Butterworth–Heinemann An imprint of Elsevier Limited © 2004, Elsevier Limited. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without either the prior permission of the publishers or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency, 90 Tottenham Court Road, London W1T 4LP. Permissions may be sought directly from Elsevier’s Health Sciences Rights Department in Philadelphia, USA: (+!) 215 238 7869, fax: (+1) 215 238 2239, e-mail:
[email protected]. You may also complete your request on-line via the Elsevier Science homepage (http://www.elsevier.com), by selecting ‘Customer Support’ and then ‘Obtaining Permissions’. First published 2004 ISBN 0 7506 5560 7 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 Medical knowledge is constantly changing. As new information becomes available, changes in treatment, procedures, equipment and the use of drugs become necessary. The author/contributors and the publishers have taken great care to ensure that the information given in this text is accurate and up to date. However, readers are strongly advised to confirm that the information, especially with regard to drug usage, complies with the latest legislation and standards of practice.
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Preface Recent years have seen an exponential growth in the field of refractive surgery. In 1996, the US Federal Drug Administration’s approval of certain excimer lasers to correct myopia lead to a rapid increase in the uptake of excimer laser procedures. Further approvals have been granted for other laser manufacturers, and for the correction of astigmatism and hyperopia. Popularity increases as patients hear about the successful outcomes for friends and relatives. Famous people who undergo this surgery are a further boost, and laser clinics are quick to state the names of famous patients who have undergone treatment. Techniques have improved, better lasers and other equipment are available and there seems to be an unlimited supply of patients willing to undertake surgical alternatives to wearing spectacles or contact lenses. Practitioners, both optometrists and ophthalmologists, seem to be split over refractive surgery. Some advocate its usefulness as a viable alternative, whereas others feel it is nothing more than cosmetic surgery for refractive error. It may be oversimplifying the issue to call refractive surgery a cosmetic procedure, as patients often state that their disability requires the use of optical aids, almost like using an aid to assist hearing. However, it would be correct to say that refractive sur-
gery is an elective procedure, as the patient chooses to undergo surgical intervention on an otherwise healthy eye, and the surgeon agrees to operate on an eye that is without pathology. In the UK refractive surgery is offered on a private basis only. There have been attempts to treat higher refractive errors on the NHS, but these schemes tend to be regional and not the norm. Patients who decide to undergo refractive surgery either book in to a refractive surgery clinic or go to see a consultant ophthalmologist who offers treatment privately. Surgeons who offer refractive surgery do not need to be consultant ophthalmologists accredited with the Royal College of Ophthalmologists, although many are. However, they must have suitable qualifications, such as Member or Fellow or the Royal College of Ophthalmologists (MRCOphth or FRCOphth), Fellow of the Royal College of Surgeons (FRCS) or be listed on the European Specialist Register. Most laser manufacturers ensure that doctors who use their lasers have attended the relevant training courses to use that particular apparatus. In the UK the most common types of refractive surgery currently employed are photorefractive keratectomy (PRK) and laser in-situ keratomileusis
(LASIK), although PRK is being replaced by laser epithelial keratectomy (LASEK). Both of these use the same currently widespread excimer laser technology. Other techniques are available, but to a lesser degree. This book examines various aspects that may be relevant to those interested in learning more about the current status of refractive surgery, with particular attention paid to patient selection, available surgical techniques and the evaluation of patients pre-operatively and post-operatively (details of some specialist instrumentation are also outlined). Clinicians with a degree of knowledge in refractive surgery may be interested in the chapters that discuss wound healing after refractive surgery and case reports from surgeons. Of general interest, the book also discusses legal issues and future trends in this fastchanging area. Notes for the CD-ROM The large size of the videos means that the loading of video clips 4 and 5, in particular, may take 1–2 minutes on some computers, and users of Mac OS9 may see a white screen while the videos are loading. When the videos finish loading, the screen will change and the Play, Pause, and other buttons will appear Shehzad A Naroo
Contributors Alejandro Cerviño DOO (EC) W Neil Charman DSc, PhD Paul MH Cherry MBBS, LRCP, FRCS(Ed), FRCS, FRCSC, FRCOphth Catharine Chisholm PhD, MCOptom Sandip Doshi PhD, MCOptom Stephen J Doyle BSc(Hons), MRCOphth Balasubraminiam Ilango FRCS(Ed), MRCOphth Mohammad Laiquzzaman MBBS, PhD Shehzad A Naroo MSc, PhD, MCOptom, FIACLE Sunil Shah FRCS(Ed), FRCOphth Baldev K Ubhi BSc(Hons)
1 Patient selection and pre-operative assessment Shehzad A Naroo
For many patients who want to find out more about refractive surgery the first port of call is often the local ophthalmic practice, while others call dedicated clinic phone lines or browse the Internet. Prospective patient interest can be classed into two categories: those who make casual enquiries to see if they are suitable and those who have decided that this is definitely something they will opt for. The first group may progress to become part of the second group when they feel they are more informed. The latter group can sometimes be difficult to dissuade from surgery if they are found to be unsuitable. Those patients who make casual enquiries often seek general advice only and can usually be referred to websites or professional bodies that produce this type of information. Whereas for patients who have decided to opt for refractive surgery, it is usually advisable to make a specific appointment to discuss the surgical options and perform the relevant tests (or else refer the patient to a colleague who is able to do this). Some patients may suspect that optometrists have their own agenda and advise against refractive surgery because they want to protect their own livelihood. Furthermore, many optometrists may feel that their knowledge about the current state of affairs is not adequate and thus choose not to become involved at all and advise patients not to proceed with this option. A proactive approach towards refractive surgery by optometrists is advised by some refractive surgery clinics, and more recently the number of optometrists who have become involved in co-management schemes with refractive surgery providers has increased (discussed in Chapter 7). However, a careful balance needs to be struck by optometrists in rou-
tine practice to ensure they are best able to serve their patient’s needs.1 Optometrists are in a unique primary care position in eye health from which they can offer an unbiased opinion. Various studies have shown the average age of prospective patients to be the mid-to-late thirties with an almost equal ratio of male to female patients.2,3 The author recently completed a study (not yet published) that shows the average patient age to be creeping up to around 40 years, and there seems to a shift towards more female patients. Since refractive surgery usually involves an initial financial outlay comparable to that for contact lenses, which in the UK are often paid by monthly bank debits, most studies seem to show a prevalence of patients from higher socio-economic groups. This may partly explain the age groups of refractive surgery patients too. Figure 1.1 shows the breakdown of the occupational groups of new patients who presenting for refractive surgery.
Retired 6%
Student 3%
Most studies highlight that many patients who present for refractive surgery are former contact lens users.4,5 Often the reasons why people want contact lenses are similar to the motivation for patients to have refractive surgery, so it is not surprising that there are some similarities in the types of patients who present for both types of refractive correction. Both groups of patients often say that they want the freedom from spectacles or they want to achieve a cosmetic look that spectacles do not allow, or perhaps the reasons are related to certain activities (work or sports, etc.). Patients who cease contact lens use in favour of refractive surgery often complain of the inconvenience of contact lenses and/or complications with contact lenses, which is a primary motivation for their decision. Often, many of the less serious complications with contact lenses that patients complain of could be minimized with improved contact lens management, which requires the appropriate input from their contact lens practitioners.
Unemployed 5%
Unskilled 12%
Professional 19% Management 15%
Semi-skilled 9% Clerical 31%
Figure 1.1 Breakdown of the occupational groups of new patients who present for refractive surgery3
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Refractive surgery: a guide to assessment and management
Also, some patients choose refractive surgery as a primary alternative to spectacles and present for surgery even though they have not tried contact lenses. This may result, in part, from the way that laser refractive surgery is marketed. In many cases it would be useful for the patient to try contact lenses first. Laser refractive surgery clinics advertise on the radio, newspapers and television. There seems to be an interesting shift in the way that advertisers have portrayed refractive surgery over the years. In the early days the convenience of refractive surgery was used to herald it as being a ‘quick’, ‘painless’ and ‘safe’ treatment that only took a few seconds or minutes to complete and the patient would return to work shortly afterwards. The next wave of advertising seemed to use people that patients could relate to, either celebrities who would advocate a certain clinic or ‘real’ people that were respected in the community, such as firemen, nurses and even priests. The most recent advertising trend seems to focus on the technology that a particular centre uses, although in the UK this approach has come under the scrutiny of the Advertising Standards Association. Patients who opted for refractive surgery gave the main factors shown in Figure 1.2 as those that influenced their decision to cease contact lens use; the values relate to the percentage of patients who offered the particular reason as an influential factor.3 Patients who are former contact lens wearers are advised to remove their contact lenses prior to their pre-operative refractive surgery consultation. The time period for lens removal depends on the type and modality of lens worn. Typically, soft lenses are removed for 7–14 days prior to the appointment and gas permeable lenses for
Costs 21%
10–21 days. Users who wear hard polymethylmethacrylate (PMMA) lenses may find that they have to leave their lenses out for a few months, especially if they are longterm wearers, to ensure that any corneal distortion is eliminated. Patients are often asked to produce past refraction details, for up to the previous 3 years, to show that they have some level of stability. A patient with a large recent change in refraction would probably be advised to wait until two or three consecutive prescriptions were similar. If patients undergo refractive surgery and then find that a year later their prescriptions naturally became worse, they will often be dissatisfied with the outcome. It has been suggested that refractive surgery may aid visual development in children with squints that are purely accommodative. This type of service would not typically be offered by most commercial refractive surgery clinics and currently it is not widely available in hospital refractive clinics either. Patients under the age of 21 years who present for refractive surgery are often advised to wait until they reach 21, or until their prescription has stabilized.6 Although there is no upper age limit for refractive surgery, it may be unwise to perform a corneal procedure on late presbyopic patients with lens sclerosis, as they may be better suited to clear lens extraction with an accurately calculated intraocular lens implant. Patients with only one ‘seeing’ eye are considered a contraindication to refractive surgery, as infection in the good eye would seriously compromise the patient’s visual function, although the risk of sight-threatening infection is extremely rare after refractive surgery.7,8 Also, in photorefractive keratectomy (PRK) surgery the two
Red eye 14%
Dry eye 18%
Overwear 17%
Intolerance to solutions 7%
Intolerance to lenses 17%
Professional advice 5% Advice from friends 1%
Figure 1.2 Main factors that influenced the decision to cease contact lens use and opt for refractive surgery (values relate to the percentage of patients who offered the particular reason as an influential factor)3
eyes are operated on over an interval of around 3 months, and the operated eye does not achieve its final prescription for a few weeks and often is quite blurred during the first week after surgery. So patients who are amblyopic in the non-treated eye may experience some degree of visual disability while the first treated eye reached its final prescription.9,10 Conical corneas, such as keratoconus or keratoglobus, are considered as contraindications to refractive surgery. Both of these conditions have associated thinning at the apex of the conical cornea, which may lead to ectasia after corneal refractive surgery. However, a corneal topography pattern that appears to indicate keratoconus without any other clinical sign of the disease may not necessarily be a contraindication.11,12 An irregular corneal surface, possibly caused by other types of disease or dystrophy such as Fuchs’ endothelial dystrophy, is also considered a reason not to proceed with refractive surgery. However, many corneal surgeons use an excimer laser to perform a phototherapeutic keratectomy (PTK) on patients with conditions such as recurrent corneal erosions or band keratopathy. In this an even layer of stromal tissue is removed to smooth off the irregularities at the anterior stroma, with a wide ablation diameter, without altering the overall corneal curvature and refraction greatly. Patients with known, current viral infections are not suitable for treatment while they have an active disease process. Patients undergoing drug therapy or treatment that may affect their corneal healing should consider refractive surgery only when they have completed their other therapy. Glaucomatous patients may be thought unsuitable for PRK, as they might require the use of corticosteroid drops post-operatively.13–15 Patients with a family history of glaucoma should be made aware that after corneal surgery the measurement of intraocular pressure (IOP) can be affected.16–19 Similarly, pregnancy is considered a contraindication to refractive surgery as there may be subtle changes in refraction during gestation, and also many patients may be wary if drug treatment is indicated after refractive surgery. Inappropriately motivated patients should not be encouraged to have refractive surgery as they may have unrealistic expectations that cannot be met. Motivation for treatment should be assessed carefully preoperatively, and patients should not feel coerced into proceeding. This can be especially difficult, as most refractive surgery takes place in a very commercial environment in which competition, pricing and advertising is often fierce. Nonetheless, it
Patient selection and pre-operative assessment
should be remembered that an unhappy patient is more likely to tell his or her friends about the experience than a happy patient. It would almost be a false economy to treat patients who were unsure about going ahead. Many refractive surgery clinics allow a cooling off period for potential patients between the time of their initial consultation and the actual surgery so that they do not feel pressurized. This tends to be the norm for laser in situ keratomileusis (LASIK) surgery, but opinions vary on this for surface-based laser treatments like PRK and laser epithelial keratomileusis (LASEK; see Chapters 3 and 4 for details of the types of surgery). Patients who are unable to comprehend the rationale of treatment should not be treated, unless for therapeutic reasons. This includes anyone who is unable to give informed consent, such as minors or mentally disadvantaged individuals. When assessed subjectively, it appears as though the majority of patients are satisfied with the outcome of refractive surgery.20, 21 The complications of refractive surgery are mentioned in patient literature and detailed in ophthalmic literature. Patients with realistic expectations are more likely to be successful candidates.22 Often it is asked why patients are willing to undergo refractive surgery knowing the potential risks associated with it and not knowing if there will be any long-term effects that are yet to be uncovered.23 Studies to carry out recognized psychometric personality tests on a group of refractive surgery patients and compare them to a control group, or maybe compare them to patients who present for other types of elective or cosmetic surgery are currently underway.24,25 Is there an underlying trait in some refractive surgery patients that leads them on a compulsive drive for perfection?26 Practitioner’s who evaluate prospective patients for refractive surgery should first assess that the patient is suitably motivated towards undergoing surgery, as highlighted above. It is usually advisable that the patient be armed with some information before attending for consultation. The actual pre-operative assessment routine may differ slightly from clinic to clinic, but the essence of the examination is the same. The individual tests that are usually performed are mentioned below, although this list is not conclusive and some tests can be omitted depending on the type of refractive surgery that the patient is to undergo. Most of these tests, unless indicated, do not require equipment additional to that currently available in the routine ophthalmic practice.
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Visual acuity
Full refraction
It is important to know the patient’s visual acuity (VA) before refractive surgery, as it can be used as a guide to post-operative success, and also to detect amblyopia. Loss of best-corrected visual acuity (BCVA) can occur after excimer laser refractive surgery and can result from one or more of the complications of the procedure mentioned here. Independent loss of BCVA may be attributed to the alteration that occurs in the magnification from the patient’s spectacle lenses. In the case of a moderate hyperope, the patient does not receive the extra magnification, after refractive surgery, in their VA that they previously had as a result of their hyperopic spectacles. Conversely, in refractive surgery for moderate myopia the patient does not have the reducing effect of their spectacle lenses after surgery. This means that the patient shows an improvement in the BCVA or, in the presence of other postoperative problems, the patient does not show a reduction in BCVA.27,28 Most clinicians use Snellen acuity, although better analyzes could be made if Bailey–Lovie charts were used. Often the figures quoted suggest that patients lose or gain lines of BCVA based on Snellen acuity. This may hold true for a Snellen chart, but it is not as accurate as quoting Bailey–Lovie charts (Figure 1.3) in which the lines of letters have equal numbers of letters and an equal rate of change exists between each line of letters.29,30
It is vitally important that an accurate prescription is measured for all prospective patients. A patient whose prescription is too minus will end up with a result that is overcorrected and thus will become hyperopic. A patient with an undetected latent hyperopia will also end up with a result that is hyperopic. This is especially important in presbyopes and pre-presbyopes, as a small hyperopic result will be more detrimental to them than a small myopic result. Cycloplegic refraction is often useful to eliminate any concerns of latent hyperopia or an over-minus of the refraction. It is not unreasonable to assume that some of the hypocorrections and hypercorrections that occur after refractive surgery result from an incorrect pre-operative refraction. The author routinely performs cycloplegia on all potential patients to avoid any refractive surprises.
Figure 1.3 High-contrast (90 per cent) distance Bailey–Lovie chart
Pupil diameter Early excimer laser refractive surgery used smaller diameter ablations of up to 3–4mm, so that the depth of the ablation keratectomy was kept to a minimum. The downside of this was noted in some patients with larger pupils, who found, at night especially, that their pupil would dilate to beyond the treatment zone.31 The result of this was a ghosting around bright objects and lights.32,33 This is very similar to the ghosting that a patient may experience from a decentred corneal contact lens, where the optic zone diameter crosses over the pupil margin. Nowadays, this is less of a problem as most excimer laser refractive surgery uses larger diameter ablations,34 but it still may be an issue in cases for which a small diameter ablation is used (possibly because the patient has a relatively thin cornea; see Corneal pachymetry below). Usually, a central stromal area is ablated with the full refractive correction and a blended zone is ablated around it, similar to the optic zone and carrier portion of a contact lens. This allows the depth of the ablation keratectomy to be kept to a minimum.35,36 However, there remains the problem that this creates substantial spherical aberration in the outer zones of the dilated pupil, so that some degradation of the retinal images occurs.37–39 Pupil diameters are measured either with a ruler under normal lighting conditions or, preferably, using a pupillometer such as the Colvard unit (Oasis Technologies, California, USA) or Keeler pupillometer (Keeler Ltd, Windsor,
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Refractive surgery: a guide to assessment and management
Berkshire, UK) or similar. One clinic in the UK exclusively uses a computerized pupillometer device to measure pupil diameter under different lighting levels.
Corneal topography Most routine optometric practices use a keratometer to assess central corneal curvature for contact-lens fitting. In the preassessment of patients for refractive surgery a keratometer is inadequate since it takes measurements from the central 3–4mm of the anterior cornea only.40 Excimer laser refractive surgery involves removal of corneal tissue by ablation over a wide area. In myopic refractive surgery this tissue is removed from the central corneal area (up to about 7mm), and in hyperopic surgery the tissue is removed from the mid-peripheral cornea (up to about 9mm). The net result of the surgery means there is a change in the anterior corneal profile. It is important to measure the full anterior corneal shape before refractive surgery, to check for any contraindications, such as corneal conditions or dystrophies, and corneal irregularities. All refractive surgery clinics use a corneal topography unit to measure the whole corneal shape to obtain baseline data for the cornea, but also a very flat cornea may prove to be more difficult in flap creation with a microkeratome.31,41,42 Contact lens users who present for refractive surgery are advised to remove their lenses for a period of time before surgery to eliminate warpage
induced by contact lenses. For a patient in whom warpage is observed, the topography measurements are repeated on subsequent visits until no further changes are seen in the topography maps; only then is the patient considered suitable for surgery. Most corneal topography units use Placido disc technology (Figure 1.4), which allows measurements of the anterior surface only. The change in the anterior curvature is dependent upon the amount of initial refractive error.43–47 Recent developments in corneal biometry include slit scan topography machines, which use light slits across the cornea to take a threedimensional image.48 Until very recently, the Orbscan corneal topography system (Figure 1.5), developed by Orbtek, Salt Lake City, Utah (Bausch and Lomb, Rochester, New York, USA), was the only commercially available machine able to assess the posterior corneal shape, but a recent unit by Oculus (Giessen, Germany) uses a rotating Scheimplug camera to take similar measurements. A map is produced by these newer devices that may be more representative of the true corneal shape, with attention given to the posterior surface topography and corneal thickness. This allows a better evaluation of anterior corneal and posterior surface astigmatism, and of residual lenticular astigmatism. More information on corneal topography is presented in Chapter 2. Recent literature shows that there can be an associated change in the posterior corneal curvature, too, which is also related to the amount of treatment.49–51 In the LASIK procedure the microkeratome cuts
Figure 1.4 Eyesys 2000 topography unit, which uses a large Placido disk and is able to give information about the radius of curvature on the anterior corneal surface
approximately one-quarter to one-third into the depth of the cornea to create a flap. Although there are no reported incidences of corneal ectasia after LASIK, there is concern over what happens to the posterior corneal curvature after this procedure, especially in high refractive corrections. It is not unreasonable to assume that an alteration in posterior corneal curvature occurs in LASIK also.52,53
Slit-lamp examination Detailed slit-lamp biomicroscopy examination is important prior to refractive surgery. Contraindications to refractive surgery should be identified, and include anterior corneal scars and opacities, clinical signs of conditions such as keratoconus (e.g., Vogt’s striae and Fleischer’s ring) and lenticular changes.54 A patient with nuclear sclerosis may be deemed unsuitable for excimer laser surgery, but may benefit from clear lens extraction with an appropriately calculated intraocular lens.55 Previous contact-lens complications, such as neovascularization, do not usually contraindicate refractive surgery.
Corneal pachymetry As mentioned above, PRK and LASIK involve the removal of small areas of corneal tissue by ablation with an excimer laser, which results in an alteration of the overall corneal curvature. If a patient has a very thin cornea, then
Figure 1.5 Original Orbscan corneal analysis unit, which uses scanning slit technology. The Orbscan allows the posterior corneal surface curvature and corneal pachymetry to be viewed. Note the acquisition head does not use Placido technology, but contains two scanning slit lights. (Courtesy of Bausch and Lomb)
Patient selection and pre-operative assessment
Ablation depth depends on correction and diameter
175 150
4.0mm
125
4.5mm
100
5.0mm
75
5.5mm
50
6.0mm
25
7.0mm
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Spherical refractive error (D)
Figure 1.6 Amount of ametropia on the horizontal axis and the estimated laser ablation depth on the vertical axis for different ablation diameters. (Courtesy of Stefan Pieger)
Intraocular pressure measurement Fundus examination To identify abnormal ocular conditions of the fundus, patients should undergo full ophthalmoscopic examination. Some clinicians warrant dilated fundus examination with an indirect ophthalmoscope, such as a Volk lens, in addition to direct ophthalmoscopy. Many patients who elect to undergo refractive surgery are high myopes. In the case of high myopia the likelihood of spontaneous retinal detachment is about 1 per cent. 57,58 After laser refractive surgery the retina is unchanged and the retina is as likely to detach spontaneously as before surgery. However, very often the patient’s lifestyle may change, especially if this was one of the primary motivations for having refractive surgery, and the patient may partake in activities and sports that before were hindered by the use of spectacles. Retinal detachment after laser refractive surgery has been reported and patients should be warned about the risks, in the same way as high myopes would be warned routinely. Authors have quoted incidences of retinal problems after LASIK of between 0.06 and 0.25 per cent of eyes, and of about 0.08 per cent after PRK.59–62 The low incidence of retinal problems after refractive surgery may reflect careful pre-operative assessment of patients to assess potential risks. Some clinics apply prophylactic treatment to patients deemed at risk of later retinal detachment problems.63,64
5
200 Theoretical ablation depth (m)
cutting a flap with a microkeratome may not leave sufficient cornea under the flap to sustain corneal strength. Most surgeons like to leave a bed of at least 250–300μm under the ablated stroma left untouched. Pachymetry is also important for cases in which repeated PRK is warranted for similar reasons. Conditions that lead to areas of corneal thinning, such as keratoconus or pellucid margin degeneration, may also be detected by carefully positioned pachymetry measurements. Corneal thickness is usually measured with an ultrasonic pachymeter using an appropriate anaesthetic, since it is a contact device. The amount of tissue removed during laser refractive procedures depends on the level of ametropia to be corrected and the diameter of the laser ablation. The relationship between diameter and depth of ablation was investigated by Munnerlyn et al., 56 and many clinicians still use their formula to estimate the amount of treatment (Figure 1.6).
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Active glaucoma is a contraindication to refractive surgery, although most refractive surgery clinics do not assess visual fields on all potential patients, unless warranted. IOP is of interest to refractive surgeons as there have been suggestions in the literature of instances of altered IOP readings after PRK. It is thought that the thinner cornea still has the same mechanical forces acting on it and that regular tonometers do not make an allowance for thinner corneas.65–71 Hence, a lower tonometer force may be required to applanate the cornea by the required amount, and so the IOP reading is falsely low. Attempts have been made by some workers to quantify the change in IOP readings with the amount of ablation received by the cornea.16,17,19,72–79 The altered IOP reading is of particular importance if a patient who has undergone refractive surgery develops glaucoma in future years. For this reason other contributing factors towards glaucoma should be noted, such as positive family history, refractive error, age, race and anterior chamber depth.
breakdown of binocularity. A patient who is a moderate-to-high myope has a base-in prismatic effect when performing near tasks with spectacles on. After refractive surgery the patient loses this additional base-in prism and may develop a fixation disparity. This is likely to be more problematic in pre-presbyopic patients, who may find the need for a reading add if their base-in prism for near is removed from their habitual state. Furthermore, in early and pre-presbyopes a change in accommodative demand when moving from spectacles to refractive surgery (or contact lenses) can occur and may be problematic for the myopic patient. As hyperopia increases, the demand on ocular accommodation increases. Hence, as the spectacle refraction is moved towards the ocular
Muscle balance Although not essential, it can be useful to check the muscle-balance status of patients. A post-surgical problem, which may only be theoretical, since it has not been described in the literature, is the
Figure 1.7 Pelli–Robson CSF chart
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Refractive surgery: a guide to assessment and management
plane the hyperope benefits from the lower demand on accommodative effort, whereas the myopic patient places a higher demand on the accommodative effort.80
Contrast sensitivity function Reduced contrast sensitivity function (CSF) has been described after refractive surgery, and hence its measurement with a suitable test, such as a Pelli–Robson chart (Figure 1.7), is useful.81 In PRK patients this may be the result of corneal haze. Haze is thought to be an immune response of the stroma and forms precisely at the level of the site of laser ablation (i.e., the epithelial–stromal interface). To combat haze, some surgeons use corticosteroids prophylactically with all patients, some use them only if haze is beginning to appear and others prefer to use them with patients who are deemed to be more likely to develop haze, such as patients with higher refractive errors.14,15 It has been suggested that newly synthesized cells cause haze, and an aggregation of keratocytes may play a part in the aetiology of
References 1 Conway R (1994). PRK counselling in the optometric practice. Optician 208, 32–34. 2 Orr D, Sidiki SS and McGhee CNJ (1998). Factors that influence patient choice of an excimer laser treatment center. J Cataract Refract Surg. 24, 335–340. 3 Naroo SA, Shah S and Kapoor R (1999). Factors that influence patient choice of contact lens or photorefractive keratectomy. J Refract Surg. 15, 132–136. 4 Tan DT and Tan JT (1993). Will patients with contact lens problems accept excimer laser photorefractive keratectomy? CLAO J. 19, 174–177. 5 Whittaker G (1996). Are contact lensassociated problems a primary motivation factor for PRK patients? J Br Contact Lens Assoc. 19, 21–23. 6 Simensen B and Thorud LO (1994). Adultonset myopia and occupation. Acta Ophthalmol. 72, 469–471. 7 Holland SP, Srivannaboon S and Reinstein DZ (2000). Avoiding serious corneal complications of laser assisted in situ keratomileusis and photorefractive keratectomy. Ophthalmology 107, 640–652. 8 Hill VE, Brownstein S, Jackson WB and Mintsioulis G (1998). Infectious keratopathy complicating photorefractive keratectomy. Arch Ophthalmol. 116, 1382–1384. 9 Price FW Jr, Belin MW, Nordan LT, McDonnell PJ and Pop M (1999). Epithelial haze, punctate keratopathy, and induced hyperopia after photorefractive keratectomy for myopia. J Refract Surg. 15, 384–387.
Supplementary tests
haze. The new stromal tissue deposited is not laid in a regular pattern, which leads to a reticular pattern of fibres. Studies have shown that severe haze is more likely with patients who have high refractive corrections, since the ablation depth is deeper. Lasers that use scanning micro-beam technology appear to produce less haze than older broad-beam lasers,82–85 but this may be partly because these newer lasers make a central optic zone and a peripheral blended zone.35,36,86 Thus, the actual change in contour profile of the corneal shape is less severe. Another factor may be the laser beam itself. If the beam is able to produce a smoother ablation, the newly synthesized cells may be able to form a more regular pattern of fibres. Haze does not appear to form on eyes that have undergone LASIK, which suggests that when the flap is replaced some smoothing of the underlying tissue occurs, although altered CSF has been described after LASIK.29,87 Reduced CSF may occur in some older patients with early lens-ageing changes, in which case laser refractive surgery may be contraindicated and lens exchange may be warranted.
Altered tear secretion has been reported after LASIK,88,89 and it is useful to assess tear-film quality and measure tear breakup time. Appropriate instruments, such as the Keeler Tearscope (Keeler Ltd, Windsor, Berkshire, UK) could be useful in identifying patients with potentially low tear volumes or break-up times. This may be important for patients who undergo PRK, for whom an incidence of recurrent erosions of about 3 per cent is quoted.90–92 Many patients find ocular lubricants useful for a period of time after corneal laser refractive surgery (corneal wound healing after these types of surgery is discussed in Chapter 3). There have been reports in the literature of changes in corneal sensation after PRK and LASIK, although most authors suggest the corneal sensation is usually at its pre-operative level within a year, or sooner.93–95 However, it is not common to take aesthesiometry measurements before refractive surgery using devices such as the Cochet–Bonnet aesthesiometer mounted on a slit lamp.
10 Whittaker G (1994). Post-treatment follow-up for the PRK patient. Optician 208, 20–26. 11 Bilgihan K, Ozdek SC, Konuk O, Akata F and Hasanreisoglu B (2000). Results of photorefractive keratectomy in keratoconus suspects at 4 years. J Refract Surg. 16, 438–443. 12 Doyle SJ, Hynes E, Naroo SA and Shah S (1996). PRK in patients with a keratoconic topography picture: The concept of a physiological displaced apex syndrome. Br J Ophthalmol. 80, 25–28. 13 O’Brart DP, Lohmann CP, Klonos G, et al. (1994). The effects of topical corticosteroids and plasmin inhibitors on refractive outcome, haze, and visual performance after photorefractive keratectomy. A prospective, randomized, observer-masked study. Ophthalmology 101, 1565–1574. 14 Corbett MC, O’Brart DP and Marshall J (1995). Do topical corticosteroids have a role following excimer laser photorefractive keratectomy? J Refract Surg. 11, 380–387. 15 Gartry DS, Kerr-Muir M, Lohmann CP and Marshall J (1992). The effect of corticosteroids on refractive outcome and corneal haze after photorefractive keratectomy: A prospective, randomized, double-blind trial. Arch Ophthalmol. 110, 944–952. 16 Rao SK, Ratra V and Padmanabhan P (1999). How and where should intraocular pressure be measured after photorefractive keratectomy? J Cataract Refract Surg. 25, 1558–1560. 17 Chatterjee A, Shah S, Bessant DAR, Naroo SA and Doyle SJ (1997). Reduction in
intraocular pressure after excimer laser photorefractive keratectomy: Correlation with pre-treatment myopia. Ophthalmology 104, 355–359. 18 Patel S and Aslanides IM (1996). Main causes of reduced intraocular pressure after excimer laser photorefractive keratectomy. J Refract Surg. 12, 673–674. 19 Mardelli PG, Piebenga LW, Whitacre MM and Siegmund KD (1998). The effect of excimer laser photorefractive keratectomy on intraocular pressure measurements using the Goldmann applanation tonometer. Ophthalmology 104, 945–949. 20 McGhee CN, Craig JP, Sachdev N, Weed KH and Brown AD (2000). Functional, psychological, and satisfaction outcomes of laser in situ keratomileusis for high myopia. J Refract Surg. 26, 497–509. 21 McGhee CN, Orr D, Kidd B, Stark C, Bryce IG and Anastar CN (1996). Psychological aspects of excimer laser surgery for myopia. Reasons for seeking treatment and patient satisfaction. Br J Ophthalmol. 80, 874–879. 22 McGhee C, Sachdev N and Craig J (1999). Photorefractive surgery – assessing patient satisfaction. Optician 218, 27–30. 23 Kahle G, Seiler T and Wollensak J (1992). Report on psychosocial findings and satisfaction amongst patients 1 year after excimer laser photorefractive keratectomy. Refract Corneal Surg. 8, 286–289. 24 West SG and Finch JF (1997). Personality measurement: Reliability and validity issues. In: Handbook of Personality Psychology, p. 143–164, Eds Hogan R,
Patient selection and pre-operative assessment Johnson J and Briggs S (San Diego: Academic Press). 25 Young FA, Singer RM and Foster D (1975). The psychological differentiation of male myopes and non-myopes. Am J Optom Physiol Opt. 52, 679–686. 26 Serano N (2000). Operation overkill. Elle 16, 250–254. 27 Applegate RA and Chundru U (1995). Experimental verification of computational methods to calculate magnification in refractive surgery. Arch Ophthalmol. 113, 571–577. 28 Applegate RA and Howland HC (1993). Magnification and visual acuity in refractive surgery. Arch Ophthalmol. 111, 1335–1342. 29 Moniz N, Fernandes T, Narayanan KK and Sreedhar A (2000). Visual outcome in high myopia after laser in situ keratomileusis. J Refract Surg. 16, S247–S250. 30 Black H (1997). Low contrast tests may more accurately determine visual acuity post-PRK. Ocul Surg News 8, 53. 31 Maloney RK (1990). Corneal topography and optical zone location in photorefractive keratectomy. Refract Corneal Surg. 6, 363–371. 32 Lohmann CP, Fitzke F, O’Brart D, KerrMuir M, Timberlake G and Marshall J (1993). Corneal light scattering and visual performance in myopic individuals with spectacles, contact lenses, or excimer laser photorefractive keratectomy. Am J Ophthalmol. 115, 444–453. 33 Lohmann CP, Fitzke FW, O’Brart D, Kerr Muir MG and Marshall J (1993). Halos – a problem for all myopes? A comparison between spectacles, contact lenses and photorefractive keratectomy. Refract Corneal Surg. 9, S72–S75. 34 Martinez CE, Applegate RA, Klyce SD, McDonald MB, Medina JP and Howland HC (1998). Effect of pupillary dilation on corneal optical aberrations after photorefractive keratectomy. Arch Ophthalmol. 116, 1053–1062. 35 Pop M and Aras M (1995). Multizone/multipass photorefractive keratectomy: Six month results. J Refract Surg. 21, 633–643. 36 Pop M and Payette Y (1999). Multipass versus single pass photorefractive keratectomy for high myopia using a scanning laser. J Refract Surg. 15, 444–450. 37 Applegate RA and Gansel KA (1990). The importance of pupil size in optical quality measurements following radial keratotomy. Refract Corneal Surg. 6, 47–54. 38 Fay AM, Trokel SL and Myers J (1992). Pupil diameter and the principal ray. J Cataract Refract Surg. 18, 348–351. 39 Maeda N, Klyce SD, Smolek MK and McDonald MB (1997). Disparity between keratometry-style readings and corneal power within the pupil after refractive surgery for myopia. Cornea 16, 517–524. 40 Bennett AG and Rabbetts RB (1989). Measurements of ocular dimensions. In: Clinical Visual Optics, Second Edition, p. 457–483, Eds Bennett AG and Rabbetts RB (London: Butterworths). 41 Hersh PS, Scher KS and Irani R (1998). Corneal topography of photorefractive keratectomy versus laser in situ keratomileusis. Ophthalmology 106, 612–619.
42 Schallhorn SC, Reid JL, Kaupp SE, et al. (1998). Topographic detection of photorefractive keratectomy. Ophthalmology 105, 507–516. 43 Dutt S, Steinert RF, Raizman MB and Puliafito CA (1994). One-year results of excimer laser photorefractive keratectomy for low to moderate myopia. Arch Ophthalmol. 112, 1427–1436. 44 Maldonado A and Onnis R (1998). Results of laser in situ keratomileusis in different degrees of myopia. Ophthalmology 105, 606–611. 45 Maguen E, Salz JJ, Nesburn AB, et al. (1994). Results of excimer laser photorefractive keratectomy for the correction of myopia. Ophthalmology 101, 1548–1555. 46 Lindstrom RL, Lineberger EJ, Hardten DR, Houtman DM and Samuelson TW (2000). Early results of hyperopic and astigmatic laser in situ keratomileusis in eyes with secondary hyperopia. Ophthalmology 107, 1858–1863. 47 Salz JJ, Maguen E, Nesburn AB, et al. (1993). A two-year experience with excimer laser photorefractive keratectomy for myopia. Ophthalmology 100, 873–882. 48 Dave T (1998). Current developments in measurements of corneal topography. Contact Lens Anterior Eye 21, S13–S30. 49 Shimmura S, Yang HY, Miyajima HB, Shimazaki J and Tsubota K (1997). Posterior corneal protrusion after PRK. Cornea 16, 686–688. 50 Kamiya K, Oshika T, Amano S, Takahashi T, Tokunaga T and Miyata K (2000). Influence of excimer laser photorefractive keratectomy on posterior corneal surface. J Cataract Refract Surg. 26, 867–871. 51 Naroo SA and Charman WN (2000). Changes in posterior corneal curvature after photorefractive keratectomy. J Cataract Refract Surg. 26, 872–878. 52 Wang Z, Chen J and Yang B (1999). Posterior corneal surface topographic changes after laser in situ keratomileusis are related to residual corneal bed thickness. Ophthalmology 106, 406–409. 53 Maloney RK (1999). Discussion of article by Wang Z, Chen J, Yang B. Ophthalmology 106, 409–410. 54 Lawless M, Coster DJ, Phillips AJ and Loane M. Keratoconus: Diagnosis and management. ANZ J Ophthalmol. 17, 33–45. 55 Colin J and Robinet A (1997). Clear lensectomy and implantation of a lowpower posterior chamber intraocular lens for correction of high myopia: A four-year follow-up. Ophthalmology 104, 73–78. 56 Munnerlyn CR, Koons SJ and Marshall J (1988). Photorefractive keratectomy: A technique for laser refractive surgery. J Cataract Refract Surg. 14, 46–52. 57 Ho P and Tolentino F (1984). Pseudophakic retinal detachment: Surgical success rate with various types of IOLs. Ophthalmology 91, 847–852. 58 Colin J, Robinet A and Cochener B (1999). Retinal detachment after clear lens extraction of high myopia. Ophthalmology 106, 2281–2285. 59 Arevelo JF, Ramirez E, Suarez E, et al. (2000). Incidence of vitreoretinal pathologic conditions within 24 months
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after laser in situ keratomileusis. Ophthalmology 107, 258–262. 60 Mansour AM and Ojeimi GK (2000). Premacular subhyaloid hemorrhage following laser in situ keratomileusis. J Refract Surg. 16, 371–372. 61 Ruiz-Moreno JM, Artola A and Alio JL (2000). Retinal detachment in myopic eyes after photorefractive keratectomy. J Cataract Refract Surg. 26, 340–344. 62 Ruiz-Moreno JM, Perez-Santonja JJ and Alio JL (1999). Retinal detachment in myopic eyes after laser in situ keratomileusis. Am J Ophthalmol. 128, 588–594. 63 Charteris D, Cooling R, Lavin M and McLeod D (1997). Retinal detachment following excimer laser. Br J Ophthalmol. 81, 759–761. 64 Farah ME, Hofling-Lima AL and Nascimento E (2000). Early rhegmatogenous retinal detachment following laser in situ keratomileusis for high myopia. J Refract Surg. 16, 739–743. 65 Dohadwala AA, Munger R and Damji KF (1998). Positive correlation between TonoPen intraocular pressure and central corneal thickness. Ophthalmology 105, 1849–1854. 66 Doughty MJ and Zaman ML (2000). Human intraocular thickness and its impact on intraocular pressure measurement: A review and metaanalysis approach. Surv Ophthalmol. 44, 367–408. 67 Ehlers N, Bramsen T and Sperling S (1975). Applanation tonometry and central corneal thickness. Acta Ophthalmol. 53, 652–659. 68 Foster PJ, Baasanhu J, Alsbirk PH, et al. (1998). Central corneal thickness and intraocular pressure in a Mongolian population. Ophthalmology 105, 969–973. 69 Mark HH (1973). Corneal curvature in applanation tonometry. Am J Ophthalmol. 76, 223–224. 70 Mills RP (2000). If intraocular pressure measurement is only an estimate – then what? Ophthalmology 107, 1807–1808. 71 Shah S (2000). Accurate intraocular pressure measurement – the myth of modern ophthalmology. Ophthalmology 107, 1805–1807. 72 Cennamo G, Rosa N, La Rana A, Bianco S and Adolfi S (1997). Non-contact tonometry in patients that underwent photorefractive keratectomy. Ophthalmologica 211, 341–343. 73 Cho P and Liu T (1998). Comparison of the performance of the Nidek NT-2000 non-contact tonometer with the Keeler Pulsair 2000 and the Goldman applanation tonometer. Optom Today 38, 28–36. 74 Damji KF and Munger R (1997). Reduction of IOP after PRK: Letter to the Editor. Ophthalmology 104, 1525–1526. 75 Garcia J and Sherry R (1997). Reduction in IOP after PRK. Ophthalmology 104, 1526–1527. 76 Montes-Mico R and Charman WN (2001). Intraocular pressure after excimer laser myopic refractive surgery. Ophthalmic Physiol Opt. 21, 228–235. 77 Patel S and McLaughlin JM (1999). Effects of central corneal thickness on
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measurements of intraocular pressure in keratoconus and post-keratoplasty. Ophthalmic Physiol Opt. 19, 236–241. 78 Rosa N, Cennamo G, Breve A and La Rana A (1998). Goldmann applanation tonometry after myopic photorefractive keratectomy. Acta Ophthalmol. 76, 550–554. 79 Tuunanen TH, Hamalainen P, Mali M, Oksala O and Tervo T (1996). Effect of photorefractive keratectomy on the accuracy of pneumatonometer readings in rabbits. Invest Ophthalmol Vis Sci. 37, 1810–1814. 80 Whittaker G (1994). Pre-assessment of prospective PRK patients by optometrists. Optician 208, 28–31. 81 Verdon W, Bullimore M and Maloney RK (1996). Visual performance after photorefractive keratectomy. Arch Ophthalmol. 114, 1465–1472. 82 Caubet E (1993). Cause of subepithelial corneal haze over 18 months after photorefractive keratectomy for myopia. Refract Corneal Surg. 9, S65–S70. 83 Corbett MC, Prydall JI, Verma S, Oliver KM, Pande M and Marshall J (1996). An in vivo investigation of the structures responsible for corneal haze after
photorefractive keratectomy and their effect on visual function. Ophthalmology 103, 1366–1380. 84 Lohmann CP, Gartry D, Kerr-Muir M, Timberlake G, Fitzke F and Marshall J (1991). ‘Haze’ in photorefractive keratectomy: Its origins and consequences. Laser Light Ophthalmol. 4, 15–34. 85 Lohmann CP, Gartry D, Kerr-Muir M, Timberlake G, Fitzke F and Marshall J (1991). Corneal haze after excimer laser refractive surgery: Objective measurements and functional implications. Eur J Ophthalmol. 1, 173–180. 86 Piovella M, Camesasca FI and Fattori C (1997). Excimer laser photorefractive keratectomy for high myopia: Four year experience with a multiple zone technique. Ophthalmology 104, 1554–1565. 87 Mutyala S, McDonald, Scheinblum KA, Ostrik MD, Brint SF and Thompson H (2000). Contrast sensitivity evaluation after laser in situ keratomileusis. Ophthalmology 107, 1864–1867. 88 Aras C, Ozdamar A, Bahcecioglu H, Karacorlu M, Sener B and Ozkan S (2000). Decreased tear secretion after laser in situ keratomileusis for high myopia. J Refract Surg. 16, 362–364.
89 Patel S, Perez-Santoja JJ, Alio JL and Murphy PJ (2001). Corneal sensitivity and some properties of the tear film after laser in situ keratomileusis. J Refract Surg. 17, 17–24. 90 Loewenstain A, Lipshitz I, Varssano D and Lazar M (1997). Complications of excimer laser photorefractive keratectomy for myopia. J Cataract Refract Surg. 23, 1174–1176. 91 Seiler T, Holschbach A, Derse M, Jean B and Genth U (1994). Complications of myopic photorefractive keratectomy with the excimer laser. Ophthalmology 101, 153–160. 92 Stevens JD and Steele ADM (1993). Indications, results and complications of refractive corneal surgery with lasers. Curr Opin Ophthalmol. 4, 91–98. 93 Murphy PJ, Corbett MC, O’Brart DPS, Verma S, Patel S and Marshall J (1999). Loss and recovery of corneal sensitivity following photorefractive keratectomy for myopia. J Refract Surg. 15, 38–45. 94 Chuck RS, Quiros PA, Perez AC and McDonnell PJ (2000). Corneal sensation after laser in situ keratomileusis. J Cataract Refract Surg. 26, 337–339. 95 Sun R and Gimbel HV (1997). Effects of topical ketorolac and diclofenac on normal corneal sensation. J Refract Surg. 13, 158–161.
2 Corneal topography and its role in refractive surgery Shehzad A Naroo and Alejandro Cervino
The cornea plays a fundamental role in both the structural integrity and the refractive state of the eye. Thus, both the determination and representation of its shape are important for refractive and surgical purposes, as well as in the diagnosis and evolution of several pathologies that express corneal shape alterations, such as keratoconus, marginal degeneration and other ectasias. The adult cornea is characterized by its specific distributions of curvature and thickness along the different meridians, distributions that are essential for the correct function of the cornea as the most important and powerful refractive element of the human eye.
but it served as a base for the development of the keratometers. In 1880 Antonio Placido introduced a flat disc with a series of concentric black and white rings, with the corneal reflections of the rings examined through a central aperture. It is illuminated from a light source above or beside the patient’s head. The Placido disc, as it became known, must be held normal to the line of sight or it will give a false impression of the toricity of the cornea. Gullstrand (1896) was the first to photograph the corneal image formed with the Placido disc.1,2
Classification of corneal topography History Early interest in corneal topography dates back to Father Christopher Scheiner, who in 1619 compared corneal images to marbles. Using daylight he viewed the image formed when daylight shone through the cross-shaped glazing frame bars of his windows onto corneas and compared the images formed to those formed on marbles of a known size. Senff introduced the first concepts about human corneal topography in 1846, reporting that the anterior corneal surface flattens towards the limbus and compared the anterior surface of the cornea with a revolution ellipsoid. Henry Goode (1847) described the first keratoscope, which comprised a small luminous square held near to the eye. Helmholtz (1853) invented the ophthalmometer, and introduced the first doubling image system to avoid the problems caused by the continuous micro-movements of the eye that existed until then. This ophthalmometer was difficult to use,
Since the early investigations of Javal and Helmholtz, a basic model of corneal topography has been established that uses the ellipse as a first-order approximation to the normal corneal profile. This classic model of the corneal contour corresponds to a surface with two zones, a central spherical zone of 4–5mm diameter and a peripheral zone that flattens towards the limbus. The central zone is responsible for the foveal image formation, and within this area of the cornea the changes in curvature are small, so often uniformity is assumed. However, it has been demonstrated by Bennett that this is not actually correct,3 but rather each point on the cornea is conical (as mentioned below). The anatomic centre of the apical zone rarely corresponds to the visual centre or the geometric centre, although most instruments assume this to be true. The position of the apex is independent of the geometric centre and is usually located 0.5mm on the temporal side with respect
to the geometric centre.4 Other classifications have also been developed, such as that of Rowsey and co-workers who considered the quantity and symmetry of peripheral flattening,5 and classified the corneas into essentially four types: • Type A: paracentral zone is symmetrical (nasal–temporal difference less than 0.2mm), peripheral zone is symmetrical and the difference in flattening between the paracentral and peripheral zones is less than 0.2mm. • Type B: paracentral zone is symmetrical, as is the peripheral zone, but the difference in flattening between both zones is more than 0.2mm. • Type C: paracentral zone has a trace asymmetry (about 0.2mm), the peripheral zone is symmetrical and the difference in flattening between them is less than 0.2mm. • Type D: paracentral zone has a nasal–temporal asymmetry and the peripheral zone is symmetrical, but the difference in flattening between the paracentral and peripheral zones is greater than 0.2mm. The classification of the cornea into anatomic zones is considered inappropriate by several authors, because the cornea is a smooth surface, the curvature of which is submitted constantly to subtle changes,3,6,7 which suggests that at any individual point the cornea is conical and represented by Equation (2.1) y2 = 2ro – px2 (2.1) where p is the shape factor of the cornea (see below) and ro is the central radius of curvature of the cornea. However, in the central 3–4mm the changes are small, as mentioned above, and hence some level of uniformity is often assumed. The anterior peripheral cornea
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flattens with respect to the central curvature, a pattern mimicked by the posterior corneal curvature. The rate of flattening may be different along different meridians. The corneal asphericity is described in mathematical terms as being a prolate shape or a flattening ellipse. This shape of the cornea partially compensates the spherical aberration of the eye and improves the quality of the retinal image. The technical requirements for a correct and reliable measurement of corneal topography were established by Bibby:8 • The units used to describe the corneal topography must not depend on the method of obtaining the values. • The instrument must measure the total area of interest. • All the information must be acquired simultaneously. • The technique employed must be precise and reproducible. Following these requirements, his work suggested mean values for the corneal shape of 0.85 ± 0.18 in 2100 eyes and, later, of 0.79 ± 0.15 in 32,000 eyes. In the 20th century, the growth of the field of contact lenses, and later of refractive surgery, led to an increased interest in corneal topography. This, along with the parallel development of the computer technologies, resulted in great advances in corneal topographical analysis. Various workers have helped to develop better designs of photokeratoscopic systems and better graphic presentation and analysis of the data. Colour-coded topography maps were introduced by Klyce and later developed further by Maguire.9,10
Corneal shape Evaluation of the corneal shape is of great importance in the monitoring and followup of corneal pathologies, contact lens fitting and refractive surgery, and in the evaluation of sequential temporal changes induced by contact lens wear, refractive surgery or orthokeratology. However, the description of the cornea may not be the same for a contact lens fit as for refractive surgery purposes, for example. Mandell described the cornea in three ways, according to the viewpoint required:11 • From a qualitative point of view, several corneal zones are considered: the central, paracentral, peripheral and limbal. Also, a division into optic and peripheral zones can be made for practical purposes, in which the central optic zone, with an almost constant curvature, is surrounded by a peripheral zone with a radius that progres-
Table 2.1 Corneal descriptors and their mathematical relations Mathematical description
Shape factor (p)
Hyperbola shape
p35 years
Surgical procedures
Surgical procedure The cornea is anaesthetized by topical anaesthetics. Usually, the non-operated eye is covered with an eye pad. The patient is made to lie on a couch and asked to focus on a flashing light. A lid speculum is inserted in the eye to be treated. A LASEK 8.0mm corneal trephine is used to create an epithelial incision. The circular blade is designed to perform a 270° incision with a blunt section at the 12 o’clock position for a hinge. A 9mm corneal ring is applied, which acts as a cup and is filled with 18% ethanol and left for 30 seconds. This 9mm corneal ring allows a 7.5mm treatment zone to be achieved, as the epithelium at the edges is still adherent. A flap can be raised in most eyes 20–25 seconds after the application of ethanol, but in some patients the epithelium is more adherent and needs more time. The ethanol is soaked up with a mercel sponge and the cornea washed with a topical nonsteroidal anti-inflammatory agent applied (diclofenac). An epithelial flap is fashioned by lifting (not debriding and not damaging the stromal bed) the edge of the loosened epithelium with a sharp beaver blade. The flap can be created horizontally or vertically, or the epithelium is cut in the centre and a flap is created in all four directions. Once the epithelial flap has been created, the
Table 4.2 Indications, absolute contraindications and relative contraindications for LASEK Indications Age 21 years and above Stable refraction Adequate central corneal thickness Myopia –3.00D to –6.00D Hyperopia up to +4.00D Astigmatism up to 4.00D Absolute contraindications Keratoconus Herpes virus infection of the cornea Deep corneal dystrophy Grossly amblyopic eye Corneal melt Unstable refraction Relative contraindications Significant cataract Certain occupations (pilots, computer programmers and heavy goods vehicle drivers, because contrast sensitivity and glare can be a handicap among these groups of patients) Patients with obsessive personality
corneal stroma is bare and laser is applied without delay, before the stroma dehydrates, as this might lead to overcorrection. The patient must be warned that the ablation usually produces a burning smell. After laser ablation the flap is replaced onto the cornea. A contact lens is then placed on the eye and removed after 4 days. This results in less pain and quicker visual recovery than for standard PRK. This procedure is especially beneficial for patients with small palpebral apertures, deep-set eyes, extremely flat or steep corneas, thin corneas or high myopia, as well as for patients who may not qualify for refractive surgery.28 Post-operative care Post-operative care includes topical antibiotics for 1 week. The patients are told to avoid swimming, contact sports, dust and smoke for about a month. Reviews of the patients are usually after 1 week, 6 weeks and 6 months. The vision gradually improves over a few days to a few weeks (at most) depending on the size of the ablation. Complications of LASEK As LASEK and PRK are essentially same procedures, the potential complications are the same.26 However, in the authors’ experience since using this technique, the incidence of complications is very low. The authors have not seen haze that affects visual acuity. The complications after LASEK can be classified into two broad groups: • Refractive; and • Miscellaneous. These are summarized in Table 4.3.
Table 4.3 Complications after LASEK Intra-operative Intra-operative loss of epithelial flap25 Refractive Early: • Induced irregular astigmatism • Primary undercorrection • Primary overcorrection Late: • Regression • Undercorrection • Overcorrection • Miscellaneous • Decentred ablation • Glare • Haloes • Ptosis • Infectious keratitis
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LASIK Overview LASIK was first performed by Pallikaris et al. in 1990,30 and is a combination of excimer laser with lamellar corneal surgery for the correction of refractive errors. LASIK is mainly carried out to correct myopia, but it is also used to correct astigmatism and hyperopia. Most refractive surgery in the USA is now LASIK. To achieve the desired refractive power the corneal thickness and shape are altered. The excimer laser is used to ablate the corneal stromal tissue to achieve the desired refractive change.30,31 Indications, absolute contraindications and relative contraindications for LASIK are given in Table 4.4. Surgical procedure The patient lies on a couch with the excimer laser delivery system above the patient’s head. The cornea of the eye to be operated is anaesthetized with topical anaesthetic drops. A lid speculum is inserted after instilling topical anaesthesia. The patient is asked to fixate on the laser bream and the cornea is marked with gentian violet to help realign the flap. A suction ring is applied to the limbus and the pressure increased to more than 65mmHg to
Table 4.4 Indications, absolute contraindications and relative contraindications for LASIK Indications Stable refraction (no change over a period of 2 years) Age ≥21 years Adequate central corneal thickness Myopia ≤–10.00D Hyperopia ≤+4.00 to 5.00D Astigmatism ≤6D Absolute contraindications Keratoconus Central corneal thickness +4.00D. In another study on 54 hyperopic eyes (range +1.00D to +6.00D), Lian et al. reported that predictability was good after 12 months: 83% eyes were within +1.00D and 66% achieved +0.5D.40 Loss of uncorrected visual acuity Lian et al. also reported that 92.6% of the eyes had UCVA of 20/40 or better and 63% had 20/20 or better.40 One eye lost two lines of BCVA and two eyes gained two or more lines. LASIK versus LASEK This topic is covered in more detail in Chapter 8, but here it is sufficient to say that in some prescription groups the end results of LASIK and LASIK are the similar. Claringbold suggests that LASEK appears to be safe and more effective than
Surgical procedures
LASIK in that complications related to the stromal flap are eliminated and it can be performed in patients for whom LASIK may be contraindicated (e.g., deep-set eyes, thin corneas, etc.).29 However, LASEK has some disadvantages with respect to LASIK: • Patients experience varying degrees of pain during the first 2 days after surgery; • Recovery of vision is slower, as vision is somewhat blurred for the first week after LASEK surgery; and • Patients may have mild recurrent epithelial erosion and so require postoperative corticosteroid for a longer period than required after LASIK. Intracorneal ring segments Overview Intracorneal ring segments (ICRS) is a procedure based on the assumption that the refractive error can be corrected by flattening the cornea using tissue added to the outer two-thirds of cornea. This extra tissue in the peripheral cornea distends the cornea, which in turn flattens the central cornea.41 This technique is used to correct low myopia and astigmatism. In this procedure half-ring segments of Perspex are inserted into channels created in the corneal stroma, which results in a flattening of cornea. The advantage is that the central cornea is not involved and the ring is positioned outside the pupillary margin. This process is easily reversible and the corneal shape remains intact. The indications for ICRS are low grade myopia +5.00 to +15.00D;45 • Thin corneas; • Previous refractive keratotomy surgery. Surgical procedures Anterior chamber lens implantation The surgery is carried out under sterile conditions to avoid intraocular infection. The pupil is dilated with mydriatics and anaesthesia, either topically or with peribulbar anaesthetics. A temporal corneal incision of about 3–3.5mm is made with the diamond blade. Sodium hyaluronate is injected into the anterior chamber to deepen it. The lens is implanted into the anterior chamber, the haptic ends are placed under the iris with a spatula and the lens is centred. Peripheral iridectomy is performed to avoid blockage by the peripheral haptic. The viscoelastic material is removed by either irrigation or aspiration with balanced solution. Postoperative antibiotic and corticosteroid drops are given for 5–7 days. Posterior chamber lens implantation The pupil is dilated with mydriatics and the eye to be treated is anaesthetized with peribulbar anaesthetics. A temporal or nasal corneal incision of about 3–3.5mm is made with the diamond blade. The silicone IOL is implanted in front of the natural crystalline lens, under the protection of a viscoelastic substance. No suture is necessary. A peripheral iridectomy is performed, either intra-operatively or by laser after surgery. At the end of the surgical procedure, gentamicin and corticosteroid are given topically or both topically and subconjunctivally. The advantages and disadvantages of PIOL procedures are given in Table 4.7. Surgical outcome, anterior chamber lens implant Loss of best-corrected visual acuity Hoyos et al. reported for anterior chamber lens implantation a mean BCVA in myopic eyes of 20/35 and in hyperopic eyes of 20/23 after 1 year in a study on 31 eyes (17 myopic and 14 hyperopic, myopia ranged from –11.8 to –26.00D and hyperopia from +5.25 to +11.00D).45 In myopic eyes, no eye lost lines of acuity, and in hyperopic eyes one eye gained one line of BCVA and one eye lost one line.
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Refractive surgery: a guide to assessment and management
Table 4.7 Advantages and complications of phakic intraocular lenses Advantages Preservation of accommodation Compatibility with proved cataract and phakic IOL implantation procedures Correction of higher levels of myopic and hyperopic refractive errors Reversibility46–48 Complications Post-surgical astigmatism Secondary glaucoma (major complication of the anterior chamber lens) Chronic intraocular inflammation Pigment dispersion Uveitis Endothelial cell damage Cataract formation Endophthalmitis Glare and poor-quality vision at night with a wider pupil
Predictability After 1 year follow-up, the MSE of refraction was –0.22 ± 0.87D in myopic eyes, with 87% within the desired refraction of ±1.00D; in hyperopic eyes the MSE was +0.38 ± 0.82D, with 79% within the desired refraction of ±1.00D.45 Surgical outcome, posterior chamber lens implant Loss of best-corrected visual acuity Brauweiler et al. evaluated 18 eyes with high myopia (pre-operative MSE –14.58 ± 3.04D).49 BCVA remained unchanged in one eye or improved by two lines or better, and three eyes lost one line of BCVA. Predictability After 2 years follow-up the MSE was –1.33 ± 0.71D. Clear lens extraction Overview Various treatments for patients with high refractive errors have been used in the past (e.g., glass spectacles, contact lenses, etc.), but the higher the refractive error the higher the dissatisfaction with these traditional treatment methods. During the past two decades refractive surgery has made much progress and become popular. More and more patients with refractive error seek life without these traditional
methods of treatment (e.g., glass and contact lenses), and refractive surgery results are promising in terms of rapid recovery and safety.29 The indications are: • High myopia >6.00D; and • Hyperopia. Surgical procedure This surgical procedure is similar to a cataract operation, the only difference being that the natural crystalline lens is removed even though it is not opaque, and an artificial lens is implanted. The IOL’s strength is calculated such that when it replaces the crystalline natural lens the required refractive power is achieved. The complications of clear lens extraction are given in Table 4.8. Results Loss of best-corrected visual acuity Usitalo et al. reported that, for 38 eyes, 71.9% gained one or more lines and 40.6% gained two or more lines in their study of highly myopic eyes (range from –7.75D to –29.00D), and 6.2% lost one line of BCVA after 1 year.50 Accuracy In the same 38 eyes, the spherical equivalent refraction was within ±1.00D in 81.6% and within ±0.5D in 71.1%, and in eyes with myopia
