To be sure, macular degeneration, which currently affects an estimated 10 million Americans, is not a fatal disorder. But it can be cruelly debilitating. For while the macula (named after the Latin word for spot) is no wider than a pencil, it is a hundred times more sensitive to small-scale features than the rest of the retina. Without a healthy macula, people cannot read a newspaper, recognize a friend, thread a needle, watch TV, safely negotiate stairs or see much of anything at all.

Until now, physicians have been able to offer only palliative care to patients with macular degeneration: more powerful eyeglasses; visual aids, such as machines that enlarge print; and, for a minority of cases--those that involve the invasive growth of blood vessels--laser therapy that sometimes slows down the disease, at least for a time. But only 10% of those in whom macular degeneration is diagnosed develop this more rapidly progressing, invasive form of the disease. For them, experts agree, intervention is needed earlier, before so much visual acuity is lost.

Macular degeneration is devastating because it kills off a small but critical patch of light-sensing cells that line the retina like the film in a camera. Known as rods and cones because of their telltale shapes, these cells record visual images as patterns of illumination and shadow, and relay that information, as electrical impulses, through the optic nerve to the brain. It is not that people with macular degeneration become completely blind; peripheral vision, which is handled by other areas of the retina, remains unaffected by the disease. But as damage to the macula builds up--probably a consequence of chemical damage that accumulates over a lifetime--central vision fades, and the external world dissolves into an indistinct blur.