Two Hours Outdoors a Day Helps Keep Myopia Away
BY DAVID A. BERNTSEN, OD, PHD, FAAO
Many optical interventions continue to be studied as potential ways to slow the progression of myopia in children. While the efficacy of bifocal spectacle lenses for slowing myopia progression has generally been limited (Smith, 2013), contact lens treatments have shown more promising results.
A randomized clinical trial found that orthokeratology slows axial eye growth by roughly 43% (Cho and Cheung, 2012). Unlike spectacle bifocal studies—in which lenses often provided no meaningful accumulation of the treatment effect after the first year—there is evidence that the treatment effect in orthokeratology continues to accumulate over at least three years (Hiraoka et al, 2012).
There is also early evidence that wearing multifocal soft contact lenses can slow myopia progression (Walline et al, 2013; Lam et al, 2014); long-term clinical trials evaluating soft multifocal contact lenses for myopia control and examining the potential mechanism of action are currently under way.
In addition to optical interventions, low-dose atropine (0.01%) has also received a great deal of attention recently. Issues involving the use of 1% atropine for myopia control have always included the side effects (e.g., cycloplegia necessitating bifocal spectacles and dilated pupils). Unlike 1% atropine, 0.01% atropine can slow myopia progression while having significantly less side effects. Low-dose (0.01%) atropine minimizes the rebound effect (accelerated myopia progression) reported when children stop using higher doses of atropine (Chia et al, 2016).
While both optical and pharmaceutical interventions continue to be studied, eyecare practitioners should not overlook one of the most simplistic interventions available—time outdoors.
Outdoor Time and Onset
Spending more time outdoors is well-established as being protective against the onset of myopia in children. There were hints that outdoor time might be involved with myopia back in the early 1990s when a study reported that time spent outdoors was associated with slower myopia progression in boys, though they found no association in girls (Pärssinen et al, 1993).
The protective effect of outdoor time against myopia onset was first reported in a large longitudinal study by Jones et al (2007). The same group also found no association between outdoor time and time spent doing near work (i.e., kids spending more time outdoors were not simply doing less reading; the effects were independent). Outdoor time was also protective against myopia onset regardless of how many myopic parents the child had. In their study, children spending greater than 14 hours per week outside (i.e., about two hours per day) had the least chance of becoming myopic.
After this initial report of the protective effect of outdoor time against myopia onset, multiple other groups conducting studies of refractive error around the world checked in with similar findings (Rose et al, 2008; Dirani et al, 2009; Guggenheim et al, 2012; French, Morgan et al, 2013). Though studies have used a variety of different survey questions to assess outdoor time (ranging from a single question about time outdoors and/or sports to much longer surveys assessing outdoor time and activities), the consistent findings across studies speak to the robustness of the protective effect of outdoor time when it comes to myopia onset.
Clinical trials have also been conducted to determine whether increasing the amount of time spent outdoors during the school day could be sufficient to reduce the incidence of new cases of myopia in school-age children. Two different studies in Taiwan and China that increased outdoor recess time during the school day by between 40 to 80 minutes per day both found a reduced incidence of new cases of myopia compared to schools where children did not have additional outdoor time (Wu et al, 2013; He et al, 2015).
The simple lifestyle modification of increasing time spent outdoors provides eyecare practitioners with a simple yet powerful recommendation for parents: send the kids outside to play. Beyond the implications for myopia onset, more time outdoors also carries with it other health benefits from the likely increase in physical activity.
Outdoor Time and Progression
So, if increasing outdoor time protects against myopia onset, does it also slow its progression? The answer to this questions is not as clear cut. Though there is evidence of seasonal variation in myopia progression with faster progression in winter months and slower progression in summer months (Gwiazda et al, 2014), studies examining the effect of outdoor time on annual myopia progression question the efficacy of outdoor time when it comes to slowing progression once a child is myopic.
Although Jones-Jordan et al (2012) found a protective effect of outdoor time against myopia onset, a subsequent analysis of their longitudinal data found no association between outdoor time and progression after a child became myopic.
Additionally, in the study by Wu et al (2013) mentioned above in which extra outdoor recess in school reduced myopia incidence, the additional outdoor time did not result in a significant slowing of myopia progression among children who were already myopic. Additional studies are currently in progress trying to better understand this disconnect.
What Is It About Being Outdoors?
Multiple potential explanations for the outdoor effect have been proposed (French, Ashby et al, 2013). Outdoor environments have a more uniform dioptric space with less defocus across the peripheral retina due to reduced differences in accommodative demand (Flitcroft, 2012). It has also been suggested that smaller pupils when outdoors due to a bright environment may play a role due to an increased depth of focus. If hyperopic defocus on the peripheral retina stimulates eye growth, an outdoor environment may decrease the amount of hyperopic defocus experienced versus an indoor environment. Another prominent theory of eye growth with strong support from animal models is that the outdoor effect is mediated by dopamine; higher light levels stimulate dopamine release, which is known to slow eye growth (Norton and Seigwart, 2013).
Research continues to better understand the underpinnings of the outdoor effect and its utility. Advances in technology now allow researchers to more accurately and objectively measure light exposure in children without the need for surveys. Using watches that contain both a light sensor and activity meter, Read et al (2014) reported that myopic children in an observational study spent less time outdoors than did emmetropic children, but found no differences in physical activity levels between myopes and emmetropes, which further supports the involvement of light intensity in the outdoor effect. Studies are also ongoing utilizing glass classrooms that greatly increase light exposure throughout the instructional day in an attempt to reduce myopia onset and progression. Stay tuned for the results. For now, you can advise parents to have their children go out and play more often (but remember the sunscreen). CLS
For references, please visit www.clspectrum.com/references and click on document #245.
Dr. Berntsen is an associate professor at the University of Houston College of Optometry. He has received research funding from Johnson & Johnson Vision Care and B+L.