Macular Degeneration Health Article

Definition

Macular degeneration (MD) is the progressive deterioration of the macula, the light-sensitive cells of the central retina, at the back of the eye. The retina is the sensitive membrane (soft layer) of the eye that receives the image formed by the lens and is connected with the brain by the optic nerve. As these macular cells malfunction and die, central vision becomes gray, hazy, or distorted, and eventually is lost. Peripheral (away from the center) vision is unaffected.

Description

Millions of people suffer from MD and it accounts for about 12% of all blindness in the United States. The macula contains the highest concentration of photosensitive cells in the retina. These cells transform light into electrical signals that are sent to the brain for processing into vision. Fine detail vision and critical color vision are located in the macula. The macula depend on nutrient diffusion from the choroid layer, a region of several delicate vascular (pertaining to blood vessels) membranes or structures behind the retina and under the macula. Anything that interferes with this nutrient supply can lead to MD.

Age-related macular degeneration (AMD or ARMD) is by far the most common type of MD. One in six Americans develops AMD between the ages of 55 and 64 and one in three Americans over 75 has AMD. About 10% of those with AMD eventually suffer severe vision loss. The incidence of AMD is expected to triple by 2025, as the population ages. Whites and Asians are more susceptible than blacks. Women and those with lighter-colored eyes are somewhat more susceptible. AMD may occur in only one eye. However there is a very high likelihood that the other eye will be affected eventually.

About 90% of AMD is the dry form. Over time, the macula thins and the pigmented retinal epithelium, a dark-colored cell layer that supports the retina, is gradually lost. About 10% of dry AMD cases progress to the wet form. In a process called choroidal neovascularization (CNV), new blood vessels proliferate in the choroid and may invade the retina. These fragile vessels can leak blood and fluid into the retina, damaging or killing macular cells and resulting in scar tissue that interferes with vision. If untreated, the macula may be destroyed. Wet AMD progresses more rapidly than dry AMD and severe vision loss typically occurs within two years.

Less common forms of MD include:

• juvenile macular degeneration (JMD), a group of inherited disorders affecting children and younger adults
• cystoid macular degeneration, the development of fluid-filled cysts (sacs) in the macular region, associated with aging, inflammation, or severe myopia (nearsightedness)
• diabetic macular degeneration
• retinal pigment epithelial detachment, a rare form of wet MD in which fluid leakage from the choroid causes the detachment or disappearance of the pigmented retinal epithelium

Causes

Age-related macular degeneration (AMD) appears to result from a combination of hereditary, environmental, and metabolic factors. Over time, highly reactive free-oxygen radicals damage and destroy macular cells. Free radicals are produced by:

• bombardment of light on the macula, particularly long-term exposure to ultraviolet and blue light, including sunlight and sunlamps
• smoking, which increases the risk of AMD two- to four-fold
• a high-fat diet

The body's antioxidant systems that destroy free radicals become less effective with aging.

Factors that contribute to the hardening and blocking of the capillaries supplying the retina and lead to AMD include:

• smoking
• diets high in saturated fat and cholesterol
• low dietary consumption of antioxidants

The cause of choroidal neovascularization (CNV) in wet AMD is unknown. However many people with AMD also have cataracts and cataract surgery increases the risk of dry AMD progressing to wet AMD.

Symptoms

AMD is painless, and in the early stages, the brain easily compensates for vision loss, particularly if AMD is restricted to one eye. Symptoms of AMD include:

• requiring more light for reading
• reduction, blurring, a blank spot, or loss of central vision while peripheral vision is unaffected
• difficulty recognizing faces
• visual distortions such as the bending of straight lines
• images appearing smaller
• changes in color perception or abnormal light sensations
• a decline of at least two lines in visual acuity as measured on a standard eye chart. For example, 20/20 vision declining to 20/80
• phantom visions, called "Charles Bonnet syndrome"