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Based on epidemiological studies, children need to spend around three hours per day with light levels of at least 10,000 lux to be protected against myopia. This is about the level experienced by someone under a shady tree, wearing sunglasses, on a bright summer day.

Why we are becoming myopic

Short-sightedness (myopia) is reaching epidemic proportions. Some scientists think they have found a reason why.

[From Nature: The Myopia Boom].

The southern city of Guangzhou has long held the largest eye hospital in China. But about five years ago, it became clear that the Zhongshan Ophthalmic Center needed to expand.

More and more children were arriving with blurry distance vision caused by myopia. With so many needing eye tests and glasses, the hospital was bursting at the seams. So the centre began adding new testing rooms — and to make space, relocated some of its doctors and researchers into a local shopping mall. Now during the summer and winter school holidays, when most diagnoses are made, “thousands and thousands of children” pour in every day, says ophthalmologist Nathan Congdon, who was one of those uprooted. “You literally can't walk through the halls because of all the children.”

East Asia has been gripped by an unprecedented rise in myopia, also known as short-sightedness.

Sixty years ago, 10–20% of the Chinese population was short-sighted. Today, that number is up to 90% of teenagers and young adults. In Seoul, a whopping 96.5% of 19-year-old men are short-sighted.

Other parts of the world have also seen a dramatic increase in the condition, which now affects around half of young adults in the United States and Europe — double the prevalence 50 years ago. By some estimates, one-third of the world's population — 2.5 billion people — could be affected by short-sightedness by 2020. “We are going down the path of having a myopia epidemic,” says Padmaja Sankaridurg, head of the myopia program at the Brien Holden Vision Institute in Sydney, Australia.

The condition is more than inconvenient. Glasses, contact lenses and surgery can help correct myopia, but can not correct the defect: a slightly elongated eyeball. This elongation of the eyeball means the lens focuses light slightly in front of the retina, rather than directly on the retina. In severe cases, the deformity stretches and thins the inner parts of the eye, which increases the risk of retinal detachment, cataracts, glaucoma and even blindness.

Because the eye grows throughout childhood, myopia generally develops in school-age children and adolescents. About one-fifth of university-aged people in East Asia have an extreme form of myopia, and half of them are expected to develop irreversible vision loss.

Scientists are challenging the old idea that myopia is the domain of the bookish child and are joining around a new idea — spending too much time indoors is placing children at risk.

“We're really trying to give this message now that children need to spend more time outside.”

Kathryn Rose, head of orthoptics, University of Technology, Sydney, Australia.

For many years scientists held that myopia was largely based in genetics. Studies in the 1960s showed myopia was more common among identical twins than fraternal twins, which suggested a strong DNA influence. Today more than 100 regions of the genome are identified as linked to short-sightedness.

But genes cannot be the whole story.

A 1969 study of Inuit people on the northern tip of Alaska identified certain lifestyle changes. Of Inuit adults who had grown up in isolated communities, only 2 out of 131 had myopic eyes. But more than half of their children and grandchildren had myopia.

Genetic changes happen slowly, too slowly to explain the rapid eye change amongst the Inuit — or the soaring rates in myopia that have since been documented all over the world.

But there was one obvious, possible, culprit: book work. That idea arose more than 400 years ago, when Johannes Kepler, the German astronomer and optics expert, blamed his own short-sightedness on all his reading. The idea took root and by the nineteenth century, some leading ophthalmologists were recommending pupils use headrests to prevent them from reading too closely to their books.

Attractive as the idea was, it did not hold up. In the early 2000s, research looked at books read per week or hours spent reading or using a computer, none seemed to be a major contributor to myopia. But a distinctly different factor did. In 2007, Donald Mutti at the Ohio State University College of Optometry in Columbus tracked more than 500 eight and nine-year-olds in California who started out with healthy vision. His team examined how these children spent their days, and “sort of as an afterthought at the time, we asked about sports and outdoorsy stuff,” explained Mutti.

It was a good thing they did. After five years, one in five of the children had developed myopia, and the only environmental factor that was strongly associated with risk was time spent outdoors.

“We thought it was an odd finding,” recalls Mutti, “but it just kept coming up as we did the analyses.” A year later, Rose and her colleagues arrived at much the same conclusion in Australia. After studying more than 4,000 children at Sydney primary and secondary schools for three years, they found that children who spent less time outside were at greater risk of developing myopia.

Rose's team tried to eliminate any other explanations — for example, that children outdoors were engaged in more physical activity and that this was the beneficial effect. But time engaged in indoor sports had no protective association; only time outdoors did, whether children played sports, attended picnics or simply read on the beach. And children who spent more time outside were not necessarily spending less time with books, screens or close work.

“We had these children who were doing both activities at very high levels and they didn't become myopic.”

Kathryn Rose

Close work might still have some effect, but what seemed to matter most was the child's eye exposure to bright light.

Based on epidemiological studies, Ian Morgan, a myopia researcher at the Australian National University in Canberra, estimates that children need to spend around three hours per day under light levels of at least 10,000 lux to be protected against myopia. This is about the level experienced by someone under a shady tree, wearing sunglasses, on a bright summer day. (An overcast day can provide less than 10,000 lux and a well-lit office or classroom is usually no more than 500 lux.) Three or more hours of daily outdoor time is already the norm for children in Morgan's native Australia, where only around 30% of 17-year-olds are myopic. But in many parts of the world — including the United States, Europe and East Asia — children are often outside for only one or two hours.

In 2009, Morgan set out to test whether boosting outdoor time would help to protect the eyesight of Chinese children. He and a team from the Zhongshan Ophthalmic Center (where Morgan also works) launched a three-year trial in which they added a 40-minute outdoor class to the end of the school day for a group of six- and seven-year-olds at six randomly selected schools in Guangzhou; children at six other schools had no change in schedule and served as controls. Of the 900-plus children who attended the outside class, 30% developed myopia by age nine or ten compared with 40% of those at the control schools. The study is being prepared for publication.

A stronger effect was found at a school in southern Taiwan, where teachers were asked to send children outside for all 80 minutes of their daily break, instead of giving them the choice to stay inside. After one year, doctors had diagnosed myopia in 8% of these children, compared with 18% found at a nearby school.

Sending children outside to play has plenty of other benefits besides those for the eyes. “It probably also increases physical activity, which decreases likelihood of obesity and enhances mood,” Rose says. “I can only see it as a win — and it's free.”

More than a century ago, Henry Edward Juler, a renowned British eye surgeon, offered similar advice. In 1904, he wrote in A Handbook of Ophthalmic Science and Practice that when “the myopia had become stationary, change of air — a sea voyage if possible — should be prescribed”. As Wildsoet points out: “We've taken a hundred years to go back to what people were intuitively thinking was the case.”

Original article:The Myopia Boom

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