Increased time spent outdoors during youth is associated with reduced myopia risk

Cambridge, UK-Available observational evidence indicates that increased time spent outdoors is inversely associated with prevalent myopia, according to the findings of a systematic review and meta-analysis presented by Anthony Khawaja, MBBS, and colleagues.

The meta-analysis, which included data from seven cross-sectional studies with a total of 9,885 participants, found a statistically significant, 2% reduced odds of myopia per extra hour spent per week outdoors. Results of a weighted mean difference analysis showed subjects with myopia spent significantly less time outdoors than those without myopia, with the average difference between groups being 3.7 hours per week, said Dr. Khawaja, PhD student, Department of Public Health and Primary Care, University of Cambridge, UK.

“Myopia is a common condition that, while easily treated with glasses or contact lenses, is associated with significant cost and significant visual morbidity,” Dr. Khawaja said. “A recent rapid increase in prevalence, particularly in eastern Asian countries, suggests a role for an environmental risk factor. If such a risk factor is found and it is reversible, there is the possibility to develop public health strategies to decrease myopia risk.

“More evidence is needed from cohort or interventional studies to confirm the association identified in our research and to establish a causal relationship between time spent outdoors and protection against myopia. In addition, further study would be needed to elucidate which aspects of time spent outdoors are important in order to inform potential public health strategies,” he noted.

The research was led by Justin C. Sherwin, MBBS, MPhil, Research Fellow, Department of Public Health and Primary Care, University of Cambridge, UK. Papers for possible inclusion in the systematic review were identified by searching four databases (Medline, EMBASE, Web of Knowledge, Cochrane). Selected papers were observational or interventional human studies with participants younger than 20 years of age. However, to be included in the meta-analysis, studies needed to report an effect estimate with 95% confidence interval of time spent outdoors as a risk factor for myopia.

The systematic review identified 20 relevant studies, of which eight met the inclusion criteria. Aside from the seven cross-sectional studies included in the meta-analysis, there was one cohort study.

“The 12 studies that did not meet the inclusion criteria for meta-analysis were mostly excluded because of different effect estimates or the presence of just a p value,” Dr. Khajawa said. “However, of the 12 excluded studies, nine suggested time spent outdoors had a protective effect while three found no significant association.”

A random-effects meta-analysis was conducted because of expected heterogeneity of the studies, but several sensitivity analyses were also conducted because of study heterogeneity. One sensitivity analysis analyzed data separately for three studies conducted in Eastern Asia and four studies conducted in another geographic region. Three other sensitivity analyses were conducted excluding 1) a single study that included only very young children (<5 years), 2) a study where myopia was self-reported, and 3) two studies in which only outdoor sports were considered as outdoor activity.

The findings of the sensitivity analyses agreed with the overall finding and consistently showed that increasing time spent outdoors was associated with significantly reduced odds of myopia by 1% to 2% for each extra hour spent per week. Dr. Khawaja noted that it cannot be determined from the research he presented whether more outdoor activity actually prevents myopia or if the inverse association simply reflects that subjects with myopia prefer spending less time outside. However, results from prospective studies have indicated a possible causal role.

He also suggested possible mechanisms that could explain an association between time spent outdoors and myopia.

“There are several possible mechanisms by which increased time outdoors may protect against myopia,” Dr. Khawaja said. “This includes increased dopamine release from the retina in response to bright light; increased light intensity outdoors resulting in pupil constriction, decreased blur, and decreased eye growth; and the low accommodative demand for distance vision when outdoors.”

He also postulated that UV light exposure or increased physical activity could play a role.

Dr. Khawaja observed that studies would need to elucidate what component of outdoor activity influences myopia development before public health suggestions could be issued and that any recommendations for myopia protection will also need to balance the benefit from spending more time outdoors against the adverse effects of UV exposure for increasing the risks of cataract and some skin cancers. Currently, there are two prospective, randomized clinical trials registered on www.clinicaltrials.gov underway in eastern Asia that are investigating the role of outdoor activity for myopia prevention.

The paper has been accepted for publication in Ophthalmology. The authors have no relevant financial interests in the material discussed.

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