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).
Since time spent indoors seemed to be such an important risk factor, Saw and Rose asked whether it might explain the extraordinarily high prevalence of short-sightedness in Asia. To find out, they compared two groups of 6 to 7-year-old children, one in Singapore and one in Australia. The team looked only at children of Chinese ethnicity, to rule out genetic differences between races as an explanation for higher myopia rates in certain countries.
The result? On average the children in Sydney spent nearly 14 hours per week outside, and only 3 per cent developed myopia. In contrast, the children in Singapore spent just 3 hours outside, and 30 per cent developed myopia. Once again, close work had a minimal influence; the Australian children actually spent more time reading and in front of their computers than the Singaporeans (Archives of Ophthalmolology, vol 126, p 527).
But why should the great outdoors stave off myopia? One possibility is that the sheer intensity of light outdoors somehow stalls eyeball growth, a theory supported by Regan Ashby's work at the University of Tübingen in Germany.
Ashby attempted to induce myopia in a group of chicks by blurring their vision using special lenses. He divided the birds into three groups and exposed them to different light levels for just 15 minutes each day. At the end of the five-day trial, those exposed to intense light - half as bright as direct sunlight and 30 times brighter than normal indoor lighting - were on average 40 per cent less short-sighted than chicks exposed to normal laboratory light levels during those 15 minutes. Actual daylight slashed the severity of myopia by a further 40 per cent (Investigative Ophthalmology and Visual Science, DOI: 10.1167/iovs.09-3419). Although the mechanism behind this remains uncertain, Ashby believes bright light may stimulate the retina to produce high levels of dopamine, a substance known to inhibit eye growth.
Other researchers, however, think this only goes part of the way to explaining the causes of myopia. "There is a 500-pound gorilla sitting in the corner of the myopia research room, and this is the three-dimensional structure of the eye," says Flitcroft.
There's a 500-pound gorilla in the myopia research room, and it is the 3D structure of the eye
He points out that peripheral vision has largely been ignored when studying myopia, since we are much more conscious of a blurred image in the centre of the retina, the fovea, than at the edges of our vision. But in a series of experiments, Earl Smith at the University of Houston, Texas, found that if he fitted monkeys with lenses that blurred only their peripheral vision, they still became myopic (Vision Research, vol 49, p 2386).
The finding is supported by other studies, which had found that people are more likely to become myopic if their peripheral vision is slightly long-sighted. It seems the eyeball elongates to compensate for this error
Quick fix
Conversely, the theory might also explain why other, less conventional treatments have turned out to be unexpectedly successful at slowing the progression of myopia. Corneal refractive therapy (CRT), for example, was originally designed as a temporary solution to myopia, in which patients wear hard lenses overnight that temporarily reshape the cornea. In the morning, they take out the lenses and can see well for the rest of the day. But new research by Jeff Walline at the Ohio State University in Columbus has shown that unlike glasses or regular contact lenses, CRT actually slows progression of myopia by around 50 per cent (British Journal of Ophthalmology, vol 93, p 1181). Walline suggests that flattening the cornea may also reduce the blur at the edges of the subjects' vision, which might explain CRT's success.
A similar explanation may be behind the small benefit that multifocal specs carry. Perhaps they don't just correct poor accommodation, but instead reduce the blur in the lower half of the peripheral vision, says Frank Schaeffel at the University of Tübingen, Germany. "If that is the case, then reading glasses that change the focusing power around the whole periphery could easily be up to three times or more effective," he says.
His group recently started working with the eyewear firm Rodenstock on a new type of lens, which aims to achieve exactly that. Unfortunately, these glasses can distort central vision if the wearer looks through the wrong part of the lens, but Schaeffel believes further improvements will minimise this.
The importance of peripheral blur is further supported by a promising study of bifocal soft contact lenses, which have a central portion that corrects myopia, surrounded by a ring that corrects the long-sighted defocus around the periphery of the retina. Thomas Aller, an optometrist based in San Bruno, California, and Christine Wildsoet at the University of California, Berkeley, found that bifocal contact lenses slowed myopia progression by 80 per cent (Ophthalmic and Physiological Optics, vol 26(s1), p 8).
The role of peripheral vision in this study isn't clear-cut, since it focused on children who tend to slightly cross their eyes when reading. Their eyes have especially poor accommodation, which may put them at particularly high risk of progressive myopia. So the success of the trial may stem from treating this problem, in addition to reducing their peripheral blur.
Flitcroft, however, is sceptical. He believes that all of these possible treatments miss a key point: in crowded environments, even people with perfect eyesight experience a significant amount of blur in their peripheral vision. "Around the edges of a book or computer screen the world is further away and blurred," says Flitcroft. "Conversely, if we look at a distant scene across a room through a window, the objects inside the room that fill our peripheral vision are then closer and [yet] also out of focus," he says.
For genetically susceptible individuals, that might be enough to trigger myopia. Time spent outdoors may benefit vision simply because it's only when we look out over large, open spaces that all objects are sufficiently far away for the eye to focus a crisp image across the whole of the retina.
The theory certainly fits in with one of Rose's observations - that living in a highly urban environment increases the risk of myopia, since the cityscape may confine our gaze to shorter distances (Investigative Ophthalmology and Vision Science, vol 49, p 3858). Unfortunately, if the theory proves correct, none of the proposed corrective lenses could help, since the blur varies depending on whether you look close by or into the distance.
Others, however, are more optimistic that the promising trials of bifocal contact lenses and specially designed reading glasses will continue to pan out. "I have noticed that there is a lot more willingness to believe that we may be able to come up with something that will change the way we treat myopia. Every contact [lens] and eyeglass company is working on something," says Aller, who is involved in one of these projects himself.
In the meantime, the safest bet to prevent myopia still seems to be to spend as much time outside as possible. That's good news for my own children: they currently go to a forest kindergarten where they spend the morning outside in the woods, and I can send them to a local primary school that also teaches many classes outside. And in the future, I'll be sure to keep an eye on the latest designs of reading glasses. I'm now hopeful that even with my poor genes, my kids stand a good chance of keeping their world in focus.
Are your eyes what you eat?
Seven years ago, evolutionary biologist Loren Cordain at Colorado State University in Fort Collins caused a stir by suggesting that myopia may be triggered by the excessive consumption of refined carbohydrates. The study compared diets and rates of myopia in different nations, and it seemed plausible that insulin levels which were raised in response to a high-carb diet could stimulate the eye to grow and become elongated, causing myopia.
This year, two independent studies, led by Frank Schaeffel at the University of Tübingen in Germany and Josh Wallman at the City College of New York, have provided further evidence that insulin can stimulate eye growth. Working with chicks that wore special lenses to provoke myopia, they found that injecting insulin into the chicks' eyes increased the deterioration in their sight dramatically.
Yet whether this explains the link between diet and myopia remains hotly debated. "Initially we just didn't believe Cordain's carbohydrate story, but now that we know that insulin can interfere so much, I am not so sure," says Schaeffel. Wallman remains more doubtful, arguing that a high-carb diet may not necessarily raise insulin levels in the eye enough to cause damage.
Cordain cites studies which found that people with high blood-sugar levels are more likely to be myopic, and says that insulin levels in the eye do seem to reflect levels elsewhere in the body. High blood sugar may also promote myopia by raising levels of the growth factor IGF-1, a substance which likewise stimulates eye growth, he says.
Drugs and eye exercises
Optical interventions are not the only way to fight myopia, but two alternatives - drugs and eye exercises - have failed to live up to expectations.
Two drugs, atropine and pirenzepine, do slow myopia, but with side effects such as increased light sensitivity, dizziness and blurred vision, so most researchers and doctors are understandably reluctant to advocate their widespread use. Another approach - using exercises to teach the eye how to focus again - has no serious side effects, but there is little convincing evidence that it works. "By practising these exercises people get better at functioning with their deficit, but six months later their prescription hasn't changed," says optometrist Thomas Aller from San Bruno, California.
One piece of advice that may make sense is to look into the distance every half hour while reading. While direct evidence is lacking, studies in which animals are fitted with blurring lenses that cause myopia show that even short periods without the lenses can be beneficial.
Nora Schultz is a writer based in Berlin, Germany
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