Why are so many children getting myopia?

Washington - Myopia isn’t an infectious disease, but short-sightedness has reached nearly epidemic proportions in parts of Asia.

In Taiwan, for example, the percentage of seven-year-old children who are short-sighted increased from 5.8 percent in 1983 to 21 percent in 2000. An incredible 81 percent of Taiwanese 15-year-olds are myopic.

If you think that the consequences of myopia are limited to a lifetime of wearing spectacles – and, let’s be honest, small children look adorable in glasses – you are mistaken.

The prevalence of high myopia, an extreme form of the disorder, in Asia has more than doubled since the 1980s, and children who have myopia early in life are more likely to progress to high myopia. High myopia is a risk factor for such serious problems as retinal detachment, glaucoma, early-onset cataracts, and blindness.

The explosion of myopia is a serious public health concern, and doctors have struggled to identify the source of the problem.

Near-sightedness has a strong element of heritability, but the surge in cases shows that a child’s environment plays a significant role. A variety of risk factors has been linked to the disorder: frequent reading, participation in sports, television watching, protein intake, and depression. When each risk factor was isolated, however, its overall effect on myopia rates seemed to be fairly minimal.

Researchers believe they are now closing in on a primary culprit: too much time spent indoors.

In 2008 orthoptics professor Kathryn Rose found that only 3.3 percent of six- and seven-year-olds of Chinese descent living in Sydney, Australia, were myopic, compared with 29.1 percent of those living in Singapore.

The usual suspects, reading and time in front of an electronic screen, couldn’t account for the discrepancy. The Australian cohort read a few more books and spent slightly more time in front of the computer, but the Singaporean children watched a little more television. On the whole, the differences were small and probably cancelled each other out.

The most glaring difference between the groups was that the Australian kids spent 13.75 hours a week outdoors, compared with a rather sad 3.05 hours for the children in Singapore.

Rose, whose research was covered in the magazine Science News, pointed out the vastly different educational cultures that prevail in Sydney and Singapore. Most Australian children participate in one year of part-time preschool, which teaches social skills and communal play more than reading or writing, followed by one year of full-time kindergarten.

During the same stage of development, the typical Singaporean child goes through three largely full-time years of education in an attempt to ensure that he or she can read before beginning school. Full-time schooling probably comes at the expense of time spent playing outside.

If the problem is just a matter of light intensity, however, you could send your child outside to read.

Over the past few years, much myopia research has focused on substantiating, quantifying, and explaining the connection between time spent outdoors and proper ocular development.

Dozens of studies have come out, mostly from Asia. Last year a systematic review paper aggregated previous studies and concluded that each hour spent outside a week reduced a child’s chance of developing myopia by two percent.

There isn’t yet broad agreement on why the outdoors may protect children from near-sightedness. One hypothesis is that kids focus their gaze on more distant objects in the outdoors, while indoor time is usually spent staring at a computer screen, book, or toy only a couple of feet away. Studies on rhesus monkeys, however, suggest that simple light exposure is the more likely explanation.

While myopia is extremely uncommon among non-human primates, researchers can easily induce myopia by depriving infant monkeys of normal outdoor lighting levels. (Outdoor light is usually about 100 times more intense than indoor lighting.)

It’s no wonder that the most extreme changes in the prevalence of myopia over recent decades have occurred in sunny places like Singapore, where the difference between outdoor and indoor light intensity is most extreme.

This is, on the whole, an encouraging finding. If children became myopic due to looking at objects too closely, then we would be stuck with an unsolvable dilemma: choosing between teaching children to read and protecting their eyesight.

Researchers have been worrying about the necessity of this trade-off for years. In the 1930s scientists observed that myopia was rare in hunter-gatherer societies, and a 1960s study of native people in Alaska showed that older generations, who had not attended school, were much less likely to have myopia than younger generations who had. Singaporean studies 20 years ago likewise linked educational attainment to myopia.

If the problem is just a matter of light intensity, however, you could send your child outside to read or purchase high-intensity light sources that mimic outdoor exposure.

The unfortunate part is that parents don’t seem inclined to send their children outside like they used to – or, alternatively, computers, video games, and improved television programming have made the indoors too delightful for a child to resist.

According to a 2004 study from the University of Michigan, the average child in 2002 spent exactly half as much time participating in outdoor activities as did children in 1981. While myopia hasn’t yet reached the levels seen in much of Asia, prevalence in the US is rising quickly.

A 2009 study showed that the prevalence of myopia among Americans aged between 12 and 54 surged from 25 percent in the early 1970s to 42 percent around the turn of the millennium.

When I was a kid, my parents used to tell me to turn off the television and go outside to enjoy the beautiful weather.

Perhaps the new mantra should be: “Go outside, you’re blinding yourself in here.” – Slate / The Washington Post News Service

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