Dose–Response Relationship of Outdoor Exposure and Myopia Indicators: A Systematic Review and Meta-Analysis of Various Research Methods

Methodological quality of the review: Low confidence

Author: Ciao-Lin Ho, Wei-FongWu and Yiing Mei Liou

Region: Australia, UK, USA, China (rural and urban) and Taiwan

Sector: Myopia

Subsector: Relationship of outdoor exposure and myopia

Equity focus: No

Study population: Children and adolescents

Type of programme: School based

Review type: Other review

Quantitative synthesis method: Systematic review and meta-analysis

Qualitative synthesis method: Not applicable

Background: Myopia has become a major public health problem worldwide, and its prevalence has increased rapidly, especially in East Asia. Myopia prevalence among children in East Asian and Western countries, respectively, is 80-90% and 2-29.4%, and it is expected to increase in the future. In addition to direct costs of approximately $148 (US dollars) per year per child. Myopia management consists of a combination of drugs, glasses, contact lenses, surgery, and behavioral interventions. It was reported that outdoor light exposure is related to myopia incidence and prevalence, and some evidence suggests outdoor light exposure slows myopia progression. However, light has less effect on lens-induced myopia. Although most studies have demonstrated that physical activity is not related to myopia, one study indicated that outdoor time and physical activity were both confirmed to be related to myopia, and outdoor time was suggested to be more effective in preventing myopia Therefore, the effect of outdoor light exposure on individuals with myopia requires further investigation. Few studies have analysed the effectiveness of outdoor light exposure intervention programs in slowing myopia progression.

Objectives: This systematic review and meta-analysis aims to evaluate the effects of outdoor light exposure on myopia.

Main findings: After exclusion based on these criteria, 13 studies were included in the analysis: four cross-sectional studies, three cohort studies, and six intervention studies. A total of 15,081 children were enrolled. Participants in these studies were school-aged children between 4 and 14 years old at baseline. 13 articles were judged to be of moderate to high quality. Comparing the high-level and low-level exposure time groups from the cohort studies revealed that the outdoor time of the high-level group resulted in a significantly reduced risk of myopia incidence (OR=0.57, 95% CI: 0.35-0.92). The intervention studies revealed that outdoor light exposure time had a significant protective effect on the risk of myopia incidence (OR=0.5, 95% CI: 0.37-0.69). The overall meta-analysis yielded a pooled OR for myopia of 0.85 (95% CI: 0.80-0.91) per additional hour of time spent outdoors per week. The pooled estimates indicated that the mean difference in the change of SER showed significant reduction (mean difference = 0.15 D, 95% CI: CI: 0.09-0.22) in Figure 3. The mean differences in the change of SER for children without and with myopia were 0.13 and 0.15 D/year, respectively, and 0.16 D/year. Regarding the effect of reducing myopic progression in intervention studies, compared with control groups, the percentages of reduction were 50% (incidence/prevalence of myopia), 32.9% (SER), and 24.9% (axial length) in the intervention groups. The analysis of the dose–response effect of intervention programs revealed linear curves. This means that the variance of the benefits in the reductions of myopia incidence, SER, and axis elongation predicted by the outdoor light exposure time were 93.78%, 71.34%, and 94.35%, respectively. These results indicate that longer outdoor light exposure time at school was related to a stronger prevention effect against myopia incidence, SER, and axial elongation. Compared with the control groups, 50% lower myopia incidence, SER, and axis elongation required increases of 7, 10, and 10 h/week, respectively, of outdoor light exposure time at school

Methodology: The literature collected comprised studies with participants aged 4 to 14 years at baseline, and studies investigating the relationship between outdoor light exposure time and myopia in terms of prevalence, incidence, axial length, or spherical equivalent refraction. Cross-sectional, cohort, and intervention studies with long-term follow-ups were selected.

The Cochrane Handbook for Systematic Reviews was used in the meta-analysis. The search was conducted using the Cochrane Library, Medline, CINAHL, PubMed, China Academic Journals full-text database and National Digital Library of Theses and Dissertations in Taiwan for articles published from 2000 to 2019. Each primary article obtained from the search was studied to determine its potential for inclusion. Article selection was conducted according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-analysis) guidelines. The quality of the intervention studies was assessed using the Cochrane Bias Risk Assessment tool. The quality of the cohort studies and cross-sectional studies was assessed based on the standard Newcastle-Ottawa Scale (NOS). For data analysis, odds ratios (ORs) with 95% confidence intervals (CIs) and the mean difference in SER and axial length was used to assess myopia incidence and prevalence as well as myopia progression. The ­X² test was used to determine the heterogeneity among the results of the included studies. When statistical homogeneity was detected in dose–response analysis, a simple linear regression model was used to explore the relationship.

Applicability/external validity: Regarding the external validity, the authors reported some limitations which affect the external applicability of the results of this review. The collected studies of this review included participants from East Asian, Caucasian and Hispanic ethnic groups; the six intervention studies identified were conducted only in Asia. Finally, the authors concluded that for more generalisable scientific evidence, future research should refer to the results of this study to develop and conduct a large-scale, multinational, blinded, and randomised controlled trial including Asian children and children from other ethnic groups.

Geographic focus: The author reported collected studies of this review included participants from East Asian, Caucasian and Hispanic ethnic groups; the six intervention studies identified were conducted only in Asia, which can affect the geographic extension of the results of this review.

Summary of quality assessment:

Overall, this review was attributed low confidence, as important limitations were identified. Literature searches were not comprehensive enough to ensure that all relevant studies were identified, and it is not clear if the searches were conducted from inception. In addition, authors do not report methods used to screen and extract data of included studies.