Bad company

One defective eye can spoil the other. Early detection and treatment helps

Published: Monday 15 November 1999

 Correct vision: a definite po two separate studies report that the visual experience of one eye influences the growth and subsequent quality of vision in the other. 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, normal eye. The studies by researchers Dolores Bradley and Alcides Fernandes from the Yerkes Primate Research Centre, Emory University, Atlanta, Georgia, further substantiate growing evidence thatright from infancy, visual development is influenced by a control system.This system integrates the two eyesand its functioning is dependentnot merely on birth but the associated environmental factors in which achild grows up ( Investigative Ophthalmology & Visual Science ,Vol 40, No 1; and Vision Research ,Vol 39, No 16).

The discovery of how this inter-ocular control system works is a major step towards prevention of problems like myopia, a condition affecting approximately a quarter of the global urban population.

Though the research was done on monkeys, the results have eventual clinical applications for humans aswell because their visual systemis designed for two eyes to act asone. Bradley, a vision research scientist, and Fernandes, a paediatric ophthalmologist, used monkeys as modelsfor conditions affecting the visualsystem in children. Their findingssuggest that infants' eyes should be checked in the first year itself, so that a problem in one eye can be detected early, and prevent it from disrupting the growth of the other eye.

Parents, even in developed countries like the uk , often wait until their children reach the schooling age to have their eyes checked. By then, subtle problems in one eye may have already damaged the healthy eye as well. "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 six months or one year is not developing properly, clinicians could intervene to get him back on track," says Bradley. Such intervention, she says, could save a child from myopia, hyperopia (farsightedness) or strabismus (crossed-eyes), all of which would otherwise affect the child for life. Such problems inhibit depth perception or the ability to see fine detail and general visual acuity, limiting an individual's activities.

Both monkeys and humans start life with their eyes not fully developed; the eyeball is too short for the eye's powerful optic capability. As babies grow, the eyes grow in length, so that by age one in monkeys and age four in humans, about 95 per cent of the growth is complete. This is about a seven millimetre increase -- the right length for normal vision with no refractive error.

Refraction refers to the deflectionof a ray of light as it enters the eye. Asit goes in, the light is bent or refracted. With normal vision, the image of adistant target comes into focus on the surface of a retina located at the backof the eye. In a myopic (nearsighted) eye, the image comes into focusbefore the retina; in the hyperopic(farsighted) eye, the image is focussed past the retina.

Earlier, 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," says Fernandes. The finding also allows to exercise at least some control over how our eyes develop.

"Instead of having people wait to develop myopia and treat it in adulthood," says Bradley, "we hope that this type of study will allow us to find out very 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 seven is even more so at 20. If eye care experts could know that a four-year-old child is going to be myopic at seven, they could impose some form of external correction to manipulate growth so the eyes don't get defective.

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