Feb. 1, 1999
Volume 51, No. 18
Two Yerkes researchers focus on myopia in monkeys
In two separate studies, vision researchers at Yerkes have discovered that the visual experience of one eye influences the growth and subsequent quality of vision in its fellow eye. Previously it was believed that a problem existing in one eye--a cataract or nearsightedness, for instance--would have no ill effect on the other.
The studies, reported in the January issue of Investigative Ophthalmology & Visual Science and the upcoming May issue of Vision Research add to the growing evidence that visual development is influenced from infancy by a control system integrating the two eyes that is dependent on environmental, not merely genetic, factors.
Discovering how this inter- ocular control system works is a major step toward the prevention of problems like nearsightedness, or myopia--a condition affecting approximately 25 percent of the U.S. population and costing billions of dollars a year in treatment. Preventing myopia during the development stage would enable more people to achieve accurate focus without wearing eyeglasses or having corrective surgery, the effects of which are not always permanent.
Though the work was done in monkeys, the results have eventual clinical applications for humans, according to Dolores Bradley and Alcides Fernandes, co-investigators of the studies. Bradley, a vision research scientist, and Fernandes, a pediatric ophthalmologist, use monkeys as models for conditions affecting the visual system in children. Their findings suggest that infants should have their eyes checked in the first year of life so that a problem with one eye can be detected early and prevent disrupting the growth of the fellow eye. Parents often wait until their children reach school age to have their eyes checked, but by then subtle problems in one eye may have already damaged the healthy eye as well.
Bradley first studied a group of 237 normal monkeys to develop a road map for exactly what "normal" visual development entails. "Our ultimate goal is to find a way to predict the course of normal eye growth--to work out a schedule so that if an infant at 6 months or 1 year is not developing properly, clinicians could intervene to get him back on track," Bradley said. Such intervention, she said, could save a child from myopia, hyperopia (farsightedness) or strabismus (cross-eyes), all of which would otherwise affect the child for life. Such problems inhibit depth perception, ability to see fine detail and general visual acuity.
After examining the normal group, the team compared it with 15 neonatal monkeys that were reared in three different treatment groups until adolescence. For each group, one "treated" eye was allowed a different type of visual input using specially made lenses. The fellow "untreated" eye had completely normal vision. The untreated eyes from the three test groups were then compared with monkeys of the same age who had normal vision in both eyes.
Results showed that normal, untreated eyes varied systematically according to the differences in the visual experience of the treated eyes. Before it was thought that each eye had its own regulatory mechanism. "Now we see that they work together, so whichever corrective prescription you give to a child, it may affect the development of both eyes," Fernandes said.
The finding does present the opportunity, however, to exercise at least some control over how our eyes develop. "Instead of having people wait to develop myopia and treat it in adulthood," Bradley said, "we hope this type of study will allow us to find out early in life if myopia is likely to occur later--so we can correct the visual input to prevent it. That's why we study eye growth-to see if it can be influenced to prevent or reduce the need for glasses and slow down the inevitable progression of myopia as a function of age."
For instance, a child who is myopic at age 7 is even more so at 20. If eyecare providers could know at 4 years of age that the child is going to be myopic at 7, they could impose some form of external correction to manipulate growth.
The surgeries available today to correct myopia do so by altering the curvature of the cornea. The Yerkes lab is attacking the same problem with a different approach-trying to influence the length of the eye. "It's just an earlier stage of intervention," Bradley said. The scientists point out, however, that clinical application is still in the future and will require further investigation.