) most commonly present with a hard nodule in the thyroid, often with a history of rapid enlargement and also frequently a past medical his­ tory of head and neck irradiation as therapy for another
  
  I able 14.
  
  pti_.il thyroid rruligruti
  
  Primary neoptam»
  
  l.jr\ •
  
  >WUI
  
  pbllii-ui
  
  AiupUstii ч.arunnma Medullary carcinoma (calcuoain aecn may form part of the syndrome* of multiple endocrine neoplaua) rnphoma \ lorosarcoma "her n o ' p U u m
  
  Secondary deposit» from extrathvroidal tumoun
  
  Wg. 18.9 Thyroid scmnscan showing a 'cold' nodule in the left inferior pole of the gland. Background levels of radioactivity are higher than in Fig. 18.8. In this case the patient was found to have a thyroid carcinoma.
  
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  м правом
  
  THYROID DYSFUNCTION
  
  will remain low, rising in the presence of tumour recur­ rence (see also Ch. 39). Medullary carcinoma of the thyroid secretes calcitonin, which mav therefore be used as a tumour marker. The tumour occurs in both sporadic and familial forms and because of the association with syndromes of multiple endocrine neoplastic disease (MEN)» patients should be screened for the other manifestations of MEN (e.g. hypcrparathyroidism and phaeochrornocytoma). If the diagno­ sis of MEN is confirmed, first degree relatives of the index patient should also be screened for these diseases. Management of thyroid cancer should be co-ordinated by those with special expertise in the area.
  
  353
  
  evated concentrations of total and free thyroid hormones, detectable plasma TSH and normal or exaggerated re­ sponse to T R H . Patients with generalized resistance to thyroid hormone arc euthvroid; the defect may lie in tis­ sue T 3 receptors. Those with selective pituitary resistance are hypcrthyroid. Suppression of T S H secretion by high doses of T 3 and the response of T S H to T R H will differ­ entiate the latter disorder from a TSH-secreting tumour. Т Ы abnormality may be in the pituitary* T 3 receptor or in the pituitary 5'-monodeiodinase enzyme. (For a recent review of this subject see Franklyn 1991.)
  
  PART 8 SCREENING PART 7 SYNDROMES OF RESISTANCE T O THYROID HORMONES
  
  Is screening for thyroid undoubtedly some groups function testing, using free urement, is justified even if
  
  These rare inherited disorders are characterized by el­
  
  Table 1S.16
  
  PT4low
  
  Interpretation
  
  rotdfun
  
  (bold type show* m
  \ - i m u o i i v \ i
  
  SH k M 1 H M U ) I I ) : repeat test when AA\ illness resold
  
  HYPOTHYKomiSM needed
  
  Hypopiiuilar i >lhrr test» of pituitary function abnormal
  
  Hvpopaui!
  
  Inadccjuati 1 1 replacement therapy
  
  Thyrotoxicosn: free T * high; check autoanubodics (jmtfcpa Lasc and ami l h\ roglobuhn)
  
  E L T H Y R O I » DO confirmation neede
  
  ILLNESS: repeat test when non-mvroidal illncaa resoh
  
  PT4 normal
  
  mnn
  
  disease justified? There are of patients in whom thyroid T 4 and sensitive T S H meas­ there is no clinical evidence of
  
  в other t e a of piraitary
  
  i .confirmation
  
  Don abnormal
  
  Adequate T-l replacement no confirmation needed
  
  COMPENSATED HTPOTHYBOmiSMj ched autoantibInadequate 1 l гер1дССШеШ them;
  
  SuKlmical thymuflocouv free T 5 nnrmali check a utoanti bodies 1 thyrotoxicot: rmal or high
  
  • T3
  
  Previous overtrcatment with 11 1T4 high
  
  T H Y U o i o \ l < O S i s free ТЭ high; check autoanhbodics
  
  SICK E l / T i l Y R O I D : repeat when non-thynndal illness resolved
  
  ERRATIC COMPLIANT 1 WITH T4 ТНВЯЛР1
  
  Thyrw uh coexistent non-thvroidal ilh ■ i maybe normal; check autountibodics
  
  In therapy
  
  Resistance to thyroid hormones: free T 3 high; normal or exaggerated response Ы TSH RHs J'amilv studies
  
  placement
  
  Autoantibod i T4 (ami T J GEM 1 * may be high; test for jutoantibodic Ю thyroid hormones; trv a ditVcrcnt method for tree T 4 Albumin variants: efectrophorc*t to detect valiants; try a different method m 11 Resistance to thvroid hormones: free high, normal or exaggerated response of TSH to TRH; umily studies TSH-secreting tumour Ircc T 3 high; no response to T R H or i other test» of pituitary function abnormal
  
  TSH-sccrenng tumour: free 1 ^h; no response I I SH to I'RH м 1 1
  
  354
  
  r.UNu:.Ai, Hitxnt-MisTKY
  
  thyroid disease. The question of screening in neonates has already been discussed; similarly, screening is warranted in all patients with a personal history of previous thyroid disease or neck irradiation, ophthalmic Graves* disease, a trial tachyarrhythmias or a history of goitre or previous thyroiditis. In view of the increased prevalence of longterm thyroid dysfunction in patients who have previously received antithyroid drugs, this group of individuals should also be offered thyroid function testing. Screening patients who have hyperlipidaemia (who have a higher prevalence of hypothyroidism than the nor­ mal population), a personal or strong family history of other organ-specific autoimmune disease is also justified, but the situation is far less clear in the case of screening larger sections of the general population» such as the eld­ erly female population or the hospital inpatient popula­ tion. In the former example, the cost of such a screening programme has to be balanced against the likely pick-up rate and in the latter, the confounding influence of nonthyroidal illness is likely to reduce the value of screening
  
  in this circumstance. Screening programmes should prob­ ably be targeted on otherwise well but 'at risk' individuals in the community, rather than on the hospital population at large.
  
  PART 9 INTERPRETATION O F RESULTS O F THYROID FUNCTION TESTS No single biochemical rest of thyroid function can be used to diagnose or monitor all forms of thyroid dysfunction. The guidance on interpretation in Table 18.16 is based on the system of T S H and free T 4 assays, with free T 3 assay performed when necessary. The most commonly occur­ ring diagnosis in each case is given in bold type. Compre­ hensive clinical information assists in deciding the most likely diagnosis and therefore what other tests should be performed.
  
  FURTHER READING Ekins R. Measurement of free htmnone* in hlood. Endocrine Review* 1990; 11:5 46. A substantial revKK of the- conecpt of free hormone as acthe entity and of the physicoehemical basis of free hormone assays. FranWyn J A. Syndrome* of thyroid hormone resistance. Clinical Endocrinolog)- 1 Ir
  
  itrophin rcleu «actor ( R H
  
  mul.i
  
  .
  
  шсгеааа г cell proliferation Enhances and suppresses natural killer cell activity Increases II.I and 11-2 production Increases II-! receptor prodacttoc
  
  Sleep Anorexia Neutrophil leukocytos» utc phase protein production I jpoprorein tipasc inhibitKKi
  
  Insulin Glucagon Prolactin
  
  GH AC 1 11 TSH AVP Corticotrophin releasing factor maiostatin
  
  contribute to the clinical picture associated with various diseases. An example of this is ILI which is produced by macrophages and other cell types (Table 19.5) in response to antigen, bacterial toxins and tissue injury. It mediates many of the responses to illness and stimulates the release of numerous hormones (Table 19.6). Another important cytokine is IL6 which is also pro­ duced by a variety of cell types (sec Table 19.5). Like ILI, it has many functions including regulation of the acute phase response, T cell proliferation and В cell differentia­ tion. It acts often syncrgistically with ILI and may even mediate some of the actions of the latter. Likewise tumour necrosis factor (TNF) reproduces many of the biological effects of ILI, including fever and hypotension. However, unlike ILI it does not direcdy activate lymphocytes. y-Interferon is another important cytokine which has a variety of actions including activation of macrophages and enhancement of natural killer cell activity and antibody production. In addition it can alter the expression of major histocompatibility complex antigens under certain situations and may play an important role in the pathogencsis of autoimmune endocrine diseases. Let us con­ sider how these cytokines might contribute to the acute phase response to trauma or sepsis. The acute phase response is a systemic response char­ acterized by fever, leukocytosis, increased vascular perme­ ability and acute phase protein production by the liver (Table 19.7). ILI, IL6 and T N F all appear in the general circulation during the acute phase response. IL6, in par­ ticular, circulates in very high concentrations and is one of the most important stimuli for hepatic acute phase protein synthesis. Another important aspect of the acute phase
  
  1 V r t_ i I
  
  taiiiv
  
  ri
  
  Ul
  
  HORMONAL ASPECTS OF NON-ENDOCRINE DISEASE Table 19.7 Human hcp-i :cin production during the acute pha*c rcvponur 10 t n u m i or аерч* Increased
  
  Decreased
  
  a, - Antich vmocrypun avAntitrypun -Acid glycoproiem ilaptoflobin Неширок] Fibrumgen Scrum amyloid A 1 iivc protein Complement components C3 & H
  
  Albumin Trarafcrnn Apolipi'pnucin AI
  
  '
  
  response is fever. It is still not known why fever occurs but it is likely to have important consequences for survival, perhaps by influencing metabolic processes. The current consensus of opinion is that fever is mediated via the pro­ duction of peripheral prostaglandins which act on the ihermoregulatory centres in the hypothalamus. The pro­ duction of these prostaglandins is regulated by cytokines such as IL1, T N F and INF, all of which are endogenous pyrogens. Fever can also be modulated by hormones, particularly those of the hypothalamo-piruitary adrenal (HPA) axis, including corticotrophin releasing factor, arginine vasoprcssin, A C T H , OLMSH, opiates and glucocorticoids. The permissive action of CRF-41 on fever appears to be mediated via prostaglandins whilst the apparent antipyretic action of ACTH is via as yet un­ determined mechanisms. The antipyretic action of glucocorticoids is probably secondary to their ability to induce lipocortin synthesis which in turn reduces phospholipase A : activity and hence prostaglandin synthesis. Thus, in the acute phase response the eicosanoids, cytokines and stress hormones all contribute to the clinical picture. The system is tightly linked together in a series of regulatory loops. There is good evidence that both prosta­ glandins and cytokines can activate the HPA axis and all the stress hormones have been shown to have immunoregulatory properties, including effects on immune cell growth and differentiation, chemotaxis, killer cell activity, immunoglobulin synthesis andcytokine production. Other hormones which arc released during the acute phase re­ sponse, such as GH and prolactin, also have profound immunomodulatory properties (see Table 19.4). In this way the cytokines activate mechanisms (e.g. glucocorticoid secretion) which help regulate their production and actions and ultimately lead to a diminution of the acute phase response. There is considerable debate at present as to the mechanisms whereby the cytokines activate the neuroendocrine system. It is generally accepted that they arctoo large to penetrate the CNS and it is thought that they must gain access via the specialized areas which are known to be outside the blood-brain barrier, e.g. the
  
  359
  
  organosum vasculosum of the lamina terminalis, where they activate prostaglandin-dcpendent mechanisms. How­ ever, another intriguing possibility is that a central cytokine network exists and there is evidence for the existence of both IL1 neurons and receptors within the CNS. A model of the way in which sepsis and trauma activate the neuroendocrine-immune axis is shown in Figure 19.2. NEUROENDOCRINE RESPONSES T O SOCIAL STRESS Stress is a difficult concept to define but can broadly be separated into physical stresses such as those discussed above and psychological stresses. Both contribute to the pathogenesis of stress-related disease states and also may hinder the interpretation of biochemical investigations. Social stresses arise as a consequence of living in a social group and give rise to three types of responses: adaptive behaviour, autonomic responses and endocrine responses. It must also be realized that social relationships may not only be a source of stress but may also help to modify* an individual's response to stress, and so may be beneficial. Reproductive system There is little doubt that social factors can influence re­ productive function and much of our knowledge on this subject has been gained from studies of non-human pri­ mates. These have shown that dominant males are more successful breeders and have higher plasma testosterone concentrations than subordinate males. Interestingly, these differences occur as the result of social interaction since they become apparent only when the monkeys are allowed to form a group. It is possible that these hormonal changes may have an important adaptive function in preventing subordinate males from mating. However, lowplasma testosterone concentrations cannot fully explain their reproductive incompetence since hormone replace­ ment therapy does not stimulate normal sexual behaviour. In men, plasma testosterone concentrations arc also sup­ pressed in response to physical and psychological stress. However, socially dominant men (defined by personality characteristics) have higher plasma testosterone concen­ trations which increase in response to psychological stress. There is also some evidence which suggests that aggres­ sive men have higher plasma testosterone concentrations than passive men. Also, it is well accepted that one of the benefits of anabolic steroid therapy in athletes is an in­ crease in aggressive behaviour. Social factors also influence female reproductive func­ tion. Subordinate female monkeys ovulate less frequently than dominant ones and, in some monkey species, the female offspring will not ovulate so long as they remain in their original family group. In women both physical and
  
  материал, за
  
  TUK» ic g vhenu'thcrapy)
  
  the body's energy requirements for survival. Central to this process is the role of the liver in integrating the meta­ bolic responses. It does this by producing glucose at the expense (if amino acids in muscle and by switching the metabolism of fatty acids from lipogenesis to [J-oxidation, thus producing ketonc bodies and ketosis. The flux of fuel through the various metabolic pathways is controlled by key enzymes all of which are under hormonal control. During starvation the plasma insulin concentration falls and this augments the ability of glucagon and adrena­ line to promote glycogenolysis. In the liver this process produces glucose. However, in skeletal muscle the glu­ cose 1 -phosphate derived from glycolysis must enter the glycolytic pathway since skeletal muscle lacks glucose 6-phosphatasc and so cannot generate free glucose. The products of glycolysis can be used for oxidativc meta­ bolism in the muscle itself or enter the circulation as pyruvate or lactatc. However, the stores of glycogen are rapidly depleted and the glucose requirements of the body must be fulfilled thereafter by gluconeogenesis. This process is augmented by glucagon in the liver which controls the concentration of fructose 1,6-bisphosphatc and hence the activity of phosphofructokinase. T h e net result is conversion of fructose 1,6-bisphosphate into fructose 6phosphatcand activation of gluconeogenesis.
  
  Protein metabolism During starvation the rate of protein breakdown exceeds that of protein synthesis and this leads to a negative nitro­ gen balance. Protein synthesis is impaired due to a lack of amino acids and the low concentration of insulin. Protcolysis is dependent upon glucagon, cortisol and G H , and the resultant amino acids are metabolized via three path­ ways. The major route is transamination to form an oxo acid; less importantly, oxidative dcamination to form an oxo acid and ammonium, and decarboxylation to form an amine, can occur. During starvation alaninc plays a key role since it is formed by transamination from pyruvate in muscle and recycled to the liver where it enters the gluconeogenic pathway. The kidney also plays an important role in glu­ coneogenesis, utilizing glutamine released from skeletal
  
  muscle. Both these amino acids also help to excrete the nitrogen generated by proteolysis and they do this by generating ammonium urns which are utilized to excrete anions (ketonc bodies) and to synthesize urea. Fat m e t a b o l i s m Much of the body's energy is stored as fat in adipose tissue and during starvation this is mobilized by increased lipolysis associated with increased activity of hormonesensitive lipase. This enzyme is activated via cAMP and is thus sensitive to those hormones which stimulate adenylatc cyclasc activity in adipose tissue (catecholamines, glucagon, cortisol, GH, ACTH and AVP). In addition, (J-adrcncrgic receptor sensitivity to catecholamines is in­ creased which allows noradrenaline to act synergistically with glucagon and vasopressin to stimulate lipolysis. The net result of this process is to release free fatty acids into the circulation, which arc utilized as a major energy source by most tissues, including muscle, during starvation. In order for the free fatty acids to leave the adipose tissue they require the permissive action of both cortisol and thyroid hormones. In the liver the free fatty acids undergo energy-yielding (J-oxidation in the process of syn­ thesizing acetyl CoA and ketonc bodies. These latter com­ pounds play a central role in survival during starvation since they provide an important source of fuel, particu­ larly for the brain. The liver produces 3-hydroxybutyratc and acctoacetate and the latter undergoes spontaneous decarboxylation to produce acetone. These compounds produce the condition known as starvation ketosis. Gluca­ gon makes a major contribution to this situation via its ability to inhibit acetyl CoA carboxylase activity, thus lowering malonyl CoA concentrations with resultant stimulation of fatty acid oxidation and hence ketogencsis. During the first few days of starvation both 3-hydroxy­ butyratc and acctoacetate serve as the major fuels for muscle metabolism. However, as starvation progresses the consumption of these compounds by muscle decreases and the consumption by other tissues, particularly brain, increases. Acetone can also contribute to gluconeo­ genesis via the metabolic intermediates propanediol and mcthylglyoxal.
  
  Hormones Figure 19.3 shows that during starvation the major hor­ monal influences arc derived from an increase in the se­ cretion of glucagon and catecholamines aided by cortisol and GH, together with a decrease in circulating insulin concentrations. AVP and aldostcronc also contribute by helping, in conjunction with catecholamines, to prevent hypovolaemia. Thyroid hormones also play a role since plasma T 3 levels fall progessively over the first 3 days of starvation, together with a reciprocal rise in rT3 conccn-
  
  HOKMONA! ASI'K I S o|- NON-HNTHH KIN* П Ы AM
  
  369
  
  T Glycogenolysis Glucose LIVER
  
  Г Gluconeogenesis
  
  )
  
  A p-oxidation to ketone bodies
  
  i
  
  T Glucagon
  
  * Brain
  
  t Adrenaline *С0Г1150*
  
  4
  
  SKELETAL MUSCLE
  
  T GH i Insulin
  
  K:
  
  AD» POSE TISSUE
  
  ' Proteolysis Glycogenolysis
  
  T Aldosterone
  
  Г Catechoiammes
  
  Г Arrano acids
  
  i
  
  T Pyruvale.lactate
  
  I Lipolysis
  
  t Free fatty adds —
  
  i
  
  TAVP KIDNEY
  
  1
  
  T Gluconeogenesis
  
  » T Glucose
  
  Rotontion of salt and water
  
  i
  
  Maintenance of BP
  
  Flf. 19.3 Hormonally activated homoeostaric mechannms during starvation. irations and a reduction in hepatic T4 5'-monoiodinase activity which has the net result of reducing mitochondhal oxidative phosphorylation. PSYCHIATRIC DISORDERS Because the hypothalamus plays a central role in the inte­ gration of behavioural and hormonal reponses it is not surprising that psychiatric illnesses influence endocrine function (see also Ch. 32). Schizophrenia Basal levels of GH and prolactin arc normal in both acute and chronic shizophrenia. However, GH responses to the dopaminc agonist apomorphinc are enhanced in the acute and blunted in the chronic form of the illness. Basal levels of gonadotrophins and T S H are normal in both acute and chronic schizophrenics with normal T S H responses to T R H and blunted LH and FSH responses to LHRH. Other endocrine abnormalities include elevated AVP levels.
  
  L InLreaped plasma conccntrati ГН and cortwol Phii ft in the arcadian rhvthm of At 1 H Hid »>гтг*о1 Abnormal low dove dcxamethasonc suppression te»t Blunted ACTH rcspemuCR] Reversal rmal i mofCH :ctionwith me hypereccretion and nocturnal hyposccrction АЬсгпшт GH raporae* i taRH md to TR1I
  
  depression. In general there appears to be a higher rate of non-suppression in endogenous than nonendogenous depression. However, the performance of the D S T is dependent upon the diagnostic criteria used and the predictive value of the test can be improved when combined with other biological markers of depression such as the sleep electroencephalogram. It must be remembered that a number of other condi­ tions can give rise to an abnormal D S T and provide a diagnostic trap for the unwary (Table 19.26). Basal prolactin levels are normal in depression with reduced responses to L-tryptophan and fenfluraminc, perhaps reflecting abnormalities in central 5-HT function.
  
  Affective disorders The most consistent biological abnormality in depression is hyperactivity of the HPA axis (Table 19.25). This hypcrcortisolism appears to cause a high rate of nonsuppression in the dexamethasone suppression test (DST) and this has been widely investigated as a potential bio­ logical marker of depression. These studies have produced conflicting results, perhaps reflecting the biological
  
  ТшЫс 19.26
  
  I-acton which may result in tupprctsKin rc*t
  
  Endocrine dnoiden (e.g. Cuahing'i) Diabetes mellitus Severe weuchr Acute medical illneaa Drugs (e.g. alcohol, barbiturates, otfbanuuxpinc)
  
  370
  
  CUNICAL BIOCHEMISTRY
  
  About 30% of patients have blunted TSH responses to TRH which normalize during remission. There is also an increased incidence of thyroid disease associated with depression.
  
  and it can enhance the prolactin response to tryptophan. Although lithium can increase the secretion of PTH, sig­ nificant hypercalcaemia is an uncommon side-effect of lithium therapy.
  
  Psychotropic d r u g s
  
  FEEDING DISORDERS
  
  Psychostimulants such as the amphetamines cause wide­ spread behavioural effects thought to be secondary to their effects on central monoamine function. As a result they also cause an increase in the secretion of various stress hormones such as ACTH, prolactin and G H . The tricyclic antidepressants have many pharmacological pro­ perties including prominent effects on central monoamine function. Acute administration of desipramine stimulates the secretion of GH, ACTH, melatonin and prolactin. However, the effects on GH and melatonin may be re­ duced in depressed patients and chronic administration of the drug may cause adaptive changes at adrenoreceptors. Selective 5-HT uptake inhibitors such as fluoxctinc have not been shown to have any significant effect upon neuroendoenne function.
  
  "1Ъс ncuroendocrinology of feeding is highly complex and appears to involve the hypothalamus which integrates the interaction of a number of neurotransmitters and neuropeptides (Fig. 19.4). Because of its central role in the feeding process it can be seen that feeding disorders may involve hypolhalamic dysfunction with consequent hormonal abnormalities.
  
  Anxiolytic drugs such as the benzodiazepines act by activating central GABA-mediated mechanisms. As such they cause an increase in the secretion of GH and a reduc­ tion in the secretion of ACTH and prolactin. Tolerance can be developed in patients on long-term therapy. The neuroleptic class of drugs such as the dopaminc receptor antagonists cause increases in the plasma concentrations of prolactin. A 3-6-fold rise in plasma prolactin is seen within the first week of standard neuroleptic treatment and this effect is sustained over a long period of time. However, tolerance can develop after 5 years of treatment. This side-effect of neuroleptic drugs can result in trouble­ some amenorrhoea and galactorrhca. Lithium is well established as a treatment for mania. However, it can cause significant neuroendocrine effects. The most significant effects are those on the thyroid gland with about 4% of patients on long-term treatment devel­ oping goitres. Following the start of lithium therapy plasma concentrations of T 3 and T 4 decrease and, in most pa­ tients, eventually return to normal although a significant proportion go on to develop hypothyroidism. Basal TSH levels are elevated in a third of patients on long-term lithium treatment and the T S H response to T R H is exag­ gerated, lithium also inhibits the peripheral conversion of T 4 to T 3 and increases the titre of thyroid autoimmune antibodies. Thus, all patients should undergo full thyroid assessment before commencing lithium therapy and should be monitored carefully during treatment. Another prominent problem of lithium therapy is nephrogenic diabetes insipidus. If this develops then the lithium should be stopped and alternative treatment insti­ tuted. Lithium does not alter basal GH or prolactin levels. However, it may reduce the GH response to clonidine
  
  Anorexia ncrvosa a n d bulimia nervosa Anorexia nervosa (AN) is a disorder which affects pre­ dominantly young females and is characterized by a fear of gaining weight associated with an altered perception of body image. It is associated with significant hypothalamic dysfunction resulting in characteristic hormonal changes. Some of these changes are compounded by the effects of weight loss. The underweight phase of AN is character­ ized by profound hypcrcortisolism and shows features similar to those of depression (Table 19.27). These fea­ tures suggest a normal feedback response with adrenal hypcrsensitivity. Hypothalamic hypogonadism is another cardinal feature of AN and in 50% of cases amenorrhoea may precede the weight loss. Other hormonal abnormali­ ties also suggest hypothalamic dysfunction (Table 19.27). Bulimia nervosa (BN) is another feeding disorder which characteristically affects young females. The cardi­ nal features are binge eating associated with a desire to
  
  Tuble 19.27
  
  Endocrine abnormalities found in anorexia nervosa
  
  The hyfk>thitiamo-puuitary-adrtnai axis Increased plasma concentration* of corti*.nl Increased cortisol production rate Decreased metabolic clearance of cortisol Abnormal dcxamcthasone suppression test (cither failure of suppression or 'early escape'.) Increased 24 h excretion of urinary free cortisol Tht hypothaiamo-pituitary gonaJai axis Secondary amenorrhoea Low plasma oestradiol concentrations Low plasma 1 Л and FSH concentrations Normal LH/FSH response to GnRH following priming Other hormonal abnormalities Exaggerated GH response to GHRH Reduced GH response to apomorphinc Abnormal release of G H in response to T R H The GH response to a glucose load may show a paradoxical rise GH response to insulin hypoglycaemia may be blunted Low plasma 14 and ТЭ concentration*High plasma гТЭ concentrations Delayed TSH response to TRH Subnormal AVP response to increased plasma Na" concentrations
  
  \n, заиц
  
  HORMONAL ASPECTS OF NON-ENOOCRINE DISEASE
  
  purge cither by vomiting, diuretics, laxatives, exercise or fasting. It may be found in conjunction with AN and up to 30% of patients with BN have a history of AN. Thus there is much overlap between the two disorders and classifica­ tion is further complicated by the fact that as many as 50% of patients with BN have concurrent depression. The endocrine disturbances in BN are milder than those in AN. They have mild hypercortisolism and basal thyroid hormone levels are normal. However, as many as 50% may exhibit an abnormal T R H test with a delayed but normal peak T S H response to TRH. Similarly, 50% have menstrual abnormalities which may be one of two types. Patients with low body weight have low plasma ocstradiol concentrations and failure of follicular development, while those with normal body weight have normal plasma oestradiol concentrations but progesterone secretion fails in the lutcal phase. Also the secretion of cholccystokinin in response to a meal is impaired in BN. Obesity Obesity is a major cause of morbidity and premature death. Endocrine disease is a recognized but uncommon cause of obesity (Table 19.28), but obesity itself can result in hormonal abnormalities. There appears to be a significant link between geneti­ cally determined causes of obesity and hypogonadism and the best example of this is the Prader-Willi syndrome (sec
  
  Tab!. H U f
  
  (
  
  human obesity
  
  Autotomal reccwive Alitrom «.yndrnme Bardct Hict.il wutrome Edwards' tyndrome I .шгелсе Moon I .yndromc Chromosomal Pradcr WUB svndromc X-lmkcd Borjeson -Vazquez syndrome Acquired Idiopathtc I ndoenne hyporhw i.hsm hypogonadism Gushing'* lymiromc potycyibc ovary syndrome growth horn. ency imulinoma I I vpochaljmic lesions trauma
  
  tumours Drugs monoaminc oxidate inhibitor» coitkoittroMJb C) prohcptadine m\ul lithium phcnothiazmcs
  
  Table 19
  
  •hcral *.•
  
  37 1
  
  rmoncs
  
  CCK Bombcsm < iuMnn-ri'li-asmg peptide Glucagon
  
  Table 19.23). The hypothalamus appears to play a role in the development of certain types of obesity since damage to the ventromedial nucleus of the hypothalamus in both animals and humans results in hypcrphagia, obesity and hypcrinsulinaemia. However, this cannot explain all types of obesity and it is probable that abnormalities in peripheral mechanisms also play a role. It is known that a peripheral satiety system exists which utilizes a number of putative satiety hormones (Table 19.29), of which CCK appears to be the most potent. It mediates its effects on feeding partly via an ability to slow gastric emptying and partly via neuronal projections to the nucleus tractus solitarius and the paraventricular nucleus of the hypo­ thalamus (see Fig. 19.4). However, on its own it only accounts for 20% of the satietv effect involved in terminating a meal and the effect of the other satiety hormones appears to be additive. Other circulating hormones which may influence feeding behaviour arc shown in Table 19.30. So far few studies have addressed the role of the satiety hormones in obesity. However, as far as the Prader-Willi syndrome is concerned it appears that these individuals fail to satiate and that this is associated with very high plasma concentrations of CCK. Hndocrinologists are often asked to see obese patients to exclude an underlying endocrine disorder. The diagno­ sis is usually not difficult but one must be aware of the endocrine changes which can accompany obesity alone (Table 19.31). Obesity is often associated with changes in reproductive function in both males and females which may respond to weight reduction (Table 19.32). In gen­ eral the evidence favours an abnormality in peripheral steroid metabolism as the most likely explanation for altered sex steroid secretion and binding in obesity. Both G H and pituitary-adrenal function may also be disturbed in obesity but this docs not usually cause any diagnostic difficulty. In the case of prolactin there is a subgroup of patients with obesity in whom the prolactin response to T R H is normal but the response to insulin is abnormal. Abnormalities in AVP control mechanisms may also be found in obesity.
  
  AGEING The process of ageing has profound effects upon the en­ docrine system and vice versa. It is generally agreed that the longevity of a species is determined by the genome
  
  1ЩИ1
  
  372
  
  - i INH AI HUH I H - M I S T M
  
  PARAVENTRICULAR NUCLEUS NUCLEUS TRACTUS SOLITARIUS Oalanin (iendorpnin
  
  Peripheral vagal inputs
  
  1
  
  CCK
  
  -
  
  TRH
  
  —
  
  1
  
  Cbolecystokinin Thyrotrop«n releasing hormone
  
  GABA —
  
  Y-Arninobutyrk: actd
  
  Carbohydrate
  
  5-HT
  
  —
  
  5-Hydroxytryptamine
  
  intake
  
  CRF
  
  —
  
  Cortocotropin releasing factor
  
  NA
  
  —
  
  Noradrenaline
  
  Fat intake
  
  И * . 19.4 The interaction of ncu retransmitters and ncuropeptidcs in the control of feeding. Table 1
  
  Ifc*.
  
  nones on i ceding la Decreased Wtfto Impotence Decreased plasma tcionc Decreased sex hormone-binding globulin
  
  uhn 4ilin
  
  I roid hormones Inhibitor* of/ittdmg Ca iin
  
  Table 19. *| besirv
  
  PUsma hormone change
  
  ВКНЯМЯ
  
  1 decreased
  
  Concentrations of: >uhn glucagon parathyroid hormone trogens T3
  
  Concentrations
  
  Conversion of androget i oestrogen* in adipose tissue
  
  may be seen
  
  -tOMCrO!
  
  LH GH
  
  Water excretion
  
  and recent evidence suggests that the endocrine system plays a role in the ageing process. It appears that the decline in a number of important body functions begins with changes in the ncuroendocrinc system. This process is initiated in the hypothalamus with a decline in both neurotransmitter function and hypothalamic hormone se­ cretion. This is compounded by changes in the pituitary gland and its target organs primarily due to a loss of receptors and to derangements in postreceptor cellular events. Thus, a number of hormones show characteristic changes with advancing age and these must be taken into account when assessing elderly patients.
  
  Suppression of plasma AVP concentrations in response to water loading
  
  Reproductive function
  
  pancreati. -sue concet
  
  pcptide asj of Г \ receptor
  
  Cortisol degradation rate jdip»»e tissue)
  
  ACTH and cortisol responses to CK
  
  ■1 prod (second* degradation)
  
  Com sol response to mcthoxamme u-adrencrgn. agt>
  
  km rat 4sed
  
  Fanakt Increased incidence of dysfunctional bleeding Secondary amenorrhoea Increased plasma testosterone and HsdroaajgnrHooc Increased ratio of plasma oestxonc/ocstradiol Decreased sex hormone-binding globulin Decreased nocturnal IJ-I secretion boocmal preovuUtory plasma PSH «nadotrophin response t.. t i n R H may be exaggerated
  
  • »H responses (e.g. to insulin. type
  
  46XX
  
  46XY
  
  Variable - т о м often
  
  Gooach
  
  Ovano
  
  Теме»
  
  Bom true lenticular and ovarian thsuc
  
  Internal genit.il dm
  
  Female
  
  Mile
  
  Variable t gonadal dependent)
  
  Kxtcrnal genitalia
  
  Vttttnd
  
  Incompletely maacubnizcd
  
  Usually ambiguous
  
  talia in an individual with a 46XY karyotypc and tcsticular development. The causes and clinical manifestations of, and appro­ priate investigations for, each of the three main classes of intersex will be discussed in turn. Female pseudohcrmaphroditism Table 20.2 lists the main causes of female pseudohcr­ maphroditism. The family of inherited disorders termed congenital adrenal hypcrplasia accounts for virtually all causes of female pseudohcrmaphroditism and is described in detail. A maternal source of androgens may virilizc the external genitalia in a female fetus, either in the form of isolated labial fusion or isolated clitoromcgaly. Maternal androgens may be from endogenous or exogenous sources (see later). The fetal adrenal produces weak androgens, (for example !6ttOH-dehydroepiandrosterone sulphate), which are normally aromatized to oestrogens by the pla­ centa. A recently described rare cause of masculinization in a female fetus is placenta! aromatase deficiency. Maternal oestriol levels arc low in the absence of evidence of signifi­ cant placemal dysfunction. A similar pattern of steroid secretion is produced by fetal adrenal insufficiency (e.g. congenital adrenal hypoplasia) and placenta! sulphatase
  
  •Ic 20.2
  
  377
  
  male pscudohermaphri'
  
  Incrtaud fetai anJrogvm Adrenal 21-hydmxylasc defiocn< AdrenaM!|VhydrA>xyia»edeT' у Adrenal Hi-livdroxytierotd d< Pi nt fetal adrenal stctxtidogencsis Placenta! aromatasc deficiency /ncnsottrf maternal andrvgens latrogenic, e.g. anabolic ds, oral progestogem Ovarian tnmoun Adrenal tumourt Vinli/mg lutcoma of pregnancy •ngeniul adrenal hypcrplasia m mother AfttOalofMW Isolated idwpathic clitoromcgaly t litoromegaly in ncurofibromatoaii bolated labial fusion nital abrionn tin v awnriated with other congenital anomalies
  
  m 4
  
  deficiency. The latter condition is a relatively common X-linked disorder characterized by prolonged gestation and the appearance of icthyosis during the first months of life in the affected male infant. Isolated clitoromcgaly may occur in some preterm in­ fants due to the persistent secretion by the fetal adrenal gland of 3[J-OH-5-enc steroids which act as weak andro­ gens. Isolated clitoromcgaly or labial fusion in some newborn female infants remains unexplained. Certain syndromes are associated with an enlarged clitoris, such as neurofibromatosis and Seckel and Beckwith Weidemann syndromes. Congenital adrenal hyperplasia This term refer» to a family of disorders inherited in an autosomal recessive fashion and characterised by ACTHinduced hypcrplasia of the adrenal cortex during fetal life as a result of primary conisol deficiency. It is intriguing why maternal conisol concentrations are insufficient to compensate for fetal adrenal insufficiency. The pathways of adrenal steroid biosynthesis and the key enzymes in­ volved arc illustrated in Figure 20.2. A deficiency of 21-hydroxylasc enzyme activity accounts for more than 90% of the causes of congenital adrenal hypcrplasia (САН). The clinical hallmark is virilization of the external genitalia in an affected female infant due to increased adrenal androgen production. The severity may range from mild isolated clitoromcgaly or isolated labial fusion to marked clitoromcgaly resembling a penis, scrotalization of the labia and the formation of a penile urethra. In this situation, the newborn infant may be mis­ taken for a male with undcsccndcd tcstcs. This enzyme defect is the commonest cause of intersex. The affected male has normal genitalia at birth apart from the occa­ sional increase in scrota! skin pigmentation. At least 7 5 % of all 21 -hydroxylase-deficieni infants have an associated defect in aldosterone synthesis. This manifests clinically in its most severe form as an acute salt-wasting state during the second or third week of life. Early biochemical fea­ tures are hypokalaemia and increased urinary sodium excretion before the plasma sodium concentration starts to fall. The most sensitive index of mincralocorticoid
  
  378
  
  MJS'ICAL BIOCHEMISTRY Cholesterol
  
  •170H-PregnercA>ne-
  
  Denydroep>andraeterooe
  
  3
  
  7 OH- Progesterone
  
  Androstenedone
  
  5
  
  Deoxyooflicoeterone
  
  11-Deoxycofltsol
  
  Testosterone
  
  Corncosterone 7| A'aosterone
  
  'i
  
  CortisoJ
  
  Oeetradto*
  
  MlneralocortlcoJd
  
  GlucocortlcoW
  
  Sex hormones
  
  JL
  
  Fig. 20.2 Pathways of adrenal ueroid biosynthesis. The number* refer to the following etuymcv 20>22-dcxmolase (II; 17ci-hydroxylase (2); 3(Miydroxv*teroid dehydrogenase 'Use (6); 1 Ы-h у droxy steroid dehydrogenase П); 17t*-hydroxyitcroid dehydrogenase (Ы); aromatase (V>.
  
  insufficiency is an elevated plasma rerun activity but the result is seldom available soon enough for diagnostic use. Hypoglycaemia may also occur with an adrenal saltwasting crisis. The affected male salt-loser may be mis­ taken for having pyloric stenosis. The latter disorder, when associated with prolonged vomiting, is characterized biochemically by a hypokalacmic aikalosis rather than the hyperkalaemic acidosis, hyponatracmia and azotacmia characteristic of the САН infant in severe salt-losing crisis. About 5% of САН cases result from a deficiency of the 1 l(J-hydroxylase enzyme. Again the clinical hallmark is virilization which tends to be more severe than in the 21-hydroxyiase defect. Salt-wasting does not occur since increased production of deoxycorticostcrone (see Fig. 20.2), a potent mineralocorticoid, compensates for aldosterone deficiency. Hypertension eventually occurs in the untreated state. The enzyme 3(3-hydroxy steroid dehydrogenase is required for both corticosteroid and sex hormone biosynthesis. Consequently, a deficiency of this enzyme causes either female or male pseudohermaphroditism depending on whether the karyotype is 46XX or 46XY. The degree of virilization in a female infant with ЗР-hydroxysteroid dehydrogenase deficiency is mild be­ cause the increased production of dehydroepiandrosterone and its sulphate produces a weak androgenic effect. In fact, it is now becoming increasingly recognized that the commonest presentation of this enzyme defect is in its partial form, manifesting in later life clinically as
  
  hirsutism, acne and menstrual dysfunction. The infant with severe enzyme deficiency will also be a salt-loser. Males with САН either due to 21-hydroxylasc or 11[Jhydroxylase enzyme deficiencies are not virilized at birth, despite plasma concentrations of testosterone which may fall within the normal adult male range. There is no ready explanation for this apparent paradox. Untreated males develop signs of virilization by the second or third year of life with penile and pubic hair growth, a muscular habitus and rapid linear growth. *1Ъс testes remain infantile in size, an important clinical clue to indicate that the early pubertal development is not centrally activated from in­ creased gonadotrophin secretion. The investigations required to confirm the diagnosis of САН are listed at the end of this chapter. The peripheral karyotype must be determined and should be available within 48 h of lymphocyte culture. Virilized external genitalia in an infant with a 46XX karyotype is invariably due to САН. The diagnosis is clinched by the measurement of the concentration of 170H-progesterone which can be determined in plasma, filter paper blood spot or saliva specimens. Plasma concentrations of this steroid are gen­ erally below 10 nmol/L in healthy fullterm infants by the second day of life, the placental influence on steroid levels having disappeared by then (Fig. 20.3). Deficiency of 2 1 hydroxylase activity is characteristically associated in the newborn with plasma 170H-progestrone concentrations greater than 300 nmol/L and not invariably, well in excess of 1000 nmol/L. Caution should be exercised with the
  
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  increase in the activity of this enzyme in the newborn). Plasma A C T H levels, as expected, are elevated in САН but this provides no information on the type of enzyme deficiency. Similarly, plasma concentrations of testo­ sterone and androstencdione arc increased in both 2 1 hydroxylase and 11 (J-hydroxylase enzyme deficiencies. Many steroid concentrations can now be determined in filter paper blood spot specimens. Consequently, it is fea­ sible DO perform newborn screening for 21 -hydroxylase deficiency by measurement of filter paper blood spot 170H-progcsterone concentration.
  
  0 4 8 1216202428323648 2 3 4 5 6 Age (days) Age (hours)
  
  i
  
  Fig. 20. J Plasma 170H-progc*teronc concentrations in normal and САН infants during the first week of life. The line represents the mean and the shaded area the range of concentrations observed m normals. Affected infants arc indicated by individual values,* representing a value of 2450 nmol/l. (reproduced with permission of Archives of Disease in Childhood).
  
  interpretation of results in sick, prcterm infants without adrenal disease who may show levels of 170H-progcstcronc elevated to 100 nmol/L or more, presumably the re­ sult of an adrenal response to stress. In the case of 11 [J-hydroxylasc deficiency, the specific marker is an elevated plasma 1 l-dcoxycortisol concentra­ tion (see Fig. 20.2), although 170H-progesterone levels may be somewhat increased. A number of candidate precursor steroid levels are expected to be elevated in 3{J-hydroxystcroid dehydrogenase deficiency, but the most specific marker is 170H-pregnenolone rather than dehydroepiandrosterone and its sulphate. There can be considerable confusion with the inter­ pretation of plasma steroid measurements in the early newborn period, particularly if the infant is premature. ЧЪегс is a maturation delay in the onset of the adult pat­ tern of adrenal steroid secretion as well as the production of a number of unknown steroids in fetal and early neonatal life which can interfere with the measurement of steroids by immunoassay techniques. It is particularly in such instances that specific analysis of steroid secre­ tion is useful by measurement of the excretion of urinary steroid metabolites using techniques such as HPLC and GC-MS. Salt balance is disturbed if mineralocorticoid secretion is inadequate. Hypcrkalacmia precedes hyponatraemia in onset, while urinary sodium excretion is inappropriately elevated. An even more sensitive index is an elevated plasma rcnin activity (after allowing for the normal
  
  It is unlikely that an adequate routine clinical examina­ tion of the newborn infant would miss a virilized female with САН, so screening programmes are targeted at detecting the affected male infant who is at risk of a life-threatening salt-losing crisis. The turnaround time for processing results must be speedy if the screening pro­ gramme is to identify the suspect case before clinical signs of a decompensated salt balance arise during the second week of life. The true incidence of САН in newborns is probably no greater than 1 in 12 000 live births with a further halving of the incidence for the target group if it is assumed that all affected female infants would be recognized at birth. False positive tests for 170H-progestcronc occur in prcterm infants for reasons previously mentioned. For all these reasons, САН newborn screen­ ing has not become widely accepted alongside the current routine screening for congenital hypothyroidism and phenylketonuria. The molecular genetics of САН associated with 2 1 hydroxylasc deficiency has been well characterized in recent years. The mode of inheritance is autosomal reces­ sive. Close linkage with the genes encoding the major histocompatibility complex on the short arm of chromo­ some 6 led to the cloning of the gene for 21-hydroxylase located on this chromosome. Two genes, identified as CYP21A and CYP2IB, were identified with only the В gene being functionally active in encoding for 21-hydro­ xylase. The CYP21A gene has acquired a series of delete­ rious mutations which cause a lack of transcriptional activity and a 2 - 3 % difference in sequence homology between the two genes. A map depicting this part of chro­ mosome 6 and adjacent genes of the HLA complex and complement is shown in Figure 20.4.
  
  С2Ы
  
  C4A
  
  CYP21A ♦
  
  C4B
  
  CYP21B
  
  -120kto
  
  Fig. 20.4 Schematic representation of the location of the 21 hydroxylase genes on chromosome I"he relationship to the components of complement and the HLA-B and HLA-D loci arc shown.
  
  \на
  
  i iINK
  
  AI BKM
  
  Hi MisiK-.
  
  Both A and В genes each contain 10 exons. Approxi­ mately 10-25% of patients with САН have a deletion of the CYP2IB and C4B gene unit. This would be possible to detect on Southern blot analysis of DNA digested by appropriate restriction enzymes and hybridized with a suitable radiolabelled probe. The majority of cases are the result of one of a scries of mutations characterizing the A gene which have been acquired by the В gene. Some of the known mutations resulting in functional 2 1 hydroxylase deficiency are listed in Table 20.3. A pattern is beginning to emerge which shows a correlation between the type of mutation and severity of the condition. For example, the simple vitalizing form of САН is specifically associated with the exon 4 mutation which changes an isolcucine to asparagine. The mutation in intron 2 causes altered mRNA splicing and is a common cause of classic САН. It has been possible to establish a prenatal diagnosis of 21-hydroxylasc deficiency for a number of years by meas­ urement of amniotic fluid steroid concentrations. The first report utilized pregnanetriol as a marker, but later it was shown that an elevated ПОН-progesterone concentration in amniotic fluid collected at 16-18 weeks of gestation was a reliable test. Concentrations of androstenedione and testosterone in amniotic fluid are also elevated at this time but the levels arc only discriminatory in an affected female fetus because of the normal surge in testosterone secretion which occurs in the male fetus during the first trimester. Genetic linkage with HLA haplotypes has also been applied in the past for the prenatal diagnosis of САН. In this situation, the haplotypes of the family must be known beforehand in order to establish the HLA haplotype of the fetus through analysis of the amniocytes. The technique of chorionic villus sampling has provided a means of obtaining DNA for analysis as early as 10 weeks of gestation. Again, it is essential to know the results of DNA analysis in family members in order to decide if the fetus is affected or not. Treatment can now be offered prenatally in order to prevent virilization of an affected female fetus. Dexamethasonc, a long-acting glucocorticoid which readily
  
  T«hl< tiydnmytoc t-
  
  mutations which result in fui
  
  1 >ctcin m o( C4B and CYP21В gene» large gene conversions (* YP21A BO CYP2IB) line to Icucfcx (codot ennvenxm in e n d I A or С nu< dc chant: a mtr An H h**c-pair iK I I" 1 Itolei.'amginc (codon П2) convcni 4 I hree mhacnr mutauom in exon о (codon* Valinc to Icucinc (cod о к п к ш in exoa A 1 besc-p*" in exon 7 'Cd с mutation (< I .\( i m exon h «ton 51 Anpmnc to trvptnphan (codconvention in e\ ■
  
  crosses the placenta, is given to the mother once the preg­ nancy is confirmed. An early start to treatment is required because the fetal adrenal gland is able to function independendy of maternal control by mid-first trimester. The diagnosis is confirmed or excluded, as well as the karyotype determined, at the time of chorionic villus sampling. Treatment is continued throughout the pregnancy in the case of an affected female fetus. Further tests are needed after birth to confirm the diagnosis. In the case of an affected male fetus, treatment is discontinued since the development of the external genitalia will not be affected by the excess adrenal androgens. The gene encoding for the 1 l(i-hydroxylase enzyme is located on the long arm of chromosome 8. Since this form of САН is far less common, there is little experience of prenatal diagnosis using molecular genetic analysis. However, the same principles would apply as those used for the prenatal diagnosis and treatment of 21-hydroxylase deficiency. Deficiency of 3(J-hydroxystcroid dehydrogenase en­ zyme activitv can lead to mild virilization of an affected female (due to excess weak adrenal androgens) or undcrvirilization of an affected male (due to decreased testoster­ one production in the testis). In either instance, there may be an associated salt loss. The gene which codes for this enzyme is located on the short arm of chromosome 1. Other causes of female pseudohermaphroditistn There is the possibility of a female fetus becoming vinlized by maternal androgens transplacentally acquired. Certain tumours may produce excessive androgens in the mother. ЧЪеве include ovarian luteoma of pregnancy, androblastoma, villus cell tumours and adrenal tumours. Appropriate investigation of the mother should be under­ taken if investigation of the virilized female infant has failed to reveal a cause. Maternal ingestion of hormones with androgenic properties is an unlikely cause now that certain progestogens derived from nortcstostcrone arc no longer used for the prevention of recurrent miscarriages. The fetal-placental unit acts as an important source of oestrogens during pregnancy. The aromatase enzyme complex of the placenta converts weak androgens pro­ duced by the placenta (e.g. 1 бОН-dchydrocpiandrosterone sulphate) into oestrogens such as oestriol. A placental sulphatase enzyme is also required for dcsulphation of these fetal steroids. A case of female pseudohermaphroditism has been described from Japan where a newborn female infant was virilized because of placental aromatase deficiency. A persistence of the normal fetal adrenal pattern of steroid production in a preterm female infant may occasionally cause virilization. Isolated clitoromegaly may occur as part of a syndrome (e.g. neurofibromatosis, Beckwith syndrome) without evidence of any apparent endocrine dysfunction.
  
  ABNORMAL SEXUAL DEVELOPMENT
  
  381
  
  Plasma testosterone (nmoVL)
  
  IX -d lesmierom ptaduaum Petal gonadotrophm deficiency Lcydig cell hypoplaAi*. anorchia Dytgcnctic testct Defect) m lenticular JOKCTICMI
  
  18
  
  Abnormal шишгот metabolism Sa-rcductase deficiency Increased лгоошаге act rmal tcsxotUTvnt: productum Complete androgen initmhiiify syndrome Partial androgen Snsa nyndrome Abnormal MulIeruMVinhibitintc (actor production or ястит
  
  Male p s e u d o h e r m a p h r o d i t i s m Male pseudoherniaphnxiitism with decreased testosterone production Table 20.4 lists the main causes of male pseudo­ hermaphroditism. Testosterone production is pituitary LH-dependent and hence any major disturbance in hypothalamo-pituitary function may result in incomplete male sexual development. For example, congenital hypopiiuitarism in the male is typically associated with micropenis due to prenatal LH and testosterone deficiency in the fetus. Differentiation of the external genitalia is usu­ ally normal because of the hCG-dcpcndcnt production of testosterone by the fetal testis. Other neonatal conse­ quences of congenital hypopituitarism may include hypothyroidism secondary to pituitary T S H deficiency and hypoglycaemia due to a combination of ACTH and GH deficiency. The latter hormone deficiency will also manifest later in infancy with signs of growth suppression. Iso­ lated LH deficiency may occur but this is more likely to cause delayed puberty in an otherwise normally developed male (see later). Stimulation with appropriate concentra­ tions of biologically active LH appears to be a prerequisite for normal l^eydig cell differentiation and testicular steroidogenesis. In the male fetus this is provided for by the effect of placental hCG and later fetal pituitary LH. There is a surge in I Л secretion after birth which is re­ flected in the male infant by a rise in plasma testosterone concentrations into the lower end of the normal adult male range. The testes are hyperresponsive at this age to exogenous hCG stimulation and abundant Lcydig cells are evident on histology. When I-H levels decline, there is a corresponding decrease in both basal and post-hCG stimulated plasma testosterone levels. Furthermore, the Leydig cells become differentiated and assume a fibroblast-like appearance during childhood, only to reappear again with the onset of puberty. Figure 20.5 depicts the pattern of plasma testosterone levels during these prenatal and postnatal phases. A biologically inactive form of the LH molecule has long been suggested as a rare form lcydig cell aplasia. This has been given further credence
  
  T
  
  i
  
  1
  
  1
  
  1
  
  r
  
  72 10 70 130190 0 Hours Days
  
  Years
  
  Fig. 20.5 Pattern of plasma testosterone profile during postnatal lite in the male.
  
  by the recent discovery of an altered circulating LH mol­ ecule in a male patient with hypogonadism, due to a single point mutation in the (J-subunit of LH. This eliminates the ability of LH to bind to its receptor. Primary testicular abnormalities comprise a large group of causes of abnormal male differentiation and develop­ ment. Sex chromosome abnormalities give rise to abnor­ mal gonadal differentiation and in turn, often decreased Leydig cell function. Examples include mixed gonadal dysgencsis with an XO/XY karyotype, Klinefelter's syn­ drome with an XXY karyotype and the XX male. The last disorder is seldom associated with incomplete genital dif­ ferentiation apart from a mild hypospadias in about 10% of patients and a small penis. The recent identification of the SRY tcstis-determining gene on the Y chromosome was achieved in no small part by the study of why XX males develop testes. It is now clear that in the majority of XX males there has been X-Y translocaiion of the SRY gene during paternal meiosis. The corollary is the XY female in whom there is pure gonadal dysgencsis and a point mutation of the SRY gene in a proportion of cases. The external genitalia are normal female in these cases, who typically arc tall and have primary amenorrhoea. In­ complete forms of XY gonadal dysgenesis occur and the external genitalia can range from truly ambiguous to predominantly male or female in appearance. Structural abnormalities of the Y chromosome include deletions of both the long and short arms, dicentric chromosomes, as well as rings or isochromosomes of both arms. *1Ъс clini­ cal phenotypc will depend on whether the sex-determin­ ing region of the Y is preserved but a variable phenotype is probably the result of sex chromosome mosaicism due to an associated 45,X cell line. Assuming the presence of a normal Y chromosome (containing an intact SRY gene) and complete testicular
  
  382
  
  CLINICAL BIOCHEMISTRY
  
  differentiation in fetal life, a defect in testosterone biosyn­ thesis may he one cause of male pseudohermaphroditism. Figure 20.6 illustrates the pathway of testosterone biosyn­ thesis which occurs in Lcydig cells. The rate-limiting step is LH-induced conversion of cholesterol to pregnenolone via a P450 side chain cleavage enzyme (P450 sec). Both this enzyme as well as 3|l-hydroxysteroid dehydrogenase and 17a-hydroxylase are required for adrenal steroid bio­ synthesis. Consequently, features of adrenal insufficiency (salt loss, hypoglycaemia, peripheral circulatory collapse) lend to predominate over signs of genital abnormalities. Typically, a male infant with 3P-hydroxysteroid dehydro­ genase deficiency has severely undervirilized external genitalia. Wolffian duct development is usually normal and Mullerian structures are absent. Partial forms of enzyme deficiency are now recognized with, for example, hirsutism in adolescent females as a clinical presentation. A definitive marker of the enzyme deficiency in plasma is an increased concentration of 170H-pregnenolone while the urinary excretion of metabolites of A4-3(i-hydroxy C19 and Д^-З^-hydroxy C21 steroids are increased. The defect is inherited in an autosomal recessive manner and now that the gene has been cloned, reports arc beginning to appear of gene mutations found in affected families. The cyiochrome P450 17a-hydroxylase enzyme is encoded by a gene on chromosome 10. A single P450 enzyme
  
  Cholesterol
  
  Pregoenotone
  
  170H- Pregnenolone
  
  -*► 170H- Progesterone
  
  41 Denydroepiandrosterone
  
  Androstenedione
  
  Testosterone
  
  i
  
  61 Dihydrotesioslerone
  
  The profound virilization which occurs at puberty without evidence of masculinizing effects in utero is an interesting paradox which is not fully explained. Clearly, sufficient testosterone is synthesized at puberty as a result of I-H-induced steroidogenesis. T h e ubiquitous distribu­ tion of 17(i-hydroxysteroid dehydrogenase enzyme may be a further explanation for enhanced androgen synthesis in later life. Most affected males have been reared as females and so it is customary to remove the testes and give oestrogen replacement when the diagnosis is established at puberty. Occasionally, there is some masculinization of the external genitalia at birth in which case it may be possible to rear the infant as a male and perform appro­ priate corrective surgery to the external genitalia. Recent mutational studies have confirmed abnormalities in the type 3 isoenzymc. Male pseudohermaphroditism with abnormal testosterone metabolism
  
  1 1
  
  Androstenediol-
  
  mediates both 17a-hydroxylase and 17,20-lyasc activities. Males affected with 17a-hydroxylasc deficiency manifest clinically with hypertension and usually female external genitalia. However, ambiguous external genitalia in an XY male has been described due to compound heterozygous mutations in the P450 17a-hydroxylase gene. The gene for the cholesterol side chain cleavage enzyme is located on chromosome 15. When this enzyme is deficient, the adrenals appear large because of lipid deposition. Affected males are invariably raised as females because of the severe degree of undcrvirilization. The final step in testosterone biosynthesis is mediated by the enzyme 17(i-hydroxysteroid dehydrogenase which, although not present in the adrenals, is widely distributed throughout the body. The enzyme is involved in reversible reactions between androstencdione and testosterone, ocstrone and oestradiol, and dehydroepiandrosterone and androstenediol. Deficiency of this enzyme in males rarely presents before puberty. Rather, affected males usually have female external genitalia at birth but then virilize at the time of expected puberty with marked clitoromegaly, deepening of the voice and increased muscle mass. Gynaecomastia may also occur. No Mullerian structures persist.
  
  7 Oestradiol
  
  Fig. 20.6 Pathway* of steroid synthesis m the tcsiis. *I"he numbers refer to the following cn/ymcv 20T22sJc*molasc (II; 17n>hydroxyla*e dmgcnase \"i); 17,20-desmohwe (4); 17(Miydruxysten»d dehydrogenase (5); Sa-reductanc 16); arorrwtase :?).
  
  Testosterone also functions as a prohormone by providing a substrate through aromatization for oestradiol formation on the one hand and amplification of the androgen signal on the other, to the reduced androgen dihydrotestosterone. The aromatase reaction is mediated by a cytochrome P450 enzyme complex which in the female is clearly most abundant in the granulosa cells of the ovary. Oestrogen formation in the male is predominantly extraglandular occurring in muscle, skin and fat- A small amount of oestrogen is synthesized directly in the testis. Dihydrotestostcrone is formed mainly by extra-
  
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  ABNORMAl. SEXUAL ПЕУЕШРМЕЧТ
  
  glandular conversion of testosterone in androgcn target tissues such as prostate gland and skin. The microsomal enzyme 5u-reductase requires NADPH as an absolute cofactor to irreversibly convert testosterone to dihydrotestosteronc. Circulating concentrations of testosterone exceed those of dihydrotestosterone by more than 10 times but the latter androgen is at least twice as active as testosterone in its biological effect. It appears that dihydrotestosterone is required to amplify the androgenic effect locally in specific target tissues. Thus, there is evi­ dence that the development of male external genitalia in the fetus and the growth of the prostate gland and hair follicles in adult life are predominantly dihydrotestosterone-dependent. There is no better clinical example of this dependence than the syndrome of 5u-reductase defi­ ciency in the human. Affected males have normal testis differentiation but undervirilized external genitalia. Typi­ cally, there is micropenis, perineal hypospadias, undescended tcstcs and the remnants of a urogenital sinus. The Wolffian ducts develop normally. Such infants are usually reared as females. However, studies in a large inbred population of patients with the disorder in the Dominican Republic indicate that spontaneous vihlization occurs at puberty in response to increased testosterone secretion. That can partly be explained by an apparent increased responsiveness of the external genitalia to testo­ sterone at puberty (in contrast to fetal life) and some residual 5a-reductase activity to account for a direct dihvdrotcstostcronc effect. *1Ъс condition is autosomal recessive and a similar population of affected cases has been described in Papua New Guinea; there are also reports of isolated individuals. The biochemical diagnosis depends on determining the ratio of plasma testosterone to dihydrotestosterone concentrations which normally exceeds 30:1 in affected individuals. Since basal androgen levels are low in child­ hood, an hCXJ stimulation test is required to demonstrate the abnormal ratio. Deficient 5a-reductase can also be demonstrated by measurement of the ratio of 5a- to 5preduced metabolites of C19 and C21 steroids in urine. That glucocorticoids are also metabolized by this enzyme is a useful means to confirm the diagnosis in an affected individual in whom the testes have already been removed. The conversion of testosterone to dihydrotestosterone can also be measured in vitro by incubating genital skin fibroblasts with radiolabeiled testosterone as substrate. However, the assay is not entirely reliable in view of the wide range of conversion values observed in genital skin fibroblasts from normals. Two genes have been isolated in the rat and human which encode for two isoenzymes of 5a-rcductasc. Type 1 is expressed in liver and intestine whereas type 2 is ex­ pressed in the testis, epididymis and vas deferens. It ap­ pears that type 2 enzyme acts as an anabolic enzyme to mediate the growth-promoting effects of dihydrotesto-
  
  383
  
  sterone (e.g. on prostate) whereas catalytic effects are mediated by the type 1 enzyme in, for example, liver metabolism of C19 and C21 steroids. The type 2 enzyme is encoded for by a gene on chromosome 5. A variety of mutations have now been identified amongst this 5 exon gene. The clinical phenotype in 5a-reductase deficiency can van,- widely and mutational analysis of large numbers of different affected pedigrees may eventually explain such heterogeneity. Male pseudohermaphroditism with normal testosterone production If testosterone and dihydrotestosterone synthesis are both normal, then resistance by target tissues to the action of these androgens will result in failure of masculinization. There are now numerous clinical syndromes recognized to be associated with hormone resistance. These include disorders as diverse as pseudohypoparathyroidism, pseudohypoaldosteronism, ncphrogenic diabetes insipidus, resistance to thyroid hormones, vitamin D, growth hor­ mone and insulin as well as male pseudohermaphroditism caused by resistance to androgenic hormones. Complete resistance to androgens in a 46XY male with normal testes presents phenotypically as a female. The term 'testicular feminization* was originally coined to describe such individuals. It is now known that degrees of resistance to the action of androgens can manifest as a wide range of genital abnormalities so that the term androgen insensitivity syndrome (complete and partial forms) is more commonly used. The complete form of the syndrome is usually recognized in adolescence when an otherwise normal looking female is investigated for primary amenorrhoea. Breast development is normal, pu­ bic hair is absent or sparse and the external genitalia have a normal female appearance. There is no development of the uterus and Fallopian rubes (due to the effect of Mullerian inhibiting factor produced by the testes) and the vagina is short and blind-ending. The testes are either situated in the abdomen or may be located within the inguinal or labial regions. Testes presenting as swellings within inguinal herniae may manifest in early infancy lead­ ing to early diagnosis if the significance of bilateral in­ guinal herniae in an apparent female infant is appreciated. Partial forms of androgen insensitivity manifest as varying degrees of virilization of the external genitalia. This can range from mild clitorumegaly alone (female phenotype) to severe perineoscrotal hypospadias and per­ haps micropenis (male phenotype). The latter phenotypic appearance is common to several other disorders previ­ ously discussed so that normal testosterone synthesis must be verified before partial androgen insensitivity can be considered as a possible cause. The hormonal profile consists of normal to elevated plasma concentrations of testosterone for age, increased
  
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  cuNHAi- Hioatr.MisTRV
  
  1,H but normal FSH levels. Breast development at adoles­ cence is the result of LH-induced tcsticular ocstradiol se­ cretion coupled with a lack of the testosterone inhibition of breast development which is the feature in a normal male. '1Ъе reason why normal or even increased concen­ trations of testosterone fail to elicit a biological response in target tissues is now reasonably well explained. Androgens, in company with other steroid hormones, enter a target cell in a passive manner and bind to an inactive receptor in the cytoplasm. The bound hormone-receptor complex becomes activated so that it now resides in the nucleus where further binding takes place to specific chro­ mosomal DNA target sites known as androgen response elements. It is postulated that the activated receptor inter­ acts with other proteins which also act as transcription factors in order to influence the putative gene expression which ultimately manifests the androgenic effect. Much is now known about the early steps in this path­ way. The androgen receptor is expressed in genital skin fibroblasts which, therefore, can be used for androgen binding studies in vitro. When cells arc incubated with increasing concentrations of radiolabelled dihydrotescostcrone, a saturation curve is obtained which, when ex­ pressed as a Scatchard plot, provides information on receptor concentration and binding affinity for androgen. In general, cells from patients with the complete form of androgen insensitivity do not bind androgen whereas binding takes place in cells derived from patients with the partial form of the syndrome. Occasionally, there is a qualitative change in binding expressed as a decreased binding affinity of the receptor or there is evidence of thermolabiliry as shown by decreased androgen binding to the receptor when the assay is performed at an increased temperature. '1Ъе results of these indirect studies of the androgen receptor indicated either the absence of the receptor protein in complete androgen insensitivity or the presence of a receptor which was somehow structurally abnormal in partial androgen insensitivity. "l"hc cloning of the androgen receptor gene has pro­ vided a considerable impetus to understanding androgen binding defects in the androgen insensitivity syndrome. The androgen receptor is a member of a superfamily of nuclear receptors which include receptors for glucocorticoids, oestrogens, progestogens, mincralocorticoids, vitamin D, thyroid hormones and the retinoids among its number. A further number of orphan receptors with a similar array of functional domains have also been discov­ ered whose ligands have yet to be identified. The andro­ gen receptor is encoded by a gene located in the region Xql 1-12 on the X chromosome and is comprised of eight exons. One large exon encodes for the N-tcrminal domain of the receptor protein which is involved in transcriptional regulation. A central domain, which is highly conserved amongst the family of nuclear receptors, is encoded by two exons and contains cysteine residues which co-ordinate
  
  two molecules of zinc. These act as 'fingers' to bind the receptor to chromosomal DNA target sites. The C-terminal end of the protein is involved with androgen binding and is encoded by five exons. Antibodies raised against specific peptide fragments of the androgen receptor can be used for the localization of androgen receptor ex­ pression. Thus immunocytochemistry has demonstrated androgen receptors in brain, prostate gland, pituitary and sebaceous glands as well as genital skin fibroblasts. l h e combined approach of studies of androgen bind­ ing in genital skin fibroblasts and mutational analysis of the androgen receptor gene is now shedding light on the causes of androgen insensitivity, particularly in the com­ plete form. Whether androgen either binds normally to fibroblasts or binding is absent, deficient or qualitatively abnormal indicates the likely location in the gene of a possible mutation. For example, patients with complete androgen insensitivity and normal binding have been described with either a complete deletion or a functional point mutation in one or other of the two exons encoding for the zinc fingers. It is assumed that the androgen re­ ceptor complex in such cases is unable to bind to specific DNA target sites. Large deletions of the gene are uncom­ mon. The majority of patients with the complete form of androgen insensitivity are binding negative and have point mutations in one of the five exons encoding for the hor­ mone binding domain. These may be in the form of pre­ mature stop exons, substitutions of one amino acid for another and mutations at intron/exon junctions leading to altered transcript splicing. Mutations in the androgen receptor gene account for most, if not all, cases of the complete androgen insensitivity syndrome. The situation with the partial form of androgen insensi­ tivity is far less clear. Point mutations have been reported in one of the five exons encoding the C-terminal hormone binding domain in a number of cases. Clues as to whether that is a likely occurrence can be gleaned from androgen binding studies which usually show decreased specific binding activity or a qualitative abnormality in binding such as an altered binding affinity of the receptor or evi­ dence of thermolability at increased incubation tempera­ ture of the binding assay. A mutational screen in these patients is also more likely to be positive when more than one member of a family is affected. However, the majority of patients with phenotypic evidence of partial androgen insensitivity demonstrate normal androgen binding in genital skin fibroblasts and a mutational screen of the androgen receptor gene is normal. Clearly there are other as yet unidentified genes involved in the complex process of normal masculinization of the external genitalia during fetal life. Functional analysis of the androgen receptor gene is possible in order to determine the effect of various identi­ fied mutations. Following the creation of an expression construct, cell lines such as the monkey kidney epithelial
  
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  ABNORMAL SEXUAL DEVELOPMENT
  
  cos cell line can be transfccted in order to studv the androgen binding characteristics of an altered receptor. Furthermore, transactivation assays utilizing both a recep­ tor expression construct and a reporter construct can be used to duplicate the process in vitH> of androgen receptor binding to DNA target sites. Thus, a mutation affecting the two exons encoding the central DNA binding domain could, when recreated in an expression construct, bind androgen normally but will not increase the activity of a reporter gene such as chloramphenicol acetyltransferase. Similar experiments can be performed with recreated receptors spanning the whole range of mutations now being reported affecting this gene. A major unresolved question is the wide spectrum of genital abnormalities observed in the partial form of androgen insensitivity and perhaps the combined results of androgen binding and mutational analysis studies may shed some light on this problem. The X-linked inherit­ ance pattern for this syndrome requires appropriate ge­ netic counselling for affected families which can now be supplemented with information from molecular analysis. An interesting highly polymorphic polyglutamine region in the N-terminal domain of the receptor can be a useful genetic marker to track a mutation through an affected family. Prenatal diagnosis by analysis of DNA obtained from a chorionic villus sample can also be performed in conjunction with sensitive techniques of ultrasound of the external genitalia.
  
  True hcrmaphroditism The word *true' in this context defines a disorder of sex differentiation in an individual in whom both tcsticular and ovarian tissue are present. Ovarian tissue in this case is defined as a gonad which contains oocytes or develop­ ing follicles and does not include the streak gonadal appearance typical of, for example, Turner's syndrome. The gonadal pattern is variable; about half the described patients have a unilateral ovotestis with a eontralateral ovary or tcstis. Approximately one third of patients have bilateral ovotestes while in the remainder, there is a testis on one side and an ovary on the other (normally left) side. In view of the gonadal mix, it is not surprising that the appearance of the external genitalia is highly variable. They are generally ambiguous with hypospadias and undescended gonads. Ovotestes may be found in labioscrotal folds. The phallus can be a reasonable size so that the majority of true hermaphrodites are raised as male. The appearance of the internal genitalia is equally variable. A uterus is usually present and the genital ducts normally develop according to the nature of the ipsilateral gonad. In the case of an ovotestis, a Fallopian tube generally pre­ dominates but rarely can be combined with Wolffian duct development as well. The minority of infants raised as females have breast development at puberty and may
  
  385
  
  also menstruate. Fertility has been reported. Karyotypc frequency is 46,XX > 46XX/46XY > 46,XY (rare).
  
  PUBERTY Physical signs of normal puberty Puberty is the transitional period between childhood and adulthood which also spans adolescence and leads to the acquisition of reproductive capacity. The time span for the physical changes to take place lasts 4-5 years on aver­ age, but the changes in an individual may be compressed into a 2-year interval or expanded into a 6-year time frame. What does vary quite considerably is the age at onset of the signs of puberty. It is important to appreciate that such an age range exists so that the limits of what constitutes early (precocious) and late (delayed) puberty are defined. The first physical sign of puberty in girls is develop­ ment of the breasts which starts, on average, at 11 years with an age range of 8-13 years. This starts as a small mound of tissue beneath the nipple manifest as a breast 'bud' and easily distinguishable on palpation from the softer and more amorphous texture of subcutaneous fat. Further development of the nipple, areola and underlying breast tissue takes place over the ensuing 4 years or so, the entire process conventionally classified into five stages (Bl-5). Almost coincident with breast development is the onset of pubic hair growth as the second physical sign of pu­ berty, although pubic hair may precede breast develop­ ment in 10% of girls. Hair growth actually starts initially on the labial skin before spreading over the mons pubis and occasionally later on, to the medial aspect of the upper thighs. Five stages of pubic hair growth are also described (PH 1-5). Axillary hair growth occurs at about 12.5 years of age and takes a further 18 months or so to reach adult configuration. Menarche, the onset of menses, is a relatively late event occurring at about 13 years of age in the UK. This would coincide with breast development stage B4. Many factors influence the age of menarche, including genetic and socioeconomic. Even in the UK, there is a North - South divide with 3 months or so separating the age at menarche in girls studied in the two geographical regions while the difference amounts to 6 months when comparing Southern with Northern Europe. Increased growth velocity is another obvious outward sign of puberty. In girls, peak height velocity (defined as the maximum rate of growth achieved during puberty) is achieved relatively early in puberty (around breast stage B2-3) and before the onset of menarche. This also implies that by the rime girls start to menstruate, the majority of adult height has been achieved. Changes in body compo­ sition also occur at puberty, with fat distribution typically being a specific feature in girls.
  
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  The physical signs of puberty in boys are also divided conventionally into five stages. The first sign is enlarge­ ment of the testes which is obviously not an outward sign to observe. Testicular volume, as determined by the use of a scries of standard ovoids (the Prader orchidometer), remains between 2-3 m L throughout infancy and child­ hood. An increase to 4 m L heralds the onset of puberty which occurs, on average, at 11.5 years with an age range of Q--14 years. Progressive enlargement of the testes takes place over Ъ years with testicular volumes reaching up to 25 mL in aduh life. Leydig cells constitute only a small part of testicular volume with virtually 9 5 % comprising seminiferous tubules. Pubic hair development, which may start as a few scrotal hairs, follows closely on testicular development and follows the same pattern of stages as in the female. Spread of hair upwards along the abdominal wall is char­ acteristic in many men and is sometimes classified as stage PH6. Growth of the penis, firstly in length and then in breadth, proceeds alongside these other changes and is given a composite stage rating (Gl 5) with the increase in testicular volume. Growth of axillary hair, moustache and beard are all later events in puberty ranging in age from 14-16 years. 'Breaking' of the voice is due to enlargement of the larynx and elongation of the vocal cords which again does not occur until genital stage 3-4. Peak height velocity in boys corresponds with genital and pubic hair stage ratings of 4 and testicular volumes ranging from 12 15 mL. 'Ihis occurs, on average, at a chronological age of 14 years as opposed to 12 years in girls. The adult male is taller than an adult female for two reasons; a later age starting the growth spun (hence taller initially) and a greater magnitude of height incre­ ment achieved (28 versus 20 cm in females). Genetic fac­ tors also play an additional role. Body composition alters in favour of increased muscle development and relatively less subcutaneous fat. Endocrine changes in puberty There is an enormous wealth of data on hormonal events occurring at the time of puberty but the precise trigger which initiates the onset of puberty in the human still remains elusive. There is activation of pituitary gonadotrophin secretion both in early fetal life and again after birth for several months. During this latter period, plasma concentrations of gonadotrophins and sex steroids may reach levels normally observed at puberty. Throughout later infancy and childhood gonadotrophin levels remain low, although evidence of pulsatile LH secretion using more sensitive immunoassays can be detected in some children. The first endocrinological evidence for puberty is an increase in LH secretion associated with nocturnal sleep. The trophic hormone is released in a pulsatile fashion in
  
  response to a bolus of GnRH released into the pituitary portal system. The neurons which release G n R H migrate to the hypothalamus from the medial olfactory placodc during fetal development. A failure of such migration is considered to be the cause of some inherited forms of hypogonadotrophic hypogonadism which arc associated with anosmia (Kailmann's syndrome). The concept of a GnRH pulse generator is now accepted to play a role in the onset of puberty by causing a progressive increase in amplitude and frequency of LH pulses which predates the onset of physical signs of puberty by more than 1 year. Numerous studies of 24 h profiles of plasma gonado­ trophin concentrations have demonstrated this phenom­ enon, while increased gonadotrophin responsiveness to acute GnRH administration is a more practical way to demonstrate the impending onset of puberty. The neuroendocrine mechanisms which cause activation of the GnRH generator at the appropriate age are extremely complex but appear to involve excitatory amino acids, catecholamincs, neuropeptidc Y and acctylcholine as well as external influences such as nutritional intake. In response to increased gonadotrophin secretion, it is not surprising that a gradual increase in the secretion of gonadal steroids occurs, principally testosterone in males and oestradiol in females. This is based on cross-sectional and longitudinal studies on groups of children. Random daytime sex steroid measurements are seldom predictive of pubertal events in an individual, although early morning testosterone levels as a reflection of nocturnal secretion are more useful. Boys show the most marked increment in testosterone levels by stages 2 and 3 of puberty. Spermatogenesis starts between 11 and 15 years of age and sperm can be detected in early morning urine specimens by 13 years of age. Thus the reproductive capacity is achieved some time before pubertal maturation is completed. The attainment of reproductive capacity in the female depends on a more sophisticated hormone control system because of the requirement for cyclical gonadotrophin se­ cretion to ensure ovulation. Increasing oestrogen secretion by developing ovarian follicles under the influence of FSH leads eventually to a positive feedback on LH secretion in the form of a midcycic surge. This is a relatively late event in female pubertal development. The onset of menses is seldom accompanied by ovulatory cycles from the outset and there is generally a 1-2 year interval after menarche before the majority of girls are ovulating regularly. Ultrasonography has provided an additional means to assess the development of reproductive function in girls. En­ largement of the uterus and endometrial thickening is readily evident after birth due to the effect of transplaccntal passage of maternal oestrogens. It is not uncommon to observe multiple ovarian follicular cysts on ultrasound examination. Throughout childhood, cystic changes in the ovary often occur and the effect of increasing secretion of oestradiol during puberty can be observed by appropriate
  
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  ABNORMAL SEXUAL DEVELOPMENT
  
  morphological changes in the appearance of the uterus. The prepubertal uterus actually starts to increase in size from 7 years of age onwards. Adrenal androgen secretion increases in late childhood and before evidence of increased gonadal activity occurs. This phenomenon, termed adrenarche, is characterized by an increase in dehydroepiandosterone (and its sulphate) and androstenedione secretion which starts in both sexes at about 7 years of age and continues for the next 7 years or so. There is no concomitant increase in adrenal glucocorticoid and mineralocorticoid secretion and so it has been postulated that a trophic factor other than ACTH may be selectively stimulating adrenal steroidogenesis at this time. No such factor has yet been isolated but there is evidence of increased activities of the 17,20-lyasc and 17-hydroxylase enzymes. A mid-childhood small growth spun has been identified at the same age which may in part be caused by the increase in adrenal androgen pro­ duction. There is no evidence to indicate that adrenarche has a role in activating the central neural-mediated onset of puberty in the normal child.
  
  Precocious p u b e r t y As a general guideline, signs of puberty which occur be­ fore the ages of 8 and 9 years in girls and boys respectively indicate abnormal early puberty. The problem is com­ moner in girls in which a cause is often not found. Table 20.5 lists some of the known causes of precocious puberty in both sexes. Structural lesions within the central nervous system arc increasingly recognized through sensitive imaging techniques of the h\pothalarnic-pituitary region. Head trauma if severe may be followed quite abruptly by the onset of puberty. It is presumed that premature activa­ tion of the GnRH pulse generator has occurred. Similar mechanisms may operate in hydrocephalus, cerebral palsy, spina bifida and intracranial infections.
  
  Table 2o.*
  
  .ifpn
  
  uspubci
  
  Gtmotimrophin-iUpendent (central) Ы CNS tumour* Head trauma CNSirmduiJ.m Hvpothyrouiiun Gonadtawpkm-ttuUpemdma McCunc Albright чуЫг.чпс Adrenal rumour* Adrenal hyperptaua Gonadal tumour* Тсмо
  
  III
  
  HCG-ftccrcting tumour* Pubsnjl tanants Premature I iche Premature adrenarche Exogenous sex steroid*
  
  387
  
  Tumours in the region of the hypothalamus known to be associated with precocious puberty include hamartoma, astrocytoma, neurofibroma and, occasionally, craniopharyngioma. While such tumours causing precocious puberty have the same prevalence in boys and girls, as a proportion of all causes of precocious puberty, a tumour is more likely to be found in a boy. Hypothalamic-pituitary damage from cranial irradiation for the treatment of such tumours and for prophylaxis in leukaemia is increasingly resulting in early onset of puberty. Endocrine investigations confirm that these central causes arc associated with a gonadotrophin-dependent increase in gonadal steroid production. The McCune-Albright syndrome comprises the triad of cafe au lait skin pigmentation with irregular margins, fibrous dysplasia of bone (affecting particularly long bones and the base of the skull) and signs of precocious puberty. The latter in girls is typically gonadotrophin-independent as shown by a prepubertal LH and FSH response to acute GnRH stimulation. Autonomously functioning multiple ovarian cysts occur as well as evidence of dysfunction in other endocrine glands such as the thyroid (thyrotoxicosis), adrenal (Cushing's syndrome), pituitary (gigantism and hyperprolactinaemia) and parathyroids (hypcrparathyroidism). The condition may present in early child­ hood and progress to gonadotrophin-dependent ovarian hyperfunction and the consequent problems of menses in a young child. The McCune-Albright syndrome is less common in boys and may manifest with asymmetrical enlargement of the tcstcs as well as the secondary sexual characteristics. Recent studies suggest an abnormality in cell signal transduction to account for the endocrine hyperfunction characteristic of this syndrome. Thus so­ matic mutations which appear to activate the a-subunit of the Gs protein have been identified in several patients with the McCune-Albright syndrome. Autonomously hyperfunctioning testes independent of gonadotrophic stimulation arc also a known cause of pre­ cocious puberty in males. Familial forms are recognized which present with signs of virilization but with testes in­ appropriately small for the size of the penis. Testosterone levels are increased while basal and GnRH-stimulated levels of gonadotrophins are suppressed. Histological ex­ amination of the testes shows Leydig cell hypcrplasia and spermatogencsis. By analogy with Graves' disease (thyrotoxicosis), the term 'testotoxicosis' has been applied but in contrast to the autoimmune thyroid disorder, no circulat­ ing stimulatory LH antibody which binds to tcstis LH receptors has been demonstrated. A circulating stimulat­ ing factor for Leydig cells with LH-like activity has been demonstrated in plasma from affected boys using a bioassay system. Another possible cause may be related to some disturbance of the intratesticular regulatory mecha­ nisms which are now known to exist in the local control of Leydig cells by Scrtoli cells. One definite cause for
  
  388
  
  CLINICAL BIOCHEMISTRY
  
  testotoxicosis has been established by mutation analysis of the LH receptor gene. Constitutive!)* activating missense mutations have been identified in that pan of the gene which is important in G protein coupling. Т Ы rationale for treatment in gonadotrophin-dependent precocious puberty is based on the effect of continuous as opposed to pulsatile secretion of GnRH. In the former mode, long-acting agonists of GnRH are used which bind to the GnRH receptor on gonadotropes and cause desensitization with loss of receptors. The effect persists with continu­ ing agonist treatment because the imracellular signalling pathway becomes uncoupled. Conditions such as the McCune-Albright syndrome and familial 'testotoxicosis* do not respond to such treatment initially because of the autonomous gonadotrophin-independent nature of these two disorders. Later, they may become gonadotrophindependent and hence GnRH agonists can be added to the treatment regimen.
  
  Variants of early puberty Other forms of early puberty arc described where gonadotrophin dependence is not a feature and the signs of puberty are incomplete. The term 'pseudoprecocious puberty* is sometimes used to encompass these variants. The most frequent form is premature thelarche or iso­ lated early breast development. The usual age of onset is 1-3 years and occasionally breast development may have persisted as a result of neonatal gynaecomastia. Theremay be unilateral or bilateral breast enlargement which typically tends to wax and wane over time. There are no other signs of puberty and the rate of linear growth is normal for chronological age. Plasma oestradiol and gonadotrophin concentrations remain prepubertal. It has been suggested thai breast enlargement is the result of enhanced tissue responsiveness to a transient increase in oestradiol which was either missed by timing of sample collection or an inadequate assay sensitivity fails to detect. Increased concentrations of sex hormone-binding globu­ lin (SHBG) have been detected in some girls with prema­ ture thelarche. The consequence of this change would be a decrease in free testosterone concentration with an in­ crease in the ratio of free oestradiol to testosterone levels. This may explain the breast development, even though total oestradiol levels arc not increased. The source of oestogen is ovarian and ultrasonography shows the ap­ pearance occasionally of a small ovarian cyst. The natural history of premature thelarche is the onset of puberty and menarche at a normal age as well as normal reproductive potential. Premature adrenarchc is the term applied to the early development of pubic hair without any other signs of puberty. It is more common in girls, occurring usually between 6 and 8 years of age. The cause is an increase in adrenal androgen production associated with elevated
  
  levels of dehydroepiandosterone and its sulphate, as well as androstenedione. Since there is no concomitant in­ crease in the secretion of C21 steroids, it has been sug­ gested that some steroidogenic trophic factor other than ACTH has an effect on adrenal androgen secretion. No precise factor has yet been identified but there is circum­ stantial evidence to indicate a change in the activity of certain steroidogenic enzymes in the zona reticularis of the adrenal gland. The natural history of premature adrenarche is progression to normal puberty at the appropriate age. Delayed p u b e r t y The wide range for the age of onset of puberty in normal children has been emphasized previously. On the basis of that information, puberty would be regarded as delayed if physical signs have not started by 13 years and 14 years of age in girls and boys, respectively. Table 20.6 lists some of the causes of delayed puberty in both sexes. ГЬе list is not an exhaustive one but encompasses the great majority of the important cause». Delayed puberty presents as a clinical problem far more often in boys than in girls. This may be an ascertainment bias perhaps because of the greater social pressures of delayed puberty (and hence short stature) in a boy than a girl. Most delayed puberty just represents the extreme lower end of the physiological range for pubertal events. Since it is inevitably accompanied by short stature due to the delayed appearance of the pubertal growth spurt, this common cause is called constitutional delay in growth and development. There is often a family history of a similar problem affecting the parents when they were teenagers. The natural outcome is for puberty to develop spontane­ ously (according to skeletal maturation as assessed by bone age examination rather than chronological age) and lead to normal, albeit delayed, final height according to genetic potential.
  
  Table 20.6
  
  t
  
  .JUSO
  
  pofcmado*9pktc кураяотл&жт • nrnmi .ugnraflli and development (тгапмеш • NS tumour» rmaoona i 11.-ad trauma I N S irradiation laicd gonadotrophin deficiency Chrome «ystct Bacaac Acquired hypothyroidnm Syndrome^ WfflQ n*rtvwkb*ropkk hypagonodium Syndromes (e.g_ Turner's, Khncfclter ladal dysgenest» •nadal ageneus Acquired gnnadal dy*fiinction (irradiation, chemotherapy) Sex ftcrohJ biobynthciic defect»
  
  \n, зашл
  
  Wi
  
  U W
  
  I VI
  
  ABNOKMAl. SHXUAI. DEVELOPMENT
  
  'ITie clinical assesment of delayed puberty led to the conclusion that this is a disturbance in the tempo of growth and pubertal maturation. Endocrine assessment suggests a delay in activation of the hypoihalamic GnRH pulse generator. Thus, random and GnRH-stimulated plasma gonadotrophin levels are low and appropriate for physiological development, as arc the levels of gonadal steroids. The problem has been to distinguish this from a similar endocrine profile which appertains in delayed puberty secondary 10 hypogonadotrophic hypogonadism. Spontaneous onset of puberty does not occur in the latter disorder, but there is clearly a time limit to how long one can wait. The growth delay component in delayed puberty is partly the result of an insufficient production of growth hormone (GH) reflected in decreased pulse amplitude. This, in turn, results in less generation of insulin-like growth factor 1 (IGF1) which may be relevant for gonadal function. '1Ъегс is clearly a complex interplay of sex hor­ mones, GH and growth factors operating at the time of puberty. Organic causes affecting the hypothalmic-pituitary area result in hypogonadotrophic hypogonadism and de­ layed puberty. Numerous examples exist such as craniopharyngioma, optic glioma, germinoma, astrocytoma, head trauma (may also lead to early puberty), effects of irradiation, intiltrativc diseases and posiinfcctious lesions. Functional gonadotrophin deficiency is usually the reason for delayed puberty arising as a result of chronic systemic disease (such as cystic fibrosis, inflammatory bowel dis­ ease, chronic renal failure, anorexia nervosa, poorly con­ trolled diabetes) and exercise-induced amenorrhoea in female athletes and ballet dancers. Several syndromes are associated with gonadotrophin deficiency; these include Prader-Willi, Lawrencc-MoonBiedl and Kallman's svndromcs. T h e last disorder is associated with anosmia or hyposmia and is now known to be due to a failure of migration of the GnRH neurons from the olfactory placodc to the medial basal hypothalamus. The syndrome exists in a variety of forms of severity due to heterogenous mutations which may affect either the X chromosome or autosomes. Contiguous gene deletions involving the distal short arm of the X chromosome can result in Kallman's syndrome associated with icthyosis (steroid sulphatase deficiency), mental retardation and chondrodysplasia punctata. Another example of isolated gonadotrophin deficiency occurs in association with congenital adrenal hypoplasia. Affected individuals present in infancy with a salt-losing crisis and masquerade as if the diagnosis was congenital adrenal hyperplasia. However, adrenal steroid levels arc low and unresponsive to ACTH stimulation. The diagno­ sis may be suspected before birth because of low maternal oestriol excretion resulting from decreased steroid sub­ strates from the affected fetal adrenals. The disorder is X-linked and affected boys fail to enter puberty spontanc-
  
  389
  
  ously because of an associated gonadotrophin deficiency. Whether the cause is secondary to hypothalamic dysfunc­ tion or is a primary pituitary defect has still not been satis­ factorily resolved. An association with glycerol kinase deficiency has been demonstrated and the congenital adrenal hypoplasia locus mapped to Xp2L3-21.2. Muta­ tions in DAX-1, a gene which encodes a new member of the nuclear hormone receptor superfamily, have recently been described in patients with X-linked congenital adre­ nal hypoplasia and hypogonadotrophic hypogonadism. The other major groups of disorders which manifest with delayed puberty are those associated with primary hypogonadism. In these, gonadotrophin levels are elevated. Chromosomal disorders as exemplified by Klinefelter's syndrome (47XXY) in the male and Turner's syndrome (45XO and variants) in the female are an important cause of delayed puberty. Turner's syndrome has an incidence of about 1 in 2500 liveborn girls although more than 9 5 % of 45XO conceptuses arc aborted spontaneously. Two constant clinical features of this syndrome arc short stat­ ure and failure to enter puberty spontaneously because of premature ovarian failure. The former feature dictates that a peripheral karyotype should be performed on any short girl in whom there is no readily apparent cause for the growth failure. The typical physical features of Turner's syndrome may not always be present. They include a short broad neck which may be webbed, a low hairline» low-set ears, high-arched palate, ptosis, hypoplastic nails, short mctacarpals, cubitus valgus (increased canrying angle), pigmented naevi and cardiovascular anomalies. Affected girls are often small at birth and some present then with peripheral lymphoedema. Growth failure is invariably present by 2-3 years of age. Ovarian failure is a progressive process starting in late gestation and appears to be an acceleration of the diminution in the number of primary oocytcs which occurs between fetal and postnatal life in the normal fe­ male. In Turner's syndrome, only streak gonads remain as remnants of ovaries. The occasional Turner patient with preservation of the long arm of the X chromosome has spontaneous breast development and menses. Man­ agement of Turner patients requires oestrogen replace­ ment at rhe time of expected puberty and the use of growth hormone to enhance the final adult potential. Chromosomal mosaicism in the form of 45XO/46XY may show some features of the Turner phenotype but the predominant physical expression is abnormal develop­ ment of the external genitalia. Klinefelter's syndrome affects about 1 in 1000 males and is characterized in adult life by tall stature, small firm testcs, gynaecomastia and infertility. Detailed survey of affected boys ascertained by cytogenetic analysis at birth indicates that the onset of puberty is not actually delayed and genital development is satisfactory in the majority of patients. Other causes of primary hypogonadism in the
  
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  UJNICiU. BIOCHEMISTRY
  
  male include testosterone biosynthetic defects, the effects of chemotherapeutic agents in the treatment of conditions such as the nephrotic syndrome and leukaemia, and anorchia. The last problem is relatively common and is sometimes referred to as the vanishing testis syndromeGenital differentiation in affected boys is normal which implies that testicular function (including testosterone and M I F production) was also normal in early gestation. The cause of testicular regression is not known although testicular torsion leading to a compromised blood supply is a reasonable explanation. Prolonged stimulation with H C G to demonstrate an absent plasma testosterone re­ sponse is a useful test to confirm anorchia and may avoid an unnecessary explorative laparotomy. More recently, measurement of MIF levels in plasma has been proposed as a useful marker of the presence or absence of tesricular tissue.
  
  INVESTIGATIONS Abnormal sexual development embraces disorders which may be the result of dysfunction in one of a number of endocrine systems. Rather than provide a catalogue of tests arranged according to endocrine glands (e.g. pitui­ tary, adrenals, gonads), the following is a brief guide to a suggested list of investigations based on a problem-orien­ tated approach. In that respect, the three major clinical problems to consider are intersex, precocious puberty and delayed puberty. It is assumed that the performance of investigations only follows and is influenced by a detailed clinical history and physical examination of the patient. Intersex Initial baseline tests:
  
  Peripheral karyotype Plasma 170H-progestcronc Plasma testosterone Pelvic ultrasound Vaginogram
  
  Karyotype 46XX Consider congenital adrenal hyperplasia: (a) 21 -hydroxylasc Plasma ПОН-progesterone f defect (normal < lOnmol/L) Plasma testosterone T (normal < 1.0 nmol/L) Plasma androstenedione Plasma 21-deoxycortisol T Plasma Na*, K', urea Plasma renin activity Plasma aldosterone Plasma A C T H Urinary prcgnanetriol (by gas chromatography) DNA for СYP21В gene mutation
  
  (b) 11 (^hydroxylasc defect
  
  Plasma 1 l-deoxycortisol T Plasma testosterone T Plasma androstenedione Urinary tetrahydro-11deoxycortisol Urinary tctrahydrodeoxycorticosterone
  
  (c) 3(J-hydroxysteroid Plasma 170H-prcgnenolonc dchydrogenase Plasma dchydroepiandrosterone defect Urinary pregnenetriol Urinary 1 бОН-pregnenolonc Urinary 1 6 0 H dehydroepiandrosterone A Synacthcn stimulation test is sometimes required to amplify the specific markers used for each of the enzyme defects. In the short-term stimulation, a standard dose is 250 ng given i.v. or i.m. with samples collected at 0> 30, 60 min. Occasionally, it may be necessary to use a depot preparation of ACTH for more prolonged adrenal stimu­ lation. Depot tctracosactrin 1 mg is given i.m. daily for 3 days. Karyotype 46XY Consider deficient androgen production or abnormal androgen action. The lynchpin investigation is the h C G stimulation test which is designed to assess the capacity of Leydig cells to synthesize testosterone. There are several protocols in use. A frequently used protocol is h C G 1500 or 2000 units daily i.m. for 3 days with samples collected on day 1 and day 4 (24 h after the last h C G injection). This is a shortterm stimulation test which can be extended longer term by continuing h C G injections twice weekly for 3 weeks with a final post-hCG sample collected 24 h after the last hCG injection. Measurements to be performed on samples collected before and after hCG injections include the following; Plasma 170H-progesterone Plasma androstenedione Plasma testosterone Plasma dihydrotestosterone Plasma dehydroepiandrosterone Plasma oestronc Plasma oestradiol Urinary androgen metabolites A defect in testosterone biosynthesis, such as 17|Jhydroxysteroid dehydrogenase deficiency, is characterized by a disproportionate increase in androstenedione and oestronc compared with testosterone and oestradiol, re­ spectively. In 5o>reduciase deficiency, the ratio of testo­ sterone to dihydrotestosterone in plasma and the ratio of 5 a to 5 p reduced androgen metabolites in urine is a particularly useful marker of the enzyme defect. When
  
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  ABNORMAI. SEXUAL DEVEI-OPMENT
  
  normal androgen biosynthesis and metabolism are dem­ onstrated, then a defect in androgen action to account for incomplete virilization must be considered. It is possible to study androgen binding in vitro by the use of fibroblasts established in culture from primary genital skin explams taken at the time of surgery to the external genitalia. Mutational analysis of the gene for the androgen receptor protein may also be informative. An acute GnRH stimulation test is often performed in cases of gonadal dysfunction. The rationale is based on the classic negative feedback mode of operation so that an exaggerated LH/FSH response (perhaps also with el­ evated basal levels) indicates primary gonadal deficiency. The GnRH dose is 100 pg given intravenously with sam­ ples for LH, FSH and testosterone collected at 0, 20, 60 and 120 min. An immunoassay for circulating M I F is now available and may become a useful marker for the presence of tcsticular tissue.
  
  Precocious puberty The following is a list of some of the endocrine in­ vestigations usually required in a case of precocious puberty: Basal LH, FSH, testosterone/oestradiol Acute GnRH stimulation test Thyroid profile Prolactin (J-hCG A thyroid profile is performed since paradoxically, severe primary hypothyroidism may sometimes lead to premature activation of the GnRH pulse generator. The
  
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  acute GnRH stimulation test usually confirms a pubcrtal response in gonadotrophin levels; some centres would also assess pulsatile LH and FSH secretion during the day and night. Occasionally, precocious puberty may be caused by a primitive gonadotrophin-secreting tumour in the liver or testis which would be indicated bv any detectable levels 3-hCG. Any case of precocious puberty also requires de­ tailed radiological imaging of the hypothalamit-pituitary region. Delayed puberty The endocrine investigation of delayed puberty is rather frustrating in view of the common occurrence of constitu­ tional delay in growth and development and the lack of a reliable and simple endocrine test for this physiological variant. The following list contains some of the tests performed: Peripheral karyotypc (delayed puberty in females) Thyroid profile Basal LH, FSH, testosterone/oestradiol Acute GnRH stimulation test Nocturnal LH/FSH profile (if facilities available) Skeletal age (bone age) Other investigations of a non-endocrine nature may be required if chronic systemic disease is suspected as a cause of delayed puberty. For example, a full blood count would demonstrate the anaemia associated with late onset cocliac disease; urea and electrolytes may indicate evi­ dence of chronic renal failure. Children with delayed puberty are also short and may need investigations for growth hormone insufficiency.
  
  FURTHER READING ln:tr*cx Batch J A, Patterson M N, Hughes 1 A. Androgen ineenwtivity syndrome. Reproductive Medicine Review 1*92; I; 131 50. Griffin J E. Androgen resistance the clinical and molecular spectrum. New England Journal of Medicine 1992; Л16: M l - 1 8 . Grumbach M M» Come F A. Disorders of sexual differentiation. In; Wilson J D, Foster D W fedsl Williams textbook of endocrinology, 8th cdn. Philadelphia: Saundcrs, 1992, p 853-951. Hughes I A. Management of congenital adrenal hyperplasia. Archives of Disease in Childhood 1988; 63: 1399-404, Hughes 1 A, Pmsky L. Sexual differentiation. In: Collu R, Ducharme J R, Guyda H J (eds) Pediatric endocrinology, 2nd edrt. New YorkRaven Press, 19Й9. P 2 5 1 93. Josso N, Boussin I., Knchclmann B, Nihoul-Fckcte C, Picard J Y. Ana-Millie л an hormone and intersex states. Trends in Endocrinology and Metabolism 1991; 2: 227-32. Wachtel S S led). Molecular genetics of sex determination. London: Academic Press, 1994. White P C, New M I. Genetic basis of endocrine disease: congenital adrenal hyperplasia due to 21 -hydroxylasc deficiency. Journal of Clinical Endocrinology and Metabolism 1992; 74- 6-11. Puberty Brook С G D, Stanhope R. Normal puberty: physical characteristic*
  
  and endocrinology. In; Brook С G D (ed) Clinical pacdiatnc endocrinology, 2nd edn. Oxford: Blackwcll Scientific I*ublications, 1989, p 169-Я8. Grumbach M M, Stync D M. Puberty: ontogeny, neurocndocrinology, physiology and disorders. In: Wilson ] I), Foster D W feds) Williamstextbook of endocrinology, 8ih edn. Philadelphia: Saundcrs, 1992, p 1139-221. Grumbach M M, Sizonenko P C, Aubcn M L. Control of the onset of puberty. Baltimore: Williams and Wilkins, 1990. Marshall ] C, Kelch R P. Low dose pulsatile gorudotrophin-releasing hormone in anorexia ncrvosa: a model of human pubcrtal development. Journal of Clinical Endocrinology and Metabolism 1979; 19:712 18. Roseniicld R L. Diagnosis and management of delayed puberty. Journal of Clinical Endocrinology end Metabolism 1990; 70: 559-62. Imeitigatwm Bcsscr G M, Hall R. Fundamentals of clinical endocrinology, 4th edn. Edinburgh: Churchill Livingstone, 1989. p 4*57-7e>. Hughes I A. Handbook of endocrine investigations in children, 2nd edn. Guildford: Buttcrwonb» 1989. Rankc M B. Functional endocrinologic diagnostics in children and adolescents. Mannheim: } 9л] Vcrlag, 1992.
  
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  C H A P T E R
  
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  Abnormalities of reproductive function John Waterstone, John Parsons, John Miell and Richard Ross
  
  INTRODUCTION In both normal males and females, gonads produce steroid hormones which affect secondary sexual characteristics, the functioning of the reproductive tract and sexual behaviour. Production of gametes and hormones by the gonads is under the control of pituitary glycoprotein hor­ mones. The hypothalamo-pituitary-gonadal axis appears to be universal throughout the vertebrates: small amounts of releasing factors from the hypothalamus elicit the release of rather larger amounts of glycoprotein gonadotrophins and yet larger increases in gonadal steroids. These steroids in turn influence the rate at which they themselves are produced (feedback control). In clinical terms, the couple presenting with impair­ ment of reproductive function should be investigated and assessed as a couple. However, the biochemical aspects of reproductive function are such that it is convenient to consider the female and male separately and this is reflected in the overall structure of this chapter.
  
  At birth some 10* immature germ cells are present in the ovaries as primary oocytes, arrested between prophase and metaphase of the first meiotic division. Each primary oocyie is surrounded by a layer of epithelial cells, the whole being known as a primordial follicle (Fig. 21.1). Primary oocytes do not complete meiosis during child­ hood; in fact the majority of them degenerate. During childhood the ovaries remain inactive, but at puberty a monthly ovarian cycle is established through the interaction of the hypothalamus and pituitary with ovarian follicles and manifests itself by the onset of menstruation. The human menstrual cycle is 23-39 days long. By definition, day 1 of the cycle is the first day of menstrual bleeding. '1Ъе cycle is divided into follicular (or prolifcrative) and luteal (or secretory) phases by the event of ovulation. In cycles of different length it is the duration of the follicular phase which varies; the length of the luteal phase is remarkably constant at 13-15 days. Follicle dynamics
  
  REPRODUCTIVE FUNCTION IN T H E FEMALE John Waicrstonc and John Parsons PHYSIOLOGY O F T H E MENSTRUA1VOVAR1AN CYCLE The ovary The human ovary produces female gametes and steroid sex hormones. Both functions depend largely on the monthly growth and rupture of single ovarian follicles.
  
  On day 1 of the cycle several 5 mm follicles are present in both ovaries. These consist largely of a fluid-filled antrum and can be visualized by ultrasonography (Fig. 21.2). Each 5 mm antral follicle has developed from a primordial follicle by proliferation of the epithelial granulosa cells and by the appearance and coalescence of fluid-filled spaces among them (Fig. 21.1). The earlier stages of follicle development do not require gonadotrophin stimulation, but the 'recruitment* of antral follicles from the preantral pool is follicle-stimulating hormone (FSH) dependent. Through a poorly understood process of selection, one of the apparently identical 5 mm follicles present on day 1 393
  
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  Cytoplasm of primary oocyte Nucleus of primary oocyte Epithelial/ granulosa ce^ls >
  
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  becomes dominant while the others degenerate. *1Ъс dominant follicle grows rapidly in the late follicular phase» reaching a maximum diameter of approximately 20 mm. Outside the basement membrane of the granulosa cell layer the wall of the dominant follicle consists of the theca intcrna and theca cxtcrna, which have developed from ovarian stromal cells. At ovulation the follicle collapses, releasing its fluid and the oocytc, which by now has com­ pleted the first meiotic division. The follicle subsequently refills with fluid and blood vessels penetrate the basement membrane and vascularizc the granulosa cells for the first time; further proliferation of the granulosa cells trans­ forms what was the follicle into the corpus Iutcum. The corpus Iutcum has a limited lifespan; in the absence of conception it involutes, another dominant follicle devel­ ops and the cycle repeats itself.
  
  P l a s m a c o n c e n t r a t i o n s of reproductive h o r m o n e s Figure 21.3 illustrates the fluctuations in plasma con­ centrations of reproductive hormones through a typical menstrual cycle. Changes in the pulsatile release of gonadotrophins in the late luteal phase (LLP) and early follicular phase (EFP) appear to bring about the growth of a group of antral follicles, one of which (the dominant follicle) enlarges greatly and secretes increasing amounts of oestrogens. Although oestrogens generally exert a nega­ tive feedback effect on gonadotrophin levels, high, rising oestrogen levels in the later follicular phase (LFP) feed back positively on the hypothalamic-pituitary axis, causing a massive release of luteinizing hormone (the LH 'surge') and a smaller release of FSH. The LH surge triggers the resumption of meiosis and follicle rupture with oocyte release. Oestrogen levels fall transiently at ovulation. The corpus luteum secretes both oestrogen and progesterone; serum levels of both hormones peak in the mid-luteal phase (MLP), falling in the L I P as the corpus luteum involutes. It is likely that ovarian proteins such as inhibin
  
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  and activin also play a role in regulating follicle develop­ ment and steroid synthesis. These proteins and other structurally similar 'growth factors* may exert important paracrine effects within the ovary. Uterine changes Rising oestrogen levels in the follicular phase induce proliferativc changes in the endometrium, causing it to thicken. Oestrogens and progesterone, from the corpus luteum, induce secretory changes in the endometrium, preparing it for pregnancy. Once deprived of hormonal support from the corpus luteum, the endometrium is shed and another cycle begins.
  
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  Conception Fertilization takes place in the ampullary part of the Fallopian tube, close to the ovary. Implantation occurs 6-7 days after fertilization. The early conceptus somehow signals the corpus luteum so that the life of the latter is prolonged, maintaining the endometrium.
  
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  Fig. 21.2 Vaginal ultrasonographic images of the ovary in the follicular phase of the menstrual cycle. (A) day 2; CB) day 8; , inhibin suppresses the release of FSH from pituitary cell cultures, while activin stimu­ lates FSH release. While these proteins may play a paracrine role within the ovary, it is uncertain whether they exert physiologically important endocrine effects on the pituitary.
  
  Prolactin Prolactin is a single chain polypeptide containing 198 amino acids. It shares a high degree of homology with growth hormone and placcntal lactogen. It had been con­ sidered a non-glycated protein, but it now appears that both non-glycated and glycated species are present in the pituitary and in plasma. Prolactin is synthesized by lactotroph cells of the anterior pituitary* Dopamine is the principal negative modulator of prolactin secretion; oestrogens and thyroid hormone-releasing hormone increase prolactin release. Prolactin has diverse actions in vertebrates, playing roles in processes as diverse as osmoregulation and meta­ morphosis. Prolactin is luteotrophic in some mammals but not in man. T h e only definite role of prolactin in women is the postparrum initiation and maintenance of milk production. REPRODUCTIVE STEROID HORMONES Structure
  
  H u m a n chorionic g o n a d o t r o p h i n (hCG) The p-subunit of h C G consists of 145 residues. h C G is synthesized by the syncytiotrophoblastic cells of the placenta. The hormone has a relatively long plasma halflife (24-36 h).
  
  Ovarian steroid hormones arc derivatives of cholesterol and contain the cyclopentanoperhydrophcnanthrcnc nu­ cleus (Fig. 21.4). Oesirogens (C-18) steroids, androgens (C-19 steroids) and progestogens (C-21 steroids) contain 18, 19 and 21 carbon atoms respectively. These com-
  
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  AHNORMAIJTIISS OF REPRODUCTIVE FUNCTION
  
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  Structural basis of steroid hormones.
  
  pounds and the pathways involved in their biosynthesis arc illustrated in Figure 21.5. osynthctic e n z y m e s 1. Cholesterol side chain cleavage. Three separate reac­ tions are involved: 20 a-hydroxylation, 22-hydroxylation
  
  397
  
  and scission of the C-20-22 carbon bond. This is the point where I-H primarily regulates ovarian stcroidogencsis. The product of the process, which occurs in a mito­ chondria! complex containing the cytochromc P450 system, is pregnenolone. 2. 3fi-Hydroxysteroid dehydrogertase Д4-' isomerase. This catalyses both the 3|J-hydroxysteroid dehydrogenation and isomcrizauon of the double bond from ring В to ring A. Pregnenolone is thereby converted to progesterone. 3/4. 17a-Hydroxylase/17,20-de$molase. This enzyme catalyses the 17ct-hydroxyIation of pregnenolone and pro­ gesterone. The desmolase reaction involves the formation of a peroxide at C-20, epoxidation of the C-17 and C-20 carbons and side chain cleavage to form the C-17 oxosteroids dehydrocpiandrostcronc from pregnenolone and androstenedione from progesterone. Both reactions are catalysed by the same enzyme. The intermediates are 17a-hydroxypregnenolone and 17ct-hydroxyprogesterone respectively. 5. 17-Oxosteroid reductase. This enzyme catalyses the conversion of an 17-oxosteroid to a 17(i-hydroxysteroid and vice versa. Androstenedione and ocstronc arc con­ verted to testosterone and oestradiol respectively. 6. Aromatase. This converts С-19Д 4 '' oxosteroids to oestrogens by hydroxylation of the C-19 angular methyl group, oxidation and cleavage of the C-19 methyl group
  
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  as formaldehyde, dehydrogenation of the A ring, and finally conversion of the 3-oxo group to a 3($-hydroxy group. FSH regulates the level of this enzyme in granulosa cells. Oestrogens are synthesized from androstenedione, the major pathway being through oestronc. Pregnenolone is converted to androstenedione cither by the Д' pathway (through progesterone) or by the Д4 pathway (through dehydroepiandrosterone). The Д* pathway is favoured in granulosa/corpus luteal cells, while the Д* is favoured in theca cells. The major secreted oestrogen is 17(J-ocstradioI; it is in equilibrium with oestronc in the circulation. Oestronc is farther metabolized to oestnol, probably for the most pan in the liver. Oestradiol is the most potent oestrogen of the three and oestriol the least. Steroid secretion through the menstrual cycle Before ovulation the granulosa cells are not vascularued. The theca cells produce large quantities of androgcns, which diffuse into the granulosa cells and are convened to oestrogens. This concept of co-operation between cell types is called the two-cell theory. Vascularization of the granulosa cells accounts for increased progesterone se­ cretion in the luteal phase. The stromal cells of the ovary secrete small amounts of androgcns. This secretion be­ comes more significant after the menopause when it adds to oestrogen levels through peripheral conversion. Steroid hormone transport and metabolism Most circulating oestrogen and progesterone is proteinbound, loosely to albumin and strongly to globulins; oes­ trogens are carried by sex hormone-binding globulin (see later) and progesterone by conicosteroid-binding globu­ lin. Degradation of both C-18 and C-21 steroids occurs in the liver and involves hydroxylation and water solubilization by conversion to glucuronidc and sulphate conju­ gates. Water-soluble metabolites are excreted in urine and in bile. Quantitative urinary tests were used in the past to assess levels of oestrogens and progestogens but have been superseded by rapid, sensitive and specific serum immunoassays. The major excreted oestrogen and progestogen are oestradiol and pregnanediol respectively. Actions of gonadal steroid hormones Oestrogens Oestrogens broadly maintain the functions of the reprodiKUvv rr.ui, ;irv rv-рипчЬк' tin мччпиЬгу ^с\ил\ J U : . K teristics and affect sexual behaviour. Oestrogens cause myometrial hypertrophy, cause the endometrium to thicken in the follicular phase, promote secretion of large
  
  amounts of watery cervical mucus around the time of ovu­ lation and maintain vaginal function; after ovaricctomy or the menopause, the uterus and vagina shrink in size. Oestrogens produce duct growth in the breasts and are responsible for breast enlargement at puberty. Oestrogens, like androgcns, increase libido in humans, apparently by a direct action on hypothalamic neurons. In general, oestro­ gens reduce secretion of I Л and I ; SH (negative feedback) but under cenain circumstances oestrogens increase LH secretion (positive feedback). In addition to their repro­ ductive role, oestrogens have imponant systemic effects; they maintain bone density and skin thickness and protect against athcromatous anerial disease in premenopausal females. Synthetic oestrogen derivatives with agonist actions are used in contraceptive and hormone replacement prepara­ tions. Ethinyl oestradiol is most commonly used; it is po­ tent and, unlike naturally occurring oestrogens, is active when given by mouth. Recently, it has become possible to use 17(J-oestradiol for transderma) hormone replacement therapy by incorporating it into skin patches. Ami-ocstrogcn preparations are also available: clomiphene citrate and tamoxifen are non-steroidal triphenyl ethylene derivatives with mixed oestrogen agonist and antagonist actions. Clomiphene citrate is generally used for induction of ovulation, tamoxifen to treat oestrogendependent breast cancers. Progestvgetis Progesterone causes secretory changes in endomctrium already primed by oestrogen. It promotes the secretion of smaller amounts of thicker cervical mucus. It is essential for the maintenance of early pregnancy. Synthetic steroids with progesterone agonist properties arc used in contraception and hormone replacement therapy. Those commonly used tend to be derivatives cither of 17a-hydroxyprogcstcronc or of 19-nonestosterone. Both types of derivative are used in combined oral contraceptive preparations; along with ethinyl oestra­ diol, they prevent follicle growth, promote an endometrial reaction unfavourable to implantation and render cervical mucus thick and impenetrable to sperm. Compounds with progesterone antagonist action have only recently been developed. RU486 is a derivative of norethisterone which blocks the actions of progesterone peripherally. It induces menstruation when given in the late luteal phase. It also induces abortion when given in early pregnancy and is already in use as an abortifacient. Andwgen metabolism in the female The two main androgens in the female are testosterone and androstenedione. Dehydroepiandrosterone (DHEA) and its sulphate (DHEAS) are less imponant androgens.
  
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  ABNORMALJTIES OF REPRODUCTIVE RJN'CTION
  
  During the reproductive years, 90% of DHEA and DHEAS is synthesized by the adrenal and 10% by the ovary. Fifty per cent of androstenedione comes from the ovary and 5 0 % from the adrenal. Only 50% of testosterone produc­ tion is glandular (roughly equal contributions from adre­ nal and ovary)» the remainder resulting from peripheral conversion of weaker androgens, mostly in adipose tissue. Ninety-nine per cent of testosterone is bound and 1% is free; the great majority (78%) is bound to sex hormonebinding globulin (SHBG). SHBG is a glycoprotem syn­ thesized in the liver, with a carbohydrate content of 34%. It is a high affinity, low capacity binding protein. Plasma SHBG concentrations arc raised when oestrogen levels are high (as in pregnancy) and in hyperthyroidism; levels arc low in women treated with testosterone. Because SHBG-bound testosterone is relatively inert biologically, it may be useful for laboratories to measure both testoster­ one and SHBG, to give an indication of how much testo­ sterone is not SHBG bound.
  
  INVESTIGATION O F OLIGO- AND AMENORRHOEA Oligomenorrhoea means menstrual cycle length greater than 6 weeks, but less than 6 months. Amenorrhoea means complete absence of menstruation or cycle length greater than 6 months. In these conditions ovulation does not occur or is very infrequent. Women with oligo- or amen­ orrhoea may seek medical assistance because their bleed­ ing pattern is abnormal, because of infertility, because of hirsutism/virilism (see below) or with a combination of these complaints. Primary ovarian failure and hyperprolactinaemia are common causes of amenorrhoea. The former is diagnosed by a raised serum FSH level (>40 i.u./L) and is irrevers­ ible. Diagnosis of the latter has been confused by a lack of consensus about what constitutes the upper limit of nor­ mal for serum prolactin concentration; values ranging from 300 to 1000 mU/L (15-50 ng/mL) have been quoted by different laboratories. Significant disease is unlikely unless the prolactin level is greater than 1000 mU/L. The possibility that hyperprolactinaemia is secondary to drugs (e.g. methyldopa, phenothiazines and mctoclopramide) must be excluded. Computed tomography (CT) of the pituitary should be performed routinely in hyperprolactin­ aemia to exclude a macroprolactinoma (a prolactinsecreting adenoma) or a non-functioning tumour; the most frequent diagnoses resulting are idiopathic disease (no C T abnormality) or a microadenoma (inhomogeneity on C T scan). Microadcnomas are so common in the normal population that the nature of their association with hyperprolactinaemia is uncertain. This topic is discussed further in Chapter 16. Increasingly, the treatment of first choice for hyper­ prolactinaemia, whether tumoral or idiopathic, is medical.
  
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  Bromocriptine has been the drug almosi invariably used, but new agents such as cabergoline and CV 205-502 show promise. The majority of patients with oligomenorrhoea and a significant minority of those with amenorrhoea have polyeystic ovaries. Today this morphological diagnosis is made by ultrasonography: polyeystic ovaries tend to be of large volume and contain a large number (> 10) of small (2-8 mm) follicles. Normal ovaries contain several small follicles in the early follicular phase and may be wrongly labelled as 'polyeystic' if diagnostic criteria are not applied strictly. The classic 'polyeystic ovary syndrome' (PCOS; synonym polyeystic ovary disease) was described by Stein and Leventhal in 1935; they associated polyeystic ovaries with oligo- and amenorrhoea in a small series of patients, most of whom were infertile or hirsute. It later became apparent that plasma levels of LH and/or androgens are elevated in many but not all women with polyeystic ova­ ries; FSH levels are normal. T o complicate the situation further, 2 3 % of women who have no gynaecological symptoms have also been found to have polyeystic ovaries. The term PCOS is best reserved for patients with poly­ eystic ovaries (diagnosed by ultrasonography) who also have oligoVamenorrhoea. The pathophysiology of PCOS is unknown; the disorder has been variously ascribed to primary abnormalities of the ovaries, of gonadotrophin se­ cretion and of the adrenal glands. It seems most likely that changes in the pattern of gonadotrophin secretion arc re­ sponsible for the failure of antral follicles to grow and for a dominant follicle to emerge and ovulatc. It is possible to induce follicle growth in patients with PCOS by therapeu­ tic administration of FSH. Obesity and insulin resistance are associated with PCOS and there is evidence that weight reduction in obese PCOS patients leads to a re­ sumption of ovulatory cycles. It is important to recognize that polyeystic ovaries and PCOS can occasionally occur as a secondary phenomenon in patients with other endo­ crine disorders. 1Ъеве include adrenal conditions such as congenital adrenal hyperplasia and Cushing's syndrome and pituitary diseases such as hyperprolactinaemia and acromegaly. Medical treatment of PCOS (as above) can bring about follicle growth so that subfertility can be temporarily over­ come. In some patients with PCOS surgical treatment such as wedge resection or ovarian diathermy returns cycle length to normal. Hypothyroidism is a rare cause of oligomenorrhoea which causes secondary hyperprolactinaemia. Once ovarian failure, hyperprolacrinaemia and hypo­ thyroidism have been excluded further investigations are unnecessary. Patients whose complaint is infertility, with or without polyeystic ovaries, are treated by induction of ovulation. An anti-oestrogen is given initially; clomiphenc citrate has been used most widely and there is no evidence that others, such as cyclofenil, are superior. If clomiphene
  
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  citrate fails, human menopausal gonadotrophins (hMG) arc substituted, with human chorionic gonadotrophin given to induce follicle rupture by simulating the ovulatory LH surge. hMG dosage is kept as low as possible to try to prevent a large number of follicles from growing. An excessive response, which tends to occur in patients with polycystic ovaries, may result in multiple pregnancy or the ovarian hyperstimularion syndrome. If safe induction of ovulation proves difficult, in vitro fertilization (IVF) treat­ ment may be preferred. INVESTIGATION O F INFERTILITY Specific investigations aimed at establishing one of the causes of infertility in Table 21.1 are indicated if a woman has failed to conceive after trying for 12 months. Semen analysis is used to exclude subfertility in the male partner, but the results arc often inconclusive; attempts at IVF establish the fertilizing capacity of semen more clearly. The causes and management of subfertility in the male partner are considered later in this chapter. Laparoscopy with hydrotubation and hysterosalpingography both assess patency of the Fallopian tubes, using direct vision and radiography respectively; intraperitoncal spillage of fluid injected through the cervix excludes tubal blockage. Ultrasonography, which is performed increas­ ingly by the vaginal route, is used to assess ovarian morphology. The postcoital test (PCT) is a traditional in­ vestigation which involves the microscopic examination of mucus aspirated from the cervical canal at mid-cycle; in­ tercourse should have taken place approximately 12 h pre­ viously. Normally, 15 motile sperm per high-power field arc observed. A negative test is often the result of mistim­ ing - cervical mucus only becomes penetrable by sperm for a couple of days around the time of ovulation. The finding of persistently negative PCTs suggests the need for measurement of antisperm antibody litres in cervical mucus, semen or male serum. Antisperm antibodies fall into two categories: immobilizing antibodies, which in high titrcs arc associated with infertility, and agglutinating antibodies, which are not. Basal serum levels of FSH and LH arc measured rou­ tinely in infertile women. Usually a single serum sample is taken during the EFP, on day 1-3 of the cycle. Inter­ pretation of apparently abnormal EFP gonadotrophin levels must take into account the pulsatile nature of re­ lease of both hormones. For LH the pulse interval in the EFP is 65-90 min with peak and trough concentrations Table 21.1
  
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  varying by as much as a factor of 5. Serum FSH levels do not fluctuate so widely - by a factor of 2 at most. It is becoming apparent that elevated FSH levels in women with normal length menstrual cycles indicate diminished fertility and possibly incipient ovarian failure. In patients undergoing IVF treatment, EFP serum FSH concentra­ tion is inversely related to the number of oocytes obtained and to the probability of conception; basal FSH appears to be a better predictor of treatment outcome than age. Measurement of FFP LH is of less certain diagnostic value; levels tend to be abnormally high in patients with PCOS. Spuriously high serum 1 Л values result when sampling coincides with a pulse peak. Basal levels of prolactin and thyroid hormones are unlikely to be of diagnos­ tic value unless menstrual cycle length is abnormal. Laboratory investigations are used to assess the ovulatory process, although it is impossible to be certain that ovulation has been normal except when conception has followed. Serum progesterone levels in the mid-luteal phase (Ml-P) (i.e. 7 days after ovulation) are measured to assess ovulation. Values in conception cycles have been found to range between 28 and 53nmol/L (8.8-16.7 ng/ ml.), and 30 nmol/L is generally taken as the lower limit of normal for MLP progesterone. Serum sampling before or after the mid-luteal peak results in spuriously low values. Serum sampling may be timed correctly by per­ forming it several times and identifying the MLP sample retrospectively (8 days before the first day of the next menstrual period). Alternatively, the patient may detect her own LH surge using a one-step urine test kit (see below) and serum sampling may be timed 8 days later. Progesterone is secreted in a pulsatile fashion during the luteal phase, concentrations varying by as much as a fac­ tor of 3; when sampling coincides with a trough the serum progesterone level may appear abnormally low. MLP pro­ gesterone levels in patients with unexplained infertility (no tubal, uterine or male factors identified) are consistently found to be lower than in conception cycles; some of these patients are presumed to be infertile because of poor luteal function. The term iuteal deficiency' is applied to such patients and it is assumed that they have a form of ovulatory dysfunction. Progesterone causes an elevation in body temperature and a biphasic pattern is apparent when the majority of women who ovulatc record their temperature daily, with temperatures in the luteal phase higher by 0.5°C. Basal temperature charts have been used in the past to diagnose normal ovulation but are becoming outdated because they are inaccurate as well as inconvenient. Some patients who ovulatc normally do not have biphasic temperature charts. Serial ultrasonography is used to examine and time fol­ licle rupture; successive scans should visualize the growth and subsequent disappearance of a dominant follicle. In fact, the vast majority of patients with normal-length men­ strual cycles exhibit normal follicle growth and rupture.
  
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  ABNORMALITIES OF REPRODUCTIVE FUNCTION
  
  The LH surge which precedes follicle rupture can be detected by qualitative urine tests which employ mono­ clonal antibodies. One-step dip-suck tests are now com­ mercially available and are used increasingly to dictate the timing of sexual intercourse, artificial insemination and frozen embrvo transfer. Superovulation treatment Increasingly, subfertility, whatever its cause, is treated using assisted conception techniques. These techniques all involve superovulation treatment, which causes a number of follicles to grow so that several oocytes mature. Oocytcs may be collected for fertilization in vitro or trans­ ferred together with sperm to the Fallopian tubes (gamete intrafallopian transfer, GIFT). Oocytes fertilized in vitro are usually transferred to the uterine cavity as 2-8 cell cleavage stage pre-embryos. Modern superovulation regi­ mens include human menopausal gonadotrophins (hMG); commercial preparations are extracted from postmenopausal urine and contain equal quantities of LH and FSH.
  
  Supenwulation without pituitary deserialization hMG has been used alone and in combination with clomiphene citrate. Ultrasonographv is used to determine the number of follicles which grow and their size. An endo­ genous IJ4 surge necessitates cancellation of the cycle and can be detected by serum sampling; LH, oestradiol (E 2 ), progesterone or a combination of these hormones may be measured. If LH is measured a surge may be detected directly. Oestrogen levels should rise continuously from day to day; a levelling off or fall indicates that a surge has occurred. Similarly, a rise in progesterone levels indicates a surge. An injection of hCG, used as a LH surrogate, is given when the largest follicles reach a diameter of 17 mm, ideally just pre-empting the endogenous LH surge; oocyte collection is performed 34 h later. Superovulation with pituitary desensitization Most superovulation protocols now include administra­ tion of a GnRH analogue, which prevents endogenous LH release, allowing oocyte collection procedures to be scheduled conveniently. Thus far, agonists such as buserelin acetate, leuprolide and decapeptyl have been used in order to achieve a functional antagonist action. Adminis­ tration of a single dose of agonist results in the release of both LH and FSH and an increase in their plasma con­ centrations. However, prolonged administration downregulates pituitary receptors and results in low plasma levels of IJH and FSH. Agonist is commenced on day 1 or 2 of the cycle; h M G is added 2 weeks later (in iong protocols') or 2 days later (in 'short protocols') and is con­ tinued until hCG administration. Because endogenous
  
  401
  
  LH release is prevented when GnRH analogues are used, serum hormone assays are unnecessary. hCG is usually administered when the diameter of the largest follicles is approximately 20 mm. HIRSUTISM AND VIR1USM Hirsutism means excessive hair growth in sites usually associated with male sexual maturity, i.e. the face, lower abdomen (giving the appearance of a male escutcheon), anterior thigh, mid-chest and periareolar region. It can be associated with acne and oily skin. Virilism refers to more extreme manifestations of androgen exposure including temporal hair recession, clitoromegaly, increased muscle mass, breast atrophy and deepening of the voice. Testosterone, after conversion to dihydrotcstostcronc, acts on hair follicles so that terminal hair (long, coarse and pigmented) grows rather than vellous hair (short, fine and poorly pigmented). Once stimulated, the hair follicle remains responsive to low-dose weak androgen exposure because it has attained the ability to convert weak androgens to testosterone. In a minority of cases, a definite cause of hirsutism can be identified (sec Table 21.2). Polycystic ovaries, with or without oligo-Iamenorrhoea, arc associated with a substanual proportion of cases. In many cases, neither polycystic ovaries nor any of the rarer causes can be diagnosed; such hirsutism is termed idiopathic. In idiopathic hirsutism total testosterone levels may be normal, but some androgen disorder can generally be identified. Androstenedione concentrations arc usually elevated and SHBG reduced; free testosterone concentra­ tion may be raised even though total testosterone is within normal limits. The investigation of hirsutism should determine whether androgen levels are indeed elevated and to what degree and rule out serious disease (particularly malignancy) of the ovary or adrenal. Grossly elevated testosterone con­ centrations and hirsutism of sudden onset (especially if accompanied by virilism), suggest malignancy. Polycystic ovaries can conveniently be diagnosed by vaginal ultrasonographv, which will also reveal all but the smallest ovarian tumours. Computed axial tomography or magnetic resonance imaging is used to visualize adrenal tumours. Laboratory investigations used to evaluate hirsutism include plasma cortisol and 17a-hydroxyprogestcronc, in addition to testosterone, androstenedione and i—
  
  —
  
  I able 21.2
  
  Some rarer cu
  
  1
  
  ■ umm
  
  Congcmi.il .iJn-ii.il hypcrptatia (adult on*• t h i n g ' s syndnimc Ovarian tumour* Adrenal tumour* Sdf-edmuii--iT.il:.4i of jnilrogens
  
  402
  
  CIJNir.AI. HIOCHI-MISTRY
  
  SHBG; steroid hormone concentrations following over­ night dexameihasone suppression or ACTH stimulation may also be useful. Adult-onset congenital adrenal h>perplasia (САН) is increasingly being recognized as a cause of hirsutism. T h e most frequent type is 21 -hydroxylase deficiency, which results in elevated plasma concentra­ tions of 17a-hydroxyprogesteronc and increased synthesis of adrenal androgens. Testosterone concentrations are significantly elevated and ACTH stimulation is followed by an excessive increase in 17a-hydroxyprogesterone (see Appendix (i), p. 410, for protocol). In Cushing's syn­ drome there is loss of the normal diurnal rhythm in plasma cortisol concentrations and failure of suppression in the overnight dexameihasone suppression test. Although 9 0 % of DHEAS is normally of adrenal ori­ gin, elevated DHEAS levels in hirsutism do not confirm an adrenal source for the excess; elevated DHEAS levels may also be found in polycystic ovarian syndrome. A DHEAS level that is more than twice the upper limit of normal suggests the possibility of an adrenal rumour. Effective suppression of elevated androgen levels in hir­ sutism by dcxamcthasonc might be expected to confirm an adrenal source; however, selective venous catheterization studies have demonstrated that elevated ovarian androgens in hirsutism may also be effectively suppressed by dexameihasone.
  
  Once the rare causes of hirsutism shown in Table 21.2 have been excluded, its treatment consists of medication to suppress androgen levels. Combined oral contracep­ tives arc widely used; they depress production of both ovarian and adrenal androgens. Glucoconicoids such as dexameihasone may also be effective, as may the antiandrogen cyproterone acetate. Medical treatment may be supplemented by cosmetic techniques such as shaving, waxing and electrolysis.
  
  REPRODUCTIVE FUNCTION IN T H E MALE John MicU and Richard Ross T H E TEST1S The lestis has two distinct roles - production of androgens and spermatogenesis. Androgen production occurs in the Ley dig cells, which appear histologically as clumps of cells lying between the seminiferous tubules. Leydig cell mass correlates positively with androgen production. The pre­ dominant male androgen is testosterone and normal male sexual differentiation in the fetus is dependent on its pres­ ence. In early gestation (12-16 weeks) fetal plasma testoМуоЮ o#ll Spermatogeness
  
  SpefTnatogonia and spefmatocytes |^«*Г1,|ФЛ1
  
  Ыоос; , i-Ut/eiaculate > 1 Mimol/cHKulale
  
  •NX ! Ю cUuincetion described in tcxT
  
  Antibodies directed against spermatozoa and constitu­ ents of seminal fluid have been described, in particular sperm-agglutinating or sperm-immobilizing antibodies, which mav be associated with infertility. Evaluation of sperm - cervical mucus interactions is described elsewhere (p. 400). Fig. 21.7
  
  Hypothalamic-pituitaiy control of icsticular function.
  
  \ [dual sperm output on a day-to-day basis often make accurate interpretation difficult. It is often more useful in clinical practice to assess the actual fertilization capacity of sperm m nm>. For semen analysis, a fresh semen sample is collected by masturbation into a wide-mouthed container and should ideally be analysed within 1 hour. The volume and viscosity of the sample is noted - normally the ejaculate should liquefy within 20 min. Determination of the pH of seminal fluid is useful in analysing the relative proportions of seminal vesicle secretion (alkaline) and prostatic fluid (acid). The presence of fructose, which is secreted by the seminal vesicles, excludes an obstruction of the ejaculatory ducts. Microscopically, sperm density, motiliry, viability and sperm morphology are assessed. Sperm density is reported as millions of sperm per millilitrc; alternatively, the total sperm count may be given as millions per ejaculate. The W H O classification of motiliry is defined as: a. b. c. d
  
  rapid and linear progressive motiliry; slow or sluggish non-linear motiliry; non-progressive motiliry; i m motile.
  
  If the immotile spermatozoa exceed 60%, the proportion of live spermatozoa (viability) can be assessed using stain­ ing techniques. The percentage of dead cells should not exceed the percentage of immotile sperm. A large propor­ tion of viable but immotile sperm may suggest structural defects in the flagcllum. Reference ranges for semen variables are given in Table 21.4.
  
  Endocrine evaluation - h y p o t h a l a m o - p i t u i t a r y gonadal axis Determination of basal plasma LH and FSH concen­ trations is a routine part of the assessment of the hypothai amo-pituitary-gonadal axis. As 1.11 and, to a lesser extent, FSH are secreted in a pulsatile fashion, more accu­ rate interpretation may be possible if serial samples are taken over a period of time, though in routine clinical practice the measurement of basal levels of LH and FSH with testosterone provides most of the information re­ quired. Elevated gonadotrophins (outside the reference range) in the presence of a low testosterone suggest pri­ mary testicular failure and low or normal gonadotrophins associated with a low testosterone suggests a hypothalamopituitary disorder. It is important to recognize that any acute illness may cause a temporary hypogonadotrophic hypogonadism. Rarely, LH alone may be elevaied with a low testosterone (the Sertoli-only syndrome) and in patients with abnormal spermatogencsis FSH may be raised with a normal testosterone. It should be remem­ bered that pituitary and gonadal dysfunction may coexist, as in haemochromatosis, and in this situation both gona­ dotrophins and testosterone are low. An h C G stimulation test (see below) may help diagnose this state. Since the isolation and characterization of GnRH, GnRH tests have been used in the evaluation of the go­ nadal axis. GnRH promotes the release of LH and FSH in a dose-dependent manner with a peak at 20-30 min. Nor­ mally, 100 ng is given intravenously and blood taken at 0, 20 and 60 min (see Appendix (iv), p. 411). GnRH regu­ lates both gonadotrophin synthesis and release and also self-regulates pituitary GnRH receptor density, so the
  
  0M
  
  ARN'OKMAIJ'ntiS Ob RKPROlKXTtVE FUNCTION
  
  G n R H test may theoretically be a valuable index of both hypothalamic-pituitary function and the releasable pool of pituitary gonadotrophins. Nevertheless, the test does have some limitations. Prolonged GnRH deficiency may lead to depleted pituitary stores of gonadotrophins, resulting in an impaired response to exogenous G n R H unless the pituitary is 'primed' with chronic GnRH administration for up to a week before the test. Furthermore, the GnRH test cannot distinguish between hypogonadotrophic hypogonadism and delayed puberty two conditions in which a prepubcrtal response to GnRH is seen (poor gonadotrophin response with higher levels of FSH than LH). Thus the GnRH test has limited use in clinical practice and provides no extra information above that given by the measurement of basal levels of LH and FSH. The GnRH test may demonstrate that there are pituitary stores of LH and FSH by direct stimulation of the pituitary, but this does not tell one about the in vivo function of the hypothalamo-pituiiary axis. Stimulation tests with clomiphene citrate have also been used in the assessment of the gonadal axis in men (see Appendix (ii), p. 410). A positive response to clomi­ phene may help in diagnosis as it suggests that there is a normal hypothalamo-pituitary axis (clomiphene blocks testosterone and oestrogen feedback). However, the test is usually negative and in this situation is of little clinical value. It may be negative because of a structural abnor­ mality at the level of the hypothalamus or pituitary or because the patient has low levels of testosterone and therefore clomiphene has little effect. Leydig cell function Measurement of 9.00 a.m. plasma testosterone concen­ tration by sensitive and reproducible immunoassays has simplified the assessment of Lcydig cell function. Lowconcentrations are found in both primary testicular disease and disorders of the hypothalamic-pituitary axis with depressed gonadotrophin secretion. Elevated levels are seen in patients with gonadotrophin- or androgensccreting tumours (e.g. Leydig cell tumours), in whom gonadotrophin levels will be low. Basal gonadotrophin es­ timations are again important, as an increase in basal LH and FSH above the reference range, in association with low testosterone levels, would suggest primary testicular dysfunction. In certain situations when gonadotrophins arc low, stimulation with hCG may be helpful in differentiating primary testicular failure from gonadotrophin deficiency (see Appendix (iii), p. 411). This test is also useful in confirming the presence of testicular tissue in cryptorchidism (sec below) and in conditions in which there may be combined pituitary and testicular dysfunction, for example iron deposition in haemochromatosis or treated thalassaemia.
  
  405
  
  MALE HYPOGONADISM Male hypogonadism may result from either priman,- tes­ ticular failure, in which the testes fail to develop properly or are injured by disease or medical manipulation, or sec­ ondary testicular failure due to hypothalarnic or pituitary lesions (hypogonadotrophic hypogonadism). Hypogonad­ ism results from developmental or acquired pituitary dis­ ease or a failure of hypothalarnic GnRH secretion. Other syndromes resulting in hypogonadism include enzymatic defects in which hormonal synthesis is interrupted despite an intact hypothalamo-pituitary-testicular axis. Clinical features The physiological effects of testosterone have been de­ scribed above (see Table 21.3). Prcpubertal testicular fail­ ure results in lack of sexual maturation with persistence of infantile genitalia and absence of pubic and axillary hair growth. Delayed epiphyscal closure results in the develop­ ment of a eunuchoid habitus with increased arm span (more than 5 cm greater than height) and increased sole to pubic symphysis length (more than 5 cm greater than pubic symphysis to head length). The reduction in testo­ sterone levels also results in diminished anabolism and a decrease in normal male muscular development. Postpubertal onset of androgen deficiency is less obvi­ ous and usually presents as decreased libido, impotence and infertility. There may be an associated decrease in growth of facial, pubic and axillary hair and a diminution in skeletal musculature. Gynaecomastia may occur as a result of an increase in the oestradiol-testostcronc ratio (see below). Despite some reduction in testicular volume, the penis and prostate do not shrink and erections and orgasm may continue. Nevertheless, spermatogenesis requires testosterone and, even when able to ejaculate, severely hypogonadal males are azoospermic. Laboratory findings If both Leydig cell function and spermatogenesis are defi­ cient, laboratory findings will be azoospermia, low testo­ sterone concentrations and high gonadotrophins. Isolated seminiferous tubule dysfunction results in a low sperm count and increased plasma FSH concentration. In hypo­ gonadotrophic hypogonadism or secondary testicular failure, levels of LH and FSH are normal or low in the presence of low testosterone. It is important to remember that a level of LH or FSH falling within the laboratory reference range may be inappropriately low for the level of testosterone. Primary testicular failure Klinefelter's syndrome In 1942, Klinefelter, Reifenstein and Albright described a
  
  Матер
  
  ■ авторским пр
  
  406
  
  CLINICAL BIOCHEMISTRY
  
  syndrome comprising cunuchoid stature, gynaccomastia, small tester and high urinary FSH levels. Later, these abnormalities were found to occur in association with un­ usual chromosomal karyotypes •- usually 47XXY but oc­ casionally other extra X states or mosaics (e.g. 48XXXY/ 46XY). A few patients are seen with a 46XX karyotype, with translocation of the male sex-determining gene from the Y chromosome to the short arm of the extra X. Leydig cell abnormalities, characterized by abnormal mitochondria and endoplasmic rcriculum, result in im­ paired testosterone secretion. The low levels of testoster­ one fail to activate negative feedback pathways, resulting in increased LH levels. FSH levels are also increased, reflecting impairment of seminiferous tubular function and decreased inhibin secretion. Patients are infertile and there is no treatment for this.
  
  vaginal pouch and inguinal or labial testes. Testosterone levels are normal, therefore Wolffian duct derivatives are found, as it is testosterone which is responsible for their development. At puberty, patients undergo virilization with some phallic enlargement, development of male habitus and psychosexual orientation and descent of the testes - occa­ sionally with normal spermatogenesis. This occurs either as a result of a massive increase in testosterone levels at puberty or by conversion of some testosterone to D H T if the enzyme deficiency is not complete. Patients may re­ spond to very high doses of testosterone or, if peripubertal virilization is minimal, may be raised as infertile females after removal of the cryptorchid testes.
  
  Anorchia
  
  Complete resistance to testosterone, probably due to absence of or deficiencies in the D H T receptor protein, results in an apparently normal female phcnotypc with absence of pubic and axillary hair. However, the uterus and Fallopian tubes are absent and testes may be found in the inguinal canal. Patients are generally gcnotypically 46XY and the condition is transmitted as an X-linked re­ cessive trait. Testosterone, 1 Л and FSH levels are normal or high. 5ot-Rcductasc activity is normal and there arc normal levels of dihydrotestosterone. Administration of supraphysiological doses of testosterone has no effect on the generation of secondary sexual hair growth.
  
  Complete absence of testes in a phenotypic male with 46XY karyotype is known as anorchia. The presentation is that of complete castration with absence of secondary sexual characteristics, low testosterone levels and high gonadotrophins. The condition should be differentiated from cryptorchidism as the undescended tcstis has a high frequency of malignancy (between 12 and 30 times that of the normally descended testis). The testes may be palpa­ ble in the inguinal canal or visualized radiologically in the abdomen. If doubt persists, an h C G test should be per­ formed and if there is a rise in testosterone then a further search including a laparotomy should be made and the testes removed to prevent malignancy. Defective hormones and h o r m o n e receptor failure
  
  Feminizing testis syndrome
  
  As patients are usually phenotypically female, with female sexual drive, treatment usually involves removal of cryptorchid testes with subsequent oestrogen replace­ ment. Full sexual activity is usual, though of course the patients remain infertile.
  
  Enzymatic defects Five enzymes are responsible for the conversion of choles­ terol to testosterone and defects in each of these enzymes have been reported. Enzymatic defects lead to pscudohermaphroditism with rudimentary or absent Wolffian duct derivatives (seminal vesicles and ductus deferens) and ambiguous external gcnitalia. *1Ъе uterus and Fallopian tubes do not develop because of the presence of M I F and there is usually cryptorchidism. Testosterone levels are low and stimulation with h C G results in elevation in levels of precursors proximal to the enzymatic block.
  
  Reifenstein s syndrome Patients with Reifenstein's syndrome are generally pheno­ typically male and have a normal 46XY karyotype. Af­ fected individuals have marked hypospadias and undergo incomplete virilization at puberty, which is often associ­ ated with gynaccomastia. This syndrome was originally thought to be due to a postreceptor defect resulting in androgen insensitivity, but recent evidence suggests that there is an abnormality in the androgen receptor itself I-evels of LH, FSH and testosterone are normal or high. Family pedigrees with various members affected accord­ ing to an X-linked pattern have been described.
  
  5&-Rcductase deficiency Deficiency of 5u-reductase, which converts testosterone to dihydrotestosterone ( D H T ) , results in pseudohermaphroditism. D H T is responsible for masculinization and normal levels are a prerequisite for the full development of the phallus, scrotum and prostate. In 5u-reductase defi­ ciency, external genitalia at birth are ambiguous with a urogenital sinus, clitoral hypertrophy, bifid scrotum, blind
  
  Hypogonadotrophic hypogonadism Hypogonadism resulting from deficient secretion of pitui­ tary gonadotrophins is known as hypogonadotrophic hypo­ gonadism (Table 21.5). This may result from either congenital inherited abnormalities such as Kallman's syndrome or isolated LH deficiency (fertile eunuch syn-
  
  иериал, защищенный авторским правом
  
  ABNORMAUT1KS OF REPRODUCTIVE FUNCTION
  
  Table 21. S Causes of hypogonadotn
  
  goo
  
  Suprasellar rumour* v raruopharyngiomak
  
  pfemlomM Instructive pituitary disease pituitary adenomas \urgcry empty sells syndrome
  
  Ггп1аг|1пппт Chronic illness Malnutrition Iron deposition haemochromatosii haemogJobinopathics Oestrofen-ftecrerinf adrenal rumour»
  
  drome) or acquired conditions affecting the hypothalamicpituitary axis, such as suprasellar tumours or destructive pituitary disease. In all these conditions, both plasma testosterone concentration and pituitary gonadotrophin levels are low. Kallman *$ syndrome Testosterone deficiency secondary to primary pituitary gonadotrophin failure may occur in the absence of pitui­ tary or suprasellar tumours. There is often accompanying anosmia, sometimes with complete absence of the olfac­ tory lobes, and occasionally other midline craniofacial developmental abnormalities, e.g. cleft lip and palate. Plasma LH and FSH concentrations are very low. The condition is transmitted either as an autosomal recessive or as an autosomal dominant trait with incomplete expres­ sion. Clinically, patients with Kail man's syndrome have scant secondary sexual development with absence of body hair, prepubertai penis and testicles, poor musculature and cunuchoid habitus. It is often impossible to distin­ guish hypogonadotrophic hypogonadism from delayed puberty, although the presence of a family history, anos­ mia or microgenitalia may give a clue. Early pubertal boys will respond to clomiphene citrate with a rise in LH, while this response is absent in hypogonadotrophic pa­ tients. Often it is necessary to treat the patient to induce puberty and then stop at a later stage. It is, of course, important to exclude pituitary or suprasellar tumours by imaging (CT scan or MRI). Fertile eunuch syndrome Selective, isolated LH deficiency was described by McCullough in 1953, who coined the term "fertile eu­ nuch syndrome". These patients have low LH levels but normal FSH levels and are capable of ejaculating a low semen volume with normal spermatozoa. Leydig cell function is unimpaired and patients respond to hCG in­ jection, but testosterone levels are not sufficient to induce full virilization. Treatment with hCG or testosterone is usually successful.
  
  407
  
  T r e a t m e n t of hypogonadism Primary testicular failure Testosterone replacement remains the mainstay of treatment. Testosterone may be given by intramuscular injections of long-acting esters every 2-4 weeks. Oral pre­ parations have, in the past, been unsatisfactory because of poor absorption and hepatic complications (cholestatic jaundice and hepatomas), but testosterone undecanoatc appears to be free of significant adverse effects (although a tendency to cause gastrointestinal upsets means that com­ pliance may be a problem). A third delivery regimen involves subcutaneous implantation of pellets which can normalize testosterone levels for up to 6 months.
  
  Hypogonadotrophic hypogonadism Even in the presence of diagnostic uncertainty, there is no benefit in withholding testosterone replacement treatment beyond the mid-teens. In prepubertai males, testosterone can be used to induce masculinization. Testosterone can be given for 6 months to induce virilization and the treat­ ment regimen then curtailed. Most boys with delayed pu­ berty (no pubertal development after the age of 14 years) will proceed normally through adolescence, whereas pcrsistendy low LH and testosterone levels would suggest a diagnosis of hypogonadotrophic hypogonadism and treat­ ment with testosterone can then be reintroduced. If fertil­ ity is desired, treatment with h C G for 6 months followed by hCG and human menopausal gonadotrophin (hMG) can be instituted. The addition of h M G to supply FSH activity will usually induce a good quality ejaculate which is quite capable of resulting in conception despite a rela­ tively low sperm count (1-20 million per mL). An alterna­ tive treatment is with small pulses of GnRH delivered via a programmed pump, mimicking physiological GnRH pulsatility. Although rather cumbersome, this regimen is effective in inducing LH, FSH and testosterone secretion and can result in adequate spermatogenesis. Long-acting GnRH analogues arc ineffective as they bind to GnRH receptors protractedly, resulting in desensitization of the gonadotroph and hypogonadism.
  
  GYNAECOMASTIA Excessive development of the male mammary glands with increases in both stromal and glandular tissue (gynaecomastia) accounts for approximately 70% of male breast disorders. In the male, breast development may vary from a small subareolar button of tissue to florid breast devel­ opment with feminization of the nipple and associated breast tenderness. Gynaecomastia may occur in neonatal life, when it is usually transient and caused by transplacental transfer of maternal oestrogens into the fetal bloodstream. During puberty, gynaecomastia of some degree may occur in up
  
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  408
  
  < ; INK AI ним ш MISIK',
  
  ГаЫс -'i .e - ,iu-.t i ■■! впш Menb -
  
  ЮОП
  
  Totit castration mump* or Kim -vndrome Ricfenstcm's syndrome араогаЪвааНЯ Adrenal Cusl syndrome 11 jfr-h \yUie dcficiei i I byroad disordc ! typcrparathyroidum
  
  Chronic renal failure Cirrhosis
  
  Tcsticular опсагсюотд
  
  Lcyctif ( &
  
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  HaerrHKhromaioaii
  
  I
  
  Adrenal
  
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  adenoma ( .и. BOOH
  
  B:
  
  malnutntion Albright's syndrome
  
  tofenk can toona
  
  pane carcinoma 1 rastnc adcnocarcmoma Renal adenocarcrooma I .ymphoprohfcranve disorders MycJoprotifOBdvc dttordt
  
  "*' ■
  
  to 60% of adolescent boys and this may reflect normal physiological development with spontaneous regression after 12-18 months. Ptiystca
  
  Physiological gynaecomastia during puberty probably occurs as a result of the change between ocstradiol and testosterone levels - in normal early puberty, oestradiol levels are high in comparison with testosterone and this increased ocstradiol-tcstosteronc ratio can induce breast development. Pathological causes of gynaecomastia can be broadly divided into endocrine, tumoral and metabolic (Table 21.6). Several drugs have also been implicated in the development of gynaecomastia (Table 21.7). frTITffTiffTffrfl
  
  A full drug history and a careful physical examination arc essential, with particular reference to the testes, thyroid, liver, lungs, cardiovascular system and nutritional status. A suggested schedule for the investigation of gynaeco­ mastia is illustrated in Figure 21.8 and the interpretation of endocrine tests in Table 21.8. Drug*
  
  Biocrwmttry
  
  Lve» function tews Uree Croatinme
  
  Endocrinology
  
  FSH.LH Taa4oatercn»oeatradlol SHBG TSH Prolactm
  
  in tin- pjihogcncsis ot gynaecomastia
  
  Fig. 21.8
  
  Investigation of gynaemmasoa.
  
  IMPOTENCE Erectile impotence is defined as the inability to achieve and maintain a penile erection of sufficient quality to per­ mit penetrative intercourse. In practice, the commonest physical cause of impotence is diabetes mcllitus - some 50% of diabetic males will become impotent after the age of 50. Endocrine causes include hypogonadism (primary or secondary), hyperprolactinaemia and thyroid disease. Other causes arc summarized in Table 21.9. A psychogenic aetiology is suggested by a history of early morning or sleep erections or full erection on mas­ turbation or with other partners. Treatment with certain drugs can result in impotence - these are summarized in Table 21.10.
  
  Мпплтча
  
  Drag
  
  Androgens Mgltalis Tctrahydri4:annabtnol riftcofulvm Spir.moUctone Cimctidinc Cyprot crone Mcthadonc Phenothianne* Reserptnc liooiaaid
  
  uss
  
  1
  
  Causes of gynaecomastia
  
  Table И71
  
  Radiology
  
  Chest X-ray Skull X-ray СТ
  
  Direct stimulation. inhibition of androgen production Aromatt/ed to oestrogen* Binding Co oestrogen rcccj Binding to oestrogen receptors Binding to it :cn receptors Increase ш tfaPSdhul testosterone ratio Anu-androgen Anu-androgen Anti-androgen
  
  \ j )
  
  Mechanism unknown
  
  Investigation The initial history and examination is of great importance. Reduced frequency of shaving, reduced libido, small tes­ tes and regression or lack of secondary sexual characteris­ tics suggests hypogonadism. Tcsticular tumours may be palpable and visual field assessment and radiology of the brain and skull should be carried out if there is a possibil­ ity of pituitary tumour. A full neurological and cardio­ vascular examination is essential. Funher evaluation may be divided into biochemical investigation, evaluation of vascular causes and neurogenic causes (Table 21.11).
  
  r
  
  '
  
  ABNORMAl JT1ES OF REPRODUCTIVE FUNCTION
  
  Table 21.1 Intcrpr changes important in diagnosis arc thown
  
  fgynaccoma
  
  in the
  
  409
  
  those
  
  Horn Condition
  
  Ti
  
  Гптагу tcsticular failure S< lary К Ear failure IVimary oestrogen-secreting tumour hCG-sccrcting tumour Chnmi< dsease Thyrotoxicosi»
  
  i i 1
  
  СГОО*
  
  LH
  
  FSH
  
  SHBG
  
  Ootra«ii>'l
  
  T T T
  
  T* i t
  
  T
  
  T T
  
  'Depends on the i p c c l k l q of the I Л assay
  
  Table 21.9
  
  A number of techniques are available for the investiga­ tion of vascular causes. The intracavcrnosal injection of papaverine and phentolamine results in an increase in intracavcrnosal blood flow and a reduction in the resist­ ance to venous filling (as a result of smooth muscle relaxa­ tion). 'I*his results in an erection in approximately 7 5 % of males. An inadequate response suggests significant vascular pathology. Alternatively, arterial blood pressure may be recorded. Penile systolic blood pressure is measured with a Doppler stethoscope and recorded as a ratio to that of the brachial artery. Low penile-brachial ratios ( пЮСПСППСш tcttt
  
  Tests for vascular causes
  
  I о is for neurogenic causes
  
  IH 1Ml «ostcronc oiactm
  
  Duplex ultrasoaography I'apavennc и i vcrnosograph у PhalloarTcriogra pi Nocturnal tumesccrv
  
  Cutaneous perception threshold measurements Pudenda! nerve evoked potentials fefMfcM sensu
  
  OmndM Try
  
  T TMI
  
  inc
  
  ung
  
  410
  
  CIJNU:AI. BIOCHEMISTRY
  
  sensory function. An electromagnetic test probe is placed on the penile skin in various sites and the intensity of vibration increased until it is perceived by the patient. The degree of vibration required for perception can he com­ pared with other sites on the patient's body. More elaborate and specific techniques are only avail­ able in specialized centres. Tests such as the bulbocavernosus reflex latency measurement (which assesses the whole reflex arc) and pudenda) nerve somatosensory evoked potential tests (which assesses the pathway between the sensory nerve ending and the cortex) may be useful in screening, but will not localize pathology-. Treatment of erectile impotence Various treatment methods for impotence have been assessed, including psychosocial counselling, oral drugs, penile prostheses, self-injection with smooth muscle relaxants (e.g. papaverine) and mechanical vacuum tumescence aids. Oral agents include the a_>-adrenoceptor blocker yohimbine (which is not available in the UK) and the dopaminc receptor agonists apomorphinc and bromocriptine. Studies suggest that yohimbine may be effective in up to 2 5 % of patients with psychogenic impotence and about 15% of patients with impotence of mixed aetiology. Penile prostheses, either semirigid or malleable rods inserted into the corpora or self-inflatable prosthetic devices, have been used with varying degrees of success. Local injection, mechanical failure and the length of NHS waiting lists have resulted in diminished use of this treatment option. Self-injection of papaverine can result in successful erections in patients without a vasculogcnic cause of impotence. The injection method needs to be adequately taught and should not be used more than once a week to reduce potential side-effects of corporeal fibrosis and priapism. More recently two types of vacuum tumescence aids have become available - ErecAid, which maintains tumes­ cence by application of a constriction band, and Synergist, which requires the condomlikc device to remain in situ. These devices have been successfully employed in a number of patients. CONCLUSION While much has been learned over the past decade about human reproductive function and its abnormalities, many questions remain unanswered. On the female side, the process whereby a single dominant follicle emerges during each menstrual cycle is uncertain, as is the pathogenetic mechanism which gives rise to polycystic ovary syndrome. On the male side, conventional semen analysis appears increasingly to have limited power in predicting the ferti­
  
  lizing capacity of semen. In both males and females, a cause for infertility may not become apparent despite ex­ tensive investigation. It is likely that the increasing appli­ cation of assisted conception techniques will cast new light on these areas over the next decade. APPENDIX: PROTOCOLS FOR E N D O C R I N E INVESTIGATIONS (i) Congenital adrenal hyperplasia - stimulation of 17a-hydrox\progesterone Perinatal diagnosis of congenital adrenal hyperplasia can be confirmed by a single assay of 17a-hydroxyprogesterone (17-OHP) in a capillary blood sample. In less clear cut cases, it is often necessary to measure levels of 17O H P before and following stimulation. Consequently, 17-OHP levels are measured before and 60 min after an intravenous injection of 0.25 mg of ACTH. The expected values 1 h after ACTH are as follows: Unaffected adults Heterozygotes for 21 -hydroxylase deficiency Patients with mild 21-hydroxylase deficiency
  
  3 30 nmol/L 6-44 nmol/I 63-470 nmol/L
  
  For further details of the diagnosis of heterozygotes and the late-onset form of 21-hydroxylase deficiency, see New e t a l 1«83 (below). Recently a similar test assessing both the ACTH and 17-OHP responses to corticotrophin-rcleasing factor (CRH) has been described. This test can also distinguish classic cases from late-onset congenital adrenal hyperplasia. REFERENCES Morcira A C, Eha* L L K . Pituitary adrenal гсгрипъс* to corueocrophin-rc leasing hormone in different degree* of adrenal 21 hydroxvlase deficiency. Journal of Clinical Kndocrinology and Metabolism 1992; 74: 198 203 New M I et al. (icnotyping steroid 21-hydroxvlase deficiency; hormonal reference data. Journal of Clinical Endocrinology and Metabolism 19Я*;57; ?20-o.
  
  (ii) Clomiphene test The clomiphene test may be helpful in distinguishing gonadotrophin deficiency from weight-related hypogonadism and idiopathic delayed puberty. Side-effects include visual disturbances, symptoms of oestrogen deficiency and depression, which may be severe enough to warrant discontinuation of the test. Clomiphene (3 mg/kg/day to a maximum of 200 mg/ day) is given in divided doses for 7 days. Serum l.H and FSH arc measured on days 0, 4, 7 and 10 and in females progesterone levels (to confirm ovulation) are assessed on
  
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  ABNORMA1JTIES OF REPRODUCTIVE FUMTTION
  
  day 2 1 . In a normal response LH and FSH rise to outside the reference range or to double the basal values. A lack of response suggests gonadotrophin deficiency due to pituitary or hypothalamic disease. Patients with anorexia nervosa may show no response. Prcpubertal children show no response and children in early puberty may actually show a reduction in gonadotrophin levels during the test.
  
  Rl-FKRKNCK Andersen D C et al, Stimulation teM* of pituitary Leydig cell function in normal male subject» and hypognnadal men. Clinical Endocrinology l ° ? 2 ; I 127-40.
  
  (iii) hCG stimulation test In the differential diagnosis of male hypogonadism, a stimulation test with human chorionic gonadotrophin (hCG) may be useful. There are no specific precautions and the test does not have to be carried out after fasting. h C G is injected i.m. in a dose of 2004) i.u. on days 0 and 2 and serum testosterone measured on days 0, 2 and 4. A normal response is defined as a rise in serum testo­ sterone to above the reference range. A failure of any rise in testosterone suggests an absence of functioning testicular tissue. If the testes are not palpable in the scrotum or inguinal canals, a rise in testosterone suggests the pres­ ence of intra-abdominal testes. In gonadotrophin defi­ ciency with normal testes, the low basal testosterone level should triple after hCG.
  
  41 I
  
  REFERENCE Andersen I) С et al. Stimulation test* of pit unary-Ley dig cell function in normal male subject* and hypojjonadal men. Clinical Endocrinology 1^72; I: 127^*0.
  
  (iv) GnRH stimulation test In the investigation of gonadotrophin deficiency, assess­ ment of the responses to exogenous gonadotrophin releas­ ing hormone (GnRH) may be useful. There is no need for the patient to fast unless the test is being combined with assessment of other anterior pituitary hormone responses to insulin-induced hypoglycaemia. GnRH 100 ^ig is given intravenously at time 0 and samples taken at 0, 20 and 60 min. Normally LH and FSH levels both rise, though the degree of increase is de­ pendent on individual laboratories. The peak response may be seen at either 20 or 60 min. The interpretation of the GnRH test is largely based on the basal values. In a patient with delayed puberty, a normal response or a response in which the LH peak is greater than that of FSH suggests that the patient is about to go into puberty. The response to GnRH may be suppressed by intervening illness. REFERENCE Bc*ser G M, Ross R J M. Arc hypothalamic releasing hormones uvefuJ in the diagnosis of endocrine disorder»? In: Edwards I R, Iincoln С R VCr feds). Recent advance» in endocrinology and metabolism. Edinburgh; Churchill Livingstone, 1989, p 135-58.
  
  h'UKTHKR KKA1MNC i Adams J, Pulton D VC\ Franks S. Prevalence of polycystic ovaries in women with anovulation and idiopathic hirsutism. KriiKh Medical Journal 1986; 2<M 1S5-9 Crosignani P G, Ferrari C. Dopaminergk treatments for hypcrprolactinaemia. Baillierc's Clinical Hndacrinology and Gynaecology 1990; 4(*i: 441 5V Filicori M et .il Physiology and pathophysiology of pulsatile gonadotrophin secretion. Gynaecology and Endocrinology 1988; 2 . 7 * Я5. de Groot L J Ted). Endocrinology. Philadelphia; W. B. Stiunder», 1989. The classic ет&кппоич*у text m three volume. containing m depth sections on disorder* of Ычк male and female reproducing function. Grossman A (ed)< Clinical endocrinology. Oxford: Blackwell Scientific Publications, I 4«2. An up-tt*-date, simple to n W text xi'ith many refervnici containing sections on the pttuitjry gonadal axiiy sexual differentiation and female infertility and oho мнит* i»; Ju.yrd^rr .»/gnncth and development, including monogrupfi* r>« premature sexual development and delayed puberty. Hull M G R. Savage P E, Bromham D R, Ismail A A A, Morris A F. The value of a single serum progesterone measurement in the midlutcal phase as a criterion of a potentially fertile cycle l.'ovulaimn") derived from treated and untreated conception cycles. Fertility and Sterility 1082; 37 355 W. McPhaut M J et al. Genetic basis of endocrine disease. 4. '1Ъс spectrum of mutations in the androgen receptor gene that cause andrvtgen resistance, Journal of Clinical Endocrinology- and Metabolism 1992; 76: 17 2 V
  
  A recent article which reviews the current knowledge on abnormalities of the human androgen receptor gene whwh are responsiblefara range oj phemKyfbc abnormalities of так sexual devehrpment. Scott R T, Toner J P, Muashcr S J, Ochmngcr S, Robinson S, Roscnwaks 7.. Follicle stimulating hormone levels on cycle day J are predictive of in vitro fertilisation outcome. Fertility and Sterility 1989;51:651 4. Stein I F, Leventhal M L. Amenorrhoea associated with bilateral polycysiic ovaries. American Journal of Obstetric* and Gynccology 1935-29: 181 91. Siync О M. Puberty and its disorders. In: Stync D M (cdl. Endocrinology and Metabolism Clinics of North America. Philadelphia: W. B. Saunders. 1991; 20(i): 1-245. Useful sections on disorder* of puberty, including a complete chapter on the pathology and treatment of the undescended testicle. WHO. laboratory manual for the examination of human semen and semen-cervical mucus interaction. Cambridge. Cambridge University Press, 1987. A useful volume which attempts to standardize teas and wvcsttgaium* in common use throughout the tumid. Wiles P G. Erectile impotence in diabetic men: aetiology, investigation and management. Diabetic Medicine 1992; 9: 888-92. A short review discussing the frequency, aetiolog\ and treatment of impotence in diabetic men.
  
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  C H A P T E R
  
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  Pregnancy, oral contraception and hormone replacement therapy R. Bradley and D. Crook
  
  INTRODUCTION Pregnancy is associated with marked hormonal changes in the maternal circulation which facilitate the metabolic, vascular and immunological adjustments necessary for the fetus to thrive. Abnormal concentrations of these hor­ mones, or of other plasma constituents of fctoplacental origin, may indicate gestational pathology. The metabolic changes associated with pregnancy arc relatively short lived and thus are rarely harmful to the healthy mother. Conversely, the administration of exogenous sex steroids to the non-pregnant woman, often involving prolonged exposure at non-physiological doses, may induce bio­ chemical changes with potentially harmful consequences. This chapter reviews the diagnostic and pathological potential of the biochemical changes observed in these situations. PREGNANCY Prior to the widespread availability of high resolution ultrasound scanning, the diagnosis of early pregnancy failure (such as spontaneous abortion and ectopic preg­ nancy), of fetal malformation and fetal growth distur­ bances relied solclv on clinical evaluation backed bv blood tests on the maternal circulation. Although these indirect and often inaccurate methods have largely been replaced by ultrasound technology, biochemical assays still form an important part of the screening of many pregnancies. Biochemical diagnosis of pregnancy Human chorionic gonadotwphin Human chorionic gonadotrophin (hCG) can be detected
  
  in maternal blood 7-9 days after conception and in urine 1-2 days later. The biochemical diagnosis of pregnancy is routinely made by the detection of (ihCG using a latex agglutination technique detecting concentrations >200 i.u./ L, while a monoclonal antibody enzyme immunoassay will detect concentrations as low as 50 i.u./L. The concentra­ tion of (ihCG in normal pregnancy rises steeply to a peak at 10 weeks' gestation and then falls and remains at a lower concentration for the remainder of pregnancy. 'ITiis fall at the end of the first trimester is peculiar to (JhCG, as concentrations of the other chemical products of the fctoplacental unit rise with increasing gestation. Highly sensitive assays hav'e shown PhCG to be present at low concentrations ( glucose tolerance test (ХУГТ;, 101 prof ocol, 3 I 3 growth hormone urinary excretion, 105 growth hormone -releasing hormone ( G H R H l ectopie secretion, 305 hypcrprolnctinntmia r 503 management, 305 multiple endocrine ncoplasia 1 I ' M K N I j . 684 phosphate (TmP/GFR) elevation, 104 pituitary function assessment. 305 pituitary tumour management, 6&4 polycysac ovaries association, 399 polyofttoiic fibrous dysplasia, 531 post-pituitary surgery polyuria, 101 presentation, 105 rheumatological disorders, 537 therapeutic response monitoring, 105 306 T R H test, 105 Aciivin, 195. 396 structural aspect*. 396 Acute phase protein», 15ft, 359, 736 inierteukm 6 (IL.6) response, 732 liver synthesis, 228. 712 metabolic response to injury, 718 pregnancy-associated changes, 418 Acute phase response, 711-737 articular disease, 540 541 clinical features, 7 1 1 cytokines 15Я-359, 718, 715 fever, 359. 72ft inflammation, 7 33 mierleukin 6 (1L6), 7 2 3 724 mechanisms. 7 34 plasma protein changes, 7 35 7 36 pruewglandin*, 359 shock. 734 735 stTcss hormones, 159 time course, 736 737 trace metal concentration changes, 737 Acyl CoA: cholesterol acyl transferase (АСЛТ) cholesterol metabolism, 627 Nicrnann-Pick disease type II, 614 PCH. inhibition. 62 3 Acylcamilinc translocate, 544 Acylgjycines urinary excretion, 554 Addisonian crisis. 321 congenital adrenal hyperplasia (L) receptor binding, 627, 628 ncphrotic syndrome-associated overproduction, 636 normotriglyceridacmic inJ3lipoproteinaemia, 631-632 very low density lipoprotein f\*LDL), 626 ApoC-1,625 scry low density lipoprotein fVI JM.), 626 ApoC-11,625 chylomicnms acquisition in plasma, 625 transfer to HDL, 626, 627 lipoprotein lipasc (I.PI.) activation. 628 LPL-roediated lipolysis of VLDL, 627 very low density lipoprotein (VLDL), 626. 627 Apo С-II deficiency chvloinicronaemia syndrome, 6*2 clinical features, 632 Apo C-III, 625 chylomicrons acquisition in plasma, 625 I J T hydrolysis regulation, 626 transfer to Н П Ц 626, 627 very low density lipoprotein (VI.DL), 626 transfer HDL, 627 Apo 0-Ш/аро A-I combined deficiency, 634-635 Apo D. 625 ApoE, 625, 627,757 chylomicrons acquisition in plasma, 625 remnants, 626 transfer to HIM- 627 genetic variant», 757» 75* high density lipoprotein (HDL) acquisition, 627 intermediate density Iipoprotcins fIDl.) hepatic uptake. 627 remnants., 627 isofonm, 625 low density lipoprotein 'LDL) receptor binding, 627, 62» very low density lipoprotein (VIJ)L). 626 remnants, 627 transfer to HDL, 627 Apo E defictency chylomicron remnants accumulation, 626, 627 intermediate density lipoprotcin ПП1.) accumulation, 627 remnant hYpertipoprotcinacmia (WHO гуле III), 632 Apo E receptor, 627 Apo K2, 625, 757-758 chylomicron remnants accumulation, 626, 627 hypothsToidism-associated dysbeialipoprotcinaemia. 6)6 intermediate density lipoprotein {IDL) accumulation, 627 receptor binding activity, 757 758 remnant hyperlipopTotcinacrma (WHO fvpe III), 6)2-633 Apo E ) , 625, 757 Apo K4, 625, 757 common hypcrcholc*tcrofocmia (polygenic hyperchole*terolaemia), 634 Apohpoprotein A, 624 Apolipoprotcin B, 624 625 chylomicron synthesis, 209 molecular biology. 625
  
  795
  
  Apolipoprotcin C, 625 Apolipoprotcin D, 625 Apolipoprotein E, 625 measurement, 639 Apolipoprotcin(a] (apofa)}, 625 Apoli poprot с ins, 62-4-625 functions, 624 measurement, 639 nephrotk syndrome, 154 Apomorphine, 410 Apopmtein В deficient"), 593 APP (p-amytoid precursor protein} Alzheimer's disease, 581 583,584 gene mutations, 583 582 lysotomal metabolism, 58 ) metabolic pathways, 583 molecular generic*, 581 trisomy 21 (Dossn's syndrome). 583 APUD cell hormone tumour markers» 714 APUD tumours, 712 hormone secretion, 712-713 Arachidonic acid metabolism, 623, 726 cyelo-oxygenase pathway, 623, 726 reactive oxygen species, 768, 769 essential fatty acids, 175 lipid pemxidatwn, 766, 767 lipoxygenasc pathway, 623, 726 malignant tissue, 694 Arginase deficiency, 448, 449, 6 94 exogenous variation, 94 ionized calcium, 93, 100 total plasma calcium, 93 tubular handling analysis, 113-114 urine calcium. 94 Calcium carbonate, hypcrphosphataemia management, 107 Calcium channel Ыосксг* elderly patient», 57 teratogenicity, 659 Calcium excretion per litre of glomcrular filtrate !Ca,),94 Calcium gluconate infusion, 58 calcitonin provocative test, medullary thyroid carcinoma, 686 hypcrmagnesacmia, 112 hypocalcacmia, 102 Calcium metabolism, У7 102 bone 89, 92
  
  chronic non-respirators* acidosis, 71 calcitonin, 89, 92-93 chronic renal failure, 138 cytokincs/growth factors, 93 depressive illness, 579 l t 2Wihydrox>-vitamin П (l,25(OH) : D). 89,91-92 gastrointestinal tract, 88-89, 94, 113 endogenous faecal calcium, 88 secretion, 88 hormonal influences. 93 intestinal absorption 88, 210 assay,. 94, 113 hypoparathyrokhsm, 99 kidney, 89, 90, 92 tubular handling assessment, 113-114 nconatc, 430 disorders, 4 30-132 paremcral nutrition, 137 parathyroid hormone (FTH), 89 91 regulation, R9 93 renal phosphate handling iTmP/GFRj, 105 transplacentaJ transport, 430 Calcium plasma concentration, 87 hacmodialysis monitoring, 141 malabsorpnon investigation, 211 measurement де Calcium assays muscle disease, 548 nephrotic syndrome, 153 nutrition monitoring, 197 osrcomalacia/rickets, 518 osteoporosis, 515 Pagct's disease of bone. 527 parathyroid bone disease, 525 polyuxia, 40 renal osteodystrophy, 522 hyperparathyroidisni supprcssivc treatment, 524 renal transplant monitoring, 142 rickets of prematurity, 431 vitamin D deficiency, 189 we u/if» Hypercalcaemia; Hypocalcacmia Calcium pyrophosphatc disease. 535, 536 anicular crystal deposinon. 5 36 chondrocaicinosis, 536 familial form, 536 metabolic disease associations, 536 synovial fluid light microscopy, 5 4 ! , 542 treatment, 536 Calcium supplements osteoporosis, 516 vitamin D-dcpendem rickets type II, 519 Calcium urinary excretion assay, 94
  
  calcium-creaunine ratio, 516 osteoporosis, 516 Pagct's disease of bone, 527 rickets of prematurity, 431 СЛ1ЛЛ marker, 500 Caloric deprivation test, Gilbert's syndrome, 232 Cancer cachcxia, 693 tumour necrosis factor a (TNFa), 698 Cannabinoids abuse in pregnancy, 424 Capillary membrane ultrafiltration, 26 Captopril, 56 Caplopri) suppression lest, 324 Carbamates poisoning, 670 Carbamazepme, 47» therapeutic drug monitoring. 650, 657 Carbamoyl phosphate synthetase deficiency, 449. 607
  
  799
  
  diagnosis, 608 enzyme assay, 608 prenatal diagnosis, 608 Carbenicilhn, 53 Carbenoxolone, 319, 325 glucose tolerance effect, 270 hypokalacmic alkalosis, 53 Carbidopa, 681 Carbimazole, 292» 308, 332, 337, 345 Carbohydrate absorption clinical aspects, 206 hydrogen breath test, 207 intestinal transport, 206
  
  investigation, 206-207 lactose tolerance test. 207
  
  xylose absorption test, 206- 207, 214 Carbohydrate, dietary, 174 175,205 deficiency. 174 high intake, 175 Carbohydrate metabolism chronic renal failure, 365 digestion, 205-206 cntcfocytc brush border enzymes, 205-206 salivary/pancreatic amylasc, 205 energy - metabolism, 174 large bowel. 206
  
  liver, 220 liver disease, 365-366 malignant dibeasc, 693 694 metabolic response to injury, 719T 720 neon ate/pre term infant, 428-430 starvation, 367-368 P-Carbolincs, 577 Carbon dioxide metabolism excretion non-respiratory acidosis, 70 pulmonary, 66, 68 pulmonary disease, 8-} glucose oxidation, 65 hydrogen ion production, 64 Carbon dioxide retention cardiovascular effects, 76 management, 76 nervous system effects, 76 rapid reduction of PCO,t 76 respiratory acidosis, 75 buffer systems, 76 renal hydrogen ion excretion, 76 Carbon dioxide transport, 63, 83-84 Carbon monoxide, neurotran&mitter actions, 730 Carbon monoxide poisoning, 660, 676 677 carboxyhaemoglobin half-life, 676 clinical features, 661, 676-677 diagnosis, 661, 676 hyperbanc oxygen therapy, 677 management, 677 toxicity, 676 Carbonate dehydratase, 63, o4 renal tubular cells, 66, 67 Carbonic anhydrasc II deficiency, 531 Carbonic anhydrasc inhibitor*, 696 Carbonic dehydratase (carbonic anhydrasc) III muscle disease associated elevation, 549 myocardial infarction, 783. 784 Carboxyhaemoglobin, 676 plasma concentration measurement. 676-677 poisoning associations, 660 Carboxypcptidasc A, 207 Carboxypcpndasc B, 207
  
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  INDEX
  
  Carcinoembryonic antigen (CEA) ACTH-dependent Cushing's syndrome» 307 breast carcinoma» 714 gastrointestinal rumours, 71 3 lung carcinoma, 71*1 mctastatic liver tumours, 240 nephrotic syndrome with malignant disease, 698 С a r a n о genesis, reactive oxygen species involvement, 772 Carcinoid crisis 680 Carcinoid syndrome» 679, 703 biochemical investigations, 681 carcinoid chub, 680 clinical features, 680 glucose intolerance, 270 nicntirumide deficiency states, 680 plasma 5-hvdrnxyindoleaccoc acid (5-Н1ЛЛ)» 681 serotonin biosynthesis, 681 serotonin effects, 729 treatment, 682 tryptophan metabolism, 680-681 tumour localization, 682 Carcinoid tumour*, 679 682, 703 characteristics, 6*10 clinical features, 679-680 cctopic ACTH secretion, 306 embryonic origins, 680 5-hydroxyindoles secretion, 679, 703 multiple endocrine neoplasia I «MEN I), 684 pathology, 679 pepiide hormone secretion, 679 serotonin biosynthesis, 681, 695 serotonin oxidativc domination, 681 treatment, 682 tryptophan metabolism, 680 681 Cardiac arrest, 55 Cardiac enzymes, 780, 781 783 myocardial infarction, 781 Cardiac glycoside antidotes, 660, 664 Cardiomyopathy, 55* Cardiotoxic poisons, 660 Cardiovascular system acidosU, 7 1 , 76 diabetic ketoacidosis, 277 hypcrmagnesacmia, 112 hypocalcaemia. 99 hypomagncsacmia, 111 respiratory alkalosis, 79 uraemic syndrome, 137 Carnitine dietary requirement, 182 fatty acid metabolism inborn errors, 446 muscle defects, 553 554 metabolism, 445 pcroxjsomal processes, 451 Carnitine acyl iransfcrases, 554 Carnitine deficiency, 554 Carnitine palmitoyl transferase deficiency, 446 muscle fatty acid oxidation defects, 554 Carnitine palmitoyl tramferasc I, 544 Carnitine palmitoyl transferasc II, 544 Carnitine translocate, 544 P-Carotenc, dietary, 177, 178 Carotenes assay, 776 dietary intake estimation, 189 free radicals scavenging. 770 plasma measurement, 189
  
  Carotenoids, cerebrospinal fluid (CS!7)» 558 Carrier state, 744, 762 DNA analysis, 761 population frequency calculation, 745 screening programmes, 753, 754 X-linked inheritance, 745 Cutalase assay, 776
  
  hydrogen peroxide removal, 771 Catamcnial variation, 9 Cataracts, 99 Catecholamines actions, 327 adrenal medulla production, 327 anxiety disorders, 569, 571 chronic renal failure, 365 glucose regulation, 269 hypoglycaemia response, 282, 429 metabolic response to injury, 9, 355, 356, 718,719 leukocytosis, 736 neonatal carbohydrate metabolism, 428, 429 phaeochromocytoma, 328, 572 urinary excretion, 328, 686 phosphate redistribution, 103 potassium distribution, ЗО, 50 receptors, 327 social stress response, 360 starvation, 368 Cation exchange resins, 51 Cavernous sinus syndrome, 302 cDNA, 762 Cellular aspects, 717-738 Cellulose, 184 Central nervous system (CKS) acidosis effects, 7 ) , 76 hypoglycaemia symptoms, 282 283 Central nervous system (CKS) diseases, 563 567 CTcatinc kinase (CK) elevation, 549 Central pontine myelinolysis (CPM), 48, 590 Central venous pressure monitoring, 131, 133 Ccphaloeporins, crcatininc plasma assay interference, 126 Ceramide trihexosidase (a-gaJactosidase) deficiency see Fabry's disease Ceramide tnhexosidase assay, 597 Cerebellar degeneration, subacute, 701 Cerebral abscess, 56 3 Cerebral hypoxia, C^SF changes, 559 Cerebroside sulphatasc (aryKulphatasc A) assay, 590, 615 Cerebroside sulphata&e (arylsulphatase A) deficiency metachromatic leukodystrophies (MLDj, 590,614 pseudodeficicncy, 591 Cerebroside sulphur sulphatasc, 591 Cerebroside (V-glucosidase ш G1 uc occrcbros i dasc Cerebrospinal fluid (CSF), 557-567 alphafetoprotem (AFP), 709 amino acid levels, 566 appearance, 558, 563 bilirubin detection, 558, 565 btoptenns, 566 С-reactive protein, 564, 565 cell count. 563, 565 cells, 558 circulation, 557 asternal/Ventricular fluid analysis, 563
  
  clot formation, 558 composition, 557 -558 creaitnine kinase brain i*oenzvme. 566 enzymes, 563 flow reduction measurement, 561 CSF albumin, 561 CSF prealbumin: albumin, 561 CSF total protein, 561 formation, 557 Gram stain/spun deposit, 56 3 haem detection, 558 homovamllic acid, 574 human chonomc Rnnadotrophin fhCO), 709, 710 Jg clectrophoresis blood-bram barrier permeability assessment, 560, 561 mtrathecal Ig synthesis assessment, 561 intrathecal IgG oligoclonal banding, 561 iransierrin lau band, 565 Cerebrospinal fluid (CSF) glucose, 558, 559, 565, 566, 567 CSF ottWrhinorrhnca, 565 CSF-plasma glucose ratio, 559 malignant leptomeningeal infiltration, 566 mechanisms of alteration, 559 meningitis differential diagnosis, 563-564 Cerebrospinal fluid (CSF) 5-hydroxyindoleaceric acid, (5-HIAA) alcoholism, 576 depression, 578 schizophrenia, 574 suicidal behaviour, 578 Cerebrospinal fluid (CSF) lactate, 559, 567 elevation, 566 meningitis differential diagnosis, 564 mitochondria] disorders, 566 pyruvate metabolism defects, 566 respiratory chain defects, 552 Cerebrospinal fluid (CSF) oto-/rhinorrhoea, 565
  
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  [SDHX Cerebrospinal fluid pmtcin*cmia, 59 3 apo A - l , 625 apo А-П, 625 apo A-IV, 624,625 apo В-4Я, 624, 625
  
  apo C-II, 625 apo C-U\r 625 apo B, 625 apolipoproteins, 625 atherogenicity, 622 gross appearance of plasma/serum sample, 638 lipoprotein lipasc (LPL) hydrolysis, 625-626 modification in plasma, 625 apobpoproteins acquisition, 625, 626 transfer of components to HDI-, 627 triglyceridcs incorporation, 62 3 mucosal cell synthesis, 209 remnants, 626 cholesteryl ester transfer from HDL., 627 removal, 625 svnihcsis, 625
  
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  802
  
  INDEX
  
  Chymotrypstn, 207 a.-Chymorrypsuwgen, 7 2 * Orcadian rhythm» 9 ttdrenocorticotrophic hormone : A C T H ) , 297, 320 aldostcronc, 324 corrieotrophin releasing factor (("RF^1, 297 cortisol, 9, 319 glucose plasma concentration, 274 insulin, 274 testosterone, 403 thyroid-stimulating hormone f J*SH/> 297» 334 urinary protein excretion, 147 Circumference measurements» 186 mid-arm circumference f M A C \ 186 waist: hip ratio, 186 Cirrhosis, 2*4-248 alcoholic Itvcr disease, 24 3, 671 ascites, 244- 246 carbohydrate intolerance, 2 4 8 chronic active hepatitis (САН) progressum, 240,241 complications» 244 cystic fibrosi*. 253 glucorepulatorv delects, 27) gtycogen storage disease type IV, 254 hepatic cnccphalopathy, 244 precipitating factors, 246 hepatocellular carcinoma, 249 hereditary fructose intolerance, 254 histological diagnosis, 244 hypcrglycacmia. 365 hypcrinsulinacmia, 365 hypoftlyeaeinia 289 lhFer function tests, normal with alcohol abstinence» 244 liver pathology. 244 pituitary dysfunction, 217 renal failure, 246 sex hormones/sex hormnnc-binding proteins, 247-248 ryrosinaemia, 25 5 vitamin D levels, *»2 Ctsplatin treatment hypomagnesaemia, 53, 696, 703 nephrotoxicitv, 696 potassium loss. 53 Citrate-glucomc acid solutions, 112 Citratcd blood products, plasma calcium as-say. 9 3, 100 Citrullinacmia (arginosuccinic acid synthetase deficiency) 449 Classification of test results, 20 Clearance, renal, 125 Clinical applications. 1 5 diagnosis, 2 - 3 management, 3—4 screening, 4 - 5 Clinical diagnosis, 2 Cliteromegaly, ixolaied, 377, 380 Clomiphene, 39* ovulaUon induction, 399 superuvulation, 401 Clomiphene test amenorrhoca, 298, 300, 310 pituitan-hypothalamic function in men, 405 protocol. 410-411 Clonazcpam. 471 Clone, 762 Clonidine, growth hormone reserve assessment, 299
  
  depressive illness, 579 Clottmg cascade, 731-732 extrinsic activation pathway, 731 732 intrinsic (contact system) activation pathway, 7 31 Clotting factor defects acute liver failure, 240 hypothyrokUsm, 348 multiple myeloma, 500 nephrotic syndrome, 153 Clozapinc, 575 CNT, 27-28 Cobalamin ж Vitamin B,, Cobalt, 204 dietary, 182 Cocaine abuse, 673 in pregnancy, 424 Cockayne's syndrome, 618 Codormnant inheritance, 745, 762 Coeliac disease, 199 delayed puberty, 391 diarrhoea, 213 iolatc malabsorption, 203 gonadal function, 367 iron malabsorption, 201 mueosal biopsy, 211 neopirnn levels. 562 secondary diabetes mcllirus, 269 secondary lactasc deficiency, 206 therapeutic diets, 194 vitamin D resistance» 518 Colchicme» 536 Cold agglunnin disease, primary, 505 Cold agglunnins, 505 Collagen articular cartilage, 533 biosynthesis» 508 bone matrix, 507, 508 509 congenital defects of synthesis, 529, 530 extracellular crosslinks wpe I, 508 Collagen crosslinks, 509 bone turnover marker, 512-513 osteoporosis, 515 maturation, 509 Paget's disease of bone, 527 post-parathyroidectomy, 525 Collecting ducts functional aspects, 123 hydrogen ion excretion, 165 structural aspects, 117, 120 Colloid oncotic pressure, 25 Colon, carbohydrate metabolism, 206 Colonic potassium excretion, I 38 Colonic segment urinary diversion, 52 Colony-stimulating factors. (CSFs), 7 2 1 , 724 726 Colorecral carcinoma cholesterol plasma concentration, 694 dietary fibre intake, 184 genetic mechanisms, 759, 761 hypennchosis lanuginosa acquistta, 699 nephrotic syndrome, 098 tumour markers. 699, 713
  
  11
  
  Coma, hypoglycaemk, 283 Complement, 732 733 acute phase response, 734, 7 36 alternative activation pathway, 732, 7 33 classical activation pathway, 7 32, 733 inflammatory response, 733 porphyric photosensitivity, 478 Compliance, drug therapy, 643 o44 therapeutic drug monitoring, 648 Compulsive water drinking, 40 Compurcr systems data interpretation, 23 report forms generation, 11 test request form, 7, Й Computerised tomography ( C T ) see C T ^ а Г | Conception, 395 luteal phase progesterone level, 400 Confuwonal state, acute uw Brain syndromes, acute Congestive cardiac failure hypogrycaemia. 284, 2 8 9 management, 37 sodium retention with oedema, 34 thirst, 40 uraemic syndrome, 137 Connective tissue lysosomal storage disease, 612, 6 1 5 616 лее also Mucopolysaccharidoses C o n n ' s syndrome glucose intolerance, 29 non-respiratory alkalosis, 78 Constant region (Cj genes, 494 Constant regions, 493 Constipation, 184 Constitutional delay in growth and development, 310, 388 endocrine investigations, 391 management, 310 Contact system, 730 731 activation, 730 731 acute phase response, 734 clinical shock, 734 fibrinolyne system activation, 732 inflammatory response» 7 И metabolic response to injury, 730-731 Continuous ambulatory peritoneal dialysis (СЛН1)), 142 chronic renal failure aklosterone, 364 anaemia, 364 cat echo! a mines, 365 iS-hydroxycortieosterone activity, 364 menstrual activity, 363 plasma remn activity, 364 testicular dysfunction, 363 thyroid function, 364 Continuous arteriovenous hacmodiafiluauon (CAVHDJ, 134 Continuous arteriovenous haemodialyvs, 665 Continuous arteriovenous huemofiliration {CAVH}, 134 Contrast media, 130 Co-oximcter, 83 Copper. 182-18 3 body stores assessment, 191 dietary sources, 182 functional aspects, IR2 Copper deficiency, 18 3 laboratory investigations, 191 Mertke't disease, 618 parenteral nutrition in neonate/preterm infant, 437
  
  i, Зс
  
  ленный авторским правом
  
  гчЩ X Copper metabolism acute phase response changes, 737 free radical reactions, 766 Indian childhood cirrhosis, 252 lipid peroxidation, 766 malignant disease. 695 Mcnkc's disease, 618 oestrogen thcrapv, 421 pregnancy-associated changes, 420 primary sclcrosmg cholangitis, 24 * Wilson's disease. 212.251 252. 4*6. 571. 593 Copper ncphropathy, 155 Соррет-depcndcm enzymes, 182 collagen assembly, 509 superoxidc dismutasc, 771 Coproporphyna. hereditary, 253, 473 acute abdomen, 787 clinical features, 473, 597 differential diagnosis, 473 investigations, 47 5 acute attack, 480 photosensmvity, 470, 47 3, 478 treatment, 473 Coproporphyrin excretion associated hcpatobilliary dysfunction, 479 Dubin-Johnson syndrome, 232 crythropoietic porphyna, 474 vancgatc porphyna, 472, 480 Coproporphynnogen oxidase haem biosynthesis, 467 harderoporphyna, 478 hereditary coproporphyna, 47 3 lead toxicity, 479 porphyria cutanea tarda. 476 variegate porphyria, 472 Coproporphyrinogcns aminolaevulinic acid dchydratase deficiency porphyna, 473 erythropoictic porphyna, 474 haem biosynthesis, 467 hereditary coproporphyna, 473, 480 variegate porphyna, 472 Coronary artery disease apo A-I deficiency, 634 atherosclerosis, 621-622 chest pain, 779 cholesterol plasma level. 16, 634 familial combined hypcrlipidacmia (FCH), 631 familial hypcrcholeMcrolacmia, 633, 6 34 hyperapohctalipoprotcinaemia, 631 nutritional factors, 193-194 obesity, 193 waist hip ratio, 186 nsk assessment, 621 apohpoprotcins measurement, 639 lipoprotein(a) measurement, 625, 639 selective hypcrcholcstcrolaemta scrcenmg, 5 Corpus luteum early pregnancy loss, 415 formation, 395 lit t r maintenance, 396 involution, 3**5 oestrogen secretion, 395 progesterone secretion, 395, 396 Corticostcroid therapy avascular necrosis, 537 depression. 57? growth impairment, 702 hvpcrcalcaemia management, congenital adrenal hyperplasia, 325-326 conversion into cortisone, 319 Cushing's disease, post-treatment replacement therapy monitoring, 308 Cushing's syndrome, 306, 308, 573 depressive illness, 573, 578 ebb phase trauma/sepsis response, 357 hirsutism, 401.402 hypenhyroidism. 343 hvpoglycaemia response, 282 hypothyroidism, 348, 351 insulin stress test responses, 298-299 metabolic response to injury/stress, 9, 297, 319,356,718,719-720,736 ottcocalcin biosynthesis regulation, 509 osteoporosis, 515 phospholipasc A; inhibition, 729 pituitary function assessment, 297, 301, 302,311 post -surgical, 301, 302 plasma level variation, 319 receptor*, 318 replacement therapy monitoring, 311 social stress response, 360, 361 starvation, 368 Consul-binding globulin (CBG), 420 Cortisol-cortisonc shuttle, 319
  
  Costs, 5
  
  screening tests, 4 C-peptidc, 259, 260 insulin-dependent diabetes mcllitus (IDDM), 265 C-peptidc plasma concentration, 280 alcohol-induced fasting hvpoglycaemia. 292 autoimmune hvpoglycaemia, 292 factitious hypogrycacmia, 287, 288 fastmg hvpoglycaemia, 285, 294 hvpoglycaemia with adrenal insufficiency, 291 hvpoglycaemia with pituiiarv insufficiency, 290 insultnoma, 288, 683 non-islet cell tumours, 290 Cranial irradiation delayed puberty, 389 endocrine sequelae, 702 precocious puberty, 387 Cranial nerve defects, 302 < 'raniopharvngh'in i delayed puberty, 389 diabetes msipidus/vasoprcssin deficiency, 311 generalised hypopstuitarism, 309 precocious puberty, 387 C-reacti\*e protein acute phase response. 228, 718, 735 time course, 7 36 articular disease, 540-541 ccrcbrospmal fluid (CSF), 562, 564, 565 hepatic synthesis, 220 interleukin 6 (1L6) response, 723 magnitude of response to injury, 735 myocardtal infarction, 784 opsonm activity, 7 36 streptococcal pneumonia, neonate, 424 Crcatinc kinasc (CK) children, 549 CSF brain isoenzyme, 562, 56 * mtracramal metastatic disease, 566 Duchenne muscular dystrophy, 756 elevation without muscle disease, 549 exercise, 10, 549 isoeruymes, 782 muscle necrosis/regeneration, 549 muscle ATP formation, 544 muscle disease, 548^549 musclc/bratn isoforms (CK-MB), 548-549, 780 myocardial infarction, 781, 782-783 cardiac isoenzyme, 781, 782, 784 CK-MB early marker, 780 tissue/plasma isoform analvsis, 782-783, 784 rhabdomyolysis, 107 surgery-related variation, 10 treatment monitoring, 549 Great mine body m a » relationship. 9 proximal convoluted tubule sccrenon, 121 Creatininc clearance calculated, 128 GFR measurement, 126 hacmo,lKil\Ms assessment, 1 II Creatininc plasma concentration acute liver failure prognosis, 240 ascites treatment monitoring, 245 246 biological/analytical variance, 127 brain syndromes, acute, 570 calculated creatininc clearance, 128 cirrhosis with renal failure, 246
  
  804
  
  INIJKX
  
  Crcatininc plasma concentration {cvntj) comparison of results with reference limit*, 18 factors influencing, 127 GFR measurement, 126-128 haemodialysis assessment, M l hypcrbilirubinaornia, estimation problems, 240 hyperkalaemia, 57 hypocalcacmia, 101 hypokalaemia, 54 malignant disease, 694 muscle m a » estimation» 188 neonatc/preterm infant, 42ft pregnancy-associated changes» 418 renal failure acute intrinsic, J 30, 131 acute prerenal, 1 30 chronic, 124, 1 37 hypcrphosphatacmia, 107 renal transplant monitoring, 142 «odium deficiency, 33 urine calcium measurement, 94, 98 Crcsccnoc glomcruloncphnris, 151, 152 CRH-41 see Corticotrophin releasing factor ehydroepiandrosterone sulphate (DHEAS) adrenal biosynthesis, 318, 399 female metabolism, 398, 399
  
  Материал
  
  авторским правом
  
  INUKX
  
  Dehydrocpiandrosicronc sulphate (amid) ovarian biosynthesis, 39Q Delayed puberty, 310, 388-390 cause*. 388 chronic renal failure, 36) clomiphenc test, 300 constitutional delay tn growth and development, 310, 388, 391 endocrine events, 389 endocrine investigations, *9\ exercise -related, 310 GnRH pulse genera юг, 388-389 GnRH test, 405 gnnadorrophin deficiency, 310, 389 hyperprolactinacmia, 304 hypogonadotrophic hypogonadism, 389 КлИгпапЧ syndrome differential diagnosis 40? low body weight, 310 primary hypogonadism, 389 systemic disease associations, 389 Delirium, 570 E>eletcd in colonic carcinoma CDCC) gene. Deletions, 742 I>clta aminolaevulinic acid if* Aminolaevulinic acid (Al.\) Delta aminolaevulinic acid dehydratasc (ALAD) sec Aminolaevulinic acid dehydrause (ALAD) Delta aminolaevuhnic acid synthaw: (Al-AS) их Aminolaevulinic acid synthasc (AI-AS; Demeclocyclinc, 38, 49 Dementia ccrcbrospinal fluid (CSF) investigation, 566-567 hypoihyroidism, 571, 573 metabolic disorders, 590- 592 sec otto Brain syndromes, chronic Dental caries, 174, 175, 194 Dental development, 99 DeoxycorticoMcronc (DOC), 316, 318 1 l-Deoxyconisol, 11 (3-hydroxylase deficiency diagnosis, 379 Deoxypyridinoline (lysytpyndinulinc), 509 bone tumo\ r er marker, 94, 512 urinary excretion, 94, 512, 513 diurnal rhythm, 511 Depression, 572-573, 578 ACTH secretion, 578 aeiiulogtcal aspects, 564 bipolar, 572 lithium response, 579 eholincrgic-adrencrgic ncurotransmission imbalance, 578 clinical diagnosis, 569 clinical features, 572 corricotrophin releasing factor (CRF), 361 cortisol hypersccrction, 578 Cushing's syndrome, 573 dcxamcthasonc suppression test (DST), 578 579 electrolyte abnormalities, 579 endogenous, 572, 579 feeding disorder association, 370, 371 growth hormone (GH) secretion, 579 5-HT neuroiransmission, 576, 578 monoamine neurotransmitters, 574 neuroendocrine changes, 369, 573, 578-579 noradrenaline ncurotrammission, 578 prolactm secretion, 579
  
  psychotic» 572
  
  thyroid function, 370, 572, 573 unipolar, 572 Dermatan sulphate, accumulation in mucopolysacchari doses, 615 Dermatological disorders, malignant disease, 699 Dermatomyositis, 700 Dcsicrroxaminc, 486, 776 aluminium toxiciry treatment, 524 iron poisoning, 669-670 safety in pregnancy, 660 Desferroxamine test protocol, 532 total body aluminium load estimation, 523 Desipramine, 370 Desmolase ovarian biosynthctic pathways, 397 prcgnenotone biosynthesis, 403 Dcsmopressin (dDAVP) diabetes insipidus (DI), 41 hypopitmtarism, 311 pregnancy-associated polyuria, 40 water deprivation test, 40, 41 Desogestrcl, 420 Dcxarnethasonc hirsutism trcatmcRC, 402 prenatal САН treatment, 380 Dcxarnethasonc suppression lest adrenal function assessment, 320, 329 affective disorders, 369 Cushing's syndrome, 306, 402 ACTH-dependent, 306-307, 308 post-surgical assessment, 308 depression, 578 579 factors resulting in non-suppression, 369 high dose, 306-307, 320, 329 regime, 307, 329 urine free cortisol, 319 hirsutism, 402 low dose, 306, 308, 320, *29 overnight, 320, 329 primary aldostcronism differential diagnosis, 324 protocols, 329 De x tropropoxyp h enc hypoglycaemia, 291 toxiciry, 060, 67 3 Diabetes insipidus (DI), J 7 - 3 8 cause», 37, 38, 298 central (cranial), 37, 38, 41, 301, 311 pituitary rumours, 311 clinical assessment, 302 head injury-associated, 309 hypcrnatracmia, 42, 590 hypenonic saline infusion test, 41, 301 management, 4) nephrogenic, 37, 38, 4 1 , 301 congenital, 38, 41 diabetes mellitus, 273 lithium therapy association, 370 pt**t-pituitary*
  
  4Ur
  
  K c O» 301
  
  prcterm infant, 427 urine/plasma osmoWity, 298 water deprivation test, 40, 41, 298, 300-301 Diabetes meltiius, 257, 264 280 advanced glycauon endpruduct (AGE) accumulation, 272 aldose rcduLtasc enzymes, 272 aldosc rcductase inhibitors, 272 angiotensm-converiing enzyme (ACE), 541 apolipoprotein glycation, 636
  
  805
  
  atherosclerosis» 271 272 biochemical tests, theoretical aspects, 278-280 blood glucose measurement, 278 b m t l e diabetes, 275
  
  C-pepude plasma concentration, 280 calcium pyrophosphate disease association, 536 cardiac dysfunction, 272 cataract, 2 7 1 , 272 cheiroarthropathy, 271, 272 classification, 264 definitions, 264 delayed puberty, 389 diabetic emergencies, 275 278 diagnostic criteria, 263, 264 diet children, 271 dietary fibre intake, 184 modification, 194, 274 dyslipidacmia, 273-274, 626 screening, 5 triglyceridc plasma level, 636, 638 endocrine disease associations, 271, 287, 305 exoenne pancreatic dysfunction, 269 fatty liver, 235, 273 foot problems, 272 free radicals overproduction, 272 gastrointestinal complications, 275 genetic aspects, 272 gestational diabetes, 267-268 glucose transporter expression, 259 giycaemic control, 274 dawn phenomenon, 274 cthanol, 275 exercise, 275 fructosaminc assay, 279 glycated albumin, 279 gtycated fibnnogen, 279 gtycatcd haemoglobin (HbA,), 279 incercurrent illness, 274 nutrient absorption unpredictability, 275 problems, 274-275 rebound from hypoglycaemia, 274 275 Somogyi phenomenon, 275 stress, 274 tests, 279 growth, 271 growth hormone, G T T response, 304 hyperinsulinacmic glucose clamp lest, 280 hypertension, 272, 273 hypoglycaemia adrcnergic symptoms, 282 exercise-induced, 287 tnsulin-/$ulphonylurea-induced, 287 postp ran dial/reactive, 293 propranolol, 291
  
  hypoglycaemic unawarcness, 287 hypomagnesaemia, 273 hyponatracmia, 273 latrogenic, 270 impotence, 408 insulin resistance, biochemical tests, 279 280 insulin therapy, 274 insulin-dependent (IDDM), 264-265 aetiology, 2 6 4 - 2 6 5
  
  genetic susceptibility, 265 fiypophosphatacmra, 108 microalbummuna, 273 nephropathv-hvpertension associations, 273
  
  Материал, защиш
  Genes DNA analysis 4, 746 752 a,-antitrypsin deficiency, 754 755 applications, 752-754 atherosclerosis- 757 758 automated analysts of genetic disease, 739 cystic fibrosis, 755 756, 761 dot blotting, 747 fetal tissue sampling technique, 416 Humington's disease, 757 index cases diagnosis, 753 muscular dystrophy, 756 -757, 761 mutations detection, 749 752 allele-spccific oligonucleotide (ASO) technique, 749 750 amplification refractory mutation svstem (ARMS). 750 cleavage mismatch detection, 750 denaturing/temperature gradient elcctrophorcsis, 750-751 DNA sequencing, 751 liguse-mcdiaied allele detection, 750 multiple mutations, simultaneous assay, 750 scanning methods, 750-751, 761 single-stranded conformational polymorphism (SSCP), 751 oncogeneVeuppresaor genes, 758- 759 phenvlalanme hvdroxylase dcficiency/PKU, 606 prenatal diagnosis, 752-753, 761 11 |3-hydroxyIa*e deficiency, 380 21-hvdroxylase deficiency, 380 screening, 753 754 somatic mutationvmalignancy detection, 754 Southern blouing,747 specific sequences detection, 746 749 hybridization probes, 746 748 poLymerase chain reaction (PCR), 748 749 restriction endonucleases, 746 terminolgy, 762-763 tracking of linked markers, 751-752 urea cycle disorders, 608 DNA damage base adduct formation, 775 careinogenesis* 772 tree radicals, 766, 772, 775 repair processes, 742, 775 DNA fingerprinting, 752 DNA hybridization probes, 746 748, 762 cloning, 748 vectors, 748, 763 labelling, 747 synthetic oligonucleottdes, 748 DNA scanning techniques, 750-751, 761 dystrophin gene mutations/muscular dystrophy, 756 DNA sequencing, 751 Dubutaminc, 1 31 Dominant inheritance, 744, 762 X-Unked genes, 745 Domperidone, 303 Doparnine brain neuronal systems, 574 schizophrenia, 575 giomerular nitration regulation, 121 mental illness pathophysiology, 574
  
  807
  
  ncurotransmission, 574 prolacrin release regulation, 297, 396 trauma/sepsis response, 356 Dopaminc agonist therapy acromegaty, 305 prolactinoma, 304 Dopaminc infusion test, 3o5 Doparnine receptor antagonists, 370 Dopaminc receptors, 575 Doparnine therapy acute renal failure, 1 31 thyroid function tests, 338 TSH release suppression, 335 Doss porphyria ш Aminolaewlmic acid dehydratase deficiency porphyria Down's syndrome (triaomy 21), 603 Alzheimer's disease, 583, 603 antenatal screening fetal tissue sampling techniques, 415, 416 human chorionic gonadotrophin (hCG), 414 maternal atphafctoproicin (AFP), 416 •triple test', 415-116 APP (p-amyloid precursor protein), 583 clinical features, 603 diagnosis, 603 enzyme activity abnormalities, 603 incidence, 603 maternal age associanons. 415, 416 mental handicap, 599 serotonin (5-Н"П system, 603 Doxombicin, 224, 683 Drug abuse, 672-673 diagnosis, 661 duration of positive urine test result, 672 intrauterine growth retardation, 423, 424 neonatal withdrawal symptoms, 424 neonatal maternal urine drug screen, 424 prctcrm birth, 424 Drug adverse reactions, 10 acute liver failure, 239, 240 acute porphyria, 470 acute renal failure, 129, 131 hepatic dysfunction, 248, 249 hypoglycacmia, 291 lupus, 539 thyroid function, 34 \ tubular nephropathy, 155 Drug interactions, 645, 647 Drug trials, 5, 10 Dry chemistry techniques, 1 3, 285 Dual porphyria tee Chester porphyria Dubin-Johnson syndrome, 232 bromosulphlhalcin elimination test, 230 diagnostic tests. 232 hyperbili ru b in acmi a/faun dice, 232 pregnancy, 248 porphyrinuria, 479 urinary coproporphyrin, 2 32 Duchenne muscular dystrophy, 547 carrier detection, 549 crcatine kinase (CKJ elevation. 540, 756 dystrophin gene mutation, 756 manifesting carriers, 756 Duplications, 742 I>ysalhuminacfnic hypenhyroxinaemia, familial, 338 Dysalphalipoproteinaemitt, 634 635 apo A-I deficiency, 634-035 apo A-I structural abnormality, 635 HDL metabolism disorder», 635 Dyshetalipoprotcinaemia, 630-634 o^Mipoproteinaemia, 630 631
  
  Материал, защиш
  s), 203 periconceptional supplements, 203
  
  pregnancy-associated changes, 420 status assessment, 187, 214 vitamin B,. deficiency-, 204 Folate deficiency, 203 assessment, 187, 204 clinical features, 200 haematologicaJ changes, 203 malignant disease, 697 laboratory investigations, 204, 205 Folate malahsorprion, 203, 211 congenital, 204 hypoihyroidism, 348 Foil tele-stimulating hormone (FSH> a -tu burnt assays 339 activin in regulation, 396 amenorrhoea investigation, 298 androgen insensmvity syndrome, 384 anterior pituitary synthesis, 296, 396 basal pituitary function test, * 11 chronic renal failure, 363 cirrhosis, 247 clomiphene test responses, 300 GnRH control of secretion, 297 female reproductive function, 396 in male, 403 tn mrp fertilization treatment monitoring, 400 infertility in women, 44.Ю mhibin secretion control, 297, 196 KJincfcltcr's syndrome, 406 male hypogonadism, 405 menopause, 373 menstrual cycle, 393, 395 oestrogen feedback regulation, 398 ovarian failure, primary, 399 ovarian follicle actions, 297, 3U6 ovarian steroid biosynthesis control, 297, 398 precocious puberty, 391 pregnancy-associated changes, 418 pulsatile secretion, 297, 404 Sertoli cell receptors, 403 sperm count in secretion assessment, 298 spermatogenesis, 247, 297, 404 structural aspects, 396 testts endocrine evaluation, 404 testosterone feedback in regulation, 403 Foiliclc-ttimutating hormone (KSH> deficiency, 302, 407 Food intake-related variation, M Forced diuresis hypcrcalcacmia management. 98 poisoning management, 664. 665, 668 Formiminoglutamic acid urinary excretion (F1GI.U), 204 10-Formyl tctrahydrofolate, 202, 201 w uUo Polyglutamatcs Fracture healing, 511 Fragile X syndrome, 599, 603 clinical features, 603 folk acid metabolism, 603 genetic mechanism, 603, 757 Kramcshift mutations, 742, 762 muscular dystrophy, 756 Free radicals, 765-776 amino acids oxidation, Too assessment methods, 774-775 DNA damage, 766, 775 exogenous spin traps, 774 hydroxyl radicals, 766 lipid pcroxidation, 766, 774-775 scavenging systems, 768, 770-772 amioxidanw, 770 771
  
  11
  
  cytoprutecuve enzymes, 771 -772 Schift'hasc formation. 766, 774 superoxidc, 766 «ducuon, 315, 316 bone mineral density (BMD) effects, 515 calcium/bone metabolism, 93 clinical effects of excess, 318 cytokines regulation, 359 fever regulation. 359 functional assessment, 319-320 muscle metabolism, 546 receptors, 518 relative potencies, 318 stress-related changes, 9, 356 357 therapy u* Corticosteroid therapy; Steroid replacement therapy Cilucotanasc. 259 gene abnormalities, Mason/MOOY type diabetes meUiius, 265, 268 Gluconeogenesis, 258, 281 hydrogen ion homeostasis, 64, 68 bicarbonate ion production, 65 inborn errors of metabolism, 73, 442
  
  lactic acid metabolism, 73 liver, 220 metabolic response to injury, 718, 719, 720 neonatal carbohydrate metabolism, 428, 429 starvation, 368 Glucose homoeostasis, 257-264 autonomic nervous system regulation, 282 fed state, 281 hormonal responses, 282 normoglycaemia regulation, 257 258 postabsorptive/fast ing state, 281 Glucose intolerance see Impaired glucose tolerance (1GT) Glucose metabolism, 258-259 hepatic, 220 acinus uptake, 219 hydrogen ion production, 65 intestinal absorption, 206 liver disease, 365 366 muUgnant disease, 693 renal tubular reabsorpuon, 121, 164 starvation, 367-368 .<ee aho Glucose homoeostasis Glucose plasma concentration acute brain syndromes, 570 circadian changes, 274 CSE-'-plasma glucose ratio, 559 entcral nutrition monitoring, 197 falciparum malaria, 291 food intake-related variation, 10 homoeostasis, 257 258 hypcrkalacmia, 57 inborn errors of metabolism, neonatal presentation, 441 insulin actions, 261 insulinoma, 288 ketoacidosis, 276 measurement, 278 interfering substances, 278 reagent stick techniques, 14, 278 metabolic response to injury, 719, 720 mixed meal tolerance test, 287 muscle disease, 548 nconatc, 4 2 9
  
  paracetamol toxicity, 240 parent era! nutrition monitoring, 197 pregnancy -a ssocia ted changes, 419 prolonged fast test, 285 see also Hyperglycacmia; Hypoglycaemia Glucose tolerance chronic renal failure, 138 oestrogen therapy-associated changes, 421 pregnancy -a swxiatcd changes, 419 420 Glucose tolerance test (G'IT) acromegaly, 304 protocol, 313 therapeutic response monitoring, 305 intravenous test (.IVGIT), 280 oral tests ( O ( i T R ) , 278 279 Glucose transporters. 259 biochemical tests. 279 P N A analytic techniques, 280 inrracellular second messenger activation, 263 malignant disease, 693 Glucose 6-phosphatase CG6P) deficiency, 74, 435,443 Cilucose 6-phosphare dchydrogenase (G6PD) deficiency, 456, 464, 745 genetic variants, 456 hacmolytic disease, 434 chronic extravascular haemolysis, 456 precipitating factors, 434, 456, 457
  
  molecular heterogeneity, 745-746 neonatal hypcrbilirubinaemia, 434 red cell glutathione concentration. 463 screening test, 462 Glucuronyl transfcrasc deficiency (Crigler-Naijar syndrome), 232 hyperbihrubinaemia/]aundice, 232 neonate, 434 molecular basis, 232 type 1.2 32,434 type 11,2 32,434 Glucuronyl transterases, 221 Glue sniffing *v Volatile substance abuse G L U T 1-5, 259 Glutamatc oxaloacetate transaminasc (GOT] see AspartBte aminotransferasc (AST) Glutamatc pyruvatc transaminasc iGPT) tee Alanine aminotransferasc (ALT) Glutaminc nutritional support, 195, 196 synthesis, 68, 219 Glutanc aciduna type II, 445 Glutaryl CoA dchydrogenase deficiency, 445 Glutathione metabolism autoimmune hypoglycaemia, 292 free radicals scavenging, 770, 771 glutarhionc/reduced glutathione assay, 776 paracetamol poisoning, 666 red cell, 454, 455 congenital defcctsyhacmolyue anaemia, 456 Glutathione pcroxidase, 771 assay, 776 Glutathione reductase, 771 Glutathione reductase deficiency drug/toxins-induced haemolysis, 456 screening test, 462 Glutathione S-transfcra&e, 227 Glutathione synthctasc deficiency drug/toxins-induced haemolysis, 456 metabolic acidosis, 463 red cell glutathione concentration, 463 Glutathione transfcrasc B, 223 Grycated albumin, 279 Glycated fibnnogen. 279 Glycated haemoglobin lllbA,), 279 Glycerol, energy metabolism, 258, 259 Gtyccryl trinitratc, 780 Gtycinc enccphalopathy (nonketotic hypcrglycinacmia), 566 Glycogen, 258 fetal liver accumulation, 429 glucose homoeostasis, 281 histochemical demonsiration, 551 hypoglycaemia-associated mobilization, 282 storage, 258 synthesis, 258 skeletal muscle. 263 Glycogen breakdown disorders, 550 enjeyme analysis, 551 histochemical investigations, 550-551 Glycogen phosphorylase, 281 myocardial ischacmia, 784 Glycogen storage disease (GSD) type 1 (von Gierkc's disease) see Glucose 6-phosphatase (G6H) deficiency Cilycogen storage diseases clinical features, 254, 550 hepatocellular carcinoma, 254 liver disease, 254 liver function tests, 254 Glycogen synthetase, 281
  
  Материал, за
  
  ценный авторским правом
  
  1NDLX Glycogcnolyins, 2ft 1 inborn error* of metabolism, 442 liver, 21V, 220 metabolic response to injury, 719 muscle, 543, 546 neonatal carbohydrate metabolism, 428 Glycolysis, 25Q hydrogen ion production, 64-65 lactic acid metabolism, 73 inherited disorders, 73 malignant tissues, 693 muscle, 544, 545, 546 red cell. 454, 455 Glycolysis disorders, 550 enzyme analysis, 551 functional dynamic tests, 550 histochemical investigations, 550 551 ischacmic exercise test, 550, 555 Glycosaminogtycan urinary excretion, 615, 616 Glycosuria, 125, 165 Fanconi syndrome, 168 hacmochromatosis, 250 ketoacidosts, 276 measurement, 278 neonatal liver disease, 435 pregnancy, 419 pretcrm infant, 427, 428 proximal convoluted tubule function assessment» 128 renal tubular acidosis type 2 (proximal), 74 Glycyrrhizic acid 11 JHiydroxysteroid dchydrogenase inhibition, 319, 325 hypoatdostcronism, 325 hypokaJacmia, 53, 319, 325 G 4 l gangliosidosis (generalized gangliosidosis), 612-613 adult form (type ИМ, 612 p-galaciosidase assay, 611 ^galactnsKJase deficiency, 612 clinical features, 612-613 diagnosis, 613 gaJactosylgalHctose-N-acyl ceramide accumulation, 612 infantile form (type I), 612 juvenile form (type II), 612 G%12 gangliosidoscs ш Hcxos&minidasc deficiency Goitre chronic renal failure, I 39, 364 congenital hypothyroidism, 349, 550 iodine deficiency, 350 iodine excess, 550 lithium therapy association, 370 Gold therapy, 542 Gonadal failure chronic alcoholism, 366, 367 hacmochromatosis, 367 systemic disease association*, 367. 368 «v also Hypogonadism GonadaJ function tests GnRil stimulation lest, 301 post-pituitary surgery, 302 Gonadotrophin a-suhumts, 396 functionlcss pituitary adenoma secretion, 303, 309 anorchia, 406 childhood levels, 3H6 chronic alcoholism, 367 chronic renal failure, I 39, 363
  
  fetal secretion, 376, 386 hypogonadism, 389, 405 menstrual cycle-related variation, 9, 395 neonatc, 386 pulsatile release, 395 schizophrenia, 369 testes endocrine evaluation, 404 Gonadotrophm deficiency, 3OQ-310 amenorrhoca, 310 associated syndromes, 389 basal hormone investigations, 298 clinical assessment, 302 clomiphene test, 300 delayed puberty, 310, 389 diagnosis, 310 hacmochromatosis, 367 hypogonadotrophic hypogonadism, 406 407 infertility management, 310 male, 405 malignant disease/malignant disease treatment, 702 pituitary-gonadal axis assessment, 298 post-pituitary surgery, 302 progression of hypopiruitarism, 298 treatment, 310 Gonadotrophin receptors, 334 Gonadotrophm replacement therapy, 302 Gonadotrophin-rclcasing hormone (GnRil), 395-396 age-associated effects. 37 3 gonadotrophin (IH/FSH) secretion control, 297, 396 in male, 403 hypothalamic synthesis, 395-306 pulsatile release, 297, 403 pulse generator, 386 delayed puberty, 388-389 precocious puberty, 387 premature activation, 391 puberty, 386 Gonadotrophin-releasing hormone (GnRH) analogues acute intermittent porphyria (AIP), 471 precocious puberty, 388 superuvulation, 401 Gonadotrophin-releasing hormone (GnRH) deficiency fetal ncuronal migration defect, 386 malignant disease/malignant disease treatment, 702 Gonadotrophin-rrleasing hormone (GnRH) stimulation test gonadal dysfunction, 391 male pituitary gonadotrophin» evaluation* 404-405 precocious puberty, 391 protocol, 411 Gonadotrophin-secreting tumour, 308 precocious puberty, 391 (toodpasture's syndrome, 542 Gordon's syndrome w? Pseud oh vpoaldostcronism type 2 Gout, 535-536* classification, 535 clinical features, 535 diabetes mcllitus association, 273 epidemiology, 535 hyperuricaemia, 535, 536 hyporeninaemic hypoaldosteronism, 57 1 -csch-Nyhan syndrome, 617 monosodhim urate (iVlSC*) crystal arthropathy, 535
  
  813
  
  renal disease. 535-536 synovial fluid light microscopy, 541, 542 treatment, 536 urate plasma concentration, 541 Cf-pTOieins, 575 Granulocyte colony-stimulating factor —J60 haemoglobinuna, 157 haemopcxin in plasma, 459, 460 haemosidenn in urine, 458, 460 haptoglohin (Hp; plasma concentration, 459,460, 465 hyperkaUemia, 56 intravascular, 453, 456 laboratory tests, 459-461, 465 abnormal haemoglobin, 461
  
  evidence for cause, 461-464, 465 immune haemolysis, 461 red cell enzyme defects, 462 464 red cell fragility>survivaJ, 460-461, 465 red cell morphology, 459 460, 465 rcticulocyte count, 459. 465 mcthacmalbumin in plasma, 459, 460 poisoning associations, 660 acute renal failure, 664 Haemolyiic anaemia, 453 acquired, 457-458 chemical agents, 458 immune, 457 malaria, 458 microangiopathK:, 458
  
  non-immune, 457 458 prosthetic heart valves, 458 congenital, 456—457 red cell enzyme defects, 456 457, 745 red cell membrane defects. 456 definition, 456 diagnostic approach, 464 465 clinical evidence, 464 laboratory tests, 464 465 eryihropoietic porphyria, 473, 474 HbH disease, 486 hcpaioerythropoictic porphyna, 477 jaundice, 231 unstable Hb variants, 489 Haemolyiic disease of newborn, 4 3 ! , 434 Haemolyiic uracmic syndrome, 458 Haemoperfusion, 665, 668 Hacmopewn, 766 hacm binding, 459
  
  haemolysis-associated levels, 459, 460 Haemophilia, 538 Haemorrhage multiple myeloma, 500 thim, 29 Haemosidenn, 200, 766 intravascular haemolysis, 458, 460 malignant disease. 697, 698 Haemosiderosis, 250 iron »tverU»ad, 486 Hagcman factor (factor XII), 730 activation to HFa, 731
  
  815
  
  Hepatic clearance tests, 229 2>0 aminopyrine (dcmethylation) breath test,
  
  factor X I activation, 731
  
  injury-mediated surface binding, 7*0 PK-HMWK complex binding, 730-731 Haloperidol, 303 Haptoglobin624, 627 apo A-1V. 624 apo C-I, 625 apoC-11,625, 627 apoC-HI, 625, 627 apo D, 625 apo E, 627 LDL receptor binding, 627 atherogenesis protective effect, 622 cholesterol, 625, 627, 638 diabetes mellitus, 272, 274 cholesteryl ester transfer protein (CIETPj deficiency, 627, 635 chronic renal failure, 139, 365 dyslipoproteinacmias, 635, 639 gencuc vanation, 758 HDL,, 627 deficiency. 635 oestrogen effects, 636 progesterone effect, 636 H D I ^ , 627 investigations, 638 metabolic disorders, 635 metabolic transformations, 627 apolipoproteins, 626 cholesterol, 627
  
  chylomicmns, 625. 626, 627 phosphotipids, 626, 627 W.DU 627 nephrotic syndrome, 154, 636 oestrogen therapy-associated changes, 421 phospholipid, 626, 627 pregnancy-associated changes. 419 subclasses, 638 High density lipoprotcin iHDL) deficiency fish eye disease, 635 Tangier disease, 598, 635 High molecular weight kminogen (IfMWK), 730 bradykinin release, 731 prckallikrein (PK) complex, 7 30 Hirsumm, 401^402 causes, 401 investigation, 401 -402 polycystic ovary syndrome (PCOS), 399, 401 testosterone metabolism, 401 treatment, 402 Histamine, 729 actions, 729 acute phase response, 734, 736 clinical shock, 734 inflammatory response, 733 Histidase assay, 609 Histidasc deficiency, 608 Histidinacmia, 60Я-609 diagnosis, 609 heterozygote detection, 609 Hisnocytos» X, 309, 694 History taking, 2 Hodgkin's disease acquired ichthyosis, 699 fever. 698 iron metabolism, 695 nephrotic syndrome, 699 peripheral neuropathy, 700 serum alkaline phosphatasc, 225 subacute ccrebellar degeneration, 701 treatment-associated infertility, 702 Но1осагЬоху1аче synthetase deficiency» 444 Homocysteinc endothcltal damage, 610 oral contraceptive pUl-associatcd elevation, 420 Homocystine metabolism, 450, 600 unne assay, 610 Homocystinuria, 448, 450, 609-610 clinical features, 450, 609-610 mental handicap, 609, 610 neurological signs, 610 thromboembolic disease, 610 cystathiorunc P-synthasc deficiency, 609 differentia] diagnosis, 610 hctcrozygoic identification, 610 hydroxycobalamin synthesis defects, 450 incidence, 609 management, 539, 610 methylmalonic aciduna, 450 plasma aminu acids levels, 609 prenatal diagnosis, 610 pyridoxine-responsive, 610 rheumatological complications, 538 screening, 610 Homogenrisic acid oxidase deficiency, 539 Homovamilic acid (HVA) antipsychotic drug monitoring, 574 ccrcbrospinaJ fluid (CSK), 574 schizophrenia, 574
  
  1ный авторским np
  
  INDEX Hornovanillic acid (HVA,1 {contd) urinary excretion, tctrahydmbiopccrin (BH,) deficiency, 005 Homozygous state, 742, 762 Hormone replacement therapy (HRV), 398, 420-121 glucose tolerance, 421 ocstrogens, 420 parenteraJ administration routes, 420 plasma lipids/lipoproteins, 420—421, 63b plasma proteins, 420 po«t-menopau»al osteoporosis, 516, 517 progestogens, 398, 420 Hormone values comparison of results with reference limits, 18 reference value* relationship, 17 Horseshoe kidney, 117 Human chorionic gonadotrophin 'hCGj tx-subunit, 396, 692 ACTH-dependent (bushing's syndrome, 307 f3-*ubunit, 396, 692 rumour expression, 692 chon ocarcinoma diagnosis, 710-711 prognosis, 706, 7 11 screening, 706, 711 corpus luteum maintenance, 396 Down's syndrome screening, 414, 416 ectopic pregnancy, 414, 787 cctopic production, 692 fetal Leydig cell regulation, 376, 396 germ cell rumours, 70R ^-subunit assay, 704 diagnosis, 709 prognosis, 710 residual disease detection, 710 treatment monitoring, 709, 710 gestations! trophoblastic disease, 414 hyperemesis gravidarum, 335 Ixydig cell differentiation, 381 male infertility treatment, 407 malignant insulinoma, ОЯ4 precocious puberty, 391 pregnancy diagnosis, 413 414 pregnancy-associated change», 41 8 structural aspects, 396, 692 supcrovutation, 401 thyroid -stimulating activity, 334 tumour marker, 692 Human chonomc gonadotrophin (hCG) stimulation test cryptorchidism, 406 haemochromatosis, 404 intersex investigation, 390 391, 406 Leydig cell functional assessment, 405 protocol, 390, 411 vanishing tesiis syndrome (anorchia}, 390, 406 Human menopausal gonadotrophin (hMG) male infertility treatment, 407 ovulation induction, 400 superovulatkm, 401 Human placenta I lactogen (hPL) fetal well-being assessment, 417 pregnancy-associated changes, 418 Hungry bone syndrome, 524, 525 Hunter's syndrome we Mucopolysaccharidosis type II Huntington's disease. 746, 757 age-dependent expressivity, 746, 757 clinical features, 757
  
  genetic analysis, 753, 757 genetic mechanisms (CAG repeat expansion), 757 presymptomatic diagnosis, 757 prevalence, 757 reactive oxygen species, 773 Hurier's syndrome ut Mucopolysaccharidosis type I Hyaline membrane disease, 424 Hvdatiform mole chonocarcmoma, 710, 711 hCG screening, 706, 711 thyroid hormone secretion, 347 Hydralazme autoimmune hypoglycaemia, 292 \itamin B 4 metabolism. 180 H ydroccphal u s C T scan, 565
  
  precocious puberty, 387 Hydrotortisone ACI"H deficiency, 301 Addison's disease (primary hypoadrenahsm), 321 congenital adrenal hypcrplasia, 326 glucocorticoid replacement therapy, 326 hypoglycaemia, 290, 291 hypepituitarism, 290, 311 mineralocorticoid replacement therapy, 326 Hydrogen breath test bacterial overgrowth, 212 carbohydrate absorption, 207 Hydrogen ion concentration» 61, 62 acid-base status assessment, 69 acidosis non-respiratory, 71 respiratory, 76 alkalosts non-respiratory, 78 respiratory, 79 base excess, 69 mixed acid-base disorders, 81 ncar-panent testing, 14 ЯСО ; relationship, 80 i& atio pH Hydrogen ion excretion, 66-68 ketoacidosis, 72 non-respinuor> acidosi* compensation, 70, 71 pulmonary carbon dioxide, 66, 68 renal, 66-68 ammonium excretion.. 66, 68 bicarbonate reabsorptum, 66 tubular mechaniMns, 164, 165 respiratory acidosis, 76 respiratory altalosis, 79 Hydrogen ion homeostasis, 61-81 buffer systems, 62-63 ammonia, 63 bicarbonate, 62-63 haemoglobin, 63 phosphate, 63 plasma proteins, 63 definitions, 61 62 disorders, 70-81 compensatory processes, 80, 81 metabolic (non-respiratory), 62 mixed, 81 respiratory, 62 hydrogen ion turnover, 63-64 excretion, 66-68 production, 64-66 intracelluLar, 61 physiological role, 61 62
  
  817
  
  Hydrogen ion production, 64-66 amino acid metabolism, 65 66 carbon dioxide, 64 glucose oxidation, 65 glycolysis, 64-65 ketogenevs, 65 (acute metabolism, 64-65, 73 renal tubular cells, or>, 67 collecting duct, 123 distal convoluted tubule, 123 proximal convoluted tubule, 121 renal tubular acidosis type 1 (classic distal), 167, 168 renal tubular acidosis type II (proximal), 167 renal tubular acidosis type IV (distal with hyperkalacmia), 168 renal tubular acidosis type IV (generalized distal), 167 tngiycende oxidation, 65 urea synthesis, 65-66 Hydrogen peroxide adrenaline metabolism, 768 atherosclerosis pathogenests 774 catalasc in removal, 771 chemical reactivity, 767 glutathionc peroxida&e in removal, 771 respiratory burst, 768 superoxidc dismutasc in formation, 771 transition metal-catalysed hydroxyl radicals production, 767 xanthinr oxidasc-catalysed generation, 768 Hydrogen sulphide poisoning, 661 Hydroperoxyeicosotctraenoic acid (HPITI"E), 623 Hydrops fetalis, 485-486, 491 18-Hydroxy cortisol, 324 18-Hydroxy dcoxycorticosteronc j IX-hvdroxv rxx:), 316,3i8 3-Hydroxacyl CoA dehydrogenase, 554 Hydroxyapatite, 507 ^- Hydroxybutyratc krtnacidosis, 276 neonatal hyperinsulinaemic hypoglycaemia, 429 starvation, 368 a-Hydruxybutyrate dehydrogenase (HBD), 781 782 3-Hydroxyburyratc plasma concentration fasting hypoglycaemia, 2R5, 286 alcohol-induced, 292 hypoglycaemia with adrenal insufficiency, 291 hypoglycaemia with pituitary insufficiency, 290 insulinoma, 288 neonatal hypoglycaemia, inborn errors of metabolism, 442, 445 25-Hydroxycholecalciferol seasonal variation, 9 vitamin D stores estimation, 189 Hydroxycobalamin synthesis defects, 450 Зр-Hydroxydchydrogenase deficiency, 32, 56 Hydroxycicosotetraenoic acid (HKTE), 623 5-Hydrox)indolcacetic acid (5-НГАА) carcinoid syndromci 679, 681, 70 3 urinary excretion, 681 ccrebrospinal fiuid (CSFj alcoholism, 576 depression, 578 schizophrenia, 574 suicidal behaviour, 578 serotonin dcamination product, 681 tetrahydrobiopterin (BH 4 ) deficiency, 605
  
  Материал, защиш
  0 molecular genetic», 380 prenatal diagnosis 380 vinlLzation of female external geniralia, 378 vinluation in male infant, 378 17a-Hydroxylas< ovarian biosymheuc pathways, 397 testosterone biosynthesis, 403 17n-Hydroxylase deficiency hypokalacmic alkalosis, 5 3 male pseudohermaphrodiusm, 582 testicular steroid biosynthesis, 382 21 -Hydroxyiase deficiency aldosterone deficiency, 377 378 congenital adrenal hyperplasia, Y2h, 377-378 diagnosis, 326, 379 l70H-proge*ieronc, 378 hirsutism, 402 hyperkalaemia, 56 investigations, 390 molecular genetics, 326, 379- 380 neonatal screening, 379, 440 prenatal diagnosis 380 renal sodium loss 32, 56, 377- 378 virtlization, 377 male infant, 378 Hydroxylysinc bone turnover marker, 513 Hydroxylysylpyridinolinc ue Pyndinoiine Hydroxymethoxymandclic acid (HMMA) uhne level, 328, 686 phacochromocyioma, 572 vanillm intake-related variation, 10 Hydroxymcthylbilanc :HMB), hacm biosynthesis, 467 Hydroxymcthylbilanc synthase (HMBSl hacm biosynthesis. 467 lead toxicity, 479 Hydroxyrnethylbilanc synthase (HMBS) deficiency acute intermittent porphyria (AIP), 470, 471 Chester porphyria (dual porphyria}, 478 variegate porphyria» 472 Hydrox>Tnethylglutaryl (HMO) CoA lyasc deficiency, 445, 446 Hydro xymethylglutaryl-t^oA-reductase (HMG-CoA reductasei, 623 17a-Hydroxypregnenolonc, ovarian biosynthctic pathway*, 397 17a-Hydroxyprogcstcrone (I70HP,1 hirsutism, 401, 402 21-hydroxyiase deficiency, 326, 402, 410 neonatal screening. 440 stimulation test protocol, 410 ovarian biosynthctic pathways, 397
  
  Hydroxyprolinc unnary excretion bone turnover estimation, 94, 512 osteoporosis 515 diaphyseal dysplasia, 531 hyperthyroidism, 343 idiopathic hyperphosphatasia, 531 malignant disease, 694 Pagefs disease of bone, 526-527 treatment monitoring, 527, 528 polyostotic fibrous dysplasia, 5 3 1 pnrnary hyperparathyroidi.tm, 525 Зр-Hydroxysteroid dehydrogenase ovarian biosynthetic pathways, 397 testosterone biosynthesis, 403 3f3-Hydmxystcroid dehydrogenase deficiency congenital adrenal hyperplasia, 378 *alt wasting form, 378 diagnosis, 379 I70H-prcgncnolonc, 37*1 investigation*, 390 male undervirilwation, 378, 380, 382 molecular generic*, 380 partial forms 382 testosterone biosynthesis defect. 382 11 (3-Hydroxystcroid dehydrogenase (ll(3-OHSl>) cortisol-cortisone shuttle. 319 testosterone biosynthesis, 103 1 l(3-Hydroxysteroid dehydrogenase (lip-OHSD)dcficicncy corttsol metabolism, 319 hypokalacmic alkalosis, 53 17(V-Hydroxysteroid dehydrogenase deficiency investigations 390 male pseudohermaphroditism, 382 virihzation at puberty, 382 5-Hydroxytryptaminc (5-H"I") see Serotonin 5-Hydroxytryptaminc (5-HT) receptors ш Serotonin (5-HT) receptors 5-Hydroxytryptophan urinary excretion, tare moid syndrome, 681 Hype raldust его nism aJkalosis 53. 318 cirrhosis, 244 hypokalaemia, 5 3, 318 primary tee Aldostcronism, primary secondary, 53, 214, 325
  
  causes. 325 sodium retention, 36, 37 Hyperammonaemia clinical features, 607 hepatic encephalopaihy, 244, 589 neonate, 448-M9 causes, 441, 448 neurological deterioration, 448 parcnteral nutrition, 437 organic acidacmtas, 444. 447-448 plasma ammonium assay, 607-608 pymvate carboxylase deficiency, 444 Reye's syndrome, 250 urea cycle disorders 606-607 differential diagnosis. 607-608 respirators- alkalosis, 607 screening, 607-608 HyperapobeialipopToteinaemia coronary- heart disease association, 631 small/triglyceride poor VLDL panicles 627 Hyperbanc oxygen therapy, 677 Hyperbilirubinacmia, 224 acute hepatitis, 238 alcoholic st cat о sis, 243 asymptomatic patients, 233
  
  cerebrospinal fluid bilirubin (xanrJiochromia), 558 congenital hypothyroidism, 4 39 conjugated, 231, 232 investigations, 231 hacmolytic anaemia, 231, 453 inherited, 232 jaundice hepatic fhcpatoicllularl, 2 И poschcpaiic (cholcstatic), 231 prehepatk, 231 Jcernicterus, 4 33 liver function tests, 231, 232 neonatal tee Neonate Wilson's disease, 251 Hypercalcaemia, 94 98 causes 95 97, 113 clinical features, 95 depressive illness, 579 dialysis patients, 522 hypergastrinaemia interaction, 685 hyperthyroidism, 343 infancy/childhood, 430 investigation. 97-98 flow chart, 97 malignant disease «re Hypercalcaemia of malignancy multiple endocrine neopla^ia I (MEN I), 684, 685 multiple endocrine neoplasia 2 (MEN1 2), 687 multiple myeloma, 497, 499, 500 neonaie/preterm infant, 430 nephrogenic diabetes insipid us (DI), 38, 41 parcnteral feeding. 197 polyuna, 38, 40, 95 post renal transplantation, 142 renal calculi, 170 renal failure acute, 133 chronic, 1 38 sarcoidosjs, 538 ihyroioxicosis, 93 treatment, 98 vitamin I) toxicity, 190 Hypercalcaemia, familial hypocalciuric, 430 Hypercalcaemia, infantile (Williams' syndrome), 4 30 Hypercalcaemia of malignancy, 96, 695-696 humoral hypercalcaemia of malignancy (HHM>, 96 investigations 98 metasratic bone destruction, °6 osteoclast-activating factors (OAFs), 695, 696 parathyroid hormone (PTH; assay, 90 parathyroid hormone-re la ted pepttdc ■PTHrP), 93, 96, 695, 696 assay, 98
  
  ectopic production, 690, 692 vitamin D metabolite a-hydroxylaUon, 695, 696 Hypcrcalcmna hyperthyroidism,, 343 idiopathic, 105 osteoporosis, 516 phosphate transport defects, 167 renal calculi formation, 169-170 renal tubular acidosi* type I (classic distal}, 168 Hypercapnia hypoxaemia, H4
  
  Материал
  
  [вторским правом
  
  INPI-X Hypcrcapnia UontJ) poisoning. 660 rapid lowering of TCO., 78 79 j*v aUo Carbon dioxide retention Hypercholesterolaemia acute intermittent porphym (AIP). 471 common (polygcmc), 6)4 frequency. 634 tschaemic heart disease association, 6)4 porphyrias, acute attacks* 479 «creening, I» 5 therapeutic diets, 194 Hypercholcitcrolacmu, familial, 2. 63)-634 atherosclerosis, 633 clinical features, 6 ) 3 - 6 ) 4 homozygou* «ate, 6Л4 lipoproteinfaj, 625 low density lipoprotein (LDL) plasma concentration, ft27 Urn- density hpoprotem (IDl.) receptor deficiency, 627 mutations, 634, 757 Hypcrcitrulhnacmia. 444 Hypcremesis gnmdarum, 3)5 H>T>erg*btrinaemia, 6Й5 Hypergiucagonaemia cirrhosis, 271
  
  diabetes mcllirus, 271 glucagonoma/MHX 1,684 Hyperajycacmia acute pancreatitis 785 adverse effects, 257 chronic liver failure, 220 cirrhosis. 2 7 1 , 365
  
  diabetes mcllirus long-term complication», 271 non-insuhn-dependent (NIDDMj, 2bt\y 267 growth hormone response, 43H insulin release, 2o0 mtulinoma removal response, 289 malignant disease, 694 metabolic response to iniurv, 355 neurolopcal disturbance, 58° osmotic diuresis, 123 parenteral nutrition in neonate/prctcrm infant, 4 37 potassium distribution/replacement, 30 Hypcrglycinaemia kctotic ш Propionit acidacmia non-ketotic, 449-450 Hypennvuhnacmij crmmic ren;»l failure, 365 cirrhosis» 271,365 diabetes me I lit us long-term complication*. 271 maternal» 429 non-insulin-dependcm CNIDDM1), 266 tasting hypoglycaemia, 285 3-hydroxvbutvrate plasma concentration, 2Я6 insulin resistance, 267 msuhnoma, 683 obesity, 266
  
  primary hyperparathyroislism, 209 Hyperinsulinaemia syndrome, 265, 267 dvsJipidaemia, 272 HypcrinsulinaemiL glucose clamp test. 279. 280 Hyperkalaemia, 55-58 acidosis. 30, 71
  
  Addison4 disease (primary hypoadrcnalism). Ъ22
  
  aJdosterone release, 26 asotcs therapy monitoring, 246 cause*, 55-57 chloroquine poisoning, 669 clinical aspect*. 55 digoxin poisoning, 669 emergency treatment, 58 hypoaldostenmism, 33, 56 57, 138, 320 ketoacidosis, 276 laboratory investigations, 57-58 management, 58, 13) ncuromuscular function, 30 potassium body More* reduction, 58 potassium retention, 56 redistribution hyperkalaemia w vitro (spunouVpscudohypcrkaUcmia), 55, 57 tn г«ч», 55- 56, 57 renal failure. 4 acute, I 3 ), I W chrome, 138 with cirrhosis, 246 renal tubular acidosis type IV, 74, 167, 168,171 172 with sodium depletion, 32, 3) surgery-related variation, 10 HvperkaJaemic periodic paralysis, 56 Hyperlactatacmia, 85 Hypcrhpidacmia, 621 -640 acquired (secondary), 6J5-637 cause», ft 36 cutaneous lipid deposition, 637 alcohol abuse, 671 classification, 628-629 diabetes mellitus screening, 279 dietary fibre intake, 184 cxtravascular manifestation srtipid deposition, 629 ncphrotic syndrome, 154 pregnancy-, 419 thyroid disease screening, 354 treatment, 640 *iv aito Dyslipidacmia Hypcrlipidaemia, combined familial (FCH), 631 coronary artery disease association, 631 nicotinic acid therapy, 180 small tnglyccridc-poor VLDL, 627 atherogenicity, 621-622 Hypcrmagnesacmia, 112 «cute renal failure, 133 causes, 112, 113 clinical implications, 112 management, 112 parathyroid hormone response, 110 Hypcmatraemia, 37, 41-43 acute sodium loading, 36, 42 cause*, 42. 590 diabetic non-ketotic hyperotmolar hyperglycaemic states, 277 managemeni, 43, 590 neonate, 427-428 clinical feature*, 428 neurological manifestation*, 590 spurious, 41 water deficiency deficit estimation, 59 with thirst, 42 without thirst (adipsicVhypodipsic), 42413 Hypcroxaluna, renal calculi formation, 169, 170 Hyperparathyroidtsm calcium pyrophosphatc disease, 536
  
  819
  
  classification, 90- 91 familial, 95
  
  McCune-AlbrighT syndrome, 387 maternal, neonatal hypocalcaemu, 100 multiple endocrine ncopla&ia 1 <MEN I), 684, 6Я5 multiple endocrine neoptasia 2 (MKN 2"), 687 parathyroid hormone (PTH) a**ay, 90 primary, 90 clinical features, 95 generic mechanisms, 95 hypcrcakacmia, 94, 95, 170, 430 hypercalciuria/renal calculi, 170 hypcfinsulirucmia, 269 infancy/childhood, 430 investigations, 98 parathyroid bone disease, 524 525 phosphate (TmP/GFRl reduction, 104 prevalence, 95 treatment, 98 renal osteodysirophy, 521, 52) bone demitometry, 523 clinical features, 522 management, 523-524 radiographic manifestations, 523 renal transplant recipient», 524 secondary, 90 chronic renal failure, 1 37, 138 hypocalcaemu investigation, 101 ncphrotic syndrome, 15) 154 Paget's disease of bone, 528 phosphate (TmP/GFR) reduction. 104 post renal transplantation, 142 rickets, 431 vitamin D deficiency, 100, 189 tertiary, 90 chronic renal failure, 138 post renal transplantation, 142 H yperphcnylalarunaemia differential diagnosis, 605-606 chromatographic techniques. 605 enzyme assays, 605 urine ptcrins analysis, 605 dihydropterine reductase (DHPR) deficiency, 605 mental handicap, 604-6O6 phcnylalaninc hydroxyiase deficiency, 604-60 5 screening, 605-60ft
  
  Guthne test, 605 tctrahydrobioptcrin (BH,j deficiency, 605 transient in pretcrm infant, 435 ш also Phcnylkctcmuria (РК1Л Н\рсфпо*ргииасгтйа, 106-107 cause», 106, 113 clinical consequences, 106 107 diagnostic approach, 107 dialysis рааепи, 522 hacmolyscd samples, 106 hypocakaemia, 100, 101 hypoparathyroidism, 99 osteomalacia/nckm, 519 renal failure acute, 1 33 chronic, 136 renal osieodystrophy, 521 therapeutic approach, 107 Hyperphosphatasacmia» transient of infancv, 432 Hyperphosphatasia, idiopathic, 531 Hyperphosphaturia, 343 Hyperpipecolic acidaemia, 451
  
  Материал, защищенный авторским правом
  
  820
  
  IN'DIX
  
  \ lyperpr olactinacroia amenorrhora, 399 causes, 303 chronic renal failure, 139, 363 diagnosis, 399 drug-induced 394 glucose intolerance/in*ulin resistance, 260 h>pothyroidism, 348, 39V impotence, 40Я McCune-Albright syndrome, 38 7 pituitary portal circulation impairment, 297,303 polycystic ovaries association, 399 proUcun secretion dynamic tests, 303 treaunent, 399 Hypertension acromegaly, 305 congenital adrenal hypcrplasia, 326 coronary heart disease, 193, 621 athcroma formation, 622 diabetes mcUitus, 272, 273 non-insulin-depcndcnt (NIDDMl, 266 screening, 279 haemolytic anaemia, 458 miners loco rticoid excess, 36, 37, 320 obesity, 193 phacochromocytoma, 327 primary aldosteromsm, 323, 324 protcinuna/microalbuminaemia, 159 renal disease, 53, 123, 124, 135, 136, 137 selective hypercholesterolaemia screening, 5 Hypcrrhyrcridisrn, 342-347 amiodarone therapy, 346 causes, 343-347 clinical features, 342-343 cctopic th>Toid tissue, 347 hypokalacmia, 342 hypolipidacmia, 637 iodine-induced, 346 liver function tests, 342 pituitary thyroid hormone resistance, 353 rheumatologkal disorders, 537 sex hormone-bindmg globulin (SHBG), 399 subclmical, 22H depression association, 573 thyroid function tests, 228, 343, 344 thyroiditis, 351, 352 thyrotoxicosis facblia, 346-347
  
  treatment response monitoring, 344 TSH receptor autoantibodies, 340 Hypcrtonic saline infusion acute dilutions] hyponatraemia, 48 central pominc myelinolysis fCPM), 48 chronic dilutkmal hyponairaemia, 49 polyuria, 41 posterior pituitary functional assessment (AVP test), 59-60, 301 Hypcrrrichiwis lanuginosa acqui&ita, 699 H ypertriglyccrid acmia diabetes melhtus, 272, 273, 274, 636 p»eudoh>-perphosphataemia, 106 pseudohyponatraemia, 44 Hypertriglyccndaemia, familial (FIiTC»), 631 insulin resistance, 631 large VLDL particle size, 626-627 Hypertrophic pulmonary osteoarthropathy (HPOA).538, 699 HvpcrryTosinaemia, transient in preterm infant, 435 Hyperuncaerrua diabetes melhtus (NIDDM;, 266
  
  glycogen storage diseases, 254 gout, 535, 536 hereditary fructose intolerance, 2^4 l.csch-Nyhan syndrome, 617 malignant disease, 696 renal failure acute. 133 chronic, 137 sarcoidosis, 538 uhc acid nephropaihy, 542 uric acid stone», 170 Hyperuncosuria, renal calculi formation, 169 calcium stones, 170 uric acid stones, 171 Hypervcntilation alveolar oxygen uptake, 82 anxiety disorders, 572 compensatory, 69
  
  non-respiratory acidosis, 70-71, 72 respiratory acidosis, 76 respiratory alkaloni*, 79, 80 hypophosphataemia, 108 Hyperviseosity syndrome investigations, 502 multiple myeloma, 500 VC-'aldcnwrom's macroglobulinacmia l"WM), 501, 502 Hypoadrenahsn cortisol plasma levels, 319 shon Svnacthcn test. 320 HypoaJbuminacmia acute pancreatitis, 785 «cites, 244 glomenilar diseaxe/nephrotic syndrome, 147, 153 h>*perthyroidism, 342 kwa^hiorkor, 192 liver disease, 227, 244 malignant disease, 694 Hyponldostcronism, 325 causes, 325 hypcrkalaemia, 56-57 metabolic disturbances, 320 renal tubular acidosis type IV, 74 sodium deficiency, 32- 3 3 Hypobctalipoprotcinaemia apo В molecular genetics, 631 familial, 631 selected deletion of apo B-48 set ChylomiCTon retention disease with truncated apo Bs, 632 Hypocalcaemia, 99-102 acute pancreatius, 785 causes, 99 100, 113,430 clinical features, 99 hypomagnesaemia, 111 investigation, 100-102 flow diagram, 101 neonate, 430-131 ostcornalacia/rickcts, 431, 518, 519 parenteral feeding, 197 post-parathyroidectomy, 524, 525 renal failure acute, 133 chronic, 138 renal ostcodystrophy, 521, 522 treatment, 102 Hypocalciunc hypercalcaemia, familial hypercalcacmia, 94-96 investigations, 98 management, 98 Hypocapnia, acute anxiety, 572
  
  CNS effects, 79 H у po с h 1 or acm i a acute intermittent porphyria (AIP), 471 non-respiratory alkulosis, 78 variegate porphyria, 472 Hypochlorous acid (HOG)), 767 respiratory burst, 768 Hypocholesterolaemia, Tangier disease (HDL, deficiency), 598 Hypodipsic hypematracmia, 42 management, 43 subtypes, 42-43 Hypoglycaemia, 281 -294 alcohol-induced, 275, 291-292, 293, 671 autoimmune, 292 classification, 283 deficient energy intake, 292 definition, 282,294 WhippJc* triad. 282, 294 diabetes mellitus, 287 diagnostic approach, 283 284, 294 clinical assessment, 284 flow chart, 284 history taking, 283-284 investigation*. 285-287 drug-induced, 287,291 dry reagent strip tests, 285 endocrine deficiencies, 290 291 exercise-related, 292-293 factitious, 2Я7-288 tasting, 283, 285, 291-292, 294 blood glucose measurement, 285 blood sample collection, 285 causes, 287 293 laboratory investigations, 285 286 prolonged fast test, 285, 288, 294 glycogen storage diseases, 254 hereditary fructose intolerance, 254 hormonal response, 282 hypcrprotactinacmia, 303 inborn errors of metabolism, 294 islet cell tumour (insulinuma), 288-289, 683, 694 liver disease, 248, 289, 365 liver failure, acute, 220, 240 malignant disease, 693, 694 muscle fairy acid oxidation defects, 553 neonatal KY Neonatal hypoglycaemia neurogenic response, 282 non-islet cell tumours, 289-290 panhypopituitarism, 302 postprandial/reactive, 283, 286-287, 293, 294 alimentary, 293 blood glucose measurement, 286-287 blood sample collection, 286 causes, 293-294
  
  idiopathvc, 293-294 impaired glucose tolerance/mild diabetes, 293 laboratory investigations, 286 287 mixed meal tolerance test, 287, 294 preventive regulatory mechanisms, 257 renal failure, 289 Reye's syndrome, 250 salt wasting congenital adrenal hyperplasia, 378 septicaemia, 292 sulphonylurea-induced, 287, 288 symptoms, 282-283, 589 adrencrgic, 282, 283 anxiety, 572 neuroglycopcnic, 257, 282-283, 589
  
  Материал
  
  [вторским правом
  
  INDEX Hypogonadism adrenomyelnneuropathy, 595 alcoholism, 366 anorexia ncrvosa, 370 impotence, 408 inherited neurological disorders, 367 male» 405—407 clinical features, 405 hormone receptor defects, 406 laboratory findings» 405 steroid biosynthctic enzyme defects, 406 treatment, 407 obesity, genetically determined, 371 primary causes, 38" 390 chromosomal disorders. 389 delayed puberty, 389 seminiferous tubule dysfunction, 405, 406 set d/ю Gonudal failure Hypogonadotropruc hypogonadism, 405, 406-407 causes, 407 delayed puberty, 384 GnRH test, 405 infertility management, 407 multiple endocrine ncoplasia type I (MEN I), 684 temporary with acute illness, 404 tcstes endocrine evaluarion, 404 treatment, 407 Hypokalaemia, 4 ° 55 acid-base status, 51 ACTH cctopic secretion, 306. 307 alkalosis, 30 non-respiratory, 54, 78 respiratory, 79 asotes therapy monitoring, 246 causes, 49-5 3 cxtrarenal, 50-51 redistribution in tntn> (spurious hypokalaemia), 49- 50 redistribution in tiw, 50 renal, 51-53 chloroquine poisoning, 668 clinical aspect», 49 drug-induced, 31** l-ancom syndrome, 169 gastrointestinal potassium losses, 51 11 p-hydroxystcroid dehydrogenase riiP-OHSD} deficiency, 319 hypenhyroklism, 342 hypomagnesaemia, 111, 112 laboratory investigations, 53 54 management, 54 55 mmeralocorticoid excess, 36, 37, 318, 320 spironolaeione therapy, 325 muscle weakness, 548 ncphrogenic diabetes insipidus , 242 constant regions, 493 heavy chains, 493 intrathecaJ synthesis, 56 L ] chain, 493 light chains, 493 paraproteinaemia, 498 proximal tubular rcab^orption/cauboliMn, 145 secretory piece, 493 structural aspects, 493 variety, 493^*94 synovia! fluid analysis, 542 urinary excretion, 159 variable regions, 493 I mm uno suppression multiple myeloma, 499 transient paraprotcinacmia, 503 Immunosuppressrve therapy chronic autoimmune hepatitis, 233, 234, liver transplantation, 255 renal transplantation, 142 Impaired glucose tolerance flGT) chronic renal failure, 365
  
  congenital conditions, 270 endocrine abnormalities, 269 270 insulin receptor mutation, 262 macnivascular disease, 272 malignant disease, 693, 694 parenteral feeding, 197 porphyna cutanca tarda, 476 porphynas, acute attacks, 479 reactive hvpoglycaemia, 29 Э Implantation, 395 Impotence, 408 410 causes, 408, 409 drug-induced, 408, 410 investigation, 408 410 psychogemc, 408, 410 treatment, 410 in litn* fertilization, 400 FSH Treatment monitoring, 400 superos'ulauon treatment, 401 Inaccuracy, 12 Inborn errors of metabolism, 73-74, 250-254, 4 3 9 ^ 5 1 , 599-618 neonatal presentation ice Neonate Incidence, definition, 20 Indian childhood cirrhosis, 252 Indications for biochemical tests, 1, 5 Indirect amiglobulin test, 461 Indirect calorimetry, 187 Individual screening, 5 Indocyamne green clearance test, 230 In dome thai: in, 4 1 , 425, 427 Infection acquired haemolytk anaemia, 458 energy requirement, 174 multiple myeloma, 499 sickle cell anaemia, 488 4K9 Infectious samples handling, 241 Inferior vena cava thrombosis, 15 5 Infertility causes, 400
  
  chronic renal failure, 363 hyperthyroidism. 343 investigation in women, 400—401 male hypogonadotrophic hvpogonadism. 407 malignant disease treatment, 702 superovulation treatment, 401 Inflammation acute phase response, 733 ana phy lot ox ins (C3a/C5a), 732 733 reactive oxygen species, 773 Inflammatory bowel disease angiotcnsin-cnnvcning enzyme (ACE), 541 arthritis, 537 delayed puberty, W liver function tests, 242 metabolic acidosis/hypokalaemia, 51 primary sclerosmg cholangitis, 242 secondary* lactase deficiency, 206 Influenza A, 436 Influenza B. 436 Inhtbin, 403 follicle-stimulatinghormone (FSH) regulation, 297, 396, 403 Klinefeher's syndrome, 406 menstrual cycle, 395 structural aspects, \9to InNertinm, 742 Insulin, 259 263 action», 261 262, 281
  
  amylin co-secretion, 266
  
  Материал
  
  [вторским правом
  
  INDEX
  
  Insulin (conttfl APUD tumour secretion, 712 biosynthesis, 25*-2*0 bone metabolism, 93 C-pcptide, 259, 260 arcadian changes, 274 deiicicncv states see Diabetes mcllitus DN'A analytical techniques, 280 exercise-associated change», 275 factitious hypoglycacmia, 287 fasting hypoglycacmia, 283 genetic abnormalities, 261 glucose transporter responses, 259 hepatic metabolism first pass effect, 260 liver disease, 565 kinetics, 260 lipid metabolism, 27 5-274, 621. 622, 62 3. 636 metabolic response to injury, 720 muscle metabolism, 546 neonatal carbohydrate metabolism, 428, 429 normoglycaemia regulation, 258. 259. 621, 622 phosphate distribution. 103, 106 plasma concentrations, 260 potassium distribution, 30 preproinsuhn, 259 proinsuhn, 259. 260 second messengers, 262-263 %ccrcti<m9 260 self-administration «*■*! Factitious hypoglyeaemia starvation, 368 structure. 260 Insulin antibodies, 270-271 autoantibodics, 271 P cell function estimation, 285 biochemical tests, 279 factitious hypogtycaemia. 287 Insulin plasma concentration cirrhosis, 248 food intake-related variation, 10 hypoglyeaemia adrenal insufficiency, 291 alcohol-induced, 202 autoimmune, 292 factitious. 287, 2R8 fasting. 285, 294 neonatal, 129, 442, 445 pituitary insufficiency, 290 msulinotna, 284, 683 non-islet cell tumours, 290 obese patients. 9 ut- alto Hypcrinsulinaemia Insulin receptor, 261, 262 activation, 263 assay, 279 DNA analytical techniques, 280 genetic abnormalities, 262, 263, 268 insulin resistance, 2b7, 279 acanthosis nigricans, 270 leprcchaunism, 270 internalizarion. 263 intracclluiar second messengers, 263 Insulin receptor antibodies, 292 Insulin resistance acanthosis nigricans, 270 associated conditions, 270 biochemical tests, 279 280 chronic renal failure, 365 cirrhosis, 271, 3b5
  
  diabetes mellitu* long-term complications, 271 non-insulin-dcpendent (N1DDMJ, 267 familial hypertnglyceridaemia (FHTG), 631 hypehnsulinaemic glucose damp test, 279, 280 hypcrprolactinaemia, 2b9 insulin-binding antibodies, 270-271. 279 leprechaumsm, 270 lipodystruphy, 270 malignant disease, 6*4 metabolic response to injury, 720 obesity, 267, 279 poiycyscic ovary syndrome (PCOS), 269, 399 pregnancy, 419 Insulin stress test (1ST) ACTH reserve assessment, 301 АСГГН-adrenal axis assessment, 298-299 chronic renal failure, 365 combined pituitary function test, 300 contraindications, 299 Cusjiing's disease treatment monitoring, 308 growth hormone reserve assessment, 299, 301 indications, 312 post-surgical pituitary function assessment, 301 protocol, 312 Insulin suppression test, 288 Insulin therapy, 265, 274 anti-insulin antibodies, 270 chronic renal failure, 138 diabetic nephropathy prevention, 158 hypoglycacmia, 589 alcohol consumption, 292 fasting, 283, 287, 289 with renal impairment, 289 lntercurrent illness, 274 kctoacidosis. 277 non-insulin-dependent diabetes mcllitus (NIDDM). 266 non-ketotic hyperownolar hyperglycaemic states, 277 overdose, 287 pregnancy, 419 redistribution hypokalaemia in twro, 50 Insulin tolerance test (ГГГ) м* Insulin stress test (1ST) Insulin-like growth factor 1 (IGF1), 263, 725 BcTomcgaly, 304, 305 actions, 725 hone matrix, 509 calciunVbone metabolism, 93 chronic renal failure, 362 delayed puberty, 389 fetal/neonatal plasma concentration, 438 O H in control of synthesis, 297 ■
  
  growth effects, 297 hypoglycacmia, 290 metabolic response to injury, 357, 720 nutritional status assessment, 187 narenteral solutions, I9o renal phosphate handling, 104-105 Insulin-like growth factor 2 (IGF2), 263, 725 bone matrix, 509 chronic renal failure, 362 hypogrycaemia, 290 Insulin-like growth factor (KiFl receptors, 261,264
  
  823
  
  Insulin-like growth factors 'IGb's; somatomedine), 263-264, 725 age-associated changes, 373
  
  chronic renal failure, 139 growth stimulation, 438 Imulinoma, 288 289 benign adenoma. 288 clinical features, 288 diagnosis, 288 differential diagnosis, 683 hormones secreted, 288 hypogjycaemia, 283, 285, 288, 694 chrome, 589 investigations, 285 localization, 288 malignancy, 288 multiple endocrine neoplasia 1 [MKN 1), 683 684 treatment, 289, 684 Intensive care patients magnesium deficiency, 100 near-patient testing, 14 Interference, 12 Interferon therapy carcmoid tumours, 682
  
  chronic viral hepatitis. 233, 241 islet cell rumour management, 683 thyroid function tests, 241 Interferon», 721, 724 726 actions, 358 endogenous pyrogen effects, 698 prosta gland ins in regulation, 359 lntehndividual variation calculation, 11 comparison of results with reference limits, 18 Interleukin 1 (ILI), 717, 721, 722, 726 actions, 358, 722
  
  acute phase response, 358, 734, 73ft time course, 736 arginine vssopresstn lAVPl release, 357 bone metabolism, 93 C5a-mcdiatcd release, 733 cellular disinhution, 358 clinical shock, 734, 735 cortisol/stres* hormone response, 719, 735 endogenous pyrogen activity» 698, 726 erythropoietic activity, 725 hypcrcalcaemta of malignancy, 96. 696 inflammatory response, 733 lipopoly*accharide (IJ*S) response, 722, 735 lymphocyte activation ( I A F ) assay, 722
  
  metabolic response to injury, 719, 735 hypoihalamic activation, 356 multiple myeloma, 499, 500 prosraglandins in regulation, 359 Interleukin l a (ILla), 722 Interleukin 1[J (IL1P>. 722 hypcrcalcaemia of malignancy, 696 Interleukin 2 (IL2), 721, 722 actions, 722 interleukin 6 (ILb) response, 723 multiple myeloma, 499, 500 Interleukin 3 (IL3J, 721. 722 actions, 722, 725 Interleukin 4 (11 A), 721, 722 actions, 722 Interleukin 5 (IL5). 722 actions, 722-723 Interleukin 6 (IL6), 717. 721, 72 3-724, 726 actions, 358, 72 3 acute phase response, 358, 723-724, 734, 736
  
  Материал
  
  [вторским правом
  
  824
  
  INDEX
  
  Inrerlcukin 6 (Пл), (cmttf) time course, 73ft arjjminc vasoprcssin (AVPJ response, 357 C-reactive protein response, 563 cellular distribution, 358 FDP* stimulation of release, 732 fever, 726 hypcrcalcaemia of malignancy, 96 Iipopolysacchandc tf-PS.) response, 721 multiple myeloma, 500 sepsis-associated hypothalamic activation, 350 Intericukm ft (IL6) receptors, 723 Interieukin 7 (IL7), 724 Interieukin 8 (1L8), 724 Interleukms, 721, 722-724 Intermediate density Itpoprotctn flDL), 624
  
  apo B-100, 624 apo C-I, 625 apoC-11, 625 apo ( M i l , 625 apo П, 625 atherogenesis, 621 chronic renal failure, 138 I 39 hepatic uptake, 627 H T G L hydrolysis to L U U 627 hypothyroidiKm, 63t» low density lipoprotcin (U)I.i receptor binding, 627 metabolic transformation, 627 remnants, 627 Intermittent peritoneal dialysis. 142 Intermittent porphyna, acute (AIP), 253, 470 4 7 1 , 7 4 6 acute abdomen, 787 acute attacks investigation, 480 triggers, 470, 471 earner detection, 471 clinical features, 470-471, 597 differential diagnosis, 471 genetic aspects, 470 investigation, 471 latent disease, 470 prevalence, 470 treatment, 471 variegate potphyria (VP) combination, 477 478 Interpretation, 15 23 comparison of observed results with previous values, 19 comparison of observed results with reference limits, 17-19 computer system», 23 critical differences calculation, 19 cut-off values, 18 19 normal range, 15-16 reference value*, 16-17 reliability of data, 15 Intersex, 375-385 classification, 376 377 investigations, 390 391 karyotype, 390 Interstitial nephritis u* Tubulointerstitial ncphxopathy Interstitial space, 25 Intestinal absorption absorptive capacity assessment, 215 ammo acids, 207 cystinuna, 165 familial renal iminoglyeinuria, 167 Hartnup disorder, 166 calcium, 88-89, 210
  
  assay, 94, 11 3 chronic renal failure, 1 38 Ktiopathic hypercalciuria, 105 osteoporosis, 516 renal osteodystrophy, 521 carbohydrate, 206 fatty acidsAriacylglycerols, 209 folatc, 203 iron, 201 vitamin C, 181 magnesium, 110, 210 oxalate, 170 peptidcs, 207 phosphate, 103, 104 ktiopathic hypercalciuria, 105 renal ostcody«trophy management, 521 vitamin B, : , 204-205 Intestinal aspirate culture, 212 Intestinal brush border pcptidascs, 107 Intesdnal carbohydrate digestion, 205 206 Intestinal tymphangiectasia, 211 IntraceJlular fluid (ICF), 25 body water content relationship, 26 electrolyte composition, 25 osmolahry, 26, 23 osmotic pressure. 25 potassium, 29 30 volume, 26 water, 28-29 Iniracramal tumour CSF markers, 563, 566 C T scan, 565 precocious puberty, 387 Inrxahcpanc cholestasis of pregnancy, 248 Intraindrtidual vanation analytical goals definition, 13 calculation, 10-11 comparison of observed results with reference limits, 18 Intrauterinc growth retardation uu Small for dates (SFD) infants Intrautcrinc infection hcpatocellular damage., 434, 435 hyperhilirubinaemia conjugated, 435 unconiugated, 434, 435 intrauterine growth retardation, 423 Intravascular space (plasma volume), 25 Intravenous glucose tolerance test (IVGTT), 280 Intrinsic factor (IF) deficiency, 205 vitamin Rl? absorption, 204 Intron, 687, 762 Inulin clearance, 125-126 Iodine dietary, 182 hypothyroidism, 350 thyroid gland metabolism, 331-332 Wolff-Chaikoff effect, 332 thyroid metabolism, 331-332 Iodotyrminc dehalogenase deficiency, 350 Ion exchange resins, 1 39 Ipecacuanha, 664 Iron dietarv sources, 200 functional aspects, 200 protcm-bound stores, 200 sutus assessment, 201, 203 Iron chelatuui therapy, 489. 669-670 Iron deficiency, 201 assessment, 187 articular disease, 541
  
  iv
  
  iron absorption, 201 laboratory investigations, 201, 203 red cell fragility, 461 Iron deficiency anaemia, 199, 200, 201 clinical features, 201 hypokalacmia, 51 intravascular haemolysis, 458 malignant disease, 697 phenylalaninc loading test interference, 606 Iron metabolism, 200-201, 203 absorption, 200-201 assessment. 203 control, 201 dietary binding factors, 200, 201 entcrocyte events, 201 gastric acidity, 200 201 combined pituttary'testicuJar dysfunction, 405 free radical reactions, 766 hacmochromacosis, 772 Uchaemic reperfusion injury, 773 lipid peroxiduuon, 766, 767 neurological disorders, 773 774 haem catabolism, 458 homoeostasis, 201 malignant disease, 695 porphyria cutanca tarda, 476, 477 Iron overload, 250-251 alcoholic liver disease, 243 P thalassaemia major, 486 chronic, 201 crythropoietic porphyria, 474 erythropoictic protoporphyria, 475 extravascular haemolysis, 458 glucose intolerance/secondary diabetes, 269 hacmosiderin, 200 hypogonadism/impotcnce, 247 laboratory investigations, 20 3, 250 liver biopsy, 251 porphyria cutanea tarda, 244 rransferrin saturation, 228 Iron pla*ma concentration acute phase response changes, 737 extravascular haemolysis, 458 genetic hacmochromatosis, 250 posture-related variation, 10 Iron poisoning, 201, 660. 669-670 clinical features, 669 laboratory investigation, 203 management, 669-670 serum analysis, 669 toxic doses, 669 Ischaemic exercise test, glvcolvsis disorders, 550 protocol, 555 Ischaemic heart disease »«■ Coronary artery disease Ischacmic reperfusion injury, 772 773 Islet amyloid polypeptidc (IAPPj t€c Amylin Islet |3 cells t£€ Pancreatic p cells Islet cell rumours cctopic ACTH secretion. 306 MEN I UT Multiple cndiKrinc neoplasm I (MKK 1) prolinsulin plasma concentration, 286 .97 ovarian receptors, 396 ovulatioivassociaied surge, 395 ovulation assessment, 400, 401 ovulation induction, 400 polycystic ovaries syndrome (PCOSi, 400 pregnancy-associated changes, 418 puberty, 386 pulsatile release, 297, *86, 396, 403, 404 precocious puberty, 391 puhatihty assessment, 300 structural aspects, 396 superovulaiion monitoring, 401 tester endocrine evaluation, 404
  
  testosterone feedback regulation, 403 testosterone secretion, 297 Lutcinuing hormone i.LH.I deficiency, ЗЙ1 hypothyroidism, 548 isolated (fertile eunuch syndrome), 407 Kallman4 syndrome, 407 post-pituitary irradiation, 302 Lutcinuing hormone-re lea sing hormone (LHRH) acromegaly, 305 chronic renal failure, 5o3 combined pituitary function test, 300 >ve cjfcii Gonadotrophin-releasing hormone (GnRH) Lymphangicctasia, 211 Lymphatic duct obstruction, 211 Lymphocyte activation (LAFj assay, 722 Lymphokines, 720 Lymphoma acantbosis nigricans, 6°9 acquired ichthyosis, 600 copper metabolism, 695 CSF&-microglobulin measurement, 563 CSF paraprotein secretion, 566 dermatomyositis/pol>Tnyositis, 700 fever, 698 heavy chain disease, 502 hypercalcaemia, 98, 696 hypertrophic pulmonary ostcoanhropathy (HPOA), 699 Mediterranean type, 502 paraproteinaemia, 502-503 peripheral neuropathy. 700 small bowel radiology, 211 subacute cerebellar degeneration, 701 uric acid nephropathy, 535-536, 696 Lymphosarcoma, 56 Lymphotoxm ut Tumour necrosis factor p CTNFpJ Lyphoid malignancy tumour marker, 695 Lysergic acid diethylamide от LSD Lysine metabolism, 451 Lysinc vasopressin, 41 Lysinuric protein mtolcrance, 448 Lysosomal storage disease. 601-602, 612-616 connective tissue disorders, 615-616 lipid metabolism disorders, 612-616 neonatal hepatomegaly, 435 treatment. 612 1 .ysozyme, urinary light chain proteinuria, 158 tubular function assessment, 156 Lysyl oxidase, 50^ Lysylpyridinohne sec Deoxypyridinoline
  
  ttj-Macroglobulin, biood-brain barrier permeability assessment, 560, 561 Macrophagc colons-stimulating factor ( M C S F \ 725 Macrosomia gcswuonal glucose intolerance, 420 insulin-dependent diabetes mellitus, 419 Magnesium dietao' sources. 109 110,210 distribution, 10** fluxes, 110 intraccllular functions, 109 Magnesium carbonate, 107 Magnesium deficiency hypucalcaemia, 100. 101 renal potassium loss, 53, 54 Magnesium metabolism, 109 112 chronic renal failure, 13K intestinal absorption, 110, 210 parathyroid hormone f FTH), 90 renal handling, 110 Magnesium plasma concentration, 109 chronic renal failure. 13ft enteral nutrition monitoring, 197 haemodialysis monitoring, 141 hypercalcaemia investigation, 98 hypcrthyroidism, 343 hypocatcaemia investigation, 101 hypokalacmia, 54 hypomagncsaemia. 111, 112 krtoacido&i*, 277 magnesium supplementation monitoring, 112 >«■ also Hypermagnci.aemia; iiypomagnesaemja Magnesium retention test, 111-112 Magnesium sulphate, 112, 213 Magnesium supplementation, 112 Magnesium tolerance test, 116 Magnesium urinary extrction hyperthyroidism, 343 hypomagnesaemia. 111 Magnesium-containing enemas, 110 Magnesium-related diarrhoea, 112 Malabsorption, 199 215 anaemia. 200 carbohydrate. 20b classification, 2 1 0
  
  clinical features, 199-200 diabetes mcUitus, 275 diarrhoea, 199-200 hypohpidaemia, 637 intestinal oxalate uptake, 170 investigation, 211 212 mechanisms, 210 211 enzymatic hydrolysis, 210 lymphatic duct obstruction, 211 mucosal enzyme activity, 211 mucosal transport, 21 1 HOiubilization processes, 210 metabolic acidosis/hypokalacmia, 51 rickets, 4 31
  
  steatorrhoea, 204) tertiary hyrxrparathyroidism. 90 Malaria anti-DNA antibodies, S40 haeinoglobinopathy hcterozygosiry protective effect, 483, 488, 745 haemolytie anaemia, 458 hypoglycacmia, 291 quinine-induced, 291 intrjvascular haemolysis 453 Male reproductive function, 402 410
  
  Материал за.
  
  нный авторским право
  
  INDliX Male reproductive function (ccntd) androgens, 403 h>-porhalamic-pituitary-testes feedback mechanisms. 403, 404 testes, 402-103 Male sexual differentiation, 402 Malignant disease acute uric acid ncphropathy, 696 anorexia, 695 ascites, 245
  
  carbohydrate metabolism, 693-694 cerebrospmal fluid (CSF) investigation, 566 dietary associations, 194 DNA analysis in detection, 754 endocrine sequelae, 701 -702 reproductive function, 702 somatic growth, 701 energy metabolism, 69) genetic mechanisms, 758-759, 761 hacmatological sequelae, 696-698 anaemia, 697-698 crythrocytosis, 696-697 leukocytosis, 698 hypercalcaemia JW Hypercalcaemia of malignancy hypcruncaemia, 696 hypomagnesacmia, 696 hyponatraemia, 696 lipid metabolism, 637, 694 minerals, 695, 696 obesity-associated mortality, 193 paraneoplastic condition*, 698-701, 701 dermatologies! disorders, 699 fever, 698 immune-mediated renal disease, 698 699 neurological symptom*, 699-701 pro-opiomclanocortin f P O M O gene expression, 689 prostaglandin metabolism, 694 protein metabolism, 694-695 syndrome of inappropriate antidiureric hormone, 696 trace elements, 695 vitamin», 695 weight loss, 693 Malignant hypcrpyTcxia, 549, 664 Malignant hypertension, 53 Malnutrition growth hormone, G T T response, 304 hypoglycaemia, 283 hypophosphataemia, 108 nutritional status assessment. 185, 186 hepatic secretory proteins, 187 immune response, 187 «Unfold thickness, 186, 187 nutritional support, 195 phcnylalanine loading test interference, 606 secondary diabetes mellirus, 268 see olio Protein-energy malnutrition Maltese, 206 Maltose, 205 Management applications, 3-4 monitoring progression of disease, 3 4 prognosis, 3 seventy assessment, 3 Manganese, 18:3-184 body stores assessment, 192 dietary sources, 183 functional aspects, 183 toxkity, 184, 192 Manganese deficiency, 183
  
  laboratory investigations, 192 Manganese-containing enzymes, 771 ■Mania, 572-573 clinical features, 572 Manic depressive psychosis, 572 Manniiol, 48, 123, HO, 131 Mannose, 260 Maple syrup urine disease, 449, 611 classic subtype, 61 1 clinical features, 441, 611 diagnosis, 449, 611 amino acids/organk acids analysis., 611 dinitrophenylhydrazme test, 611 enzyme assay, 611 heterozygoic detection, 611 intermittent subtype, 611 leueine plasma concentration, 611 neonatal screening, 441, 611 prenatal diagnosis, 611 ihUmine-responsive subtype, 611 treatment, 441,449, 611 Marasmus, 192 March hacmoglobinuna. 458 Mason type diabetes mellitus, 268 Mastocytosis, systemic, 52 McCune-Albright syndrome, 387 polyostobc fibrous dysplasia, 531 precocious puberty, 387, 388 MDMA (ecstasy), 576, 673 adverse effects, 673 overdose, 67 3 Mean corpuscular haemoglobin (MCH) hacmoglobinopathics, 490 thalassacmia, 485 Mean corpuscular volume (MCV) alcohol abuse, 244, 671 haemoglobinopathies, 490 thalassaemia, 485 Mechanical ventilation
  
  hypoxia, 86 neonatal respiratory distress, 425 poisoning, 664, 670, 675 Mcconium aspiration, 424 Meconium ileus, 253, 440 Medical audit, 14 Medicolegsl aspects, 5 Medium chain acyl CoA dehydrogenase (MCAD), 544 muscle fatty acid oxidation, 554 Medium chain acyl dehydrogenase (MCAD) deficiency, 446, 447, 554 Mcdroxyprogesteronc acetate, 636 MeduUary thyroid carcinoma calcjtonin tumour marker, 93, 353, 685, 686.715 family screening, 686, 715 provocative tests, 686 clinical features, 685-686 ectopic ЛСТН secretion, 306 multiple endocrine neoplasia 2 (MEN 2), 353, 682, 685 686 treatment, 686 Megaloblastic anaemia foUte deficiency, 203, 204 folaie supplements, 203 vitamin B , : deficiency, 203, 204, 205 Meiosis, 743 crossover events, 744 MELAS, mitochondrial transfer RNA mutations, 553 Melatonm, 370 Membranoprolifcratjve glomerulonephrins, 152
  
  829
  
  Membranous giomeruloncphrnis, 1 SO, 151, 152 microscopic haematuria, 150 selective proteinuria, 150, 152 Menadione, 179 Mcnaquinonc (vitamin K,)t 178, 179 Meningitis activated neurrophil metabolites, 559 CSF С-reactive protein, 562, 564 CSF lactaie, 559, 564 CSF lactatc dehydrogenase, 563, 564 CSF protein/glucose, 563- 564 CSF-plasma glucose ratio, 559 fungal, 559 investigations, 563
  
  metabolic effects of cerebral hypoxia, 559 viral, 559, 563-564 Meningococcal septicaemia, 356 МспксЧ disease, 618 cJinicaJ features, 4 37, 529, 618 copper transport defect/copper deficiency, 183, 437,618 Menopause alkaline phosphatasc (ALP) plasma values, 511 bone mineral density parBcrines, 726- 729 recovery phase, 718, 720 remn-angiotensin system, 357 sympathoadrenal system responses, 356 vasoacme substances, 729-7 30 Mctachromanc leukodysirophv, 590-591, 614-615 adult form, 590 cerebroside sulphatase (aryUulphatasc A) assay, 590-591,615 cerebrowde sulphatase (arylsulphatasc A) deficiency, 614 cerebroside sulphate sulphatase as&av, 591 clinical features, 615 diagnosis, 590-591 galactosyl 3-sulphate ccramide accumulation. 614 heterozygote identification, 591, 615 infantile form, 615 juvenile form, 590, 615 late infantile form, 590 prenatal diagnosis, 591, 615 Mctall о thi on e in Wilson's disease, 593 zinc metabolism, 182 Mctancphrines, urinary, 328 Methadonc abuse in pregnancy, 124 toxictty, 67 *
  
  Methaemalbumin, 459, 460 Methaemoglobin, ccTebrospinal fluid (CSF), 55Я Methaemoglobin reductase deficiency, 458 screening test, 462 Mcthae m oglob i naemia acute necroti/шц pancreatitis, 785 poisoning, 660, 677 Mcthanol (methyl alcohol; poisoning, 659, 671,672 clinical features, 671 haemodialysis, 665 lactic acidosis, 74 treatment, 671 Mcthicillm-induccd acute interstitial nephritis, 32 Methimazole, 332, 345 Methionine malabsorption syndrome ste Oast house urine syndrome Mcchionine metabolism, 450 paracetamol poisoning, 667 Mcthinninc synthase deficiency, 450 Methotrexate folic acid metabolism, 203, 695 rheumatoid arthritis, 534 therapeutic drug monitoring, 651-652 baseline changes, 647 clinical audit, 657 indications, 652 urgent analyses, 656 toxicity, 647 3- Mcthoxy-4-hydroxyphenylcthy lene glycol, CSF, 588 i-Methoxy-4-hydroxyphenylglycol (MIIPO) depression, 578 panic attacks, 577 Methyl histidine, 694 5-Methyl tetrahydrofolaie, 202, 203 *лг also Poryglutamate* Methylamphetamine, 672 Mcthyldopa, 399. 461, 681 Mcthylenc blue therapy, 677 Meihylcne chloride poisoning, 676 3,4-MethyIene,methampheiamine set MDMA (ecstasy) Methylmalonrc acidaemia, 448 Methylmv tonic aciduria, 450 Methylmalonyl CoA mutase deficiency, 448 Methylxanmines, metabolism in preterm infant, 425, 426,427 Mctoclopramide, 674 hypcrprolactinacmia. 303, 399 Mctoclopramide test, 30 * Metoprolol, 291 Meryraponc adrenal adenoma, 323 antiadrenal activity, 323 Cushing's disease, 308 Gushing 4 syndrome, 684 depression, 573 Meryraponc test, 299, 307 Mcryrosine, 686 Mianscrin poisoning, 675 Microangiopathic hacmolyric anaemia, 458 malignant disease associations, 698 red cell morphology» 459 P.-Microglobulin, 500, 562 bone metabolism, 93 CSF measurement, 562-563 haemodialysis-aseociaied deposition, 538 multiple myeloma, 499, 500 protemuru, 154, 156 cadmium exposure monitoring, 155
  
  M
  
  light chain proteinuna, 158 tubular function assessment, 128, 156 Microprolacunoma, 302 Microproteinuria, 143» 147 MicrosHlelhtc, 762 Microsatellite polymorphisms, 752, 753 Mid-arm circumference (MAC), 186 cnrcral nutrition monitoring, 197 parcnteral nutrition monitoring, 197 Midline fibrosis syndromes, 309 Midline granulomas, 309 Migration inhibitory factor (M1F), 720 Mineralocorticoid deficiency causes, 325 renal tubular acidosis type IV, 168 Mineralocorticoid excess causes, 32) congenital adrenal hypcrplasia, 326 hypokalaemia, 53, 54 metabolic disturbances, 320 non-respiratory alkalosis, 53, 77-78 sodium retention, 36 Mineraloconicoid (non-aldo«erone) secreting tumour, 325 Mineralocorticoid replacement therapy, 326 Mincralocorucoids, adrenal, 315, 316, 31H assessment tests, 320-321 basal aldosteronc measurement, 321 basal rcnin measurement, 321 potassium plasma concentration, 320 potassium urinary concentration, 320 receptors, 318 relauve potencies, 318 Minerals, 182-184 malignant disease, 695 nutritonal status assessment, 191-192 parcnteral feeding, 196, 197 short gut syndrome, 213 Minimal change giomeruloncphntis, 150, 151 children, 150 management, 150 proteinuna. 150 selective, 148, 152 Minisaiollite. 672 MinisateUite polymorphisms, 752. 753 Missensc mutations, 742 Mitochondria! disorders, C S F lactate, 566 Mitochondrial DNA mutations, 553, 745 Mitochondrial 26-hydroxylasc deficiency, 594 Mitochondrial oxidativ* metabolism cytochromes, 553 electron transfer flavoprotein/ dchydrogenase (ETF/ETFDH), 544, 546 enzyme cytochemistry, 552 muscle, 541, 544, 546 polarographic measurement, 552 reactive oxygen species leakage, 768 respiratory control ratio (RCR), 552 Mitochondrial respiratory chain, 544, 546, 551 blockade by poisons, 661 complex activities measurement, 552-553 complex 1 gene mutations, 553 complex Western blotting, 553 flux measurement, 552 mitochondrial genome mutations, 553 muscle ATP regeneration process, 546 Mitochondrial respiratory chain delects, 444, 552 CKS symptoms, 551, 552 CSF examination, 552 d>-namicVfunction tests in blood, 551-552
  
  i, 3ci
  
  нный авторским правом
  
  INDKX Mjtochundrul respirator>' chain defects (contd) enzyme cytochemistry, 552 mitochondnal cytochemistry, 552 mitochondria! cytochromc concentrations measurement, 553 mifochondrial morphology fragged -red fibres). 552 mitochondnal oxidations measurement, 552 molecular biology techniques 554 respiratory chain complex activities measurement, 552 553 Mitotanc, \2h 684 Mixed acid-base disorders, 81 Mixed connective tissue disease, 5 34 .Mixed gonadal dysgenesis (XO/XY), 381 Mixed meal tolerance test, 287, 2 ° 3 . 294 Mixed venous oxygen saturation (SvO,), 85 MODY type diabetes mclhtus, 268 glucokmase genetic abnormalities, 265, 2h8 Molecular clinical biochemistry, 2, 739-763 Molybdenum, 183 body stores assessment, 191 functional aspects, 183 high intake, 18) Molybdenum cofactnr deficiency, 183» 450 Molybdenum deficiency, 183, 191 Monoamine oxidase inhibitors, 578 toxjcity. 675 Monoamine oxidase (MAO), 681 Monoamincrgic ncimnransmitters, 574 Monoclonal gammopathy of unknown significance {MGUSj, 495 Monosodium unite (MSLH arthropathy see Gout Mood disorders, 572 574 Morphine overdose in ncooatc, 6f>0 therapeutic drug monitoring, 652 MR scan Addison't disease (primary hypoadrcnali&rn), 322 Cushing's svndromc ACTH-dependent, 307 adrenal tumours, 322 lumbar spine lesions, 565 metachromatic leukodystrophy, 590 multiple sclerosis, 565 phaeochn^mocytoma, 328, 686 pituitarv imaging, 303 pituitary tumours, 684 primary aldosteronism, 324 proluctinoma, 304 TSH-secrcttng pituitary adenoma, 346 Mu chain disease, 503 Mucin-likc glycop rote ins, 714 Mucopolysaccharidoscv 539 differential diagnosis, 6 1 6
  
  hcterozygotc detection, 616 mental handicap. 615-616 prenatal diagnosis, 616 rheumatological complications, 539 Mucopolysaccharidosis type I (Huricr's syndrome), 615 MucopolysacchandoMs type II (Hunter's syndrome), 615, 616 Mucopolysaccharidosis type HI (Sanfilippo syndrome), 615-616 Mueosal biopsy, 211 MuHerian inhibiting factor <MIF), 375, 403 age-associated changes, 375 anorcma (vanishing tcsus syndrome), 389 assav, 375
  
  male sexual differentiation, 375 Multiple acyl CoA dehydrogenation defect (glutanc acidunu type II), 445 Multiple carboxylasc deficiency, 448 Multiple drug resistance (MDR) phenotype, 500-501 Multiple endocrine neoplasia 1 (MEN I; Werner'* syndrome), 682, 683 685 adrenal cortical tumours, 684 carcinoid rumours 684 generic aspects, 682 hypercalcaemia management, ЗД hypoglycuemia, 2&4 insulin о ma, 288 pancreatic disease, 682, 683-4)84 gastrin-secreting tumours, 681 glucagonoma, 684 insulinomas, 683-684 management, 681 PPomas, 684 tumour localization, 683 vipomas, 684 parathyroid disease, 95, 684-685 generic mechanisms, 95 pituitary tumours, 684 prevalence, 68* tumour types, 682 frequency, 683 Multiple endoenne neoplasia 2 (MEN 2; Sipple syndrome). 682, 685-687 clinical features, 682, 685 family screening, 686 genetic aspects, 682 medullary thyroid carcinoma, 353, 682, 685 686 cakitonin tumour marker, 685, 686 management, 686 provocative test* for family screening, 686 MEN 2A, 327, 682, 685, 686, 687 MEN 2B, 327, 682, 685, 686, 687 parathyroid disease, 685, 687 phaeochromocytoma, 327, 682, 685, 686 treatment, 686 Multiple endocrine neoplasia, 682-687, 703 hereditary forms, 682, 6 « i sporadic forms, 682 Multiple myeloma, 497-501 albumin plasma concentration, 500 anaemia, 4*>9 p.-miiTOglobulin plasma concentration, 499, 500 Bence Jones proteinuria, 157 bone marrow biopsy, 499 bone/joint involvement, 96, 499, 538 chromosomal abnormalities, 501 coagulation defects, 500 copper metabolism, 695 diagnosis. 497-498 hypcrcalcaemia, 96, 98, 498, 499, 500 hyperkalaemia, 57 hypcrviscosity syndrome. 500 lg amyloidosis, 504 immunosuppression, 499 laboratory investigations, 498, 500 501 immunocytochemistry, 499 oncogenc abnormaliues, 501 paraprotcinaemia, 495, 496, 497 paraproteins, 3, 498-499 Bence Jones protein (BJP), 157, 498-499, 504 clinical effects, 5O0 CSF measurement, 566
  
  831
  
  immunoglobulin isotypes, 498 prognostic value, 498 peripheral neuropathy, 700 phenotypic markers^ 500-501 presenting features, 497 prognostic factors, 499-500 renal dysfunction, 132, 157-158. 498, 499. 500 thrombosis, 500 mgfyccnde/cholesterol plasma concentration, 694 Multiple sclerosis CSF immune response detection, 565 IgG ohgoclonal bands, 565, 566 CSF protein elevation, 567 diagnosis, 565 intrarhecal Ig synthesis assessment, 561 intrathecal IgG index, 561 ncopTcnn C S F level monitoring, 562 reactive oxygen species, 773 specific!ry/prcdictive value of positive test, 22 Multiplex PCR, 762 muscular dystrophy prenatal diagnosis, 756 Munchausen's syndrome, 287 Muscle ATP regeneration process, 543 544 glycogen breakdown, 545, 546 glycolysis, 544, 545, 546 mitochondria! respiratory chain cofactors, 516 oxidative carbohydrate catabolism, 544, 546 transphosphorytatton reactions, 544 ATPase activity, 543 carbohydrate metabolism glycogen synthesis, 263 insulin actions, 261 - 262 fibre types, 543 lactate production, 544, 545 mitochondnal metabolism (3-oxidation of fatty acyl CoA esters, 544, 546 fatty acid catabolism, 544 fatty acid/acyl CoA ester transport, 544, 546 respiratory chain, 544, 546, 551 phosphocreatme conversion to crcatine, 544 physiology, 543-546 structural aspects, 543, 544 Muscle biopsy, 547-548 enzyme analysis, 551 Muscle carbohydrate metabolism disorders, 547, 550 551 biochemical investigations, 551 functional dynamic tests, 550 histochemical investigations, 550-551 Muscle damage crcatine lunasc (CK) activity, 548-549 myogiobinuna, 549 Muscle disease, 543 555 classification, 548 genetically determined mvopathies, 549-550 investigations, 547
  
  muscle biopsy, 547-548 myoglobinuna, 549 plasma crcatine kinase activity, 548-549 plasma ептуте levels, 549 routine biochemical tests, 548 presentation, 547
  
  Материал, защиш
  crme ncoplasia 1 fMEN I), 685 renal failure patients, 106 renal ostcodystrophy management, 524 Parenteral nutrition, 195 anabo]ism-promoting additives, 196 hiotin deficiency, 181 change to cntcral nutntion, 196 chromium deficiency. 1^4 complications, 197 copper deficiency, 183
  
  essential fatty acids, 176 glutamine supplements, 196 hypophosphataemia, 108, 109 lactic actdosis type B, 74 lipid emulsions, 134 liver function te*t abnormalities, 249 manganese deficiency, 183 manganese toxjeiry, 1R4 molybdenum deficiency. 18* monitoring. 197 neonate, 436 437 cholestatic liver disease, 435 selenium deficiency, 183
  
  short gut syndrome, 213 sites, 195 solutions composition, 196 zinc deficiency, 182 Parkinson's disease, 77* Paroxetine poisoning, 675 Paroxysmal nocturnal haemoglobinuria (PNH>, 456, 458, 460 laboratory tests, 461-462, 465 red cell membrane protein defects, 458 Patent ductu» artcriovus (PDA), 425 Patient identification, 8 Patient preparation, 8 PCO» 63. 6Я acid-base status assessment, 69 derived units, 69 hydrogen ion concentration relationship, 80 mixed acid-base disorders. 81 non-respiratory abdosis, 71 respiratory aeidosis, 75. 76 respiratory alkalosis, 79 Pellagra, 180 carcinoid syndrome, 680, 703 llartnup disorder, 611 Pendrcd's syndrome, 349 Penctrance, 746, 762 PemciUaminc autoimmune hypoglycacmia, 292 copper binding, 182 cyuinuria, 166 Indian childhood cirrhosis, 252 monitoring, 10, 252, 542 rheumatoid arthritis, 534 vitamin B, metabolism, 180 Wilson's disease, 252, 593 Penicillin» proximal convoluted tubule secretion, 121 renal potassium loss, 53 Hentagastrin test, 686 Pentamidinc pcell toxicity, 291 hypoglycacmia, 283, 291 Pentose phosphate pathway, red cell, 454, 455 congenital defccta/haemolyttc anaemia, 456 Pepsin, 207 Peptide absorption, 207 Peptidc digestion, 207 Peptidc hormone*, 721 ACTH-dependent Gushing 4 svndromc^ 307 calcitonin response, 92 camnoid tumour secretion, 679 ectopic production, 687 immune function regulation, 357, 358 Perchloratc discharge test, 337 congenital hypothyroidism, 349 Percutaneous endoscopic gasrxostomy (PEG), 195 Percutaneous nephrostomy, 132
  
  835
  
  Pericarditis, 137 Perinatal asphyxia, 431 Periodic acid-SchifT (PAS) technique, 551 Peripheral neuropathy» 595 596 malignant disease associations, 700-701 Peritoneal dialysis glucose ahaorption/insulin requirement, 138 poisoning management, 665, 676 principles, 142 protein losses, 139 renal failure acute, 134-1*5 chronic, 142 hvpogrycaemia, 289 Pernicious anaemia bilirubin formation, 223 intrinsic factor (IK) deficiency, 205 Peroxide ion, 760 Peroxisomal disorders, 450-451, 591, 617 618 diagnosis, 451 treatment, 451 Pcthidinc poisoning, 673 Peirosal sinus sampling catheter, 307 p-glycoprotcin, 501 pH. 61-62 blood reference range, 61 Phaeochromocytoma, 327-328 catecholamines, 572 clinical features, 327 328 anxiety state, 571, 572 diagnosis 328, 572 ectopic ACTH secretion, 306 glucorcgulation abnormalities, 269 invcsugatkms, 328 multiple endocrine ncoplasia 2 (MEN 2), 682, 685, 686 treatment, 328, 686 Pharraacokinetics, 641-643 absorbed dose estimation, 642 absorption rate, 642 active metabolites, 643 apparent volume of distribution (Vd), 642 distribution half-life, 641 elimination, 641-642 elimination halt-life, 642 initial concentration, 641, 642 metabolism, 642 steady state conditions, 642-64* Op**, 64 * dose requirement calculations, 658 rapid achievement, 647 toxins, 659 volume of distribution, 641 Phenacctin ncphropathy, 155 Phenforrnin-mduced lactic audosis, 73 Phenobarbitone ghicuronyl transfcra&e deficiency type II, 434 therapeutic drug monitoring. 643, 652-653 Phenolphthalein, 213 Phenothia/me-induced hypcrprolactinaemia, ЗОЗ, 399 Phenotypc, 762 Phenoxybenzamine, 328 Phcnylalanine metabolism defects, CSF biopterins measurement, 566 nutritional aspects, 604 Phcnylalanine hydroxylase assay, 605, 606 Phcnylalanine hydroxylase deficiency clinical features, 604-605
  
  Материал, защищенный авторским правом
  
  836
  
  INDFA
  
  Phenylalaninc hydroxylasc deficiency (лад&О D N A analysis 00t>
  
  enzyme assay, ft05, (>l)o heterogeneity of gene mutations, 745 hctcroiygolcs detection, loading test*. 606 incidence, 604 phe no types hypcTphenyUUninaemia, 004 non-PKU hypcrphcnyialamnacmia, r>04 phcnylkcionuria IPKL'J .«л* Phenylkemnuna iTKU) phenylalaninc plasma concentration, 604 phenylpsruvk avid urinary excretion, 604 Phenytalanine loading tent, 606 Phenviketonuna (PKL'>, 604 clinical features, 604-605 mental handicap, 6Л4 diagnosis, 605, 606 differential diagnosis, 605 DNA analysis, b06 genetic heterogeneity, 745 hetcro;cygotcs detection, 606 mtrauicrinc growth retardation^ microcephaly. 42 3 management, 439 preventive phe n >M anine-frce diet, 144, 605 residual neurological deficit, 43*) maternal, 604. 605 dietarv control. 605 teratogeme effect, 605 neonatal screening, 4, 22, 43ч, си)4 phenylalaninc hydrox) lase deficiency, 604 phenylalanine plasma concentration, 604 Phenytoin dosage adiustment, 64H dosage prediction protocol, 658 dose-plasma concentration relationship, 648 drug-induced acute porphyria, 47*» free drug assays. 649 intestinal calcium transport impairment, 418 paracetamol interaction, ььь poisoning management, 664, 66**, 675 teratogenicily, 660 ihcrapcutic drug monitoring, 6 4 3 , 645, 646, 653 indications. 653 urgent analyses, 05t> thyroid function effects, 341 U>xiciry, 647 valproate interaction, 649 Phosphate dieiar>> ' 0 4 chronic renal failure, 139, 140 distribution, 102 fluxes, 103, 104 intracclluJar, 102 103 cytosohc, 102, 103 inorganic. 102- 103 organic compounds, 102 redistribution, 103 parcnteral feeding, 197 Phosphate buffer system. 0 3 non-respiratory acidosis, "1 respiratory acidosis, 76 uritWi 66, 71 Phosphate deficiency acute «yndromc, 108, 109 rickets of prematurity, 4 31 tissue oxygenai ton, 102 Phosphate enema, 106
  
  Pho&phate infusion hypcrcalcacmia management, 98 hyperphosphaiaemia, 106, 10? Phosphate metabolism, 102 109 bone, 103 chronic renal failure, I 38 homeostasis. 103-104 intestinal absorption, 103, 104 neonate, 4 30—132 disorders 430-432 puirenteral nutrition, 437 regulatory factors, 104 105 renal 25COIDD la-hydroxyUtion, 9 1 , 103 114 renal osteodystrophy, 521 renal tubular transport uv Renal phosphate reabsorption trunsplacental transport, 430 urinary excretion, 103 Fancnni syndrome, 168 renal tubular acidoftis type 2 i proximal h 74 nckrt* of prematurity, 4 31 Phosphate plasma concentration, 105-106 childhood, 105 chronic renal failure, 138 diumal variation, 105 106 enteral nutrition monitoring, 197 haemodialysis monitoring, 141 hypocalcaemia investigation, 101 hypoparathyroidism, 101 hypophosphataemia diagnosis, 108 inorganic ions, 102 keioacidosis, 277 malabsorpiion investigation, 211 muscle disease, 548 obteomaJacu/rickcts, 518 osteoporosis, 515 parenteral nutrition monitoring, 147 plasma protein bound, 102 postprandial variation, 106 renal osteodystrophy, 522 523 renal transplant monitoring, 142 respiratory alkalosi*, 74 ш abv Hyperphosphataemia Phosphate replacement therapy, 109 complications, 109 Fanconi syndrome, 169 hypophosphatacxnic osteomatacia, 520 Phosphate-binding agents aluminium hydroxide, 522 aluminium toxiciTy, 140 chronic renal failure, 107, 140 hypcrmagnesacrma, 112 hypophosphataemia, 108 phosphopenu; osicomalaeia. 519, 520 renal osteodystrophy management, 524 PhosphoothamMamine urinary excretion, 521 Ph dynamic tests, 300-301 protocols, 301, 311-313 releasing hormone tests, 300 water deprivation test, 300-301 Pituitary imaging, 302-303 hyperprolacunacrnia, 309 precocious puberty, 391 Pituitary irradiation, 302, 304, 308 Pituitary replacement therapy, 311 Pituitary surgery acromeg&ly, 305 Cushmg's syndrome, ACTH-dcpendent, 306 post-surgical assessment, 308 functionless pituitary adenomas, 30R prolacunoma, 304 TSH-secrcting pituitary adenoma, 308 vasopressin deficiency following, 311 Pituitary tumour a subunit assay*, 330 h>*pogonadotrophic hypogonadism, 407 impotence, 400 lateral extension with nerve defects, 302 metastatic, 300 multiple endoenne neoplasia 1 (MEN 1), 682, 684 pituitary function monitoring, 302 post-treatment, 301 302 pituitary irradiation, 302 portal circulation interference, 206 visual field loss/optic chiasma pressure, 206,302 uv aho Pituitary adenoma PlYKA, vitamin К deficiency, 100 PLacental aromatasc deficiency, 377, 380 Placcntal sulphatasc deficiency, 377 Plant toxin poisoning, 677, 678 Plasma proteins acute phase response, 735 736 buffering capacity, 63 cerebrospinal fluid 2 Fanconi syndrome, 160 hypcrcalcaemia, 05 hypcrthyroidism, 543 laboratory investigations, 40-41 hypertonic saline infusion, 41 water deprivation tesi, 40-41 management, 41 post-piturtary surgery, 301 pregnancy'» 38-40 primary, 57-38 secondary» 37-38 polydipsia see Polydipsia Population gene frequencies, 745 Population screening, 4 Porphobilinogcn (PBti) acute intermittent porphyria (All*:, 470, 471 acute porphynas, 460 neurovisceral attacks, 478 haem biosynthesis. 467 Porphobilinogcn (PBCi) dcaminasc, druginduced inhibition, 470 Porphobilinogcn fPBG) urinarv excretion, 787 acute intermittent porphyna (AIP). 471 acute porphyria attack diagnosis, 480 hereditary coproporph>Tia, 473 lead toxkity, 470 variegate porphyria, 472 Porphyria cutanca tarda, 470, 475 477 acquired aetiology, 460 alcohol abuse, 24 3, 244 liver disease association, 476 associated medical conditions, 477 asymptomatic carrier detection, 476 biochemical features, 244 clinical features, 476 cutaneous lesions, 476, 480 differential diagnosis* 476—477
  
  Материал
  
  [вторским правом
  
  838
  
  INDEX
  
  Porphyria cutanca tarda (ouuJ) hereditary form, 475, 476 investigations, 476 photoscnsuizfttion, 476, 478 porphyrins accumulation, 475 476 7-carboxylatc porphynn, 476 5-carboxytate porphynnogen, 476 uroporphyiin, 476 sporadic (type I), 475, 476 precipitating factors, 476 toxic form, 476 treatment, 477 uroporphyrin-produang liver tumours, 476 Porphyries, 2 5 3 , 46Q-480 acute, 469 clinical feature». 470 acute neuroviaceral attack» acute abdomen, 787 diagnosis 480 mechanisms, 478 4 7 0 biochemicaJ tests, 479 480 classification, 470 cutaneous lesion*, 480 differential diagnosis. 567 genetic aspects. 469 latent case* identification, 479 non-acute, 469, 470 peripheral neuropathy, 507 photoscnsmviry, 469, 470, 472, 473, 474, 4 7 5 , 476, 477 mechanism, 478 prenatal diagnosis, 480 terminology, 469 treatment, 597 Porpbyrin gallstones, 474, 475 Porphynnogcns, haem biosynthesis, 468, 469 Porphyrin? faecal, 479 porphyria cutanca tarda, 480 variegate porphyria, 480 haem metabolism, 467 photosynthetic activity. 468 urinary, 479 erythropoietic porphyria, 480 hepatocrythropoieac porphyria, 477 porphyria cutanca tarda, 480 Porphyrtnuria, asymptomatic, 25 5 Portal п у р е п с т ю п , 244 Portal vein thrombosis, 234 Ponocaval shunt, 244 Positional cloning, 76 2 Postan a lyrical sources of error, 14 Postcoital test ( P C T ) , 400 Postheparin lipolyhc activity fPHLA), 639 Postnatal depressive illness, 340 Postpartum haemorrhage acute tubular necrosis, 132 Shcehan's syndrome, 3l>9 Postpartum psychosis, 573- 574 Postpartum thyroiditis hypothyroidism, 352 thyroid peroxidase autoantibodics, 340 transient hypcrthyroidism, 351, 352 Postprandial syndrome, 293 idiopathic hypogtycaemia, 293-294 Post-translational modiGcation, 688, 742 pro-opiomelanoo>rtin (PC)M(*J gene products. 690 Postural proteinuria, 146 Posture-related variation, 9-10 Potassium, 2 5 - 6 0 chronic renal failure, 138
  
  colonic secretion/retention, 31 comparison of results with reference limits, 18 dietary chronic renal failure, 139, 140 deficiency, 320 disorders of metabolism, 49-58 distribution, 25, 30 with acidosis, 71 catecholammes, 30 insulin. 30 extracellular fluid fECF). 30 intraccllular concentrarion, 29 30 nconatc, 426 neuromuscular function, 30 non-respiratory alkalo&is, 77, 78 parenteral feeding. 197 renal metabolism i** Renal potassium handling retention» 56 urine concentration, 31 Potassium chlorate poisoning, 676 Potassium hydrogen phosphate, 109 Potassium nitnte/nttratc poisoning, 676 Potassium plasma concentration, 29 acute brain syndromes, 570 acute renal failure, 133 Addison's disease (primary hypoadrenalismj, 322 aldosterone regulation, 316 ascites treatment monitoring, 245, 246 enteral nutrition monitoring, 197 haemodialysis monitoring, 141 ketoacidosis, 2 7 6 , 2 7 7
  
  mincralocorucoid* assessment, 320 muscle disease, 548 parenteral nutrition monitoring, 197 polyuna, 40 renal transplant monitoring, 142 w aiio Hyperkalaemia; Hypokalaemia Potassium replacement therapy, 54 familial hypokalaetnic periodic paralysis, 50 intravenous therapy, 5 4 - 5 5 non-respiratory alkalosis, 78 oral preparations, 54 renal tubular acidosis type 1 (classic distal), 168 Prader-Willi syndrome, 371, 746 gonadotrophin deficiency, 389 Pnihdoxime, 670 Prealbumin ccrebrospinal fluid (CSF), 560 flow reduction measurement, 561 nutritional status assessment, 187 pregnancy-associated changes, 419 stress/acute phase response, 718 Prcanalytical source* of error, 8 11 biological factors, 8-11 age, 8 9 body mass, 4 sex, 9 drug intake, 10 exercise, 10 food intake, 10 mrxuuic biological variation, 10 11 posture, 9-10 stress-related changes, 9 technical factors, 8 timc-dependeni changes, 6 Precision, 12, 13, 15 Precocious puberty, 387-388 adrenal androgens, 318 cause», 387
  
  endocrine investigations. 391 G n R H agonist rrcaiment t 388 polyostotic fibrous dysplasia, 5 31 variants, 388 Predictive value of lest», 3, 19-2) calculation, 2 1 , 22 definition, 20 multiple testing, 22 practical applications, 22 prevalence relationship. 21-22 receiver operating characteristic (К(Х1) curves, 22-23 relation to true/false positives/negative results, 20 selection of cut-off point, 21 sensitivity relationship, 21 specificity relationship, 21 Prcdnisolonc, 150, 1 2 1 , 6 8 2 l*re-cclamp*ia fetal well-being assessment, 4 1 7 - 4 1 8 inrrauterine growth retardation, 423 liver damage, 2 4 8 proteinuria, 420 sodium retention, 35 Pregnancy, 41 * 420 acute fatly liver, 248 alkaline phosphatasc plasma concentration, 225, 2 3 4 , 5 1 1 isoenzymes, 226 argmmc vasopressin (AVP), 3R, 39-40 associated biochemical changes, 418-420 biochemical diagnosis, 413 414 biochemicaJ monitoring, 414 416 chronic renal failure, 363 diabetes mcUttus gcstational diabetes, 267-268 glycaemic control. 268, 279 dietary' protein requirement, 176 Dubin-Johnson syndrome, 248 endocrine change*, 418 energy requirement, 174 fetal malformation screening, 415 416 fetal well-being assessment, 417—418 tolate requirement, 203 gcstational age assessment, 41 3, 414 glucose tolerance, 4 ) 4 - 4 2 0 Grave*" disease management, 345 human chononic gonadotrophin (hCG) assay, 413-414 hyperprolactinacmia, 303 hypothyroidism management, 351 hypouricaemia, 536 intestinal calcium absorption, 88 intruhepatic c-holestusu, 248 intrapartum fetal monitoring, 418 iron absorption, 201 iron requirement, 201 jaundice. 248 liver function change*, 248 liver function test*. 248 osteocalcin plasma values, 512 placental 1,25-dihydroxyvitanun Г> synthesis, 92 plasma lipiuMipoprotein, 419 plasma proteins, 4 1 8 419 poisoning, 659 660 polydipsia, 39 polyuria, 38 4 0 pregnancy-associated plasma protein A (PAPPA), 414 progesterone effects. 398 piolacran plasma concentration, 297 pfotcinuria, 147, 420
  
  Материал з
  414 sex hormone-bin ding globulin (SHBG), 399 sodium retention, 55 3b thyroid function tests. 334-335 thyroid pcroxidase autoantibodics, 340 zinc requirement, 182 Pregnancy loss, early biochemical monitoring, 414 415 human chononic gonadotrophin ihCG), 413-114 s5 androgen inscnsitivity ss-ndrome, 385 DNA analysis, 752-75 3, 761 in fertilized embryo*, 753 Fabry's disease, V'7 Gaucher's disease, 592, 614 G 4 , gangliosidnsis, о I 3 hacmoglobinopathics, 491 homocysrinuria, 6 1 ft 11 (5-hydroxylaw deficiency, 380 21 -hydroxylase deficiency, 380 hypophosphatasia, 521 Krabhe's leukodystrophy. 591 Ixsch-Nyhan syndrome, 617 maple syrup urine disease, 61 I mctachromatic leukodystrophv, 591, 615 mucopolysaccharidoses, 616 muscular dystrophy, 756 Sandhoft disease, 613 Tar-Sachs disease. 61 3 tracking of mutant genes, 751 urea cycle disorders, 608 Preterm infant. 423 acidosis, 428 alkaline phosphatase (Al-P) levels, 432 amino acid plasma concentration, 449 ammoaciduna, 449 apnoea of prematurity, 425 blood sampling-associated anaemia, 425 calcium/phosphate metabolism, 430 carmtinc, dietary requirement, 1Я2 fl u id/el ectro I yt cs drugs affecting balance, 427 requirements, 426, 427 hypernatracmia, 427 hypocalcacrma, 100, 431 hypnglycacmia, 429 insensible water loss, 427 kemicterus, 233, 4 33 maternal drug abuse, 424 parenteral nutrition. 436—437 hcpatoccllular necrosis, 249, 437 infant preparations, 437 metabolic bone disease. 437 trace element deficiencies, 437 physiological jaundice, 233, 433 poisoning, 660 renal function, 425, 426
  
  acid-base status, 428 test interpretation, 428 rickets of prematurity, 431 thyroid hormones, 440 total body water, 426 transient hyperphenyialaninacmia/ tyrosinaemia, 435 vitamin E deficiency, 178 zinc requirement, 182 Prevalence calculation, 21, 22 definition, 20 predictive value of tests, 21 22 Primidonc, active metabolite Cphenobarbitonc} monitoring, 643, 652 Probenecid, 536 ProcainAmide autoimmune hypogtycacmia, 292 therapeutic drug monitoring, 653 Prochlorpcnvinc» 303, 471 Procollagen biosynthesis, 508 Procollagen fragment bone formation marker, 512 Procollagen peptide III (PII1P) assay, 230 Progesterone adrenal secretion, 315 bulimia nervosa, 371 chronic renal failure, 36 3 corpus lutcum secretion. 596 earty pregnancy loss, 415 ectopic pregnancy, 787 endometrial effects, 395 1-H-induced ovarian secretion, 396 lipoprotein plasma concentration effects, 636 luteal deficiency, infertility association, 400 menstrual cycle. 9, 396 hitcal phase, 395, 400 oral contraccptis'c pill.. 420 ovarian biosynthebe pathways, 397, 398 ovulation assessment, 400 pituitary-gonadal axis assessment, 298 poscpartum psychosis, 573 pregnancy-associated changes, 418 pulsatile sccrcnon, 400 superovulation monitoring, 401 transport, 398 170H-Progestcrone, congenital adrenal hypcrplasia marker, 378-379 neonatal screening, 379 prenatal diagnosis, 380 Progesterone agonist», synthetic, 398 I'rogcsterone antagonists, synthetic, 398 Progestogens actions, 398 excrenon, 398 hormone replacement therapy (HRT), 420, 421 oral contraceptive pill, 420, 421 structural aspects, 396 Progestogens, synthetic, 420 early pregnancy loss prevention, 415 lipoprotein plasma concentration effects, 636 Pregnanediol, 398 Prognosis, 3 assessment acute liver failure, 235 chronic liver disease, 234-235 Prognostic index (PI), 235 Progressive external ophthalmoplcgia, chronic, 553 Proinsulin, 259
  
  839
  
  hypennsuJuraemia assessment, 266 metabolic activity, 262 plasma concentration, 280 fasting hypogtycacmia, 286 insuhnoma, 288, 683 structure, 260 Prolactin affective disorders, 369 anterior pituitary synthesis, 295, 396 basal pituitary function investigation, 293, 311 depressive illness, 579 diurnal variation, 9 dynamic tests of secretion, 303 hormonal regulation of release, 297, 396 in male, 403 portal circulation dependence, 297 immunomodulatory properties, 359 lactation, 297, 396 pregnancy, 297, 418 psychotropic drug effects, 370 schizophrenia, 369 stress-induced secretion, 9, 2^7 structural aspects, 396 tryptophan-induced secretion, 579 Prolactinoma, 303-304 dopamine agonist therapy monitoring, 304 hyperprolactinacmia, 303 management, 304, 684 multiple endocrine ncoplasia 1 <MKN 1), 684 pituitary hormome assessment, 303 304 presentation, 304 Proliferativc glorncruloncphritis, 151, 152 microscopic haemamna, 152 selective protcinuria, 152 Prolonged fast test, 285, 294 insulinoma, 288 Promoter, 688, 741, 762 Pro-opiomclanocortin (POMC) gene, 689 ACTH ectopic production, 689-690 gene products, 689 non-pituitary tissue expression, 689-690 pituitary tissue expression, 689 post-translational modification of gene products, 690 transcription control, 690 Pro-osteocalcin, 509, 512 Propionic acidaemia, 448 Propionyl CoA carboxylasc deficiency, 448 Propranolol, 50, 328, 342, 674 drug-induced hypoglycacmia, 291 Propyithiouracil, 332, 337, 345 Prostacyclin (PG1.), 623 actions, 623, 727 biosynthesis, 623, 727 metabolism, 727 Prostaglandin D , ( P G D J , 694 Prostaglandin E. (PGE J actions, 727 glomemlar filtration regulation, 121 endogenous pyrogen activity, 727 half-life, 727 malignant disease, 694 Prostaglandin endoperoxide synthase, 623 Prostaglandin F : , 121 Prostaglandin G 3 (PGGj) biosynthesis, 623 reactive oxygen species, 768, 769 Prostaglandin H, (PGH\) biosynthesis, 623 Prostaglandin synthase inhibitors elderly patients, 57
  
  Материал, защищенный авторским правом
  
  840
  
  INDEX
  
  ProstagUndin synthase inhibitors ' intake, 176, 207 chronic renal failure, (39 endogenous, dietary contribution, 207 nutritional status assessment total body protein, 1 KB 189 visceral proteins, 187 posture-related variation, 10 Protein C . 179 Protein catabohc rate [PC-RA 141 Protein metabolism absorption, 207 diiuca! aspects. 207-208 investigations, 20H chronic renal failure, 119 digestion, 207 liver, 220-221 malignant disease, 694 695 metabolic response to miury, 719, 720 renal conservation mechanism, 143-146 starvation, 368 Pmtem S. 179 Protein synthesis gene transcription, 687-688 post-translational modification, 688, 742 secretory proteins, 688 RNA translation, 6Й8, 711 initiation site, 741 Protein-deficient pancreatic diabetes, 268 Protein-energy malnutrition, 192-19* hypogUvurrnia, 292 third world children, 192-19* clinical classification, 192-19* nutritinnal d*arf, 19 5 western adults, 19* zinc deficiency, 182 Protein-losing enteropathy, 208 Proteinuria, 123, 124, 143-160 appearance of urine, 124 classification» 147 clinical correlates, 148 clinical investigation, 159 protocol, 160 diabetes mellitus. 158, 273
  
  drug-induced, 542 exercise-induced, 146 glomcrular, 125, 147-154 ftlomerulonephritis, 150, 152 mechanism's, 147 148 pathophysiological consequences, 151-154 renal histology relationship, 152 153 selectivity, 148, 150, 152 steroid-responsive, 150, 192 nephrogeme, 125 non-renal disease, 15Я-159 overflow, 125 postrcnal pathology, 159 postural in healthy patient, 146 pregnancy, 420 prerenal origin, 156-158 proteins of renal origin, 145 renal disease, 147-158 acute intrinsic renal failure, 131 chronic renal failure, 135 renal transplant monitoring, 142 tubular, 125, 154 renal disorders, 154 155 urine specific gravity, 124 u n n c stick tests. 160 ш alw Urinary protein excretion /Vwfuf infection, 170 Prothrombm time (PT), 228 acute liver failure, 219, 240 prognosis assessment, 240 hepatitis. 3 chronic active (САН), 241 progression li> acute liver failure, 239 hvrr function tests, 227 228 chronic disease prognosis assessment, 235 malubsorption investigation, 211 neonatal hepatocellulax disease, 434 paracetamol poisoning, 666 667 Rcye's syndrome, 250 sahcylate poisoning, 66H vitamin К status assessment, 190 Proto-oneogcncs, 758 Protoporphynn, 469 aminolacvuhnic acid dchvdratase dcficicncv porphyna, 47 3 eTythropojctic protoporphyria, 474 haem biosynthesis, 467. 468 -469 lead toxicity, 479 variegate porphyna, 472, 480 Proioporphyrtnogen IX haem biosvnthcsis, 4o7 variegate porphyna, 472 Protoporphyhnogen oxidase. 467 Protoporphyhnogen oxidase deficiency Chester porphyna (dual porphyna), 478 variegate porphyna, 471 Proximal convoluted tubule albumin catabolism, 15 ? a m m o acids reabsorption, 164 bicarbonate reabsorption, 1б5 functional aspects, 121-122 functional assessment, 128 glucose reabsorption, 164 hydrogen ion excretion, 164, 165 phosphate reabsorption, 164 protein catabolism. 145 protein reabsorption, 145 sodium reabsorption. 164 structural aspects, 117, 118-119 Pruritus, 242, 248 Pseudo-Cushing*» syndrome chronic alcoholism, 36b, 671
  
  secondary diabetes mellitus, 268 Pseudogout, 536 Pscudohermaphroditism, Female, 376, 377-380 causes, 377 congenital adrenal hypcrplasia set Adrenal hypcrplasia, congenital feminizing testis syndrome, 406 maternal androgen-secreting tumours, 380 5(t-rcducusc deficiency, 406 testosterone hiosynthetic ctuyme defects, 406 Pseudohermaphroditism, male, 376- 377 decreased testosterone production, 381 382 33-hydroxystcToid dehydrogenase deficiency, 378, 380 normal testosterone production, 383- 385 testosterune metabolism abnormality, 382 383 Pseudohyperaldosteronism, 53 Pseudohyperkalaemia, 320 familial. 55, 57 Pscudohypcrphosphatacmia, 106 Pscu dohypoaldostcron ism type 1, 33, 57 type 2 (tiordon's syndrome), 57 P«eud«ihyponatraemi4, 43-44 Pseudohypoparathyroidism classification, 100 Ellsworth-Howard test, 100, 102 protocol, 114 hypocalcaemia, 104.», 101 type I, 100 type II, 100 vitamin D deficiency management, 102 Picudtwu>nai sp. infection, 170 Pseudoporphyria, drug-induced, 476-477 Pbeudoprecocious puberty, 388 Psychiatric disorders, 569 584 aetiological aspects. 569 biochemical investigations. 571-572 endocrine aspects, 369 370 functional disorders, 569 hypcrvcntilation syndrome, 572 mental state examination, 569 mood disorders, 572- 574 neurotransmitter studies, 574-578 doparmncrgic systems, 574-575 serotoncrgic systems. 575-577 organic brain syndromes, 570-571 organic disease associations, 569-570 postpartum psychosis, 573-574 Psychogeruc polydipsia, 4 0 acute diluuonal hyponatracmia, 45 see ahtr Compulsive water drinking PsychtJtropic drugs, endocrine effects, 370 Ptcroyl glutamic acid ffolic acid) ш Folace Puberty, ЗЯ5-387 adrenarchc, 387 alkaline phosphatasc I'AIJ*) levels, 432 body composition, 385, 386 breaking of voice, 386 breast development, 385 delayed, 388-390, 391 endocrine changes, 386 387 facial hair growth, 386 growth spurt, 385. 386, 702 gynaecomasna, 40K menarche, 385, 386. 393 penile growth, 386 physical signs, 385- 386 precocious, 387-388, 391
  
  Материал, защип
  
  t . авторским п
  
  INDEX Puberty {coiud) pubic/axillary hair growth, 185, 386 testicular volume, 386 Pubic hair growth, 385, 386, 388 Pulmonary disease joint involvement, 538 tissue oxygenation, 8*3 85 A C O „ 8V84 ГаО ; ,'» 3-Я4 shunting, 83 ventilation-pcrfusion imbalance, 83, 84 Pulmonary embolism, 785 chest pain, 779 ncphrotic syndrome, 153 Pulmonary surfactant, 424 administranon to at-nsk nconatc, 425 Pulmonary wedge pressure monitoring, 131, 133 Pulse oximetry, 83, 425 Pure red cell aplasia, 697-698 Putrescine excretion, 694 Pyelonephritis, acute, 125 Pyloric stenosis, 78 Pyloroplasty, 29 3 Pyridinolinc fhydroxylysylpyridinohnc), 509 bone turnover marker, 94, 512 urinary excretion, 512 diurnal rhythm, 51 3 Pyridoxal 5-phosphatc plasma concentration, 190 hypophosphatasia, 521 PyridoxaJ, 180 Pyridoxarmne, 180 4-Pyndoxic acid urinary excretion, 190 Pyridoxinc set Vitamin B> Pyrimeihamine, 203, 477 Pynmidine 5'~nucleotidase deficiency chronic hacmorync anaemia, 457 haemolysis investigation, 459-460 red cell glutathionc concentration, 463 screening test, 462 Pyruvatc cerebrospinal fluid {CSF), 559, 566 energy metabolism, 258, 259 plasma concentration non-insulin-dependent diabetes mellitus ftflDDM), 266 respiratory chain defects, 552 Wcrnickc-KorsakorT syndrome, 571 Pyruvatc carboxykinase deficiency, 443 Pyruvate carboxylase deficiency, 444 lactic ackkms, 74
  
  type В (neonatal presentation), 444 hypcrammoiiacmia, 448 Pyruvatc dehydrogenase deficiency, 443, 444 clinical features, 444 CSF investigation, 566 lactic acidosis, 74 management, 444 Pyruvatc kinasc (PK) muscle disease-associated elevation, 549 red cell 2,3-DPG/ATP relationship, 463 Pyruvatc kinasc (PK) deficiency chronic haemolytjc anaemia, 456-457 enzyme activity measurement, 464 glycolytic intermediates elevation, 463 red cell 2,3-DPG concentration, 462, 463 screening test, 462 Pyruvatc metabolism disorder, 443 444 C S F investigation, 559, 566 Wemicke's encephalopathy susceptibility, 571 Pyruvate tolerance test, 571
  
  6-Pyruvoyltetrahydroptenn synthase (6-PTS) assay, 605 Quality assurance, 13-14 external, 13 internal, 13 therapeutic drug monitoring, 656-657 Quinagolide, 304 Qumidine, therapeutic drug monitoring, 653 Quinine drug-induced hypoglycaemia, 283, 291 infant accidental overdose, 660
  
  Radiation enteritis, 206 Radiographic contrast dyes, 100 Radioiodine treatment, 345, 346, 351 Radioisotope techniques bone scanning, 98 calcium intestinal absorption, 94 glomerular filtration rate (GFR) measurement, 128 Graves' disease, 344 Hashimoto's thyroiditn, 351 hyperthyroidism, 345, 346 osteoporosis, 516 Pagct's disease of bone, 527 phacochrornocytoma, 328 primary aldosteronism differential diagnosis, 324- 325 renal function assessment, 124 thyroid gland imaging, 331 thyroid starus evaluation, 337 Rapid tests, 3, 12, 13 ras mutations, 758 colorectal cancer, 761 RB gene mutations, 758, 759, 761 Reactive oxygen species, 767-768 assessment, 774-776 antioxidants assays, 776 cytoprotectrvc enzymes measurement, 776 DNA base adduct formation, 775 exogenous spin traps, 774 lipid pcroxidation measurement, 774 775 protein oxidation measurement, 774 associated pathology, 772-774 atherosclerosis, 774 carcinogencsis, 772 hacmochrornatosis, 772 inflammatory response, 73 ischaemic reperfusion injury, 772-773 neurological disorders, 773-774 eicosanoid metabolism, 768 endothelium-dcnved relaxing factor (EDRF), 768 leakage from mitochondria! electron transport chain, 768 respirator)* burst, 767-768 Reaven's syndrome itt Hypcnnsutinacmia syndrome Receiver operating characteristic (ROC) curves, 22-23 Recessive inheritance, 744, 762 X-linked inheritance, 745 Recombtnant DNA diagnostic techniques, 618 Records, 14 Rectal villous adenoma, 51 Recumbent position, 9, 10 Red cell casts, 160 acute intrinsic renal failure, 131
  
  Мг
  
  841
  
  Red cell enzyme defects, 464 congenital haemolytic anaemia. 456-457 enzyme activity measurement, 464 glycolytic intermediates, 463 464 laboratory investigations, 462-464 metabolite concentrations, 462-463 screening tests, 462 Red cell folate folatc deficiency, 204 maUbsorption investigation, 211 vitamin В.. deficiency, 204 Red cell fragility/survival, 461 haemolysis investigation, 460-461, 465 Red cell indices, malignant disease, 696, 697 Red cell membrane defects» 464 acquired haemotytic anaemia, 458 congenital haemolytjc anaemia, 456 Red cell morphology, haemolysis investigation, 459-460, 465 Red cells aminotransferase, 190 anaerobic grycolysis congenital defects, 456-157 ATP levels, 462, 463 coenzyme A, 191 2,3-DPG concentration, 462, 463 free protoporphyrin, 201 functional aspects, 453 454 glutathione concentration, 463 glutathionc rcductase, 190 haemoglobin, 453, 454 membrane composition, 454 metabolic pathways, 454, 455 anaerobic gjycolysis. 454 glutathionc metabolism, 454 nucleottde metabolism, 454 pentose phosphate pathway, 454 structure, 454 5o>Rcductasc. 403 5a-Reductasc deficiency, 406 diagnosis, 383 external genital phenotype, 376 investigations, 390 male pseudohenruiphmditism. 376. 383 management, 406 molecular genetics, 383 virilization at puberty, 383, 406 Reference distribution, 17 Reference interval, 17 Reference population, 17 Reference ranges, 3 food intake-related variation, 10 intrinsic biological variation. 10-11 nconatc, 441
  
  amino acid plasma concentration, 449 posture-related variation. 9 10 Reference sample, 17 Reference values, 16-17 age-related variation, 8-9 body mass-related variation, 9 calculation, 17 comparison with observed results, 17-19 disadvantages, 17 posture-related variation, 9 10
  
  pregnancy-related changes, 9 results outwith limits without disease, 17 sex-related variation, 9 stress-related changes, 9 time-dependent changes, 9 Reflux nephropathy, 136 Rcfsum's disease, 596-597,617 clinical features, 5 % , 617 neurological disturbance, 596
  
  ;:иал, защищенный авторским правом
  
  842
  
  iNDHX
  
  Rcfsum's disease [, 1 38 assay, 90, 97-98 pathogenesis, 135-136 phenylalanme loading test interference, 606 phosphate metabolism, 104, 138 plasma renin activity, 364 polyuria, 37, 40 potassium metabolism, 56, 138 pregnancy, 418 progression, 135 136 creatininc plasma concentration against time, 127 protein metabolism, 1 39 renal lu-hydrolasc deficiency, 518 renal osteodystrophy, 138, 518 salt-losing nephropathy, 32 sex hormones, 363 «odium metabolism, 137, 364-365 therapeutic drug monitoring, 64 3, 645, 647 thirst, 40 thyroid disease diagnosis, 364 thyroid function, I 39, 364 uraemic encephalopathy, 589 uraemic svndrome, 1 >5, 1 36-139
  
  Материал, защиш
  
  y>oglycaemia, 292 clinical features, 534 C-reactive protein, 541 erythrocyte sedimentation rate (ESR), 540 incidence, 534 management, 534 pleura] fluid analysis, 542 rheumatoid factor (RF)« 539 scrum amyloid protein A (SAA), 541 Rheumatoid factor (RF)> 505, 539 clinical applications, 539 immunoassay, 539 latex agglutination test, 539 Rose Waaler test, 539 synovial fluid analysis, 542 Riboflavin 180 body stores assessment, 190 dietary sources, 180 functional aspects, 180 intestinal absorption, 179 urinary concentration, 190 Riboflavin deficiency, 180 laboratory investigations, 190 Rickets, 517 childhood, 431-132 Fancuni syndrome, 169 hypophosphatacimc «# Hypophosphataemic rickets inherited forms, 431 432 of prematurity, 431 presentation, 431 pnvarional vitamin D deficiency, 517-518 renal lu-hydrolase deficiency, 518 vitamin D-dependent type I, 102,518,519 type II, 518.519 Riedel's thyroidius, 351-352 clinical features, 336 RNA translation, 688, 741 initiation site, 741 Rose Bengal dye test, 233 Rose Waaler test, 539 Rotor's syndrome, 232 hypcrbilirubiruiemia/jaundice, 232 porphyrinuria, 479 Routine tests, 1, 5 hypomagnesaemia, 112 RU486, 398 Rubella, congenital infection, 423 neonatal hypcrbilirubinaemia, 434
  
  Salazopynne, 534 Salbutamol, 50, 56, 5Я Sahcylate albumin-bound bilirubin displacement, 433 hypogrycaemia, fasting, 283 in diabetics, 187 with renal impairment, 289 Reye's syndrome, 436 therapeutic drug monitoring, 653-654, 656 Sahcylate poisoning, 660, 667-468 activated charcoal treatment, 664 clinical features, 667 elimination techniques, 665 hypoglycaemia, 291 laboratory measurements, 656, 667 mechanisms, 667, 668
  
  respiratory alkalosis, 108 toxic dose, 667 Saline infusion suppression test, 324 Sab vary amylase, 205 Saimtmeita diarrhoea, 213 Salt wasting congenital adrenal hyperplasia ut Adrenal hyperplasia. congenital Salt-losing ncphropathy, 32 Sample collection, 8 Sample handling, 8 SandhofT disease, 594, 61 3 clinical features, 613 (>*: gang'ioMdesA^ligosaccharides accumulation, 613 heterozygore identification, 613 hexosaminidase oVp subunit defects, 594, 613 hexosarmnidasc deficiency, 613 infantile form, 594 molecular genetics, 594 prenatal diagnosis, 61 3 SanfiUppo -iyndromc $te Mucoporywecharidosis type Ш SarcokJosis nngiotensin-c\>nvcrting enzyme (ACE), 541 bone/joint involvement, 538 granuloma K25-dihydroxyvitamin О synthesis, 92 hyperealcaemia, 95, 96, 98, 538 hypcruncacmia, 538 hypopituitansm, 309 Satiety* hormones, 371 Schiff base measurement, 774 Schilling test, 205 terminal lleal resection, 212 Schizophrenia acoologicaJ aspects, 569 cerebral atrophy/ventricular enlargement» 574 cholecystokimn (CCK) neurotransmission, 575 clinical diagnosis, 569 CSF homovaniliic acid, 574 CSF 5-hydroxyindoleacenc acid (5-HIAA), 574 dopaminergic systems, 574-575 dopaminc receptors, 575 temporal lobe, 575 endocrine abnormalities, 369 GABA neurotransmission, 575 growth hormone (OH) secretion, 579 plasma homovanillk acid, 574 postmortem biochemical studies, 574-575 serotonergtc (5-HT) systems, 576 site of cerebral abnormality, 574, 575 Schwangerschaftsprotein 1 (SP1), 414 SUeroderma, 211, 534, 540 Sclerosing choiangitis, primary, 242-243 Screening, 4-5 individual, 5 low prevalence conditions, 4 population, 4 predictive value of positive result, 4 selective, 4-5 strategies, 4 test cut-off values, 18-19 Scurvy ut Vitamin С deficiency Seasonal variation, 9 Seckel syndrome, 377 Secretin, 92, 208 Sccretin-cholecystokirun test, 214-215 Secretm-рипстеохугпш test, 212 Secretory IgA, 146
  
  Материал, за
  
  ценный авторским правом
  
  !N!>KX Seizures. 587-590 alcohol withdrawal, 588 uraemic encephalopathy, 589 Selective screening, 4 5 Selenium, 183 body stores assessment, 1Q1 cancer protective effect, 194 dietary sources, 183 functional aspects, 183, 333, 771 toxicity (sclcnosis), 183, 191 Selenium deficiency, 183 laboratory investigations» 191 paremeral nutrition in neonate/preterm infant, 437 Selenium-containing ermrnes, 333, 771 Sella turcica (pituitary fossa), 295 Semen analysis, 400, 403-404 Sensitivity, 2, 15 calculation, 2 1 , 22 definition, 20 in practical situations, 22 receiver operating characteristic (ROC) curves, 22, 23 Sepsis acute intrinsic renai failure, 131 acute phase response *-187 Sm antibody, 540 Small bowel disease, 3,211 Small bowel fistula. 212 Small bowel radiology» 211 Small bowel resection, 213 malabsorption, 211 Small bowel stasis, 212 Small cell carcinoma of lung ACTH production, 306, 690 arginine vaoprcssin (AVP.) secretion, f»90, 69o chronic diluiional hyporuttraemia, 46 syndrome of inappropriate antidiuretic hormone, 696 bombesin secretion, 6^5 insulin secretion, 694 neuron-specific enolase secretion, 695, 715 peripheral neuropathy, 700 tumour markers, 714-715 Small for dates (SFD) infants, 423 causes, 423 drug abuse, 424 intntuierine infection, 423 postnatal investigaturns, 423-424 Smoking bone mineral density (BMD), 515 ischaemic heart disease association, 193. 621 selective hyperehoie«erolaemia screening, 5 vitamin С requirement, 181 Smooth muscle antibody (SMA), 242 Snake bite antidotes, 664 Sodium, 25-60 arginine vasoprcssin (AVP) regulation. 28 body content, 26 diabetes mellitus, 273 neonate, 426
  
  Материал
  
  [вторским правом
  
  846
  
  INDfcX
  
  Sodium (conui1, body fluid distribution, 25 comparison of results with reference limits, 18 disorders of mctabohsm s 31 -49 extracellular Ouid (BCF). 26-28 mincralococticoid actions, 31К osmolaliryVtonicxy «f body fluids, 2ft renal failure acute, 1 s * chrome, 1 37 renal metabolism see Renal sodium handling urine concentration, 2ft Sodium bicarbonate infusion see Bicarbonate infusion Sodium bicarbonate ingestion see Bicarbonate ingestion Sodium chlorate poisoning, 676 Sodium deficient-)', 31 - 34 causes, 31-33 extrarenal sodium loss, 31-32 primary- renal sodium loss, 32 secondary renal sodium loss, "32 33 clinical aspects, 31 esumation of deficit, 59 extracellular fluid (ECF) volume reduction, 31 fluid replacement, 34 laboratory investigations 33 management, 3 3- 34 x c aho Hyponatraemij Sodium exccsVretcmion, 34-37 a «cites, 244 causes, 34 36 chrome renal failure. 136 clinical aspects, 34 diabetes meilitus, 27 3 hcpatorcnal syndromcj 24ft laboratory investigations., 3(»-37 management, 37 menstrual cycle, 3b nephrotic syndrome, 15 3 in pregnancy, 35 36 with oedema, 34-3(», 155 without oedema, 36 sec also Hypcrnatmemia Sodium hydrogen phosphate, 109 Sodium intake, 26 chronic renal failure, 139. 140 hypertension association, 1°3 rcmn/aldosterone basal level, 321 sodium appetite. 28 Sodium loading acute, 3b
  
  long-term loading test, 324 Sodium nitrate poisoning. 676 Sodium nitnte, 676 Sodium plasma concentration, 26 acute brain syndromes, 570 acute renal failure, 1 33 Addison's disease (primary hypoadrenalism), 322 ascitrs treatment monitoring, 245 mineralocorticoid excess, 320 muscle disease. 544 polyuna, 40 renal transplant monitoring, 142 sodium deficiency. 33 stv also Hypeniairacmia, Hyponatraemia Sodium polystyrene sulphonate fKcsonium A), 51 Sodium pumps ( N a \ K-ATPasc), 25 Sodium replacement therapy, 57, 133
  
  Sodium thinsutphate antidote, 676 Sodium valproate, 471 neonatal hyperammonaerma, 418 phenytoin interaction. 649 Rcye's syndrome, 136 therapeutic drug monitoring, 654 Soft tissue calcification, 106 Solvents abuse see Volatile substance abuse Somatic cells, 762 Somacumedins at Insulin-tike growth factors riGFi) Somatostatin Alzheimer's disease, 584 growth hormone regulation, 297, 438 Somatostatin analogues, 682. 683, 684 Somatostatin suppression test, 288 Somatosjatinoma, 270 Somogyi phenomenon, 275 Southern blotting, 747, 762 Specificity, 2, 12, 15 calculation, 2 1 , 22 definition, 20 in practical situations, 22 receiver operating characteristic (ROC) curves, 22, 2 3 Specimen container labelling. 8 Specimen reception/handling protocols, 8 Sperm count, 404 Sperm motihry classification, 404 Spermatogcncfis, 403 assessment, 403 404 FSH abnormalities, 404 FSH in control, 297 male hypogonadism, 405 Spermidine excretion, malignant disease, 694 Spcrmine excretion, malignant disease, 694 Spherocytosis, hereditary, 456, 465 haemocytometry, 459 neonatal hypcrbilirubinaemia/jaundicc, 231, 433 red cell fragility, 461, 465 red cell morphology, 459 Sphingolipidoses, 254 Sphingomyclinaxe a**ay, 614 SphingomyeHnase deficiency, 614 Spinal cord disorder», 594-595 Spinal cord subacute combined degeneration, Spironolactooc, 37, 50, 56, 74, 168, 246, 325 creaurune plasma concentration assay interference, 126 Spleen bilirubin metabolism, 223 red cell destruction, 456 Splencctomy, 456 Squamous cell carcinoma-associated antigen, 715 Standard bicarbonate, 6*1 Standards of performance, 12 13 analytical goals, 13 Siaphviecoecuf aureus, toxic shock syndrome, 249 Starch dietary, 174, 205 enzymatic digestion, 205 intestinal absorption efficiency, 206 Starling forces, 26 Starvation, 367-369 acute intermittent porphyria (AID, acute attack, 470,471 energy metabolism, 281 fat metabolism, 368
  
  glucose metabolism, 281, 367 368 glucose transporter expression, 259 hormonal influences, 368 369 hypogtycaetnia, 292 ketoneVketosis, 368 neonatc/pretcrm infant, 436 protein metabolism, 368 Steatorrhoea, 200, 209, 211 Stcrcobihn, 224 Steroid card, 327 Steroid replacement therapy, 326-327 Addison's disease, 321 Cushing's disease, post-treatment, 308 dosage with inrcrcurrcm illness, 327 hypopicuitamm, 311 insulin $TTCS% test, 298-299 myxoedema coma, 350 patient education, 326 327 pituitary surgery, penoperativc, 301, 302 stressful illness, 299 trauma/sepsis response failure, 356 Steroid sulphatasc deficiency, 389 Stcrols, dietary energy source, 175 Streak gonads, 389 Streptococcal infection gjnmcnilonephritis, 147, 152 neonatal respiratory distress, 424 Sfrrf>ttK*4xw mutant t 194 Sirepto/nrotin, 289, 683, 684 Sirens, 9 ACTH secretion, 297 adrenal conical response, 360 assessment, 298 299 cortisol levels, 247, 3l(> diabetes melhrus glycaemic control, 274 growth hormone secretion, 297 hyperprolactinaetma, 303 hypothalamic amenorrhoca, 310 metabolic response sc< Metabolic response to imury physical damage-associated, 718-719 pituitary' hormone plasma aesay, 298 prolactin secretion, 297 psychological, 719 response with chronic renal failure, I 39 social, 360 feor^anxiety, 361 neuroendochne response, 359-361 reproductive function, 359-360 steroid replacement, 299 Strontium absorption test, 94, 113 Subacute bacterial endocarditis, 147 Subacute sclerosing panencephalitis (SSPE), 565 Subarachnoid haemorrhage cerebrospinal fluid (CSF), 565 bilirubin Cxanthochromia), 558, 565 cell count, 558, 565 oxyhaemoglnbin, 558, 565 pigmentation, 565 C T scan, 565 Subdural empyema, 563 Subscapular skinfold thickness, 186 Substance abuse, 672 673 childhood poisoning, 660 diagnosis, 661 Substance К 726, 7 30 actions, 7 30 Succmic dchydrogenase i SDH; cytochemistry, 552 Succinyl acetoacetate/succinylacetonc, unnary. 254 Sucrase-isomahasc, 206
  
  Матери,
  
  защи!
  
  торским правом
  
  1МЖХ Sucrase-iwmaltasc deficiency» hereditary, 206 Sucrose, 205 Sudden infant death (SID), fatty acid metabolism mbom errors, 447, 553 Sugars dental caries, 174, 194 dietary, 174,205 high intake, 175 energy metabolism, 174 Suicidal behaviour, 578 Suicidal drug overdose, 660 Sulphasalazine, 203 Sulphatidoscs s& Metachromatic leu kodystrophy Sulphite oxidase deficiency, 450 with xanthinc oxidase deficiency, 171 Suiphonamides drug-induced acute porphyna, 479 hypoglycacmia. 291 Sulphonylurcas contraindications with liver disease, 248 hypoglycaemia, 283, 291 alcohol consumption, 292 diabetics, 287 neonatc, 2 9 | renal impairment, 289» 291 inadvertent intake, 288 insulin release response, 260 overdose» 291 self-administration tec Factitious hypogrvcacmia syndrome i»f inappropriate antidiuretic hormone secretion <S1ADH>, 273 Sulphur-containing amino acids metabolism. 609 Supcrovulation treatment, 401 Superoxide, 76 adrenaline metabolism in formation, 768 atherosclerosis pa the agenesis, 774 chemical reactivity, 767
  
  DNA base adduci formation, 775 leakage from mitochondria! electron transport chain» 768 nitric oxide reaction, 768
  
  respiratory burst, 768 superoxide dismutase in removal, 771 transition metal catalysed hydroxyl radicals production, 767 xanthine oxidase catalysed generation, 768 Superoxide dismutase, 771
  
  Addison's disease (primary hypoadrenalism), 321 adrenal function tests. 328, 329 congenital adrenal hyperplasia, )90 conisol secretion assessment, 320 chronic renal failure. 365 Cushtng** disease, post-treatment monitoring, 308 hyponatraemia, chronic dilutional, 48 long test, 310s 320, 329 protocol, 329 pituitary function assessment, 320 short test, 299, 320, 321. 328 indications, 312 protocol, 312, 328 Synarthroses, 533 Syndrome of inappropriate A1)H secretion (S1ADH) AVP-sccrcting tumour, 690, 696 clinical features, 696 diagnostic criteria, 47
  
  hyponatraemia, 46-17, 589-590 II.1/11.6 responses, 357 neurological causes, 589 porphyrias, acute attacks, 479 sulphonyJureas association, 273 Syndrome X se<e Hypcrinsulmaemia syndrome Synovia! fluid. 534 Synosial fluid analysis, 541-542 light microscopy, 541 542 naked eye appearance, 541 Synosial joints, 533-534 Syphilis, congenital, 434 Systemic lupus erythematosu* anti-ONA antibodies, 540 anunuclear antibodies (ANA), 539, 540 articular disease, 5 34 autoimmune hypoglycacnua, 292 (--reactive protein, 541 epidemiology, 534 glomemlonephmis, 148 focal and scgmcntal, 152 membranous, 150 hypcrkalaemia, 57 I>: cell test, 540 organ involvement, 534 porphyria cutanca larda association, 477 renal tubular dysfunction, 57 Sm antibody, 540 treatment, 534
  
  away, 776 free radicals scavenging, 770 Suppressor genes, 758, 7o3 function, 759 mutations, 758 reduction to homorvgositv in tumours, 758 759 Supraihac skinfold thickness, |8n Surgery, 10 prcoperative nutritional support, 196
  
  proteinuria, 159 vitamin С requirement, 181 Swan-Ganz catheter placement, 1 3 3 Sweat calcium. 89 magnesium, 110 phosphate. Ю5 potassium, 51 Sweat test, cystic fibrosis, 440 441 Synacthen test, 319 32U ACTH isolated deficiency. HO ACTH-adrenal axis assessment, 299 post-pituitary surgery* W2
  
  T cell leukaemia-lymphoma, acute (IITLV I associated disease], 96 Tamm-Horsfall glycoproicin, 145 functional aspects, 145 multiple myeloma, 158 renal tubular secretion, 145 urinary sediment/casts, 125 Tamoxifen, 398, 702 Tangier disease (HDI. deficiency) 598 clinical features 598, 635 H D L metabolism 635 Tartratc-resistant acid phosphatase 509, 51 3 Tau protein Alzheimer's disease, 580-581 cerebrospinal fluid (GSF), 560 Tay-Sach* disease, 593» 612, 745 clinical features, 613 0 M j gangliosides accumulation, 613 hcterozygote identification, 613 hcxA a suburnt defect, 593, 613 hexosaminidase deficiency, 593, 613
  
  847
  
  infantile f o r m , 594
  
  molecular genetics, 593 prenatal diagnosis, 5, 613 ад,и * Тс scan, thyroid маш* evaluation, 3 37 T c - D T P A renal scan acute intrinsic renal failure, 131 with GFR measurement, 128 TCDD-induced porphyna cutanea tarda, 476 Telephone reports, 14 Temporal lobe epilepsy, 302 Teratoma, alphafetoprotein (AFP) tumour marker» 706-70? Terminal ileal resection, 212 Test request, 7 Test request form, 7, 8 computer systems, 8 Testcs, 402-403 androgen production, 402-403 development, 375 FSH effects, 403 functional assessment, 403 405 endocrine evaluation, 404-405 Leydig cell function, 405 semen analysis, 403-404 h>pothalamic-pituicary feedback mechanisms, 403, 404 Mullenan inhibition factor {MIF") production, 403 spermatogcncsis, 403 Testicular cancer peripheral neuropathy, 700 rumour markers, 3» 694, 695, 707 Testicular failure, primary, 404, 405 406 anorchia, 406 gorutdotrophin levels, 405 Klinefelter's syndrome, 405—W6 treatment, 407 Testicular feminization «v Androgen in&ensiuvity syndrome Testosterone, 247 aggressive behaviour, 359 albumin binding, 247 androgen insensitivity syndrome, 383 anorchia (vanishing testis syndrome), 389, 390 basal hormone investigations, 298 basal pituitary function test, 311 biological actions, 403
  
  biosynthesis, 382, 403 enzymes, 403 from adrenal androgens, 318 calcium/bone metabolism, 93 chronic alcoholism, 366 chrome renal failure, 139, 36 3 Orcadian rhythm, 403 cirrhosis, 247-248 cochac disease, 367 congenital adrenal hyperplasia, 379, 402 dihydrotesiosterone (DHT) biosynthesis, 382, 403 ebb phase of trauma response, 357 female metabolism, 398, 399 ovarian biosynthetic pathways, 397 u*x hormone-binding globulin (SHBG) transport, 399 fetal genital structures development, 376, 396 fetal icstes secretion, 375, 376, 396, 402-403 gonadotrophin deficiency treatment, 310 hair growth effects, 401 hCG stimulation test, 390 hypogonadism, 405, 406
  
  Материал, защиш
  , 334 combined pituitary function test, 300 T S H secretion control, 297 Thyrotrophin-releasing hormone (TRH) test, 339 acromegaly, 305 hyperprolactinaemia, 303 Thyroxinc (T4) basal pituitary function test, 311 biological acuons, 333 cataholism, 333 chronic renal failure, 335. 364 congenital hypothyroidism screening, 440 deiodinases, 333 fetal levels, 334 free plasma level, 333, 337 338 half-life, 333 lithium therapy, 370 liver metabolism, 183, 366 muscle metabolism, 546 neonaic, 334, 440 non-thvroid illness, 335
  
  Материал, защиш
  own*s syndrome Trophoblastic tumour», 347 Tropical «pruc, 2 0 3 , 206 Troponin CT 783 Troponin I, 7 8 3 , 784 Troponin T mvocardial ischaemia, 7R 3-784 unstable angina, 780, 7Я4 Troponins, myocardial ischaemia, 783 784 True negatives ( T N ) , 20 True positives i T P ) , 20 Trypstn acute pancreatitis, 786 protein digestion, 207 I rypsinoger», 786 Tryptophiin malignant disease, 645 mcutinamidc synthesis, 180 serotomn biosynthesis. 6Й0-6Н1 Tryptophan load test, vitamin B n deficiency. 190 Tuberculosis Addison's diMrtsr, 321, 322 ascites, 245 glucocvrticoid stress response attenuation. 357 granuloma l«25-dihydroxyvitamtn П synthesis. 92 hypcrcalcacmia, 92, 96 hypopuuitarism, 309 meningitis, 55ч, 564 Tubular necrosis, acute, 131 1 32 diuretic phase, I 32 fluid/sodium replacement, 133 hcpatorcnal syndrome, I 33 natural history, 1 32 oltgurtc phase, I 32 paracetamol poisoning, 240, 666 pathogenesis, 131- 132 recovery phase, 1 32 urinary sediment/casts, 125 Tubulointerstitial nephropathy causes, 154 hyperkalaemia. 57 tubular protctnuna. 154 Tumoral cakinosis, 106 Tumour lysis syndrome acute renal failure, 100 hypcrphosphataemia, 106, 107
  
  hypocalcacmia, 100 T u m o u r markers, 7 0 5 716 ACTH-dependent Cushing's syndrome, 307 alkaline phosphatasc isocnzymcs, 694 aJphafcroprotcin (AFP), 714 applications, 706-707 A P U D cell hormones, 713, 7 M assays. 7 0 8
  
  breast carcinoma, 714 СЛ125, 711 CAI9.Q, 713 CA-50, 713
  
  CAI95, 713 calcitomn, 715 carcinoembryonic antigen (OEA), 7 1 3 , 714 chohocarcinoma, 710-711 clinical usefulness assessment, 705-706, 708-715,716 definition, 705 diagnosis, 706 cctopic hormone products, 692.. 693 follow-up. 707 gastrointestinal tumours. 712-713 germ cell rumours, 708-710, 714 hepJitoblastoma, 714 human chonoruc gonadotrophin ( h C G ) , 692 lactatc dehydrogenase ( Ш Н ) , 695, 715 lung carcinoma, 714 715 monitoring response to therapy, 707 mucinlike glycoproteim, 714 multiple myeloma, 500 neonatal tumours, 714 neuroblastoma, 714 neuron-specific enolasc (NSEh 695, 714, 715 oestrogen-binding receptors (ERs>, 714 ovarian tumours, 711 712 paediatnc tumours, 714 paraprotcinaemia/Bencc Jones proteins, 495 predictive value, 705 primary hepatoccllular carcinoma, 713 714 prognostic value, 706 prostate-specific antigen (PSA), 712 pnistatic acid phosphatase (PAPJ, 712 prostatic carcinoma, 712 residua] disease detection, 706-707 screening, 706 sensitivity* 705 specific markers, 707-708 specificity, 705 squamous cell carcinoma-associated antigen, 715 thyroglobulin, 715 thyroid ncoplaeia, 352-353, 715 tumour-associated, 705 tumour-derived, ?05 tumour-specific, 705 Tumour necrosis factor a f I"NFn; cachcctin), 7 2 1 , 7 2 4 , 726 actions, 358. 724 acute phase response, 358. 734, 736 acute phase proteins, 7 i 6 time course, 736 bone destruction, 499 (!5a-mediated release, 733 clinical shock, 734, 735 endogenous pyrogen activity, 698, 726 bpopolysaccharidc gout, 535 636 malignant disease, 646, 703 Unc acid overproduction, Lesch-Nvhan syndrome. 616 Uric acid stones, 170 171, 536. 537 prevention, 171 Urmalysis, 124-125 acute renal failure intrinsic, 131 prcrcnaJ, 130 appearance of urine, 124 articular disorder*. 542 glucose, 125 p H , 125 protein urw, 124, 125 specific gravity/osmolality, 124-125 urinary sediment, 125 Urinary acidification test, 172 Urinary amino acid chromatography, 128 Unnary calculi, 123, 169 Urinary casts, 125, 160 acute renal failure intrinsic, 131 prcrcnaJ, 1 30 multiple myeloma, 158 Tamm-HorsiaU glycoproicin, 145 Urinary frequency, 12 3 Urinary protein excretion, 4 . 146 147 age-associated venation, 146 diurnal variation, 147 exercise-related variation. 146 ■ 147 normal, 146 posture-related variation, 146 pregnancy, 147 w€ abo Proteinuria Urinary sediment, 125 acute intrinsic renal failure, 131 haemosidenn iron, 458 Urinary tract infection neonatal hyperammonaemia, 448 triple phosphate stone formation, 170 urine turbidity, 124
  
  Urinary tract obstruction acute renal failure, 129 intrinsic, 131 obstructive (posirtnal}, 132 diagnosis, I 31 hypokalaemia following relief, 5 ) renal sodium loss following relief, 32 Urinary- urea nitrogen, 188 Urine buffer systems, 63, 66 colour, 124 foaming, 124 odour, 124 pH/hydrogen ion excretion, 66 protein content, normal, 146 sediment, 125 specific gravity/osmolahty, 124 125 turbidity, 124 U n n c calcium measurement, 94 hypo^aratnyroidism, 99 intestinal calcium absorption, 94. 11 3 Urine cortisol measurement, 319 Unnc formation collecting duct, 12 3 distal convoluted tubule, 123 l o o p o f H e n l e , 122-123 proximal convoluted tubule, 121 Urine oemolality, 29, 123, 124-125 chronic renal failure, 137 collecting duct function, 123 diabetes insipidu* (DO, 298 nconate, 427 sodium deficiency, 33, 47 Urine stick tests, 160 Urobilmogen, 224 haemolysis-associated increase, 458 Urobilmogen urine concentration biliary obstruction, 231 haemolysis investigation, 460 Umkirusc, 145-146 renal tubular secretion, 145 Uroporphyrin excretion, 469 porphyria cutanea tarda, 476 Uroporphynnogcn cosynthasc, 467 Uroporphynnogcn cosynthase deficiency crythropoietic porphyria, 473 hcpatocrythropoietic porphyria, 477 porphyria cutanea tarda, 476 Uroporphynnogen dccarboxylasc hacm biosynthesis, 467 lead toxicity, 479 Uroporphynnogcn dccarboxylase deficiency, porphyria cutanea tarda, 244. 475 Uroporphynnogcn I cnthropoietic porphyria, 474 hacm biosynthesis, 467 Uroporphynnogcn 111 eryihropoietu: porphyria, 474 hacm biosynthesis, 467 Ursodeoxycholic acid therapy, 253 Uterus developmental changes, 3K6 menstrual cycle changes, 395 Vaginal function, 398 Vaiinc metabolism, 447 Valine metabolism defect, 449 Vanillin effect, 10 Vanishing test» syndrome, 389-390 Variable number of tandem repeats (VNTRs), 752, 763
  
  Variable regions, 493 Variable (Vl gene s e g m e n t , 494 Varicella, 436 Varicose vein*, 193 Variegate porphyria, 253, 471 4 7 2 , 745 acute abdomen, 787 acute attack* investigation, 4HO acute intermittent porphyna ГА1Р combination, 477-478 clinical features, 472 cutaneous lesions, 472, 480 penphera! neuropathy, 597 differential diagnosis, 472 genetic aspects, 471 investigations, -172 latent disease, 4 7 1 , 472 photoscnsirivity, 470, 472, 4 7 8 management. 472 plasma fluorescence spectrum, 472 prevalence, 471 treatment, 472 Vasoactivc intestinal peptide :VIP>, 726, 730 actions, 730 insulin release response, 260 tumour secretion uc VIPoma Vasopressin от Arginine vasopressin (AVP) Vasopressinasc, placvntal, 39-40 Venesection, 319 Ventilation, alveolar, 82 VcntilaiiOfi-perfusion imbalance, 82, 83, 84 Verner-Morrison syndrome glucose intolerance, 269 от also VlPorna Very long chain fatty acids (VLCFA) adrenoleukodystrophy» 5 9 1 , 592 adrcnomyeloneuropathy, 595 peroxisomal disorders, 450, 451 Very low density lipoprotem (VLDI.), 624 a p o B - 1 0 0 , 624. 626 apo C-I, 625, 62o apo U-II, 625, 626 а р о С - Ш , 625, 626 apo Г), 625 apo E, 6 2 5 , 626 atherogenesis, 621 chronic renal failure, 138-139, 365 diabetes mclhtus, 272, 274 dyslipoprotcinaemias abctalipoprotcmaemia, 593 familial combined hvpcrhpidacmia ( F C H ) , 631 familial hypcrtnglyccndaemia ( F H T G ) , 631 fish eye disease, 635 K T G L deficiency, 635 hs-pcrapobetaJipoprotcinaemia, 631 remnant hyperiipoproteinacmia ( W H O type Ш ) , 632 gross appearance of plasma serum sample, 638 hepatic secretion, 626 LPL-rncdiated lipolysis, 627 metabolic transformation in plasma, 626-627 тисоча! cell synthesis, 209 nephrotic syndrome, 1 5 4 , 6 3 6 oestrogen effects, 4 2 1 , 6 30 particle size variation, 626-627 remnants, 627 cholesteryl ester transfer from HDL~, 627 removal, 625 small tnglvceride-poor, 621
  
  Материал, защип
  
  t . авторским п
  
  INDI-X Vibrio ihoitrwe diarrhoea, 213 Villus cell tumours, 380 Vincristine, 686 VfPotna APUD tumours, 71 3 clinical features, 21 ), 684 glucose intolerance» 270 metabolic acidosis, 51 multiple endocrine ncoplasia 1 (MEN 1), 684 VIP plasma concentration, 684 Viral encephalitis, 563 Viral hepatitis ice Hepatitis, viral Viral meningitis CSF-plasma glucose ratio, 559 investigations, 563 tl.Sr" G-reactive protein, 564 CSF lactate, 564 CSF lactate dehydrogenase, 564 OSF protein/glucose, 563-504 Virilizaiion, 401 402 acanthosis nigricans, 270 congenital adrenal hyperplasia 11 {V-hydroxylase deficiency', 378 21 -hydroxylase deficiency, ?77 3l*-hydroxysteroid dehydrogenase deficiency, 378 maternal androgen-secreting tumour*, 380 partial androgen insensitrvity syndromes, 383 placcntal aromatasc deficiency, 380 Visual field loss hyperprolactinaemia, 304 pituitary/hypothalamic mass lesions, 296. 302, 308, 684 Vitamin A (retinol), 177 178, 189 actions, 177 body store assessment, 189 cancer protective effect, 194 carotenoid pigments, 177 dietary sources, 177 hepatic storage, 177, 219 structure, 177 tcratogcnicity» 178 toxicity, 177-178 Vitamin A (retinol) deficiency, 177 abetalipoproteinacmia, 630 laboratory investigations, 189 Vitamin B, (thiamin), 179-1 HO body stores assessment» 190 Wcrnickc-Korsakoff syndrome, 571 dietary source», 179 functional aspect», 179 intestinal absorption, 179 plasma concentration, 190 tenacity, 179-1 HO urinary concentration, 190 Vitamin B, (thiamin) deficiency, 179 alcohol abuse, 588 heritable susceptibility, 571 laboratory ins*cstigations, 190 red cell transketolesc activity, 190, 588 Wcrntcke-Korsakoff syndrome, 571, 588 Vitamin B, (thiamin) therapy dialysis patients, 179 maple syrup urine disease (MSUD), 449, 611 Wemicke-Korsakoff syndrome, 588 Vitamin BK, 180, 181 body stores assessment, 190 drug-associated disorders of metabolism, 180 intestinal absorption, 179
  
  malignant tissue metabolism, 695 toxicity, 180 Vitamin Bft deficiency, 180 hyperoxaJuna, 170 laboratory investigations, 190 sulphur-containing amino acids metabolism, 609 Vitamin B„ supplements, 180 cystathioninuria, 610 homocystinuna, 450, 610 Vitamin K,^ 204 205 absorption. 204-205 assessment (Schilling test), 205 gastric acid actions, 204 lleal active transport, 204 205 intrinsic factor (IF) complex, 204 R binding protein, 204 dietary sources, 204 functional aspects, 204 homocystinuria, 450 malahsorpnon investigation, 211 pregnancy-associated changes, 420 propionyl CoA metabolism, 447 status assessment, 205 structural aspects, 204 toxicity, 205
  
  transcobalamin II carrier protein, 205 Vitamin Bjj deficiency, 205 assessment, 187 haematologicut changes, 203 folate deficiency relationship, 203, 204 hypothyroidism, 348 laboratory investigations, 205 malignant disease, 697 Vitamin B12 malabsorption clinical features, 200 small bowel resection, 211 spinal cord subacute combined degeneration, 594 Vitamin B,, supplements, 205 methylmalonic acidacrma, 448 Vitamin C, 181 annoxidant activity assay, 776 ascorbaic urinary* concentration, 191 cancer protective effect, 194 dietary folate availability, 203 free radicals scavenging, 770-771 functional aspects, 181 high dose intake, 181 hyperoxaluria, 170 intestinal absorption, 179 leukocyte concentration, 191 malignant tissue metabolism, 695 plasma concentration, 191 status assessment, 191 Vitamin С deficiency, 181 folate requirement, 203 laboratory investigations, 191 Vitamin С supplements with desferrioxamine, 486 Vitamin О actions, 92 bodv store assessment. 189-190 calcium absorption, 92 hypercalcacmia of malignant disease. 695, 6% hypomagncsaemta, III idiopathic hypcrcalciuria, 105 metabolism, 91-92 in granulornatous tissue, 96 hepatic 25-hydroxylation, 91 renal I tx-hydroxylation, 91 -92 metabolite measurement, 92
  
  853
  
  phosphate transport defects. 167 synthesis, 91 toxicity, 97, 98 srr л/л» 1,25-Dihydroxyvitamin D (l,252 genetic variant*, 514 Vitamin D receptor defect, 518 Vitamin D, supplements Pancom syndrome, 169 laboratory monitoring, 190, 519 parcntcral feeding, 197 rickets of prematurity, 431 toxic levels, 190 Vitamin D-binding protein, 91 Vitamin D-dcpcndenl rickets type I renal Itt-nydroUsc deficiency, 518 response to therapy, 519 vitamin D deficiency management, 102 Vitamin D-depcndent rickets type II l,25(OH).D receptor abnormality, 518 response to therapy, 519 Vitamin D_. (crgooitciferol), 91 Vitamin D, (cholccalcuerol), 91 Vitamin D , (cholccaJcifcrol), supplements, 516 Vitamin deficiency fat soluble abetatipoproteinaemia, 630 chylomicron retention disease, 631 short gut syndrome, 21 3 Vitamin H (ii-tocopherol), 178 ci-rocophcrol assay, 776 body store assessment, 190 cancer protective effect, 194 functional aspects, 178 nutritional requirement, 178 Vitamin К (u-tocopherol) deficiency. 178 abetalipoproteinacmia, 593, 630 laboratory investigations, 190 nconates, 458 Vitamin E , 228 Vitamin К mal absorption, сф Upoproteinaerma, 593 Vilamm K.-dependent clotting factors, 17Я liver disease, 228 malahsorption investigation, 211 pmthrnmbin time i'1'l >, 22ft Vitamin K, (phytomenadioncl, 17M, IT*) Vitamins chronic renal failure, 140 dietary requirements, | 7 o |H2 fat-soluble, 176, 177 170 digestion, 20» 209 malignant disease, 695 nuthtonal status assessment, IK9-I91 parcnteral feeding, 197 parcnteral solutions 196 water-soluble, 176, 17* 182 Volatile substance abuse, 6 7 2 , 6 7 3 cardiotoxicity, 660
  
  childhood poisoning, 660 redistribution hypokalacmia, 50 Volume of distribution (Vdj, M l , M 2 Volume repletion acute renal failure, I 33, 1 34 acute tubular necrosis, 132 obstructive (postrcual), 132 prerenal, 131 diabetic non-ketotic hvpcrosmolar states, 277 hypercalcaemia management. 98 ketoacidosis 277 non-respiratory alkalosis, 77, 7Й renal calcium metabolism, 89 short gut syndrome, 213 sodium deficiency. 33, 34 Vomiting hypokalacmia, SO, 52 non-respiratory alkalosis, 7Й von СЬегксЧ disease see Glucose 6phosphata.se (CifiP) deficiency
  
  U'aldensrrom's macroglobulinaemia (WM), 501 502 Bcnce Jones protein. 502 hyperviscosity syndrome, 5 0 1 , 502 paraprotcmv, 502 Warfarin, 178 Water body content. 25 collecting duct rcabvorption, 123 disorders of metabolism, 17-49 intake control, 29 mrraccUular fluid ( K : R J . 28-29 loop of Henlc rcabsorption, 122 osmorcgulation, 28 29 proximal convoluted tubule reab*orpnon, 121 Water deficiency hypernatiaemia, 42 4 3 estimation of deficit, 59 with thirst, 42 without thirst, 42—4 3 sodium deficiency, 31 fluid replacement, 34
  
  Water deprivation test, 4 0 - 4 1 , 300 301 desmopresbin response, 300 301 diabetes tmipidus, 298 indications, 312 protocol, '59, 312-3 И vasoprensin test, 59 Water excess, primary, 17 Water intoxication, 37, 44 46 neurological manifestations, 46, 58** 590 sodium requirement estimation, 59 Water load test, 60 Water losses, preterm infant, 427 Water retention chronic chlutional hyponatraemia, 46 4 7 chronic renal failure, 1 36 nephrotic syndrome, 153 Water)- diarrhoca-hypokalaemta-achlorhydria (WDHA) syndrome glucofte intolerance, 269 270 metabolic acidosn, 51 Werner's syndrome lee Multiple endoenne ncoplasia 1 (MEN 1) Wernickc-KorsakofT s>-ndrome, 179, 588 diagnosis, 588 treatment, 588 Wemicke's encephalopaihy, 571 biochemical investigations, 571 clinical features, 588 neuropath о logical changes, 588 treatment, 588 see aiw Wernickc-KorsakorT syndrome Western bloc, 763 Whipplc's disease arthritis, 538 mucosal biopsy, 211 protein-losing entcropathv» 208 Wild type, 763 Williams' syndrome see Hypercalcaemia, infantile Wilms* tumour, 40 Wilson's disease, 251-252, 592 593 acute liver failure, 2 39 eacruloplasmin plasma concentration. 228, 2 4 2 , 2 5 1 , 252, 4 3 6 , 5 7 1 , 593 calcium pyrophosphatc disease association, 536 earner detection, 593 chronic active hepatitis (САН), 242 clinical features, 436 copper metabolism, 242, 251-252, 436, 5 7 1 , 593 serum concentration, 252 tissue accumulation, 251 2 5 2 . 5 9 2 urinary concentration, 252 diagnosis, 242, 436 geneuc aspect», 592 hypouncacmia, 536 Kayscr-Fleischcr rings, 242, 2 5 1 , 592, 59 3 laboratory invesriganons, 251 252, 593 liver biopsy, 252 liver disease, 2 5 1 , 436 management, 252 liver function tests, 242, 251 neurological manifestation)., 592 pathophysiology, 593 psychiatric manifestations, 5 7 1 , 592 renal dysfunction, 251 treatment, 593
  
  urinary Pj-micrcglobulin, 156 Wolfram syndrome, 37
  
  X chromosome mapping, " 4 0 X-linked hypophosphataemic rickets/ osteomalacia. 105, 167. 431 4 3 2 , 519 X-linked inheritance, 744, 745 dominant traits, 745 X chromosome inactivation effect, 745 X-porphyrin, faecal, 472 Xanthinc oxidasc free radicals generation, 768, 773 ischaemic rcperfusion injury. 773 Xanthine oxidate deficiency. 171 with sulphite oxidate deficiency, 171 Xanthine stones, 171 Xanthinuria hereditary, 171 hypouncacmia. 536 sulphite oxidasc deficiency, 171 Xanthochromia see Bihrubin, cercbrospinal fluid Xcrodcrma pigmentosum, 750 XX male, 375, ЗЯ1 XY female (pure gonadal dysgenesisv, 375, 381 Xylose absorption test, 206 207, 211 protocol. 214
  
  Y chromosome sexual differentiation, 375 structural abnormalities, 38I test» determining gene (SRY), 375 point mutation, 381 X-Y translocation, 381 Yohimbme, HO, 577
  
  Zellweger syndrome (ccrebro-hepato-renal syndrome), 451 clinical features. 451 neonatal liver disease, 4 3 5 peroxisome function failure. 451 Zinc, 182 acute phase response changes, 737 body stores assessment, 191 dietary sources 182 cntcral nutrition, 197 functional aspects 182 malignant disease, 695 parcnteral feeding, 197 plasma concentration variation. 191 tissue distribution, 192 toxicity, 182 Zinc deficiency, 182 clinical features, 437 laboratory investigations, 191 parenteral feeding, 197 nconatc/pretcrrn infant, 437 Zinc sulphate therapy, 593 Zmc supplements, 191 Zinc-dependent enzymes, 467, 771 Znllingcr-Kllikon syndrome glucose intolerance, 269 metabolic alkaloMsAiypokalacmia, 5 1 , 52 multiple endocrine ncoplasia 1 (MKN i j , 6 8 3 co-existent hyperparaihyrnidisrn, 685
  
  Материал з