Close up on myopia: U.S. prevalence climbs

New study shows jump in myopia rate

Pentacam image in patient with high myopia/astigmatism Source: Jeffrey Horn, M.D.

The American population is becoming significantly more myopic, with prevalence jumping from 25% in the early 1970s up to 41.6% in the recent period from 1999-2004, according to Susan Vitale, Ph.D., research epidemiologist, National Eye Institute, Bethesda. Results published in the December 2009 issue of Archives of Ophthalmology show that occurrence of myopia has substantially grown in this country over the past several decades.
This recent study was driven by the availability of modern data on the incidence of myopia in the U.S. “In 2008 we published a paper on refractive error in the U.S. based upon the currently used methods for defining whether someone is myopic or not, or has other kinds of refractive error,” Dr. Vitale said. “There were U.S.-wide figures and they were higher than the ones that had previously been determined from similar data 30 years ago.” However, since the approach taken in the 2008 study was different from the one taken in the 1970s, investigators felt that they were not necessarily comparable. Fortunately, however, the same type of data that had been used in the 1970s was available from the 2008 study. “We noted that we had the same information in the newer data set that the researchers used in the old data set to come by their [myopia] definition,” Dr. Vitale said. “This was the first chance we had to use high quality data that represent the entire U.S. to be able to examine the question of whether the prevalence was changing over time.”

Duplicating methods

Investigators did their best to duplicate the approach used in the earlier study. “One of the authors on the paper, Bob Sperduto, was involved in the original study, so I worked closely with him to make sure that I was applying the method in the same way as they had back then,” Dr. Vitale said. In the old study, investigators noted that refractions were not available on everyone. “The researchers took visual acuity when people came in and did lensometry on their glasses and contact lenses if they had them,” Dr. Vitale said. “The researchers measured these and if there was a negative spherical equivalent they would say the patient was myopic.” For those with lower levels of vision but no glasses, pinhole acuity was used as a surrogate measure to see if their vision was corrected. “If researchers saw improvement in the patient’s vision with the pinhole then they would say, ‘It’s a refractive error, but we don’t know what kind of refractive error we are seeing here,’” Dr. Vitale said. These patients weren’t considered in either study.
It was only those who had 20/50 acuity or worse in the initial study who underwent refractive testing. “We had visual acuity and lensometry in the 1999 to 2004 NHANES [National Health and Nutrition Examination Survey] study and we had refraction on everyone,” Dr. Vitale said. “We basically pretended that we didn’t have refraction on everyone so that we would be in the same situation as 30 years ago.”
When investigators compared the resulting data they saw that prevalence of myopia was significantly higher in the recent study, with 41.6% of patients considered to be myopic versus just 25% in 1971-1972. This dramatic jump held true for various racial groups. In 1971-1972 just 13% of black individuals were identified as myopic versus 33.5% in 1999-2004. Likewise, just 26.3% of white individuals were considered myopic compared with 43% in the new study.
These results are in keeping with investigators’ expectations. “We suspected this because results have been coming in from other countries saying that they saw an increase in myopia,” Dr. Vitale said. “We thought it was possible that there was an increase but it was nice to be able to confirm it with solid data that represented the whole U.S. and to be able to get the methods as close as possible [to the original study].”

Considering the cause

Investigators are unsure why this jump in myopia has occurred. “There are a lot of different hypothesis out there, and unfortunately the NHANES survey didn’t have enough data to be able to look at any of those in depth,” Dr. Vitale said. “For example, we didn’t have information on how many hours a day people spent doing x, y, or z close up.”
One hypothesis to explain this may be less time spent outdoors. “There’s a group in Australia that looked at this carefully and they found that being outdoors more did seem to be an important protective factor,” Dr. Vitale said. “They’re not sure why that is. It could be that people are viewing distant objects more or that the light is different.”
Delving into the possible causes is something to which the National Eye Institute is committed. “This affects a high proportion of the American public,” Dr. Vitale said. “Currently we’re funding over $10 million worth of research into myopia looking at things like a possible biological basis for this.”
Overall, Dr. Vitale stresses that there is an upside. “I think the good news here is that this is easily and effectively treated,” she said. “The more difficult side is that we don’t really know what’s causing the increase. We hope in a few years, when these studies that we’re funding have results, we’ll be able to have a better understanding of why the increase is happening.”

Editors’ note: Dr. Vitale has no financial interests related to her comments.

Contact information

Vitale: sev@nei.nih.gov