Etiology of myopia hard to pinpoint

Baltimore-Over the past several years, researchers of numerous studies have tried to pinpoint the exact cause of myopia, to no avail. The jury is still out to what degree the causes are hereditary or environmental, said Elliott H. Myrowitz, OD, MPH, at the 3rd annual Evidence-Based Care in Myopia Control, Retina and Vision Enhancement meeting.

Dr. Myrowitz of the Wilmer Eye Institute at Johns Hopkins University, Baltimore, MD, chaired the meeting, which was held in conjunction with the Maryland Optometric Association.

Nature versus nurture?

The role of inherited factors and environmental factors on the presence of myopia is a topic that has long been studied by researchers.

"Is it nature or is it nurture?" Dr. Myrowitz asked. "These two areas are still being studied. Heritability has been examined in twin and family studies, while 'nurture' or environmental factors, such as reading, are also under investigation."

Dr. Myrowitz reviewed several of the studies that have been conducted to better elucidate what the role of heritability and environment are in the etiology of myopia. He began with the Singapore Cohort Study that showed that there were some correlations between socioeconomic factors and myopia.²

"Higher family income was associated with higher myopia in this study, as was higher education," Dr. Myrowitz noted. "Also in this study, the number of books read was significant, and correlated with higher myopia. In later studies, it was not."

Also in 2002, researchers of The Orinda Longitudinal Study found that parental myopia was the most powerful predictor.³ But they also found that many children with non-myopic parents were reading as much as children of myopic parents, and this became a confounding factor.

The Genes in Myopia (GEM) study looked a genetic variations, and they concluded that both heritability and environmental factors play a role in the etiology of myopia, and attributed 55% of myopia to heritability and 33% to environmental factors.⁴

In 2008, the Singapore Cohort Study on the Risk Factors for Myopia looked at two cross-sectional samples of children aged 6 and 7 years, all of Chinese ethnicity, with similar parent refractive errors.⁵ Some of these children lived in Singapore, and others lived in Sydney, Australia. Of those living in Sydney, only 3% were myopic, compared to 29% in Singapore.

"It was the same heritage in these children, but such difference in the prevalence of myopia," he said. "Also interesting in this study was that the children in Sydney, with the lower rate of myopia, were actually reading significantly more books, at the p = 0.001 level."

A key variable these researchers found was the children in Sydney were spending 13 hours per week doing outdoor activities, compared with only 3 hours per week in the children from Singapore.

"This study has garnered a lot of interest, and it is why being outdoors has become a hot topic in myopia research," Dr. Myrowitz noted.

Studies in twins have been numerous, he continued. The most recent one, done in 2009, found that heritability accounted for 77% of myopia, while environmental factors accounted for 33%.⁶

"If myopia is heritable, what causes it? Is there a 'myopia gene?' Can we do something about this gene?" Dr. Myrowitz asked. "All the data says that we don't have an exact idea of which genes they are. Many people are researching this as reported in a recent review."⁷