Treating "lazy eye" is more likely to be successful the younger the child, but even older kids can benefit more than was previously believed, according to a large new study.

Researchers reviewed data on nearly 1,000 children treated for amblyopia, or lazy eye — diminished vision in one eye that can stem from a number of causes. They found that kids between the ages of three and seven were much more responsive to treatment than kids between seven and 13, especially in moderate to severe cases.

Despite older kids achieving less dramatic improvements than the younger groups, however, they did get somewhat better and in a few cases saw significant vision gains.

"Earlier is still better, but you still can treat late," said study co-author Dr. Michael Repka, an ophthalmologist at Johns Hopkins University in Baltimore, Maryland.

A lazy eye is caused either by shortsightedness in one eye, or a misalignment of the eye that affects vision. If the condition is left untreated, the brain will eventually begin to ignore the images from the lazy eye, leading to permanent vision problems.

It is the leading cause of vision problems in children, affecting between two and four percent of all children.

Treatment begins by first addressing any underlying eye conditions that are responsible for the poor vision through surgery, eye exercises or eyeglasses. Next, the good eye is covered with a patch for two hours a day or given vision-blurring eye drops to force the weaker eye to work.

The treatment has no significant side effects and with it, "85 percent of children get to 20-30 vision or better," said Repka.

To see whether age makes a difference in how successful treatment will be, he and his colleagues in the multi-institution Pediatric Eye Disease Investigator Group reviewed the results of four previous studies that included 996 children.

The kids were divided into three groups: ages three to five, five to seven and seven to 13. They were also separated into two categories, moderate and severe, depending on the quality of vision in their affected eye.

Younger children showed a better response to treatment in both moderate and severe cases of lazy eye. Among moderate cases, children aged three to five improved their vision by 39 percent more than the oldest children and the kids between five and seven improved their vision 46 percent more.

Measuring vision improvements by a scale known as logMAR lines, which roughly reflects rows on a standard eye chart, kids between three and five gained an average of 2.29 lines, kids five to seven gained 2.41 lines and kids seven to 13 gained 1.65 lines.

More dramatic differences were observed in children with a severe lazy eye, although the authors caution that the study did not include many children with severe disease so the numbers may not be broadly representative.

In that category, children in the youngest age group improved their vision, on average, by 4.16 lines of visual acuity -- more than twice as much the oldest children who gained 1.99 lines.

The response to treatment for a lazy eye "is only important if they maintain that level" of vision, noted Dr. Norman Medow, director of Pediatric Ophthalmology at Montefiore Medical Center in the Bronx, New York, who was not involved in the study.

The new results are important, but the patient's age may not predict whether they are likely to see long-term benefits from the treatment, he told Reuters Health.

Despite the advantage of earlier treatment, Repka observed that the benefits of the treatment for older children were better than previously thought. "A group that we didn't think is treatable is moderately treatable," he said.