Myopia, or short-sightedness, now affects between 80% and 90% of school-leavers in major East Asian cities, such as those in China, Taiwan, Hong Kong, Japan, Singapore, and South Korea. This imposes a huge disease burden currently, and poses further future problems, not least because 10-20% 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 suggests that it is lack of exposure to bright light outdoors that could play a major role in this enormous problem. This second paper in The Lancet Series on Ophthalmology is by Professor Ian Morgan, Australian Research Council Centre of Excellence in Vision Science, Australian National University, Canberra, Australia, and colleagues.
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 to the risk factors to which they are all exposed. Migrant studies have also been vital. Children of South Asian ancestry in the UK and Australia show higher prevalences of myopica 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 centres in east and southeast Asia. Children of European origin in Sydney have less than half the myopia of those in the UK.
As many studies have shown a link between schooling and myopia, close reading of books and intensive study have been implicated as causes of myopia. 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 shows that the only environmental factor that correlated with the much higher prevalence of myopia in Singapore was time spent outdoors. Trials are ongoing in Singapore and China to test the effect of increased outdoor exposure as a preventive measure.
These studies suggest that it is the different experiences of children at different places which 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. In addition, some forms of high myopia have a clearer genetic basis, even though the prevalence of high myopia is rising due to environmental pressures.
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 neutralise peripheral hyperopic errors, or to impose localised myopic defocus. However they add that given the common reduction or disappearance of protective effects of these devices with longer use, and the small effect sizes reported, caution suggests that widespread adoption of any of these devices be delayed until the results of long-term trials, including analyses of side-effects, have been reported.
Atropine is well known to slow progression of myopia in children, and has been used frequently in East Asia. But due to side effects, it has not been adopted on a larger scale. However, recent evidence suggesting a positive effect with a lower dose, thus mitigating the side effects, could mean more clinicians adopt the drug.
The authors believe that research priorities should include methods to prevent myopia progressing to high myopia, and stress the importance of ensuring that people with this serious form of the disease are aware of its blinding potential, so that they can seek prompt treatment.
They conclude their paper by saying: "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. 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."
Professor Ian Morgan, Australian Research Council Centre of Excellence in Vision Science, Australian National University, Canberra, Australia.
T) Australia +61 417450746, but currently in China on: +86 13610311267 E) ian.morgan@anu.edu.au
Journal
The Lancet