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Findings on Myopia Call for Action

Myopia Boom Seen in Many Parts of World

According to an article in the journal Nature, there has been a dramatic increase in myopia worldwide. Myopia now affects about 50% of young adults in the US and Europe – double the prevalence of a half century ago. Sixty years ago, 10–20% of the Chinese population was short-sighted, while up to 90% of teens are today. In Seoul, South Korea, a whopping 96% of 19-year-olds are myopic (1).

Myopia (near-sightedness) is a condition in which light coming into the eye doesn’t focus correctly. People with myopia aren’t able to see distant objects in focus, but they can focus properly on objects up close. The underlying defect is a slightly elongated eyeball. That means that the lens focuses light from far objects slightly in front of the retina, rather than directly on it. Glasses, contact lenses or surgery usually help correct myopia. But in severe cases, myopia raises the risk of cataracts, glaucoma, and retinal detachment. With short-sightedness reaching near epidemic proportions, many scientists are trying to identify the factors that may cause it – from genetics to the environment.

Can a Tendency for Myopia Be Inherited?

Looking for other myopia-related risk factors, scientists noted that the modern rise in myopia mirrored a trend for children in many countries to spend more time engaged in reading, studying or glued to computer and smartphone screens. It seemed biologically plausible that constant up close work could alter growth of the eyeball as it tries to focus close-up images exactly on the retina. While some believe that excess close work could influence myopia development, the association hasn't held up well over time. In 2007, a study turned attention to light exposure as a potentially important contributor. Following 500 eight and nine-year olds with healthy vision over 5 years, the study found that the only factor strongly associated with myopia risk was time spent outdoors. Subsequent studies seem to rule out physical activity as an explanation for the outdoors link, since similar activities done indoors didn’t afford protection. The outdoors does offer longer viewing distances, which could help counter too much close work. However, what seems to matter most is bright light exposure.

How Might Bright Light Be Protective?

How bright light may offer protection is not yet understood. The leading theory is that light triggers the release of dopamine in the retina, and that this neurotransmitter in turn blocks the elongation of the eye during development (2). Another explanation involves vitamin D, as sunlight stimulates the body’s production of this vitamin. It’s thought that vitamin D influences ocular growth through interactions with vitamin A. So far, some studies report that myopia is associated with lower vitamin D status in young adults, while others question low ‘D’ levels as an independent risk factor. Based on studies to date, one researcher estimates that children need to spend around 3 hours daily under light levels of at least 10,000 lux to be protected against myopia. This is about the level experienced when sitting under a shady tree, wearing sunglasses on a bright summer day. In contrast, a well-lit class-room is usually no more than 500 lux.

Diet and Diabetes

Another risk factor proposed is diet. The increasing commonness of myopia in countries that have adopted Western ways of eating – bigger portions with a higher glycemic index – has led to the hypothesis that insulin resistance affects growth factors influencing the shape of the outer layer of the eyeball. Some studies report that myopia is more common in diabetics than non-diabetics, and that myopia is more common than farsightedness in those with diabetes.

Bottom Line: While scientists sort out which factor or factors are important, all of these actions – sending kids outside, making sure they eat well, and insuring good vitamin D intake – have plenty of health benefits beside those for the eyes.

References

1. Dolgin E. The myopia boom. Nature 519:276-78, 2015.
2. Goldschmidt E & Jacobsen N. Review: Genetic and environmental effects on myopia development and progression. Eye 28:126-33, 2014.