Environmental Factors
Less controversial is the evidence suggesting that time outdoors protects against the development of myopia. "Most ophthalmologists and optometrists were taught that myopia was a [strictly] genetic disorder, but the evidence has stacked up solidly against this idea," Morgan says. He points to the ambitious work of the Consortium of Refractive Error and Myopia, which carried out a meta-analysis on more than 55,000 individuals from four continents, has explained only a small part of the variation. In contrast, he says, the dramatic increases in prevalence of myopia in East Asia cannot be due to genetic change, but must result from environmental and/or social changes.
Morgan's work, instead, has focused on the role of ambient light on development of myopia, specifically the impact of time spent outdoors. He and his colleagues proposed that any protective effect of time outdoors was most likely to be mediated by bright light stimulating the release of a transmitter dopamine from the retina; light is known to stimulate dopamine release, and drugs that mimic the effects of dopamine reduce eye growth. He was one of the leaders of the seminal Sydney Myopia Study. As part of that study, Kathryn Rose of the University of Sydney developed a comprehensive questionnaire to pinpoint more accurately how much time children spend indoors and outdoors, and what types of activities they do in both settings.
Morgan says his hypothesis is supported by work showing that experimental animals, raised under conditions that normally lead to myopia but with the addition of bright lights, did not become myopic. In addition, drugs that block dopamine have also been shown to block the protective effect of light.
Seang-Mei Saw, an ophthalmic researcher with the National University of Singapore who led the Singapore Cohort Study of the Risk Factors for Myopia, teamed with Rose to conduct a key comparison of 6- and 7-year-old children of Chinese ethnicity living in Sydney and Singapore. Although the two groups presumably shared a similar genetic predisposition to myopia, their measured prevalence of this condition contrasted sharply: 3.3% among those living in Sydney versus 29.1% for those in Singapore. A comparison of the children's lifestyles further revealed that the Sydney group engaged in just as much, if not more, near work than their Singapore counterparts.
The Sydney group had already detected protective effects of time outdoors against the development of myopia using a detailed questionnaire. From the Singapore side, Saw recalls that these findings represented a turning point in her perception of environmental influences on myopia. While she and her colleagues were actively seeking near-work conditions that might have an effect on eye development, the researchers were surprised by the glaring point of contrast between the two groups: time spent outdoors. "Before we did this study, we did not know that the main difference would be outdoors," she admits. "We had about twenty questions on reading and writing; we only had one question on outdoors, and the most striking difference was the outdoor time."
The children in Sydney were found to be outdoors outside of school hours for an average of almost 14 hours of every week, while those in Singapore averaged only 3 hours outdoors. Saw adds that more detailed studies have been unable to draw distinctions between any specific type of outdoor activity, from hard exercise to lounging under the sun. "This is not the inverse of near work," she emphasizes. "It wasn't because they were spending more time outdoors and less time on near work. We do not know exactly what activity protects from myopia. But we do know from the studies that the total time outdoors was protective."
Clinical trials support outdoor time as an effective intervention. A pilot trial in Taiwan reported a 50% reduction in new cases of myopia by simply locking classroom doors during school recess, which prevented the children from staying indoors and working. And in a larger trial in Guangzhou, adding 45 minutes of structured time outdoors each day was associated with a 25% reduction in new cases of myopia. This study also included an information campaign targeted at parents.
Morgan, one of the leaders of the Guangzhou study, says, "The epidemiology indicates that there is a dose–response relationship between time outdoors and protection, so the expectation is that if we can lift the amount of time outdoors up to closer to the Australian norm, then greater protection would be achieved." He says children in Australia get outside an average of 4.5 hours per day, both through general activities and at school, compared with about 1.5 hours per day for children in Guangzhou and Taiwan.
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Since 2001 the Singaporean government has conducted a myopia prevention program of public education and vision screening starting in preschool. Other Asian countries have tried interventions such as atropine eye drops and special eye exercises in attempts to prevent or postpone myopia in children, but the most successful interventions so far have involved getting children to spend more time outdoors.
Top: © Health Promotion Board, Singapore; bottom: © Mijang Ka/Getty Images