BritishJournal ofOphthalmology 1994; 78: 344-348

Epidemiology of blindness and visual impairment inthe kingdom of Tonga

H S Newland, A J Woodward, L A Taumoepeau, N S Karunaratne, I GM Duguid

AbstractData on the prevalence and causes ofblindnessand visual impairment in Polynesians are notreadily available nor are they population based.This survey was designed to obtain an accurateestimate of blindness and its causes in Tonga.A sample of 4056 persons, aged 20 years andover, was selected by stratified cluster samp-ling. Participants received a screening, visualacuity examination, and, if visually impaired,were referred for detailed ophthalmic examina-tion to determine the cause. The prevalence ofbilateral blindness in the study population was0-47% and all affected were aged over 50 years.It is estimated that the national prevalence ofbilateral blindness, adjusted for the sampleweight applied in the selection procedure, is0 56% (95% confidence interval 0-1.13). Mono-cular blindness was three times more frequent.Cataract was responsible for 68*4% of bilateraland 30*3% of monocular blindness. Risk fac-tors for life time experience of cataractincluded age and diabetes (self-reported).Neither smoking nor the presence ofpterygiumwere independently associated with cataract.Increasing years of education were protectiveagainst cataract for women, but not men.Corneal opacity from infection or trauma, anddiabetes were responsible for most of theremaining visual impairment. While theseresults do not represent a significant publichealth problem by world standards they doprovide a basis for planning blindness preven-tion programmes in the region.(BrJ Ophthalmol 1994; 78: 344-348)

International CommunityEye Care, FlindersMedical Centre, SouthAustralia 5042, AustraliaH S Newland

Department ofCommunity Medicine,University of Adelaide,South Australia 5000,AustraliaA J Woodward

Ministry of Health,Nuku'alofa, TongaL A Taumoepeau

ASPECT FoundationLtd (Australian SouthPacific Eye ConsultantTeam), Ford Street,Ringwood, Victoria 3134,AustraliaN S KarunaratneI G M DuguidCorrespondence to:H S Newland, FlindersMedical Centre, BedfordPark, South Australia 5042,Australia.Accepted for publication21 October 1993

There are few reliable statistics on the magnitudeand causes of visual impairment in Polynesians.In 1981, it was estimated by the World HealthOrganisation that the prevalence of bilateralblindness in Tonga was 0-4% (385 persons in apopulation of approximately 96000).1 In thesame year, the prevalence of bilateral blindnessin Vanuatu, a Melanesian population, was esti-mated to be 0 35%. Later, an overall prevalenceof bilateral blindness of 0-4% was found inVanuatu after a population based survey wasconducted in 1989.2 Cataract was responsible for85% of this blindness. Population based surveysof visual impairment have been conducted indeveloping countries in Asia and Africa in recentyears. In 1985, Brilliant et al estimated theprevalence of blindness in Nepal to be 0-84%with 67% of bilateral blindness being due tocataract.3 Several African surveys conductedaccording to WHO guidelines have suggestedbilateral blindness rates of 0 3% in the Congo,41-26% in Tanzania,5 and 1-5% in Saudi Arabia.6In all surveys, cataract was responsible for more

than half of all blindness. However, populationbased data are lacking for Tonga which posessignificant problems in planning prevention ofblindness programmes. In order to obtain anaccurate estimate of blindness and its causes, thissurvey was initiated by ASPECT and conductedwith the government of Tonga.The kingdom of Tonga has a population of

approximately 100 000, half of whom are agedless than 20 years. Tonga consists of 170 islands,40 of them populated, spread across 360 000square kilometres of ocean. Two thirds of thepopulation live on the main island, Tongatapu,which includes the capital of the kingdom,Nuku'alofa. One national ophthalmologist,based in Nuku'alofa, is responsible for tertiaryeye care services and the education of nurses andtrainee health officers. There are budgetaryrestrictions on travel to many, especially themore remote, islands.

Cataract is a particularly important cause ofblindness and low vision, since it is a commoncondition and is usually treatable. Cataract hasbeen reported in some studies to be more com-mon in women than men, even with adjustmentfor age differences, and possible differences inaccess to surgery. Smoking has been found to beassociated with cataract in several studies, bothcohort and case-control investigations,7 althoughthe possible mechanism is not known. In devel-oped and developing countries cataract is dispro-portionately common among the poor anddisadvantaged. This may be due to differences intreatment, or differences in incidence of disease(for example, through sun exposure, dietarydifferences [high intakes of dietary antioxidantare reported to be protective], or as a result ofsevere dehydrating diseases in early life).8 Hard-ing claims that pterygium and cataract are notrelated (if so, this would tend to count against thehypothesis that sunlight is a major aetiologicalfactor for cataract, since pterygium is well knownto be associated with cumulative ultraviolet Bexposure.8 This study provided an opportunityto test whether such an association existed in apopulation living at low latitudes.

SamplingThe study was restricted to persons aged 20 yearsand over, owing to the anticipated low prevalenceof blindness and low vision in younger agegroups. The study aimed to obtain information onapproximately 3500 persons, based on estimatesof the prevalence of blindness and the resourcesavailable for conduct ofthe study. Anticipating aresponse rate of approximately 70%, a sample of5000 (10% of the national adult population) wasselected. The study population was selected bystratified cluster sampling, based on the results of

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Epidemiology ofblindness and visual impainment in the kingdom ofTonga

the 1986 national census. The national popula-tion was first broken down into the five statisticaldivisions that correspond to the principal islandgroups. The northern most group (the Niuas)was excluded from the study because of itsinaccessibility and its small population (2*1% ofthe national total). Within each division, weexcluded islands with populations of less than100, and those without permanent residents,most ofwhich are tourist resorts.

In each of the four remaining divisions, vil-lages were selected randomly until the accumu-lated number of adults aged 20 and overexceeded 10% of the total in the division. In thecase of the two island groups with widely dis-persed populations (Ha'apai and Vava'u), vil-lages were grouped before sampling into thosethat could be reached by road from the airstrip('central'), and those that could be reached onlyby boat ('remote'). To conserve limitedresources, three quarters of the study populationin each island group were selected from thecentral villages.The principal town, Nuku'alofa, was sampled

separately. A quota of 10% of the town's popula-tion aged 20 years and over was calculated, andthe town divided into two areas of equal popula-tion (Kolofo'ou and Kolomotu'a plus Ma'ufuga).In each area a list ofcensus blocks was drawn up,and one block selected randomly. An adjacentblock was also selected at random, and thenfurther adjacent blocks were included, movingclockwise around the index block, until theaccumulated number of adults aged 20 years andover exceeded half the quota for the town.The study sites, together with numbers of

adults aged 20 and over and the number ofhouseholds (as recorded in the 1986 census) areshown in Table 1. The sample consisted of 18villages and 10 census blocks in Nuku'alofa,which altogether contained 5175 adults aged 20and over.

Blindness surveyDuring July 1991, two teams examined a total of

Table I Study sites, adults aged 20 and over, andhouseholds (as recorded by the 1986 census)

Study site Population 20+ Households

Nuku'alofa: 1140 348Kolofo'ou 502 140Kolomotu'a and Ma'ufuge 638 208

Tongatapu: 2035 698Longoteme 301 108Nukuleka 109 41Makaunga 160 54Laphaha 825 280Kolovai 391 139Masilamea 126 37Ha'avakatolo 123 39

Eua: 229 92Futu 118 44Mata'aho 111 48

Ha'apai: 889 307Hihifo 453 164Fotua 119 41Pukotala 87 30Ha'afeva Island 230 72

Vava'u: 882 328Neiafu (census block 9) 153 52Mataika 242 87Taoa 221 82Falevai 99 38Hunga Island 167 69

Total 5175 1773

4056 persons in all sites. The study teamsincluded nurses and trainee health officers fromthe Department of Health, kingdom of Tongaand two registrars in ophthalmology (NSK andIGMD). The teams participated in a 3 daytraining course before field work. Questionnairesand eye examination techniques were piloted inNuku'alofa.At each village or census block, all houses were

visited and the number of occupants in eachhousehold was documented. Each occupant aged20 years or more was questioned regarding age,education, smoking status, diabetes, and anyhistory of ocular disease.

Visual acuity was then measured beside thehouse. The health worker placed a plastic coatedE type or Snellen chart at 6 metres and thesubject was asked to identify the direction of theE or the letter. Ifvisual acuity was 6/18 or better,the examination ended. If the identification wasnot correct, optotypes for 6/60, 3/60 (at 3metres), and 1/60 (at 1 metre) were shown until asuccessful result was obtained. If the examineecould not see the optotype at a distance of 1 metre(that is, counting fingers), the health workerrecorded the subject's perception of light orassessed the ability to fix and follow a focusedlight source during the ophthalmic examination.Participants unable to see at least 6/18 wereoffered a pinhole correction and any improve-ment was noted. All participants were testedusing the best correction (that is, pinhole orglasses), and those with visual impairment werereferred to the ophthalmologist for assessment ofthe cause.A basic eye examination was then conducted

by the health worker using a hand torch. Patientswith any abnormality were referred to theophthalmologist for assessment and treatment.The ophthalmic examination included an

assessment of the anterior segment and of theposterior pole through a pupil dilated with 10%phenylephrine and 1% tropicamide. The diag-nosis of the cause of any visual impairment andaetiology of any ocular disorders were recorded.

All data were recorded on the WHO/PBL eyeexamination record.9 The data collection formswere checked, sorted, and forwarded to Adelaidefor coding and entry. The d-Survey programwas used for entering and checking the data.Statistical analysis was carried out using SYSTATand SAS-PC (SAS institute, NC, USA).

Statistical methodsThe study population represented a stratifiedcluster sample, with few clusters selected perstratum, some oversampling (among the 'central'villages in Ha'apai and Vava'u), and variableresponse rates.

Prevalence estimates were calculated using themethod ofCochran, to adjust for the effects ofthesampling method.'0 Confidence intervals werederived from estimates ofvariance, also based onCochran.

Stratified analyses were carried out using theSYSTAT and sAs-pc statistical packages and theappropriate standard statistical tests (Cochran-Mantel-Haenszel X2 test). The multivariableanalysis was undertaken using the LOGISTIC pro-

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Table 2 Distribution ofsurvey participants by district andsex (percentage of 1986 census population shown in brackets)

District Men Women Total (%)

Nuku'alofa 379 492 871 (8 5)Tongatapu 721 852 1573 (8-4)Eua 90 92 182 (8-9)Vava'u 314 394 708 (9 9)Ha'apai 315 407 722 (16-8)Total 1819 2237 4056 (9 5)

(Sampling fraction for total population excludes Niuas)

Table 3 Age and sex distribution ofpersons surveyed inTonga, 1991

Age group (years) Male Female Total (%)

20-24 339 358 697 (17-2)25-29 226 267 493 (12-2)30-34 175 245 420 (10-4)35-39 159 221 380 (9 4)40-44 131 202 333 (8-2)45-49 137 186 323 (8-0)50-54 151 193 344(8 5)55-59 131 156 287 (7-1)60-64 122 129 251 (6-2)65-69 88 90 178 (4 4)70-74 64 84 148 (3 6)75-79 54 48 102 (2 5)80-84 20 26 46(1-1)85+ 22 32 54 (1-3)Total 1819 2237 4056 (100-0)

cedure in SAS. All main effects were includedinitially, and then a stepwide procedure was usedto study significant interactions.

ResultsTable 2 shows the distribution of the studypopulation by district and sex. The number ofpeople included in the study is 9 5% of the 1986census population of Tonga. The age distribu-tion of study participants is shown in Table 3, bythe age categories reported in the 1986 Tongancensus. The proportion of the census number ineach age group that was included in the surveyvaried from 7-8% (20-24 years) to 11-6% (50+years). As the survey progressed the percentageof the 1986 census population examined in eachdistrict increased. The overall prevalence ofbinocular blindness in the 20 years and overpopulation was 0-56% (95% confidence interval0-1 13) and all those affected were aged morethan 50 years. Blindness rates increased steeplyamong those aged-60 and over (Fig 1). Monocu-lar blindness was more than three times morecommon than bilateral blindness.The best estimate of the national prevalence of

20

El Monocular| Binocular

10)0D

CL

Age (years)

monocular blindness, calculated by the methodof Cochran, was 16 7% per 1000 (95% CI 11 0-22-5). The adjusted prevalence estimate forbinocular blindness was 5 6% per 1000 (95% CI0-11-3).Table 4 shows the causes of monocular and

bilateral blindness in the surveyed population.Cataract was responsible for 68-4% of bilateral

Table 4 Causes ofmonocular and binocular blindness*

Monocular blindness Bilateral blindness

Cause No (%) No (%)

Cataract 20 (30 3) 13 (68-4)Aphakia 3 (4-6) 0Pseudophakia 1 (15) 0Phthisical eye 19 (28-8) 2 (10-5)Comeal opacity 8 (12-1) 3 (15-8)Glaucoma 2 (3-0) 0Optic atrophy 2 (3-0) 0Diabetes 3 (4-6) 1 (5-3)Amblyopia 1 (15) 0Other 7 (10-6) 0Total 66 (100-0) 19 (100-0)

* Bilateral blindness defined according toWHO criteria of visualacuity <3/60 in the better eye. Monocular blindness defined asvisual acuity in one eye of <3/60.

Table S Causes oflow vision in the survey (bilateral)*

Cause No (%)

Cataract 29 (83-0)Refractive error 1 (3-0)Pseudophakia 1 (3-0)Corneal opacity 1 (3-0)Diabetes 1 (3-0)Macular degeneration 1 (3 0)Amblyopia 1 (3 0)Total 35

* Bilateral visual impairment defined as acuity less than 6/18 butbetter than or equal to 3/60 in the better eye.

Table 6 Needfor cataract surgery by district

No of Prevalenceoperable (per 1000

District cataracts* (%) persons)

Nuku'alofa 16 (15-1) 18-4Tongatapu 48 (45-3) 30 5Eua 4 (3-8) 22-0Ha'apai 12 (11-3) 16-6Vava'u 26 (24-5) 36-7Total 106 (100-0) 26-1

* In a total of 72 persons and one or both eyes.

Table 7 Factors associated with lifetime experience ofcataract (includes persons with cataract present, or a historyofcataract surgery)

Cataract No(%) cataract Total p* Value

Age:20-24 years 2 (0-29) 695 69725-34years 2 (0-22) 911 91335-49 years 9 (0-87) 1027 103650+years 112 (7-94) 1298 1310 <0-0001

Sex:male 50 (2-75) 1769 1819female 75 (3-35) 2162 2237 0-268

Diabetes:present 19 (16-2) 98 119absent 106 (2-71) 3827 3933 <0 0001

Pterygium:present 40 (3-81) 1010 1050absent 85(2 83) 2921 3006 0-113

Ever smoked:yes 65 (3 90) 1600 1665no 60 (2-51) 2328 2388 0-012

Years ofeducation:(mean) 7-3 years 9-2 years 0-0001

* p Value calculated from X' test or t test.

Figure I Blindnessprevalence by age, Tonga1991.

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Table 8 Factors associated with lifetime experience ofcataract - results oflogistic regression

Parameter 95% ConfidenceVariable estimate Standard error Odds ratio interval

Age (years) 0 10 0 008 1 11 1-09-1-12Education (years)

male -0 01 0-062 0.99 0 87-1 12female -0-21 0-074 0-81 0 70-0-94

Ever smoked(yes v no) 0-21 0-226 1-24 0-791-92

Diabetes(yes v no) 1-46 0-313 4-28 2-32-7-91

Pterygium(yes v no) -0-48 0-322 0-62 0-40-0 95

District*2 v 1 0-91 0-588 2-49 0-797-893 v 1 male 0 53 0 573 1-70 0-55-5-21

female 1-72 0 539 5-61 1-95-16-14v 1 1-62 0-637 5 06 1-45-17-6S v 1 0 93 0 535 2-54 0-897-256v 1 1-15 0 540 3-17 1-109-13

* Districts 1=Kolofo'ou; 2=Kolomotu'a; 3=Tongatapu; 4=Eua; 5=Ha'apai; 6=Vava'u.

and 30 3% of monocular blindness. Cornealopacity was the cause of 15-8% (three cases) ofbilateral blindness, due to trachoma, infection,and recurrent pterygia obscuring the visual axis.Bilateral phthisical bulbi was found in two cases,one due to complications following Stevens-Johnson syndrome while in the other, traumaand endophthalmitis were responsible. One per-son was bilaterally blind from diabetic retin-opathy. Nineteen persons (28-8%) weremonocularly blind with phthisical eyes, most dueto trauma. Pseudophakia and aphakia accountedfor four blind eyes (6 1%) while corneal opacitywas found in eight persons (12 1%). Diabeticretinopathy caused monocular blindness in threepersons (4-6%). Glaucoma, optic atrophy,amblyopia, and others were implicated in theremainder of cases. Many of the blind were oldand infirm and unable to recall the cause of theirblindness.Table 5 shows the causes of low vision in the

survey (1 8% ofpersons had acuity less than 6/18but better than or equal to 3/60 in the better eye),29 (83%) were caused by cataract while othercauses included diabetic retinopathy and pseudo-phakia.Table 6 shows the number of operable cata-

racts, as determined by the attending ophthal-mologists, by district. A total of64 (60 4%) werein Nuku'alofa or on the main island Tongatapuwhere eye services are most easily available.

Factors associated with lifetime cataract areshown in Tables 7 and 8 (logistic regression).

DiscussionThe results of this survey provide the firstobjective measure of the prevalence of blindnessand visual impairment in a Polynesian popula-tion. The prevalence of bilateral blindness inthose aged 20 years and over in Tonga isestimated to be 0-56%. Persons aged under 20years make up approximately 54% ofthe Tonganpopulation, and probably include very few per-sons bilaterally blind. Therefore, the prevalenceof blindness in Tonga, all ages included, is likelyto be close to 0-26%. This is the relevant figurefor health service planners, who must estimatethe likely burden of blindness nationally. Inter-national comparisons of blindness prevalencemust allow for the different age structures of

national populations. The Tongan blindnessestimate (bilateral, aged 20 and over) stan-dardised using the age structure of the standardworld population," was 0 33%.The number of participants in the study

represented 78% of the population in the studyareas, as recorded in the 1986 census. It is likelythat older residents are overrepresented in thestudy group, compared with the national popula-tion. (The 50 and over age group made up 27-7%of all Tongans over 20 in the 1986 census. In thisstudy, the 50 and over category comprised 34-7%ofthe total.) Males were underrepresented in thestudy population (sex ratio 0-813, ratio ofmen:women in 1986 census 0-941). The ageprofile of participants, and the clear relation ofblindness to increasing age suggest that the studymay have overestimated the true national blind-ness prevalence rates. The most common reasonsfor non-response, especially among males, wereabsence overseas or elsewhere in Tonga, onholiday or temporary employment, and absencelocally, fishing or gardening. Only four personswho were contacted refused to take part in thesurvey. There were no known instances of non-response due to absence for medical treatmentof serious eye disease. In these circumstancesblindness may be less common among personswho were eligible to take part in the study but didnot participate, compared with those who didtake part.

In the only other population based survey inthe region, in Vanuatu, the prevalence of bilat-eral blindness (all ages) was found to be 0.4%.2The lower prevalence in Tonga would cer-

tainly be due in part to the permanent presence ofan ophthalmologist in Nuku'alofa, whereasspecialised care in Vanuatu is only short term.Logistically, Vanuatu is more isolated thanTonga, with more islands, many of which areaccessible only by sea. This may result in much ofthe population not being able to make use oflimited eye health care.These two surveys provide the baseline data

for planning blindness prevention programmesfor the South Pacific. The prevalence of blind-ness rate of0X26% is half the 0 5% rate that is thegoal of theWHO programme for the preventionof blindness to eliminate the burden of avoidableblindness.'2 However, it is reasonable to assumethat cataract blindness is readily reversible inTonga and therefore more than 60% of bilateraland 30% ofmonocular blindness could be cured.This survey did not attempt to identify all cases

of cataract in the study population. Healthworkers carried out a general eye examinationwith hand torch, and noted lens opacities thatwere visible. A full eye examination was per-formed by one of the ophthalmologists only inpersons whose corrected vision was less than 6/18in either eye. Therefore, the cataracts identifiedin this study are chiefly those associated with(and generally, responsible for) visual impair-ment.Among 4056 persons surveyed, 52 were

reported to have lens opacities in one eye, and 53had opacities in both eyes (prevalence of personswith cataract of 2 6%). The examining doctorjudged that cataract surgery was indicated in 72out of 105 persons reported to have cataract (106

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eyes). On the basis ofthis result, we estimate thatthere are in Tonga approximately 760 personswith untreated cataract associated with visualimpairment or blindness, and suitable for sur-gery, among persons aged 20 and over. Assum-ing that the average duration of this condition(the time from diagnosis to death) is about 5years'3 we estimate that there are in total approx-imately 150 new cases of disabling cataractarising each year, in addition to the 100-150 casesthat are currently recognised and treated. Boththe prevalence and absolute number of theoperable cataracts are likely to be highest inTongatapu and Vava'u, the two most denselypopulated islands. When considering the ser-vices available to respond to this morbidity, it isinteresting that almost two thirds of the operablecataracts identified in this study were on the mainisland, where there is a permanent ophthalmolo-gist. There is a need for improved case finding inTongatapu and Vava'u. It appears that, in thepast, the smaller populations on outlying islandsmay have received a better service from visitingeye teams than the larger populations in themajor centres.

In the survey populations the number ofpersons judged to require cataract surgery (72)was three times greater than the number who hadreceived surgery for the condition (24). This isstrong evidence of unmet need for eye services.Among all persons who had once suffered fromcataract, the probability of receiving surgery washighest in Tongatapu (0X31) and lowest in Vava'u(0 05).

Studies elsewhere have shown that social fac-tors may be important in determining whoreceives treatment, regardless of distance fromservices. For example, in Nepal it was found menwere more likely to receive cataract surgery thanwomen, and wealth was also associated withsurgery. 14 In Tonga, the probability of receivingsurgery was the same for men and women. Thestudy included no direct measure of disposableincome or wealth. Those requiring surgery hadfewer years of education than the recipients ofsurgery, but the difference was small and notstatistically significant. Neither was age a rele-vant factor (mean ages 71X5 years for personsreceiving surgery, 73-1 years for those requiringsurgery, p=0 4).

Risk factors for lifetime cataract were exam-ined. Cases included persons found to havecataract at survey and persons who had receivedcataract surgery in the past. In bivariate analysesage and diabetes were strongly associated withexperience of cataract; weaker associations werefound for sex, pterygium, and years ofeducation.The prevalence of cataract was 50% higheramong those who had ever smoked than amonglifetime non-smokers (Table 7). The associationwith a history ofsmoking was due principally to ahigh proportion (11%) of ex-smokers experienc-ing cataract.

In the multivariate analysis, the associationwith age and diabetes status remained statis-tically significant, but sex and smoking status nolonger acted as independent risk factors that were

statistically significant (Table 8). The associationwith pterygium was reversed (that is, the pres-ence ofpterygium was associated with absence ofcataract, when adjustment was made for otherrisk factors). Therefore, these data are consistentwith the view that cataract and pterygium do nothave common aetiologies.

Increasing years of education was associatedwith reduced frequency of cataract in women,but not in men. Compared with the capital city(as represented by district 1), lifetime cataractwas observed more frequently in Tongatapu(females only), Eua and Vava'u. It is not clearwhat environmental conditions or lifestyle fac-tors may explain these differences.

It is of some concern that diabetes was moststrongly associated with cataract (adjusted oddsratio 4-28) as this condition is likely to becomeincreasingly important in Tonga as a cause ofcataract and other serious eye conditions.(Already diabetic retinopathy is responsible for5% of blindness in one or both eyes, and 3% oflow vision.) There is a need for further researchon the natural history of diabetic eye disease inthis population.The low prevalence of blinding glaucoma is in

keeping with the findings in Vanuatu and withthose of a survey of Australian aborigines.'5 Thepresent survey did not attempt to screen for earlystages ofglaucoma.

We thank the following for their support and assistance: Austra-lian International Development Assistance Bureau; ASPECTFoundation; Department of Health nurses - Kelela Taufa,Florence Tupou, Mele Pongi, Lesieli Paliulu; trainee healthofficers - Losaline Faleta, William Falevai, Primrose Finau,Tupou Katiloha, Paeahe-Lotu Langoia, Liliani 'Otutaka, SioneTu'ipulotu, Ofa Tu'itavulu; Ministry of Health - Dr Sope Foliaki,Dr Maika Kinahoi, Ms Krystyn Willson, and Dr Philip Ryanassisted with statistical analysis.

This study was conducted in full accordance with provisions ofthe Declaration of Helsinki; and the free and informed consent ofthe subjects was obtained.

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2 Newland HS, Harris MF, Iwasaki W, Momomura K, WallandM, McKnight D, etal. Epidemiology of blindness and visualimpairment in Vanuatu. Bull WHO 1992; 70: 369-72.

3 Brilliant LB, Pokhrel RP, Grasset NC, Lepkowski JM,Kolstad A, Hawks W, et al. Epidemiology of blindness inNepal. Bull WHO 1985; 63: 375-86.

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5 Rapoza PA, West SK, Katala SJ, Taylor HR. Prevalence andcauses of vision loss in central Tanzania. Int Ophthalmol1991; 15: 123-9.

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9 Coding instructions for the WHO/PBL eye examination record(version III). UnpublishedWHO document PBL/88. 1.

10 Cochran WG. Sampling techniques. 3rd ed. New York: JohnWiley, 1977.

11 Waterhouse J, Munt C, Shanmugaratnam K, Powell J, eds.Cancer incidence in five continents. Vol IV. Lyon: Interna-tional Agency for Research on Cancer, 1982.

12 Guidelinesforprogrammesfortheprevention ofblindness. Geneva:World Health Organisation, 1979.

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14 Brilliant GE, Brilliant LB. Using social epidemiology tounderstand who stays bLind and who gets operated forcataract in a rural setting. Soc SciMed 1985; 21: 553-8.

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