Bright light outdoors stimulates the release of the retinal transmitter, dopamine, which has a protective effect.
Myopia, or short-sightedness, is a condition in which distant objects appear blurred, but closer objects can usually be seen in sharp focus.
Its biological basis is an eye that, during childhood, has grown too long for its optical power. The focal plane for images of distant objects ends up in front of the retina, causing out-of-focus perception.
Fortunately, mild to moderate levels of myopia can be readily corrected with spectacles, contact lenses or laser surgery, which flattens the front of the eye.
But prevention is better than correcting the optical defocus. Fortunately, spending more time outdoors may decrease children’s chances of developing myopia.
Finding the cause
Myopia was once regarded as almost totally genetically determined. But its prevalence has increased spectacularly in urban mainland China, Hong Kong, Taiwan, Singapore, Japan and South Korea, where 80-90% of those completing high school are now short-sighted. This is up from 20-30% only two generations ago.
Since gene pools do not change that fast, these massive changes must be due to environmental change.
In 2005, we comprehensively reviewed the research on myopia and found a correlation with education. (This was not a particularly novel insight; such a link was postulated as far back as Kepler in 1604.) We found locations with a high prevalence of myopia were all top performers in surveys of international educational outcomes.
Fortunately, not all high-performing locations, Australia among them, showed a high prevalence of myopia. This shows that high educational outcomes do not necessarily lead to myopia.
We also hypothesised that all human population groups had a tendency to develop myopia under particular environmental conditions. Indeed, North America and Europe have seen growing rates of myopia, although they are still nowhere near as high as in East and Southeast Asia.
Prevention is the key
A common cutoff for high myopia is -5 diopters. This means vision is blurred beyond 20cm from the eyes. Such severe or high myopia increases with age and can lead to visual impairment that can’t be corrected.
The prevalence of high myopia has now reached 20% in young adults in East and Southeast Asia, which foreshadows major increases in visual impairment and blindness as these young adults age. So prevention of myopia has become crucial, particularly for East and Southeast Asia.
Three clinical trials in East Asia have shown that increasing the amount of time children spend outside at school reduces the risk of myopia. Experimental studies suggest that bright light outdoors stimulates the release of the retinal transmitter, dopamine, which has a protective effect.
Australia has naturally low levels of myopia with a lifestyle that emphasises outdoors activities. Young children report spending two to three hours a day outside, not counting time outdoors at school. However, there are formidable barriers to achieving this benchmark in locations where spending time outdoors is seen as a distraction from study.
Policy responses must therefore also aim to slow the progression of myopia, the phenomenon in which mild to moderate myopia becomes more severe during childhood. There is currently controversy over whether time outdoors slows progression, but strong seasonal effects on progression suggest that it may.
School regimes which give a sufficient place to time outdoors may reduce both the onset and progression of myopia. These school-based interventions will need to be supplemented by clinical interventions, such as the use of atropine eye drops.
Recently, the Brien Holden Vision Institute released projections on the future prevalence of myopia and high myopia, based on these well-documented trends. While projections are inevitably based on uncertain assumptions, they confirm that a critical situation could emerge in the next few decades in more than just East and Southeast Asia, if preventive measures are not put in place.
Commentators have recently emphasised the role of digital devices, and specifically tablet computers, in the emergence of an epidemic of myopia. In Taiwan, this has prompted limits on the use of such devices by young children.
However, a simple historical perspective suggests that their role is minor. The prevalence of high myopia in young adults in Singapore and Taiwan was already high by the early 1990s, well before digital devices became common. The world wide web was not launched until 1993, and smart phones and tablets were not developed until just a few years ago. These devices cannot have been causal.
There is, in fact, no evidence that digital device are harmful in themselves. They may add to “near” workloads, or lead to children spending less time outdoors, which could exacerbate current problems. But the root causes of the myopia epidemic lie in the imbalance between educational pressures and the amount of time children spend outdoors in bright light.
The epidemic needs to be addressed at this level, through school-based preventive interventions, combined with a more active preventive approach in clinical practice.
This article originally appeared on The Conversation.
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