Prevalence of the different types of age-related cataract in an African population

PURPOSE. To describe the prevalence of different types of cataract and thei r association with visual acuity in a Tanzanian population aged 40 years an d older. METHODS. A prevalence survey for lens opacity, glaucoma, and visual impairm ent was carried out on all residents age 40 and older of six villages in Ko ngwa, 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% (3268/36 41). The prevalence of cataract was as follows: NSC, 15.6%; CC, 8.8%; and P SC, 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%; an d PSC, 5.9% for those aged 70 years and older (P < 0.0001 for all cataract types, chi (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 < 6/1 2), US blindness (less than or equal to6/60) and WHO blindness (<6/120) for this population were 13.3%, 2.1%, and 1.3%, respectively. Older age and ea ch of the major types of pure and mixed cataract were independently associa ted 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 seemi ng lower prevalence of CC may to some extent be explained by different grad ing schemes, differential availability of cataract surgery, the younger mea n age of the Tanzanian subjects, and a higher prevalence of NSC in this pop ulation.