Low Vision Manual

© 2007, Elsevier Limited. All rights reserved. First published 2007 No part of this publication may be reproduced, stor...

Author: Jonathan Jackson PhD | James Wolffsohn BSc PhD MCOptom

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© 2007, Elsevier Limited. All rights reserved. First published 2007 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 the prior permission of the Publishers. Permissions may be sought directly from Elsevier’s Health Sciences Rights Department, 1600 John F. Kennedy Boulevard, Suite 1800, Philadelphia, PA 19103-2899, USA: phone: (+1) 215 239 3804; fax: (+1) 215 239 3805; or, e-mail: [email protected]. You may also complete your request on-line via the Elsevier homepage (http://www.elsevier.com), by selecting ‘Support and contact’ and then ‘Copyright and Permission’. ISBN-13: 978-07506-1815-1 ISBN-10: 0-7506-1815-9 British Library Cataloguing in Publication Data A catalogue record for this 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. Note Neither the Publisher nor the Editors assume any responsibility for any loss or injury and/or damage to persons or property arising out of or related to any use of the material contained in this book. It is the responsibility of the treating practitioner, relying on independent expertise and knowledge of the patient, to determine the best treatment and method of application for the patient. The Publisher

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Foreword

It may seem paradoxical, but visual impairment and the need for its rehabilitation are becoming more important because of improving health care. Worldwide, longevity is increasing. Having a larger proportion of the population in the older age groups means a higher prevalence of visual impairment because vision loss predominantly results from age-related diseases and disorders. As new or improved treatments are developed for various disabling eye diseases, there will inevitably be some shifts in the relative importance of different eye diseases, but for the foreseeable future, we can expect the incidence of age-related vision loss to continue to rise. Low vision care will continue to become increasingly important. The goal of low vision rehabilitation is to minimise any functional impediments imposed by vision loss. In part, this is done by gaining as much use as possible from the patient’s remaining visual abilities. In this process, the ophthalmic clinician’s first task is to identify and understand the functional difficulties that the individual experiences as a direct or indirect result of their visual limitations. Tasks associated with reading, face recognition and mobility are likely to remain around the top of the list. Second, the clinician seeks a more detailed assessment of the individual’s vision through examining the eyes in order to better understand the causative pathology and the prognosis, evaluating the refractive characteristics of the eyes, and performing a range of tests of visual function. Visual acuity, visual fields, contrast sensitivity, vii

Foreword

colour vision, light and dark adaptation, oculo-motor control and glare disability are all relevant and clinically-quantifiable visual functions. However, it is not enough to simply measure thresholds. Clinicians should also evaluate the ease, efficiency and accuracy of task performance, and dependencies on more subtle variation in the visual stimulus, such as lighting levels and visual clutter. As the third part of the process, the ophthalmic clinician’s job is to consider ways in which the performance of the patient’s visual tasks can be optimised or facilitated though optical manipulations by magnification, minification, prisms, filters, lighting control and through the use of electronic display systems or through training specific visual skills. Taking care of the visual aspects of the patient’s tasks is only part of the overall rehabilitation process for the visually impaired patient. Accessing other rehabilitative services and support can be crucial. Rehabilitation specialists, mobility instructors, occupational and physical therapists, special educators and social workers are all important parts of the team of rehabilitation professionals that can provide strategies, techniques and training for improving task performance, or facilitating the use of devices that can make some tasks less dependent on vision. Often the psychological, social and recreational needs of visually impaired individuals warrant considerable attention and support from family, friends and, sometimes, professionals. For many, especially those who acquire their vision loss in adult life, there will be a need for vocational counselling and training, along with accommodations being made in the workplace environment. Technological developments are presenting exciting new opportunities but also new challenges for low vision rehabilitation. In particular, display technologies are allowing a lot more scope for optimising the display of printed material to our visually impaired patients. Electronic display systems offer much more flexibility than the more traditional optical low vision aids. Most videomagnifiers allow easy variations in print size, contrast, luminance and colour of the printed or pictorial displays. The same set of visual parameters can be varied on computer-controlled display screens, but computerisation expands the range of modifications of the visual display through reformatting by changing font, style, columns, rows, spacing, highlighting, controlled scrolling, streaming, and other modes of visual presentation. Computers can also enable the information to be displayed as speech output or tactile output that may be used to supplement or replace the usual visual viii

Foreword

screen images. In most societies, the information technology revolution is changing the visual demands of daily life. Mobile phones, automatic bank telling machines and similar displays for business transactions are becoming more commonly encountered by people from all walks of life. In Western societies at least, personal computers are becoming more important to individuals as the favoured means of communicating with friends, paying bills, checking bank and business records, as well as using the web-based technologies to access information for a wide variety of recreational, occupational, spiritual and intellectual purposes. Despite their difficulties in acquiring skills to use a keyboard or a mouse, the elderly are rapidly expanding their use of computers. In the least technologically-developed countries, there is currently an exponential increase in the use of mobile phones and the associated expansion of access to information technology through both visual and auditory displays. With many of these electronically-controlled displays, there is scope for the user to experiment and make their own choices of display parameters. However, because ophthalmic clinicians understand eye diseases and their effects on vision, and they know how the visual system works and how visual images and displays may be manipulated, these practitioners should be able to provide well-informed advice and guidance on the selection of the display parameters, and on which methods and strategies are best for the individual. Whether the displayed image comes from an optical system or a display screen, the skills of ophthalmic clinicians are needed to provide the optical corrections so often required to ensure that the retinal image is in satisfactory focus. Only a relatively small fraction of optometrists and ophthalmologists are specialising in low vision rehabilitation, maintaining a high level of expertise, and developing close working associations with other low vision rehabilitation specialists. Fewer still are active in research into relationships between functional performance, visual functions and quality of life, or in the development of improved methods for assessing visual functions or methods for training visual skills, or in creating new optical and electronic systems to assist visually impaired patients. But it is not just the experts who need to know about low vision rehabilitation. All optometrists and ophthalmologists engaged in clinical practice have a responsibility to be informed about low vision rehabilitation and what it can do. All eye-care practitioners need to be knowledgeable about today’s newly emerging pharmacological, ix

Foreword

surgical and genetic treatments that hold promise for reducing the occurrence or severity of certain eye diseases. They need to be able to answer their patients’ questions on such matters, and to make appropriate referrals. As society’s experts in vision, all optometrists and ophthalmologists have a similar responsibility to be knowledgeable about methods for assessing functional vision, and the possibilities of patients benefiting from the use of the various optical and electronic low vision aids. As part of a healthcare delivery system, they all have a responsibility to be acquainted with the range of rehabilitation services and support systems that are available to visually impaired persons. This Low Vision Manual presents a technically sound, comprehensive and up-to-date account of low vision rehabilitation that will serve as an excellent guide and resource for students and clinicians wishing to develop their knowledge and skills in low vision care. For those who simply wish to familiarise themselves with the state of the art in low vision care today, this Manual will be an accessible and valuable source of information. For clinicians who are already expert, this Low Vision Manual will provide new information and new insights from its knowledgeable team of authors. Ian L. Bailey

x

Preface

The population is ageing, particularly in ‘developed’ nations. This, together with the lack of treatment options for conditions such as atrophic age-related macular degeneration, has resulted in a substantial increase in the number of visually impaired people requiring ‘vision care’. In addition, as our standard of living rises, there is an expectation that we will maintain a high quality of life into old age. These two factors continue to create an ever increasing demand on low vision rehabilitation services, especially those that involve multidisciplinary integrated care. This book has been written by clinical and research experts in the fields of disease detection and management, primary and secondary optometric care, low vision optics and prescribing, counselling and rehabilitation. In writing this text we have attempted to disseminate the latest research findings in a digestible format for the clinician. The book is intended to be a comprehensive guide and up-to-date reference source. It is presented in an easy to access format, which should enable the front-line eye care professional to provide patients with sound, research-based, clinical care and rehabilitation. It is unique in presenting both the latest evidencebased knowledge and in covering the full gamut of issues relevant to comprehensive low vision rehabilitation. In Section 1, low vision and its epidemiology are defined. The signs, symptoms and clinical management of the range of conditions that cause visual impairment in the three stages of life – childhood, working age and advancing years – are presented in xi

Preface

detail. The measurement of visual function of the visually impaired is covered in Section 2, not just in terms of traditional measures such as visual acuity and contrast sensitivity, but importantly in considering the psychology of visual loss and functional visual measures such as quality of life. In Section 3, low vision aids, from simple hand magnifiers to electronic vision enhancement systems, are described from their optics to practical tips on prescribing. Last, but by no means least, rehabilitation strategies and techniques are discussed in Section 4, embracing the treatment of the visually impaired as a whole person. This book aims to be an essential read and reference text for all professionals involved in the care of the visually impaired, including ophthalmologists, optometrists, dispensing opticians, orthoptists, ophthalmic nurses, rehabilitation workers, occupational therapists, social workers, peer workers and psychologists, to name but a few. Although the book mentions, where appropriate, the situation in the UK, it also covers the worldwide status, and the contents should prove valuable to those wishing to push back the frontiers of the field, irrespective of location. We hope that this book will be of use in routine eye care practice to enhance patient care from diagnosis to rehabilitation and, in particular, to optimise the quality of life of visually impaired people. James S Wolffsohn A Jonathan Jackson

Dedication Collectively, we would wish to dedicate this book to those who have come along to see us as patients, or indeed as the parents or guardians of our ‘smaller’ patients, and who have placed their trust in us to help and advise on the optometric, medical and rehabilitative management of visual impairment. To our many visually impaired friends we would wish to express our genuine thanks for all that you have taught us over the years. Jonathan, James, Giuliana and Owen

Acknowledgements This text, which has been a labour of love over many years, could not have evolved to this point had it not been for the long suffering xii

Preface

support of Carolyn, Rachel, Gordon and Pauline, our respective wives and husband. Their support, and sacrifice, is very much appreciated, as too is that of our children, Daniel, Lauren, Joshua, Peter, Laura and Adam. We would also wish to acknowledge the support of clinical colleagues at the Royal Victoria Hospital and, in particular, the secretarial support of Miss Elizabeth Elliman and Ms Amanda Macfarlane. Our thanks also go to the members of the Photographic Team at the RVH and QUB, Mr Mark Tierney, Mr David McCallum, Ms Stephanie O’Connor and Mr Vittorio Silvestri and to our many friends and colleagues from GDBA and RNIB who have assisted with many of the illustrations. Finally, we would wish to thank Ms Barbara Ryan for having reviewed the text and providing helpful suggestions concerning layout and content. Prof AJ Jackson, Dr JS Wolffsohn, Dr G Silvestri, Mr OF Adams

xiii

Editors

Dr Jonathan Jackson studied Ophthalmic Optics/Optometry at Glasgow College of Technology (UK), achieving a 1st class Honours Degree (1981). Upon completion of a pre-registration year at Moorfields Eye Hospital, London he obtained membership of the College of Optometrists and was awarded both the Scottish and Colebrook prizes (1982). After returning to Belfast to establish a hospital optometry department at the Royal Victoria Hospital, he completed a PhD entitled ‘An Analysis of Corneal Endothelial Morphology under Normal and Traumatic Conditions’ at Queen’s University (1993). He is currently Principal Optometrist at the Royal Victoria Hospital and is Head of Professional Ophthalmic Services at the Northern Ireland Central Services Agency. Professor Jackson holds an Honorary Senior Lectureship in the School of Biomedical Sciences/Centre for Vision Science, Queen’s University and a visiting Professorship at the Department of Optometry, University of Ulster, Coleraine. His research interests include visual disability and corneal physiology/contact lenses with particular emphasis on paediatrics and learning disability. Professor Jackson has contributed to, as either principal or senior author, in excess of 60 peer reviewed scientific papers and has presented research findings at a broad range of national and international multidisciplinary meetings. The Belfast visual impairment team which he leads holds a large number of research grants from national and regional funders. xiv

Editors

Dr James Wolffsohn studied optometry at UMIST, Manchester, UK, achieving a 1st class degree. He qualified to practice independently following a pre-registration year at Moorfield’s Eye Hospital, London. Following this, James undertook a PhD on ‘the effects of visual imagery on the oculomotor system’ at Cardiff University and funded by British Aerospace. He then took up a clinical/ research position at the Victorian College of Optometry/University of Melbourne, Australia in 1997. In 2000, he returned to the UK and a lectureship at Aston University, being promoted to Senior Lecturer in 2002 and Reader in 2006. He is now Head of Optometry. James’ research and teaching interests mainly revolve around contact lenses, low vision and the measurement of accommodation, having published over 65 peer reviewed academic papers and given numerous international presentations. James is also the past President of the British Contact Lens Association.

xv

Contributors

Owen F Adams MA Cert in Technical Work for the Blind Low Vision Consultant, Downpatrick, UK Nicholas J Rumney MScOptom FCOptom FAAO Bishop, Bishop & Rumney, Hereford, UK Janet Silver OBE, DSc, FCOptom London, UK Giuliana Silvestri MD FRCS FRCP(Ed) FRCOphth Senior Lecturer & Consultant Ophthalmic Surgeon, Head of Division of Surgery & Perioperative Care, Department of Ophthalmology, Queen’s University Belfast, Royal Victoria Hospital, Belfast, UK

xvi

Plates 1 & 2 (Fig. 2.2) Example of a ‘flecked’ retina. Both colour plates show fundus flavimaculatus at different stages of the disease. Plate 1, Early changes with well defined yellow ‘fish-tail’-like flecks and mild macular atrophy. Visual acuity is relatively well preserved at this stage. Plate 2, More advanced stage of the disease, with significant macular atrophy.

Plates 3 & 4 (Fig. 2.4) Advanced retinitis pigmentosa with extensive retinal pigmentary changes, vascular attenuation and optic disc pallor. Plate 3, right eye, Plate 4, left eye.

Plates 5 & 6 (Fig. 3.1) Treated diabetic retinopathy in a 24-year-old woman. The patient has been treated with panretinal photocoagulation for proliferative retinopathy (scars in the peripheral retina of the left eye – Plate 6) and with bilateral grid laser for maculopathy (subtle scars in the macular areas – Plates 5 & 6).

Plates 7 & 8 (Fig. 4.5) Anterior segment view of an intraocular miniaturised telescope (IMT) implanted within the capsular bag (Plate 7) with corresponding view through the implant of the retina showing a disciform scar (Plate 8).

Plate 9 (Fig. 6.2) Corneal topographical maps illustrating the difference between regular with-the-rule astigmatism (upper panel) and keratoconus (lower panel). The steep, inferiorally positioned, cone in the lower panel gives rise to a distorted scissor-like retinoscopy reflex, irregular astigmatism and a progressively increasing myopic refractive correction.

Plate 10 (Fig. 7.4) The Thomson Test Chart 2000 computer-generated visual acuity assessment system. (Courtesy of Professor D Thomson; reproduced with permission from Macnaughton 2005.)

Plate 11 (Fig. 7.11B) Accurate positioning of targets by the Nidek MP1 microperimeter, which is used to assess retinal sensitivity within, and adjacent to, retinal areas of specific interest. (Reproduced with kind permission of Nidek Technologies.)

Plate 12 (Fig. 7.12) Selection of colour vision tests: a, City colour vision test; b, Ishihara plates; c, Jumbo D15 (PV16) (buttons), which are of particular use when testing patients with a visual acuity of 6/20 or less.

Plates 13 & 14 (Fig. 9.2) Television viewing at 2 m (Plate 13), resulting in a doubling of size compared with viewing at 4 m (Plate 14). In this case the change in accommodative demand (0.5D for 2-m viewing and 0.25D for 4-m viewing) is likely to be within the patient’s depth of focus (tolerance to blur), and hence a distance prescription is still appropriate.

Plate 15 (Fig. 14.1) A range of EVES.

Plate 16 (Fig. 15.1) High contrast, large print, tactile cooking items.

Plate 17 (Fig. 15.2) Other items for the kitchen.

Plate 18 (Fig. 15.3) Marked up cooker, cup with liquid level indicator, and high contrast kettle.

Plate 19 (Fig. 15.5) Large print, high contrast and tactile games.

Plate 20 (Fig 15.7) paper.

Writing guide, typoscope, and raised and thick lined

Plate 21 (Fig. 15.8) Range of writing implements and the visibility of their ink against a white paper background.

Plate 22 (Fig. 15.9) Mobility and symbol canes.

Plate 23 (Fig. 15.10) Electronic reader.

Plate 24 (Fig. 16.4) Guide dog and user.

Plates 25 & 26 (Fig. 18.1) Place setting as seen by someone with normal vision (Plate 25) and how it may be seen by someone with visual impairment (Plate 26).

Plates 27–32 (Fig. 18.2) Real world scenes and how they may be seen by someone with visual impairment: street scene (Plates 27 & 28);

Plates 27–32 (Fig. 18.2) countryside view (Plates 29 & 30);

Plates 27–32 (Fig. 18.2) difficulties encountered in a supermarket environment (Plates 31 & 32).

SECTION ONE

Ophthalmology for low vision Section Editor: Giuliana Silvestri

CHAPTER

1

Epidemiology of low vision A. Jonathan Jackson

Epidemiology has been defined as ‘the study of the distribution, determinants and control of diseases in human populations’.1 When applied to the world of ophthalmology and optometry, it concerns the identification, management and prevention of eye disease, in different populations, so as to promote normal vision, to prevent blindness and visual impairment, and to preserve ocular health. Fundamental to the process of providing low vision care to visually impaired persons is an understanding of the terminology surrounding visual impairment. This chapter seeks to highlight a range of regularly quoted definitions and to review the causes of visual impairment from a global perspective.

1.1 Definitions of visual impairment Many of the definitions used to describe visual impairment have evolved as organisations responsible for healthcare management 1

Ophthalmology for low vision

and the delivery of services in the developed world have attempted to classify disability in such a way as to control, or alternatively enhance, access to services or benefits. The term ‘blind’, which depends on the context in which it is used, may be preceded by the words ‘educationally’, ‘legally’ or ‘functionally’, and is often used to indicate profound visual impairment requiring specialist services or financial assistance. This term, which is highly emotive, has also been used in the title deeds of many voluntary sector service providers that depend on the generosity of the public at large, such as the Royal National Institute for the Blind (RNIB), Guide Dogs for the Blind Association (GDBA) and the Blind Centre for Northern Ireland (BCNI). Regrettably the term now carries with it the perception that ‘all sight is gone’ and that the individual thus affected is helpless and, to some extent, to be pitied. This in itself can cause embarrassment and difficulty for those labelled with the term, as at some later date they may be identified by peers and others in society, including shopkeepers, bus drivers and volunteer helpers, as having some form of usable vision. The perception thus taken is that they have falsely acquired the title. Terminology used to describe less severe forms of visual impairment includes ‘partial sight’, ‘low vision’ and ‘subnormal vision’. A review of the literature illustrates how these terms have been used almost synonymously and that preference for one as opposed to another changes with time. In peer-reviewed ophthalmic literature published in the UK, the terms ‘blind’ and ‘partial sight’ are usually linked with data on registration, whereas the terms ‘low vision’ and ‘subnormal vision’ have been used when referring to the provision of optometric services for the visually impaired. The term ‘visual impairment’ has been used to describe a broader spectrum of sight loss. In recent years, in an attempt to de-stigmatise visual impairment, more general descriptive terms referring to ‘sight loss’ and ‘problems associated with vision’ have been advocated by some. Unfortunately many of these terms are not clearly defined and their introduction contributes to confusion in the epidemiological world, leading at times to greatly overestimated prevalence values for severe visual impairment.

1.1.1 World Health Organization definitions Since its inception in the 1940s the World Health Organization (WHO) has shown an interest in the prevention of blindness. In 1973 the WHO highlighted that one of the major problems associ2

Epidemiology of low vision

1

Table 1.1 WHO Definitions of Visual Impairment3,4 Category of visual impairment

Visual acuity with best possible correctiona Maximum less than

Minimum equal to or better than

1

6/18 (20/60) [0.5]

6/60 (20/200) [1.0]

2

6/60 (20/200) [1.0]

3/60 (20/400) [1.3] CF at 3 m

3

3/60 (20/400) [1.3] CF at 3 m

1/60 (20/1200) [1.8] CF at 1 m

1/60 (20/1200) [1.8] CF at 1 m

Light perception (PL)

Low vision

Blindness 4

a

5

No perception of light (NPL)

9

Undetermined or unspecified

Values are Snellen metres, (Snellen feet) and [LogMAR]. CF, count fingers.

ated with the collection of definitive data on visual disability was the non-standardisation of definitions.2 It is estimated that, on a worldwide scale, 65 different definitions of blindness and poor vision are in existence. By 1978, in an attempt to bring a degree of consistency to the classification of visual disability and blindness, the WHO proposed a standard classification that could be used on a worldwide basis.3 The proposals were subsequently included in the tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10).4 Regrettably the classification has not been universally adopted, although references to it are now common (Table 1.1).

1.1.2 Blind and partially sighted registration definitions In the UK the statutory definition of blindness, as defined in the 1920 Blind Persons Act5 and subsequently incorporated into the 1948 National Assistance Act6 states that a person is ‘so blind as to be unable to perform work for which eyesight is essential’. It is the 3


Table 1.2 Quantifiable Categories of Blindness, UK5,6 Group

Description

1

Visual acuity of less than 3/60 [1.3] or less than 1/18 [1.25] if tested at a closer distance, in a patient with full visual fields

2

Visual acuities ranging from 3/60 [1.3] to less than 6/60 [1.0] with significantly contracted visual fields

3

Visual acuity of 6/60 [1.0] or better with a gross visual field constriction, particularly in the lower field

[LogMAR equivalent acuities]

responsibility of the consultant ophthalmologist to certify a visually impaired person and, thus, to open the door for them to access community social services. With respect to registration, the following points are of note: 1 The definition regarding blind registration is not whether the person is unable to pursue his or her ordinary occupation, but rather whether he or she is too blind to be able to perform any work for which eyesight is essential. 2 Although information concerning other contributory factors is requested on the blind registration form, advisory notes that accompany the form clearly state that in defining vision impairment these conditions should be disregarded, with only the visual problems being taken into account. Clinical guidelines issued subsequent to the 1948 Act indicate that blind registration should be restricted to those whose visual acuities fall within one of three groupings (Table 1.2). If the extent of the visual field is taken into account, patients with a visual field radius no greater than 10° but greater than 5° around central fixation should be placed in category 2, and those with a field no greater than 5° around central fixation should be placed in category 3, even if the central acuity is not impaired. No indication is given, however, with regard to the methods used to quantify visual field loss. Interestingly, those with homonymous or bitemporal hemianopia, who have retained a central acuity of 6/18 [LogMAR 0.5] or better, are to be excluded from blind registration. Guidance concerning the impact of recent, as opposed to long standing, visual impairment is considered by most to be ambiguous. 4


1

Table 1.3 Quantifiable Categories of Partial Sight, UK6 Group

Description

1

Visual acuity of 3/60 [1.3] to 6/60 [1.0] with full fields

2

Visual acuity of up to 6/24 [0.6] with a moderate field constriction, medial opacities or aphakia

3

Visual acuity of 6/18 [0.5] or better with a gross field defect (i.e. hemianopia)

[LogMAR equivalent acuities]

Within the statutory context of the 1948 Act, no exact definition of less profound visual impairment is given, although subsequent guidelines define the partially sighted as ‘those who are substantially and permanently handicapped by defective vision caused by congenital defect, illness or injury’. Guidelines concerning visual acuity requirements are generally considered to be more flexible than those adopted for blind registration. Further clarification is given to those who are responsible for the certification of children, in that those with visual acuities of 6/24 [LogMAR 0.6] or better should be considered candidates for mainstream schooling. Children, ‘unless obviously blind’, should only be classified as ‘partially sighted’, and at the age of 4 years and over the binocular vision should be considered as the determining criterion. The clinical guidelines for partially sighted registration, released subsequent to the 1948 Act, are outlined in Table 1.3. Until recently three different types of registration form have been used in the UK: BD8 in England and Wales, BP1 in Scotland and A655 in Northern Ireland. The BD8, the most recent version of which was introduced in 1990, was a four-part document that included information on visual status in part A, information relevant to ocular health in part B, a section on proposed registration status in part C, and treatment plans and education or employment recommendations in part D. The BP1 and A655 were broadly similar in concept, although the layout of the forms was entirely different. All forms included ophthalmological data, designed to be held on an epidemiological database. The year 2003 brought changes to the registration system in England. These changes were designed to: • Assist with the early identification of visual impairment for social care intervention 5


• Increase registration uptake • Improve the accuracy of data collection on the incidence and type of eye disease resulting in visual impairment. Three new forms were proposed: the CVI 2003, which will replace the conventional certification/registration form; the LVI 2003, which is designed to initiate referrals from optometrists to social services; and the RVI 2003, which is designed for use by nonconsultant hospital eye service staff. Interestingly all three forms contain sections on disability and the impact of visual impairment on daily living. Information targeted specifically at those who may have a driving licence has been highlighted, drivers having been warned of the consequences of driving when vision fails to meet the standard requirements (http://www.dh.gov.uk/PolicyAnd Guidance/HealthAndSocialCareTopics/fs/en). In Scotland the recommendations of a review on registration, carried out in 2001, are in the process of implementation.7 In Northern Ireland the Department of Health and Social Services introduced new certification and registration forms, based on the CVI, LVI and RVI mainland equivalents, to the province in the spring of 2005. In the USA, state aid became available to the visually impaired in the 1930s, whereas the legal definition of blindness was introduced under the Social Security Act in 1935.8 Blindness is defined as a best corrected visual acuity in the better eye of less than or equal to 20/200 [LogMAR 1.0] or, if the visual acuity in that eye is better than 20/200 [LogMAR 1.0], a visual field of less than or equal to 20° in the widest diameter. Having been classified as legally blind, the visually impaired person becomes eligible to receive supplementary security income and social disability insurance. In Canada those classified as legally blind are eligible to receive all of the services available through the Canadian National Institute for the Blind (CNIB).9 Similar statutory registration systems exist in many other developed countries. Within a European context, research has indicated that of 47 states only 33 have a legal definition of blindness or visual impairment. Of these, only two sets of definitions were entirely consistent with WHO guidelines. Thirty-seven states utilised quantifiable data on visual acuity, whereas only 16 used definitions that included reference to visual field size, to define levels of visual impairment. Other states use functional definitions based on employment and social welfare law. In the Netherlands, for example, the blind are defined as ‘those who are obliged to read 6


1

Braille or make use of the spoken word’, whereas in Norway the definition is applied to ‘those who have reduced vision to the extent that it is impossible or difficult to read normal writing and or orientate themselves with the help of sight’.10 Information on European definitions, although not always available in a consistent and comparable format, can be obtained from the European branch of the World Blind Union (http://www.worldblindunion.org).

Registration anomalies The astute reader and experienced low vision practitioner will be acutely aware of the problems associated with definitions linked to registration. Visual field specifications are of course dependent on the target size and brightness, whereas visual acuity measurements fluctuate widely depending on testing conditions and, in particular, lighting conditions. The visually impaired individual, tested by a zealous clinician in a properly illuminated consulting room, may be unfairly classified as being partially sighted, whereas, if tested by another more sympathetic practitioner in suboptimal conditions, may be registered as blind. It is also important to remember that visual acuity can vary from day to day with some conditions such as diabetic retinopathy and multiple sclerosis. These factors become particularly important if registration is being embarked upon as a process through which access to benefits will be gained. The all or nothing concept linking registration to benefit entitlement is a particular cause of concern. These factors, together with others associated with psychological attitudes to registration, help to explain some of the epidemiological anomalies inherent in registration data, as highlighted in articles by Bunce et al,11 Evans & Wormald12 and Robinson et al.13 Bunce et al concluded that registration data are biased towards permanent non-treatable causes of central visual loss. Robinson et al highlighted the need to remove the ‘negative perceptions’ associated with registration held by professionals, and stressed the need to target the elderly and those with chronic sight-threatening eye disease.

1.1.3 Disorder, impairment, disability and handicap For many years the terms impairment, disability and handicap have been used inappropriately. These terms are neither synonymous nor interchangeable. They represent different aspects of the 7


Table 1.4 International Classification of Impairment, Disability and Handicap Introduced by the WHO in 1980 (ICIDH-2)14 Disorder

Usually used to describe the impact of the disease or injury on the anatomical structure of visual function within the organ or, in the case of vision, a component of the visual pathway

Impairment

The consequence, in terms of measurable loss or departure from functional capacity, to the bodily organ, affected by disorder or disease, of an anatomical or physiological function

Disability

The consequence to the patient in terms of the effect of the impairment on the patient’s abilities

Handicap

The consequence of the disability in terms of how it affects the patient’s ability to interact with society

problems that result from a disturbance of human functioning. The International Classification of Impairment, Disability and Handicap (ICIDH-2/1980), introduced by the WHO in 1980, attempted to standardise the terminology in terms of the functional consequences of the disease process (Table 1.4).14 Table 1.5 indicates how the 1980 classification applies to a number of ophthalmic conditions. An understanding of the concept helps to bridge the gap between the results obtained when recording quantifiable visual function data in the clinical environment and the statements made to describe the impact that sight loss has on an individual patient. Two individuals with exactly the same degree of impairment may experience entirely different levels of disability and handicap. The taxi driver with a best corrected visual acuity of 6/18 [LogMAR 0.5] resulting from macular oedema of recent onset will find the withdrawal of a driving licence devastating from both an economic and social perspective, whereas an adult with congenital nystagmus, who never drove in the first place, may cope with impaired visual function without great difficulty. In 2000, the classification was upgraded to ICIDH-2/2000 and renamed the International Classification of Functioning, Disability and Health.15 The new classification provides a multiperspective approach to the classification of functioning and disability as an interactive and evolutionary process. It allows both scientific 8

Age-related macular degeneration

Retinitis pigmentosa

Congenital cataracts (aphakia)

Oculocutaneous albinism

Example 1

Example 2

Example 3

Example 4

Reduced VA and CS, and photophobia

Reduced VA and CS, glare disability and reduced near acuity

Contracted VF, reduced CS and impaired night vision

Reduced VA and CS

Reduced functional performance

Impairment

CS, contrast sensitivity; VA, visual acuity; VF, visual field.

Disease or injury

Description

Disorder

The organ

Orientation difficult in bright light. Problems with distance vision

Near vision tasks including reading more difficult than distance tasks

Walking speed reduced, Mobility problems

Reading speed and fluency reduced

Reduced skills or abilities

Disability

Restrictions to certain travel tasks

Difficulties with educational development

Limitations to unaccompanied travel and resulting social isolation

Difficulty in assessing written material and correspondence

Limitations on social participation

Handicap

The person

Table 1.5 Classification of Loss or Impairment – Disorder and Impairment Relate to the Organ whereas Disability and Handicap Relate to the Person


1

9


investigations and clinical/rehabilitation providers to map different aspects of the process. Table 1.6 illustrates how disruption of bodily function has an impact on a person’s abilities and how these can have difference consequences, depending on the impact of both environmental and personal factors on the situation. The concept is more complex to grasp than that outlined in the original ICIDH and, as such, the author feels, within the world of visual impairment, that it does not supersede the original concept.

1.1.4 Low vision Those approaching the problem of visual impairment from an optical or optometric background advocate the term ‘low vision’, which to a large extent has evolved from the term ‘subnormal vision’. This term is almost synonymous with visual impairment, with the added provision that the residual vision is usable. Those persons who are totally blind, having a visual acuity of no perception of light in both eyes, make up less than 6% of the visually impaired population, and those with a visual acuity of perception of light only, a further 5%. Some 11% of visually impaired persons are therefore not included in the category of low vision patient. Low vision could thus be defined as ‘vision that, when corrected by optimal refractive correction, is not adequate for the patient’s needs’. Low vision is thus a functional definition that can be applied easily to any patient with a disease or disorder affecting the visual system. Some authorities advocate the term ‘residual vision’, emphasising that attention needs to be paid to that which has been retained rather than that which has been lost. In dealing with rehabilitation issues, this is particularly important to the patient.

1.2 Epidemiology Although epidemiology began with the study of infectious disease outbreaks such as cholera, it has expanded in such a way that the methodology can be applied to the study of any disease process affecting human populations.1,16

1.2.1 Epidemiological methodologies Strategies used in epidemiological research can essentially be grouped into two main categories: those that involve 10

Bodily functions Body parts

Physiological change Anatomical change

Functional and structural integrity

Impairment Disability

Domains

Constructs

Positive aspects

Negative aspects

Function and structure

Activity limitation Participation restriction Disability

Activity participation

Capacity and performance

Life areas (i.e. tasks and actions)

Activities and participation

Functioning and disability

Barriers and hindrances

Facilitators Functioning

Hindrances of features in the physical or social world

External influences

Not applicable

Not applicable

Impact of attributes on person

Internal influences

Personal

Contextual factors Environmental

Table 1.6 ICIDH-2 Overview of the Functional Classification of Disability and Health15


1

11


interventional experimental design (randomised controlled trials or community interventions) and those that can be described as utilising observational design (cross-sectional, case-controlled and longitudinal studies). In the case of the randomised controlled trial, individuals invited to take part in the study, using predetermined inclusion or exclusion criteria, are allocated randomly to a treatment type or intervention, whereas others are allocated to a control or placebo-type group. Outcomes are assessed by individuals who are, where possible, unaware of which arm of the study patients have been allocated to. For studies involving recruitment of a small sample, allocations may have to be stratified to help ensure similarity between the groups. The nature of randomised controlled trials necessitates that the treatment under evaluation be withheld from one group. Although this can be difficult for patients to accept, it is essential for proper evaluation of the therapy. It is important to explain the process carefully to the patient and to enable them to understand that the treatment is not yet proven and may have significant side-effects. Observational studies differ in that populations are examined according to whether they have, or have not, been exposed to factors that may have an impact on health and disease. A population exposed to high levels of ultraviolet radiation over prolonged periods of time, in whom cataract is prevalent, may be compared, for example, with a group of individuals who have had no such exposure. The major problem with this type of study is the need to compensate for confounding factors such as environmental exposures, genetic predisposition, or a combination of the two. An understanding of the terms ‘prevalence’ and ‘incidence’ is crucial to the appreciation of epidemiological studies.

Definition of Prevalence and Incidence No. of cases or events Prevalence =

Incidence = Total population at risk

12

No. of new cases over a given time interval Total population at risk at the beginning of the specified time


1

A more sophisticated way of expressing incidence is, however, to use the incidence rate, which considers the number of new cases occurring over the course of a follow-up study in relation to the total number of persons at risk during the same period. Practitioners involved in the provision of low vision services may be more interested in the prevalence of age-related macular degeneration (AMD) in the community, whereas the contact lens practitioner may be interested in the incidence of soft contact lens-related microbial keratitis. AMD is a chronic, essentially untreatable, condition that requires long-term rehabilitative intervention, whereas contact lens-related microbial keratitis is an acute condition that, if treated effectively, usually resolves rapidly, leaving the patient visually unaffected. Other methods of expressing epidemiological data, relevant to visual impairment, include the specification of the regional burden of blindness (RBB) and the age blindness burden (ABB). Definition of Regional Burden of Blindness and the Age Blindness Burden Percentage of the world’s blind population living within a region of interest RBB =

Percentage of the world’s blind population in a specified age group ABB =

Percentage of the world’s total population living within the designated region of interest

Percentage of the world’s total population that falls within that age group

The RBB equation essentially attempts to determine whether there is an equitable distribution of blindness within an area in comparison to other regions. If, for example, there are 510 million people living in sub-Saharan Africa, 7.1 million of whom are blind, the prevalence of blindness in the region is 1.4%. This compares to a global prevalence figure of 0.7%. The RBB in sub-Saharan Africa, which has an overall population of 510 million and represents 9.7% of the world’s population of 5.26 billion, is thus 1.9. A value greater than 1 indicates a need to prioritise the treatment and prevention of blindness in the specified region. Application of the ABB equation to global figures of blindness illustrates the impact of degenerative eye disease on an ageing population. On a global basis, ABB rises from 0.12 in children 13


under the age of 15 years to 2.68 in adults between the ages of 45 and 59 years. Differences in the elderly population become more marked when comparing ABB values for the developed world with those for the developing world. Another unit of measurement, worthy of note, is the disability-adjusted life-year (DALY). This measurement combines the influence of premature loss of life, which is more prevalent in the developing world, with the loss of healthy life-years from disability.17

1.2.2 Global epidemiology Although analysis of registration data from developed countries with adequate social care facilities provides a basis for comparison, an entirely different approach has to be taken when looking at the incidence and prevalence of visual impairment in the developing world. In these cases prevalence has to be established from representative community-based studies, the results of which need to be extrapolated to the population as a whole. Thylefors et al18 divided world data into eight economic regions and examined the prevalence accordingly. In established market economies, including western Europe, the USA, Australia, New Zealand and Japan, the prevalence of blindness is estimated as 0.3%. In Latin America, China and the Middle East it rises to 0.5–0.7%, whereas in Asia and India the proportion rises to almost 1.0%. Most strikingly the prevalence in sub-Saharan Africa is approximately 1.4%. The overall global estimate of blindness is 45 million, with a further 135 million individuals classified as having low vision (Table 1.7, Fig. 1.1).19 Another striking factor is the relative distribution of blindness according to age across the world. In the developed world blindness is most likely to be associated with essentially untreatable degenerative processes associated with ageing, whereas in the developing world there is a much higher prevalence of preventable childhood blindness. In China, sub-Saharan Africa and the Middle East an estimated 16 million people are blind as a result of cataracts, whereas 5.9 million are blind from trachoma. A further 0.3 million residents in Central and West Africa are blind from onchocerciasis. The latter two conditions have, to all intents and purposes, been eliminated from countries with established market economies. Blindness resulting from cataract represents less than 4% of the overall burden of blindness in the developed world, whereas lack of access to cataract surgery in the 14


1

Table 1.7 The Global Distribution of Blindness1,19 Estimated no. of blind people (millions)

Regional prevalence of blindness (%)

Regional Major blindness causes by burden region

Established market economies

2.4

0.3

0.41

AMD 50% Glaucoma 8%

Former socialist block

1.1

0.3

0.41

AMD 84% Glaucoma 7% Cataract 8.3%

Latin and Central America

2.3

0.5

0.72

Cataract 57% Glaucoma 8%

China

6.7

0.6

0.82


Middle East

3.6

0.7

0.74

Cataract 45% Trachoma 25%

Asia

5.8

0.8

1.18


India

8.9

1.0

1.46


Sub-Saharan Africa

7.1

1.4

1.93


37.9

0.7

1.0

Overall

developing world ensures that cataract remains a major cause of blindness. Foster & Gilbert20 estimated that, of the 1.5 million blind children worldwide, 1.3 million reside in Asia and Africa, and that 75% of this blindness could have been prevented or is curable. The prevalence rate for childhood blindness in Europe and North America (0.03%) contrasts markedly with that in Africa (0.11%). Similarly, causes differ according to region. Genetic causes account for the majority of childhood visual impairment in the developed world, whereas in the developing world infection, including measles and rubella, causes intrauterine and infant visual 15


EME 0.3% FSE 0.3% LAC 0.5% CHI 0.6%

MEC 0.7% ACI 0.8% IND 1.0% SSA 1.4%

Figure 1.1 Estimated prevalence of blindness according to World Bank economic regions. EME, established market economies; FSE, former socialist market economies; LAC, Latin America and Caribbean countries; CHI, China; MEC, Middle Eastern crescent; ACI, Asian countries and islands; IND, India; SSA, sub-Saharan Africa. (Adapted from Johnston & Foster 1998.1)

impairment. Distressingly, between 60% and 80% of children who become blind die within 2 years of having become blind.21

1.2.3 Epidemiology in the UK Epidemiological data on visual impairment in the UK is generally taken from one of three sources. The most widely quoted figures, which most authorities agree underestimate the true extent of the problem, are those obtained from regional blind and partially sighted databases. More detailed data, which tend to be collected by locality and relate to population subgroups (the elderly, children, or those with specific disease entities), appear in peerreviewed medical and scientific publications. In addition, voluntary sector organisations and those dealing with the rehabilitative requirements of the visually impaired often use information extracted from government census returns and population surveys on self-perceived levels of visual impairment. 16


1

Table 1.8 Regional UK Blind and Partially Sighted Registration Data (2004) Region

Registration status (n) Blind

England

PS

Total

Estimated prevalence (%) Blind

PS

Total

156 675

155 230

311 905

0.32

0.31

0.63

9 643

10 565

20 208

0.33

0.36

0.69

Scotland

23 557

14 443

38 000

0.46

0.28

0.74

Northern Ireland

2 273

3 122

5 395

0.14

0.18

0.32

Wales

PS, partially sighted.

Registration data Within the UK, registration data have been scrutinised since the 1950s when Arnold Sorsby published the first of his comprehensive reviews on the subject.22 Data pertaining to the past 15 years have been analysed extensively by Evans and co-workers.23,24 Current data from all four UK regions can be accessed via departmental websites: • England – http://www.dh.gov.uk/PublicationsAndStatistics/ Statistics/StatisticalWorkAreas/StatisticalSocialCare/fs/en • Wales – http://www.dataunitwales.gov.uk/eng/Data. asp?cat=252 • Scotland – http://www.scotland.gov.uk/stats/bulletins/ 00292-00.asp • Northern Ireland – http://www.dhsspsni.gov.uk/ comstats_04.pdf Recent figures indicate that there are approximately 377 000 individuals registered as blind or partially sighted in the UK (Table 1.8). The ratio of those registered as partially sighted to those registered as blind has, over a 20-year period, changed from approximately 1 : 2.5 to almost 1 : 1 in England and Wales, whereas in Scotland partially sighted registration still appears to be unrepresentative of the true prevalence of moderate visual impairment (Fig. 1.2). The levelling off in blind registration and the progressive increase in partially sighted registration is in keeping with 17

Number of individuals


350000 300000 250000 200000 150000 100000 50000 0

England

82 84 86 88 90 92 94 96 98 00 02 04 19 19 19 19 19 19 19 19 19 20 20 20 Year Number of individuals

25000

Wales

20000 15000 10000 5000 0

82 84 86 88 90 92 94 96 98 00 02 04 19 19 19 19 19 19 19 19 19 20 20 20 Year Number of individuals

40000

Scotland

30000 20000 10000 0

Number of individuals

82 84 86 88 90 92 94 96 98 00 02 04 19 19 19 19 19 19 19 19 19 20 20 20 Year 7000 6000 5000 4000 3000 2000 1000 0

N. Ireland

82 84 86 88 90 92 94 96 98 00 02 04 19 19 19 19 19 19 19 19 19 20 20 20 Year Partially sighted

Blind

Total registered

Figure 1.2 Changing trends in blind and partially sighted registration in the UK, recorded over a 20-year period.

18


Blind

Partial sight 9.7%

20%

1

8.5%

3.3% 3.4% 3.4%

23.1%

11.7%

48.5%

No information on main cause Cataract Optic atrophy Diabetic retinopathy

7% 2.3% 3% 9.6%

46.5%

Glaucoma Macular and post polar degeneration Other conditions

Figure 1.3 Causes of blind and partially sighted registration by primary ophthalmic disease. (Adapted from Evans 1995. 23)

changing population demographics (ageing) and an associated increase in degenerative ophthalmic pathology, which results in moderately severe central visual loss (AMD). Detailed analysis of registration data is, however, undertaken only sporadically. Evans & Wormald,25 in a review of English blind registration data collected since the 1950s, found that, once corrected for changing age profiles in the population, only registration rates attributable to AMD showed an increasing trend (from 6% in 1933–1943 to 49% in 1990–1991). Registration rates resulting from glaucoma and optic atrophy showed a small but significant decline, whereas blind registrations resulting from cataract decreased by a factor of 12 over the same time interval. Analysis of the causes of blind, as opposed to partially sighted, registration in England and Wales, undertaken in 1990 and 1991, indicated that only for cataract did the trends for blindness and partial sight differ significantly (Fig. 1.3):23,24 • Blind registrations – AMD 48.5%, glaucoma 11.7%, diabetic retinopathy 3.4%, optic atrophy 3.4%, cataract 3.3% • Partially sighted registrations: AMD 46.5%, glaucoma 9.6%, cataract 7.0%, diabetic retinopathy 3.0%, optic atrophy 2.3%. New registration data for Northern Ireland, collected over a 15year period (1984–1996), illustrate similar trends.26 19


Analysis of 1980–1981 and 1990–1991 data published by Evans et al23–25 highlights the extent to which both blindness and partial sight disproportionately affect the elderly: • Blind registrations – age 0–15 years, 3 per 100 000 population; 16–64 years, 5 per 100 000; 75–84 years, 200 per 100 000; 85 years and over, 530 per 100 000 • Partially sighted registrations – age 0–15 years, 4 per 100 000 population; 16–64 years, 7 per 100 000; 65–74 years, 66 per 100 000; 75–84 years, 231 per 100 000; 85 years and over, 416 per 100 000. This burden of age-related visual impairment is likely to increase as the population ages.

Population-based data Most data relating to population-based studies are specific to the age-related degenerative conditions that affect the elderly. The reason for this is that it is disproportionately expensive to collect prevalence data, using screening methodologies, for conditions that occur relatively infrequently. Important UK studies include those by Cullinan27 in 1978, Gibson et al28 in 1986, Lavery et al29 in 1988, Wormald et al30 in 1992, Reidy et al31 in 1998, Van der Pols et al32 in 2000 and Evans et al33 in 2002. Lavery et al,29 in a general practice-based survey of 529 individuals aged 75 years and older, living in Melton Mowbray, found that 18% of men and 30% of women had a visual acuity of 6/18 [LogMAR 0.5] or worse. Only 3.8% of this population group had a best corrected acuity of less than 6/36 [LogMAR 0.8]. Interestingly, 95% could achieve N10 or better with an appropriate near correction. Wormald et al,30 examining 207 individuals aged 65 years and older, recruited from general practice lists in inner London, found the prevalence of blindness (WHO) to be 1% and the prevalence of visual impairment (visual acuity less than 6/15 [LogMAR 0.4]) to be 7.7%. Van der Pols et al,32 as part of the National Diet and Nutrition Survey of those aged 65 years or more living in either private homes or nursing homes in the UK, found that 9.3% had a habitual or pinhole visual acuity of less than 6/18 [LogMAR 0.5]. Interestingly, the prevalence of visual impairment was 3.5 times greater amongst those living in nursing homes.32 This finding confirms what is known about the increased prevalence of visual impairment amongst those with other forms of 20


1

disability.34 In the largest and most recent study, conducted on 14 600 subjects aged 75 years and over, from 53 general practice lists across the UK, Evans et al33 found that the prevalence of low vision (visual acuity less than 6/18 [LogMAR 0.5] to 3/60 [LogMAR 1.3]) was 10.3% and blindness (visual acuity less than 3/60 [LogMAR 1.3]) 2.1%. Acuity was measured using Glasgow Acuity Cards and recorded with both the habitual correction and, when indicated, a pinhole.33 A review of Gibson’s Melton Mowbray data indicated the extent of sight-limiting pathology in the elderly (cataract 46%, AMD 41%, primary open angle glaucoma 6.6%).28 Wormald et al30 further highlighted the fact that almost 75% of visual impairment in the elderly was potentially treatable through the provision of an appropriate refractive correction or cataract surgery. In a parallel study, Das et al35 provided evidence of a disproportionate degree of potentially treatable visual impairment in ethnic communities in the UK. Recent results from Evans et al,36 which complement previous registration reviews, indicate that of 1742 visually impaired elderly persons recruited from 49 general practices 32.0% had AMD, 20.4% cataract, 6.4% glaucoma and 2.1% diabetic eye disease. Most strikingly, 31.6% of this population subgroup were found to be visually impaired as a result of uncorrected refractive errors.36 Other regional and local studies from which UK data are available include those from Bristol,37 Bradford,38 Nottingham,39 Avon40 and Leicestershire.41 In addition to studies carried out in the elderly, a number of studies have examined the prevalence of visual impairment in children. Few studies have involved the screening of large population groups, although Stewart-Brown & Haslum42 investigated 15 000 children born between 5 and 11 April 1970, and found the prevalence of blindness to be 3–4 per 100 000 and that of partial sight to be 5–9 per 100 000. Research by Rahi & Cable,43 using alternative case-finding methodology, found higher prevalence rates, with a cumulative incidence of visual impairment in young children of 5.9 per 100 000 by the age of 16 years. Most importantly, 77% of these children were noted to have additional nonophthalmic disabilities and disorders. Local studies from Oxfordshire,44 Liverpool45 and Belfast46 have confirmed these findings.

Government survey data An alternative approach to collecting data from registration databases or from population-based studies involves the use of 21


large-scale household surveys. The most well known of these, within the UK, is the Office of Population Censuses and Surveys (OPCS) Disability Survey.47 In the survey, which was sent to 100 000 private households, questions on visual disability asked about ‘difficulty reading newspaper print and recognising a friend from across the road’. Questions were always appended with the phrase ‘even if glasses or contact lenses are worn’. Of the 20 415 individuals who responded ‘yes’ to one of the disability questions, 2534 were noted to have a self-reported visual disability. Results recorded from communal institutions, including nursing homes, were subsequently included in the analysis. Estimated age-specific prevalence rates for visual impairment in the UK were calculated as 0.8% for those aged 16–59 years, 5.6% for those aged 60–74 years and 26.2% for those over 75 years. Using these data, the estimated number of individuals in the UK who are visually impaired approaches 1.5 million. Analysis of similar Family Resource Survey data by Grundy et al48 indicates that this figure could be as high as 1.9 million. This represents almost 25% of all disabled adults. A supplementary survey on those identified from previous government-based surveys carried out by the RNIB and using self-reported questionnaires found the prevalence of significant visual impairment (difficulty reading newsprint) to be 2.8% of all adults and 14.4% of those over 75 years of age.49

1.3 Summary This chapter seeks to highlight the importance of epidemiology in the detection, treatment and long-term management of eye disease resulting in visual impairment. Fundamental to the process is the need to use internationally agreed definitions of disease, impairment, disability and handicap as outlined by the WHO, and of working towards agreed definitions of visual impairment, blindness and low vision. The situation concerning data collection in the UK has been reviewed and important issues concerning blind and partially sighted registration highlighted. Readers have, in addition, been shown how causes of visual impairment can vary greatly depending on location and sampling methodology. Differences between the UK and other global geographical locations have been highlighted. 22


1

References 1. Johnston GJ, Foster A. Prevalence, incidence and distribution of visual impairment. In: Johnston GJ, Minassion DC, Weale R, eds. The epidemiology of eye disease. London: Chapman & Hall; 1998:7–30. 2. World Health Organisation. The prevention of blindness. Report of a WHO study group. WHO Technical Report Services 518. Geneva: WHO; 1973. 3. World Health Organisation. Guidelines for programmes for the prevention of blindness. Geneva: WHO; 1979. 4. World Health Organisation. International statistical classification of diseases and related health problems (ICD-10). Geneva: WHO; 1992. 5. The Blind Person’s Act 1920. London: HMSO. 6. National Assistance Act 1948. London: HMSO. 7. Report of the Certification and Registration Working Group. Social work services for people with a sensory impairment. Edinburgh: Scottish Executive; 2001. 8. Goldstein H. The demography of blindness throughout the world. New York: American Foundation for the Blind; 1980. 9. Leat SJ, Legge GE, Bullimore MA. What is low vision? A reevaluation of definition. Optometry and Vision Science 1999; 76:198–211. 10. Jackson AJ, Gallagher B, Hart PM. European blindness: a review of national definitions and currently available epidemiological data. Ophthalmic Research 2000; 32(Suppl 2):117. 11. Bunce C, Evans J, Fraser S, Wormald R. BD8 certification of visually impaired people. British Journal of Ophthalmolgy 1998; 82:72–76. 12. Evans JR, Wormald RPL. Epidemiological function of BD8 certification. Eye 1993; 7:172–179. 13. Robinson R, Deutsch J, Jones HS et al. Unrecognised and unregistered visual impairment. British Journal of Ophthalmology 1994; 78:736–740. 14. World Health Organisation. ICIDH-2. International classification of impairment, disability and handicap. Geneva: WHO; 1980. 15. World Health Organisation. ICIDH-2. International classification of functioning, disability and health. Geneva: WHO; 2000. 16. Minassian OC. Epidemiological research methods. In: Johnston GJ, Minassion DC, Weale R, eds. The epidemiology of eye disease. London: Chapman & Hall; 1998:33–61. 17. Bos E, Vu MT, Levin A, Bulatao RA. World population projection 1992–1993. Estimates and projections with related demography statistics. Baltimore: Johns Hopkins University Press; 1992. 18. Thylefors B, Negrel AD, Pararajasegaram R, Dadzie KY. Global data on blindness. Bulletin of the World Health Organisation 1995; 73:115–121.

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19. World Health Organization. Global initiative for the elimination of avoidable blindness. WHO/PBL 97:61, Rev 2. Geneva: WHO; 2000. 20. Foster A, Gilbert C. Epidemiology of childhood blindness. Eye 1992; 6:173–175. 21. Cohen N, Rahman H, Sprague J, Jalil MA, Leemhuis de Regt E, Mitra M. Prevalence and determinants of nutritional blindness in Bangladeshi children. World Health Statistics Quarterly 1985; 38:317–329. 22. Sorsby A. The causes of blindness in England and Wales. Medical Research Council Memorandum No. 24. London: HMSO; 1950. 23. Evans JR. Causes of blindness and partial sight in England and Wales 1990–1991. Studies on Medical and Population Subjects No. 57. London: HMSO; 1995. 24. Evans JR, Rooney C, Dattani N, Ashwood F, Wormald RPL. Causes of blindness and partial sight in England and Wales. Health Trends 1996; 28:5–12. 25. Evans JR, Wormald R. Is the incidence of registerable age-related macular degeneration increasing? British Journal of Ophthalmology 1996; 80:9–14. 26. Canavan YM, Jackson AJ, Stewart A. Visual impairment in Northern Ireland. Ulster Medical Journal 1997; 66:92–95. 27. Cullinan TR. Epidemiology of visual disability. Transactions of the Ophthalmological Society of the UK 1978; 98:267–269. 28. Gibson JM, Lavery JR, Rosenthal AR. Blindness and partial sight in an elderly population. British Journal of Ophthalmology 1986; 70:700–705. 29. Lavery JR, Gibson JM, Shaw DE, Rosenthal AR. Vision and visual acuity in an elderly population. Ophthalmic and Physiological Optics 1988; 8:390–393. 30. Wormald RPL, Wright LA, Courtney P, Beaumont B, Haines AP. Visual problems in the elderly population and implications for services. British Medical Journal 1992; 304:1226–1229. 31. Reidy A, Minassian DC, Vafidis G et al. Prevalence of serious eye disease and visual impairment in a North London population: population based, cross-sectional study. British Medical Journal 1998; 316:1643–1646. 32. Van der Pols JC, Bates C, McGraw PV, Thompson JR, Reacher M, Prentice A, Finch S. Visual acuity measurements in a national sample of British elderly people. British Journal of Ophthalmology 2000; 84:165–170. 33. Evans J, Fletcher AE, Wormald R et al. Prevalence of visual impairment in peopled aged 75 years and above in Britain: results from the MRC trial of assessment and management of older people

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34.

35.

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39. 40.

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45. 46.

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in the community. British Journal of Ophthalmology 2002; 86:795–800. Warburg M. Visual impairment in adult people with intellectual disability: literature review. Journal of Intellectual Disability Research 2001; 45:424–438. Das BN, Thompson JR, Patel R, Rosenthal AR. The prevalence of eye disease in Leicester – a comparison of adults of Asian and European descent. Journal of the Royal Society of Medicine 1994; 87:219–222. Evans JE, Fletcher AE, Wormald RPL. Causes of visual impairment in people aged 75 years and above in Britain: an add on to the MRC trial of assessment and management of older people in the community. British Journal of Ophthalmology 2004; 88:365– 370. Clark JB, Grey RHB, Lim KKT, Burns-Cox CJ. Low of vision before ophthalmic referral in blind and partially sighted diabetics in Bristol. British Journal of Ophthalmology 1994; 78:741–744. Yap M, Weatherill J. Causes of blindness and partial sight in the Bradford metropolitan district from 1980–1989. Ophthalmic and Physiological Optics 1989; 9:289–292. Aclimandos WA, Galloway N. Blindness in the city of Nottingham (1980–1985). Eye 1988; 2:431–434. Grey RHB, Burns-Cox CJ, Hughes A. Blind and partial sight registration in Avon. British Journal of Ophthalmology 1989; 73:88–94. Thompson JR, Rosenthal AR. Recent trends in the registration of blindness and partial sight in Leicestershire. British Journal of Ophthalmology 1989; 73:95–99. Stewart-Brown SL, Haslum MN. Partial sight and blindness in children of the 1970 birth cohort at 10 years of age. Journal of Epidemiology and Community Health 1988; 42:17–23. Rahi JS, Cable N, British Childhood Visual Impairment Study Group. Severe visual impairment and blindness in children in the UK. Lancet 2003; 362:1359–1365. Croft BJ, King R, Johnson A. The contribution of low birth weight to severe vision loss in a geographically defined population. British Journal of Ophthalmology 1998; 82:9–13. Rogers M. Vision impairment in Liverpool: prevalence and morbidity. Archives of Disease in Childhood 1996; 74:299–303. Flanagan NM, Jackson AJ, Hill AE. Visual impairment in children: insight from a community based survey. Child Health Care and Development 2003; 29:493–499. Martin J, Meltzer H, Elliot D. The prevalence of disability among adults. OPCS surveys of disability in Great Britain,

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Report 1. London: Office of Population Censuses and Surveys; 1988. 48. Grundy E, Ahlburg D, Ali M, Breeze E, Sloggett A. Disability in Great Britain: results from the 1996/1997 disability survey. Technical Report 94. London: Department of Social Security; 1999. 49. Royal National Institute for the Blind. Survey of the needs and lifestyles of visually impaired adults. Technical Report (1998/1999). London: Office of National Statistics; 2000.

26

CHAPTER

2

Visual impairment in the young Giuliana Silvestri

Visual loss in childhood and the early teens is predominantly congenital or hereditary in nature.1 In this chapter the various conditions causing visual impairment in this age group are discussed, with particular emphasis on the diagnostic features that are helpful in making the correct diagnosis. The three most common clinical scenarios presenting to the ophthalmologist are discussed in detail: • The neonate or young baby who appears to be visually impaired • The visually impaired baby with no obvious ophthalmic abnormality • The child with visual difficulty. Recent advances in the understanding of the molecular genetics of many of these conditions have been truly exciting, and a brief review of the recent molecular discoveries is given where relevant.

2.1 Visual development The refractive state of the normal neonate is 2–3 dioptres of hypermetropia, accompanied by a high prevalence of astigmatism. At birth the globe measures 16.5 mm in diameter and over the first year the diameter increases rapidly to reach an almost adult 27


diameter of 24.5 mm.2 The spherical equivalent refractive error of full-term newborn infants is normally distributed about a mean of +2 dioptres with a standard deviation of 2 dioptres.3 The retina is well developed at term; however, the foveal region is immature, not reaching adult maturity until 4 months of age. The neural pathways are also immature at term, with cells in the lateral geniculate ganglion reaching adult size at age 2 years and the optic nerve becoming fully myelinated at 7 months. Coupled with increasing maturation of the visual system, the hypermetropic state reduces steadily and visual acuity improves. By the age of 20–30 months the visual acuity is estimated to be 6/6 [LogMAR 0.0]. In young children with a refractive error, the standard deviation of the error is greater the earlier the onset of the visual impairment; for example, the refractive error in tyrosinase-negative albinism would be much greater than that in Stargardt’s disease where near-normal variability is expected.

Practical advice A baby who is emmetropic or myopic at birth is likely to become progressively more myopic during childhood and adolescence.

2.2 Assessment of visual function Assessment of the visual acuity of a neonate or a small baby requires patience, attention to detail and a modification of the techniques used for adults. The examiner should be prepared to spend a substantial amount of time on the examination. At birth the full-term neonate can see colours and faces at arm’s length, and fixation should be present. In the term infant, fixation is the most reliable clinical test of visual function. The type of fixation target used, however, is of paramount importance.

Practical advice Eighty-three per cent of neonates will follow a face but not a white light. A smile in response to a silent smile should be present at 6 weeks of age and indicates good central vision.

28

Visual impairment in the young

2

Visually directed reaching, although possible at 3–4 months, is usually not a useful clinical test until approximately 6 months of age. Other important signs in a baby who is suspected of being visually impaired include the presence of abnormal ocular movements, abnormal pupillary reactions (although often difficult to illicit in the neonate), the presence of eye rubbing or poking, lack of facial mobility or expression, and the absence of optokinetic nystagmus.

Practical advice Abnormal eye movements and visual inattention are the most common signs of poor vision. The more uncoordinated the movements, the more impaired the visual acuity.

The parents are often the first to sense that there may be a visual problem with a young baby. Experienced mothers tend to present their children earlier. It is important not to dismiss parental concerns, as this may backfire. A useful adage is: ‘If in doubt, believe the mother and either re-examine or refer’. Even if the eyes look normal, the parents may complain of the child not fixing or reacting appropriately to visual stimuli. There may be an obvious abnormality such as a white pupillary reflex ‘leucocoria’ or a squint. Occasionally family snapshots using flash photography show up the absence of a red reflex and bring this to the parent’s attention.

Practical advice If in doubt, believe the mother and either re-examine or refer.

On examination, the position and steadiness of the eyes in primary gaze is important. The presence of a persistent squint may indicate that there is an opacity of the media. It must be remembered that transient losses of binocular fixation can be normal for the first 3–4 months of life; after this age, most infants demonstrate consistent binocular ocular alignment over a range of stimulus distances.4 Fusion in infants develops between 4.5 and 6 months of life. Any abnormal ocular movements such as nystagmus, or more rare abnormalities such as saccadomania or ‘dancing eyes’, may be indicative of a midline cerebral tumour. 29


The presence of leucocoria (white reflex) points to the diagnosis of cataract, primary hyperplastic vitreous or retinoblastoma. Enlargement of the globe may indicate congenital glaucoma. If there is no opacity of the media, detailed examination of the fundus, and in particular of the optic discs, is possible. Optokinetic nystagmus is not a test that is specific for visual acuity, but one that is representative of the integrity of the visual system. It is normally absent in blind and severely brain-damaged children. In children with a profound visual problem, the parents often notice that the baby appears to be unresponsive to visual stimuli.

2.2.1 Electrophysiological testing Electrophysiological testing is extremely important in the assessment of poor vision in neonates and children. Experience and patience are required for the acquisition of good quality results in the very young. These tests not only help to secure specific diagnoses, but by systematic assessment of function along the visual pathways can also localise the problem underlying the visual defects. Among children, development as well as disease can affect electrophysiological parameters (Fig. 2.1); therefore, the diagnosis of abnormality depends critically on knowledge of the normal responses for age.5 A brief description of the most common electrophysiology tests and their function is given below. • Visually evoked potential (VEP) detects dysfunction of the visual pathways from the optic nerve to the visual cortex • Electroretinography (ERG) assesses the function of the neuroretina. Different stimuli can be used to assess the function of the rod and cone photoreceptors separately. The ERG waveform can also be used to differentiate between dysfunction in different layers of the neuroretina and is an indicator of whether the problem lies within the macular area or the optic nerve. The P50 component is representative of macular function and the N95 component of optic nerve function, although an abnormality in one may influence the other • Electro-oculography (EOG) assesses the integrity of the retinal pigment epithelial (RPE) cells The interpretation of electrophysiological data is a complex issue and must always be made with reference to the clinical findings. Examples of expected electrophysiology abnormalities in inherited disorders are given below (see Table 2.2, p. 39) 30

2


(a)

Pattern ERG 40’ check

(d)

P50

5

OD

0

uV

uV

b wave

200

OD N95

0

OS

OS

–5

–200 0

50

100

P50 max (ms) (b)

150

200

0 (e)

150

200

Maximal ERG Bright white flash b wave

500 OD uV

a wave

0 OS

100

b wave max (ms)

b wave

OD

50

N95 max (ms)

Cone ERG Bright red flash

500 uV

Rod ERG Dim blue flash (subject dark adapted)

Oscillatory potentials

0 a wave OS

–500

–500 0

20

a wave max (ms) (c)

40

60

80

100

b wave max (ms)

0 (f)

30-Hz Flicker ERG Moderate white flashes

100 b wave max (ms)

EOG Light peak/Dark trough %

500

b wave

200 OD

50

a wave max (ms)

uV

OD

0

Dark trough

0

Light peak

OS

OS –200

–500 0

50

100

150

200

b wave max (ms)

0

5

10

15

20

25

30

Dark adaptation/Light adaptation (min)

A Figure 2.1 A, Normal waveforms for standard electrophysiology tests (a–f). The ERG represents the combined electrical activity within the retina. The ‘a’ wave represents the activity of the photoreceptors; the ‘b’ wave has its origin in the Muller (glial) cells. (a) The pattern ERG demonstrates good macular function (P50) peak and normal optic nerve function (N95) trough. Normal cone function is shown by a normal cone ERG (using a bright red flash) (b) and a normal 30 Hz flicker ERG (c). Normal rod function is illustrated by a normal rod ERG tested under conditions that preferentially stimulate rod function (dim blue flash) (d). The maximal ERG response gives an indication of total photoreceptor function (e). The EOG waveform, which reflects retinal pigment epithelial function, indicates a normal ‘dark to light’ rise which reflects the comparison of amplitudes under dark and light adapted states (f).

31


(a)

Pattern ERG 40’ check

(d)

5

200 RE uV

uV

RE 0

0 LE

LE

–200

–5 0

100

50

P50 max (ms) (b)

150

200

0

50

100

N95 max (ms)

150

200

b wave max (ms) (e)

Cone ERG

200

Maximal ERG

500

RE

RE uV

uV

Rod ERG

0 LE

–200

0 LE

–500 0

20

40

a wave max (ms) (c)

60

80

100

b wave max (ms)

30-Hz Flicker ERG

50 RE

0

50

a wave max (ms)

100 b wave max (ms)

(f) 333

EOG

uV

RE

0

0 LE

LE

–333

– 50 0

50

100

150 200 b wave max (ms)

0

5

10

15

20

25

30

Dark adaptation/Light adaptation (min)

B Figure 2.1 B, Electrodiagnostic tests from a patient with retinitis pigmentosa showing ‘flat’ tracings for the maximal ERG (e), cone ERG (b) and rod ERG (d), indicating profound loss of photoreceptor function. There is also absence of the ‘light rise’ on the EOG (f). Interestingly, the patient’s central vision remains 6/9 [Log MAR 0.2] in either eye, indicating some preservation of central photoreceptors. This is confirmed by the pattern ERG (a), which shows an abnormal but relatively preserved waveform.

32


2

2.3 Clinical scenarios 2.3.1 The neonate or young baby who appears to be visually impaired When a baby with a suspected visual problem is presented to the ophthalmologist, important clinical signs to look for are those detailed in Section 2.2. If there appears to be a visual problem, the working differential diagnosis is as follows (this list is given in alphabetical order and is not all inclusive, as many other rare conditions exist): • • • • • • • • • • • •

Albinism Cerebral blindness Congenital cataract Congenital glaucoma Congenital idiopathic nystagmus High refractive error Leber’s congenital amaurosis Macular colobomata Optic atrophy or hypoplasia Primary hyperplastic vitreous Retinoblastoma Retinopathy of prematurity.

Retinopathy of prematurity (ROP) is unlikely in a full-term infant, but if the baby was of low birthweight or pre-term (born earlier than 37 weeks) ROP should be considered. Babies with a birthweight of less than 1500 g are at risk of ROP and should be screened routinely. Total visual loss from ROP has been reported in 2–4% of babies who weigh less than 940 g.6 Examination of the optic discs may reveal the diagnostic double halo of hypoplastic discs or the normal-sized but pale discs of optic atrophy. If an intracranial tumour is present, papilloedema may be seen. The heredofamilial optic atrophies are characterised by their patterns of inheritance, and hereditary optic atrophy can present in several different ways. The clinical findings and age of onset are different in each group. A simplified list of the spectrum of the inherited optic atrophies is shown in Table 2.1. The presence of leucocoria is often reported by the parents and, although the most common cause of leucocoria is cataract, retinoblastoma must always be excluded promptly. Retinoblastoma is a rare life-threatening tumour with complex genetic inheritance. 33


Those affected demonstrate a predisposition to retinoblastoma, which can be either heritable or non-heritable. Childhood cataract, although surgically treatable, causes significant visual morbidity because of associated amblyopia, even with early surgical treatment. Cataracts that are present at birth or become apparent in the first year of life account for just under 20% of all causes of blind registration in children under the age of 15 years in England and Wales. Genetically determined cataracts are a heterogeneous group of disorders.7 Approximately 25% of congenital cataract is inherited in an autosomal dominant manner; autosomal recessive cataract is uncommon in Britain and is more prevalent in communities where consanguinity is common. X-linked cataracts are rare. They usually occur in isolation, although congenital cataract can be associated with other ophthalmic manifestations such as microphthalmos, glaucoma and coloboma. Congenital cataract can also be part of a systemic syndrome, of which there are many. Congenital glaucoma occurs in 1 in 10 000 births in Western countries. Although several modes of inheritance have been documented, in most families the disorder is transmitted in an autosomal dominant manner with full penetrance. A full account of the molecular genetic details can be found at the Online Mendelian Inheritance in Man (OMIM) website (http://www.ncbi.nlm.nih.gov/omim).8 Even though the ocular globe is often enlarged at birth in congenital glaucoma owing to raised intraocular pressure in utero, an early diagnosis is advantageous in limiting field loss and preventing increasing buphthalmos. Evidence also exists indicating that, owing to the plasticity of the visual system, young babies may have some potential for optic nerve recovery if treated early. In the absence of a positive family history, the diagnosis can sometimes be overlooked in the early stages. Congenital glaucoma often presents with a ‘watery eye’. The crucial question on history taking is to determine whether the ‘epiphora’ was present at birth or developed later. Epiphora from nasolacrimal duct obstruction is usually not present at birth but begins when tear production becomes well established. A watery eye at birth should flag up the diagnosis of congenital glaucoma. Congenital idiopathic nystagmus and macular colobomata are rare causes of poor central vision, whereas primary hyperplastic vitreous presents as an opacity in the media. High refractive errors should always be kept in mind in these cases. 34

Dominant

4–8

Mild/moderate

6/12 [LogMAR 0.3] to 6/60 [LogMAR 1.0]

Blue/yellow defect. Note: if acquired red/green defect – wrong diagnosis

Rare

Mild temporal pallor

Pattern of inheritance

Age of onset (years)

Visual loss

Final visual acuity

Colour vision

Nystagmus

Optic discs

Juvenile (infantile)

Moderate

1–9

Recessive

Behr’s syndrome

Marked diffuse pallor

Usual

Severe dyschromatopsia

Mild temporal pallor

In 50%

Moderately severe dyschromatopsia

6/60 [LogMAR 1.0] 6/60 [LogMAR 1.0] to hand motion

Severe

3–4

Recessive

Congenital (simple)

Table 2.1 Clinical Features of the Inherited Optic Atrophies

Marked diffuse pallor

No

Severe dyschromatopsia

3/60 [LogMAR 1.3] to 1/60 [LogMAR 1.8]

Severe

6–14

Recessive

Optic atrophy with diabetes ± deafness

Disc swelling and telangiectatic vessels in acute stage. Moderately diffuse pallor

No

Dense central scotoma for colours

6/60 [LogMAR 1.0] to 1/60 [LogMAR 1.8]

Moderate to severe, depending on the mutation (see section 2.5.1)

11–30

Mitochondrial inheritance

Leber’s optic neuropathy


2

35


Practical advice As epiphora from nasolacrimal duct obstruction is usually not present at birth but begins when tear production becomes well established, a watery eye at birth should flag up a diagnosis of congenital glaucoma.

Children with multiple handicaps Children who suffer from congenital conditions such as rubella, cytomegalovirus, toxoplasmosis or fetal alcohol syndrome often have multiple handicaps that can involve hearing as well as vision. Recent studies have indicated that almost half of visually impaired children have cerebral palsy and the majority will exhibit behavioural problems (63%), learning disabilities (50%) and other sensory deficit (hearing impairment 18%) early in childhood.9

2.3.2 The visually impaired baby with no obvious ophthalmic abnormality If the baby is visually impaired but no obvious ophthalmological problem is detectable clinically, the following diagnoses should be strongly reconsidered and the baby re-examined. These conditions can be difficult to detect in the early stages: • Albinism – oculocutaneous (tyrosinase positive) and ocular albinism • Cerebral blindness • Delayed visual development • Leber’s congenital amaurosis • Optic atrophy • Optic disc hypoplasia. In the early stages, Leber’s congenital amaurosis often shows no abnormality on fundoscopy; the optic discs, however, can sometimes appear slightly pale, and early thinning of the arterioles is an important early sign in this condition. The pupillary reflexes, although sometimes normal, are often sluggish or absent. The baby is often photophobic. Electrophysiological testing is helpful for diagnostic purposes. Recently, Leber’s congenital amaurosis has been mapped to five genetic loci, indicating a spectrum of disease manifestation or phenotypic variation. Exact details can 36


2

be located on the OMIM website.8,10 Leber’s amaurosis is listed as disorder no. 204000. Optic disc hypoplasia can be subtle and is easily missed unless specifically looked for with the direct ophthalmoscope. Comparison of the optic disc size with the large aperture on the ophthalmoscope can be useful in diagnosing optic disc hypoplasia. Indirect ophthalmoscopy alone is not sufficiently sensitive to pick up hypoplasia of the optic disc. Other helpful signs include the presence of nystagmus, sluggish pupillary reflexes and the characteristic optic disc double ring sign.

Practical advice The large target on the Welch Allyn direct ophthalmoscope is the same size as an average optic disc.

Cerebral blindness manifests with normal ophthalmic findings, including the presence of normal pupillary reflexes. In cerebral blindness, the electroencephalogram (EEG) is abnormal; the baby often has other signs of developmental delay and may have midline defects including cleft lip or palate. Albinism can be very evident and a family history may be present, although the signs can be subtle in tyrosinase-positive ocular albinism. The pupillary reflexes are normal, as are the optic discs, but the baby may be photophobic. Iris transillumination is present, but may be difficult to elicit in a young baby using the slit lamp. The fundus will appear albinotic or blond.11

Practical advice An easy way of detecting iris transillumination in a baby is to place the transilluminator from the ophthalmoscope on the lower lid in a darkened room. Iris defects will be seen easily.

Delayed visual development involves no ophthalmological or electrophysiological abnormality. The baby is often premature or small for dates (smaller than the expected weight for age). Except in children with multiple handicaps, it is unusual for delay to persist 37


beyond the age of 4 months.12 Although the prognosis for vision is generally good, a small proportion of patients are left with a residual deficit. The electrophysiological findings in the conditions described above are summarised in Table 2.2.

2.3.3 The child with visual difficulty History and examination These children have usually been normally sighted in infancy and have thus had a relatively normal early educational history. Problems usually become apparent late in the first decade. During the early years of primary school, these children begin to have progressive problems with seeing the blackboard and subsequently with small print and low contrast material. In this age group a history of the visual symptoms and a positive family history of inherited conditions may also be present. Children with decreased distance acuity with good near acuity may simply be myopic. The normally sighted myope will, however, use a conventional near working distance, whereas the visually impaired myopic child will tend to hold things closer than expected. Other details on history taking that can be helpful include the following. Children suffering from optic atrophy complain of reducing vision but usually have no other specific symptoms. Those suffering from cone dystrophies, however, characteristically complain of intense photophobia and difficulty with colour vision.

Practical advice Interestingly, patients with cone dystrophy may remark that their visual acuity is better in dim lighting. Visual acuity can increase by two to three lines in mesopic conditions.

The parents of children with early-onset retinitis pigmentosa may report night blindness or restriction of fields, which is often manifested as clumsiness. Parents who suffer from retinitis pigmentosa are often acutely sensitive to the presence of night blindness in their children. In retrospect, when asked, people with retinitis pigmentosa will admit to having been night blind from as far back as they can remember. 38

AR

AD, AR or X-linked

Sporadic

Sporadic

Sporadic

Leber’s amaurosis

Optic atrophy

Optic disc hypoplasia

Cerebral blindness

Delayed visual development

Inheritance pattern

Normal vision often develops

Poor

Good or poor, unilateral or bilateral

Variable 6/12 [LogMAR 0.3] to hand motion

Very poor or = 20/20 in 74%, 20/25 to 20/40 in 18%,

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