Macular degeneration is a degenerative disease of the retina (a thin layer of nerve cells that lines the back of the eyeball) that causes progressive loss of central vision.
Central vision loss is due to the deterioration of the central part of the retina, known as the macula. The macula is involved in detailed vision. Light sensing cells in the macula, known as photoreceptors, convert light into electrical impulses. Then the impulses are transferred to the brain via the optic nerve. Central vision loss from macular degeneration occurs when photoreceptor cells in the macula degenerate.
The risk of developing macular degeneration increases with age. The disease most often affects people in their sixties and seventies. Macular degeneration is the most common cause of vision loss in individuals over the age of 60.
Individuals with macular degeneration may first notice a blurring of central vision that is most apparent when performing visually detailed tasks such as reading and sewing. Blurred central vision may also make straight lines appear slightly distorted or warped. As the disease progresses, blind spots form within central vision. In most cases, if one eye has macular degeneration, the other eye will also develop the disease. The extent of central vision loss varies according to the type of macular degeneration. Individuals can adapt to peripheral vision, or vision out of the corners of the eye, although some visual impairment will exist without central vision.
Age-related macular degeneration is the most common type of macular degeneration and is the leading cause of legal blindness in people older than 60 years in the United States. Most individuals with macular degeneration retain peripheral vision and learn to optimize the use of their remaining vision. Legally blind individuals are those whose best visual acuity or sharpness (with glasses or contact lenses, if needed) is 20/200 or worse in their better eye, or whose visual field, regardless of acuity, is restricted to a 20 degree diameter (10 degree radius).
The disease affects more than 10 million Americans, including 23% of Americans older than 90 years. Because overall life expectancy continues to increase, age-related macular degeneration has become a major public health problem.
Deficiencies in antioxidants (specifically zinc and vitamins A, C, and E) have been noted in some people with age-related macular degeneration. Antioxidants may protect against age-related macular degeneration by preventing free radicals or unstable oxygen from damaging the retina. Antioxidants can be found in foods such as green, leafy vegetables, vegetables of color (oranges, yellows, purples), and fruits. Dietary and lifestyle changes are necessary, including eating less fat (from meats and dairy products) and eating more vegetables and fruits, increasing exercise, and weight control.
There are several forms of macular degeneration that affect children, teenagers, or adults that are known as early onset or juvenile macular degeneration. Many of these forms are hereditary and are more accurately called macular dystrophies, instead of degenerations. Stargardt macular dystrophy is the most frequent type of juvenile-onset form of macular dystrophy. Other types include Best's vitelliform macular dystrophy, Doyne's honeycomb retinal dystrophy, Sorsby's fundus dystrophy, malattia levintinese, fundus flavimaculatus, and autosomal dominant hemorrhagic macular dystrophy.
Individuals affected with Stargardt's first present with the complaint of a decrease in central vision, generally within the first 20 years of life. In some instances, persons may not note visual impairment until their 30's or 40's. In the majority of instances, individuals do not experience a loss in peripheral vision or night-blindness, although it has been reported that persons with Stargardt's have some problems seeing in the dark.
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types of age-related macular degeneration
There are two types of age-related macular degeneration (AMD): dry and wet.
Dry form: Dry macular degeneration
is sometimes called atrophic, nonexudative, or drusenoid macular degeneration. More than 90% of all people with AMD have the dry form. With dry
macular degeneration, yellow-white deposits, called drusen, accumulate in the retinal pigment epithelium (RPE) tissue beneath the macula. Drusen deposits are composed of waste products from photoreceptor cells. For unknown reasons, RPE tissue can lose its ability to process waste, which is normally flushed out of the eye into lymphatic tissue. As a result, drusen deposits accumulate in the RPE. Drusen deposits are typically present in patients with dry macular degeneration. These deposits are thought to interfere with the function of photoreceptors in the macula, causing progressive degeneration of these cells. Drusen deposits can, however, be present in the retina without vision loss.
Vision loss from dry macular degeneration occurs very gradually over the course of many years (sometimes decades). Central vision, or straight ahead vision, in the background may remain stable between annual eye examinations. Individuals with macular degeneration do not usually experience a total loss of central vision. However, fine focus associated with vision may be lost, causing difficultly seeing small objects, such as print in books, computers, and television, and performing tasks requiring fine vision.
Most individuals with AMD begin with the dry form, and it progresses slowly. Not all people with AMD experience significant vision loss in both eyes. Only 59% of those who lose vision in one eye will lose vision in the other eye as well. Vision can be partial or total loss, depending upon the individual and their disease progression.
Dry AMD has three stages, all of which may occur in one or both eyes. These stages are including early AMD, intermediate AMD, and advanced dry AMD. Individuals with early AMD have either several small drusen (accumulated deposits) or a few medium-sized drusen. At this stage, there are no symptoms and no vision loss. Individuals with intermediate AMD have either many medium-sized drusen or one or more large drusen. Some people see a blurred spot in the center of their vision. More light may be needed for reading and other tasks. In addition to drusen, individuals with advanced dry AMD have a breakdown of light-sensitive cells and supporting tissue in the central retinal area. This breakdown can cause a blurred spot in the center of the vision. Over time, the blurred spot may get bigger and darker, taking more of the central vision. The individual may have difficulty reading or recognizing faces, or in severe cases can loose complete central vision causing legal blindness.
The dry form also can suddenly turn into the wet form, even during early stage AMD. All individuals with the wet form of AMD had the dry form initially.
Wet form: Wet macular degeneration
is also called choroidal neovascularization (CNV), subretinal neovascularization, exudative, or disciform degeneration. In the wet form, newly created abnormal blood vessels grow under the macula (called choroidal neovascularization). These blood vessels leak, bleed, and scar the retina, distorting vision or destroying central vision. The blood and fluid raise the macula from its normal place at the back of the eye. Damage to the macula occurs rapidly. Vision distortion usually starts in one eye and may affect the other eye later. An early symptom of wet AMD is that straight lines appear wavy. Wet macular degeneration affects only 10% of people who have age-related macular degeneration but accounts for two thirds of the people who have significant visual loss. More than 200,000 new cases of wet age-related macular degeneration occur each year in the United States. The wet form of AMD is a leading cause of irreversible legal blindness.
Retinal pigment epithelial detachment: Another form of wet macular degeneration, called retinal pigment epithelial detachment, occurs when fluid leaks under the RPE even though it appears that no abnormal blood vessels have started to grow. The fluid collects under the retinal pigment epithelium, causing what looks like a blister or a bump under the macula. This kind of macular degeneration causes similar symptoms to typical wet macular degeneration, but the vision can remain relatively stable for many months or even years before it deteriorates. Eventually, however, this form of macular degeneration usually progresses to the more common wet form of macular degeneration that includes newly growing abnormal blood vessels.