Overall, about 90 percent of the children were in the normal range (measured in eye exams as less than 1 diopter of myopia and greater than 4 diopters of hyperopia). African-American children were more likely to be myopic (6.6 percent) than Hispanic children (3.7 percent), and rates declined with age in both groups. MEPEDS results suggest that low-level myopia that improves with age may be normal, especially in African American infants: 14 percent were myopic at 6 to 11 months, but only 4 percent at 48 months and older. Other studies show that myopia increases again in both groups in school-aged children. Hyperopia was more prevalent in Hispanic than in African-American children (26.9 versus 20.8 percent, respectively). Prevalence declined between ages 6 and 24 months, then stabilized or increased, indicating that not all children "grow out" of hyperopia. The 2-to-3 year old time period is also when eye misalignment (esotropia, one eye turned inward) is likely to occur, and the researchers think persistent hyperopia and the onset of esotropia may be related.

Anisometropia, defined as a difference of more than 1 diopter of refractive error between the two eyes, was found in 4 to 6 percent of preschoolers in both ethnic groups. Prevalence of this vision disorder declines between 6 months and 2 years, but remained fairly stable after age 2. Anisometropia is associated with strabismus (misaligned eyes) and with amblyopia, also called "lazy eye," in which one eye increasingly does the work of seeing while the other loses vision. If a child with amblyopia receives early and consistent treatment, he or she usually regains normal vision.

"Preschool refractive error screening could detect many children with amblyopia related to anisometropia, so that treatment could be started early," Dr. Varma said. "Studies are needed to further assess anisometropic changes over time and relate anisometropia levels to risk for developing amblyopia and strabismus, so that effective vision screening protocols can be developed."

Source: American Academy of Ophthalmology