Public Health
© The Society of Public Health, 1991
(1991), 105,455-462
The Incidence and Prevalence of Amblyopia Detected in Childhood J. R. Thompson, G. Woodruff, F. A. Hiscox, N. Strong and C. Minshull Department of Ophthalmology, Leicester Royal Infirmary, PO Box 65, Leicester LE2 7LX
We present incidence estimates for amblyopia using data from a study of a cohort of 364 children from a single English county who were referred during 1983 for occlusion therapy. Using a criterion of a visual acuity of 6/12 or worse to define amblyopia, we estimate that 3.0% of the county's children develop the condition. Successful treatment of some of these children means that 1.9% will remain amblyopic as adults. Introduction The prevalence of amblyopia is high enough for the condition to be of public health importance. Estimates of the percentage of people affected vary f r o m less than 1% to around 5% i T h e condition almost always develops during childhood, but there are few data on the exact pattern of incidence with age, information that is needed when deciding on the age at which to screen for amblyopia. Surveys of the prevalence and incidence of a m b l y o p i a are difficult to organise and as a result very few have b e e n satisfactorily completed.1 D u e to the vague definition of amblyopia, there is often uncertainty over the exact diagnosis of patients and, as with all but the m o s t c o m m o n conditions, c o m m u n i t y - b a s e d studies need to screen very large n u m b e r s of people in order to obtain reliable information. A m b l y o p i a is a t e r m that covers a n u m b e r of ocular disorders with different aetiologies. V o n N o o r d e n 2 suggests the following definition, 'Amblyopia is a unilateral or bilateral decrease in visual acuity caused by form vision deprivation and/or abnormal binocular interaction for which no organic cause can be detected by physical examination of the eye and which in appropriate cases is reversible by therapeutic measures.' For prevalence surveys a tighter definition is needed and this is usually achieved by requiring a specific loss of visual acuity. A Snellen visual acuity of 6/12 or less has been suggested as being a clinically significant cut-off point, but some studies have used 6/9 or worse and others have required a difference in visual acuity b e t w e e n the two eyes of two Snellen lines.l,3 In surveys of very young children it m a y not be possible to m e a s u r e the visual acuity and in such circumstances amblyopia is usually defined in terms of behavioural differences when one eye is covered, as c o m p a r e d to the other. The fact that a m b l y o p i a may have different causes requires the subdivision of the incidence estimates to reflect the aetiology. Hillis 3 suggests that amblyopia associated with late-onset esotropia should be separated out as a major category and suggests that previous studies of aetiology and t r e a t m e n t have been obscured by a failure to Correspondence to: Dr J, R. Thompson.
456
J . R . Thomson et al.
distinguish between different forms of amblyopia. He defines late-onset esotropia as occurring when the squint develops in a child aged between one and five years. Others have divided amblyopes into strabismic, anisometropic, mixed strabismic and anisometropic and form deprivation. Prevalence studies of amblyopia divide into three main types: surveys of specific adult groups (typically military conscripts), surveys of children linked to screening and hospital-based studies of referrals. The studies of military conscripts are difficult to interpret because of selection bias and the possibility of malingering. They have produced prevalence estimates that vary between 1.0% and 4.0% .4 Most of the studies of adult clinic patients are even more unreliable. They typically give prevalences of about 5% but these are almost certainly inflated by self-selection. 1,3,4 Screening studies of schoolchildren have been conducted in several parts of the world. Such studies typically report prevalences of amblyopia of 1% or less. In the USA the National Society to Prevent Blindness is reported by HiUis, Flynn and Hawkins 1 as regularly reporting a prevalence below 1%. A survey of over 10,000 Danish schoolchildren aged seven and over estimated the prevalence to be 2.6%, using a criteria of a vision 6/9 or worse plus strabismus. 5 In the USA a prevalence of 1% was found in 1,500 four- and five-year olds using a test based on projected single letter E's and a criterion of 20/40 or worse. 4 Using a definition based on behavioural differences, large screening-based surveys of young children in Israel have reported prevalences of 0.5% in children aged under three rising to 1.8% in five-year-olds. 6 In this study we have used detailed information on a historical cohort of children from a well-defined geographical area to estimate the age-specific presentation incidence of amblyopia, that is the rate at which children are referred for treatment at different ages.
Methods
All patients first seen in Leicestershire orthoptic clinics during 1983 were reviewed using their notes. Those aged under 15 years at presentation who were diagnosed as having amblyopia were selected to form a historical cohort and the progress of their treatment was followed through to the beginning of May !990. Detailed information was extracted from the notes of each child. For the purposes of this analysis only children with an address in Leicestershire are included. Incomplete information is almost always a problem in studies that depend on patient's records. To omit such patients from the analysis would cause us to underestimate the incidence and so we have allowed for missing data by allocating those children into diagnostic groups in proportion to the recorded data for other children with the same age. In every case the number of children so allocated is small and is noted together with the results. Population data for Leicestershire in 1983 were inferred from the 1981 Census together with Office of Population Census and Survey data on births in subsequent years. 7,8 The age-specific incidences were calculated by dividing the number of new cases by the corresponding population at risk, that is, the population less the estimated number who already had detected amblyopia. The cumulative incidence of amblyopia by age 15 is used to approximate the prevalence of the condition. This estimation is based on the reasonable assumptions
The Incidence and Prevalence of Amblyopia detected in Childhood
457
that the rates are stable over time and that amblyopia is not linked to mortality or migration. For the purpose of this study, anisometropia was defined as 1.0D or more difference of astigmatism or spherical equivalent between the subject's two eyes, and the child's age was calculated from the day treatment started, For use in the analysis we define categories of suspected, confirmed and unsuccessfully treated amblyopia. Suspected cases are all children treated for the condition. Confirmed cases are those for whom we have a recorded visual acuity of 6/12 or worse. Unsuccessfully treated children are those who still had a vision of 6/12 or worse at the end of their period of occlusion. Results
We traced 368 children who started treatment for amblyopia in Leicestershire during 1983. Four had addresses outside the county and are excluded from the following analysis. Of the remaining 364, 253 (70%) were treated at the Leicester Royal Infirmary and 111 (30%) were treated at one of five peripheral clinics. The first block of Table I shows the age-specific incidences for the full cohort of 364 children. There are approximately 12,000 children in each age group in Leicestershire. Some prevalence studies have required that the visual acuity in the affected eye be 6/12 or worse before the child fulfils their definition of amblyopia. Five (1.4%) of our treated cases never had a vision taken at any visit, and a further 19 (5%) had discharge visual acuities but no earlier visions. All but one of these children were aged three years or under when their treatment started. The second and third blocks of Table I are adjusted for these missing values. If we select those children who had, on any test, at any time, a visual acuity of 6/12 or less, then we obtain the results shown in the second block of Table I. We find that 2.96% (standard error 0.17%) of the county's children developed confirmed amblyopia. The difference between the first two blocks of Table I arises because of 42 children treated despite only ever having measured visual acuities over 6/12. As some of these children were initially too young to have a vision taken, it may be the case that they had already improved by the time that their first visual acuity measurement was recorded or that they were discharged before having a measured visual acuity. All of the children studied here were treated with occlusion and some had their vision restored to better than 6/12 at discharge; consequently they could no longer be considered as amblyopic. It is the remaining children, together with any that are never treated, who carry their amblyopia into adulthood. The treatment failures are shown in block 3 of Table I and represent people who will remain amblyopic as adults. These results suggest that the prevalence in adults will be 1.94% (standard error 0.13%). The sample of 364 cases consisted of 215 (59%) with strabismic amblyopias, 66 (18%) with anisometropic amblyopias, 76 (21%) with both strabismus and anisometropia and 7 (2%) others. Table II shows the total incidences for strabismic, anisometropic and mixed amblyopia and the average ages of the children at the time of presentation. Table III shows the incidence rates for late-onset esotropic amblyopia categorised by the age when the squint was first noticed. There were 206 esotropias of which 32 (16%) had notes that contained no information on the age at which the squint was
J. R. Thomson et al.
458
Table I The incidence of amblyopia for Leicestershire in 1983 by
age at start of treatment
Age
Suspected (n = 364)
Confirmed (n - 322)
Unsuccessfully treated (,7 = 211)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.12 0.23 0.48 0.57 0.48 0.53 0.41 0.20 0.20 0.07 0.03 0.02 0.00 0.01 0.00
0.10 0.22 0.42 0.51 0.44 0.44 0.37 0.18 0.18 0.06 0.02 0.02 0.00 0.01 0.00
0.07 0.14 0.26 0.29 0.33 0.29 0.26 0.13 0.13 0.04 0.01 0.01 0.00 0.00 0.00
Total S.E.
3.35 0.18
2.96 0.17
1.94 0.13
Incidence rates are per 100 persons per year.
Table II The estimated prevalence and average age at presentation (years) for strabismic
amblyopia, anisometropic amblyopia and combined strabismic and anisometropic amblyopia (mixed) for Leicestershire in 1983 (%) Number
Prevalence (Standard error)
Average age
Type of amblyopia Strabismic:
Suspected Confirmed Unsuccessfully treated
215 183 115
1.95 (0.13) 1.66 (0.12) 1.04 (0.10)
4.0 4.0 4.1
Anisometropic:
Suspected Confirmed Unsuccessfully treated
66 60 37
0.62 (0.08) 0.56 (0.07) 0.35 (0.06)
6.8 6.8 6.7
Mixed:
Suspected Confirmed Unsuccessfully treated
76 72 56
0.68 (0.08) 0.65 (0.08) 0.51 (0.07)
4.4 4.4 4.6
first n o t i c e d . W h e r e an age was g i v e n , 39% w e r e first n o t i c e d at birth o r in t h e first y e a r o f t h e c h i l d ' s life. T h e r e m a i n i n g 61% w e r e n o t i c e d w h e n t h e child was a g e d b e t w e e n o n e a n d five years. T h e i n c i d e n c e s o f l a t e - o n s e t e s o t r o p i a a r e a p p r o x i m a t e l y e q u a l f o r c h i l d r e n with an age o f o n s e t o f o n e to t h r e e y e a r s , with 0 . 2 5 % o f c h i l d r e n a f f e c t e d at e a c h age. T h e i n c i d e n c e d e c r e a s e s in f o u r - a n d f i v e - y e a r - o l d s .
The Incidence and Prevalence of Amblyopia detected in Childhood
459
Table lII The incidence of late-onset esotropic amblyopia for Leicestershire in 1983 by age at which the squint was first noticed Age at onset
Suspected (n = 123)
Confirmed (n = 109)
Unsuccessfully treated (n = 68)
1 2 3 4 5
0.30 0.26 0.30 0.14 0.05
0.26 0.25 0.27 0.10 0.05
0.18 0.14 0.16 0.08 0.04
Total S.E.
1.05 0.09
0.93 0.09
0.58 0.07
Incidence rates are per 100 persons per year. In order to study the geographical variation in the incidence rates, the children were allocated to the county's nine census districts using their postcodes and the P O S T Z O N F I L E created by the E S R C D a t a Archive at the University of Essex. The numbers of expected cases were then calculated in proportion to the numbers of children aged nought to five years as given in the 1981 Census. O n this basis two rural areas were found to be under-represented in the total cohort. T h e s e were Melton CD and Rutland C D with nine and two suspected cases respectively, while the expected numbers were 18.6 and 12.5. This suggests a total shortfall of a b o u t 20 cases. Using a chi-squared test, the other seven districts do not differ significantly in their referral rates (Z2 = 10.5, d.f. = 6, P = 0.11). Discussion If we a d o p t the 6/12 cut-off as our definition of amblyopia, the results of this study suggest that 3.0% of people develop amblyopia during their childhood. The prevalence of a m b l y o p i a in adults will be less because of treatment. In Leicestershire, the t r e a t m e n t given during the 1980s had the effect of reducing our prevalence estimate to 1.9%. It is likely that t r e a t m e n t was less widely available in the past, so that the prevalence amongst today's adults m a y be closer to the 3.0% figure. Some studies of the t r e a t m e n t of amblyopes have suggested that better results are possible than those obtained in Leicestershire. 9 It is difficult to be sure that such studies are dealing with entire populations but, if they are, then the prevalence amongst adults in the areas covered by such studies might be expected to be less. T h e r e are a n u m b e r of factors with the potential to cause us to underestimate the true prevalence and incidence, the most important being that a few children m a y have b e e n treated outside the county and s o m e others m a y not have had their a m b l y o p i a detected. We feel confident that very few patients f r o m Leicestershire had t r e a t m e n t outside our institutions. T h e pattern of referral in our region has always tended to follow catchment area boundaries because screening policies respect those boundaries. Some patients m a y have presented to their G e n e r a l Practitioner prior to detection by screening and these patients might theoretically have asked to be referred to another centre. W e think this h a p p e n e d to a very small extent, partly because of the longer
460
J . R . Thomson et ah
distances to alternative facilities and partly because our waiting list to be seen in Leicestershire at that time was only four weeks. In 1983 all children were invited for vision screening by a Health Visitor at age seven months and again at three and a half years, after which the children suspected of having problems were referred direct to a consultant ophthalmologist. Further vision screening was conducted through the schools when the children were aged five years and seven years. The combined coverage of these four screens is thought to be high, but we have no information on the sensitivity of the screens. Our faith in the completeness of the coverage is supported by the geographical uniformity in referrals. Under-representation occurs only in the small rural districts of Melton and Rutland. Both of the districts have peripheral orthoptic clinics in their main towns, and as these are served by the same orthoptic staff as the central hospital, this suggests that the referral process could be improved in these two districts. The under-represented areas fall short of their expected numbers by about 20 children in total. This represents about 5% of the total cohort. Adjustment for this shortfall would cause the total incidence of suspected cases given in Table I to be increased only slightly from 3.35 to 3.53, The bias due to poor coverage would therefore seem to be small and there is no evidence of decreased referral from the districts on the county borders. Our results confirm the late presentation of anisometropic amblyopia.l° This needs to be taken into account when planning a screening programme. The figures for late-onset esotropic amblyopia show that these children form a sizeable group, accounting for about one-third of all amblyopes. The pattern of onset of their squints shows a concentration in years one to three, although it must be r e m e m b e r e d that onset of squint and onset of amblyopia will not necessarily occur at the same time. The results presented here are in line with some screening-based studies of the prevalence of amblyopia. In Israel the prevalence was found to increase to 1.8% at the age of five. We cannot be certain how the prevalence in Leicestershire varies with age because we only know the child's age at referral and not when the amblyopia started. However, by summing the incidences from the second column of Table I over the years 0 to 5, we find a similar cumulative presentation incidence of 2.1%. Our study demonstrates that amblyopia is, by public health standards, very common. The prevalence in Britain is much greater than that of any other treatable condition which causes long-term functional disability. The c o m m o n childhood febrile illnesses have a higher incidence but the prevalence of continuing disability is extremely low. Other long-term disabling conditions such as cerebral palsy lI or multiple sclerosis n are both less c o m m o n and arguably untreatable. The relative lack of public awareness of amblyopia may reflect doubts in the minds of medical staff about the importance of the condition and its treatability. The results of the treatment of this cohort of patients will be more closely analysed elsewhere, but our data do demonstrate an improvement with treatment. The question of whether unilateral loss of vision is a significant disability is a personal judgement. However, even if the unilateral visual loss is discounted, the possibility of subsequent loss of the other eye has to be considered. The risk of this occurring is hard to measure and only a few papers have attempted to do so. In Finland, Tommila and T a r k k a n e n 13 examined data covering a period of 20 years and traced 35 amblyopes who subsequently lost vision in the healthy eye, most from trauma. Even allowing for some under-recording the extent of the problem does not,
The Incidence and Prevalence o f Amblyopia detected in Childhood
461
in public health terms, appear to be great. However, the authors estimate that amblyopes have a considerably increased personal risk of blindness compared to the general population, and any form of preventable blindness reflects a failure of the health services. Vereecken and Brabant, 14 in a paper which perhaps selects patients who do well, found that, in the event of loss of the good eye, only 28.5% of their sample showed an improvement in vision in the amblyopic eye to 6/20 or better. In addition to the risk of losing the other eye, it must be r e m e m b e r e d that many surgeons are more reluctant to operate on patients who have only one good eye. If a surgeon varies his or her practice, offering cataract surgery at 6/18 to a patient with two functioning eyes, but only at 6/60 in a patient with dense amblyopia in the other eye, then the amblyopic patient will suffer more visual morbidity from c o m m o n bilateral conditions such as cataract. Although difficult to quantify, this may be the greatest cause of added morbidity from amblyopia. In the light of the relatively high incidence of amblyopia and its potential for treatment if detected early enough, it is important that we raise the public awareness of this condition and ensure that our screening is as effective as possible. Given the observed pattern of incidence with age, it is clear that we need to maintain our vigilance throughout early childhood.
Acknowledgements We should like to thank the staff of the Orthoptic Department of I~icester Royal Infirmary for their cooperation with this study. Material from Crown-copyright records has been made available through the Post Office and the ESRC Data Archive. Clerical assistance was provided by Ms K Robinson and Ms R Gowing, whose salary is supported by the Anne Allerton Fund.
References 1. Hillis, A., Flynn, J. T. & Hawkins B. S. (1983). The evolving concept of amblyopia: A challenge to edpidemiologists. American Journal of Epidemiology, 118, 192-205. 2. Von Noorden G. K. (1977). Mechanisms of amblyopia. Advances in Ophthalmology, 34, 93. 3. Hillis, A. (1986). Amblyopia: Prevalent, curable, neglected. Public Health Review, 14, 213-235. 4. Flom, M. C. & Neumaier, R. W. (1966). Prevalence of amblyopia. Public Health Report, 81,329-341. 5. Fransden, A. D. (1960) Occurrence of Squint: a clinical statistical study on the prevalence of squint and associated signs in different groups and ages of the Danish population. Copenhagen: H. K. Krisenson . 6. Oliver M. & Nawratzki, I. (1971). Screening of pre-school children for ocular abnormalities 1I amblyopia: Prevalence and therapeutic results at different ages. British Journal of Ophthalmology, 55,467-471. 7. Office of Population Census & Surveys. (1982). Census 1981: County Report, Leicestershire. London: HMSO. 8. Office of Population Census & Surveys. (1984). Birth Statistics 1982: England & Wales. London: HMSO. 9. Garzia, R. P. (1987). Efficacy of vision therapy in amblyopia: A literature review. American Journal of Optometry & Physiological Optics, 64,393-404. 10. Shaw, D. E., Fielder, A. R., Minshull, C. & Rosenthal A. R. (1988). Amblyopia--factors influencing the age of presentation. Lancet, ii, 207-209.
462
J. R. Thomson et al.
11. Stanley, F. & Alberman, E. (eds) (1984). The Epidemiology of the Cerebral Palsies, Spastics International Medical Publications. Oxford: Blackwell Scientific. 12. Acheson, D. E. (1985). The epidemiology of multiple sclerosis. In Matthews, W. B. (ed.) McAlpine's Multiple Sclerosis. Edinburgh: Churchill Livingstone. 13. Tommila, V. & Tarkkanen, A. (1981). Incidence of loss of vision in the healthy eye in amblyopia. British Journal of Ophthalmology, 65,575-577. 14. Vereecken, E. P. and Brabant, P. (1984). Prognosis for vision in amblyopia after the loss of the good eye. Archives of Ophthalmology,102, 220-224.
© The Society of Public Health, 1991
(1991), 105,455-462
The Incidence and Prevalence of Amblyopia Detected in Childhood J. R. Thompson, G. Woodruff, F. A. Hiscox, N. Strong and C. Minshull Department of Ophthalmology, Leicester Royal Infirmary, PO Box 65, Leicester LE2 7LX
We present incidence estimates for amblyopia using data from a study of a cohort of 364 children from a single English county who were referred during 1983 for occlusion therapy. Using a criterion of a visual acuity of 6/12 or worse to define amblyopia, we estimate that 3.0% of the county's children develop the condition. Successful treatment of some of these children means that 1.9% will remain amblyopic as adults. Introduction The prevalence of amblyopia is high enough for the condition to be of public health importance. Estimates of the percentage of people affected vary f r o m less than 1% to around 5% i T h e condition almost always develops during childhood, but there are few data on the exact pattern of incidence with age, information that is needed when deciding on the age at which to screen for amblyopia. Surveys of the prevalence and incidence of a m b l y o p i a are difficult to organise and as a result very few have b e e n satisfactorily completed.1 D u e to the vague definition of amblyopia, there is often uncertainty over the exact diagnosis of patients and, as with all but the m o s t c o m m o n conditions, c o m m u n i t y - b a s e d studies need to screen very large n u m b e r s of people in order to obtain reliable information. A m b l y o p i a is a t e r m that covers a n u m b e r of ocular disorders with different aetiologies. V o n N o o r d e n 2 suggests the following definition, 'Amblyopia is a unilateral or bilateral decrease in visual acuity caused by form vision deprivation and/or abnormal binocular interaction for which no organic cause can be detected by physical examination of the eye and which in appropriate cases is reversible by therapeutic measures.' For prevalence surveys a tighter definition is needed and this is usually achieved by requiring a specific loss of visual acuity. A Snellen visual acuity of 6/12 or less has been suggested as being a clinically significant cut-off point, but some studies have used 6/9 or worse and others have required a difference in visual acuity b e t w e e n the two eyes of two Snellen lines.l,3 In surveys of very young children it m a y not be possible to m e a s u r e the visual acuity and in such circumstances amblyopia is usually defined in terms of behavioural differences when one eye is covered, as c o m p a r e d to the other. The fact that a m b l y o p i a may have different causes requires the subdivision of the incidence estimates to reflect the aetiology. Hillis 3 suggests that amblyopia associated with late-onset esotropia should be separated out as a major category and suggests that previous studies of aetiology and t r e a t m e n t have been obscured by a failure to Correspondence to: Dr J, R. Thompson.
456
J . R . Thomson et al.
distinguish between different forms of amblyopia. He defines late-onset esotropia as occurring when the squint develops in a child aged between one and five years. Others have divided amblyopes into strabismic, anisometropic, mixed strabismic and anisometropic and form deprivation. Prevalence studies of amblyopia divide into three main types: surveys of specific adult groups (typically military conscripts), surveys of children linked to screening and hospital-based studies of referrals. The studies of military conscripts are difficult to interpret because of selection bias and the possibility of malingering. They have produced prevalence estimates that vary between 1.0% and 4.0% .4 Most of the studies of adult clinic patients are even more unreliable. They typically give prevalences of about 5% but these are almost certainly inflated by self-selection. 1,3,4 Screening studies of schoolchildren have been conducted in several parts of the world. Such studies typically report prevalences of amblyopia of 1% or less. In the USA the National Society to Prevent Blindness is reported by HiUis, Flynn and Hawkins 1 as regularly reporting a prevalence below 1%. A survey of over 10,000 Danish schoolchildren aged seven and over estimated the prevalence to be 2.6%, using a criteria of a vision 6/9 or worse plus strabismus. 5 In the USA a prevalence of 1% was found in 1,500 four- and five-year olds using a test based on projected single letter E's and a criterion of 20/40 or worse. 4 Using a definition based on behavioural differences, large screening-based surveys of young children in Israel have reported prevalences of 0.5% in children aged under three rising to 1.8% in five-year-olds. 6 In this study we have used detailed information on a historical cohort of children from a well-defined geographical area to estimate the age-specific presentation incidence of amblyopia, that is the rate at which children are referred for treatment at different ages.
Methods
All patients first seen in Leicestershire orthoptic clinics during 1983 were reviewed using their notes. Those aged under 15 years at presentation who were diagnosed as having amblyopia were selected to form a historical cohort and the progress of their treatment was followed through to the beginning of May !990. Detailed information was extracted from the notes of each child. For the purposes of this analysis only children with an address in Leicestershire are included. Incomplete information is almost always a problem in studies that depend on patient's records. To omit such patients from the analysis would cause us to underestimate the incidence and so we have allowed for missing data by allocating those children into diagnostic groups in proportion to the recorded data for other children with the same age. In every case the number of children so allocated is small and is noted together with the results. Population data for Leicestershire in 1983 were inferred from the 1981 Census together with Office of Population Census and Survey data on births in subsequent years. 7,8 The age-specific incidences were calculated by dividing the number of new cases by the corresponding population at risk, that is, the population less the estimated number who already had detected amblyopia. The cumulative incidence of amblyopia by age 15 is used to approximate the prevalence of the condition. This estimation is based on the reasonable assumptions
The Incidence and Prevalence of Amblyopia detected in Childhood
457
that the rates are stable over time and that amblyopia is not linked to mortality or migration. For the purpose of this study, anisometropia was defined as 1.0D or more difference of astigmatism or spherical equivalent between the subject's two eyes, and the child's age was calculated from the day treatment started, For use in the analysis we define categories of suspected, confirmed and unsuccessfully treated amblyopia. Suspected cases are all children treated for the condition. Confirmed cases are those for whom we have a recorded visual acuity of 6/12 or worse. Unsuccessfully treated children are those who still had a vision of 6/12 or worse at the end of their period of occlusion. Results
We traced 368 children who started treatment for amblyopia in Leicestershire during 1983. Four had addresses outside the county and are excluded from the following analysis. Of the remaining 364, 253 (70%) were treated at the Leicester Royal Infirmary and 111 (30%) were treated at one of five peripheral clinics. The first block of Table I shows the age-specific incidences for the full cohort of 364 children. There are approximately 12,000 children in each age group in Leicestershire. Some prevalence studies have required that the visual acuity in the affected eye be 6/12 or worse before the child fulfils their definition of amblyopia. Five (1.4%) of our treated cases never had a vision taken at any visit, and a further 19 (5%) had discharge visual acuities but no earlier visions. All but one of these children were aged three years or under when their treatment started. The second and third blocks of Table I are adjusted for these missing values. If we select those children who had, on any test, at any time, a visual acuity of 6/12 or less, then we obtain the results shown in the second block of Table I. We find that 2.96% (standard error 0.17%) of the county's children developed confirmed amblyopia. The difference between the first two blocks of Table I arises because of 42 children treated despite only ever having measured visual acuities over 6/12. As some of these children were initially too young to have a vision taken, it may be the case that they had already improved by the time that their first visual acuity measurement was recorded or that they were discharged before having a measured visual acuity. All of the children studied here were treated with occlusion and some had their vision restored to better than 6/12 at discharge; consequently they could no longer be considered as amblyopic. It is the remaining children, together with any that are never treated, who carry their amblyopia into adulthood. The treatment failures are shown in block 3 of Table I and represent people who will remain amblyopic as adults. These results suggest that the prevalence in adults will be 1.94% (standard error 0.13%). The sample of 364 cases consisted of 215 (59%) with strabismic amblyopias, 66 (18%) with anisometropic amblyopias, 76 (21%) with both strabismus and anisometropia and 7 (2%) others. Table II shows the total incidences for strabismic, anisometropic and mixed amblyopia and the average ages of the children at the time of presentation. Table III shows the incidence rates for late-onset esotropic amblyopia categorised by the age when the squint was first noticed. There were 206 esotropias of which 32 (16%) had notes that contained no information on the age at which the squint was
J. R. Thomson et al.
458
Table I The incidence of amblyopia for Leicestershire in 1983 by
age at start of treatment
Age
Suspected (n = 364)
Confirmed (n - 322)
Unsuccessfully treated (,7 = 211)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
0.12 0.23 0.48 0.57 0.48 0.53 0.41 0.20 0.20 0.07 0.03 0.02 0.00 0.01 0.00
0.10 0.22 0.42 0.51 0.44 0.44 0.37 0.18 0.18 0.06 0.02 0.02 0.00 0.01 0.00
0.07 0.14 0.26 0.29 0.33 0.29 0.26 0.13 0.13 0.04 0.01 0.01 0.00 0.00 0.00
Total S.E.
3.35 0.18
2.96 0.17
1.94 0.13
Incidence rates are per 100 persons per year.
Table II The estimated prevalence and average age at presentation (years) for strabismic
amblyopia, anisometropic amblyopia and combined strabismic and anisometropic amblyopia (mixed) for Leicestershire in 1983 (%) Number
Prevalence (Standard error)
Average age
Type of amblyopia Strabismic:
Suspected Confirmed Unsuccessfully treated
215 183 115
1.95 (0.13) 1.66 (0.12) 1.04 (0.10)
4.0 4.0 4.1
Anisometropic:
Suspected Confirmed Unsuccessfully treated
66 60 37
0.62 (0.08) 0.56 (0.07) 0.35 (0.06)
6.8 6.8 6.7
Mixed:
Suspected Confirmed Unsuccessfully treated
76 72 56
0.68 (0.08) 0.65 (0.08) 0.51 (0.07)
4.4 4.4 4.6
first n o t i c e d . W h e r e an age was g i v e n , 39% w e r e first n o t i c e d at birth o r in t h e first y e a r o f t h e c h i l d ' s life. T h e r e m a i n i n g 61% w e r e n o t i c e d w h e n t h e child was a g e d b e t w e e n o n e a n d five years. T h e i n c i d e n c e s o f l a t e - o n s e t e s o t r o p i a a r e a p p r o x i m a t e l y e q u a l f o r c h i l d r e n with an age o f o n s e t o f o n e to t h r e e y e a r s , with 0 . 2 5 % o f c h i l d r e n a f f e c t e d at e a c h age. T h e i n c i d e n c e d e c r e a s e s in f o u r - a n d f i v e - y e a r - o l d s .
The Incidence and Prevalence of Amblyopia detected in Childhood
459
Table lII The incidence of late-onset esotropic amblyopia for Leicestershire in 1983 by age at which the squint was first noticed Age at onset
Suspected (n = 123)
Confirmed (n = 109)
Unsuccessfully treated (n = 68)
1 2 3 4 5
0.30 0.26 0.30 0.14 0.05
0.26 0.25 0.27 0.10 0.05
0.18 0.14 0.16 0.08 0.04
Total S.E.
1.05 0.09
0.93 0.09
0.58 0.07
Incidence rates are per 100 persons per year. In order to study the geographical variation in the incidence rates, the children were allocated to the county's nine census districts using their postcodes and the P O S T Z O N F I L E created by the E S R C D a t a Archive at the University of Essex. The numbers of expected cases were then calculated in proportion to the numbers of children aged nought to five years as given in the 1981 Census. O n this basis two rural areas were found to be under-represented in the total cohort. T h e s e were Melton CD and Rutland C D with nine and two suspected cases respectively, while the expected numbers were 18.6 and 12.5. This suggests a total shortfall of a b o u t 20 cases. Using a chi-squared test, the other seven districts do not differ significantly in their referral rates (Z2 = 10.5, d.f. = 6, P = 0.11). Discussion If we a d o p t the 6/12 cut-off as our definition of amblyopia, the results of this study suggest that 3.0% of people develop amblyopia during their childhood. The prevalence of a m b l y o p i a in adults will be less because of treatment. In Leicestershire, the t r e a t m e n t given during the 1980s had the effect of reducing our prevalence estimate to 1.9%. It is likely that t r e a t m e n t was less widely available in the past, so that the prevalence amongst today's adults m a y be closer to the 3.0% figure. Some studies of the t r e a t m e n t of amblyopes have suggested that better results are possible than those obtained in Leicestershire. 9 It is difficult to be sure that such studies are dealing with entire populations but, if they are, then the prevalence amongst adults in the areas covered by such studies might be expected to be less. T h e r e are a n u m b e r of factors with the potential to cause us to underestimate the true prevalence and incidence, the most important being that a few children m a y have b e e n treated outside the county and s o m e others m a y not have had their a m b l y o p i a detected. We feel confident that very few patients f r o m Leicestershire had t r e a t m e n t outside our institutions. T h e pattern of referral in our region has always tended to follow catchment area boundaries because screening policies respect those boundaries. Some patients m a y have presented to their G e n e r a l Practitioner prior to detection by screening and these patients might theoretically have asked to be referred to another centre. W e think this h a p p e n e d to a very small extent, partly because of the longer
460
J . R . Thomson et ah
distances to alternative facilities and partly because our waiting list to be seen in Leicestershire at that time was only four weeks. In 1983 all children were invited for vision screening by a Health Visitor at age seven months and again at three and a half years, after which the children suspected of having problems were referred direct to a consultant ophthalmologist. Further vision screening was conducted through the schools when the children were aged five years and seven years. The combined coverage of these four screens is thought to be high, but we have no information on the sensitivity of the screens. Our faith in the completeness of the coverage is supported by the geographical uniformity in referrals. Under-representation occurs only in the small rural districts of Melton and Rutland. Both of the districts have peripheral orthoptic clinics in their main towns, and as these are served by the same orthoptic staff as the central hospital, this suggests that the referral process could be improved in these two districts. The under-represented areas fall short of their expected numbers by about 20 children in total. This represents about 5% of the total cohort. Adjustment for this shortfall would cause the total incidence of suspected cases given in Table I to be increased only slightly from 3.35 to 3.53, The bias due to poor coverage would therefore seem to be small and there is no evidence of decreased referral from the districts on the county borders. Our results confirm the late presentation of anisometropic amblyopia.l° This needs to be taken into account when planning a screening programme. The figures for late-onset esotropic amblyopia show that these children form a sizeable group, accounting for about one-third of all amblyopes. The pattern of onset of their squints shows a concentration in years one to three, although it must be r e m e m b e r e d that onset of squint and onset of amblyopia will not necessarily occur at the same time. The results presented here are in line with some screening-based studies of the prevalence of amblyopia. In Israel the prevalence was found to increase to 1.8% at the age of five. We cannot be certain how the prevalence in Leicestershire varies with age because we only know the child's age at referral and not when the amblyopia started. However, by summing the incidences from the second column of Table I over the years 0 to 5, we find a similar cumulative presentation incidence of 2.1%. Our study demonstrates that amblyopia is, by public health standards, very common. The prevalence in Britain is much greater than that of any other treatable condition which causes long-term functional disability. The c o m m o n childhood febrile illnesses have a higher incidence but the prevalence of continuing disability is extremely low. Other long-term disabling conditions such as cerebral palsy lI or multiple sclerosis n are both less c o m m o n and arguably untreatable. The relative lack of public awareness of amblyopia may reflect doubts in the minds of medical staff about the importance of the condition and its treatability. The results of the treatment of this cohort of patients will be more closely analysed elsewhere, but our data do demonstrate an improvement with treatment. The question of whether unilateral loss of vision is a significant disability is a personal judgement. However, even if the unilateral visual loss is discounted, the possibility of subsequent loss of the other eye has to be considered. The risk of this occurring is hard to measure and only a few papers have attempted to do so. In Finland, Tommila and T a r k k a n e n 13 examined data covering a period of 20 years and traced 35 amblyopes who subsequently lost vision in the healthy eye, most from trauma. Even allowing for some under-recording the extent of the problem does not,
The Incidence and Prevalence o f Amblyopia detected in Childhood
461
in public health terms, appear to be great. However, the authors estimate that amblyopes have a considerably increased personal risk of blindness compared to the general population, and any form of preventable blindness reflects a failure of the health services. Vereecken and Brabant, 14 in a paper which perhaps selects patients who do well, found that, in the event of loss of the good eye, only 28.5% of their sample showed an improvement in vision in the amblyopic eye to 6/20 or better. In addition to the risk of losing the other eye, it must be r e m e m b e r e d that many surgeons are more reluctant to operate on patients who have only one good eye. If a surgeon varies his or her practice, offering cataract surgery at 6/18 to a patient with two functioning eyes, but only at 6/60 in a patient with dense amblyopia in the other eye, then the amblyopic patient will suffer more visual morbidity from c o m m o n bilateral conditions such as cataract. Although difficult to quantify, this may be the greatest cause of added morbidity from amblyopia. In the light of the relatively high incidence of amblyopia and its potential for treatment if detected early enough, it is important that we raise the public awareness of this condition and ensure that our screening is as effective as possible. Given the observed pattern of incidence with age, it is clear that we need to maintain our vigilance throughout early childhood.
Acknowledgements We should like to thank the staff of the Orthoptic Department of I~icester Royal Infirmary for their cooperation with this study. Material from Crown-copyright records has been made available through the Post Office and the ESRC Data Archive. Clerical assistance was provided by Ms K Robinson and Ms R Gowing, whose salary is supported by the Anne Allerton Fund.
References 1. Hillis, A., Flynn, J. T. & Hawkins B. S. (1983). The evolving concept of amblyopia: A challenge to edpidemiologists. American Journal of Epidemiology, 118, 192-205. 2. Von Noorden G. K. (1977). Mechanisms of amblyopia. Advances in Ophthalmology, 34, 93. 3. Hillis, A. (1986). Amblyopia: Prevalent, curable, neglected. Public Health Review, 14, 213-235. 4. Flom, M. C. & Neumaier, R. W. (1966). Prevalence of amblyopia. Public Health Report, 81,329-341. 5. Fransden, A. D. (1960) Occurrence of Squint: a clinical statistical study on the prevalence of squint and associated signs in different groups and ages of the Danish population. Copenhagen: H. K. Krisenson . 6. Oliver M. & Nawratzki, I. (1971). Screening of pre-school children for ocular abnormalities 1I amblyopia: Prevalence and therapeutic results at different ages. British Journal of Ophthalmology, 55,467-471. 7. Office of Population Census & Surveys. (1982). Census 1981: County Report, Leicestershire. London: HMSO. 8. Office of Population Census & Surveys. (1984). Birth Statistics 1982: England & Wales. London: HMSO. 9. Garzia, R. P. (1987). Efficacy of vision therapy in amblyopia: A literature review. American Journal of Optometry & Physiological Optics, 64,393-404. 10. Shaw, D. E., Fielder, A. R., Minshull, C. & Rosenthal A. R. (1988). Amblyopia--factors influencing the age of presentation. Lancet, ii, 207-209.
462
J. R. Thomson et al.
11. Stanley, F. & Alberman, E. (eds) (1984). The Epidemiology of the Cerebral Palsies, Spastics International Medical Publications. Oxford: Blackwell Scientific. 12. Acheson, D. E. (1985). The epidemiology of multiple sclerosis. In Matthews, W. B. (ed.) McAlpine's Multiple Sclerosis. Edinburgh: Churchill Livingstone. 13. Tommila, V. & Tarkkanen, A. (1981). Incidence of loss of vision in the healthy eye in amblyopia. British Journal of Ophthalmology, 65,575-577. 14. Vereecken, E. P. and Brabant, P. (1984). Prognosis for vision in amblyopia after the loss of the good eye. Archives of Ophthalmology,102, 220-224.
