CONTRARY to popular belief, people in east Asia are no more genetically susceptible to short-sightedness than any other population group, according to researchers who have analysed past studies of the problem.
The epidemics of myopia in countries such as Singapore and Japan are due solely to changes in lifestyle, they say, and similar levels could soon be seen in many western countries as lifestyles there continue to change. "As kids spend more time indoors, on computers or watching telly, we are going to become just as myopic," says Ian Morgan of the Australian National University in Canberra. Myopia is on the increase in most places, but in countries such as Singapore it has reached extraordinary levels.
There, 80 per cent of 18-year-old male army recruits are myopic, up from 25 per cent just 30 years ago.
Employers such as the police are having problems finding people who meet their requirements. There is also an increasing incidence of extreme myopia, which can lead to blindness.
There is little doubt about at least one underlying cause. Children now spend much of their time focusing on close objects, such as books and computers. To compensate the eyeball is thought to grow longer. That way less effort is needed to focus up close, but the elongated eye can no longer focus on distant objects. The argument is about why the rate of myopia is so much higher in east Asia than elsewhere.
The conventional view is that people from the region have genetic variations that make them more susceptible. But after reviewing over 40 studies, Morgan and Kathryn Rose of the University of Sydney argue that there is no evidence to support this. The pair, whose work will be published in Progress in Retinal and Eye Research, use several lines of evidence to debunk the idea that genes can explain the Asian epidemics.
For instance, 70 per cent of 18-year-old men of Indian origin living in Singapore have myopia, while in India itself the rate is roughly 10 per cent. Another study found myopia rates of 80 per cent in 14 to 18-year-old boys studying in schools in Israel that emphasise reading religious texts. The rate for boys in state schools was just 30 per cent. "The simplest explanation is that you have a massive environmental effect that is swamping out the genetic influence," says Morgan.
In other words, given the wrong lifestyle, everyone is susceptible to myopia. And it looks as if those lifestyle changes are beginning to be felt in some western countries too. In Sweden, for instance, 50 per cent of children aged 12 now have myopia. It is expected that when these children reach 18 the rate will be more than 70 per cent.
"It is an impressive piece of work," says Karla Zadnik of Ohio State University College of Optometry in Columbus. But if the increase really is due to too much reading, she points out, then lenses that eliminate the stress of focusing on near work should help stop myopia getting any worse. In fact, studies show the lenses are of little help.
But that might be because we have not identified all the lifestyle factors involved, including ones that protect against myopia, says Morgan. For example, children who read less also tend to spend more time outdoors, where better light may reduce the need to focus precisely for near vision. Studies show that children who play sport are less susceptible to myopia.
One group of researchers has even proposed that diet is one of the factors contributing to the rise in myopia. They argue that eating too much refined starch affects the growth of the eyeball (New Scientist, 6 April 2002, p 9). But even if there are no big differences between population groups, genetic studies are important, says Christopher Hammond of St Thomas' Hospital in London. There might be some people whose vision remains perfect whatever their lifestyle.
"If we can identify the genes involved, we have a better chance of understanding the mechanisms involved and developing treatments," he points out. In a study of 506 pairs of twins, Hammond found that when you take environment out of the equation, genes account for 87 per cent of the variation in short and long-sightedness. The team has identified several genes that may be involved, including PAX-6, which is known to be important in the development of the eye (The American Journal of Human Genetics, vol 75, page 294).
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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