The key informant method: a novel means of ascertaining blind children in Bangladesh

BACKGROUND

Most information on the causes of blindness has come from examining children in special education. To obtain a more representative population-based sample of children, a novel method was developed for ascertaining severe visually impaired (SVI) or blind (BL) children by training local volunteers to act as key informants (KIs).

OBJECTIVE

To compare the demography and cause of blindness in children recruited by KIs with other ascertainment methods.

METHOD

Children with SVI/BL were recruited in all 64 districts of Bangladesh. Three sources for case ascertainment were utilised: schools for the blind (SpEdu), community-based rehabilitation (CBR) programmes and KIs. All data were recorded using the standard WHO/PBL Eye Examination Record.

RESULTS

1935 children were recruited. Approximately 800 KIs were trained. The majority of the children were recruited by the KIs (64.3%). Children recruited by KIs were more likely to be female (odds ratio (OR) 1.6, p<0.001), of pre-school age (OR 14.1, p<0.001), from rural areas (OR 5.9, p<0.001), be multiply impaired (OR 3.1, p = 0.005) and be suffering from treatable eye diseases (OR 1.3, p = 0.005) when compared with those in SpEdu. Overall a child with an avoidable causes of SVI/BL had 40% (adjusted CI 1.1 to 1.7, p = 0.015) and 30% (CI 1.0 to 1.7, p = 0.033) higher odds of being ascertained using the KIs compared with SpEdu and CBR methods, respectively.

CONCLUSION

Using this innovative approach has resulted in one of the largest studies of SVI/BL children to date. The findings indicate that KIs can recruit large numbers of children quickly, and that the children they recruit are more likely to be representative of all blind children in the community.

Cataract prevalence, cataract surgical coverage and barriers to uptake of cataract surgical services in Pakistan: the Pakistan National Blindness and Visual Impairment Survey

AIM

To estimate the prevalence of visual impairment and blindness caused by cataract, the prevalence of aphakia/pseudophakia, cataract surgical coverage (CSC) and to identify barriers to the uptake of cataract services among adults aged >or=30 years in Pakistan.

METHODS

Probability proportional-to-size procedures were used to select a nationally representative sample of adults. Each subject underwent interview, visual acuity measurement, autorefraction, biometry and ophthalmic examination. Those that saw <6/12 in either eye underwent a more intensive examination procedure including corrected visual acuity, slit lamp and dilated fundus examination. CSC was calculated for different levels of visual loss by person and by eye. Individuals with <6/60 in the better eye as a result of cataract were interviewed regarding barriers.

RESULTS

16 507 Adults were examined (95.5% response rate). The crude prevalence of blindness (presenting <3/60 in the better eye) caused by bilateral cataract was 1.75% (95% CI 1.55%, 1.96%). 1317 Participants (633 men; 684 women) had undergone cataract surgery in one or both eyes, giving a crude prevalence of 8.0% (95% CI 7.6%, 8.4%). The CSC (persons) at <3/60, <6/60 and <6/18 were 77.1%, 69.3% and 43.7%, respectively. The CSC (eyes) at <3/60, <6/60 and <6/18 were 61.4%, 52.2% and 40.7%, respectively. Cost of surgery (76.1%) was the main barrier to surgery.

CONCLUSION

Approximately 570 000 adults are estimated to be blind (<3/60) as a result of cataract in Pakistan, and 3,560000 eyes have a visual acuity of <6/60 because of cataract. Overall, the national surgical coverage is good but underserved populations have been identified.

The Inheritance of Peripapillary Atrophy

PURPOSE

To estimate the relative importance of genes and environment in peripapillary atrophy type beta (beta-PPA) in a classic twin study.

METHODS

Female twin pairs (n = 506) aged 49 to 79 years were recruited from the St. Thomas' UK Adult Twin Registry. Peripapillary atrophy was identified from masked grading of stereoscopic optic disc photographs. Structural equation modeling was performed using Mx with polychoric correlations of beta-PPA and refractive error (divided into deciles).

RESULTS

Beta-PPA prevalence was 25.1% and did not vary with zygosity. Case-wise concordance for right eyes was 0.76 (95% CI, 0.57-0.88) for monozygotic (MZ) and 0.37 (95% CI, 0.15-0.56) for dizygotic (DZ) pairs. Multivariate modeling suggested additive genetic effects and individual environment, with no shared environment or dominant genetic effect. Beta-PPA heritability was 0.70 (95% CI, 0.54-0.83), and spherical equivalent 0.88 (95% CI, 0.85-0.91); age had no significant effect on variance. The genetic correlation between beta-PPA and spherical equivalent was -0.21. However, only 3% of the genetic variance of beta-PPA was explained by genetic factors in common with refractive error, with 67% explained by specific genetic factors for beta-PPA. Of the 30% of variance explained by unique environmental factors, only 3% was explained by these factors in common with environmental factors involved in refractive error.

CONCLUSIONS

The presence of beta-PPA, a frequent ocular finding known to be associated with open-angle glaucoma, appears to be under strong genetic control, with only a small amount of this genetic effect shared with genes involved in myopia.

Causes of severe visual impairment and blindness in Bangladesh: a study of 1935 children

OBJECTIVE

To identify the anatomical site and underlying aetiology of severe visual impairment and blindness (SVI/BL) in children in Bangladesh.

DESIGN

A national case series.

METHODS

Children were recruited from all 64 districts in Bangladesh through multiple sources. Causes were determined and categorised using standard World Health Organization methods.

RESULTS

1935 SVI/BL children were recruited. The median age was 132 months, and boys accounted for 63.1% of the sample. The main site of abnormality was lens (32.5%), mainly unoperated cataract, followed by corneal pathology (26.6%) and disorders of the whole eye (13.1%). Lens-related blindness was the leading cause in boys (37.0%) compared with corneal blindness in girls (29.8%). In 593 children, visual loss was due to childhood factors, over 75% being attributed to vitamin A deficiency. Overall 1338 children (69.2%) had avoidable causes. Only 2% of the country's estimated SVI/BL children have access to education and rehabilitation services.

CONCLUSIONS

This is the first large-scale study of SVI/BL children in Bangladesh over two-thirds of whom had avoidable causes. Strategies for control are discussed.

The role of heredity in determining central retinal thickness

AIMS

To examine the relative roles of genetic and environmental factors in central retinal thickness, by performing a classical twin study.

METHODS

310 subjects were recruited from the TwinsUK adult registry at St Thomas' Hospital. Optical coherence tomography (Zeiss, stratus OCT3) was used to measure the average retinal thickness in the central 1 mm diameter area. The covariance of central retinal thickness (CRT), within MZ and DZ twin pairs, was compared and genetic modelling techniques were used to determine the relative contributions of genes and environment to the variation in CRT observed in this population.

MAIN OUTCOME MEASURE

CRT (average retinal thickness in the central 1 mm diameter area, centred on the fovea).

RESULTS

The mean CRT of all subjects was 212.1 microm (range 165-277). CRT was statistically related to refractive error, with increasing myopia associated with a thinner CRT. CRT was more highly correlated within MZ twin pairs (r = 0.88) than with DZ twin pairs (r = 0.58), suggesting a genetic role. A model combining additive genetic and unique environmental factors provided the best fitting model and gave a heritability estimate of 0.90.

CONCLUSION

Genetic factors appear to play an important role in CRT, with a heritability estimate of 0.90.

Causes of blindness and visual impairment in Pakistan. The Pakistan national blindness and visual impairment survey

OBJECTIVE

To determine the causes of blindness and visual impairment in adults (> or =30 years old) in Pakistan, and to explore socio-demographic variations in cause.

METHODS

A multi-stage, stratified, cluster random sampling survey was used to select a nationally representative sample of adults. Each subject was interviewed, had their visual acuity measured and underwent autorefraction and fundus/optic disc examination. Those with a visual acuity of <6/12 in either eye underwent a more detailed ophthalmic examination. Causes of visual impairment were classified according to the accepted World Health Organization (WHO) methodology. An exploration of demographic variables was conducted using regression modeling.

RESULTS

A sample of 16 507 adults (95.5% of those enumerated) was examined. Cataract was the most common cause of blindness (51.5%; defined as <3/60 in the better eye on presentation) followed by corneal opacity (11.8%), uncorrected aphakia (8.6%) and glaucoma (7.1%). Posterior capsular opacification accounted for 3.6% of blindness. Among the moderately visually impaired (<6/18 to > or =6/60), refractive error was the most common cause (43%), followed by cataract (42%). Refractive error as a cause of severe visual impairment/blindness was significantly higher in rural dwellers than in urban dwellers (odds ratio (OR) 3.5, 95% CI 1.1 to 11.7). Significant provincial differences were also identified. Overall we estimate that 85.5% of causes were avoidable and that 904 000 adults in Pakistan have cataract (<6/60) requiring surgical intervention.

CONCLUSIONS

This comprehensive survey provides reliable estimates of the causes of blindness and visual impairment in Pakistan. Despite expanded surgical services, cataract still accounts for over half of the cases of blindness in Pakistan. One in eight blind adults has visual loss from sequelae of cataract surgery. Services for refractive errors need to be further expanded and integrated into eye care services, particularly those serving rural populations.

Prevalence of Blindness and Visual Impairment in Pakistan: The Pakistan National Blindness and Visual Impairment Survey

PURPOSE

To determine the prevalence of blindness and visual impairment in adults aged 30 years and older in Pakistan and to assess socio-demographic risk factors.

METHODS

Multistage, stratified (rural/urban), cluster random sampling, with probability proportional-to-size procedures, was used to select a nationally representative, cross-sectional sample of adults 30 years of age or older. Each subject was interviewed; had visual acuity measured (logMAR; logarithm of the minimum angle of resolution); and underwent autorefraction, biometry, and fundus-optic disc examination. Those with less than 6/12 acuity in either eye underwent a detailed ophthalmic examination, including corrected distance visual acuity measurement and dilated ophthalmoscopy.

RESULTS

A nationally representative sample of 16,507 adults (95.5% of those enumerated) was examined. The age- and gender-standardized prevalence of blindness was 2.7% (95% confidence interval [CI], 2.4%-2.9%). It has been estimated that there are 1,140,000 (962,000-1,330,000) blind adults in Pakistan (2003 statistics). Blindness prevalence varied throughout the country, being highest in the provinces of Punjab and Baluchistan and lowest in the North West Frontier Province. Rural areas had a higher prevalence of blindness than did urban areas (3.8% vs. 2.5%, P < 0.001). Increasing age and being female were significantly associated with presenting visual acuity of <6/60 (odds ratio [OR], 2.5; 95% CI, 2.3-2.7 and 1.3; 95% CI, 1.1-1.5, respectively). Educational status was also associated with presenting visual acuity of <6/60. Subjects who had attended primary school were 60% (P < 0.001) less likely to have acuity of <6/60 than were subjects who had never been to school.

CONCLUSIONS

This comprehensive survey provides reliable estimates of the prevalence of visual impairment and blindness in Pakistan. A significant excess of visual impairment was found among the elderly and the uneducated. After adjustment for age differences, women were found to have a significant excess of severe visual impairment and blindness. Regional variations in the prevalence of blindness were also identified.

Heritability of macular pigment: a twin study

PURPOSE

Several studies have reported higher levels of macular pigment (MP) in association with reduced risk for age-related macular degeneration (ARMD), a disease to which there is a genetic predisposition. A classic twin study was performed to determine the heritability of MP in the healthy eye.

METHODS

One hundred fifty twin pairs (76 monozygotic [MZ] and 74 dizygotic [DZ]), aged 18 to 50 years, participated. MP optical density was measured psychophysically with heterochromatic flicker photometry (HFP) and also with an imaging method involving fundus autofluorescence (AF). The covariance of MP within MZ and DZ twin pairs was compared, and genetic modeling techniques were used to determine the relative contributions of genes and environment to the variation in MP.

RESULTS

The mean MP optical density, measured using HFP, was 0.43 +/- 0.21. Using AF, the mean MP optical density, measured at 1 degrees eccentricity, was 0.28 +/- 0.11. MP optical densities correlated more highly in MZ twins than in DZ twins, according to both HFP (MZ: 0.65; DZ: 0.24) and AF (MZ: 0.83; DZ: 0.50). A model combining additive genetic and unique environmental effects provided the best fit and resulted in MP heritability estimates of 0.67 (95% CI, 0.52-0.77) and 0.85 (95% CI, 0.78-0.90) for HFP and AF readings, respectively.

CONCLUSIONS

This classic twin study demonstrates that genetic background is an important determinant of MP optical density, reflected in heritability estimates of 0.67 and 0.85 for HFP and AF measures, respectively.

Central retinal thickness is positively correlated with macular pigment optical density

Macular pigment (MP) has been suggested to have a protective role in age-related macular degeneration by reducing the amount of oxidative stress on the retina. MP levels peak at the foveal center, where it is found predominantly in the receptor axon and inner plexiform layers of the retina. The purpose of this study was to investigate the relationship between central retinal thickness and macular pigment optical density in a group of healthy subjects. We report that macular pigment optical density (MPOD) has a significant and positive relationship with central retinal thickness as measured by optical coherence tomography. The strength of the observed relationship (r approximately 0.30) was independent of the technique used to measure MPOD, whether heterochromatic flicker photometry (HFP) or 2-wavelength autofluorescence (AF). Of note, there was no statistically demonstrable relationship between MPOD at an eccentricity of 1- or 2-degrees and central retinal thickness. This finding has important implications for future studies investigating MPOD, and its response to dietary modification/supplementation.

Central corneal thickness is highly heritable: the twin eye studies

PURPOSE

A classic twin study was performed to determine the heritability of central corneal thickness (CCT), an important parameter in glaucoma assessment.

METHODS

The concordance of CCT between monozygotic (MZ) and dizygotic (DZ) twins was compared. A total of 256 twin pairs (131 MZ and 125 DZ) were recruited from three centers: the Twin Eye Study in Tasmania, the Brisbane Adolescent Twin Study, and the Twins U.K. Adult Registry held at St. Thomas' Hospital in London. As part of an extensive ophthalmic evaluation, CCT was measured by ultrasound pachymetry. Structural equation modeling with the Mx program (Department of Psychiatry, Medical College of Virginia, Richmond, VA) was used to determine the heritability of CCT.

RESULTS

The mean age of subjects was 38 years (range, 8-81). The mean CCT of all eyes examined was 544.5 +/- 37.3 mum (SD). The CCT measurements correlated more highly in MZ twins than in DZ twins, with intraclass correlation coefficients of 0.95 and 0.52, respectively, suggesting a strong genetic influence. A model of additive genetic and unique environmental effects provided the best fit, yielding a heritability of 0.95 (95% confidence interval [CI], 0.93-0.96) with the remaining variation being attributable to unique environmental factors.

CONCLUSIONS

In this study of Australian and U.K. twins, genetic factors were shown to be of major importance in CCT, with a heritability of 0.95.

Eye birth defects in humans may be caused by a recessively-inherited genetic predisposition to the effects of maternal vitamin A deficiency during pregnancy

Congenital malformations of the eye can cause blindness in children. They occur throughout the world and in most cases the aetiology is unknown. Linkage studies have largely been unsuccessful and the risk to siblings is generally low. Epidemiological and laboratory evidence support a hypothesis that there may be genetic (recessive) predisposition to the teratogenetic effects of mild to moderate maternal vitamin A deficiency (VAD) during pregnancy. This may explain the higher prevalence of congenital eye anomalies in a part of Asian countries, where maternal VAD is common and consanguineous marriages are popular. Other congenital malformations commonly found in association with ocular coloboma (e.g. oesophageal fistulae and heart defects in CHARGE association) may also be VAD related. Mutations in a gene involved in the cellular access to vitamin A that normally protects the tissue or embryo from natural variation in dietary vitamin A intake, could render that individual intolerant of conditions of VAD. An interaction of this kind could also explain a proportion of "sporadic" cases in locations where VAD is uncommon. If this interaction is shown to be true, there are public health implications for the prevention of blindness due to congenital eye malformations. The hypotheses proposed above are reminiscent of the research leading to the discovery that folic acid supplementation could prevent neural tube defects. However, this form of intervention would be much more difficult with vitamin A, which is itself a powerful teratogen if present in excess.

Causes and temporal trends of blindness and severe visual impairment in children in schools for the blind in North India

AIMS

To describe the causes of severe visual impairment and blindness (SVI/BL) in children in schools for the blind in north India, and explore temporal trends in the major causes.

METHODS

A total of 703 children were examined in 13 blind schools in Delhi. A modified WHO/PBL eye examination record for children with blindness and low vision which included sections on visual acuity, additional non-ocular disabilities, onset of visual loss, the most affected anatomical part of the eye concerning visual impairment, and the aetiological category of the child's disorder based on the timing of insult leading to visual loss was administered in all children.

RESULTS

With best correction, 22 (3.1%) were severely visually impaired (visual acuity in the better eye of <6/60) and 628 (89.3%) children were blind (visual acuity in the better eye of <3/60). Anatomical sites of SVI/BL were whole globe in 27.4% children, cornea 21.7%, retina 15.1%, and lens 10.9%. The underlying cause of visual loss was undetermined in 56.5% children (mainly abnormality since birth 42.3% and cataract 8.3%), childhood disorders were responsible in 28.0% (mainly vitamin A deficiency/measles 20.5%), and hereditary factors were identified in 13.4%. Study of temporal trends of SVI/BL by comparing causes in children in three different age groups-5-8 years, 9-12 years, and 13-16 years-suggests that retinal disorders have become more important while childhood onset disorders (particularly vitamin A deficiency) have declined.

CONCLUSIONS

Almost half of the children suffered from potentially preventable and/or treatable conditions, with vitamin A deficiency/measles and cataract the leading causes. Retinal disorders seem to be increasing in importance while childhood disorders have declined over a period of 10 years.

The familial contribution to non-syndromic ocular coloboma in south India

AIMS

To identify the proportion of familial cases of isolated ocular colobomatous malformations in a case series from south India.

METHODS

Children with ocular coloboma without systemic features were recruited from multiple sources in Andhra Pradesh, India. Their families were traced, pedigrees drawn, and family members examined.

RESULTS

56 probands, 25 females (44.6%) and 31 males (57.4%) with a colobomatous malformation were identified. In 12 cases (21.4%) another family member was affected. The risk to siblings was 3.8%. The parents were consanguineous in 25 cases (44.6%).

CONCLUSIONS

21.4% of cases of isolated ocular coloboma in this highly consanguineous population of south India were familial, with both autosomal dominant and autosomal recessive mechanisms likely in different families.

Prevalence and causes of severe visual impairment and blindness in children in Mongolia

BACKGROUND

Reliable epidemiological data on the prevalence and causes of visual loss in children are difficult to obtain, but are essential for planning. No such data are available from Mongolia.

AIM

To determine the prevalence and causes of severe visual impairment and blindness (SVI/BL) in children from a defined area of Mongolia, using several methods of identification.

METHODS

Children with presenting visual acuities of <6/60 in the better eye who lived in 10 of the 18 provinces (Aimaks) were identified 1) by family doctors 2) in the school for the blind 3) by visiting eye departments in the capital. All eligible children were examined (or data extracted from hospital records) and the cause of visual loss determined using the WHO classification system.

RESULTS

Sixty-four children with SVI/BL before refraction were identified who lived in the 10 study Aimaks. They were recruited by family doctors (52); by home visits (3); from hospital records (4); or from the school for the blind (5). The prevalence of SVI/BL before refraction was 0.19/1,000 children (95% CI 0.16-0.22), decreasing to 0.16/1,000 after refraction (95% CI 0.13-0.19) but there was considerable variation from Aimak to Aimak. The major causes of SVI/BL were lesions of the lens (34%), central nervous system disorders (19%), lesions of the whole globe (e.g. microphthalmos) (14%), and retinal conditions (12.5%). Hereditary factors were responsible for 27% of causes, and 17% of children were blind following acquired conditions of childhood. The underlying cause could not be determined in 48%. The causes of SVI/BL was analysed in a further 16 children who lived outside the study Aimaks to compare the causes in children in special education with those not in schooling, and by age.

CONCLUSION

The prevalence estimate obtained was lower than anticipated, and possible reasons are discussed. The pattern of causes of SVI/BL is similar to that in children in schools for the blind in China, but is very different from other Asian countries. Meningococcal meningitis was the most common preventable cause of SVI/BL, and immunisation is being considered. Other preventable causes were rare, and the majority of children needing surgical intervention had already been identified and referred for treatment. The control of blindness in children could possibly be improved by better management of conditions requiring surgery, and by the provision of low vision devices.

Genetic influence on early age-related maculopathy: a twin study

OBJECTIVE

Age-related macular degeneration (AMD) is the most common cause of blindness in industrialized countries. There has been considerable interest in the genetics of early age-related maculopathy (ARM) and AMD, because they have phenotypes similar to inherited diseases where mutations have been identified, but the heritability of ARM and AMD is unknown.

DESIGN

A classical twin study was performed to compare the concordance in monozygotic (MZ) and dizygotic (DZ) twins in an unselected sample of female volunteer twins.

PARTICIPANTS

Five hundred six twin pairs, 226 MZ and 280 DZ, with a mean age of 62 years, were examined.

METHODS

ARM was graded from stereoscopic macular photographs of 501 of the twin pairs (99%) according to the International ARM Epidemiologic Study Group grading system. The casewise concordance was calculated for twin pairs from 2 x 2 contingency tables of affected/unaffected twins, and these tables were used in maximum likelihood genetic modeling to estimate the heritabilities of phenotypes graded.

MAIN OUTCOME MEASURES

Prevalence of ARM; concordance in MZ and DZ twins of the phenotypes of ARM, soft drusen >63 microm and > or =125 microm diameter, pigmentary changes and hard drusen (<20 and > or =20 in number); heritability of ARM and subphenotypes.

RESULTS

The overall prevalence of ARM was 14.6% (95% confidence interval [CI], 12.4%-16.8%). The concordance for ARM in MZ twins was 0.37 compared with 0.19 in DZ twins, suggesting a role for genes. Modeling confirmed a genetic effect for phenotypes of ARM, soft drusen, pigmentary changes, and > or =20 hard drusen, although there was little genetic effect for scattered (<20) hard drusen. The heritability of ARM was estimated as 45% (95% CI, 35%-53%). The most heritable phenotypes were soft drusen > or =125 microm (57%) and > or =20 hard drusen (81%), with the latter being dominantly inherited.

CONCLUSIONS

This study confirms a significant genetic influence in ARM and suggests that future genetic studies should examine phenotypes of large (> or =125 microm) soft drusen and > or =20 hard drusen, because these seem to be the most heritable components.

Environmental risk factors in congenital malformations of the eye

Developmental eye defects such as coloboma are a significant cause of visual morbidity in children, and are more common in India than elsewhere. The possible role of environmental factors in the aetiology of these conditions was investigated by studying birth order, symptoms of vitamin A deficiency (night blindness), drug use and maternal illness in pregnancy, rubella antibodies and exposure to agricultural chemicals. Through hospital records and community-based rehabilitation programmes in Andhra Pradesh, children with colobomata were recruited from schools for the blind. Eighty-three mothers of affected children were interviewed. The results showed that 43% of parents were consanguineous, that 19% had a positive family history and that the frequency of coloboma was highest in second-born children. Eleven (16%) mothers had a history of night blindness while pregnant with the affected child; seven (8%) took medication during the 1st trimester, abortifacients in two cases; three reported fever in the 1st trimester; and 11 (13%) reported exposure to agricultural chemicals.

Clinical findings, consanguinity, and pedigrees in children with anophthalmos in southern India

This study aimed to describe clinical findings, pedigrees, and possible environmental risk factors in children with clinical anophthalmos and remnant microphthalmos in either eye in southern India. Twenty-four children (14 male, 10 female; mean age 10.3 years, age range 1.3 to 18 years,) were recruited from schools for the blind, hospitals, and community-based rehabilitation programmes in Andhra Pradesh, India, over 1 year. Family members were examined, and mothers interviewed. Fifteen children had anophthalmos and nine had remnant microphthalmos in one or both eyes. Twelve children had associated systemic findings, of which six were major and six were minor abnormalities. Information on consanguinity was available in 19 children, 12 of whom had consanguineous parents. Five children had a positive family history. Two mothers had a history of night blindness, and one had a history of pesticide exposure during pregnancy. High rates of consanguinity suggest a genetic recessive aetiology.

Genes and environment in refractive error: the twin eye study

PURPOSE

A classical twin study was performed to examine the relative importance of genes and environment in refractive error.

METHODS

Refractive error was examined in 226 monozygotic (MZ) and 280 dizygotic (DZ) twin pairs aged 49 to 79 years (mean age, 62.4 years). Using a Humphrey-670 automatic refractor, continuous measures of spherical equivalent, total astigmatism, and corneal astigmatism were recorded. Univariate and bivariate maximum likelihood model fitting was used to estimate genetic and environmental variance components using information from both eyes.

RESULTS

For the continuous spectrum of myopia/hyperopia, a model specifying additive genetic and unique environmental factors showed the best fit to the data, yielding a heritability of 84% to 86% (95% confidence interval [CI], 81%-89%). If myopia and hyperopia (< or = -0.5 D and > or = 0.5 D, respectively) were treated as binary traits, the heritability was 90% (95% CI, 81%-95%) for myopia and 89% (95% CI, 81%-94%) for hyperopia. For total and corneal astigmatism, modeling showed dominant genetic effects are important; dominant genetic effects accounted for 47% to 49% of the variance of total astigmatism (95% CI, 37%-55%) and 42% to 61% of corneal astigmatism variance (95% CI, 8%-71%), with additive genetic factors accounting for 1% to 4% and 4% to 18%, respectively (95% CIs, 0%-13% and 0%-60%, respectively).

CONCLUSIONS

Genetic effects are of major importance in myopia/hyperopia; astigmatism appears to be dominantly inherited.

The heritability of age-related cortical cataract: the twin eye study

PURPOSE

A classical twin study was performed to establish the relative importance of genes and environment in cortical cataract.

METHODS

Five hundred six pairs of unselected female twin volunteers (226 monozygotic and 280 dizygotic) with a mean age of 62 years (range, 49-79 years) were examined. Cortical cataract was assessed using the slit-lamp-based Oxford Clinical Cataract Classification and Grading System (clinical grading) and the Wilmer Automated Grading System, which analyzed digital retroillumination images of subjects' lenses (digital grading). The worse eye categorized score for each individual was used in maximum likelihood path modeling of the correlations within twin pairs. These correlations were used to determine the underlying liability to cortical cataract.

RESULTS

Prevalence of significant cortical cataract (>/=5% of lens area) was similar in monozygotic and dizygotic twins, occurring in 19.4% and 20.6% with the clinical grading system and 24% and 23% using the digital grading system, respectively. Modeling suggested liability to cortical cataract is explained by additive and dominant genes, individual environment, and age. Estimates of the broad sense heritability of cortical cataract were 58% (95% confidence interval [CI], 51%-64%) for the clinical grading system and 53% (95% CI, 45%-60%) for the digital system. Dominant genes were estimated to contribute to 38% (95% CI, 1%-64%) of the genetic effect with the clinical grading and 53% (95% CI, 28%-60%) with the digital grading. Individual environment explained 26% and 37% and age 16% and 11% of cortical cataract variance in clinical and digital gradings, respectively.

CONCLUSIONS

Genetic effects are important in the development of cortical cataract and involve the action of dominant genes.

Factors affecting pupil size after dilatation: the Twin Eye Study

BACKGROUND/AIMS

Well dilated pupils make eye surgery easier. A classic twin study was established to examine the relative importance of genes and environment in the variance of pupil size after mydriasis, and to examine the effects of other factors such as age, iris colour, and refractive error.

METHODS

506 twin pairs, 226 monozygotic (MZ) and 280 dizygotic (DZ), aged 49-79 (mean age 62.2 years, SD 5.7) were examined. Dilated pupil size was measured using a standardised grid superimposed over digital retroillumination images taken 50-70 minutes after mydriasis using tropicamide 1% and phenylephrine 10%. Univariate maximum likelihood model fitting was used to estimate genetic and environmental variance components.

RESULTS

Dilated pupil size was more highly correlated in MZ compared with DZ twins (intraclass correlation coefficients 0.82 and 0.39 respectively). A model specifying additive genetic and unique environmental factors showed the best fit to the data, yielding a heritability of 78-80%. Individual environmental factors explained 18-19% of the variance in this population. Age only accounted for 2-3% of the variance and refractive error and iris colour did not significantly contribute to the variance.

CONCLUSIONS

Pupil size after mydriasis is largely genetically determined, with a heritability of up to 80%.

Evaluation of children in six blind schools of Andhra Pradesh

PURPOSE

1. To determine the anatomical site and underlying causes of severe visual impairment and blindness in children in special education in Andhra Pradesh, India. 2. To compare the causes of blindness in two different regions in the state. 3. To evaluate improvement with correction of refractive error and low-vision devices (LVDs).

METHODS

Children in 6 schools for the blind and in 3 integrated education programmes were examined by one ophthalmologist, and were refracted and assessed for LVDs by an optometrist. The major anatomical site and underlying aetiology of severe visual impairment and blindness (SVI/BL; < 6/60 in the better eye) were recorded using the standardised WHO reporting form.

RESULTS

Two hundred and ninety one students under 16 years were examined, and after refraction, 267 (91.7%) were classified as being severely visually impaired or blind. The most common anatomical sites of SVI/BL were retina in 31.1% children; cornea in 24.3%; and whole globe in 20.2%. The aetiology was unknown in 38.2%, hereditary in 34.8% and childhood causes in 24%. 114 children (39.2%) had functional low vision (i.e. visual acuity < 6/18 to light perception with navigational vision). In this group, 36 children improved with spectacles and 16 benefited by LVDs. 41 children (15.4%) were able to read N10 point though they were studying Braille.

CONCLUSION

Overall 37.4% of children had "avoidable" causes of blindness. The major avoidable causes were vitamin-A deficiency and cataract. Vitamin-A deficiency and congenital anomalies were more common in the dry plateau areas of the state. One in seven children could read normal print with optical support.

Genetic and environmental factors in age-related nuclear cataracts in monozygotic and dizygotic twins

BACKGROUND

Age-related cataracts are a major public health problem. The relative importance of genes and environment in the causation of nuclear cataracts, the most common form of age-related cataracts, is not known.

METHODS

We studied 506 pairs of female twins (226 monozygotic and 280 dizygotic) who were 50 to 79 years old (mean, 62). The amount of nuclear cataract in the right and left eyes was determined objectively by analysis of Scheimpflug lens photographs (yielding three measures) and subjectively with use of the Oxford Clinical Cataract Classification and Grading System (yielding one measure). All eight measures (four in each eye) were subsequently combined in one summary measure of nuclear cataract for each woman. A univariate maximum-likelihood model was used to estimate the variance of the genetic and environmental contributions to each of the measures.

RESULTS

The different measures of cataract formation were highly correlated (correlation coefficients, 0.71 to 0.94). The mean scores were similar for the right and left eyes and for monozygotic and dizygotic twins. Quantitative genetic modeling of each of the nuclear-cataract scores invariably resulted in a best-fitting model that involved additive genetic effects, unique environmental effects, and age. The common environmental and dominant genetic effects could be removed from the models without significant loss of fit. The overall heritability in the combined nuclear-cataract score (the proportion of the variance explained by genetic factors) was 48 percent (95 percent confidence interval, 42 to 54 percent); age accounted for 38 percent of the variance (95 percent confidence interval, 31 to 44 percent) and unique environmental effects for 14 percent (95 percent confidence interval, 12 to 18 percent).

CONCLUSIONS

Genetic effects are important even in such a clearly age-related disease as nuclear cataract, explaining almost 50 percent of the variation in the severity of this disease.

Regional variation in blindness in children due to microphthalmos, anophthalmos and coloboma

BACKGROUND

The prevalence and causes of blindness in children vary widely between regions. Few epidemiological data are available on the relative importance of the major congenital anomalies of the globe (i.e., microphthalmos, anophthalmos, coloboma) as causes of blindness in children. The aim of this study was to determine the re-gional variation in the proportion of severe visual impairment and blindness due to congenital abnormalities of the globe in children in schools for the blind and in those identified through Community Based Rehabilitation programs. Other objectives were to estimate the prevalence of blindness due to major congenital abnormalities, and to investigate their etiology.

METHODS

Data on the causes of blindness in children were collected between 1990 and 1998 using standard methods, definitions and reporting form in 26 countries. Children were examined in schools for the blind and in Community Based Rehabilitation programs.

RESULTS

Of 7,113 children aged 3-15 years with severe visual impairment and blindness examined, 762 (10.7%) had microphthalmos, 161 (2.3%) had anophthalmos, and 96 (1.3%) had coloboma. There are large regional differences in the proportion of severe visual loss in blind school children, ranging from 1.4% in Cuba to 33.2% in Sri Lanka. Severe visual loss due to congenital abnormalities of the globe is estimated to affect between 0.4 and 16.2/100,000 children in the countries studied. An underlying cause could not be identified in 84.2%.

CONCLUSIONS

Major congenital abnormalities of the globe are important causes of severe visual loss in children, particularly in Asian countries. Further research into etiology is warranted in order to plan prevention programs.