There’s no doubt that the simplest lifestyle intervention we can advise for both our at-risk pre-myopes and progressing myopes is to spend more time outdoors. This data first came to prominence in Rose et al’s 2008 paper, published in Ophthalmology, entitled conclusively that Outdoor activity reduces the prevalence of myopia in children. Over 4000 children (half aged 12 and the other half aged 6) participating in the Sydney Myopia Survey demonstrated that 12 year olds who spent more time outdoors and less time on near work had the most hyperopic refractions, with the reverse also being true – more near work and less outdoor time means a lower level of plus. This relationship was not shown for the 6 year old group. There was also no association between indoor sport and myopia, so the question begs – what defines the beneficial effect of outdoor time?
Outdoor light exposure - UV or brightness?
On discussion with savvy parents in clinical practice, there is a concern raised about UV exposure, especially here in Australia. One parent recently asked about indoor light – both her and her husband are myopic, and with her six-year-old only demonstrating R&L +0.25, he’s mostly likely heading the same way. This mum asked if she should ask for her son to be sat next to the window in class? Is brighter is better for indoor light? And what were the implications for an at-risk pre-myope wearing sunglasses, even if she couldn’t get her six year old to wear them yet?
It would appear that brightness rather than wavelength and hence UV exposure is the likely stronger link with myopia, with light levels outdoors (up to 100,000 lux) being up to 500 times brighter than indoor light. In their 2009 IOVS paper, which is available for free download from the IOVS website, Ashby et al revealed that chickens, when reared under strong indoor light without UV, showed dose-dependent propensity to retard development of form deprivation myopia.
In their 2012 IOVS paper, also available for free download, Sherwin et al showed that myopic adults had lower levels of long term ocular UV exposure, as measured by conjunctival autofluorescence, compared to non-myopic adults, which is indirect evidence of these myopes spending less time outdoors - drawing the conclusion of this indicating less outdoor time is complicated by use of UV blocking clear and tinted spectacle lenses and contact lenses by people needing vision correction. Interestingly, Rose et al found in their Sydney Myopia Study that children of myopic parents spent less time outdoors. Lower amounts of outdoor time could be a consequence as much as a developmental factor in myopia.
So for our savvy concerned parent, it’s likely that sitting closer to the window will be helpful to her son, even though clear glass does block a majority of incident UVA and UVB light. Wearing sunglasses blocks UV exposure and reduces brightness, but on a sunny day it’s likely that an 85% tint sunglass lens is still able to let through as much brightness, and likely more than typical bright domestic light. So if brightness of light is the 'magic sauce' of outdoor time for myopia prevention, though, sitting near a large window may not be enough, but sun protection is definitely advisable.
Why does outdoor time delay or prevent myopia?
It could be the brightness of the light, as described above. A clear retinal image could also be just as important as bright light, though – in another chicken-blinding experiment published in the open access journal PLoS ONE, Weizhong et al reared chickens with monocular diffusers to induce form deprivation myopia to find that they only received a protective benefit from bright light (15,000 lux) exposure when greater than 5 hours per day. This speaks to the role of retinal blur in myopia development and progression in children, particularly where due to binocular vision disorders like accommodative lag.
The alternative viewpoint is that of Ian Flitcroft, published in Progress in Retinal and Eye Research, who describes how the dioptric environment varies dramatically indoors. Sitting at this computer while I type this blog, focussed on the screen, my keyboard and fingers induce hyperopic blur on my inferior retina while the world beyond my screen induces myopic defocus. Several dioptres of variation from the optical infinity across my office to at least 2-3D of blur from my keyboard is possible. However once you’re outdoors and everything is beyond 3m distance, your retina enjoys a full field of optical infinity. Given the strong understanding of how the peripheral retina drives eye growth in both animal and human models, the benefit of outdoor time is likely to be a combination of the dioptric environment as well as brightness.
The clinical culmination of the above is that increasing outdoor time for children is beneficial to delay or prvent myopia onset. UV protection is important and must be balanced with the visual and retinal stimulation provided by bright natural light. And for our bookworm myopic kids, to leverage all of the outdoor time benefits, we do need to tell them that taking your book or ipad outdoors probably doesn’t count as outdoor time!
A review entitled How does spending time outdoors protect against myopia? examined eight potential mechanisms. The authors found "solid evidence that exposure to brighter light can reduce risk of myopia. Peripheral defocus is able to regulate eye growth but whether spending time outdoors substantially changes peripheral defocus patterns and how this could affect myopia risk is unclear. Spectrum of light, circadian rhythms and spatial frequency characteristics are plausible factors, but there is a lack of solid evidence from human studies. Vitamin D, physical activity and near work appear unlikely to mediate the relationship between time spent outdoors and myopia."
How much outdoor time is needed?
Xiong et al attempted to find a dose-response effect in their 2017 meta-analysis and found that less than 13 hours a week (just under 2 hours a day) was associated with the highest odds ratio for incident myopia. On the basis of five studies investigating a dose response, they found that "an increase of 76 min/day, was needed to obtain a 50% reduction in incident myopia, while an increase of 1 hr/day or 7 hr/week will result in a 45% reduction in incident myopia compared with controls." This is a simple, effective and achievable intervention which also may have positive effects on other child health factors such as reduction in body mass index (BMI), and less sedentary behaviours.
Research indicates we should aim to increase outdoor time to a total of around two hours per day, average, to have the greatest effect on delaying or preventing myopia onset in children.