Authors: Eppenberger LS, Sturm V.
Geographical coverage: India, China, Taiwan, Europe and the USA
Sub-sector: Risk factor/progression of myopia
Equity focus: Children and adolescents
Study population: General population aged 6-18 years.
Review type: Other review
Quantitative synthesis method: Narrative synthesis
Qualitative synthesis method: Not applicable
Background: The development of myopia as a refractive disorder seems to hold multifactorial causes. Among others, increased time exposed to natural light outdoors is regarded as a possible effective preventive measure against myopia development.
Objectives: The objective of this review is to analyse and summarise the evidence investigating the association between time outdoors and myopia prevalence and progression.
In summary, authors found that increasing time exposed to outdoor light seems to be a simple and effective measure to decrease myopia prevalence.
Authors included two cross-sectional studies, seven prospective cohort studies and three intervention studies, representing the data of a total of 32,381 participants. It should be noted that an exception to inclusion concerning age range of participants (6 to 18 years) was made for one article, due to the association between outdoor time and myopia prevalence, and it has been frequently cited and the mean age of participants still feeds into the predefined criterion of age range.
11 of the 12 included in this study was published by the review Sherwin et al. The included studies represent the data of a total of 32,381 study participants. Forty-five per cent of the study participants came from China and 30% from India. Only 10% represent both Europe (6%) and the USA (3%). The two interventional studies were conducted in China and the third study was carried out in Taiwan.
Authors found that the majority of the studies found an inverse association between myopia incidence/prevalence and increased time outdoors. The association between time outdoors and myopia progression on the other hand remains debatable; one recent randomised controlled trial indicated a protective value of increased time outdoors for further progression in myopic children. Authors further note the need of future investigation to better define and quantify outdoor time and its effects on myopia.
Inclusion criteria were defined for observational studies, that is, cross-sectional and cohort studies, and intervention studies. Other types of studies such as review publications and comments, as well as experimental studies with animals were treated separately. For cross-sectional studies, the number of participants in the individual study needed to be 1,000 or more, whereas cohort studies were included if their sample size was n ≥ 500. Children and adolescents aged 6-18 years were included. Only papers in English or German language were selected. For inclusion, the association between outdoor time in relation to prevalence or progression of myopia had to be precisely described. Lastly, the measurement procedure of myopia needed to be objective, therefore, authors excluded studies in which myopia was determined by means of survey or indirectly assessed (for example, spectacle wearing).
The literature search was performed in two databases, PubMed and the Cochrane Library. Articles published between October 2008 and January 2019 were considered. The references of retrieved publications were used and alerts for new publications were saved in PubMed to assure that no relevant new studies were missed between the main search point and the analysis of the literature.
Authors extracted all necessary data of included studies and assessed the quality of studies based on the following variables: (1) the process of selection of study participants, (2) measurement methods of exposure (outdoor time) and outcome (myopia); (3) description and number of possible biases; and (4) the effect estimate of the association.
To highlight the findings of this review, authors conducted a comparison to the two previously performed systematic reviews on the association between time spent outdoors and myopia in children. The articles which were selected for this review were further labelled if they had already been part of one or both of the earlier reviews. Retrieved comments or statements on some of the highly cited studies in this field of research were summarised and also included in this review.
Most of the studies were conducted in Asian countries, which can be explained by the region’s relatively greater extent of the myopia.
Geographic focus: Authors include studies from different income settings. Given the findings of studies, they generalise that outdoor time may have a protective influence in the progression of myopia.
Summary of quality assessment:
This review appropriately synthesised findings of included studies narratively given the heterogeneity of studies. However, important limitations were identified. Literature searches were not comprehensive enough to ensure that all relevant studies were identified and therefore included in the review. Furthermore, authors did not avoid language bias and searched for studies covering the previous 10 years. Authors did not report methods used to screen studies for inclusion and extract data of included studies, to understand if bias was avoided. Although authors describe narratively limitations of included studies, it is unclear which studies are at high or low risk of bias. Therefore, low confidence was attributed to the conclusions of this study.