Time spent in outdoor activities in relation to myopia prevention and control - a meta-analysis and systematic review

25 articles pooled from 51 with relevant data; 23 of 25 involved children

Mostly school-aged children across East Asia, Australia, Europe and North America

Pools clinical trials, longitudinal cohorts and cross-sectional studies together, so the aggregate inherits the confounding of the observational majority - graded C on that basis. Reports clinical-trial and observational strata separately, which is what makes it usable. Dose-response analysis performed.

Protective effect on INCIDENT myopia in clinical trials (RR 0.536, 95% CI 0.338-0.850) and in longitudinal cohorts (RR 0.574, 95% CI 0.395-0.834); cross-sectional prevalent myopia OR 0.964 (95% CI 0.945-0.982). Pooled trials show a reduced myopic shift of -0.30 D over 3 years (95% CI -0.18 to -0.41). The critical dissociation: when analysis is restricted to children who are ALREADY myopic, dose-response finds no relationship between time outdoors and myopic progression (R-squared = 0.00064). Outdoor time prevents onset; it does not treat established myopia.

High for the cross-sectional and cohort strata pooled here - children who spend more time outdoors have parents and genotypes that differ from those who do not, and parental myopia is one of the strongest predictors of child myopia. The clinical-trial stratum is the only genuinely unconfounded estimate in the paper.