title: Population density and refractive error among Chinese children.
abstract: OBJECTIVE
  China is urbanizing rapidly, and the prevalence of myopia is high. This study was conducted to identify the reasons for observed differences in the prevalence of myopia among urban versus rural Chinese children.
  
  
  METHODS
  All children with uncorrected acuity of 6/12 or worse and a 50% random sample of children with vision better than 6/12 at all secondary schools in mixed rural-urban Liangying Township, Guangdong, underwent cycloplegic refraction, and provided data on age, gender, parental education, weekly near work and time outdoors, and urban development level of respondents' neighborhoods (12-item questionnaire). Population density of 32 villages and urban zones in Liangying was calculated from census figures (mean population density, 217 persons/km(2); range, 94-957; mean for Guangdong, 486).
  
  
  RESULTS
  Among 5844 eligible children, 4612 (78.9%) had parental consent and completed examinations; 2957 were refracted per protocol, and 2480 (83.9%) of these had questionnaire data. Those with completed examinations were more likely to be girls (P < 0.001), and questionnaire respondents were more myopic (P = 0.02), but otherwise did not differ significantly from nonrespondents. In multivariate models, older age (P < 0.001), more near work (P = 0.02), and higher population density (P = 0.003), but not development index, parental education, or time outdoors were significantly associated with more myopic refractive error.
  
  
  CONCLUSIONS
  Higher population density appears to be associated with myopia risk, independent of academic activity, time spent outdoors, familial educational level, or economic development, factors that have been thought to explain higher myopia prevalence among urban children. Mechanisms for this apparent association should be sought.
authors.affiliation: Joint Shantou International Eye Center, Shantou, Peoples Republic of China.
authors.forename: Mingzhi
authors.initials: M
authors.lastname: Zhang
authors.forename: Liping
authors.initials: L
authors.lastname: Li
authors.forename: Lizhen
authors.initials: L
authors.lastname: Chen
authors.forename: Jack
authors.initials: J
authors.lastname: Lee
authors.forename: Jiasi
authors.initials: J
authors.lastname: Wu
authors.forename: Amy
authors.initials: A
authors.lastname: Yang
authors.forename: Connie
authors.initials: C
authors.lastname: Chen
authors.forename: Daocheng
authors.initials: D
authors.lastname: Xu
authors.forename: Dennis S C
authors.initials: DS
authors.lastname: Lam
authors.forename: Abhishek
authors.initials: A
authors.lastname: Sharma
authors.forename: Sian
authors.initials: S
authors.lastname: Griffiths
authors.forename: Yang
authors.initials: Y
authors.lastname: Gao
authors.forename: Nathan
authors.initials: N
authors.lastname: Congdon
mesh_terms: D000293:Adolescent; D044466:Asian People / Q000208:ethnology; D002648:Child; D002681:China / Q000453:epidemiology; D005260:Female; D006801:Humans; D008297:Male; D009216:Myopia / Q000453:epidemiology*; D011156:Population Density; D015995:Prevalence; D012029:Refraction, Ocular / Q000502:physiology; D012307:Risk Factors; D012424:Rural Population / Q000706:statistics & numerical data*; D011795:Surveys and Questionnaires; D014505:Urban Population / Q000706:statistics & numerical data*; D014792:Visual Acuity / Q000502:physiology