AMD, Vision & Vision Loss
Age-related macular degeneration (AMD) affects the macula, which is the central, most sensitive part of the retina. The macula is responsible for sight in the middle of your field of vision and for fine details and colors. As the condition progresses, the eye must rely on peripheral vision because central vision is compromised.
There are two types of AMD: dry (non-neovascular) and wet (neovascular). In the non-neovascular form, which makes up about 90 percent of cases, the retina may become thinner, and small yellow deposits (drusen) develop beneath the retina. People with the non-neovascular type usually do not lose vision unless they develop advanced non-neovascular AMD or the neovascular form. In neovascular AMD, abnormal blood vessels develop and may bleed. The body heals these blood vessels, but the remaining scar tissue prevents the macula from functioning properly.
There is no cure for AMD, although only 1 to 2 percent of people who have it will lose vision. The progression of AMD can be arrested in some people with vitamin and mineral supplements that reduce the risk of the advanced forms of AMD. If the neovascular form develops, treatment with medications injected into the eye is highly effective in minimizing vision loss.
In the United States, macular degeneration is the leading cause of severe and irreversible loss of central vision in people over age 40, affecting approximately 1.8 million Americans. The number of AMD cases is expected to reach almost three million by 2020.
The prevalence of severe vision loss from AMD increases with age, and most people with impaired vision from AMD are age 60 or older. Fortunately, most cases of AMD do not result in severe vision loss, which is commonly defined as a decline in visual acuity to 20/200 or less. Visual acuity of 20/200 corresponds to being able to see only the big "E" on an eye chart. People with corrected vision of 20/200 or less in both eyes are considered to be legally blind.
Role of the Retina
The retina is a light-sensitive layer of nerve tissue lining the inner eye. It is made up of millions of tiny nerve receptor cells called cones and rods. Light rays reflected from an object are focused onto the retina by the cornea and lens. The cones and rods then send impulses through the optic nerve to the brain in response to the light.
The most sensitive portion of the retina is a small area at its center, called the macula, which is about one fiftieth of an inch in diameter. In its middle is a small indentation, the fovea, which contains the highest concentration of cones (the most sensitive receptors of light) and provides the sharpest vision. When you look directly at an object, the light rays focus onto the fovea.
Central and detailed vision require an intact macula. If function of the macula is lost, the eye provides only peripheral vision, which is far less sensitive to detail. As a result, activities such as reading normal-sized print are impossible without the use of low-vision aids.