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The environment and the eye: an evolutionary perspective
Influences as universal as sunlight and as ubiquitous as plastic are seen as potential threats to healthy vision.
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BETHESDA, U.S.A. — Exploring the ways in which the environment affects the eye involves factors as likely as sunlight and nutrition, as well as less likely contributors such as the prevalence of computers and products made from plastic. Macular degeneration, cataract and dry eye syndrome all have been known to be associated with these environmental influences, but in most cases — so far — no hard and fast cause and effect relationship has been proved.
Despite global warming, a diminishing ozone layer and increasing ultraviolet (UV) rays, people’s evolutionary response — that is, wearing a hat and sunglasses — has diminished the toll that sunlight takes on vision. In 1988, ophthalmologists studied 838 fishermen who worked on the Chesapeake Bay. The fishermen who did not wear glasses or a brimmed hat had three times as many cataracts as those who did, according to the American Academy of Ophthalmology.
Other studies show that long-term exposure to UV light also can result in damage to the eye’s surface, specifically the cornea and conjunctiva, and may hasten the development of macular degeneration. Retina specialist and study author Vincent J. Giovinazzo, MD, recommended that people select glasses that block at least 99% of all UV rays up to 400 nm.
Risk reduction
In looking at the various ocular disease risk factors, Frederick L. Ferris III, MD, says in many ways environmental influences are ideal ones with which to experiment “because in most instances you can do some form of intervention. Unfortunately,” he concedes, “the associations are pretty weak. If you tried to do an analysis observing risk attributable to sunlight exposure, it would appear to be quite small,” said Dr. Ferris, who is director of the Division of Biometry and Epidemiology at the National Eye Institute (NEI). “By far the overwhelming risk is aging.”
But even treatments that have a small to moderate affect can make a significant contribution from the perspective of their impact on the entire population. For instance, the NEI’s studies of age-related eye disease has followed 4,757 patients for 5 years in an attempt to determine the effects of vitamin C, vitamin E and beta carotene. One half of the patients had early macular degeneration; they are being followed to determine the incidence of cataract and macular degeneration.
Free radicals in the eyes have been associated with vision reduction, and both vitamins C and E and beta carotene quench free radicals. According to Dr. Ferris, the study’s data monitoring committee, thus far, has not observed any clear-cut beneficial or harmful effects, nor has it been suggested that the study’s protocol be changed in any way.
“To me, it doesn’t seem unusual or unexpected that the vitamins are not going to have a large treatment affect like, for instance, penicillin. They’re probably going to have a small affect like aspirin for cardiovascular disease, where you see a 10% to 20% reduction in disease. This might seem small from an individual perspective, but from a public health perspective, it makes a great difference. So reducing macular degeneration by 10% for the individual would be a modest reduction in risk, but for the population, it would make a great deal of difference,” Dr. Ferris said. “The same thing would be true with cataract. If you could slow the development of cataract by 10 years, I believe the projection is that it would be possible to cut the amount of cataract surgery in half.”
AMD
Robyn H. Guymer, MD, of Melbourne, Australia, spent 2 years at Moorfields Eye Research Hospital working with Alan C. Bird, MD. According to Dr. Guymer, the factors that determine the risk of developing age-related macular degeneration (AMD) are not known, but clinical studies imply that both genetic and environmental factors are involved.
“It is likely that AMD is a complex genetic disorder with multiple genes influencing the individual’s susceptibility to environmental factors,” she says. “The lesions leading to loss of vision are thought to occur in response to the accumulation of debris in Bruch’s membrane. The deposits alone do not cause severe visual symptoms, but predispose the eye to the complications such as new vessel formation, which ultimately leak, causing destruction of the macular or the gradual decrease in central vision due to progressive death of central retinal cells.”
Computers
Almost as ubiquitous as sunlight and plastic in today’s environment are computers. A recent study at the Schepens Eye Research Institute in Boston looked at computer vision syndrome and the effects of lubricant eye drops on its signs and symptoms. Eight computer users underwent baseline examinations including slit lamp, fluorescein staining, tear break-up time, ocular discomfort, blink rate and visual acuity evaluations.
Subjects were then placed in a controlled adverse environment to produce the signs and symptoms associated with computer vision syndrome for 45 minutes. Four subjects were administered two lubricant eye drops 15 minutes after exposure and the remaining four received no treatment at all. During exposure, both groups used personal computers to complete a visual task while evaluating ocular discomfort at 15, 20, 25, 30 and 45 minutes.
After exposure, subjects underwent exit examinations including slit lamp, fluorescein staining, tear breakup time, blink rate and visual acuity evaluations. All eight subjects demonstrated an increase in keratitis, an increase in conjunctival fluorescein staining, a decrease in tear breakup time and a decrease in blink rate by 57%. Treated subjects reported an improvement in ocular discomfort lasting 15 to 30 minutes after drop instillation. Untreated subjects’ ocular discomfort remained the same or worsened during the same time period.
The Schepens team concluded that a controlled adverse environment caused normal subjects using a personal computer to develop accelerated signs and symptoms associated with computer vision syndrome. Treatment with a lubricant eye drop was shown to provide temporary relief in ocular discomfort, according to the study.
For Your Information:Reference:
- Frederick L. Ferris III, MD, is director, Division of Biometry and Epidemiology, National Eye Institute, National Institutes of Health. He can be reached at 31 Center Drive, MSC 2510, Bldg. 31, Room 6A52, Bethesda, MD 20892-2510 U.S.A; +(1) 301-496-6583; fax: +(1) 301-496-2297; e-mail: RickFerris@nei.nih.gov. Dr. Ferris has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.
- Robyn H. Guymer, MD, can be reached at Melbourne University, 32 Gisborne St., Royal Victorian Hospital, Ophthalmology Department, Melbourne, VIC 3002, Australia; fax: +(61) 3-9662-3859. Dr. Guymer did not participate in the final preparation of this article.
- Taylor HR, West SK, Rosenthal FS, et al. Effect of ultraviolet radiation on cataract formation. N Engl J Med. 1988; 319:1429-1433.