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vii
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 disor-ders. 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 foresee-able 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 func-tional 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 fi rst task is to identify and understand the functional diffi culties 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 refrac-tive characteristics of the eyes, and performing a range of tests of visual function. Visual acuity, visual fi elds, contrast sensitivity,
colour vision, light and dark adaptation, oculo-motor control and glare disability are all relevant and clinically-quantifi able visual functions. However, it is not enough to simply measure thresholds. Clinicians should also evaluate the ease, effi ciency 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 magnifi cation, minifi cation, prisms, fi lters, lighting control and through the use of electronic display systems or through training specifi c 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, occupa-tional 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 voca-tional counselling and training, along with accommodations being made in the workplace environment.
Technological developments are presenting exciting new oppor-tunities 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 fl exibility than the more traditional optical low vision aids. Most video-magnifi ers 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 modifi cations of the visual display through reformatting by changing font, style, columns, rows, spacing, highlighting, controlled scrolling, stream-ing, 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
ix screen images. In most societies, the information technology revo-lution 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 com-puters 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 tech-nologies to access information for a wide variety of recreational, occupational, spiritual and intellectual purposes. Despite their diffi culties 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 expo-nential 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 cli nicians 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 ophthalmolo-gists are specialising in low vision rehabilitation, maintaining a high level of expertise, and developing close working associ-ations with other low vision rehabilitation specialists. Fewer still are active in research into relationships between functional per-formance, visual functions and quality of life, or in the develop-ment 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 rehabilita-tion and what it can do. All eye-care practirehabilita-tioners need to be knowledgeable about today’s newly emerging pharmacological,
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 optome-trists and ophthalmologists have a similar responsibility to be knowledgeable about methods for assessing functional vision, and the possibilities of patients benefi ting 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, compre-hensive 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.
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 sub-stantial increase in the number of visually impaired people requir-ing ‘vision care’. In addition, as our standard of livrequir-ing 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 fi elds of disease detection and management, primary and sec-ondary optometric care, low vision optics and prescribing, coun-selling and rehabilitation. In writing this text we have attempted to disseminate the latest research fi ndings 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 evidence-based knowledge and in covering the full gamut of issues relevant to comprehensive low vision rehabilitation.
In Section 1, low vision and its epidemiology are defi ned. The signs, symptoms and clinical management of the range of condi-tions that cause visual impairment in the three stages of life – childhood, working age and advancing years – are presented in
xi
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 magnifi ers to electronic vision enhancement systems, are described from their optics to practical tips on prescribing. Last, but by no means least, rehabilitation strategies and tech-niques 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 valu-able to those wishing to push back the frontiers of the fi eld, irre-spective 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 par-ticular, 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
Preface
xiii support of Carolyn, Rachel, Gordon and Pauline, our respective wives and husband. Their support, and sacrifi ce, 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
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 Moorfi elds Eye Hospi-tal, 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 hospi-tal optometry department at the Royal Victo-ria 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 Ophthal-mic 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 scientifi c papers and has presented research fi ndings 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.
xv
Editors
Dr James Wolffsohn studied optometry at UMIST, Manchester, UK, achieving a 1st class degree. He qualifi ed to practice independently following a pre-registration year at Moorfi eld’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/Uni-versity 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 measure-ment 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.
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
Plates 1 & 2 (Fig. 2.2) Example of a ‘fl ecked’ retina. Both colour plates
show fundus fl avimaculatus at different stages of the disease. Plate 1, Early changes with well defi ned yellow ‘fi sh-tail’-like fl ecks and mild macular atrophy. Visual acuity is relatively well preserved at this stage. Plate 2, More advanced stage of the disease, with signifi cant macular atrophy.
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).
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 refl ex, 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.)
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 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 20 (Fig 15.7) 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.
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) countryside view (Plates 29 & 30);
CHAPTER
Epidemiology has been defi ned 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 identifi cation, 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 ter-minology surrounding visual impairment. This chapter seeks to highlight a range of regularly quoted defi nitions and to review the causes of visual impairment from a global perspective.
1.1 Defi nitions of visual impairment
Many of the defi nitions used to describe visual impairment have evolved as organisations responsible for healthcare management
SECTION ONE
Ophthalmology for low vision
Section Editor: Giuliana Silvestri
Epidemiology of
low vision
A. Jonathan Jackson
1
1
1
1 Ch001-H1815.indd 1 9/15/2006 11:35:35 AMand 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 benefi ts. 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 fi nancial 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 diffi culty for those labelled with the term, as at some later date they may be identifi ed by peers and others in society, including shopkeepers, bus drivers and volunteer helpers, as having some form of usable vision. The per-ception thus taken is that they have falsely acquired the title.
Terminology used to describe less severe forms of visual impair-ment 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 litera-ture 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 spec-trum 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 advo-cated by some. Unfortunately many of these terms are not clearly defi ned 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 defi nitions
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
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Epidemiology of low vision
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1
3 ated with the collection of defi nitive data on visual disability was the non-standardisation of defi nitions.2 It is estimated that, on a worldwide scale, 65 different defi nitions of blindness and poor vision are in existence. By 1978, in an attempt to bring a degree of consistency to the classifi cation of visual disability and blindness, the WHO proposed a standard classifi cation that could be used on a worldwide basis.3 The proposals were subsequently included in the tenth revision of the International Statistical Classifi cation of Diseases and Related Health Problems (ICD-10).4 Regrettably the classifi cation has not been universally adopted, although refer-ences to it are now common (Table 1.1).
1.1.2
Blind and partially sighted registration defi nitions
In the UK the statutory defi nition of blindness, as defi ned 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
Table 1.1 WHO Defi nitions of Visual Impairment3,4
Category of visual Visual acuity with best possible correctiona impairment
Maximum less than Minimum equal to or better than Low vision 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] 1/60 (20/1200) [1.8] CF at 3 m CF at 1 m Blindness 4 1/60 (20/1200) [1.8] Light perception (PL) CF at 1 m 5 No perception of light (NPL) 9 Undetermined or unspecifi ed
a Values are Snellen metres, (Snellen feet) and [LogMAR]. CF, count fi ngers.
responsibility of the consultant ophthalmologist to certify a visu-ally impaired person and, thus, to open the door for them to access community social services. With respect to registration, the fol-lowing points are of note:
1 The defi nition 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 defi ning 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 acu-ities fall within one of three groupings (Table 1.2). If the extent of the visual fi eld is taken into account, patients with a visual fi eld radius no greater than 10° but greater than 5° around central fi xa-tion should be placed in category 2, and those with a fi eld no greater than 5° around central fi xation 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 fi eld loss. Interestingly, those with homonymous or bitemporal hemi-anopia, 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.
Table 1.2 Quantifi able 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
fi elds
2 Visual acuities ranging from 3/60 [1.3] to less than 6/60 [1.0] with signifi cantly contracted visual fi elds
3 Visual acuity of 6/60 [1.0] or better with a gross visual fi eld constriction, particularly in the lower fi eld
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Epidemiology of low vision
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5 Within the statutory context of the 1948 Act, no exact defi nition of less profound visual impairment is given, although subsequent guidelines defi ne the partially sighted as ‘those who are substan-tially and permanently handicapped by defective vision caused by congenital defect, illness or injury’. Guidelines concerning visual acuity requirements are generally considered to be more fl exible than those adopted for blind registration. Further clarifi cation is given to those who are responsible for the certifi cation of children, in that those with visual acuities of 6/24 [LogMAR 0.6] or better should be considered candidates for mainstream schooling. Chil-dren, ‘unless obviously blind’, should only be classifi ed as ‘par-tially 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 rele-vant to ocular health in part B, a section on proposed registration status in part C, and treatment plans and education or employ-ment 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 identifi cation of visual impairment for social care intervention
Table 1.3 Quantifi able Categories of Partial Sight, UK6
Group Description
1 Visual acuity of 3/60 [1.3] to 6/60 [1.0] with full fi elds 2 Visual acuity of up to 6/24 [0.6] with a moderate fi eld
constriction, medial opacities or aphakia
3 Visual acuity of 6/18 [0.5] or better with a gross fi eld defect (i.e. hemianopia)
[LogMAR equivalent acuities]
• 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 certifi cation/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 non-consultant hospital eye service staff. Interestingly all three forms contain sections on disability and the impact of visual impairment on daily living. Information targeted specifi cally 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 certifi -cation 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 defi nition of blindness was intro-duced under the Social Security Act in 1935.8 Blindness is defi ned 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 fi eld of less than or equal to 20° in the widest diameter. Having been classifi ed as legally blind, the visually impaired person becomes eligible to receive supplementary security income and social disability insur-ance. In Canada those classifi ed 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 defi nition of blindness or visual impair-ment. Of these, only two sets of defi nitions were entirely consistent with WHO guidelines. Thirty-seven states utilised quantifi able data on visual acuity, whereas only 16 used defi nitions that included reference to visual fi eld size, to defi ne levels of visual impairment. Other states use functional defi nitions based on employment and social welfare law. In the Netherlands, for example, the blind are defi ned as ‘those who are obliged to read
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Epidemiology of low vision
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7 Braille or make use of the spoken word’, whereas in Norway the defi nition is applied to ‘those who have reduced vision to the extent that it is impossible or diffi cult to read normal writing and or orientate themselves with the help of sight’.10 Information on European defi nitions, 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 defi nitions linked to registration. Visual fi eld specifi cations are of course dependent on the target size and brightness, whereas visual acuity measure-ments fl uctuate widely depending on testing conditions and, in particular, lighting conditions. The visually impaired individual, tested by a zealous clinician in a properly illuminated consult-ing room, may be unfairly classifi ed as beconsult-ing partially sighted, whereas, if tested by another more sympathetic practitioner in suboptimal conditions, may be registered as blind. It is also impor-tant to remember that visual acuity can vary from day to day with some conditions such as diabetic retinopathy and multiple sclero-sis. These factors become particularly important if registration is being embarked upon as a process through which access to bene-fi ts will be gained. The all or nothing concept linking registration to benefi t entitlement is a particular cause of concern.
These factors, together with others associated with psychologi-cal attitudes to registration, help to explain some of the epidemio-logical 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’ asso-ciated 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 synony-mous nor interchangeable. They represent different aspects of the
problems that result from a disturbance of human functioning. The International Classifi cation of Impairment, Disability and Handicap (ICIDH-2/1980), introduced by the WHO in 1980, attempted to standardise the terminology in terms of the func-tional consequences of the disease process (Table 1.4).14
Table 1.5 indicates how the 1980 classifi cation applies to a number of ophthalmic conditions. An understanding of the concept helps to bridge the gap between the results obtained when recording quantifi able visual function data in the clinical environ-ment and the stateenviron-ments 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 cor-rected visual acuity of 6/18 [LogMAR 0.5] resulting from macular oedema of recent onset will fi nd 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 fi rst place, may cope with impaired visual function without great diffi culty.
In 2000, the classifi cation was upgraded to ICIDH-2/2000 and renamed the International Classifi cation of Functioning, Disabil-ity and Health.15 The new classifi cation provides a multiperspec-tive approach to the classifi cation of functioning and disability as an interactive and evolutionary process. It allows both scientifi c
Table 1.4 International Classifi cation 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
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Epidemiology of low vision
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9 T a b le 1 .5 Cl as si fi ca ti on of Loss or I m p air me nt – Dis o rd er and I m p air me nt R e la te t o t h e Org an w h er e as Dis abilit y and H an d ic ap R e la te to t h e P e rs on The orga n The per son Disorder Impairm en t Disa bili ty Han d ic ap De script ion Dis e as e o r inj u ry R e duc e d func ti on al R e duc e d sk ills or abilit ie s Limit at ions on s o ci al p e rf o rma n ce pa rt ic ipa ti on E x am p le 1 A ge -re la te d m ac u la r Re d u ce d V A a n d C S Re ad in g s p e e d a n d fl u e n cy D if fi cu lt y i n a ss e ss in g deg ener at io n re duc ed writ ten ma terial an d co rr e spon de nc e E x am p le 2 Re ti n it is p ig m e n to sa C o n tr ac te d V F, re d u ce d W al k in g s p e e d re d u ce d , L im it at io n s t o u n acco m p an ie d C S an d imp air ed night M o bilit y pr oble m s tr av el an d r e sulting s oci al vision is ol at ion E x am p le 3 C o n ge n it al c at ar ac ts Re d u ce d V A a n d C S , N e ar v is io n t as k s i n cl u d in g D if fi cu lt ie s w it h e d u ca ti o n al (a p h ak ia ) gl ar e d is ab ili ty a n d re ad in g m o re d if fi cu lt t h an d eve lo p m e n t re d u ce d ne ar acu it y d is ta n ce t as k s E x am p le 4 O cu lo cu ta n e o u s Re d u ce d V A a n d C S , O ri e n ta ti o n d if fi cu lt i n Re st ri ct io n s t o albinism and photophobi a bright light . Pr oble ms wit h ce rt ain t ra vel t ask s dist anc e vision C S , c o ntr ast s e nsitivit y ; V A , vis u al a cuit y ; V F, vis u al fi el d. Ch001-H1815.indd 9 9/15/2006 11:35:36 AM
investigations and clinical/rehabilitation providers to map differ-ent 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 percep-tion 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 defi ned as ‘vision that, when corrected by optimal refractive correction, is not adequate for the patient’s needs’. Low vision is thus a functional defi nition that can be applied easily to any patient with a disease or disorder affecting the visual system. Some authorities advocate the term ‘residual vision’, empha-sising 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
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11 T a b le 1 .6 IC ID H -2 O ver vie w of t h e Func tional Cl as si fi ca ti on of Dis abilit y and He alt h 15 Fu nc ti o n in g a n d d is a b il it y Context ua l fa c tor s Fu nc ti o n a n d A c tivit ies a n d En v ir o n m e n tal Pe rs o n a l st ru ctur e pa r ticipat ion Dom ains B o dily func ti ons Life ar e as ( i.e . t ask s and E x ter n al infl ue nc e s Inter n al infl ue nc e s Bo d y p ar ts ac ti o n s) Construc ts Ph ysiologic al chang e Capa cit y and per fo rman ce Hin d ra n ces of f e at u res Impa ct of a ttributes A n at o mic al ch ang e in t h e ph ysic al or s oci al on per son wo rl d Po si ti ve a sp e ct s F u n ct io n al a n d A ct iv it y p ar ti ci p at io n F ac ili ta to rs N o t a p p lic ab le struc tu ral integ rit y F u n cti on in g Neg ativ e asp ec ts Impa ir m e nt A ctivit y l im it ati on B arri ers a n d h in d ra n ces Not appl ic abl e Dis abili ty P ar ticip at ion re st ri ct ion Dis abilit y Ch001-H1815.indd 11 9/15/2006 11:35:36 AM
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 stratifi ed to help ensure similarity between the groups. The nature of randomised con-trolled trials necessitates that the treatment under evaluation be withheld from one group. Although this can be diffi cult 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 signifi cant 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 popula-tion exposed to high levels of ultraviolet radiapopula-tion 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.
Prevalence =
Defi nition of Prevalence and Incidence
Incidence = No. of cases or
events
Total population at risk
No. of new cases over a given time interval Total population at risk at the beginning of the specifi ed time
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13 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. Practitio-ners 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, con-dition 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 specifi cation of the regional burden of blindness (RBB) and the age blindness burden (ABB).
RBB =
Defi nition of Regional Burden of Blindness and the Age Blindness Burden
ABB = Percentage of the world’s
blind population living within a region of interest Percentage of the world’s total population living within the designated region of interest
Percentage of the world’s blind population in a specifi ed age group 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 com-parison 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 fi gure of 0.7%. The RBB in sub-Saharan Africa, which has an overall population of 510 million and repre-sents 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 specifi ed region.
Application of the ABB equation to global fi gures of blindness illustrates the impact of degenerative eye disease on an ageing population. On a global basis, ABB rises from 0.12 in children
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 measure-ment, worthy of note, is the disability-adjusted life-year (DALY). This measurement combines the infl uence 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 compari-son, an entirely different approach has to be taken when looking at the incidence and prevalence of visual impairment in the devel-oping 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 strik-ingly 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 classifi ed 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 blind-ness 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 prevent-able 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
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15 developing world ensures that cataract remains a major cause of blindness.
Foster & Gilbert20 estimated that, of the 1.5 million blind chil-dren 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 devel-oped world, whereas in the developing world infection, in-cluding measles and rubella, causes intrauterine and infant visual
Table 1.7 The Global Distribution of Blindness1,19
Estimated Regional Regional Major no. of blind prevalence blindness causes by people of blindness burden region (millions) (%) Established 2.4 0.3 0.41 AMD 50% market Glaucoma 8% economies Former 1.1 0.3 0.41 AMD 84% socialist Glaucoma 7% block Cataract 8.3%
Latin and 2.3 0.5 0.72 Cataract 57%
Central Glaucoma 8%
America
China 6.7 0.6 0.82 Cataract 32%
Glaucoma 8%
Middle East 3.6 0.7 0.74 Cataract 45%
Trachoma 25% Asia 5.8 0.8 1.18 Cataract 39% Trachoma 24% India 8.9 1.0 1.46 Cataract 51% Glaucoma 12% Sub-Saharan 7.1 1.4 1.93 Cataract 43% Africa Trachoma 19% Overall 37.9 0.7 1.0 Ch001-H1815.indd 15 9/15/2006 11:35:37 AM
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 fi gures, 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 specifi c disease entities), appear in peer-reviewed medical and scientifi c 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.
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)
EME 0.3% FSE 0.3% LAC 0.5% CHI 0.6% MEC 0.7% ACI 0.8% IND 1.0% SSA 1.4%
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17 Registration data
Within the UK, registration data have been scrutinised since the 1950s when Arnold Sorsby published the fi rst of his comprehen-sive 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 depart-mental 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 fi gures indicate that there are approximately 377 000 indi-viduals registered as blind or partially sighted in the UK (Table 1.8). The ratio of those registered as partially sighted to those reg-istered as blind has, over a 20-year period, changed from approxi-mately 1 : 2.5 to almost 1 : 1 in England and Wales, whereas in Scotland partially sighted registration still appears to be unrepre-sentative 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
Table 1.8 Regional UK Blind and Partially Sighted Registration Data
(2004)
Region Registration status (n) Estimated
prevalence (%) Blind PS Total Blind PS Total England 156 675 155 230 311 905 0.32 0.31 0.63 Wales 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 2 273 3 122 5 395 0.14 0.18 0.32 Ireland PS, partially sighted. Ch001-H1815.indd 17 9/15/2006 11:35:37 AM
Figure 1.2 Changing trends in blind and partially sighted registration in
the UK, recorded over a 20-year period.
350000 300000 250000 200000 150000 100000 50000 0 25000 20000 15000 10000 5000 40000 30000 20000 10000 0 0 1000 2000 3000 4000 5000 6000 7000 0 1984 1982 1986 1988 199 0 199 2 1994 1996 1998 2000 2002 2004 1984 1982 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 England Wales Year Year 1984 1982 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 Year 1984 1982 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 Year Scotland N. Ireland
Partially sighted Blind Total registered
Number of individuals
Number of individuals
Number of individuals
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19 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 cor-rected for changing age profi les in the population, only registra-tion 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 signifi cant 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 signifi cantly (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 15-year period (1984–1996), illustrate similar trends.26
Figure 1.3 Causes of blind and partially sighted registration by primary
ophthalmic disease. (Adapted from Evans 1995.23)
No information on main cause Glaucoma Macular and post polar degeneration Other conditions Cataract Optic atrophy Diabetic retinopathy 20% 23.1% 9.7% 8.5% 7% 2.3% 3% 9.6% 3.3% 3.4% 3.4% 11.7% 48.5% 46.5%
Blind Partial sight
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 specifi c 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 indi-viduals 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]. Interest-ingly, 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 fi nding confi rms what is known about the increased preva-lence of visual impairment amongst those with other forms of
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21 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 popula-tion 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-fi nding 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 non-ophthalmic disabilities and disorders. Local studies from Oxford-shire,44 Liverpool45 and Belfast46 have confi rmed these fi ndings. Government survey data
An alternative approach to collecting data from registration databases or from population-based studies involves the use of
large-scale household surveys. The most well known of these, within the UK, is the Offi ce of Population Censuses and Sur-veys (OPCS) Disability Survey.47 In the survey, which was sent to 100 000 private households, questions on visual disability asked about ‘diffi culty 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 dis-ability. Results recorded from communal institutions, including nursing homes, were subsequently included in the analysis. Estimated age-specifi c 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 fi gure could be as high as 1.9 million. This represents almost 25% of all disabled adults. A supplementary survey on those identifi ed from previous government-based surveys carried out by the RNIB and using self-reported questionnaires found the prevalence of signifi cant visual impairment (diffi culty 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 defi nitions of disease, impair-ment, disability and handicap as outlined by the WHO, and of working towards agreed defi nitions 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. Differ-ences between the UK and other global geographical locations have been highlighted.
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23
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