The authors of the new study call for more pre-school testing of childhood vision in the US.
Such a test could enable parents to plan eye testing schedules and glasses, and even opens up the research opportunities into therapies that could prevent the need for glasses.
A study of over 4,500 children published in JAMA Ophthalmology identified 414 who became nearsighted (had myopia, in which further distances are seen as blurry) by the time they were between 7 and 13 years of age.
The participants, who were aged 6-11, all had normal vision at the start of the study. The researchers followed potential risk factors among the participants and nearsightedness was tested over the 20-year study period.
The strongest single predictor of myopia among the 13 factors in the study was refractive error, found the authors led by Prof. Karla Zadnik, dean of the College of Optometry at The Ohio State University in Columbus.
The refractive error is the eyeglasses prescription and is determined in an eye exam at the optometrist's office when they change lenses while you view a distant eye chart. You answer which lens gives better vision, one or two?
The simple test - which the study scientists did using an autorefractor to measure the children's eyes - picks up the fact that children growing up with normal vision are slightly farsighted at around 6 or 7 years old.
Future myopia was found to be reliably predictable by using the test to reveal little or none of this farsightedness in young children, explain the researchers.
They add that near work - such as reading, sitting close to the television or computer work - was among the potential risk factors tested, and that the results do not support the notion of it being a cause of nearsightedness.
Prof. Zadnik explains:
"Near work has been thought to be a cause of myopia, or at least a risk factor, for more than 100 years. Some of the studies that led to that conclusion are hard to refute.
In this large dataset from an ethnically representative sample of children, we found no association."
Study widened to cover greater diversity
The study began in California in 1989, but more evaluation sites were added in the mid-1990s to expand the ethnic diversity of the study sample - the latest analysis is based on a sample split equally by sex, among the following ethnicities:
- 36.2% white
- 22.2% Hispanic
- 16.2% African American
- 1.6% Native American
- 13.7% Asian American.
The findings applied across ethnicities, Prof. Zadnik says, adding: "The prevalence of nearsightedness differs among ethnicities, but the mechanism is the same. If you become nearsighted, it's because your eyeball has grown too long. This prediction model works."
The eye researchers explain this phenomenon: in people with normal vision, the eyeball, growing with the rest of the body, is programmed to stop at a point that gives clear vision. The eyeball in myopia, however, rather than having the typical spherical shape, becomes elongated - shaped more like a grape or olive than a ball.
Less time outdoors, parents with nearsightedness
Other findings among the potential predictors of myopia included the follow-up of results from the same study 9 years ago that suggested nearsightedness was less likely for children who spent time outdoors.
The association remained in this analysis, but is a mystery, says Prof. Zadnik, and is not strong enough to be used as a reliable predictor of nearsightedness.
Another risk factor that was statistically predictive of myopia was parents also having nearsightedness, although part of this association, the paper notes, could be a result of parents with the sight problem being more likely to have their kids tested.
Prof. Zadnik calls for more pre-school testing of childhood vision in the US, which varies by state - "but if everybody went to a mandatory eye exam before school entry, refractive error would be an important variable to report."