This study was a part of the Jiangsu Eye Study, a population-based epidemiological study. Jiangsu province is located in the Yangtze River Delta. According to the different levels of economic development, Jiangsu province is divided into southern and northern areas. To estimate the prevalence of blindness and low vision among older adults aged ≥50 years, one district or county was selected as the sampling area from both southern and northern Jiangsu: Binhu district and Funing county, respectively. The surveys were carried out by randomly selecting individuals within each district or county, which was similar to the method we described previously.
23 The sampling frame was constructed using geographically defined clusters based on village register data. Cluster boundaries were defined so that each cluster would have a population of approximately 1000 individuals (all ages). Sample size was based on estimating an anticipated 4% prevalence for visual impairment <20/200 within an error bound (precision) of 20% with 95% confidence. Assuming an examination response rate of 85%, and a design effect of 1.5 to account for inefficiencies associated with the cluster sampling design, a sample of 4068 persons ≥50 years of age was required for each district or county.
23 Depending on the percentage of population ≥50 years of age, 28 to 30 clusters were randomly selected (with equal probability) by the Chinese Ophthalmological Society from the sampling frame for each district/county. A total of 12,867 persons aged ≥50 years were enumerated in Binhu district and Funing county. Geographically defined cluster sampling initially included 6722 individuals aged ≥50 years in Binhu district from January to December 2010. Actually 6106 persons were examined with the response rate of 90.8%. The same sampling was initially applied to 6145 individuals aged ≥50 years in Funing county from September 2010 to May 2011 and 5947 individuals were examined for a response rate of 96.8%. The covered area of Jiangsu Eye Study has a stable and ethnically homogenous population. All participants were unrelated self-identified Han Chinese (all four grandparents were ethnically Han Chinese). Participants were brought to village clinics or offices for general physical and full ophthalmic examinations. Cataract was defined as opacification of the ocular lens and best corrected visual acuity of <20/40.
23 Lens opacities were determined according to the Lens Opacities Classification System III (LOCS III) in 0.1-unit steps for each opacity up to a maximum of 6.9 for N, 5.9 for C, and PSC subtypes.
24 Presence of more than one cataract type in at least one eye or different pure types in both eyes was classified as mixed type.
25 We identified a total of 2208 cataract patients from Binhu district and Funing county. The prevalence of cataract was 18.3%. In this study, we selected ARC patients as research subjects. The inclusion criteria for ARC included (1) opacification of the ocular lens, (2) age ≥50 years, (3) best corrected visual acuity <20/40, (4) no other clear reasons to cause cataract. The exclusion criteria were (1) complicated cataract due to glaucoma, high myopia, uveitis, diabetes, ocular trauma, or other known causes, (2) either eye being pseudophakic or aphakic, and (3) previous treatment with radiation therapy or steroids. Based on these criteria, 1144 patients were excluded, and 1064 ARC patients (C = 335, N = 470, PSC = 42, M = 217) were included. A further 163 participants were excluded; specifically, ARC patients with systemic diseases such as diabetes, kidney diseases, and cancers and ARC patients with macular diseases and other retinal diseases. Another 67 ARC patients of all subtypes were excluded, with their worse eye having LOCSIII grade <2. As a result, there were total 834 ARC patients eligible to participate in this study. Of these 834 ARC patients, DNA extraction failed in 29 and 25 could not be genotyped. We finally examined 780 ARC patients, among which there were 257 with C type, 368 with N type, 34 with PSC type, and 121 with M type of ARC.