Significantly more Americans are nearsighted today than in the early 1970s, a report released this week indicates.

Nearsightedness, also called myopia, is when the eyes focus incorrectly to make distant objects appear blurred. This common problem can be treated by corrective eyeglasses or contact lenses or refractive surgery.

Researchers don't know why more Americans are becoming nearsighted, and "at this time, we really don't know how to prevent myopia," Dr. Susan Vitale of the National Eye Institute, part of the National Institutes of Health in Bethesda, Maryland, noted in a telephone interview with Reuters Health. "It's really important to get regular eye examinations from an eye care professional," she said.

Vitale and colleagues used data from the National Health and Nutrition Examination Survey (NHANES) to compare the percentage of black and white Americans aged 12 to 54 with myopia in 1971-1972 and 1999-2004.

They used the same methods to determine myopia during both time periods. "We wanted to make it as similar as possible to get a feel for whether the prevalence was actually increasing or not," Vitale explained.

They found that in 1971 to 1972, 25 percent of Americans aged 12 to 54 had myopia. This figure had jumped to 42 percent in the 1999-2004 period.

The prevalence of myopia was roughly 66 percent higher in the 1999-2004 period than in the 1971-1972 period, the investigators note in the latest issue of Archives of Ophthalmology.

This wasn't all that surprising, Vitale told Reuters Health, given reports from Asia, Australia, Africa, and Israel indicating that the prevalence of myopia is increasing in those regions. "This is something that has been on the radar for a while," Vitale said, "but it's the first time that we have tried to nail it down as carefully as possible in the US."

Differences in myopia rates were "striking" for black people, in whom the 1999-2004 rates were "more than double" the rates in the earlier time period. In white participants, the 1999-2004 rates were 63 percent higher than those in 1971-1972. In 1999-2004, myopia rates in men and women were 64 percent and 69 percent higher, respectively, than in 1971-1972.

While the exact cause of myopia is unknown, there are several risk factors, including being born prematurely or genetics. "If your parents are myopic, you are more likely to be myopic," Vitale said.

There have also been studies linking myopia to "more close-up work" such as reading, sitting at a computer screen, or using small electronic devices. This is a "reasonable" possibility, Vitale said, given how work and entertainment habits have changed in the past 30 years.

"An interesting study" from Australia, Vitale noted, found evidence that children who spent the most time outdoors were the least likely to suffer from myopia. "Outdoors you have different lighting conditions and you are looking at distant objects instead of near objects," Vitale noted, and both of these factors may have an effect on the risk of myopia.

While myopia can be treated relatively easily with glasses and contacts, its costs are substantial on a population basis owing to its high prevalence, Vitale and colleagues note in their report. "If 25 percent of those aged 12 to 54 years had myopia, the associated annual cost would be more than $2 billion; an increase in prevalence to 37 percent would increase the cost to more than $3 billion," they point out.

Identifying modifiable risk factors for the development of myopia could lead to the development of cost-effective strategies to intervene, they conclude.

SOURCE: Archives of Ophthalmology, December 2009.