Correlations in refractive errors between siblings in the Singapore Cohort Study of Risk factors for Myopia.

BACKGROUND The prevalence of myopia in parts of South East Asia has risen dramatically over the past 1-2 generations, suggesting that environmental factors may be particularly important determinants of refractive development in these populations. AIM To assess the contribution of familial factors (shared genes and/or shared family environment) to refractive error and ocular component dimensions of school-aged children in Singapore. METHODS Data were available for 315 children who had one or more siblings also participating in the Singapore Cohort Study of the Risk factors for Myopia (SCORM). Refractive error and ocular biometric parameters were measured under cycloplegia at baseline when children were 7-9 years, and at yearly follow-up sessions for the next 3 years, using consistent clinical procedures. The time children spent performing a variety of nearwork-related tasks was obtained from questionnaires. Familial influences were assessed by calculating between-sibling correlations. RESULTS After adjusting for age and sex, the between-sibling correlation in refractive error was 0.447 (95% CI 0.314 to 0.564), suggesting that familial factors account for 63-100% of the variation in the cohort. The between-sibling correlation for 1-year change in refractive error was similarly high, at 0.420 (95% CI 0.282 to 0.543). All ocular component dimensions were correlated significantly between siblings, especially for corneal curvature and vitreous chamber depth--the major structural determinants of refraction. The amount of time siblings spent engaged in nearwork tasks (reading, watching TV, playing video games, computing) and in outdoor activities was also highly correlated between siblings (p<0.001). CONCLUSION Shared genes and/or shared environment are important factors in the refractive development of children in Singapore. Because the time spent in nearwork tasks is highly correlated between siblings, epidemiological studies will benefit from precise, quantitative measures of refractive error in parents and more distant relatives in order to begin to dissociate genetic and environmental sources of variation.