Paper title: Time spent in outdoor activities in relation to myopia prevention and control: a meta‐analysis and systematic review
Authors: Shuyu Xiong (1)(2), Padmaja Sankaridurg (3)(4), Thomas Naduvilath (3), Jiajie Zang (5), Haidong Zou (1)(2), Jainfeng Zhi (1) Minzhi Lv (1), Xiangui He (1)(6), Xun Xu (1)(2)
- Department of Preventative Ophthalmology, Shanghai Eye Disease Prevention and Treatment Centre, Shanghai Eye Hospital, Shanghai, China.
- Department of Ophthalmology, Shanghai General Hospital, Shanghai, China.
- Brien Holden Vision Institute, Sydney, New South Wales, Australia
- School of Optometry and Vision Science, University of New South Wales, Sydney, New South Wales, Australia
- Department of Nutrition, Shanghai Municipal Centre for Disease Control and Prevention, Shanghai, China
- Department of Maternal and Child Health, School of Public Health, Key Laboratory of Public Health Safety, Ministry of Education, Fudan University, Shanghai, China.
Date: Mar 2017
Reference: Acta Ophthalmol. 2017;95:551-66 [Link to open access paper]
Across the eyecare community there is a generally accepted belief that increasing time spent outdoors can be protective against progression of myopia. Xiong et al set out to better understand the research by performing a meta-analysis of 51 clinical trials and longitudinal studies that investigated the relationship between time spent outdoors and the risk of either developing myopia, progression of existing myopia or a myopic shift in refractive error.
Spending time outdoors was found to have a protective effect for the onset of myopia where a 50% reduced risk corresponded with 76 minutes of extra outside time daily. More time outdoors also corresponded with a reduced myopic shift of -0.30D for both myopes and non-myopes. For those who were already myopic, increased time outdoors had little effect in halting myopic progression.
Further research could confirm why outdoor time appears most effective in reducing the risk of myopia onset and what an optimum duration of outdoor time would be for use as an intervention step.
Increased time outdoors appears to provide significant protection for myopia onset
- An inverse relationship was demonstrated where spending time outdoors meant reduced risk of developing myopia
- This relationship was not linear however, and so the benefit of outdoor time reduced with the increasing time spent outside
- A reduction of 50% in the incident myopia could be achieved with an extra 76 minutes outside time per day. This corresponds to 8.9 hours per week
- A 45% reduction is found with 1 hour increase per day
Where the biggest impact here is for the non-myopes, this will apply to those children who are classed as pre-myopic and susceptible to developing myopia1. This may be children who are demonstrating fast reducing hyperopic refractive error or who have a lower hyperopic refraction expected for their age. Taking the current evidence into consideration, the author’s advice is for parents to increase the time their child spends outside by 1 hour daily.
Age of child
- The results of this study suggested that 6yr olds would benefit more from the time outdoors compared to 11-12yr old children due to a greater protective effect
- The suggestion is that their eyes could be at a crucial development stage which leaves them more susceptible to ocular changes from a change in environment.
- The protective effect seems to favour non-myopes - Parents of children younger than 11yrs can still be advised on the benefits of outdoor time as the authors found the protective effect to favour non-myopes.
Progression of myopia
This meta-analysis was unable to provide evidence for increased time outdoors having a protective effect against progression in eyes which were already myopic, however there were limitations in the study design. Despite these limitations we can still advise parents that helping their children engage in more outdoor play could be beneficial.
- Even where some studies in the meta-analysis found no protective effect from more time outside, they also discovered that overall, myopes spent more time indoors than non-myopes. More time outside may be beneficial from simply meaning they spend less time indoors concentrating on closer tasks
Limitations and future research
Using questionnaires to gather information on how long a child spends outdoors does give a possibility for either recall bias or inaccurate reporting from study participants.
- Ideally, the information would be collected objectively and the authors suggested that options could include using light metres, systems based on GPS and biomarkers such as Vitamin D and conjunctival UVAF
One of the studies excluded from the meta-analysis was by Ramessur et al (2015)2 where they investigated identical twins and the environmental risk factors for myopia.
- This study was excluded on the basis that the difference in time spent outdoors was explored in the context of any spherical equivalent differences the twins had, rather than the impact on myopia incidence or progression specifically.
- Future twin studies investigating environmental influences on myopia could help reveal if differences in preference for indoor versus outdoor activities has an impact on myopia onset, incidence or progression.
Of the 25 studies included in the meta-analysis, 17 were from East Asian countries where there is a higher prevalence of myopia compared to many other countries. The remainder were from Australia, the U.K, the U.S.A and Turkey.
- It would be interesting to repeat investigations into outdoor time and myopia onset in countries with differing climates and myopic prevalence to confirm any association.
The authors identified further related research could shed light on several factors:
- What ocular mechanisms are involved if outdoor time is more effective in reducing the risk of myopia onset
- What a definite amount of outdoor time would be before it is effective in reducing the risk of onset and if prolonged time is needed as a reinforcing influence
- Why time outdoors was effective in reducing myopic progression in non-myopes but had limited impact on progression in myopes
This study aimed to explore the association between time spent outdoors and the risk of onset of myopia, the risk of myopic shift in refractive error and myopic progression (in myopic eyes).
Xiong et al collated data from 51 studies ranging from 2002 to 2015 from PubMed, the Cochrane library and EMBASE for their systematic review before including 25 in meta-analysis and dose-response analysis.
The 25 studies used in the meta-analysis included 4 clinical trials, 8 cohort studies and 13 cross-sectional studies with a total of 34,420 participants who were mostly children aged between 6 and 18yrs old. Only 2 studies had adult participants and a further 2 featured children between 6 months and 72 months.
The data was examined to ascertain a connection between time spent outdoors and the risk of incident or prevalent myopia, the risk of a myopic shift in refractive error and the risk of progression in myopic eyes. The authors used a random-effects model to pool the information for the meta-analysis and calculate the confidence interval. Dichotomous data were assessed using the combined risk ratio/odds ratio with 95% confidence interval (CI) and any continuous data was analysed using weighted mean difference and 95% CI.
Where there was a difference in follow-up times for the clinical trials, the estimate of myopic incidence and myopic shift in refractive was based on 3 years. Studies which had a 1yr follow-up interval had the data multiplied by 2.3 with the assumption that myopic progression reduces as age increases. Where cross-sectional studies had defined the time outdoors in hours per day, the data was standardised into hours per week and the standard error was adjusted accordingly.
For the dose-response analysis, the increase in the time spent outside was determined for the clinical trials and a risk ratio was calculated from the longitudinal cohort studies odds ratios.
The statistical heterogeneity was assessed and sensitivity and sub-group analyses were carried out.
The association between outdoor time and the risk of incident or prevalent myopia
The combined risk ratio from 3 clinical trials was calculated at 0.536, suggesting that increased time spent outdoors during play-times at school had a significant protective effect by reducing the chance of onset of myopia:
- From the cohort studies that compared high and low levels of outdoor time, it was found that the higher levels of outdoor time were associated with a lower risk of developing myopia with a risk ratio of 0.574.
- With the pooled estimates from the cross-sectional studies, the final odds ratio was calculated to be 0.964 for myopia with each extra hour which was spent outside per week.
For the dose-response analysis, the results from 6 cohort studies revealed that, overall, the risk of developing myopia could be reduced if time was spent outdoors. One study from Australia investigated the relationship of varying amounts of outdoor time and found that the risk for myopia onset was higher in their younger cohort who spent 13 hours or less per week outside compared to those of the same age who spent an average of 22.5 hours or more per week outside. The remaining 5 cohort studies included in the dose-response analysis discovered an inverse relation where increased time outside was associated with less chance of incident myopia. After plotting the risks of incident myopia from these 6 studies against the time spent outdoors in hours per week, an equation was formulated that gave the figures of:
- 76 minutes per day being needed to achieve a 50% reduction in incident myopia (compared to baseline values), and
- an increase of 1 hour per day would give a 45% reduction in incident myopia (compared to control values)
When the sub-group analysis was undertaken, the cohort studies indicated there was stronger protective effect for children aged 6yrs old compared to 11-12yr old children. The cross-sectional studies, however, found no significant difference between 3 age groups of 6yrs and younger, between 6 and 18yrs and over 18yrs as well as no difference between 3 classifications of prevalence (20% or less, between 20 and 80%, or over 80%).
The association between outdoor time and the risk of myopic shift in refractive error
The pooled estimates of the relationship for myopes and non-myopes was calculated from 3 clinical trials and found that more time outdoors was related to a -0.30D less myopic shift for those in the intervention group compared to those in the control group. The clinical trial studies displayed more of a reduction in myopic shift (-0.34D) than the randomised controlled studies (-0.17D).
Out of the 6 studies included in the dose-response analysis, only 1 found a statistical association between increased time outside and myopia progression, with a -0.14D mean difference between the control group and the test group.
The other studies included in the dose-response analysis demonstrated no significant statistical evidence that outdoor time had a treatment effect for myopia progression.
The combined results from the authors meta-analysis and systematic review reveal there is a protective element to myopia incidence or prevalence when there is increased time spent outdoors. This still seemed to be the message even where there was a high heterogeneity between studies as some extent of protective effect was still seen.
However, the dose-response analysis demonstrated this did not seem to apply to eyes which were already myopic, and spending more time outdoors seemed to have little impact on their myopia progression. Those studies which didn’t find a protective effect were able to show that, on average, myopic children still spent more time indoors than outdoors.
Based on these findings, a conclusion can be drawn that encouraging young children to play outside for at least 1hr per day can be beneficial in terms of reducing their risk of developing myopia and reducing the chance of their refractive error taking a myopic shift. Being outdoors is also reducing the time spent indoors on near work activities which could be playing a part too.
Where the benefit of time outdoors seems to be highest for children approx. 6yr old or younger, this seems coincident with what we know about the eye’s development at this stage and future research could tell us to what extent environmental influences can have. For now, it does no harm to ensure children have outdoor time and find a balance with how much near work they do indoors.
Title: Time spent in outdoor activities in relation to myopia prevention and control: a meta‐analysis and systematic review
Authors: Xiong S, Sankaridurg P, Naduvilath T, Zang J, Zou H, Zhu J, Lv M, He X, Xu X
Outdoor time is considered to reduce the risk of developing myopia. The purpose is to evaluate the evidence for association between time outdoors and (1) risk of onset of myopia (incident/prevalent myopia); (2) risk of a myopic shift in refractive error and c) risk of progression in myopes only. A systematic review followed by a meta‐analysis and a dose–response analysis of relevant evidence from literature was conducted. PubMed, EMBASE and the Cochrane Library were searched for relevant papers. Of the 51 articles with relevant data, 25 were included in the meta‐analysis and dose–response analysis. Twenty‐three of the 25 articles involved children. Risk ratio (RR) for binary variables and weighted mean difference (WMD) for continuous variables were conducted. Mantel–Haenszel random‐effects model was used to pool the data for meta‐analysis. Statistical heterogeneity was assessed using the I 2 test with I 2 ≥ 50% considered to indicate high heterogeneity. Additionally, subgroup analyses (based on participant's age, prevalence of myopia and study type) and sensitivity analyses were conducted. A significant protective effect of outdoor time was found for incident myopia (clinical trials: risk ratio (RR) = 0.536, 95% confidence interval (CI) = 0.338 to 0.850; longitudinal cohort studies: RR = 0.574, 95% CI = 0.395 to 0.834) and prevalent myopia (cross‐sectional studies: OR = 0.964, 95% CI = 0.945 to 0.982). With dose–response analysis, an inverse nonlinear relationship was found with increased time outdoors reducing the risk of incident myopia. Also, pooled results from clinical trials indicated that when outdoor time was used as an intervention, there was a reduced myopic shift of −0.30 D (in both myopes and nonmyopes) compared with the control group (WMD = −0.30, 95% CI = −0.18 to −0.41) after 3 years of follow‐up. However, when only myopes were considered, dose–response analysis did not find a relationship between time outdoors and myopic progression (R 2 = 0.00064). Increased time outdoors is effective in preventing the onset of myopia as well as in slowing the myopic shift in refractive error. But paradoxically, outdoor time was not effective in slowing progression in eyes that were already myopic. Further studies evaluating effect of outdoor in various doses and objective measurements of time outdoors may help improve our understanding of the role played by outdoors in onset and management of myopia.
Ailsa Lane is a contact lens optician based in Kent, England. She is currently completing her Advanced Diploma In Contact Lens Practice with Honours, which has ignited her interest and skills in understanding scientific research and finding its translations to clinical practice.