New perspective posits amblyopia secondary to binocular problems

Take-home message: Recent developments imply it may be time to change the current view of amblyopia. Laboratory findings show that the adult cortex is more adaptable than previously thought and that amblyopia may be secondary to binocular problems.

By Nancy Groves; Reviewed by Robert F. Hess, PhD, DSc

Montréal-Long-held beliefs about the etiology of a medical condition and its treatment change as new evidence develops-and this shift is happening with amblyopia.

The current view is that amblyopia is the primary problem and that treatment typically should consist of occluding the good eye to force the amblyopic eye to work-an approach applicable mainly to children.

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However, a new perspective is that amblyopia is secondary to binocular problems, that a binocularly based treatment is preferable to patching, and that treatment could be extended to all ages, said Robert F. Hess, PhD, DSc, professor and director, McGill Vision Research Unit, Department of Ophthalmology, McGill University, Montréal, Québec, Canada.

Paradigm shift

A number of laboratory advances over the past decade have called the widely accepted paradigm of amblyopia into question, Dr. Hess said.

One of these advances is the theory that the adult cortex is plastic or adaptable.

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“There’s plenty of evidence now that in adults, the visual cortex is still capable of substantial change,” Dr. Hess said.

Therefore, reliance on a treatment restricted to children makes no sense.

The amblyopic cortex is also plastic, as demonstrated by the restoration of visual function in adults following a few minutes of magnetic or current stimulation. Evidence shows that the more severe the amblyopia, the greater the improvement.

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“I’m not suggesting that this is a practical means of treatment, but as a principle it shows that the function is not lost, it’s just suppressed, and so there is hope that we can recover this function at any age,” Dr. Hess said.

Secondary to binocular vision

Another important laboratory advance alters the presumption that amblyopia is the primary problem to be corrected, with binocular vision a secondary consequence. One of the flaws with designating amblyopia the target of treatment is that when vision in the amblyopic eye is improved by patching, binocular vision is not automatically regained, Dr. Hess explained.

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“There’s something wrong with the way we’re thinking about it. There is good evidence now from both human psychophysics and also from animal work that it’s the other way around,” he continued. “The primary problem in amblyopia is a binocular one, and as a secondary consequence we have amblyopia.”

If that’s the case, then ophthalmologists should be treating the binocular problem first, and then expecting the amblyopia to improve as a consequence, he added.

Measuring suppression

Evidence for binocular being the primary problem includes the relationship between suppression and amblyopia, in which the greater the suppression, the greater the amblyopia.

“Suppression, then, is an important player, and we need to measure it,” Dr. Hess said.

Next: Strengthening binocular vision

A key laboratory finding that Dr. Hess and colleagues made after measuring suppression is that if the contrast is reduced in the fellow eye, a point can be reached at which the two eyes combine information normally. This only occurs under artificial viewing conditions of low contrast in one eye and high contrast in the other. Under normal conditions, there is significant suppression.

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In tests conducted with strabismic adults, the investigators learned that the longer the patients were able to combine information normally, the stronger the binocular vision capabilities became.

“After only a few weeks of doing this a few hours a day, they could combine vision at all contrasts,” Dr. Hess said.

Strengthening binocular vision

This formed the basis of a treatment for amblyopia in which the investigators tried to arrange viewing conditions so that the patient would use the binocular vision that had been hidden because of suppression and strengthen it over time.

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The dichoptic therapy was administered using video games played on an iPod or iPad. One of the games used in the therapy was the falling blocks game, Tetris. The amblyopic eye saw the falling blocks at high contrast, whereas the fellow eye saw the ground plane stationary blocks at low contrast. The contrast was set for each eye’s image by the use of red-green glasses or a lenticular screen and was tailored to each subject’s deficit to best allow them to combine information.

“Unless they combine information between the two eyes, they’re not going to be able to play the game, and that is our measure that their binocular vision is functioning,” Dr. Hess said.

Results of this therapy in adults have shown that over a relatively short time, binocular vision can be restored so that in the end both eyes can combine information at the same overall contrast levels. Benefits in adults and children with amblyopia have included improved visual acuity, achieved solely by improving binocular vision-which proves the concept, Dr. Hess said. It also improved stereopsis in adults to varying degrees.

Next: Future of therapy

This therapy of binocular computer activities for treatment of amblyopia is undergoing further testing. The Pediatric Eye Disease Investigator Group is enrolling participants for a study, and a U.S. company, Amblyotech, is presently seeking regulatory approval.

Robert F. Hess, PhD, DSc

P: 514/934-1934

E: robert.hess@mcgill.ca

This article was adapted from Dr. Hess’ presentation during Pediatric Subspecialty Day at the 2014 meeting of the American Academy of Ophthalmology. McGill University has U.S. patents on which Dr. Hess is a named inventor that have been licensed to Amblyotech.