For the roughly 10 million Americans who suffer from age-related macular degeneration (AMD), there are much greater worries than having it take longer than it once did to adapt to the darkness of a movie theater, or not being very good at identifying when a light source has a faint flicker. They’re worried, instead, about legal blindness, or difficulty driving, or being unable to tell the difference between the faces of their loved ones and those of strangers.
Yet it’s those relatively minor visual impairments that may prove essential, if Max Snodderly is right, to predicting one’s susceptibility to AMD years or even decades before the disease becomes manifest. And that early detection, in turn, may prove the most potent weapon we have in delaying or preventing AMD, which is caused by a deterioration of the ability of the retina to process light and transmit visual information to the brain.
“There are a few treatments that we have right now for people whose degeneration is really advanced,” says Snodderly, a professor of nutritional sciences, “but they’re of very limited value. Once things get bad, you’ve got degeneration of very delicate tissue and it’s pretty hard to do much about it. It would be far better to be able to prevent it, and to do that, to a considerable degree, we need to be able to predict it.”
At the moment, says Snodderly, the best predictor we have of a person’s chances of developing macular degeneration is the presence of a mutated version of what’s known as the Complement Factor H (CFH) gene, which in its un-mutated form helps to regulate inflammation in the retina. It’s a crude predictor, however. Not everyone who has the mutated gene develops the condition. Many people who have the normal gene do develop AMD, and very little is known about how various external factors—diet, weight, race, age, gender or lifestyle—interact with the gene or influence the development of the disease.
For Snodderly, the genetic marker is useful on its own terms as an indicator of risk, but even more useful as a means of identifying a good pool of people who can be studied in order to evaluate how predictive other potential biomarkers are. He’s interested, in particular, in what can be learned from two perceptual tests—the dark adaptation test and the flicker test— that measure sensitivities that are already known to be particularly degraded in people who suffer from macular degeneration.
In the dark adaptation test, subjects are exposed to a bright light, then put in a dark room and gauged on how quickly and how well they recover their ability to see. In the temporal contrast sensitivity test—the flicker test—participants are measured on how big an amplitude a flicker has to have before they can identify that the light they’re seeing is, in fact, flickering.
By selecting a pool of people with the mutant gene who haven’t yet suffered macular degeneration, and by comparing them over time to people who have the normal gene—both in terms of how well they perform on these tests, and in terms of who develops AMD—Snodderly hopes to begin to be able to tease out the important relationships between genes, demographics, behavior and nutrition.
“I’m using these genetic markers to try to get ahead of the game,” he says. “It’s very likely that some of these things start in teenage years and progress over time. I want to use the sensory tests to figure out what’s going on at various ages, what the modifying influences are, and which ones might make the difference.”
The ultimate question, says Snodderly, is whether a workable battery of biomarkers can be devised, and if so—if, in fact, people can learn early on that they’re at high risk for AMD—what then? Over the long term, he says, a good set of predictive biomarkers will help enormously in the study of the disease, and in the development of more refined, and more preventive, courses of treatment. Even over the short term, however, the research is producing good, if rather prosaic, answers to the question of “what then?”
Eating healthy is important. The carotenoids lutein and zeaxanthin, in particular, seem to be significant to maintaining retinal health, so you should eat your fruits and vegetables (and perhaps take vitamin supplements, if you don't absorb the nutrients well from food). Smoking is bad for visual health. Obesity is almost certainly bad as well.
“You can’t change your genes very easily,” says Snodderly, “but you can change your diet, your lifestyle. What we’re finding, in fact, is that these basic lifestyle choices are incredibly important for a variety of things—cancer, heart disease, cognition, everything. People are going to have to face up to the fact that being a couch potato with a poor diet has a high price.”
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