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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.
PURPOSE. To describe the prevalence of different types of cataract and their association with visual acuity in a Tanzanian population aged 40 years and older.
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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