Macular degeneration
Macular degeneration is the progressive deterioration of a critical region ofthe retina called the macula. The macula is a 3-5 mm area in the retina thatis responsible for central vision. This disorder leads to irreversible lossof central vision, although peripheral vision is retained. In the early stages, vision may be gray, hazy, or distorted.
Macular degeneration is the most common cause of legal blindness in people over 60, and accounts for approximately 11.7% of blindness in the United States. About 28% of the population over age 74 is affected by this disease.
Age-related macular degeneration (ARMD) is the most common form of macular degeneration. It is also known as age-related maculopathy (ARM), aged macular degeneration, and senile macular degeneration. Approximately 10 million Americans have some vision loss that is due to ARMD.
ARMD is subdivided into a dry (atrophic) and a wet (exudative) form. The dryform is more common and accounts for 70-90% of cases of ARMD. It progresses more slowly than the wet form and vision loss is less severe. In the dry form,the macula thins over time as part of the aging process and the pigmented retinal epithelium (a dark-colored cell layer at the back of the eye) is gradually lost. Words may appear blurred or hazy and colors may appear dim or gray.
In the wet form of ARMD, new blood vessels grow underneath the retina and distort the retina. These blood vessels can leak, causing scar tissue to form onthe retina. The wet form may cause visual distortion and make straight linesappear wavy. A central blind spot develops. The wet type progresses more rapidly and vision loss is more pronounced. Treatments are available for some, but not most, cases of the wet form.
Other less common forms of macular degeneration include:
- Cystoid macular degeneration. Loss of vision in the macula due to fluid-filled areas (cysts) in the macular region. This may be a result of otherdisorders, such as aging, inflammation, or high myopia.
- Diabetic macular degeneration. Deterioration of the macula due to diabetes.
- Senile disciform degeneration (also known as Kuhnt-Junius macular degeneration). A specific and severe type of the wet form of ARMD that involvesleaking blood vessels (hemorrhaging) in the macular region. It usually occursin people over 40 years old.
Age-related macular degeneration is part of the aging process. There may be ahereditary component. Having a family member with ARMD increases a person'srisk for developing it. There is a slightly higher incidence in females. Whites and Asians are more susceptible to developing ARMD than blacks, in whom the disorder is rare.
ARMD is thought to be caused by hardening and blocking of the arteries (arteriosclerosis) in the blood vessels supplying the retina. Some of the same things that are bad for the heart are thought to contribute to the development ofmacular degeneration. These risk factors include smoking and a diet that isrich in saturated fat. Smokers have a risk of developing ARMD that is approximately 2.4-3 times that of non-smokers. Smoking increases the risk of developing wet-type ARMD, and may increase the risk of developing dry-type as well.Dietary fat also increase the risk. In one study of older (age 45-84) Americans, signs of early ARMD were 80% more common in the group who ate the most saturated fat compared to those who ate the least. Low consumption of antioxidants, such as foods rich in vitamin A, is associated with a higher risk for developing ARMD. Consumption of moderate amounts of red wine and foods rich invitamin A is associated with a lower risk. It is generally believed that exposure to ultraviolet (UV) light may contribute to disease development, but this has not been proven.
The main symptom of macular degeneration is a change in central vision. The patient may notice blurred central vision or a blank spot on the page when reading. The patient may notice visual distortion such as bending of straight lines. Images may appear smaller. Some patients notice a change in color perception and some experience abnormal light sensations. These symptoms may come on suddenly and become progressively more troublesome. Sudden onset of symptoms, particularly vision distortion, is an indication for immediate evaluationby an ophthalmologist.
To make the diagnosis of macular degeneration, the doctor dilates the pupil with eye drops and examines the interior of the eye, looking at the retina forthe presence of yellow bumps called drusen and for gross changes in the macula such as thinning. The doctor also administers a visual field test, lookingfor blank spots in the central vision. The doctor may call for fluorescein angiography (intravenous injection of fluorescent dye followed by visual examination and photography of the back of the eye) to determine if blood vesselsin the retina are leaking.
A central visual field test called an Amsler grid is usually given to patients who are suspected of having ARMD. It is a grid printed on a sheet of paper(so it is easy to take home). When looking at a central dot on the page, thepatient should call the doctor right away if any of the lines appear to be wavy or missing. This may be an indication of fluid and the onset of wet ARMD.Patients may also be asked to come in for more frequent checkups.
While loss of vision cannot be reversed, early detection is important becausetreatments are available that may halt or slow the progression of the wet form of ARMD. Treatment for the dry form is not available as of 1998, but celltransplantation studies are under study.
In wet-type ARMD and in senile disciform macular degeneration, new capillaries grow in the macular region and leak. This leaking of blood and fluid causesa portion of the retina to detach. Blood vessel growth, called neovascularization, can be treated with laser photocoagulation in some cases, depending upon the location and extent of the growth. Argon or krypton lasers can destroythe new tissue and flatten the retina. This treatment is effective in abouthalf the cases but results may be temporary. A concern with laser therapy isthat the laser also destroys the photoreceptors in the treated area. If the blood vessels have grown into the fovea (a region of the macula responsible for fine vision), treatment may not be possible. Because capillaries can grow very quickly, this form of macular degeneration should be handled as an emergency and treated quickly. Patients who are experiencing visual distortion should seek help immediately.
Another form of treatment for the wet form of ARMD is radiation therapy witheither x rays or a proton beam. Blood vessels that are proliferating (growing) are sensitive to treatment with low doses of ionizing radiation. Nerve cells in the retina are not growing and are insensitive, so they are not harmed by this treatment. External beam radiation treatment has shown promising results at slowing progression in limited, early trials. An alternative treatmentis internal beam radiation therapy. For this treatment, the patient is givena local anesthetic and an applicator containing strontium 90 is inserted intothe affected eye. This brief and localized radiation therapy prevents the growth of blood vessels.
Other therapies that are under study include treatment with alpha-interferon,thalidomide, and other drugs that slow the growth of blood vessels. Subretinal surgery also has shown promise in rapid-onset cases of wet ARMD. This surgery carries the risk of retinal detachment, hemorrhage, and acceleration of cataract formation. Other experimental treatments include photodynamic therapy(PDT). For this treatment, a photosensitizing dye is injected, followed by irradiation of the area of new blood vessel growth with a special, low-intensity diode laser. This treatment damages the cells in the blood vessel walls and causes them to stop growing.
A controversial treatment called rheotherapy involves pumping the patient's blood through a device that removes some proteins and fats. As of 1998, this had not been proven to be safe or effective.
Consumption of a diet rich in antioxidants (beta carotene and the mixed carotenoids that are precursors of vitamin A, vitamins C and E, selenium, and zinc), or taking antioxidant nutritional supplements, may help prevent macular degeneration, particularly if started early in life. Good dietary sources of antioxidants include citrus fruits, cauliflower, broccoli, nuts, seeds, orangeand yellow vegetables, cherries, blackberries, and blueberries. Research hasshown that nutritional therapy can prevent ARMD or slow its progression onceestablished. Some doctors recommend taking beta carotene and zinc as a precautionary measure. Some vitamins are marketed specifically for the eyes.
The dry form of ARMD is self-limiting and eventually stabilizes. The loss ofvision is permanent. The vision of patients with the wet form of ARMD often stabilizes or improves even without treatment, at least temporarily. However,after a few years, patients with the wet form of ARMD are usually left with only coarse peripheral vision remaining.
Many patients with macular degeneration lose their central vision permanentlyand may become legally blind. However, macular degeneration rarely causes total loss of vision. Peripheral vision is retained. The patient can compensate, to some extent, for the loss of central vision, even though macular degeneration may render them legally blind. Improved lighting and special low-visionaids may help even if sharpness of vision (visual acuity) is poor. Vision aids include special magnifiers that allow the patient to read and telescopic aids for long-distance vision. The use of these visual aids plus the retainedperipheral vision usually allow the patient to remain independent. Registration as a legally blind person will enable a patient to obtain special servicesand considerations.
Avoiding the risk factors for macular degeneration may help prevent it. Thisincludes avoiding tobacco smoke and eating a diet low in saturated fat. Someother behaviors that may help reduce the risk of wet-type ARMD are eating a diet rich in green, leafy vegetables and yellow vegetables such as carrots, sweet potatoes, and winter squash; drinking moderate amounts of alcohol, such as one or two glasses of red wine a day; and taking an antioxidant vitamin supplement, especially vitamin A. Some vitamins may be toxic in large doses, sopatients should speak with their doctors. Vitamins C and E have not been shown to reduce risk, nor did selenium in one large study. The use of zinc is controversial: some studies showed a benefit, others showed no benefit, and oneactually showed an increased risk of ARMD with increased levels of zinc in the blood. Some doctors suggest that wearing UV-blocking sunglasses reduces risk. Use of estrogen in postmenopausal women is associated with a lower risk ofdeveloping ARMD.
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