Time outdoors and physical activity as predictors of incident myopia in childhood: a prospective cohort study

Abstract

Purpose: Time spent in "sports/outdoor activity" has shown a negative association with incident myopia during childhood. We investigated the association of incident myopia with time spent outdoors and physical activity separately.

Methods: Participants in the Avon Longitudinal Study of Parents and Children (ALSPAC) were assessed by noncycloplegic autorefraction at ages 7, 10, 11, 12, and 15 years, and classified as myopic (≤-1 diopters) or as emmetropic/hyperopic (≥-0.25 diopters) at each visit (N = 4,837-7,747). Physical activity at age 11 years was measured objectively using an accelerometer, worn for 1 week. Time spent outdoors was assessed via a parental questionnaire administered when children were aged 8-9 years. Variables associated with incident myopia were examined using Cox regression.

Results: In analyses using all available data, both time spent outdoors and physical activity were associated with incident myopia, with time outdoors having the larger effect. The results were similar for analyses restricted to children classified as either nonmyopic or emmetropic/hyperopic at age 11 years. Thus, for children nonmyopic at age 11, the hazard ratio (95% confidence interval, CI) for incident myopia was 0.66 (0.47-0.93) for a high versus low amount of time spent outdoors, and 0.87 (0.76-0.99) per unit standard deviation above average increase in moderate/vigorous physical activity.

Conclusion: Time spent outdoors was predictive of incident myopia independently of physical activity level. The greater association observed for time outdoors suggests that the previously reported link between "sports/outdoor activity" and incident myopia is due mainly to its capture of information relating to time outdoors rather than physical activity.

Figure 1.

Kaplan-Meier survival curve for full dataset (N = 9109). The shaded region shows the 95% CI. Note the widening of the 95% CI beyond the age of 15 years, due to markedly reduced subject number for this period.

Figure 2.

Kaplan-Meier survival curves for subgroups of children from the full dataset. Subjects were grouped according to: (A) Time spent outdoors (N = 6961). (B) Mean CPM physical activity in whole week (N = 5733). (C) Minutes with sedentary counts (N = 5733). (D) Minutes of moderate/vigorous physical activity (N = 5733). (E) Time spent reading (N = 6947). (F) Number of myopic parents (N = 5675).