Professor Ian Morgan, from the Australian Research Council Centre of Excellence in Vision Science, and the Australian National University in Canberra, and team report in the second paper in The Lancet Series on Ophthalmology that this extremely high prevalence of myopia is linked to increased schooling, according to recent data, which indicates that the lack of daylight exposure could have a major impact on this gigantic problem.
It was previously believed that myopia was mainly based on genetics. However, more recent evidence shows that environmental factors are involved as well. For instance, the three key ethnic groups in Singapore, i.e. Chinese, Indians and Malays have all experienced steep increases since 1996, indicating a similar sensitivity to the risk factors they were all exposed to.
Migrant studies have also proved crucial as they revealed that in the UK and Australia children of South Asian ancestry have a higher prevalence of myopia compared with those in India, although not as high as compared with those in Singapore.
In Australia for instance, students of Chinese origin have lower levels of myopia compared with those in urban centers in East and Southeast Asia, whilst the number of children of European origin in Sydney with myopia is less than half of those in the UK.
Numerous studies have demonstrated that an association exists between schooling and myopia, with reading books close-up and intensive study having been determined as the causes of myopia. According to more recent epidemiological surveys, longer periods outdoors protect against developing myopia. A similar study of children from Chinese ancestry from Singapore and Sydney has demonstrated that the only environmental factor that correlated with the significantly higher prevalence of myopia in Singapore was the time spent outdoors.
To examine the impact of higher outdoor exposure as a preventive measure, Singapore and China are currently undertaking trials. These studies demonstrate that the causes of ethnic differences in school myopia are children's different experiences at different places, and not ancestry genetics. However, it remains to be determined to what extent many genes of small effect and gene-environment interactions contribute to variations in school myopia within ethnic groups. Furthermore, even though the occurrence of high myopia is increasing because of environmental pressures, some forms of high myopia have a more distinct genetic basis.
The researchers point out that various trials of optical aids, such as contact lenses and spectacles can help in preventing myopia or its progression. These optical devices include lenses that can neutralize peripheral hyperopic errors, or impose localized myopic defocus, although the team add that prolonged use of these devices leads to a reduction or disappearance of the device's protective effects. They caution doctors to delay widespread use of any of these devices until the results from long-term trials, including their side effects are published.
Atropine, which is frequently used in East Asia, is well known to slow progression of myopia in children. However, it has not been adopted on a larger scale because of its side effects. According to recent evidence, lower doses were likely to be result in a positive effect, outweighing the drug's side effects, which could lead to more clinicians using the drug.
The researchers state that further research needs to be conducted in order to identify methods to prevent myopia from progressing into high myopia. They highlight that those affected should be aware of the condition's potential risk of eventual blindness, and should immediately seek treatment.
The researchers conclude:
"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."
Written by Petra Rattue