Time outdoors prevents myopia in hyperopic children, but protection is weaker in premyopic children: a post-hoc analysis of a cluster-randomised trial

Aims: To investigate the effect of time outdoors on myopic shift among premyopic children. Methods: Post-hoc analysis was nested in a cluster-randomised trial of the Shanghai Time Outside to Reduce Myopia (STORM) study. 6- to 9-year-old participants without myopia from the STORM study, who wore wristwatches to monitor time outdoors from 2017 to 2018, were included. Participants were all examined with cycloplegic refraction. Daily time outdoors was objectively monitored with the wearable smartwatch. Premyopia was defined as a cycloplegic spherical equivalent (SE) from −0.50 to +0.75 (inclusive) dioptres (D). Myopic shift was SE change from baseline to 1 year follow-up. Results: Among 3194 participants (1369 premyopic; mean age 8.2±0.6 years; 49.5% boys), there were no statistical differences between premyopic and hyperopic children in time outdoors (p=0.303). Hyperopes showed reduced myopic shift with increasing outdoor time (plateau at about 120 min/day). However, premyopes exhibited a J-shaped relationship between time outdoors and myopic shift. In comparison to the subgroup with daily time outdoors <60 min, the difference in SE change in the other subgroups was not statistically significant (61–90 min/d: −0.03 (95% CI −0.10 to 0.05); 91–120 min/d: −0.03 (95% CI −0.11 to 0.05)). The reduced myopic shift was only observed with time outdoors >120 min/d, although it was still not statistically significant (>120 min/d: 0.04 (95% CI −0.05 to 0.14)). Conclusions: Among premyopic children, increased time outdoors has a limited protective effect on myopic shift, suggesting longer duration of time outdoors or additional interventions to prevent or delay myopia onset in this population.