PHILIPP J OPHTHALMOL VOL 32 NO. 2 JULY - DECEMBER 2007 41PHILIPPINE ACADEMY OF OPHTHALMOLOGY

ORIGINAL ARTICLE

PHILIPPINE JOURNAL OF

Ophthalmology JULY – DECEMBER 2008VOL. 33 • NO. 2

ABSTRACTObjective

A study was performed to assess levels of visual impairment and blindnessamong a representative sample of older members of the Aeta, an indigenoushunter-gatherer population living on the island of Luzon in the Philippines.

MethodsUnrelated older Aeta couples from 4 villages were randomly invited to

participate in a visual-screening study. All participants had complete ophthalmicexamination and their ocular and medical history, height, and weight wererecorded.

ResultsA total of 225 individuals were screened from 4 villages. The mean height

for the entire screened population was 54.6 inches (approximately 4.5 feet),56.8 for men and 53.7 for women. Visual acuity, both uncorrected and pinhole-corrected, was significantly worse among older vs. younger age groups forwomen, men, and when combined (p < 0.001). Visual impairment, defined asbest-corrected VA > 20/40 and < 20/200, was present in 48% (100/208) ofuncorrected eyes and 43% (90/208) of pinhole-corrected eyes in the olderage group. Of all individuals screened, 6% (14/225) were bilaterally blind,defined as best-corrected VA ≥ 20/200. The major causes of blindness werereadily treatable. The most common etiologies as a proportion of blind eyeswere cataract 66% (39/59), refractive error 20% (12/59), and trauma 7%(4/59). No cases of primary open-angle (POAG), primary angle-closure(PACG), or pseudoexfoliation-associated glaucoma were observed.

ConclusionVisual impairment and blindness were common in the Aeta population.

Cataract was the most common cause of vision loss. Interestingly, POAG andPACG, both major causes of blindness in most population-based studies, werenot observed in this population. The absence of these forms of glaucoma mayreflect random-sampling error. However, the Australian Aborigines, a relatedindigenous population, also have a very low prevalence of these forms ofglaucoma. These data suggest that these populations may share genetic and/orenvironmental factors that are protective against primary forms of glaucoma.

Keywords: Aeta, Visual-status assessment, Visual impairment, Blindness

PHILIPP J OPHTHALMOL 2008; 33(2): 41-46 © PHILIPPINE ACADEMY OF OPHTHALMOLOGY

Cecilia Santiago-Turla, MD1

Mary Katherine Rivera-Francia, MD2

Bernardita Navarro, MD3

Jose Joel Eclarinal, MD4

Benjamin Dizon, MD4

Paul Francia, MD4

Sandra Stinnett, MD1

R. Rand Allingham, MD1

1Department of OphthalmologyDuke University Medical CenterDurham, North Carolina

2Department of OphthalmologyIloilo Doctors College of MedicineIloilo, Philippines

3Department of OphthalmologySanto Tomas University HospitalManila, Philippines

4Department of SurgeryIloilo Doctors College of MedicineIloilo, Philippines

Correspondence to

R. Rand Allingham, MD

2351 Erwin Road

Durham, NC 27710, USA

Fax : +1-919-6819801

E-mail : allin002@mc.duke.edu

This report is an abridgement of a thesis submitted to

the American Ophthalmological Society in partial

fulfillment of the requirements for membership, May 2008.

Assessment of visual statusof the Aeta, a hunter-gathererpopulation of the Philippines

42 PHILIPP J OPHTHALMOL VOL 32 NO. 2 JULY - DECEMBER 2007 PHILIPPINE ACADEMY OF OPHTHALMOLOGY

IT IS ESSENTIAL to assess the medical status of apopulation to determine the need for medical services.This report describes a study to evaluate the types andrelative frequency of vision-related disorders among theAeta, an isolated population living in a remote region ofthe Philippines.

The Aeta (also Ayta or Agta), until recently a hunter-gatherer population, are indigenous to the main islandof Luzon in the Philippine archipelago.1-4 They live inisolated regions along the foothills of Mount Pinatubo inthe Zambales Range and in the province of Pampanga.The massive eruption of Mount Pinatubo in 1991 forcedthe relocation of thousands of members of this populationto less-affected areas. Many of these families moved tosettlement villages. The establishment of these settlementshas enabled a variety of social agencies to have greateraccess to this population.5 Recently, medical clinics havebeen established adjacent to settlement villages to providebasic medical care; however, vision services are notcurrently available and there is little information availablefor health agencies to utilize for planning vision-relatedhealth-care interventions. To assess the need for visionservices among the Aetas, we determined the prevalenceand major causes of vision loss in older members of thispopulation.

METHODOLOGYParticipants were recruited from 4 villages (barangays)

within the Pampanga province on the island of Luzon.Barangays selected for this study were exclusively occupiedby the Aeta population and comprised groups of smallerfamily subunits. A census performed in 2001 was used tocoordinate selection of participants as evenly as possiblethroughout the region.6 The census data were very limited,containing only population estimates for each barangay.Age, sex, and other demographic data were not recordedin this census. The National Commission on IndigenousPeoples (NCIP) coordinator contacted the communityleaders (kapitans) of each barangay to assist the researchteam in the conduct of the study according to customarypractices. The number of participants selected at eachlocation was determined from the available census datawith a target of 1% of the census total (estimated 175 to200 participants).

The Aeta do not follow a calendar. To estimate age olderthan 40 years, a historical reference, World War II (1941–1945), an event well remembered by all older Aeta, wasemployed as a substitute calendar. Using this as referencedate, Aeta who were alive or whose parents orgrandparents were alive during WW II were invited toparticipate. Since generations are typically separated by15 to 20 years we used this method to divide the screenedpopulation into 3 age groups: the younger age group

under age 40, the middle-aged group between 40 and 60,and the older age group 60 years and above.

Facility and equipmentThe St. James Parish Church, located in Betis, Guagua,

Pampanga was used as the venue for vision screening.Electricity was available to power examination equipment.Automated perimetry was not available and in cases whereperimetry was indicated a tangent screen was utilized.

Screening and examination processDemographic data and general health information

were obtained, and height and weight were measured forall subjects. Each individual was tested for visual acuity(uncorrected and pinhole), intraocular pressure (IOP),slitlamp, gonioscopy, and dilated-fundus examination.Visual acuity was assessed using tumbling Es at a distanceof 20 feet. IOP was assessed in a sitting position using acalibrated Tonopen (Mentor, Norwell, MA, USA). Allparticipants who had a vertical cup-to-disc ratio (VCDR)≥ 0.6, asymmetry of VCDR ≥ 0.2, presence of focalneuroretinal rim defects, disc hemorrhages, or IOP > 21mm Hg in either eye had tangent-screen visual-fieldtesting. Tangent-screen visual-field testing was performedat 1 meter for each eye with a standard 3 mm white targetfor patients who met these criteria.

Definition of visual impairment and blindnessVisual impairment was categorized using criteria

commonly applied in most US studies that define lowvision as acuity worse than 20/40 and better than 20/200and blindness as visual acuity of 20/200 or worse.

Statistical analysisDescriptive statistics were computed for all variables in

all patients and separately for men and women. Thesignificance of the difference in medians among agegroups was assessed using the Kruskall-Wallis test. Pair-wise comparisons of medians between age groups usedthe Wilcoxon rank sum test. To assess the differencebetween men and women with respect to age, the chi-square test was used.

The study was conducted in accordance with theDeclaration of Helsinki and the customary ways andpractices allowed by the Aeta community leaders and NCIPregional office. Informed consent was obtained from allparticipants.

RESULTSFour towns were chosen based on accessibility and

willingness of the population to take part in the study.They were Angeles, Floridablanca, Mabalacat, and Porac—

PHILIPP J OPHTHALMOL VOL 32 NO. 2 JULY - DECEMBER 2007 43PHILIPPINE ACADEMY OF OPHTHALMOLOGY

all in the province of Pampanga. The census populationsof the barangays ranged from 1,800 to 7,219. The totalsampled population was 17,600. The percent of thepopulation screened from each barangay ranged from0.85% in Floridablanca to 2.1% in Angeles. Approximately1.3% of the total census populations of the selectedbarangays was enrolled and screened.

Height and weightThe mean height for the entire screened population

was 54.6 inches (approximately 4.5 feet), 56.8 for menand 53.7 for women. The mean height for women betweenage groups was not significantly different. This varied fromthe mean height in men, which was significantly greaterin both younger and middle-aged groups compared withthe older group (p < 0.001). The mean weight for allparticipants was approximately 85 lbs; 84 lbs and 94 lbsfor women and men, respectively. The mean weight forthe combined group was significantly greater in theyounger and middle-aged groups compared with the oldergroup (p < 0.002). Weight increased from the older toyounger age groups for both women (p < 0.05) and men(p < 0.001).

Visual dataA total of 225 study participants were enrolled and

examined. The proportions of eyes with uncorrected andpinhole-corrected visual acuity of 20/40 or better, lowvision (worse than 20/40 and better than 20/200), andblind (20/200 or worse) were assesssed. As expected, thevast majority of the younger age group had visual acuitybetter than or equal to 20/40 in both uncorrected andpinhole-corrected (90% and 96%, respectively) vision.The one exception was an individual with refractive errorin one eye, correctable with pinhole to 20/50, andamblyopia in the fellow eye (CF vision). In the middle-aged and older groups, both uncorrected and correctedvisual acuity fell dramatically. Uncorrected visual acuity≤20/40 dropped to 55% in the middle-aged group and33% in the older group. Similarly, pinhole-corrected visualacuity ≤20/40 decreased from 70% to 41%, respectively,in the middle-aged and older groups.

The prevalence of low vision rose from 4% in theyounger group to 48% in the older group, while pinhole-corrected visual acuity in both groups increased from 2%to 43%, respectively. Blindness increased from 2 to 19%in uncorrected eyes and 2 to 16% of pinhole-correctedeyes in the younger and older age groups, respectively.Interestingly, only 19 participants (11 female, 8 male)specifically complained about blurred or reduced vision.Of these, only 6 had uncorrected vision worse than 20/40in at least 1 eye. The majority of participants, even thosewho had significant visual impairment, did not report a

functional complaint.Visual acuity, after conversion to LogMAR units, was

analyzed for men, women, and combined by age group.Overall visual acuity was significantly different betweenage groups, with the younger group having better visionthan the older group. This pattern was consistent whenanalyzed by sex and when data were combined (p < 0.001).

Mean IOP was 15.6 ± 3.1 for right eyes and 16.0 ± 4.6for left eyes. Gonioscopy revealed 2 cases of narrow butnonoccludable angles, 3 cases of heavily pigmented angles(without corneal endothelial pigmentation or iristransillumination defects), 1 case of uveitis, and 5 cases ofperipheral anterior synechiae related to trauma or cornealscarring. Ocular pathology found on anterior segment andfundus examination, excluding cataracts, is summarizedin Table 1. There were also 2 cases of exotropia and a singlecase each of entropion, blepharospasm, allergic conjunc-tivitis, panuveitis, and optic neuropathy. Pseudophakia andaphakia were observed only in male participants. Subjectswith exotropia had normal vision in both eyes.

Blindness, excluding refractive error (pinhole-corrected), was determined in 9% of right eyes and 11%of left eyes. Bilateral blindness was determined in 6% ofparticipants, the incidence increasing dramatically by agegroup. No individual was blind in both eyes in the youngerage group, although 1 subject had 20/200 uncorrectedvisual acuity (phVA 20/50) and CF from amblyopia in thefellow eye. There were 10 nonrefractive blind eyes in themiddle-aged group (5% of eyes) in which 2 individualswere blind bilaterally (2%). In the older age group therewere 33 nonrefractive blind eyes (16% of eyes) of which14 were bilaterally blind (6% of individuals). The causes

Table 1. Ocular pathology (eyes), excluding cataract, identified on slitlamp

and fundus examinations.

Pterygium

Corneal scarring

Phthisis or penetrating trauma

Pseudophakia/ aphakia

Exfoliation syndrome

Retinal disease

Female

8

1

1

0

8

5

Male

4

3

4

7

5

2

Combined

12

4

5

7

13

7

Table 2. Causes of blindness by gender (female, n = 308; male, n = 142).

Percent is by total eyes per gender (columns 1 and 2), combined

number of eyes (column 3), and total number of blind eyes (column 4).

Female

(n = 308)

25 (8%)

10 (3%)

1 (<1%)

1 (<1%)

1 (<1%)

0

Cause of Blindness

Cataract

Refractive error

Trauma

Corneal scarring

Amblyopia

Optic neuropathy

Male

(n = 142)

14 (10%)

2 (1%)

3 (3%)

1 (<1%)

0

1 (<1%)

Total

(n = 450)

39 (15%)

12 (4%)

4 (1%)

2 (<1%)

1 (<1%)

1 (<1%)

As % of Blind

Eyes (n = 59)

66%

20%

7%

3%

2%

2%

44 PHILIPP J OPHTHALMOL VOL 32 NO. 2 JULY - DECEMBER 2007 PHILIPPINE ACADEMY OF OPHTHALMOLOGY

of blindness in all cases are summarized in Table 2 andpresented as a percentage of total eyes per gender (female= 308, male = 142), total eyes (n = 450), and total blindeyes (n = 59). As expected, cataract, found in 15% of allexamined eyes, was the most common cause of blindness(66% of blind eyes). This was followed in prevalence byrefractive error and trauma (20% and 7% of blind eyes,respectively). Corneal scarring and amblyopia accountedfor 1 case each. Three cases of blindness were caused byocular trauma with associated glaucoma. These cases wereaccompanied by opacified corneas and/or flat anteriorchambers. Highly elevated IOP measured by Tonopen ortactile tension was present in these cases. A history ofpenetrating trauma (e.g. arrow injury) preceded visionloss in 2 cases. All cases were long-standing. There wereno cases of blindness from any cause that had occurredrecently.

No definite cases of primary open-angle glaucoma wereidentified in any study subject. There were 6 cases classifiedas open-angle glaucoma suspects based on optic-nerveappearance alone. The VCDR was between 0.6 and 0.85in all cases. In one case, the VCDR asymmetry was greaterthan 0.2. Notching of the neuro-retinal rim, nerve-fiber-layer defects, and disc hemorrhages were not observed.Tangent-screen assessment did not reveal visual-field lossconsistent with the diagnosis of glaucoma in any case. IOPwas normal in all glaucoma suspects.

Exfoliation syndrome was identified in 8 individuals. Nocases of exfoliation glaucoma were observed. One femaleparticipant had unilateral exfoliation in the middle-agedgroup. The remaining 7 cases were identified in the olderage group. Five cases of exfoliation syndrome werebilateral (3 females, 2 male) and 2 were unilateral cases(1 female, 1 male). There was one suspected case of exfo-liation in a pseudophakic eye of a male subject. IOP waselevated in 1 of 13 eyes with definite exfoliation syndrome(26 mm Hg). Optic-nerve appearance and tangent screenswere normal in all eyes with exfoliation syndrome.

DISCUSSIONThe primary intent of this study was to assess the status

of visual function in a population that has not beensystematically examined in the past, and as a result of theirisolation, has largely been outside the purview oftraditional medical care.

The Aeta are characterized by small stature and have adistinctive Sub-Saharan African appearance with dark skinand fine curly hair, in striking contrast to the majoritypopulation of the Philippines. They have lived as hunter-gatherers in the mountainous interior of Luzon untilrecent times. The social structure of hunter-gatherersocieties comprises collections of small family units thatare widely scattered. Family units migrate frequently as a

function of food gathering. In the case of the Aeta,migrations often occur every 2 to 3 years when land usedto grow basic food crops is depleted of nutrients. The Aetasystem of slash-and-burn farming called kaingin has beena practice for centuries. Although sustainable over longperiods of time, it can only support the low populationdensity of hunter-gatherer societies.7

Due to their relative isolation and social structure,traditional health-care delivery has been problematic forthe Aeta, and for the most part nonexistent. Recently, theAeta have started living in larger villages with higherpopulation densities, a process accelerated by the massiveeruption of Mt. Pinatubo in 1991. Mt. Pinatubo is reveredby the Aeta and occupies the center of the Aeta homelandboth geographically and figuratively. The eruption causedtwo-thirds of the Aeta land to become uninhabitable fromthe direct effect of pyroclastic flow or ash fall, forcing themass evacuation of a large number of the Aeta.5

Recently, more consistent efforts have been made todeliver medical care to the Aeta, primarily throughintermittent medical missions. There has been norecorded ophthalmic mission for this population. For thisreason, it was felt appropriate that a vision survey wouldbe useful as a first step to gather information for futureuse by government and other health-care agencies.

The primary focus of this project was to perform visualscreening of a representative sample of the Aeta withemphasis on the older population. The goal was to gaugethe prevalence and major causes of visual compromise.This was not a formal population-based study that wouldrequire resources and personnel far beyond what wasavailable. Due to the remoteness of this population andlimitations of the census data, a large-scale study wouldhave been problematic, if not impossible. It is importantto note that we could only screen members of thispopulation who lived in resettlement centers. A largepercent of the Aeta live in areas inaccessible by motorizedvehicles. Therefore, the results of this study may not applyto those living in more remote areas. We were able tocircumvent some of these challenges by working in closeassociation with personnel from the NCIP.

The available government census provided estimatesof Aeta village populations in the Pinatubo region.6 Thecensus provided no data on sex, age, or village maps. Wearranged for the NCIP social workers to discuss the studywith kapitans months prior to our arrival. Kapitans weregiven instruction by social workers on how to enrollfamilies and the need to select elder family couplesrandomly and not based on perceived medical need. Thenature of the project, a vision screening rather than atreatment study, was explained to the kapitans and studyparticipants.

We felt that it was feasible to screen 1% of this

PHILIPP J OPHTHALMOL VOL 32 NO. 2 JULY - DECEMBER 2007 45PHILIPPINE ACADEMY OF OPHTHALMOLOGY

population, which had an estimated 17,500 individualsliving within the selected villages. We successfully recruitedand examined 225 individuals or 1.3% of the estimatedtarget population; between 0.85% and 2.1% of eachvillage. Female participants (67%) outnumbered themales (33%). This was true for all age groups with thelowest percent in the older age group (61%) and thehighest percent of female participants in the middle-aged(75%) and younger (72%) groups. The precise methodthat kapitans used to select older household members isnot known. We requested that older couples be invited toparticipate in an effort to equalize gender representationand reduce familial relatedness. We were unable todetermine what caused the differential representation byfemales and males.

Since we were unable to monitor the activities of thekapitans, it was not possible to assess to what degreeselection bias may have occurred during the recruitmentprocess. Furthermore, there was no method to record whatpercent of those asked to participate were ultimatelyscreened—another potential source of bias. However,considering the time and resources that were availablefor a project of this type, we feel the results provide usefulinformation for future medical intervention or studies.

In our sample, the older Aeta averaged 4.5-feet tall andweighed less than 80 lbs. We found that the middle-agedand younger Aeta were significantly taller than the olderage group, an increase in height found almost exclusivelyamong men. Along with increased height was an increasein weight among younger Aeta. Again, it was the malegroup that accounted for most of the change in weight.Presumably, this trend reflects changes in diet and/oractivity between older and younger Aeta which were alsosuggested as contributing causes of reduced stature byClavano-Harding and colleagues.8 Why the increases inheight and weight were primarily observed among themale Aeta is unclear.

One of the primary goals of this study was to estimatethe prevalence of low vision in older members of the Aetapopulation. The prevalence of low vision (VA > 20/40 and< 20/200) and blindness (≥ 20/200) increaseddramatically with older age. Although the younger agegroup was smaller in number, an intended consequenceof the study design, low vision from all causes was relativelyuncommon, present in 4% of eyes without correction. Themost frequent cause of low vision in the younger age groupwas refractive error. Other than refractive error, there wasone case of amblyopia which accounted for the only blindeye in younger age group. Low vision was present in 36%of eyes without correction in the middle-aged group and48% in the older age group. Blind eyes accounted for 9%and 19% of uncorrected eyes in the middle-aged and oldergroups, respectively. Since corrective glasses were not

available to this population, uncorrected visual acuity mostaccurately reflected the visual function for this population.However, even if treatment for refractive error wasavailable, low vision and blindness in this study would havebeen reduced by only 20%.

The prevalence of visual impairment and blindness inthe world varies greatly between populations. Resnikoffand colleagues noted in a World Health Organizationreport on global rates of visual impairment that theprevalence of blindness (defined as visual acuity > 20/400 in the better-seeing eye) varied between 0.4 and 9%in populations aged 50 or more worldwide.9 The lowestprevalence was in developed countries including Europeand the US, while the highest was in several regions ofAfrica. The prevalence of blindness was 6.3% in thepopulation over age 50 in the subregion that includes thePhilippines, Malaysia, Indonesia, and Thailand. In ourstudy, the prevalence of blindness was estimated to be 8%in men and women. If blindness had been definedsimilarly, the prevalence would have been lower. It ischallenging to compare studies that used differentmethodologies and sample sizes. This study was notpowered to provide a precise estimate of blindness orvisual impairment; but designed to examine the commoncauses of vision loss in a representative sample. It isinteresting, however, to find that the estimated prevalenceof blindness obtained in this sample was similar to thatobserved in other studies in this region.

The leading cause of blindness in developing nationsis cataract; it was, therefore, not surprising to find thatcataract was also the largest single cause of visualimpairment and blindness in the Aeta population. Anumber of the Aeta had obtained ophthalmic treatment,mostly for cataracts. Seven, all of them men, had cataractsurgery; 6 were pseudophakic and 1 was aphakic. It is notclear whether this represented a statistical aberration ormay be reflective of uneven access to the limited healthcare available.

The major causes of blindness in this population weresimilar to those reported in other populations fromdeveloping nations.9 It is important to note for thepurposes of continuing health care that the vast majorityof cases of low vision and blindness is readily treatable.Other causes of treatable blindness, such as trachoma orother infectious diseases, were uncommon. In thispopulation, less than 3% of the observed blindness wouldbe essentially untreatable.

Interestingly, glaucoma was conspicuously absentamong the causes of blindness. Worldwide glaucoma isthe second leading cause of blindness.10 Only 3 cases ofblindness associated with glaucoma were observed in thisstudy and in each case a history of major ocular traumawas present which was readily verified by examination.

46 PHILIPP J OPHTHALMOL VOL 32 NO. 2 JULY - DECEMBER 2007 PHILIPPINE ACADEMY OF OPHTHALMOLOGY

There were no cases of blindness from primary forms ofopen-angle or angle-closure glaucoma. Six cases wereclassified as glaucoma suspects, all based on increasedvertical cup-to-disc ratio. One case had optic-cup asymmetrygreater than 0.2. None had other stigmata of glauco-matous optic neuropathy, such as focal neuroretinal-rimthinning, disc hemorrhages, or nerve-fiber-layer defects.Similarly, visual-field loss was not detected by tangent-screen assessment in these cases. No glaucoma suspecthad associated elevated IOP. Similarly, cases of chronicangle-closure glaucoma were not observed in this studypopulation. Two individuals had narrow angles ongonioscopy, but neither case was occludable. There wereno cases of appositional closure or posterior synechiaerelated to narrow angles.

It is possible that the absence of observed open-angleand angle-closure glaucoma among the Aeta was due tothe limited sample size. However, it is also possible thatthe prevalence of these common forms of glaucoma isvery low in this population compared to others. Open-angle glaucoma is a complex inherited disorder with threeknown genes and many associated chromosomal loci.11

Likewise, angle-closure glaucoma is likely to have a strongunderlying genetic component.12

If the Aeta do have a very low prevalence of thesemajor forms of glaucoma, then one would anticipate thatclosely related populations might have similarly low ratesof these inherited forms of glaucoma.13 In an Australianreport, reference is made to one of the examiners (FCH),“who, in spite of a substantial background in theepidemiology of glaucoma and in spite of examining moreAborigines than any other examiner, did not find onecertain case of primary-open or closed-angle glaucoma inAborigines.”13

In summary, we were able to perform vision screeningon a representative sample of older members of the Aeta,

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2. Tima RG. Leaves on the Water. Olongapo City, Philippines: Printing Press, 2005.

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GH-IGF axis and nutritional status of the Ati negritos of the Philippines. Clin

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year 2002. Bulletin of the World Health Organization 2004; 82: 844-851.

10. Quigley HA, Broman AT. The number of people with glaucoma worldwide in 2010

and 2020. Br J Ophthalmol 2006; 90: 262-267.

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pathogenesis. Int Ophthalmol Clin 2004; 44: 167-185.

12. Alsbirk PH. Corneal diameter in Greenland Eskimos. Anthropometric and genetic

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Scand 2002; 53: 635-646.

13. The Royal Australian College of Ophthalmologists. National Trachoma and Eye

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an isolated population of the Philippines. Visual impair-ment and blindness were relatively common among theAeta, particularly among the older age groups. The mostcommon causes of vision loss in this population werecataract and refractive error, both of which are readilyamenable to treatment.

Although primary open- and closed-angle glaucomasare major causes of visual impairment and blindness inmost populations, no cases were identified in this sample.While this may be due to chance, it is possible, given thedata on Australian Aborigines, that this lack of glaucomacases may reflect differences in genetic susceptibility orenvironmental factors that lower the risk for primary formsof glaucoma in these populations. For these reasons,further study of this and related populations is warranted.