Time spent in outdoor activities in relation to myopia prevention and control: a meta-analysis and systematic review

Methodological quality of the review: High confidence

Author: Shuyu Xiong, Padmaja Sankaridurg, Thomas Naduvilath, Jiajie Zang, Haidong Zou, Jianfeng Zhu, Minzhi Lv, Xiangui He and Xun Xu

Region: China, Taiwan and Singapore, Australia, UK, France, USA, Turkey

Sector: Myopia

Subsector: Myopia prevention and control

Equity focus: No

Study population: School children

Type of programme: School based

Review type: Other review

Quantitative synthesis method: Meta-analysis and systematic review

Qualitative synthesis method: Not applicable

Background: Myopia is already a major public health concern in many countries in East and South-East Asia. In addition, progressive myopia is associated with increased risks of retinal detachment, cataract, glaucoma and even blindness. In contrast, with respect to preventing the onset of myopia, to date, outdoor time has been the only factor that was found to be protective. However, its role in controlling progression in already myopic eyes is not conclusive. We therefore performed a systematic review, followed by a meta-analysis and a dose-response analysis, that considers and includes data from more recently published clinical trials.

Objectives: The objective of this review was to evaluate the effect of outdoor time on the risk of incident/prevalent myopia, the risk of a myopic shift in refractive error and the risk of progression in myopic eyes.

Main findings: A total of 51 references met the criteria for inclusion in this meta-analysis. Of the 51 articles with relevant data, 25 were included in the meta-analysis and dose–response analysis. Briefly, the articles included four clinical trials (2,945 participants), eight cohort studies (8,363 participants) and 13 cross-sectional studies (23,112 participants). Twenty-three of the 25 articles involved children. Risk ratio (RR) for binary variables and weighted mean difference (WMD) for continuous variables were conducted. Mantel-Haenszel random-effects model was used to pool the data for meta-analysis. Statistical heterogeneity was assessed using the I² test, with I² ≥ 50% considered to indicate high heterogeneity. Additionally, subgroup analyses (based on participant’s age, prevalence of myopia and study type) and sensitivity analyses were conducted. A significant protective effect of outdoor time was found for incident myopia (clinical trials: risk ratio (RR)=0.536, 95% confidence interval (CI)=0.338 to 0.850; longitudinal cohort studies: RR=0.574, 95% CI=0.395 to 0.834) and prevalent myopia (cross-sectional studies: OR=0.964, 95% CI=0.945 to 0.982). With dose–response analysis, an inverse nonlinear relationship was found, with increased time outdoors reducing the risk of incident myopia. Also, pooled results from clinical trials indicated that when outdoor time was used as an intervention, there was a reduced myopic shift of -0.30 D (in both myopes and non-myopes), compared with the control group (WMD=-0.30, 95% CI=-0.18 to -0.41) after three years of follow-up. However, when only myopes were considered, dose-response analysis did not find a relationship between time outdoors and myopic progression (R²=0.00064).

Methodology: Inclusion criteria consisted of: 1) human studies that investigated the relationship between outdoor time and myopia; 2) investigated effect of outdoor time in relation to the prevalence and incidence of myopia and/or myopic shift or progression; 3) reported an effect estimate with a 95% CI or standard error (SE) or provision of sufficient data to calculate these values; and 4) reported specific increased amounts of time spent on outdoor activities (or the ability to calculate this parameter from the data provided) and the incidence of myopia and/or myopia progression; these studies were included in dose-response analysis. Two reviewers (S.Y.X. and X.G.H.) independently assessed the studies for possible eligibility. A search of PubMed, Embase and the Cochrane Library was undertaken for articles published up to 30 December 2015, including the search terms: outdoor*, outside, in combination with myopia, nearsightedness, shortsightedness, near-sight, near-sighted, near-sightedness, short-sight, shortsighted, short-sightedness and refractive error. Each primary article obtained from the search was studied to determine its potential inclusion in the review, with no restriction placed on the language of the article. Data was extracted by two reviewers (S.Y.X. and J.J.Z.) independently for authors, year of publication, location, sample size, subject age, follow-up duration, method of assessment of outdoor activities, adjusted covariates in multivariable analysis, outcomes and their 95% CIs or standard deviations, and information needed to evaluate study quality. The methodological qualities of the included studies were assessed independently by both reviewers using an adapted Downs and Black checklist. Analyses were performed to determine the association between outdoor time and 1) risk of incident/prevalent myopia from pooled estimates and dose-response analysis; 2) risk of a myopic shift in refractive error (both myopes and non-myopes) from pooled estimates; and 3) risk of progression of myopia (in myopic eyes) from dose-response analysis. Statistical analyses were performed with STATA version 12.0 software. Statistical heterogeneity was assessed using the I² test, with I2≥50% considered to indicate high heterogeneity. For estimates with high heterogeneity, ±2xTau, which is the standard deviation across studies, is reported to present the approximate 95% range of the underlying effects. Statistical significance was maintained at P<0.05.

Applicability/external validity: The authors highlighted some limitations which need to be considered for external use of data: first, the heterogeneity was high among the studies, especially the cross-sectional studies. Dose-response analysis of myopia onset in our review only suggested the existence of an inverse nonlinear relationship with increased time outdoors and myopia onset; however, the threshold or the amount of time outdoors required to reduce the incidence of myopia remains to be determined. Finally, conversion of the original effect estimates based on hours of exposure per day to estimates based on hours of exposure per week, might have potentially resulted in a bias, as the time spent outdoors could have varied between weekdays versus weekends, and such information was available from a limited number of studies.

Geographic focus: This review was conducted in high income countries.

Summary of quality assessment:

There is high confidence in the conclusions about the effects of this study, as rigorous methods were used to conduct the review.