In the study, published in the American Journal of Ophthalmology, researchers reviewed the medical records of 135 Minnesota children with intermittent exotropia -- where one eyeball sometimes moves outward (away from the nose) when a person focuses on an object.

They found that 91 percent of the children became nearsighted by the time they were 20, including those who'd had surgical correction of eye misalignment.

Intermittent exotropia is one form of strabismus, a disorder in which the two eyes fail to focus on the same image. In Western countries, the most common form of strabismus is esotropia, where the eyes turn inward; about 1 percent of U.S. children have intermittent exotropia.

However, the disorder is twice as common among Asian children as esotropia is, which means it may be the most common form of strabismus worldwide. Still, the effects of intermittent exotropia on children's vision had not been well studied, according to the researchers on the work, led by Dr. Brian Mohney of the Mayo Clinic in Rochester, Minnesota.

For their study, the researchers reviewed the records of 135 children who were diagnosed with intermittent exotropia between 1975 and 1994.

Overall, they found, 7 percent of the children developed nearsightedness by age 5, 46 percent by age 10, and 91 percent by age 20.

Those rates are much higher than the norm for the U.S., where, research suggests, roughly one-third of 12- to 17-year-olds are nearsighted, Mohney and his colleagues point out.

Strabismus can develop due to a problem in the brain's coordination of the eyes or a disorder in the muscles that control eye movement. Eye muscle surgery is one treatment option.

Of the children in the current study, 40 percent had surgery to correct the eye misalignment. However, surgery had no bearing on whether they developed nearsightedness, the researchers found.

The findings, Mohney and his colleagues write, do not prove that intermittent exotropia itself causes nearsightedness. They do, however, suggest that it is a risk factor for the vision problem, the researchers add.

They say the results also underscore the importance of having children with intermittent exotropia regularly visit their eye doctor, so that any vision problems can be detected early.