Influence of outdoor time on the spherical equivalent and axial length in childhood myopia: A meta-analysis

Abstract

This meta-analysis investigates the effectiveness of outdoor activities in reducing the onset of myopia in children and adolescents by analysing changes in axial elongation and spherical equivalent refractive error. Following PRISMA guidelines and registered in PROSPERO (CRD42024592971), the study included randomized controlled trials (RCTs) and observational studies. The eligibility criteria targeted children and adolescents aged 6 to 18 years with varying levels of outdoor exposure. Primary outcomes were axial elongation and spherical equivalent change. Studies were assessed for quality using GRADE and AMSTAR-2 tools, and data were analysed using Review Manager 5.4, with random-effects models applied when heterogeneity was significant. Fifteen studies (9 RCTs and 6 observational) were included, with a total of 16 597 participants. Outdoor activities significantly reduced or delayed the onset of myopia, with a mean axial length difference of -0.08 mm per year (95% CI: -0.09 to -0.07) and a spherical equivalent difference of 0.16 diopters per year (95% CI: 0.07 to 0.25). These differences were observed after 1 year of intervention and sustained for up to 3 years, with daily outdoor exposure ranging from 40 to 120 min. Heterogeneity was moderate to high, but sensitivity analyses confirmed the robustness of the results. While higher outdoor exposure appeared more effective in reducing myopia progression, the certainty of this evidence was rated as low due to suspected publication bias, as indicated by the GRADE analysis. Outdoor activities, especially with prolonged exposure to intense light, effectively reduce or delay the onset of myopia in children and adolescents. This study emphasizes the importance of light intensity in maximizing the benefits of outdoor interventions and highlights regional differences in effectiveness, suggesting that environmental factors play a significant role in the outcomes.