Middle- and lower-income children don't visit eye doctors as often as wealthier kids, and as a result, thousands of them may have undiagnosed sight-threatening conditions, U.S. researchers say.

All of the nearly 900,000 children in the study were covered by a national health insurer, but still, there were disparities in their access to eye care, researchers report in Health Affairs.

Experts advise that all children under age 5 be screened for two eye diseases, strabismus and amblyopia. In strabismus, the eyes are not aligned with each other, causing double vision. To get rid of the double vision, the brain will ignore sight from one of the eyes, which can lead to the development of amblyopia, or so-called "lazy eye," in which vision from that eye is permanently reduced.

"The earlier in life strabismus is detected and properly treated, the less likely the eye will become 'lazy' and the more likely any vision loss that may have occurred can be reversed," said lead author Dr. Joshua Stein of the University of Michigan.

People need to know the importance of testing for these sight-threatening diseases in children, Stein added by email.

To determine the effect of wealth on eye care visits and diagnoses of these two conditions, Stein and his colleagues used healthcare data on 890,090 U.S. children between 2001 and 2014. The children were from families of varying wealth levels, but all had the same type of health insurance.

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The researchers found that children in the lowest wealth category had 16 percent fewer visits to any eye care professional than those at the middle wealth level, while children at the highest wealth level had 19 percent more visits than middle-income kids.

A similar trend was seen for visits to ophthalmologists, medical doctors who treat eye diseases like strabismus and amblyopia.

For visits to optometrists, who are more likely to conduct screenings and provide glasses, the middle wealth category had the highest number of visits.

Children from less wealthy families were also older at their first visit to the eye doctor.

Compared to those in the middle wealth category, lower income children were 15 percent less likely to have their first eye doctor visit during the study period, while higher net worth children were 19 percent more likely to have a first visit.

Wealthier children were 64 percent more likely to be diagnosed with strabismus by age 10 than the lowest income group and 55 percent more likely to be diagnosed with amblyopia.

Assuming the wealthiest kids were not being misdiagnosed, the authors calculate that the differences mean a lot of cases of eye disease are being missed in less-wealthy children.

Specifically, they estimate there were nearly 13,000 missed strabismus diagnoses and 5,000 missed amblyopia diagnoses over a 10-year period just in their sample group.

Even for families with health insurance, having a lower income can cause logistical issues in seeking eye care, said Cathy Williams, a senior researcher at the National Institute for Health Research.

Williams, who was not involved in the study, said eye care providers might be located far from low-income housing sites and that lower income families may not understand the need for this type of vision screening.

"It may be more difficult for parents to take time off from work to take their children to an eye care professional, compared with more affluent families," Stein said, adding that copayments and deductibles might also be an issue for lower-wealth families.

Williams said vision screenings are important to catch other types of sight problems as well. "If having blurred vision in both eyes reduces a child's ability to learn and engage with their education, as it may, this could lead to lasting reductions in their life chances - all for the want of glasses," she said by email.

Stein noted that vision screenings sometimes take place at schools or health fairs. "If one's child has a failed vision screening, it is essential to promptly take him or her to an eye care professional so they can further evaluate the child to check for these serious eye diseases," he said.