About 4 percent of preschoolers have myopia (nearsightedness), 21 percent have hyperopia (farsightedness), and 10 percent have astigmatism (irregular curvature of the eye), according to a study funded by the National Institutes of Health. The largest of its kind, the study identified related risk factors, including age, ethnicity, exposure to smoking, and access to health insurance.
Uncorrected refractive errors, such as myopia, hyperopia, and astigmatism are the most common causes of vision impairment in children. Caused by slight irregularities in eye shape, refractive errors affect how light focuses on the retina, the part of the eye that converts light into signals the brain perceives as images. Refractive errors are correctible with eyeglasses, but uncorrected they can lead to lasting vision problems such as strabismus (misalignment of the eyes) and amblyopia (decreased vision in one or both eyes that cannot be corrected with eyeglasses).
The study identified strabismus in about 2 percent of children and amblyopia in about 5 percent of children. Previous research showed that children with moderate to severe refractive errors had an increased risk of these disorders; however, the current study found that risk increases even with mild refractive errors—a potentially important finding because strabismus and amblyopia can become permanent if not treated during childhood. Furthermore, mild differences in hyperopia or myopia between the eyes increased the risk of amblyopia. Additional research investigating changes in refractive error and vision as children develop may help define thresholds of refractive error at which treatment is appropriate.
The cross-sectional study suggests that astigmatism and myopia, but likely not hyperopia, may resolve with age. Although 23 percent of children 6 months to 1 year old had astigmatism, only 9 percent of children 5 to 6 years old had astigmatism. A similar trend was seen with myopia (6 percent versus 3 percent). Hyperopia, by contrast, was more common in older children: 18 percent of children 2 to 3 years old were hyperopic, versus 23 percent of children 5 to 6 years old.
Ethnicity was also associated with risk of refractive errors. Myopia was more common among African-American children (6 percent), compared to Hispanic children (3 percent) and non-Hispanic white children (1 percent). But African-American children (17 percent), compared to Hispanic children (23 percent) and non-Hispanic white children (25 percent), were least likely to have hyperopia. Astigmatism was more common among Hispanic children (13 percent) and African-American children (9 percent) than non-Hispanic white children (6 percent).
Smoking and lack of health insurance increased the risk of vision problems. Children whose mothers smoked during pregnancy had a 1.5 times greater risk of hyperopia and astigmatism and twice the risk of strabismus, compared to children whose mothers didn't smoke during pregnancy. Children whose parents lacked health insurance were almost 3 times more likely to have amblyopia than those whose parents had health insurance.
Follow-up studies may help researchers better understand these risk factors.
The study was a collaboration between the Multi-Ethnic Pediatric Eye Disease Study (MEPEDS) and the Baltimore Pediatric Eye Disease Study (BPEDS), which are funded by the National Eye Institute. Together, MEPEDS and BPEDS recruited almost 10,000 children, ages 6 months to 6 years, from communities in Los Angeles County, Calif., and Baltimore, Md. Evenly split by gender, the study population was about 44 percent African-American, 32 percent Hispanic, and 25 percent non-Hispanic white. Data for this age group, and especially for minority children, were previously lacking.
The study's findings, which were published in four reports that appeared online today in the journal Ophthalmology, provide a springboard for future research aimed at understanding, correcting, and preventing common pediatric vision problems.