AsianScientist (May 8, 2012) – Myopia, or short-sightedness, now affects a startling 80-90 percent of school-leavers in major East Asian cities, such as those in China, Taiwan, Hong Kong, Japan, Singapore, and South Korea, says a new paper in The Lancet series on opthamology.
In addition to the huge disease burden imposed by this group, 10-20 percent of these people have ‘high’ myopia, which can eventually lead to vision loss, visual impairment, and even blindness.
While increased schooling is associated with this very high prevalence, recent data from researchers at the Australian Research Council Center of Excellence in Vision Science, Australian National University, suggests that it is lack of exposure to bright light outdoors that is to blame.
While myopia was previously thought to have a mainly genetic basis, more recent evidence implicates environmental factors. For example, in Singapore, the three major ethnic groups (Chinese, Indian, Malays) have all seen sharp rises since 1996, suggesting similar sensitivity and exposure to the risk factors.
Migrant studies have also been vital. Children of South Asian ancestry in the U.K. and Australia show higher prevalences of myopia than those in India, though not as high as those in Singapore. Students of Chinese origin in Australia show lower levels of myopia than those in urban centers in East and Southeast Asia.
More recent epidemiological surveys have shown that increased amounts of time outdoors protect against the development of myopia. For example, a comparative study of children of Chinese ancestry from Singapore and Sydney showed that Singaporean kids spent much less time outdoors than those in Sydney. Trials are ongoing in Singapore and China to test the effect of increased outdoor exposure as a preventive measure.
These studies suggest that environmental pressures are the cause of ethnic differences in school myopia, rather than genetic aspects of ancestry. However, to what extent many genes of small effect and gene-environment interactions contribute to variations in school myopia within ethnic groups remains to be established.
The authors note a number of trials of optical devices (spectacles or contact lenses) to help prevent myopia or prevent its progression. These include lenses designed to neutralize peripheral hyperopic errors, or to impose localised myopic defocus. However they caution that given the common reduction or disappearance of protective effects of these devices with longer use, and the small effect sizes reported, more long-term trials and analyses of side-effects are needed.
Atropine is also well known to slow progression of myopia in children, and has been used frequently in East Asia despite its side effects. Recent evidence suggests that a positive effect with a lower dose may be possible, thus mitigating the side effects, and could lead to more widespread adoption.
The authors believe that research priorities should include methods to prevent myopia progressing to high myopia, and education of people with high myopia to ensure they are aware of its blinding potential, so that they can seek prompt treatment.
“Even if successful prevention is possible, east Asia will still be faced, for close to the next 100 years, with an adult population at high risk of developing pathological [high] myopia,” the authors write. “Further progress in our understanding of the natural history of pathological myopia is thus essential, and while there have been some promising developments in treatment, more effective treatments are still required.”
The article can be found at: Morgan IG et al. (2012) Myopia.
Source: The Lancet.
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