NIn this population-based study, we observed an overall iERM prevalence of 1.02 in Beixinjing Blocks, Shanghai, China, which included 0.63 for CMR and 0.39 for PMF. Our study suggests that iERM is less frequent in urban Chinese than reported in samples of Asians from the Handan Eye Study (3.0 ) [25] and Singapore Malay Eye Study (9.5 ) [2], Caucasians from the Melbourne Visual Impairment Project Study (5.4 ) [23] and Latinos from the LALES (17.5 ) [8]. Therefore, the prevalence of iERM differs among population-based studies, for reasons unknown. One possible reason may be ethnic differences, as mentioned by some previous studies [22,24]. Interestingly, not just the prevalence of iERM but the prevalence of DR and 58-49-1 cost agerelated macular degeneration in our previous studies [39,40] were lower than in Western countries. Another possible reason is different inclusion criteria for eligible participants. In the present study, urban residents aged 60 years or older were randomly selected, while most of the other studies used an inclusion criterion of 40 years or older [8,23,26]. The prevalence of diabetes [4,8,27], a risk factor for iERM, was 20.4 among persons who were aged 60 years in China [41], and Shanghai, as the largest city and one of the most economically developed areas, has a higher prevalence (aged 60?9 years: 22.4 /male, 22.3 /female; aged 70?4 years: 25.6 /male, 27.2 /female) [42], which was close to the results from the Singapore Malay Eye Study (21.8 ) [43], but much lower than the LALES (34.5 ) [44]. Therefore, we cannot rule out the possible association between the prevalence of diabetes and the lower prevalence of iERM in Beixinjing Blocks. In addition, cataract surgical rate (CSR) in Beixinjing Blocks (aged 60 years) was 7,790/million in 2007 [45]. Approximately 8000 cataract surgeries (an exclusion criteria for iERM) per yearVariable Sex* MaleTotal (n) CMR (n, ) PMF (n, ) Any iERM (n, ) 1481 12 (0.8) 9 (0.5) 8 (0.6) 11 (0.7) 2 (0.5) 21 (0.6) 5 (0.3) 8 (0.4) 6 (0.4) 7 (0.5) 0 (0) 13 (0.6) 17 (1.1) 17 (0.9) 14 (1.0) 18 (1.2) 2 (0.5) 34 (1.0)Female 1845 Age (y) 60?9 70?9 80+ Total* 1409 1507 410CMR, cellophane macular reflex; PMF, preretinal macular fibrosis; iERM, idiopathic epiretinal membrane. *Age-standardized prevalence using the 2000 Chinese national census. doi:10.1371/journal.pone.0051445.tPrevalence and Risk Factors of iERM in ShanghaiTable 2. Demographic and clinical characteristics among the participants (n = 3326) with or without idiopathic epiretinal membrane.*Characteristic Participants [No. ( )] Mean age 1516647 (SD, 95 CI), years 60?9 [No. ( )] 70?9 [No. ( )] 80+ [No. ( )] Male [No. ( )] Mean BMI (SD, 95 CI) Mean education (SD, 95 CI) years Illiterate [No. 
( )] Primary school [No. ( )] ML-264 Junior high school [No. ( )] Senior high school [No. ( )] College or higher [No. ( )] Systemic comorbidities suffered Hypertension [No. ( )] Diabetes [No. ( )] Cardio-cerebrovascular diseases [No. ( )] Hypermyopia [No. ( )] Mean logMAR presenting VA (SD, 95 CI) Mean logMAR UCDVA (SD, 95 CI)iERM 34 (1.02) 71.53 (6.11, 95 CI, 69.40 to 73.66) 14 (41.2) 18 (52.9) 2 (5.9) 17 (50.0) 
24.15 (3.02, 95 CI, 23.10 to 25.20) 9.38 (5.38, 95 CI, 7.51 to 11.26 ) 4 (11.8) 6 (17.6) 9 (26.5) 6 (17.6) 9 (26.5)No iERM 3292 (98.98) 70.84 (7.34, 95 CI, 70.59 to 71.09) 1395 (42.4) 1489 (45.2) 408 (12.4) 1464 (44.5) 23.90 (3.27, 95 CI, 23.79 to 24.02 ) 7.42 (4.47, 95 CI, 7.27 to 7.58 ) 468 (14.2) 1143 (34.7) 814 (24.7) 551 (16.7) 361 (9.6)Statistic val.NIn this population-based study, we observed an overall iERM prevalence of 1.02 in Beixinjing Blocks, Shanghai, China, which included 0.63 for CMR and 0.39 for PMF. Our study suggests that iERM is less frequent in urban Chinese than reported in samples of Asians from the Handan Eye Study (3.0 ) [25] and Singapore Malay Eye Study (9.5 ) [2], Caucasians from the Melbourne Visual Impairment Project Study (5.4 ) [23] and Latinos from the LALES (17.5 ) [8]. Therefore, the prevalence of iERM differs among population-based studies, for reasons unknown. One possible reason may be ethnic differences, as mentioned by some previous studies [22,24]. Interestingly, not just the prevalence of iERM but the prevalence of DR and agerelated macular degeneration in our previous studies [39,40] were lower than in Western countries. Another possible reason is different inclusion criteria for eligible participants. In the present study, urban residents aged 60 years or older were randomly selected, while most of the other studies used an inclusion criterion of 40 years or older [8,23,26]. The prevalence of diabetes [4,8,27], a risk factor for iERM, was 20.4 among persons who were aged 60 years in China [41], and Shanghai, as the largest city and one of the most economically developed areas, has a higher prevalence (aged 60?9 years: 22.4 /male, 22.3 /female; aged 70?4 years: 25.6 /male, 27.2 /female) [42], which was close to the results from the Singapore Malay Eye Study (21.8 ) [43], but much lower than the LALES (34.5 ) [44]. Therefore, we cannot rule out the possible association between the prevalence of diabetes and the lower prevalence of iERM in Beixinjing Blocks. In addition, cataract surgical rate (CSR) in Beixinjing Blocks (aged 60 years) was 7,790/million in 2007 [45]. Approximately 8000 cataract surgeries (an exclusion criteria for iERM) per yearVariable Sex* MaleTotal (n) CMR (n, ) PMF (n, ) Any iERM (n, ) 1481 12 (0.8) 9 (0.5) 8 (0.6) 11 (0.7) 2 (0.5) 21 (0.6) 5 (0.3) 8 (0.4) 6 (0.4) 7 (0.5) 0 (0) 13 (0.6) 17 (1.1) 17 (0.9) 14 (1.0) 18 (1.2) 2 (0.5) 34 (1.0)Female 1845 Age (y) 60?9 70?9 80+ Total* 1409 1507 410CMR, cellophane macular reflex; PMF, preretinal macular fibrosis; iERM, idiopathic epiretinal membrane. *Age-standardized prevalence using the 2000 Chinese national census. doi:10.1371/journal.pone.0051445.tPrevalence and Risk Factors of iERM in ShanghaiTable 2. Demographic and clinical characteristics among the participants (n = 3326) with or without idiopathic epiretinal membrane.*Characteristic Participants [No. ( )] Mean age 1516647 (SD, 95 CI), years 60?9 [No. ( )] 70?9 [No. ( )] 80+ [No. ( )] Male [No. ( )] Mean BMI (SD, 95 CI) Mean education (SD, 95 CI) years Illiterate [No. ( )] Primary school [No. ( )] Junior high school [No. ( )] Senior high school [No. ( )] College or higher [No. ( )] Systemic comorbidities suffered Hypertension [No. ( )] Diabetes [No. ( )] Cardio-cerebrovascular diseases [No. ( )] Hypermyopia [No. ( )] Mean logMAR presenting VA (SD, 95 CI) Mean logMAR UCDVA (SD, 95 CI)iERM 34 (1.02) 71.53 (6.11, 95 CI, 69.40 to 73.66) 14 (41.2) 18 (52.9) 2 (5.9) 17 (50.0) 24.15 (3.02, 95 CI, 23.10 to 25.20) 9.38 (5.38, 95 CI, 7.51 to 11.26 ) 4 (11.8) 6 (17.6) 9 (26.5) 6 (17.6) 9 (26.5)No iERM 3292 (98.98) 70.84 (7.34, 95 CI, 70.59 to 71.09) 1395 (42.4) 1489 (45.2) 408 (12.4) 1464 (44.5) 23.90 (3.27, 95 CI, 23.79 to 24.02 ) 7.42 (4.47, 95 CI, 7.27 to 7.58 ) 468 (14.2) 1143 (34.7) 814 (24.7) 551 (16.7) 361 (9.6)Statistic val.