Key message
An intervention of spending more time outdoors may potentially reduce the onset of myopia. However, we are uncertain if the intervention might reduce the progression of myopia.
What is myopia?
Myopia, or nearsightedness, is a disease in which individuals see near objects clearly, but distant objects appear blurred. In recent years, myopia has become a major public health problem all over the world. Myopia is the main cause of poor sight globally. In addition, as myopia gets worse, other eye diseases that cause poor sight also increase; thus, an increase in myopia means an increase in the number of people with low visual acuity (that is, low clarity of vision resulting in an inability to see small details with precision). So, stopping the onset and worsening of myopia is important.
What did we want to find out?
Research has shown that more time spent outdoors could stop myopia from getting worse or prevent myopia from developing. Therefore, we performed this review to find out whether interventions to increase time spent outdoors could prevent the onset and slow the progression of myopia in children.
What did we do?
We searched the medical literature for randomised controlled trials comparing interventions to increase time spent outdoors with usual lifestyle in children. These are clinical trials in which participants are randomly assigned to treatment and control groups. They are known to provide the most reliable evidence on the effectiveness of a treatment. We compared the results from the literature and summarised the evidence. We assessed the level of confidence for each piece of evidence based on factors such as study methods, sample size and consistency of the results across studies.
What did we find?
Five studies met the criteria. The total number of participants was 10,733. Four of these studies involved school-based interventions to encourage children to spend more time outdoors. In those studies, the schools, which were selected to be as balanced as possible in terms of area and educational level, were randomly allocated to the intervention and control groups. The interventions included classroom time outdoors, routine for spending breaks outdoors, motivational tools for spending time outdoors and encouragement through messages via electronic media.
In summary, the results of this study suggest that interventions to increase the time spent outdoors may potentially reduce the onset of myopia. Although the results showed that interventions may slow myopia progression, our certainty in the results was low. The onset of myopia was assessed as the incidence of myopia (how frequently it happened), and the progression of myopia was assessed as the change in refractive errors (such as an abnormal-shaped eyeball, which prevents incoming light from focusing correctly on the retina (the back of the eye) to form a clear image) and axial lengths (the distance from the front to the back of the eye).
What are the limitations of the evidence?
The studies in this review monitored the children in the intervention and control groups for different times. Most school children were in the first and second grades of primary school, so it is unclear if the results are applicable to children younger or older than this. In addition, all studies were conducted in China and Taiwan, so it is not possible to conclude whether the results can be directly applied to other countries.
How up to date is this evidence?
The evidence in is up to date to August 2022.
The intervention methods varied from adopting outdoor activities as part of school lessons to providing information and motivation for encouraging outdoor activities.
The results of this review suggest that long-term interventions to increase the time spent outdoors may potentially reduce the development of myopia in children. However, although the interventions may also suppress the progression of myopia, the low certainty of evidence makes it difficult to draw conclusions. Further research needs to be accumulated and reviewed.
Myopia or nearsightedness is a type of refractive error. It causes people to see near objects clearly but distant objects as blurred. Good vision can be obtained if the refractive error is corrected properly but, where this is not possible, impaired vision will remain. The remaining myopia imposes a considerable personal and societal burden. In addition, the progression of myopia is more likely to be accompanied by other ocular diseases such as cataract, glaucoma and retinal detachment.
Myopia has emerged as a significant global public health problem in recent years. The World Health Organization (WHO) reported uncorrected or undercorrected myopia to be a major cause of visual impairment worldwide.
From both an individual and social perspective, it is important to prevent the onset of myopia and slow down its progression.
Observational studies have shown that children who spend more time outdoors have a lower incidence of myopia. Several other non-Cochrane systematic reviews have focused on the association between increasing children's outdoor activity time and the prevention of myopia. However, none of these systematic reviews were limited to randomised controlled trials (RCTs), as they included all types of study designs, including observational studies and non-RCTs, in addition to RCTs.
To assess the effects of interventions to increase outdoor time on the incidence and progression of myopia in children.
We searched CENTRAL, MEDLINE Ovid, Embase Ovid, ISRCTN registry, ClinicalTrials.gov, and the WHO ICTRP with no language restrictions. The databases were last searched on 24 June 2022.
We included RCTs and cluster-RCTs in which interventions were performed to increase the outdoor time for children with the aim of preventing the incidence and progression of myopia.
We employed the standard methods recommended by Cochrane and assessed the certainty of the evidence using GRADE. We considered the following outcome measures: mean change in refractive error from baseline, incidence of myopia, mean change in the axial length from baseline, mean change in unaided distance visual acuity from baseline, quality of life and adverse event.
We included five RCTs in this review, four of which were cluster-RCTs. The total number of participants was 10,733. The included participants were primary school children, most of whom were in first or second grade (aged six to nine years). Four cluster-RCTs involved school-based interventions to encourage children to spend more time outdoors. The interventions included classroom time outdoors, routine for spending recess outdoors, motivational tools for spending time outdoors, and encouragement through electronic information tools.
The intervention groups had less change in refractive errors in the direction of myopia; however, 95% confidence intervals (CIs) included no benefit or both benefit and harm at years one and three, and differences at year two included both clinically important and unimportant benefits (at 1 year: mean difference (MD) 0.08 dioptres (D), 95% CI −0.01 to 0.17; 4 studies, 1656 participants; low-certainty evidence; at 2 years: MD 0.13 D, 95% CI 0.06 to 0.19; 4 studies, 2454 participants; moderate-certainty evidence; at 3 years: MD 0.17 D, 95% CI −0.17 to 0.51; 1 study, 729 participants; low-certainty evidence). Our protocol defined a difference of 0.1 D in the change in refractive error as clinically important. At one year, the difference was less than 0.1 D, but at two and three years it was more than 0.1 D.
The incidence of myopia was lower in the intervention groups compared to the control groups, but 95% CIs included no change or clinically unimportant benefits (at 1 year: 7.1% with intervention versus 9.5% with control; risk ratio (RR), 0.82, 95% CI 0.56 to 1.19; 3 studies, 1265 participants; low-certainty evidence; at 2 years: 22.5% with intervention versus 26.7% with control; RR 0.84, 95% CI 0.72 to 0.98; 3 studies, 2104 participants; moderate-certainty evidence; at 3 years: 30.5% with intervention versus 39.8% with control; RR 0.77, 95% CI 0.59 to 1.01; 1 study, 394 participants; moderate-certainty evidence). Our protocol defined a difference of 3% in the incidence of myopia as clinically important. At one year, the difference was 2.4%, but there were clinically important differences between the two groups at two (4.2%) and three years (9.3%).
The intervention groups had smaller changes in axial lengths in the direction of myopia than the control groups; however, 95% CIs included no benefit or both benefit and harm at years one and three (at 1 year: MD −0.04 mm, 95% CI −0.09 to 0; 3 studies, 1666 participants; low-certainty evidence; at 2 years: MD −0.04 mm, 95% CI −0.07 to −0.01; 3 studies, 2479 participants; moderate-certainty evidence; at 3 years: MD −0.03 mm, 95% CI −0.13 to 0.07; 1 study, 763 participants; moderate-certainty evidence).
No included studies reported changes in unaided distance visual acuity and quality of life. No adverse events were reported.