Macular degeneration is a progressive eye disease that affects the sharp central vision you need for important activities like driving and reading. It is caused by damage to the macula, a part of the light-sensitive retina at the back of the eyeball. The retina is responsible for picking up images and transmitting them as electrical impulses to the brain.
The most common form of macular degeneration develops as a person ages and is referred to as age-related macular degeneration (AMD). AMD is one of the leading causes of severe vision loss in people age 40 years and older. According to the Eye Diseases Prevalence Research Group, approximately 1.8 million Americans age 40 and older have advanced AMD with vision loss, and over 7 million Americans have a high risk of developing vision loss from AMD. The chance of developing the condition increases with age. Advanced AMD is predicted to afflict almost 3 million Americans by 2020.
AMD usually goes undetected until it reaches the advanced stage, since the intermediate form of the disease is typically asymptomatic. At the advanced stage, noticeable changes in central vision become apparent, such as blurring, waviness, or a loss of color. The deterioration in the eye has progressed to the point where blind spots are usually noticeable; eventually there's a complete loss of central vision. AMD almost never results in complete blindness. People with the advanced stage of the disease almost always retain their peripheral vision.
There are two types of age-related macular degeneration: dry AMD and wet AMD. About 85 percent to 90 percent of people have the dry type, which occurs when the retina begins to thin and deteriorate, and deposits of debris form under the macula. The much more severe wet type occurs when abnormal blood vessels form under the macula and leak fluid.
There is no cure for AMD, but a number of treatments are available that can slow the progression of the disease and restore some lost vision. The appropriate treatment depends on the stage of the disease and whether the AMD is in the wet or dry form. While both forms can lead to vision loss, wet AMD usually progresses faster and is more likely to result in advanced-stage AMD. All AMD begins as the dry form.
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All of the light we see gets processed by the retina, a thin layer of special cells that lines the back surface of the eye. The retina is made up of a number of cell types including rods and cones, light-sensitive receptor cells that are responsible for converting light into electrical signals that can be interpreted by the brain. Rod cells are primarily used for low light levels and allow a person to see grays. Cone cells operate in higher light levels, and are responsible for the vision of fine detail and the ability to see color.
If you've ever tried looking for a dim star, you may have noticed that it is easier to see the star if you look at it sideways, with your peripheral vision, than if you try to look straight at it. This is because the retina's rod and cone cells are distributed differently. Cone cells can be found throughout the retina, but they are particularly concentrated in a tiny (1.5-mm) yellow spot called the macula. The macula is a specialized part of the retina at the center of the back of the eye. Because of its location and the preponderance of cone cells, the macula is responsible for high-acuity vision in the center of the visual field.
Like all cells in the body, the rod and cone cells that make up the retina need oxygen and nutrients to live. The cells also create waste that needs to be eliminated. A single layer of cells under the rods and cones, the retinal pigment epithelium, is responsible for transporting material to and from the rod and cone cells. Beneath the RPE, blood vessels in the choroid, an area behind the eye, provide oxygen.
When this system breaks down, deposits form under the macula and the light-sensitive macular cells degenerate. In some cases, new blood vessels grow and leak fluid, causing severe vision loss.
Although the exact cause of age-related macular degeneration is unknown, doctors have a good understanding of how the condition progresses once it begins to develop. AMD begins in its dry form, which occurs when the retina begins to thin and deteriorate, and deposits of debris form under the macula, blurring or distorting vision. If the dry form progresses into the advanced stage, with significant cell death and atrophy, it can cause vision loss.
The dry form can also turn into the wet form of AMD, which is usually more severe. The wet type occurs when abnormal blood vessels form under the macula and leak fluid; those who develop the wet form make up the majority of patients who experience serious vision loss from the disease. It's impossible to predict whether any individual with the dry form will end up with the wet form, so it is very important for people with AMD to monitor their eyesight carefully and visit their eye doctor on a regular basis.
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Doctors recognize the dry form of AMD when they see the presence of drusen, round, fatty, yellow deposits that sit under the retinal cells in the macula, during an ordinary dilated eye exam. Drusen are made up of hundreds of compounds, and many doctors think that they are the accumulated waste products of the retina.
In most people, drusen do not cause changes in vision. But when they grow in size and increase in number, they may lead to a dimming or blurring of vision that people find most noticeable when they read. Drusen can also interfere with the proper functioning of retinal cells in the macula. In more advanced stages of dry AMD, the thin layer of retinal cells in the macula can become thin and die. This atrophy can lead to blind spots and eventually a total loss of central vision.
Most patients with drusen do not develop vision loss. The connection between drusen and the risk of progression to advanced AMD or vision loss is not predictable in single patients—though it is known that approximately 25 percent to 30 percent of people with intermediate or more advanced AMD can be expected to progress to serious vision loss over five years. But a person's risk of developing advanced dry AMD or wet AMD increases with the size or number of drusen. It is estimated that there are over 7 million Americans with drusen large enough to put them at a substantial risk of developing vision loss.
The more severe form of AMD occurs when blood vessels break through the membrane separating them from the rest of the eye and begin to grow underneath the macula. These blood vessels are fragile and easily broken; over time, they will leak blood and fluid into the eye, causing blind spots and loss of central vision, as well as a distortion of vision that makes straight lines look wavy. These abnormal blood vessels eventually scar, leading to permanent loss of central vision. This form of AMD is also known as neovascular form.
Most people develop AMD in middle age, and the risk increases with age. According to the National Eye Institute, additional risk factors include smoking, obesity, race, family history, and gender. Causasians are much more likely to lose sight from AMD than African-Americans, and women seem to be at a greater risk than men. Research has hinted at a connection between being obese and progressing from early and intermediate AMD to advanced AMD.
Last reviewed on 3/23/2010
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