August 6, 2012 — Increased time spent outdoors may be an effective strategy for reducing the risk for myopia and myopia progression among children and adolescents, according to the findings of a meta-analysis.
Justin C. Sherwin, MBBS, MPhil, from the University of Cambridge in the United Kingdom, and colleagues published their findings online July 20 Ophthalmology.
"This manuscript is the first article to summarize, both qualitatively and quantitatively, the relationship between time spent outdoors and myopia in children and young adults," Dr. Sherwin told Medscape Medical News by email. "Overall, our findings extend results of individual studies by providing an overall pooled estimate and comprehensive synthesis of the available literature on this subject."
The authors searched 4 databases (Medline, Web of Science, Embase, and Cochrane) for studies that examined the association between time spent outdoors and the development or progression of myopia among children and adolescents aged 20 years or younger. They included studies in the meta-analysis on the basis of 4 criteria: time spent outdoors was reported in a similar manner as defined by the authors, reported myopia (prevalent or incident) or myopic progression was the outcome measure, the association was reported either as an effect estimate with 95% confidence intervals (CIs) or standard errors or calculated from raw data contained in the article, and the study included participants no older than 20 years.
Among the 2912 articles identified in the database search, 7 cross-sectional studies met the criteria for inclusion in the meta-analysis. The authors excluded studies that did not precisely define myopia, animal studies, randomized trials, prospective cohort studies, and conference abstracts. The included studies comprised 9885 participants, and all but 1 study included school-age children.
A random effects meta-analysis revealed that an additional hour spent outdoors per week reduced the risk for myopia (pooled odds ratio [OR], 0.981; 95% CI, 0.973 - 0.990; P OR < .001; I 2, 44.3%; P heterogeneity = .092). If the benefit associated with 7 additional hours outside per week is equal to that of 1 additional hour per day, the authors estimate that the benefit of 1 additional hour per days is 0.867 (95% CI, 0.811 - 0.930).
In subgroup analysis, a stronger protective association was found between outdoor time and myopia risk in non–East Asian studies (OR, 0.966; 95% CI, 0.944 - 0.989; P OR = .003) than in East Asian studies (OR, 0.987; 95% CI, 0.979 - 0.995; P OR = .002), although greater heterogeneity was observed among the non–East Asian studies (I 2, 64.9% [P heterogeneity = 0.036] vs I 2, 0.0% [P heterogeneity = 0.608]). Additional subgroup analysis also revealed slight effects of the age group of the study and the method by which refractive effort was measured.
Publication bias was not suggested by either Begg's test (P = .652) or Egger's test (P = .293). The limitations of the study included the cross-sectional design of the included studies and the low number of studies included in the analysis.
Despite the modest but significant benefits, Dr. Sherwin cautions that additional evidence of a relationship between time spent outdoors and myopia is needed. "At this stage, it is still too early to make recommendations to parents and schools about increasing outdoor exposure to prevent myopia," Dr. Sherwin told Medscape Medical News.
"We need additional information about the risk of developing myopia in people without myopia according to outdoor activity level. We also need further data about the association between increasing outdoor activity and progression of myopia."
Don Mutti, OD, PhD, from the Ohio State University School of Optometry in Columbus, echoes the concerns of Dr. Sherwin: "It is still not clear whether time outdoors only influence the risk of onset of myopia, the rate or progression of myopia once you have it, or both," Dr. Mutti told Medscape Medical News by email. "The actual source of benefit needs to be understood before effective therapies can be developed." Dr. Mutti was not involved in the study.
Supported by the Centre for Eye Research Australia and the Victorian Government. One coauthor is a recipient of the Pfizer Australia Senior Research Fellowship. The authors and commentator have disclosed no other relevant financial relationships.
Ophthalmology. Published online July 20, 2012. Abstract
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