Breakdown of the light-sensitive cells in the macula, the central part of the retina responsible for sharp, central vision. Age-related macular degeneration (AMD) is the most common cause of vision loss in people over the age of 55 in the United States. More than 8 million Americans have AMD, and the number is expected to increase as the population ages.
There are two forms of macular degeneration: dry and wet. About 90% of all cases of AMD take the dry form. It is not known exactly what causes the condition, but it develops as the light-sensitive cells of the macula slowly deteriorate, leading to a gradual loss of central vision. Fatty deposits called drusen, which are actually debris left by deteriorating cells, build up in the macula, generating further cell loss. Wet AMD, the more aggressive form, affects only 10% of people with AMD but accounts for 90% of the severe vision loss associated with the disease. About 10% of cases of dry AMD progress to wet AMD. In this form, new blood vessels begin to form behind the retina and grow into the macula. The new vessels, which are very fragile, burst easily, leaking blood and fluid under the macula and causing rapid damage and central vision loss.
The most common symptoms of dry AMD are slightly blurred vision, difficulty recognizing faces, and the need for brighter light to see. As more light-sensitive cells are destroyed, people with dry AMD develop a blind spot in the center of their field of vision that expands as the disease progresses. For people with wet AMD, straight lines may begin to appear wavy. This is because leaking fluids raise the macula from its normal position on the back of the eye. People with wet AMD may also develop a blind spot in the middle of their visual field.
To diagnose AMD, eye-care professionals use eyedrops to dilate a person’s pupils and examine the back of the eye for signs of macular damage. Sometimes the person is asked to view an Amsler grid, a chart with a graphlike pattern of lines and a black dot at the center. For people with central vision changes from AMD, the lines on the grid will appear warped, and some lines may be missing. If the doctor suspects wet AMD, he may administer a test called fluorescein angiography, in which a dye injected into the arm travels to the retina, enabling the doctor to photograph and examine leaking blood vessels.
There is no treatment for dry AMD, but because it progresses so slowly, most people are able to function normally for some time, especially with use of low-vision aids. (The macula does not control peripheral vision, so total blindness does not occur with either type of AMD.) Some studies suggest that consumption of certain antioxidants called carotenoids, found in deep green leafy and yellow-orange fruits and vegetables, is associated with a reduced risk of AMD. One study showed that participants who highest dietary intake of the carotenoids lutein and zeaxanthin had a 43% lower risk of AMD than those who consumed the least of these nutrients. Some studies also suggest that consumption of high levels of the vitamins C and E and the mineral zinc has a protective effect against AMD. Your eye doctor can tell you more about the potential benefits and risks of taking supplements of antioxidants and zinc.
Some cases of wet AMD can be treated with laser surgery, or photocoagulation, which uses intense light to destroy the abnormal new blood vessels. Laser surgery is a fairly quick procedure that can be performed in the eye doctor’s office. The treatment stems vision loss, but it doesn’t cure the condition. There is a relatively high chance that new blood vessels will grow back, requiring more surgery. In 2000, the Food and Drug Administration approved a drug called verteporfin (brand name Visudyne) for treating certain types of wet AMD. Treatment with verteporfin is called photodynamic therapy, because the drug is effective only when activated by light of a certain wavelength. Injected into a person’s arm, the drug reaches and binds to the abnormal blood vessels of the eye. A laser shone into the person’s eye activates the drug, which spares the normal tissue but stops the leakage of blood from the abnormal ones. While neither of these treatments restores lost vision, they can sometimes preserve the remaining vision. In 2006, the Food and Drug Administration (FDA) approved Lucentis (ranibizumab injection), which is injected into the eye, for the treatment of wet age-related macular degeneration.