Significance of Outdoor Time for Myopia Prevention: A Systematic Review and Meta-Analysis Based on Randomized Controlled Trials

Methodological quality of the review: Medium confidence

Author: Kai Cao; Yue Wan; Mayinuer Yusufu; Ningli Wang

Region: China, Singapore

Sector: Myopia

Subsector: Myopia prevention

Equity focus: No

Study population: Children

Type of programme: Community and school based

Review type: Other review

Quantitative synthesis method: Systematic review and meta-analysis

Qualitative synthesis method: Not applicable

Background: Myopia, especially high myopia, would cause damage in the choroid, retina and sclera, thereby leading to vision loss. Although refractive error correction can help improve visual acuity, the pathology of myopia – a global issue – remains unclear and myopia progression, as well as concomitant fundus progression, remains uncontrolled. Under such circumstances, prevention of myopia is of great significance and thus should be prioritised.

Objectives: The objective of this study was to explore whether outdoor time has positive significance for myopia prevention.

Main findings: Five RCTs with 3,014 subjects were included. Subjects’ age ranged from 6 to 12 years, and the follow-up duration ranged from 9 to 36 months. Spherical equivalent error (SER) of the outdoor group was larger than that of the control group, and the pooled mean difference (MD) was 0.15 (95% CI 0.06-0.23) diopter (D). The change in SER of the outdoor group was smaller than that of the control group, with a pooled MD of 0.17 (95% CI 0.16-0.18) D. New myopia cases in the outdoor group were fewer than that of the control group, and the pooled risk ratio was 0.76 (95% CI 0.67-0.87). The change in axial length of the outdoor group was smaller than that of the control group, and the pooled MD was -0.03 (95% CI -0.03 to -0.03) mm. For all analysed outcomes, there was no heterogeneity across included studies (I²=0%) and there was no publication bias either.

Methodology: Inclusion criteria consisted of: 1) RCTs; 2) in which participants accepted outdoor activity as an intervention method to increase outdoor time for myopia prevention. Randomisation of participants at an individual level or a school cluster level was both acceptable; 3) as this review only focuses on the effect of outdoor time on myopia prevention, only trials using outdoor activity as an intervention method were included. The primary outcome for this review was refractive error (SER) that measured at the year of last follow-up. Secondary outcomes 1. Mean change in SER; 2. Number of new myopia cases; 3. Change of axial length.

Databases of PubMed, ScienceDirect, the Cochrane Library, the Chinese National Knowledge Infrastructure and the Wanfang Database were searched. The following terms or their combinations were used: myopia, prevention, control, random, randomised, randomisation, intervention, outdoor. Two review authors, one clinician and one methodologist, assessed the titles and abstracts of all papers independently in the initial screening process. Two reviewers extracted the data for the primary and secondary outcomes according to data collection forms designed by the authors’ team. Data measured at the last follow-up year were collected. The Risk of Bias tool recommended by the Cochrane Collaboration was applied to assess risk of bias of each study. Analysis: Mean differences (MDs) were reported for continuous outcome measures, and risk ratios (RRs) for dichotomous outcomes. Before estimating the pooled effect, the heterogeneity across studies was assessed first by Q test and I² statistic. MD with 95% CI was applied to estimate continuous outcomes, and RR with 95% CI was extracted to estimate categorical outcomes. All the analysis was performed with open-source R programme.

Applicability/external validity: The authors highlighted some potential bias, mainly selection bias.

Geographic focus: This review just included articles from two countries: China and Singapore.

Summary of quality assessment:

Overall, medium confidence was attributed in the effects of this study. Although authors used rigorous methods to analyse findings of included studies, authors did not conduct thorough searches of the literature to ensure that all relevant studies were identified. In addition, it is not clear if the search period was comprehensive enough that relevant literature was not omitted in the review.