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That lazy eye
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Amblyopia can cause unilateral vision loss in about four per cent of the population
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NO CO-ORDINATION: Amblyopia is the most common cause of visual impairment in childhood. PHOTO: MURALIKUMAR K.
Richard, now 72, was 11 when an eye examination at school revealed that the acuity in his left eye was a mere 20/200, far less than the 20/40 in his right eye. Though glasses improved the vision in his left eye, they could not restore what he lost — depth perception gleaned from binocular vision — because this defect had not been found and corrected much sooner.
By the time Richard was 11, his brain had learned to ignore the blurry image from his left eye, and he was unable to coordinate the images from both eyes to form a view of the world that clearly shows how near or far objects are. As a result, he could never learn to catch or hit a ball, and he had to concentrate hard to navigate highway exit ramps.
Richard was an early and avid reader who excelled in school, so no one suspected his eyes were not functioning normally. But without depth perception, he never would have made it as an airline pilot or interstate truck driver. Also, for unknown reasons, as an adult with amblyopia, Richard is at higher than normal risk of suffering an injury in his good eye and, in effect, becoming functionally blind.
The visual cortex of the brain develops rapidly in babies and young children until about age six. Interference with the image that forms on the retina during this critical time can cause the brain to favour one eye over the other. The great disparity in acuity in Richard's eyes caused the visual cortex of his brain to rely only on the input from his right eye.
This condition — amblyopia, which is sometimes called lazy eye — can result from strabismus, crossed eyes that turn in or eyes that turn out. When eyes turn in, double vision results and the brain discards the image from one eye to "fix" the problem. When eyes turn out, the brain receives input from only one eye at a time and learns to favour the better eye.
An eye that drifts off centre by just a degree is enough to cause amblyopia, but the untrained eye will not notice a disparity until one eye turns in or out by about five degrees.
David G. Hunter, an ophthalmologist at Children's Hospital Boston, noted in The Journal of the American Medical Association that "amblyopia is an important public health problem, causing unilateral vision loss in two per cent to four per cent of the population" and "may be the leading cause of monocular vision loss in children and adults up to the age 70."
If a "lazy eye" is not detected and treated at a young age, the vision in that eye gets worse because it is not being used. Chances are, when Richard was a toddler, the acuity in his left eye was better than 20/200, but it lost ground through years of disuse.
Dr. Hunter's article concerned the results of a 49-centre study sponsored by the National Eye Institute that sought to determine whether treatment of amblyopia after age seven might be effective. The study found that some improvement in visual acuity was possible in the "lazy eye" up to age 12, and in some children up to age 17. But, Hunter said, it is not yet known whether the improvement will last once the treatment stops or whether the loss in depth perception can be reversed.
The New York Times
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