The myopia boom : Nature News and Comment
The condition is more than an inconvenience. Glasses, contact lenses and surgery can help to correct it, but they do not address the underlying defect: a slightly elongated eyeball, which means that the lens focuses light from far objects slightly in front of the retina, rather than directly on it. In severe cases, the deformation 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 now have this extreme form of myopia, and half of them are expected to develop irreversible vision loss. But it was obvious that genes could not be the whole story. One of the clearest signs came from a 1969 study of Inuit people on the northern tip of Alaska whose lifestyle was changing2. Of adults who had grown up in isolated communities, only 2 of 131 had myopic eyes. But more than half of their children and grandchildren had the condition.
Genetic changes happen too slowly to explain this rapid change — or the soaring rates in myopia that have since been documented all over the world see 'The march of myopia'. “There must be an environmental effect that has caused the generational difference,” says Seang Mei Saw, who studies the epidemiology and genetics of myopia at the National University of Singapore. The modern rise in myopia mirrored a trend for children in many countries to spend more time engaged in reading, studying or — more recently — glued to computer and smartphone screens. This is particularly the case in East Asian countries, where the high value placed on educational performance is driving children to spend longer in school and on their studies. A report last year3 from the Organisation for Economic Co operation and Development showed that the average 15 year old in Shanghai now spends 14 hours per week on homework, compared with 5 hours in the United Kingdom and 6 hours in the United States. Attractive though the idea was, it did not hold up.
In the early 2000s, when researchers started to look at specific behaviours, such as books read per week or hours spent reading or using a computer, none seemed to be a major contributor to myopia risk5. But another factor did. In 2007, Donald Mutti and his colleagues at the Ohio State University College of Optometry in Columbus reported the results of a study that tracked more than 500 eight and nine year olds in California who started out with healthy vision6. The team examined how the children spent their days, and “sort of as an afterthought at the time, we asked about sports and outdoorsy stuff”, says Mutti. Rose's team tried to eliminate any other explanations for this link — for example, that children outdoors were engaged in more physical activity and that this was having the beneficial effect.
But time engaged in indoor sports had no such protective association; and time outdoors did, whether children had 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 and close work. “We had these children who were doing both activities at very high levels and they didn't become myopic,” says Rose. Close work might still have some effect, but what seemed to matter most was the eye's exposure to bright light. Some researchers think that the data to support the link need to be more robust.
Most epidemiological studies have estimated children's time outdoors from questionnaires — but Christine Wildsoet, an optometrist at the University of California, Berkeley, says that such data should be treated with caution. In a small, pilot study of wearable light sensors8, she found that people's estimates often do not match up with their actual exposure. And Ian Flitcroft, a myopia specialist at Children's University Hospital in Dublin, questions whether light is the key protective factor of being outdoors. He says that the greater viewing distances outside could affect myopia progression, too. “Light is not the only factor, and making it the explanation is a gross over simplification of a complex process,” he says. Yet animal experiments support the idea that light is protective.
Researchers first demonstrated this in chicks, a common lab model for studying vision. By fitting chicks with goggles that alter the resolution and contrast of incoming images, it is possible to induce the development of myopia while raising the birds under controlled conditions in which only light intensity is changed. In 2009, Regan Ashby, Arne Ohlendorf and Frank Schaeffel from the University of Tübingen's Institute for Ophthalmic Research in Germany showed that high illumination levels — comparable to those encountered outside — slowed the development of experimentally induced myopia in chicks by about 60% compared with normal indoor lighting conditions9. Researchers elsewhere have found similar protective effects in tree shrews and rhesus monkeys10. 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. Morgan is buoyed by the preliminary findings, but thinks that he can do even better. “We've got proof of principle that increasing the amount of time children spend outside actually works,” he says. “The question then is how do we make this work in practice at a level that would have a significant impact?” He recognizes that many schools do not have the flexibility to add time outdoors. So last year, in collaboration with Congdon, he began piloting the idea of teaching kids in a classroom made of glass to let in more natural light.
“This glass classroom idea is quite applicable for whole swathes of China,” Congdon says.