Early diagnosis, timely treatment and appropriate follow-up care are essential to preventing irreversible vision loss, U.S. health officials said. Dr. Paul A. Sieving, director of the National Eye Institute, part of the National Institutes of Health, said common eye diseases such as glaucoma, diabetic retinopathy and age-related macular degeneration threaten millions of Americans, potentially robbing them of vision, mobility and independence. Early stages of common eye diseases typically have no symptoms and can only be detected through a comprehensive dilated eye exam, Sieving said. Glaucoma causes damage to the optic nerve, which relays visual information from the retina to the brain. The retina is the light-sensing layer of tissue in the back of the eye, Sieving said. Diabetic retinopathy causes swelling, leakage and blockage of the blood vessels that nourish the retina, while age-related macular degeneration occurs when cells in the center part of the retina, called the macula, break down. Recent clinical trials determined that taking high levels of antioxidants and zinc reduces the risk of developing age-related macular degeneration by about 25 percent. People at higher risk of glaucoma include African-Americans age 40 and older; everyone age 60 and older, especially Mexican Americans; and people with a family history of the disease. People at risk of diabetic retinopathy include people with type 1 or type 2 diabetes, while people age 50 and older, whites, smokers and those with a family history of age-related macular degeneration are at greater risk of the disease.