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Glaucoma in Zulus
A Population-Based Cross-sectional Survey in a Rural District in South Africa
Arch Ophthalmol. 2002;120:471-478.
ABSTRACT
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Objectives To determine the prevalence and the main types of glaucoma in a representative
adult population in rural Zululand, and to describe the distribution of glaucoma-related
variables in healthy subjects and those with glaucoma.
Design A population-based, cross-sectional study.
Setting Hlabisa district, Northern KwaZulu-Natal Province, South Africa.
Participants Resident individuals of Zulu ethnic origin, 40 years or older.
Main Outcome Measures Glaucoma was diagnosed by means of strict objective criteria, based
on binocular indirect ophthalmoscopic optic disc appearances validated by
results of disc photography and threshold visual field testing.
Results From an eligible sample of 1115 subjects, 1005 (90.1%) were examined
in the survey. The adjusted prevalence of glaucoma of all types was 4.5%,
and primary open-angle glaucoma accounted for 2.7%. Secondary glaucoma occurred
with an adjusted prevalence of 1.7%, of which the principal contributors were
exfoliative and aphakic glaucoma. The prevalence of primary angle-closure
glaucoma was low. Normal tension (intraocular pressure,  21 mm Hg) was
measured in 16 (57.1%) of 28 cases of primary open-angle glaucoma. Age- and
sex-adjusted prevalence of bilateral blindness was 3.2%, which was exclusively
due to glaucoma in 9 (22.0%) of 41 cases.
Conclusions Primary and secondary glaucoma constitute a significant public health
problem in rural Zululand. The prevalence and types of glaucoma vary among
different black populations.
INTRODUCTION
EVIDENCE IS EMERGING that the prevalence and the proportions of different
types of glaucoma vary widely between ethnic groups and geographical areas
throughout the world. Considerable data have been collected among white subjects
and increasingly in those of East Asian origin. However, studies of the distribution
of glaucoma in subjects of African derivation have focused mostly on island
populations in the Caribbean, where prevalence rates are extremely high, and
on the genetically heterogeneous population of the United States. A recent
population-based study in rural Tanzania1 reported
an age-adjusted prevalence of primary glaucoma very close to that in African
Americans, and with a similar preponderance of the open-angle type. Given
the genetic diversity that exists among African ethnic groups2-3
and the exclusion of the population of East Africa from the transportation
of slaves to the New World, this consistency is surprising. It cannot yet
be assumed that these findings will be consistent across the whole continent
of Africa.
Previous work among the indigenous populations of South Africa has,
in contrast, given rise to inconsistent results, and their interpretation
is hampered by the lack of random sampling methods.4-5
Uncertainty remains concerning the prevalence of glaucoma and the distribution
of its various forms in black South Africans. In this study, our primary aim
was to determine the prevalence of glaucoma in a representative indigenous
population in rural Zululand. A secondary goal was to describe the distribution
of glaucoma-related variables in healthy subjects and those with untreated
glaucoma.
SUBJECTS AND METHODS
SETTING
The study was performed in Hlabisa, a typical rural district in Northern
KwaZulu-Natal Province, South Africa, with an estimated population of 200 000.
The population is almost exclusively Zulu, an anthropologically homogeneous
Bantu tribe that constitutes the country's largest single ethnic group. Approximately
17% of the population are 40 years or older.6
In most of the district, the population lives in scattered homesteads rather
than villages, although there is a small urban center. Health care is provided
at a number of fixed and mobile clinic points and a 300-bed hospital that
has had a resident ophthalmic surgical service since 1997. Unemployment levels
are very high, and a significant proportion of the adult population of Hlabisa
migrate outside the district to work, often living away from home for months
at a time.
POPULATION SAMPLING
We used a 2-stage, nonstratified, cluster-based, random-sampling technique
to select a sample of 1115 residents 40 years or older of the Hlabisa district.
Sample size was based on a mean (SD) predicted prevalence of primary open-angle
glaucoma (POAG) of 2.5% ± 1.0%.
No detailed census mapping information was available for this age group.
Since more than 90% of children in the district enter first grade,7 the local education department register was used as
a sampling frame. This approach had been successfully used in earlier cross-sectional
work in this district.7 In the first sampling
stage, a list of all registered first-grade students (n = 9296) was used to
randomly select 27 children. In stage 2, for each of the 27 pupils, a randomly
selected third-grade student at the same school was chosen from the register.
Third-grade children were used as secondary sampling units because they were
old enough to direct the fieldworker to their homes. They were not used in
the primary sampling stage in an attempt to minimize the selection bias that
might have resulted from the fact that a far smaller proportion of children
enter third than first grade. The selected child's house was taken as the
index point for each cluster. Starting at the physically nearest household
to the index house, a census was performed of residents 40 years or older
who were then invited to undergo examination. The next physically nearest
household was approached until 40 subjects had been selected. Information
on the residents of empty households was gathered from the neighbors and,
where necessary, each house was visited on at least 3 separate days to identify
and to recruit the residents. Given the mobility of the population of Hlabisa,
for the purposes of this study, residents were defined as household members
who normally spent at least 1 night in 2 weeks in the house.
We examined documentary evidence to confirm the age of a subject when
possible. Most of the population has identity documents. When this method
failed, an interview using a calendar of locally significant events was performed.
In this way, 27 clusters of approximately 40 subjects were selected.
CLINICAL ASSESSMENT
Participants were examined at their local primary care clinic between
April 28, 1998, and March 18, 1999. A series of questions was put to each
subject to elicit a family history of blindness or glaucoma or a previous
diagnosis of glaucoma.
Visual acuity was measured using a tumbling E chart at 6 m in ambient
illumination, with distance correction if normally worn and with a pinhole
if the acuity was less than 20/40 (6/12). Objective refraction was measured
by means of retinoscopy in subjects in whom the visual acuity ranged from
6/18 to 6/60 and improved by 2 lines or more with the pinhole. Monocular central
visual field was assessed in every eye with a visual acuity of 20/200 (6/60)
or better using a computerized field analyzer (Henson CFA3000; Tinsley Medical
Instruments, Newbury, Berks, England). This central 25° static, semiautomated
visual field analyzer has been reported to be sensitive and specific for detection
of moderate and advanced glaucomatous visual field loss.8
All visual field testing was performed in accordance with a standard protocol
by a trained Zulu-speaking technician. Appropriate spherical near correction
was worn according to age and adjusted for distance refraction if determined.
Threshold sensitivity was determined by repeated testing until 2 identical
results were obtained for each eye in an attempt to minimize the learning
effect. A 66-point threshold-related suprathreshold test was then performed
on the right eye first.
Subjects with an equivocal or abnormal suprathreshold visual field result
(false-positive rate in a healthy population, <1.0% in age-matched healthy
subjects) were reexamined using a 52-point threshold test, with automatic
retesting of any retinal locations at which the threshold was found to be
more than 4 dB below the expected age-matched value. Subjects with a normal
suprathreshold test result but with a vertical cup-disc ratio (CDR) of at
least 0.7 or a CDR asymmetry of at least 0.3 between their 2 discs also underwent
threshold visual field analysis.
The remainder of the examination was performed by a single ophthalmologist
(A.P.R.). All subjects underwent slitlamp anterior-segment assessment for
media opacities and signs of secondary glaucoma. Goldmann applanation tonometry
was performed with the subject under topical anesthesia with benoxinate hydrochloride,
and 2% fluorescein sodium was used to obtain a single reading at the midpoint
of the pulse for each eye. Only readings where minimal force was necessary
to widen the palpebral aperture sufficiently were recorded as valid. The intraocular
pressure (IOP) was also measured by means of a Tono-Pen XL (Mentor Ophthalmics,
Santa Barbara, Calif). The result of a single automatic series of readings
was recorded, but the test was repeated if the SEM was greater than 5%. Both
tonometry instruments were calibrated daily.
Gonioscopy was performed in all subjects using a Goldmann 2-mirror lens.
Angle grading was modified from the method described by Spaeth9
to include the angle of approach, the level of iris insertion, and the iris
profile in each quadrant. Indentation with a Sussman 4-mirror lens was performed
if necessary to examine the trabecular meshwork and to detect synechiae. If
the drainage angle was judged to be not occludable, the pupils were dilated
with 1% tropicamide and 2.5% phenylephrine hydrochloride.
The optic disc was assessed stereoscopically for vertical CDR, rim notching,
defects of the nerve fiber layer, and marginal hemorrhages using a Volk +78-diopter
lens (Volk Optical Inc, Mentor, Ohio). In the case of the CDR, the largest
value from the 11- to 1-o'clock and the 5- to 7-o'clock positions was recorded.
In a subgroup of 114 consecutive participants examined at the most easily
accessible clinic, stereoscopic optic disc photography was attempted to validate
the assessment of disc cupping. Using the 30° field setting of a fundus
camera (Nikon Retinapan 45-II; Nikon Corp, Tokyo, Japan), 2 photographs were
taken through dilated pupils for each eye, rotating the camera through 15°
between the shots to allow stereoscopic examination. Pairs of transparencies
were examined stereoscopically, and the CDR was measured at its widest point
from a projected image. The photographic assessment was performed with the
assessors unaware of the ophthalmoscopic measurements. Validation was performed
for only 1 eye of each subject.
DIAGNOSTIC CRITERIA AND DEFINITIONS
A scheme based on evidence of end-organ damage (ie, damage to the structure
and function of the optic nerve) was adopted to diagnose all forms of glaucoma.
This scheme was based on a prototype diagnostic scheme developed specifically
for cross-sectional prevalence surveys by the Working Group for Defining Glaucoma
of the International Society of Geographical and Epidemiological Ophthalmology.10
Definite glaucoma was diagnosed if the eye fell into 1 of the following
3 categories: (1) Structural and functional evidence of a definite and reliable
glaucomatous visual field defect was found in the presence of a CDR of at
least 0.7 or CDR asymmetry between fellow eyes of at least 0.2. These values
represented the 97.5th percentiles in this population. (2) Advanced structural
damage was found on a threshold visual field test result that was not completed
satisfactorily or that indicated a suspicious field defect but with a CDR
of at least 0.9 or a CDR asymmetry of at least 0.3. These values represented
the 99.5th percentiles in this population. (3) Assessment of the optic disc
was not possible because of media opacity. The visual acuity was light perception
or worse, with an IOP of at least 30 mm Hg and an afferent defect (if the
pupil was visible). Only in these cases of end-stage glaucoma was IOP included
as a diagnostic criterion.
A threshold visual field test was defined as definitely glaucomatous
if a defect at least 12° wide (2 adjacent points) and 5 dB below threshold
sensitivity, adjusted for the expected hill of vision, in a nerve fiber bundle
pattern was detected and confirmed on results of retesting. Visual field defects
were not attributed to glaucoma in the presence of media opacification or
a nonglaucomatous optic nerve lesion that would explain the field abnormality.
If a less dense defect was found in a still-typical distribution, the visual
field test result was defined as a suspicious field defect. A test result
was considered reliable if less than 33% false-positive responses, less than
33% false-negative responses, and less than 25% fixation losses were found.
Defective points adjacent to the blind spot were ignored.
Glaucomatous eyes were categorized as having open-angle glaucoma (OAG)
or angle-closure glaucoma (ACG) on the basis of gonioscopy findings. A drainage
angle was defined as narrow (occludable) when the pigmented trabecular meshwork
was visible on gonioscopy with the eye in the primary position for less than
90° of the circumference without indentation. The same diagnostic criteria
were applied for ACG as were used for OAG but in the presence of a narrow
angle. When signs of a precipitating factor (eg, exfoliation, uveitis) were
in evidence, the term secondary glaucoma was applied.
Otherwise, the condition was termed primary.
DATA MANAGEMENT
Unless otherwise indicated, data are given as mean (SD). Data were double
entered from standard forms into a customized EpiInfo database (Centers for
Disease Control and Prevention, Atlanta, Ga) with validation, range, and consistency
checks. Analysis was performed using EpiInfo and Stata 6 statistical software
(Stata Corp, College Station, Tex). In the calculation of confidence intervals
(CIs) for prevalence, the binomial distribution was used and allowance for
the design effect (ie, the excess variance of the estimates under the cluster-based
selection procedure used instead of simple random sampling) was included.11 Adjusted figures were derived by direct age and sex
standardization to the 1996 indigenous population structure of Hlabisa.6
ETHICAL CONSIDERATIONS
Ethical approval was granted by the Ethics Committee of the Faculty
of Medicine at the University of Natal, Durban, and the Hlabisa district tribal,
health, and education authorities. Free and informed consent was obtained
from each participant.
RESULTS
Of a selected sample of 1115 subjects, 1005 (90.1%) completed the examination.
The numbers of participants and recruitment proportions categorized for age
and sex are shown in Table 1.
Participation was above 85% in all categories, with the exception of men aged
40 to 49 years. The mean age of the sample was 59.5 ± 12.1 years (men,
60.5 ± 12.3 years; women, 58.9 ± 12.0 years). There was a considerable
female preponderance (72.1%), which was most marked in younger participants
and reflects sex differences in recruitment rates and the demographic impact
of labor migrancy.
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Table 1. Age and Sex Distribution of Subjects Examined
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OPTIC DISCS
Disc images of sufficient quality for assessing CDR stereoscopically
were obtained in 90 participants in the subgroup undergoing optic disc photography
(78.9%). The CDRs represented in this sample ranged from 0.15 to 0.68. Vertical
CDR was underestimated by indirect ophthalmoscopy relative to that by the
photographic method, but the mean difference (± 2 SD) was only 0.05
± 0.15. Paired measurements were within 0.1 in 66 (73.3%) of 90 participants,
and in no case was the difference as high as 0.2.
The optic discs were assessed ophthalmoscopically in 962 right eyes
and 952 left eyes. The distribution is illustrated in Figure 1. Mean vertical CDR was 0.34 ± 0.19 for both right
and left discs. A value of 0.7 was found to represent the 97.5th percentile
for CDR in both right and left eyes, with or without the inclusion of subjects
with glaucoma.
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Figure 1. Frequency distribution of the
vertical cup-disc ratio (CDR) (n = 1914 eyes).
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Optic disc margin hemorrhages were notably absent in all subjects, with
or without glaucoma.
VISUAL FIELD TESTING
Suprathreshold visual field testing was completed in at least 1 eye
of 867 participants (86.3% overall; 91.5% of those with a visual acuity of
20/200 [6/60] or better in at least 1 eye). Inability to perform visual field
testing was independently associated with increasing age and visual impairment
(P<.001). The 66-point suprathreshold test result
was abnormal (P<.01) in 184 eyes (11.0% of eyes
undergoing testing). With the use of category 1 diagnostic criteria, 22 of
these were classified as glaucomatous, giving a positive predictive value
to suprathreshold testing of 12.0%. The specificity was 90.1% (1478/1640).
Table 2 depicts the proportion
of eyes with a vertical CDR of at least 0.7 in which visual field testing
could be performed reliably that were found to have a threshold visual field
defect.
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Table 2. Proportion of Eyes With Visual Field Defects According to
Vertical CDR*
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INTRAOCULAR PRESSURE
The arithmetic mean Goldmann IOP was 14.2 ± 4.2 mm Hg (95% CI,
13.9-14.5 mm Hg) for all right eyes for which applanation tonometry was recorded
(n = 928) and 14.2 ± 4.1 mm Hg (95% CI, 14.0-14.5 mm Hg) for all left
eyes (n = 914). When glaucoma cases were excluded, then the mean values became
13.9 ± 3.4 mm Hg (95% CI, 13.7-14.1 mm Hg) for both right and left
eyes. Figure 2 illustrates the characteristic
right-skewed Gaussian distribution of Goldmann IOP in eyes not classified
as glaucomatous. Of these 1790 healthy eyes, 3.5% had an IOP above 21 mm Hg
(2 SDs above the mean in this population) and in 40 (4.6%) of 870 healthy
subjects, the IOP was above 21 mm Hg in at least 1 eye (defined as ocular
hypertensive cases).
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Figure 2. Frequency distribution of Goldmann
intraocular pressure (IOP) in nonglaucomatous eyes (n = 1790).
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GLAUCOMA PREVALENCE
Glaucoma of any type was diagnosed in 51 subjects, giving an age- and
sex-adjusted prevalence of 4.5% (95% CI, 3.2%-6.1%). Twenty-eight cases of
POAG were found, excluding those with evidence of exfoliation syndrome, giving
an age- and sex-adjusted prevalence of 2.7% (95% CI, 1.7%-4.0%) and making
it the most frequently encountered type (Table 3). In 11 of these (39.3%), POAG was bilateral. Excluding
cases with POAG or primary ACG, there were 21 glaucoma cases with an identifiable
secondary cause (Table 4). Secondary
glaucoma thus accounted for 21 (41.2%) of all 51 cases of glaucoma with an
age- and sex-adjusted prevalence of 1.7% (95% CI, 0.9%-2.9%).
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Table 3. Glaucoma Type for 51 Subjects With Glaucoma*
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Table 4. Glaucoma Prevalence by Age Group*
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The distribution of glaucoma types is detailed in Table 3. Among the secondary cases, the most frequent underlying
cause was exfoliation syndrome. Typical deposits of exfoliated material were
identified in the anterior segment of 11 glaucomatous subjects, representing
21.6% of all 51 glaucoma cases.
Secondary glaucoma in association with aphakia was identified in 6 eyes
of 4 subjects. Among the eyes that had undergone cataract surgery, these represented
33.3% of 18 aphakic eyes. No glaucoma was found in the 8 pseudophakic eyes
of 8 subjects. In all 6 eyes with aphakic glaucoma, the drainage angle was
open, and no evidence of pupil block was seen. Five had undergone intracapsular
surgery 5 to 15 years ago, which appeared to have been uneventful in 4 cases
and which was complicated by a small vitreous strand adherent to the wound
in the other. One eye had undergone extracapsular cataract extraction more
recently, which was complicated by vitreous loss. In only 1 eye was the IOP
elevated above 21 mm Hg, but all 6 eyes were blind as a result of untreated
glaucoma.
Angle-closure glaucoma was diagnosed in 5 subjects, giving a crude population
prevalence of 0.5% (95% CI, 0.2%-1.2%). Of these, a secondary cause was present
in 4 (exfoliation [n = 2], lens subluxation [n = 1], and uveitis [n = 1]),
leaving only a single case of primary ACG.
Angle narrowing to the extent that less than 90° of the trabecular
meshwork was visible occurred in 5 additional cases without evidence of glaucoma,
giving an overall observed prevalence of narrow angles of 1.0% (95% CI, 0.5%-1.8%).
In 1 case, the IOP was elevated in the absence of disc cupping or visual field
loss. For the whole population, maximum angle width was Shaffer12
grade 4 in 38.2%, grade 3 in 48.8%, grade 2 in 9.9%, and grade 1 or 0 in 3.1%.
The mean age of subjects with glaucoma was 69.5 ± 11.1 years.
Prevalence increased significantly with age for primary and secondary glaucoma
(P<.001). In those older than 80 years, 15.4%
were affected (Table 4), and 78.4%
of all glaucoma occurred in those 60 years or older. Prevalence of POAG increased
exponentially from 1.2% to 7.7% from the fifth to ninth decades of life.
The crude prevalence was higher in men than women for POAG (4.6% vs
2.1%) (P = .05), but this difference was not significant
after adjusting for age (P = .10). For all glaucoma
cases, men were significantly more likely than women to have glaucoma (8.6%
vs 3.7%), with an age-adjusted odds ratio of 2.0 (95% CI, 1.0-4.4) (P = .04).
A previous diagnosis of glaucoma had been made in only 5 (9.8%) of the
51 cases, of which 3 (5.9%) had received any disease-modifying treatment.
Only 1 case of POAG (3.6%) was previously recognized.
The cumulative IOP distribution for subjects with glaucoma is shown
in Figure 3. This refers to Goldmann
applanation tonometry except in eyes with corneal irregularity, in which the
Tono-Pen tonometry reading has been used. For bilateral cases, the higher
value of the 2 eyes was taken. In the case of POAG, the mean and median (interquartile
range) IOPs were 20.8 ± 8.6 mm Hg and 18.0 mm Hg (15.5-24.5 mm Hg),
respectively. These values are significantly higher than the IOP in nonglaucomatous
subjects (mean, 13.3 ± 3.3 mm Hg, and median, 13.5 mm Hg [12.0-15.5
mm Hg]) (Mann-Whitney, P<.001). In 16 (57.1%)
of 28 affected cases and 24 (61.5%) of the 39 eyes, the IOP was 21 mm Hg or
less in the affected eyes. Most POAG in this population is, therefore, normal-tension
glaucoma and emphasizes the degree of overlap with the healthy population
observed in IOP distribution. This finding is in contrast to findings with
glaucoma of the non-POAG type; for these subjects, only 4 (17.4%) of 23 had
an IOP of 21 mm Hg or less (P = .004), and the median
pressure was 44 mm Hg.
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Figure 3. Cumulative intraocular pressure
(IOP) curves showing the IOP of the glaucomatous eye of 28 subjects with primary
open-angle glaucoma (POAG). For comparison, 23 subjects with non-POAG and
the 51 subjects combined are shown. For bilateral glaucoma, the higher pressure
was used.
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PREVALENCE AND CAUSES OF BLINDNESS
Using the World Health Organization criteria (International
Classification of Diseases, 10th Revision categories of visual impairment
3, 4, and 5),13 41 subjects were bilaterally
blind, giving an age- and sex-adjusted blindness prevalence in this population
of 3.2% (95% CI, 2.2%-4.6%). In those older than 60 years, blindness prevalence
was 7.8% (95% CI, 5.6%-10.9%) and increased rapidly above this age, reaching
25.7% (95% CI, 14.1%-41.2%) in those older than 80 years. An additional 81
subjects were unilaterally blind, so the adjusted prevalence of blindness
in 1 or both eyes was 10.8% (95% CI, 8.7%-13.4%).
The principal causes of blindness are given in Table 5. Age-related cataract that had not undergone operation was
the most frequent cause of unilateral and bilateral blindness. However, in
9 (22.0%) of the 41 subjects who were bilaterally blind, the principal cause
in both eyes was glaucoma. In 13 (31.7%), blindness was due to glaucoma in
at least 1 of the 2 blind eyes. In the population 40 years and older, the
age- and sex-adjusted prevalence of blindness in at least 1 eye exclusively
attributable to glaucoma was 2.1% (95% CI, 1.3%-3.2%), and it was the cause
of blindness in both eyes in 0.9% (95% CI, 0.5%-1.7%).
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Table 5. Causes of Blindness
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In eyes with secondary glaucoma, 18 (75.0%) of 24 were blind (excluding
4 eyes primarily blind due to trauma) compared with 16 (41.0%) of 39 eyes
with POAG (Fisher exact test, P = .01). This difference
in disease severity is reflected in the criteria by which the diagnosis was
made. Without the results of visual field testing, a diagnosis of glaucoma
would still have been made in 20 (95.2%) of the 21 subjects with secondary
glaucoma under diagnostic categories 2 and 3 (advanced structural damage)
compared with only 18 (64.3%) of the 28 with POAG (P
= .007).
COMMENT
The results of this cross-sectional survey support the views that glaucoma
prevalence is generally higher in those of African origin than in other racial
groups and that it is principally POAG in type. However, real variations appear
in the type and prevalence of glaucoma between different black populations.1, 4, 14-19 Table 6 compares the methods and results
of population-based prevalence studies specifically for POAG in black populations
in Africa, the Caribbean, and the United States. Even allowing for differences
in methods and diagnostic criteria, the age-standardized prevalences in a
range of African communities are remarkably similar to one another, but are
less than in the US black population, and considerably lower than in Barbados
and St Lucia in the West Indies.
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Table 6. Crude and Standardized Prevalence of POAG in Population-Based
Surveys in Black Subjects*
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Important differences in the prevalence of other types of glaucoma in
different parts of the continent of Africa are also becoming apparent. Primary
ACG was an uncommon finding in this Zulu population, with a prevalence in
line with that in most reported white populations. This finding contrasts
with the results of some of the other cross-sectional studies in African and
African-derived populations in which prevalences as high as 1.0% have been
reported1, 14-15,20
and in which up to 28.9% of primary glaucoma was narrow angle in type.
Glaucoma in association with exfoliation syndrome was found to be the
principal contributor to the relatively high rate of secondary glaucoma in
the Hlabisa district and was the next most prevalent form after POAG. Exfoliative
glaucoma is said to be rare in the US black population,21-22
and no case was found in rural Tanzania.1 This
confirms previous impressions that exfoliative glaucoma is particularly common
in the Bantu peoples of South Africa.4, 23
These results stress the importance of avoiding the assumption that
findings in one population of African origin can necessarily be extrapolated
to others.
Recent investigators have demonstrated that the prevalence of POAG is
sensitive to changes in definitions and diagnostic criteria, and so comparisons
of prevalence rates between surveys must take into account differences in
the diagnostic tests and their interpretation.24
In the Hlabisa study, a simple, objective diagnostic scheme was used that
had a number of strengths. It was easy to apply and flexible enough to take
account of the relatively large number of subjects with advanced glaucoma
in whom visual field testing could not be performed and in whom evidence of
more severe structural damage was required to make the diagnosis. The scheme
used clear and concise cutoff points that could easily be transposed onto
data from other studies. The use of more than 1 level of severity of structural
damage to the optic nerve allowed us to estimate a prevalence of glaucoma
had visual field testing not been performed. This method has the potential
to be extremely useful for direct comparison with studies in similarly inaccessible
populations where visual field testing may not be possible for logistical
reasons.
The diagnostic criteria used in Hlabisa were strictly applied and were
quite conservative in relation to those of other studies. As a result, it
is fairly certain that virtually all cases identified as glaucoma were genuine,
and, if anything, the prevalences presented herein are minimum estimates.
From the point of view of a resource-poor country in which early disease is
almost never diagnosed or treated in a clinical practice, it is more useful
from the public health perspective to err in this direction. The conservative
nature of the diagnostic criteria, in addition to the lack of treatment, may
explain the observed high proportion of blindness in glaucomatous eyes.
The results from the Hlabisa district, although somewhat sensitive to
changes in the diagnostic algorithm, appeared to be more robust than those
previously reported. Thus, shifting the cutoff point for the minimum disc
requirement from a vertical CDR of at least 0.7 down to at least 0.5 increased
the prevalence of POAG only from 2.7% to 3.3%, and that for secondary glaucoma
was unchanged. In the study in Tanzania, the same alteration to the diagnostic
criteria increased the prevalence from 1.7% to 3.1%.1
Migrant workers were excluded from the study population for logistical
reasons, since many rarely return home. Assuming that they were healthy, the
study sample might be expected to have disproportionately more eye disease.
This effect was mitigated by the low prevalence of glaucoma and blindness
in subjects younger than 50 years, who constitute most of the migrants.
The results of this study indicate that glaucoma is a significant health
problem in this rural Zulu population. Although cataract not treated by operation
remains the principal cause of blindness, glaucoma was a contributory factor
in almost one third and was exclusively the cause in more than 20%. In an
area free of onchocerciasis and trachoma, glaucoma was thus the second most
important blinding disease. With considerable emphasis now being placed on
the provision of cataract surgery among the indigenous people of South Africa,
and with the population aging at a rapid rate, the prevalence and proportion
of blindness due to glaucoma are likely to increase still further.
Case finding in glaucoma remains a considerable challenge. In this virtually
untreated group of subjects with glaucoma, more than half of those with POAG
had an IOP of no greater than 2 SDs above the mean for this population, which
confirms findings reported previously that IOP measurement has only limited
predictive value in the detection of primary glaucoma. The high proportion
of normal-tension glaucoma in this population is in keeping with that in Tanzania,1 where the mean IOP in POAG was only 17.7 mm Hg, but
contrasts with findings in, for example, Barbados. In the Barbados Eye Study,
only 30% of cases had normal tension.25 The
IOP in the nonglaucomatous population of Barbados ranged from 3 to 4 mm Hg
higher than in healthy Zulus or Tanzanians.
Identifying cases early in the course of the disease requires sophisticated
psychophysiologic tests,26 and those currently
available are not appropriate for use in this type of community. Despite the
relatively high level of visual loss among the subjects with glaucoma identified
in this study, very few had sought a clinical opinion or had received a previous
diagnosis. Given the limited resources, a realistic short-term goal should
be the prevention of a greater proportion of patients becoming blind in both
eyes because of a failure to seek treatment when sight is lost in the first
eye. This could be approached by increasing awareness of the need for early
treatment, and further studies to examine why the population fails to seek
health care would be useful.
AUTHOR INFORMATION
Submitted for publication May 31, 2001; final revision received November
7, 2001; accepted December 11, 2001.
This study was supported by the British Council for the Prevention of
Blindness and the International Glaucoma Association, London, England.
We thank Piet Becker, PhD; Emma Cartwright; Colin Cook, FCS(Ophth)(SA);
Paul Foster, FRCOphth; Alpheus Khoza; James Kirwan, FRCOphth; Pak-Sang Lee,
MSc; Temba Mhlaba; Lindiwe Mthethwa; Ian Murdoch, FRCOphth; Londiwe Ntombela;
Karen Rotchford, MRCGP; David Wilkinson, PhD; Victoria Xolo; and the staff
of Hlabisa Hospital for their assistance.
Alan P. Rotchford, MA, MSc, FRCOphth;
Gordon J. Johnson, MA, MD, FRCSC
From the International Centre for Eye Health, Institute of Ophthalmology,
London, England.
Corresponding author: Alan P. Rotchford, MA, MSc, FRCOphth, International
Centre for Eye Health, Institute of Ophthalmology, 11-43 Bath St, London EC1V
9EL, England (e-mail: rotchford{at}supanet.com).
REFERENCES
1. Buhrmann R, Quigley H, Barron Y, West SK, Oliva MS, Mmbaga BB. Prevalence of glaucoma in a rural East African population. Invest Ophthalmol Vis Sci. 2000;41:40-48.
FREE FULL TEXT
2. Payne R, Feldman M, Cann H, Bodmer JG. A comparison of HLA data of the North American black with African black
and North American caucasoid populations. Tissue Antigens. 1977;9:135-147.
ISI
| PUBMED
3. Olerup O, Troye-Blomberg M, Schreuder GM, Riley EM. HLA-DR and -DQ gene polymorphism in West Africans is twice as extensive
as in north European Caucasians: evolutionary implications. Proc Natl Acad Sci U S A. 1991;88:8480-8484.
FREE FULL TEXT
4. Bartholomew R. Glaucoma in a South African Black Population. S Afr Arch Ophthalmol. 1976;3:135-150.
5. David R, Duval D, Luntz M. The prevalence and management of glaucoma in an African population. S Afr Arch Ophthalmol. 1984;10:55-62.
6. Population Census, 1996. Pretoria, South Africa: Census Products, Statistics South Africa;
1996.
7. Wilkinson D, Cutts F, Ntuli N, Abdool Karim SS. Maternal and child health indicators in a rural South African health
district. S Afr Med J. 1997;87:456-459.
ISI
| PUBMED
8. Sponsel W, Ritch R, Stamper R, et al for the Prevent Blindness America Glaucoma Advisory Committee. Prevent Blindness America visual field screening study. Am J Ophthalmol. 1995;120:699-708.
ISI
| PUBMED
9. Spaeth G. The normal development of the human anterior chamber angle: a new system
of descriptive grading. Trans Ophthalmol Soc U K. 1971;91:709-739.
PUBMED
10. Foster P, Buhrmann R, Quigley H, Johnson G. The definition and classification of glaucoma in prevalence surveys. Br J Ophthalmol. 2002;86:238-242.
FREE FULL TEXT
11. Kish L. Survey Sampling. New York: John Wiley & Sons Inc; 1965.
12. Shaffer R. Gonioscopy, ophthalmoscopy and perimetry. Trans Am Acad Ophthalmol Otolaryngol. 1960;64:112-125.
PUBMED
13. World Health Organization. International Classification of Diseases, 10th Revision
(ICD-10). Geneva, Switzerland: World Health Organization; 1992.
14. Tielsch JM, Sommer A, Katz J, Royall RM, Quigley HA, Javitt J. Racial variations in the prevalence of primary open-angle glaucoma:
the Baltimore Eye Survey. JAMA. 1991;266:369-374.
ABSTRACT
15. David R, Stone D. The prevalence of glaucoma in a homogeneous African population. In: Ticho U, David R, eds. Recent Advances in Glaucoma. Amsterdam, the Netherlands: Elsevier Science Publishers; 1984:145-153.
16. Mason RP, Kosoko O, Wilson MR, et al. National survey of the prevalence and risk factors of glaucoma in St
Lucia, West Indies, I: prevalence findings. Ophthalmology. 1989;96:1363-1368.
ISI
| PUBMED
17. Leske MC, Connell AM, Schachat AP, Hyman L. The Barbados Eye Study: prevalence of open angle glaucoma. Arch Ophthalmol. 1994;112:821-829.
ABSTRACT
18. Neumann E, Zauberman H. Glaucoma survey in Liberia. Am J Ophthalmol. 1965;59:8-12.
ISI
| PUBMED
19. Wallace J, Lovell H. Glaucoma and intraocular pressure in Jamaica. Am J Ophthalmol. 1969;67:93-100.
ISI
| PUBMED
20. Johnson G, Minassian D, Weale R. The Epidemiology of Eye Disease. New York, NY: Chapman & Hall; 1998:159-180.
21. Hiller R, Sperduto RD, Krueger DE. Pseudoexfoliation, intraocular pressure, and senile lens changes in
a population-based survey. Arch Ophthalmol. 1982;100:1080-1082.
ABSTRACT
22. Cashwell LF Jr, Shields MB. Exfoliation syndrome in the southeastern United States, I: prevalence
in open-angle glaucoma and non-glaucoma populations. Acta Ophthalmol Suppl. 1988;184:99-102.
23. Luntz MH. Prevalence of pseudo-exfoliation syndrome in an urban South African
clinic population. Am J Ophthalmol. 1972;74:581-587.
ISI
| PUBMED
24. Wolfs R, Borger P, Ramrattan R, et al. Changing views on open-angle glaucoma: definitions and prevalences—the
Rotterdam Study. Invest Ophthalmol Vis Sci. 2000;41:3309-3321.
FREE FULL TEXT
25. Leske MC, Connell AM, Wu SY, Hyman LG, Schachat AP. Risk factors for open-angle glaucoma: the Barbados Eye Study. Arch Ophthalmol. 1995;113:918-924.
ABSTRACT
26. American Academy of Ophthalmology. Preferred Practice Pattern: Open-Angle Glaucoma. San Francisco, Calif: American Academy of Ophthalmology; 1995.
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