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1 From the Dana Center for Preventive Ophthalmology, Wilmer Ophthalmological Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland; 2 University of Ottawa Eye Institute, Ottawa, Canada; and the 3 Kongwa Trachoma Project, Kongwa, Tanzania.
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Abstract |
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METHODS. A prevalence survey for lens opacity, glaucoma, and visual impairment was carried out on all residents age 40 and older of six villages in Kongwa, Tanzania. One examiner graded the lens for presence of nuclear (NSC), posterior subcapsular (PSC), and cortical cataract (CC), using the new WHO Simplified Cataract Grading System. Visual acuity was measured in each eye, both presenting and best corrected, using an illiterate E chart.
RESULTS. The proportion of eligible subjects participating was 90% (3268/3641).
The prevalence of cataract was as follows: NSC, 15.6%; CC, 8.8%; and
PSC, 1.9%. All types of cataract increased with age, from NSC, 1.7%;
CC, 2.4%; and PSC, 0.4% for those aged 40 to 49 years to NSC, 59.2%;
CC, 23.5%; and PSC, 5.9% for those aged 70 years and older
(P < 0.0001 for all cataract types,
2 test for trend). Cataract prevalence was higher among
women than men for NSC (P = 0.0001), but not for CC
(P = 0.15) or PSC (P = 0.25),
after adjusting for age. Prevalence rates of visual impairment
(BCVA < 6/12), US blindness (
6/60) and WHO blindness (<6/120)
for this population were 13.3%, 2.1%, and 1.3%, respectively. Older
age and each of the major types of pure and mixed cataract were
independently associated with worse vision in regression modeling.
CONCLUSIONS. Unlike African-derived populations in Salisbury and Barbados, NSC rather than CC was most prevalent in this African population. The seeming lower prevalence of CC may to some extent be explained by different grading schemes, differential availability of cataract surgery, the younger mean age of the Tanzanian subjects, and a higher prevalence of NSC in this population.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Cataract is a major cause of visual disability throughout the African continent.7 8 9 To date, however, no population-based study in Africa has examined the distribution of cataract types using a standardized grading system. We report the prevalence of the different types of age-related cataract based on the WHO grading system10 from a survey of ocular disease among adults in central Tanazania.11 Comparison is also made with previous results from African-derived populations in Barbados4 and Maryland.5
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Examination Procedures
Sampling strategy and examination techniques have been described
in detail elsewhere11
and are reviewed here. Six of the 44
villages of Kongwa district were selected at random, excluding 5
villages with active primary eye care programs, 1 with a foreign-funded
clinic, 11 that were considered inaccessible for purposes of this
study, and Kongwa town itself. After an initial census, all adults aged
40 years and older were invited to a central examination site in their
village. Visual acuity was measured at 4 m using a tumbling-E
Early Treatment Diabetic Retinopathy Study (ETDRS) chart (Lighthouse,
New York, NY) in ambient illumination with presenting correction, if
any. In persons with acuity of <6/18 in either eye, retinoscopy and
refraction were performed by an ophthalmic optician.
The study protocol was approved by the Johns Hopkins University Joint Committee on Clinical Investigation and the National Blindness Prevention Committee of Tanzania. It followed the tenets of the Declaration of Helsinki.
Cataract Grading
After dilation of the pupil (in individuals judged not to have
occludable angles), the ophthalmologist (RRB) graded nuclear, cortical,
and posterior subcapsular cataract by comparison with standard
photographs based on the WHO adaptation10
of the Lens
Opacity Classification III System.12
Briefly, the WHO
Simplified Cataract Grading System grades cortical cataract (CC) as 0
(definite cortical opacity covering less than one eighth of lens
circumference); 1 (one eighth to one fourth of lens
circumference); 2 (one fourth to one half of cortical circumference);
or 3 (greater than one half of lens circumference). In all cortical
grading, definite cortical opacity not actually at the lens
circumference is treated for grading purposes as involving that portion
of the circumference included between radial lines extended from either
edge of the opacified area. An additional code is used to indicate if
CC affects the central optical zone.
Nuclear sclerotic cataract (NSC) is graded with reference to three photographic standards. Grade 0 represents no nuclear opacity or less extensive than standard photograph 1. Grade 1 is nuclear opacity at least as extensive as standard photograph 1 and less extensive than standard photograph 2. Grade 2 is defined in analogous fashion with regard to standard photographs 2 and 3, whereas grade 3 is at least as extensive as standard photograph 3.
For posterior subcapsular cataract (PSC), grade 0 represents an opacity < 1 mm in vertical diameter, grade 1 falls between 1 and 2 mm, grade 2 between 2 and 4 mm, and grade 3 > 4 mm.
For all types of opacity, a grade of 9 represents cataract that could not be graded, whether because of poor pupillary dilation, media opacity, or other reason.
Statistical Analysis
The prevalence of cataract grade 1 or higher was calculated,
based on the highest grade by type in either eye. Prevalence was also
calculated by gender and for each decade from 40 to 70 years and older.
Significance of the change in cataract prevalence with age by decade
was examined using the 2 test for trend.
Gender differences for the different types of cataract, adjusted for
age, were analyzed using logistic regression models with cataract
(present or absent) as the outcome variable. Significance of the
regression coefficient for gender was tested with Wald’s
2. The prevalence of visual impairment
(best-corrected visual acuity [BCVA] < 6/12), blindness according to
US standards (BCVA 6/60), and blindness according to WHO
standards (BCVA < 6/120) was calculated according to the vision
in the better-seeing eye. A linear regression model was used to study
the association between the outcome of BCVA and age, gender, and
various types of pure and mixed cataract as dependent variables. In
this model, data on both eyes were included, with the SE being
corrected for the correlation between eyes.13
The outcome
variable for this regression model was logmar acuity (i.e., the
negative log of the BCVA). Thus, a negative ß-coefficient indicated
that the factor in question was protective, whereas a positive value
indicated an association with worse vision. The kappa statistic was
used to calculate interobserver reliability, with a 
0.8
considered to represent good agreement.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The prevalence of cataract (grade 1 or above in either eye) was
15.6% for NSC, 8.8% for CC, and 1.9% for PSC (Table 1)
. The prevalence of all types of cataract increased with age,
from 1.7% for NSC, 2.4% for CC, and 0.4% for PSC among persons in
their 40s to 59.2% for NSC, 23.5% for CC, and 5.9% for PSC for those
70 years and older (P = 0.0001 for all cataract types,
2 test for trend, Table 1
). A greater
proportion of severe cataract grades also occurred among older persons
(Table 2)
.
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2), but
not for CC (P = 0.15, Wald’s
2) or PSC (P = 0.25, Wald’s
2), when adjusted for age (Table 1) . For subjects whose lens opacity could be graded, the prevalence of visual impairment (BCVA < 6/12 in the better seeing eye) was 13.3% (386/3127, 95% CI, 12.0–14.6%), whereas the prevalence of blindness according to the US and WHO definitions was 2.1% (67/3127, CI 1.7–2.7%) and 1.3% (41/3127, CI 0.9–1.8%), respectively.
In regression modeling, the effect on best-corrected vision of each type of pure cataract was considered separately from the effect of two types of mixed cataract, because mixed cataract may be more visually disabling. The first type was mixed NSC and PSC, including all eyes with both NSC and PSC, whether or not CC was present. The second type included eyes with mixed CC and either PSC or NSC, but not both. In this model, advancing age, and all the types of pure and mixed cataract were significantly associated with worse vision (Table 3) .
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The prevalence of visual impairment and blindness is high in this population, in accordance with previous studies in the area.14 Certainly not all visual impairment and blindness in this population are due to cataract. In fact, a previous study of the causes of vision loss in this area of central Tanzania found corneal opacities to be the leading cause of blindness (responsible for 44% of bilateral blindness), whereas cataract was second (22% of bilateral blindness).14 Corneal opacity was thought to be secondary to trachoma, vitamin A deficiency, and keratoconjunctivitis.14 15
Increasing prevalence of the various types of cataract with older age was to be expected. The fact that a large proportion of severe grades also occurred among older persons is different from the pattern observed in many US-based populations, where the most severe grades of lens opacity often undergo cataract extraction and are thus less prevalent.5
Higher age-adjusted prevalence and incidence rates of nuclear4 16 and cortical4 17 18 cataract has been reported for women in previous population-based studies among both African and European-derived populations. This finding is not completely understood. One hypothesis is that changes in the hormonal milieu at menopause somehow increase the risk of lens opacity among women. Evidence in favor of this theory includes a decreased risk of nuclear sclerosis among current users of estrogen replacement therapy19 20 21 and a protective effect of younger age at menarche and older age at menopause against nuclear and cortical opacities, respectively.19
The most prevalent form of cataract in this African population was NSC. This differs from the preponderance of CC, which has been reported for African-derived populations in Barbados4 and Maryland.5 One reason for this observed difference appears to be a low prevalence of CC in our study (8.8%) compared with either Barbados (34%) or Salisbury (54% for African-American subjects). Several different types of explanations may be considered for this.
One possibility is grader error, or artifact, in the assessment of CC.
The fact that all grading for this study was carried out at the slit
lamp, without a permanent photographic record, does not permit review
of the grading. However, all grading was performed by a single
ophthalmologist (RRB), after a period of standardization with one of
the original designers of the grading system (SKW). At the end of the
training period, reliability testing was carried out using a set of
photographs that were graded separately by the two investigators. The
interobserver agreement between the observers for NSC and CC grades was
excellent (CC: = 0.82; NSC: = 0.83; data not shown).
Good agreement in the grading of photographs does not preclude the
possibility of error resulting from uncontrolled factors in a field
setting. For example, inadequate dilation of the pupil could result in
an underestimate of the prevalence of CC, because it is often seen
primarily at the equator of the lens. Pupil dilation for each subject
was scored by an ophthalmic nurse as adequate (6 mm), inadequate (3
and <6 mm), or pinpoint (<3 mm) with reference to standard pupillary
outlines. In grading of the right eye, 2639 subjects (95.7%) had
adequate pupillary dilation, and 118 (4.3%) had inadequate or pinpoint
pupils. The age-adjusted proportion of persons with CC among those with
adequate dilation (7.4%) was no different from those with
less-than-adequate dilation (7.5%; 2 = 0.22,
P = 0.91). It does not seem likely that inadequate
pupillary dilation could explain the low prevalence of CC in this
population.
Alternatively, the lower observed prevalence of CC in our study may have resulted from differences in cataract grading systems, especially if the WHO System categorizes less severe opacities as 0. The definition of CC used in the Barbados study was grade 2 or higher on the LOCS II22 scale; that is, a cortical opacity whose area was greater than that represented by photographic standard CI. This definition of CC may be compared with the WHO System definition of grade 1 or higher, that is, an opacity occupying one eighth or more of the circumference of the lens.10 The LOCS II CI standard photograph itself depicts an opacity with close to one-fourth circumference of the lens involved, but approximately 6% of the total area opacified.23 Thus, the opacity depicted in this photograph would meet the criterion for inclusion as a case of CC in our study.
However, it is certainly possible to imagine a wedge-shaped opacity that covers an area slightly greater than standard CI and yet occupies less than one eighth of the circumference. That is, there are CC that might count as cases under LOCS II in the Barbados study but that would not have been counted in the present study under the WHO system. However, it is clear that all opacities with areas equal to or greater than LOCS II Standard CII (21% of total lens area by computerized measurement22 ) would circumscribe more than one eighth of the lens circumference if extended centrifugally, as required under the WHO grading system.
In summary, some eyes graded as having CC in the Barbados study (e.g., LOCS grade 2) might not have been counted as CC in the present study. However, no opacities grade 3 or higher in LOCS II would have failed to be counted in our study. Assuming the most conservative interpretation in comparing CC prevalence in the two studies, namely that the WHO grading system would have missed all CC classified as grade 2 under LOCS II, the prevalence of grade 3 and higher CC in Barbados was still 17.5%, more than twice that reported in our study.
In comparing our results with those of African-American participants in
the Salisbury Eye Evaluation (SEE) Project5
in Salisbury,
Maryland, it does not appear that differences in grading systems can
explain the observed difference in prevalence of CC. In the SEE
Project, all cortical opacities occupying 3/16 of the total
area of the lens were considered to represent CC.5
Any
opacity of this size would clearly affect at least one eighth of the
lens circumference if radial lines were extended outward from its
margins, as called for by the WHO grading scheme.10
Thus,
all opacities classified as CC in the SEE project would have been
considered as CC in the present study.
Thus, it does not appear that the lower prevalence of CC observed in Tanzania compared with Barbados and Maryland can be explained completely in terms of differences between grading systems.
A final reason for the apparent differences in CC prevalence between our African population and African-derived populations in Barbados and Maryland may relate to differences between the populations themselves. A higher incidence of cataract extraction in Tanzania could theoretically lead to lower observed prevalence of CC. However, <0.5% of the subjects in our study were aphakic, and there were no pseudophakes. This is well below the prevalence reported for Barbados4 and Maryland.5
Another obvious difference between these populations is that of age. The Tanzanian population (mean age, 53.3 ± 10.9 years) was significantly younger than the Barbados (mean age, 59 ± 12 years, P < 0.0001, t-test) and Salisbury (mean age, 72.1 ± 5.6 years, P < 0.0001, t-test) populations. Age adjustment applying the Barbados prevalence rates for CC (only considering opacities greater than standard CII as discussed above) to the Kongwa population gives an age- and gender-adjusted prevalence for CC of 11.0%, lower than the observed prevalence for Barbados but still higher than reported in our study. Age adjustment in which prevalence rates for the present study were applied to the SEE population structure gives a prevalence of CC of 54.0%, exactly the same as observed in SEE.5
In summary, it would seem that the major reason for higher observed prevalence of cortical cataract in Salisbury, Maryland, versus the present study was the pronounced difference in age between the two populations. The difference between Barbados and the present study was partly due to age and partly due to differences in grading systems, where very early cortical opacities were classified in this population as not present.
Another aspect of the different patterns of opacity seen in the Tanzanian population appears to be a higher than expected prevalence of NSC. Analyses similar to those presented above were carried out for NSC. Applying the observed prevalence rates of NSC in Kongwa to the SEE population structure gives an age-adjusted prevalence of 47.8%, considerably higher than the 33.5% actually observed among African-Americans in Salisbury. This comparison can be made directly, in that the cutoff for significant NSC between these two studies was of comparable severity. Although the definition of NSC used in the Kongwa Eye Project (KEP) was more severe than that in the Barbados study, it is interesting to note that the adjusted prevalence obtained by applying the KEP prevalence rates to the Barbados population structure, 23.8%, was still somewhat higher than the prevalence figure of 19% reported for Barbados.
One reason for the somewhat higher age-adjusted prevalence of NSC in Kongwa than in Salisbury and Barbados may well be the greater availability of cataract surgery, with the prevalence of bilateral pseudophakia among African-Americans in Salisbury being 4.8%,5 and 3% of subjects in Barbados having aphakia or pseudophakia in at least one eye.4 However, these numbers suggest that minimal access to surgery cannot necessarily explain the entire observed excess age-adjusted risk for NSC in the Kongwa population. Smoking, a well-described risk factor for NSC,24 25 26 27 is not at all widely practiced in the Kongwa region. Although there is much conflicting evidence, the preponderance of epidemiologic studies suggest that reduced intake of antioxidant substances such as vitamins A, C, and E may increase risk for NSC.28 The arid climate in central Tanzania is not suitable for growing many of the plant sources of antioxidants, and it is likely that intake of these substances is lower there than intake in either Salisbury or Barbados, which may partially explain the excess age-adjusted prevalence of NSC in Kongwa.
Another difference that must be considered between the populations in Tanzania, Barbados, and Maryland is ethnic. The people of central Tanzania were not included in the slave trade to the New World to the same extent as the West African ancestors of participants in the Barbados and Salisbury studies. Although most Tanzanians share a common ancestry with present inhabitants of West Africa,29 some ethnic differences across Africa do exist and may underlie the apparent differences in prevalence of the different types of cataract. Further work on the excess of nuclear opacity in this East African population may be warranted.
As with all other populations studied to date, the prevalence of PSC in this Tanzanian population was low compared with CC and NSC. As rates of cataract surgery were lower in this population than any for which cataract prevalence has been previously reported using a standardized system, our data provide new evidence that PSC prevalence is low not simply because of rapid progression to visual disability and cataract extraction.
The relative frequency of cataract types observed in this Tanzanian population, with NSC more prevalent than CC, is actually more similar to that reported in several studies for white populations. Higher rates of NSC than CC are reported for white populations in both the Beaver Dam16 and Blue Mountains Studies.17 However, as the above discussion outlines, the disparity of these populations with regard to known risk factors for the different types of lens opacity and access to cataract surgery makes any direct comparison by race difficult.
Differences in the prevalence rates of the different types of age-related lens opacity are of more than theoretical interest. The degree of visual disability associated with the different types of cataract has been reported to vary, with PSC and NSC in particular being more likely to result in vision loss requiring cataract surgery.6 30 Our own results are generally consistent with the idea that CC is less strongly associated with vision loss than are the other types of cataract.
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Acknowledgements |
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Footnotes |
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Commercial relationships policy: N.
The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked "advertisement" in accordance with 18 U.S.C. 
1734 solely to indicate this fact.
Corresponding author: Nathan Congdon, Wilmer 120, 600 N. Wolfe Street, Baltimore, MD 21287. ncongdon{at}jhmi.edu
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