Health

The decline was rapid. I got my first pair of glasses aged 9, and by my mid-teens could no longer read the title on the cover of New Scientist at arm's length. With my mum's eyes just as bad, I always assumed that I'd inherited my short-sightedness from her and that I could do little to stop my vision from becoming a little blurrier each year.

Around the same time, however, rates of short-sightedness, or myopia, were rising to epidemic proportions around the world. Today, in some of the worst-affected countries such as Singapore, Hong Kong and Taiwan, around 80 per cent of young adults are myopic, compared to only 25 per cent a few decades back.

Rates are lower in western countries - between 30 and 50 per cent - but myopia seems to be rising steadily here too. What could be causing this mysterious epidemic? It is clear that genetics alone can't explain the condition, and the long-standing theory that reading was to blame has failed to play out in subsequent studies.

Large-scale epidemiological surveys ensued, which have pinned down the specific aspects of modern lifestyles that cause children's eyesight to deteriorate. With just a few simple measures, it now looks like we could easily prevent future generations from descending into my blurry world.

While the causes have been elusive, the anatomy of myopia has been well understood for decades. In the normal eye, the lens focuses light squarely on the retina, which records the image and sends it to the brain. We myopes, however, have eyeballs that are elongated, increasing the distance between the light-sensitive retina at the back of the eye and the lens at the front. The result is that light from distant objects is focused in front of the retina, so a blurred image is transmitted to the brain.

Near work, such as reading, had always seemed like an obvious contributor, since short-sightedness appears more common among highly educated people. According to this idea, the lenses in some children's eyes are not very good at "accommodating", or adapting their curvature to focus clearly on near objects. Because small print, for example, would appear slightly blurred, the eyeball elongates to compensate, improving near sight at the cost of distance vision.

The theory sounds plausible, but while myopia does correlate with how well educated you are, frustratingly, researchers have tried and failed to find a strong link to specific activities like reading. Worse still, attempts to correct poor accommodation have been only marginally successful.

One of the most promising ideas was to deal with incipient myopia by preventing the blur from bad accommodation. Bifocal or multifocal lenses, with weaker power in the bottom half of the lens, were given to kids to help them focus on near objects. "The idea was that if near work is bad, you can convert it into far work by putting on the right glasses," says Ian Flitcroft, a consultant ophthalmologist at the Mater Misericordiae University Hospital in Dublin, Ireland.

Yet in 2003, the results from a large-scale trial of multifocal versus single-vision glasses, involving 469 children aged 6 to 11, found that the multifocal treatment slowed the progression of their myopia by just 0.2 dioptres over three years (Investigative Ophthalmology and Visual Science, vol 44, p 1492). Their myopia increased by -1.28 dioptres over this time, and since most people start wearing prescription lenses at -0.75 dioptres, the treatment was of little practical benefit.

Clearly, some important factor was missing from the equation. Lisa Jones-Jordan at Ohio State University in Columbus stumbled upon the next lead in a study published two years ago (Investigative Ophthalmology and Visual Science, vol 48, p 3524). Analysing the lifestyle of 514 children aged 8, her team found that within four years 111 had become short-sighted. Crucially, those children spent less time engaging in outdoor and sporting activities than those who did not become myopic - 8 hours compared to 12 hours per week.

"One idea might be that these children are outside less because they are doing more near work," says Jones-Jordan. This turned out not to be the case. Importantly, the amount of time a child spent on near work did not correlate with their level of myopia, nor with the time spent outside or playing sports.

If near work is not the determining factor, could it be that good eyesight is simply another benefit of physical exercise? Kathryn Rose at the University of Sydney put paid to that idea in a study of 2367 12-year-old Australians. Playing indoor sports turned out to have no benefits for the eyes, whereas even physically inactive time spent outside was beneficial (Ophthalmology, vol 115, p 1279).

"Our findings suggest that being outdoors, rather than sport per se, may be the crucial factor," says Rose. The theory has since been backed up by a study of 1249 teenagers in Singapore, led by Seang-Mei Saw at the Yong Loo Lin School of Medicine, National University of Singapore (British Journal of Ophthalmology, vol 93, p 997).

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.