prevalence and causes of blindness and low vision in tehran province: methodology and implementation program

purpose: to describe the methodology and implementation of the investigation for prevalence and causes of visual impairment in tehran province. methods: this population-based cross-sectional study was conducted on all urban and rural residents of tehran province in 2003. a stratified cluster random sampling with probability proportional to size procedure was used. first, best pinhole-corrected visual acuity (bpcva) was determined according to habitual visual acuity (va) by an optometrist. cases of bpcva £ 20/60 in either eye were referred to an ophthalmologist for detection of best spectacle-corrected visual acuity (bscva) and main cause of visual impairment (va<20/60) including low vision (va< 20/60-20/400) and blindness (va< 20/400) according to bpcva and bscva. epidemiologic aspects and response rates in each phase of the study are presented. results: of 13 248 invited subjects, 11 975 persons participated in the first step of the study (primary response rate: 90.4%). of these, 760 cases (6.3%) with bpcva £ 20/60 were referred for ophthalmologic examination of which 557 participated in the study (secondary response rate: 73.3%). the study population in the first step consisted of urban subjects in 85% (approximately similar to the reference population) and female subjects in 61.4% (greater than the reference population, p<0.001). the proportion of the age group above 20 years, especially 20-29 years group in the study population was greater than that of the reference population. (p<0.001) referral rate to ophthalmologists was 8.3% in urban and 6.0% in rural populations (p<0.001); 2.1% under age 20 years, 4.1% in age 20-49 years, and 24.7% in age ³ 50 years. (p<0.001) no children below the age 4 years were referred. the referral rate was not different between male (6.8%) and female (6.0%) subjects. secondary participation rate was greater in male (85.8%) than female (64.4%) subjects (p<0.001), but there was no difference between rural and urban residents or between the age groups in this aspect. conclusion: despite good primary participation rate, the age and sex distribution of the study population was different from the reference population which highlights the need for planning an effective sampling procedure.the prevalence rates should be standardized for age and sex.