The measurement of time spent outdoors in child myopia research: a systematic review

Methodological quality of the review: Low confidence

Author: Jing Wang, Xian-Gui He, Xun Xu

Region: Japan, USA, China, Singapore

Sector: Myopia

Subsector: Measurement of time spent outdoors and myopia

Equity focus: No

Study population: Children

Type of programme: Community, School and clinical based

Review type: Other review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable

Background: Early onset myopia is associated with high myopia later in life, requiring expensive treatment for myopia and multiple associated sight-threatening pathologies, such as cataract, glaucoma, retinal detachment and myopic retinal degeneration. However, knowledge of the specific pathological mechanism of myopia occurrence is limited. Thus, identifying modifiable risk factors that may promote myopia onset, and establishing practical and economical intervention measures, are important. Recent clinical trials showed that increasing the number of hours of outdoor activities could provide a protective effect against the onset of myopia. Therefore, it has become particularly important to select a method to collect accurate and rigorous data on time spent outdoors or time spent performing outdoor activities when children are under natural conditions, and minimise the risks of error and bias.

Objectives: The objective of this review was to summarise the methods most commonly used for outdoor (activity) time data collection, particularly focusing on the steps of implementation, strengths and limitations of each method.

Main findings: A total of 70 articles were included in our systematic review. In total, three kinds of outdoor time measurements were discussed. Questionnaires have the longest history and are the most thoroughly revised instruments for assessing time spent outdoors, but recall bias is their most substantial drawback. Global positioning system (GPS) tracking can distinguish between indoor and outdoor locations, but its utility is limited due to several factors, such as subject compatibility. Light exposure measurement devices are newly emerging, but all of these devices require good subject cooperation. Further efforts and exploration are needed to develop better methods, and new tools to record exposure to the outdoors in real time.

Methodology: Studies were included if they met the following criteria: 1) outdoor activity was investigated in relation to the prevalence, incidence or development of myopia; 2) the specific measurement of time spent outdoors was described; 3) the study was a clinical trial that evaluated the progression of myopia. Literature search was performed on PubMed, Embase and the Cochrane Library from 1 January 1990 to 31 January 2017. The terms outdoor, outside, outdoor activity, outside activity, outdoor time, outside time, outdoor AND measurement, and outside AND measurement, in combination with myopia, nearsightedness, shortsightedness, near-sight, near sighted, near-sightedness, short-sight, short-sighted, short-sightedness and refractive error were included in the keyword search. The reference list of each primary article identified in the initial search was scrutinised for additional potentially relevant studies. Two reviewers independently assessed studies for possible eligibility, and any inconsistencies were resolved by consensus.

Applicability/external validity: Not discussed.

Geographic focus: Not discussed.

Summary of quality assessment:

Low confidence was attributed in the conclusions about the effects of this study, as important limitations were identified. Literature searches were not comprehensive enough to ensure that all relevant studies were identified and included in the review. In addition, data extraction processes were not rigorous enough to ensure that biases were avoided; quality assessments of included studies were not conducted; and a summary on the characteristics of included studies was not provided.