Age-related macular degeneration (AMD) is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field (themacula) because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults (>50 years). Macular degeneration can make it difficult or impossible to read or recognize faces, although enough peripheral vision remains to allow other activities of daily life.
Starting from the inside of the eye and going towards the outer surface, the three main layers at the back of the eye are the retina, which is light-sensitive tissue that is considered part of the central nervous system and is actually brain tissue; the choroid, which contains the blood supply; and the sclera, which is the white, outer, layer of the eye.
The macula is the central area of the retina, which provides the most detailed central vision.
In the dry (nonexudative) form, cellular debris called drusen accumulates between the retina and the choroid, and the retina can become detached. In the wet (exudative) form, which is more severe, blood vessels grow up from the choroid behind the retina, and the retina can also become detached. It can be treated with laser coagulation, and with medication that stops and sometimes reverses the growth of blood vessels.
Although some macular dystrophies affecting younger individuals are sometimes referred to as macular degeneration, the term generally refers to age-related macular degeneration (AMD or ARMD).
Age-related macular degeneration begins with characteristic yellow deposits (drusen) in the macula, between the retinal pigment epithelium and the underlyingchoroid. Most people with these early changes (referred to as age-related maculopathy) have good vision. People with drusen can go on to develop advanced AMD. The risk is higher when the drusen are large and numerous and associated with disturbance in the pigmented cell layer under the macula. Large and soft drusen are related to elevated cholesterol deposits and may respond to cholesterol-lowering agents.
Central geographic atrophy, the “dry” form of advanced AMD, results from atrophy of the retinal pigment epithelial layer below the retina, which causes vision loss through loss of photoreceptors (rods and cones) in the central part of the eye. No medical or surgical treatment is available for this condition; however, vitamin supplements with high doses of antioxidants, lutein and zeaxanthin, have been suggested by theNational Eye Institute and others to slow the progression of dry macular degeneration and, in some patients, improve visual acuity. However, recent advancements within the field of stem cell research in theUnited States have led to the first human embryonic stem cell trial for dry AMD, which reports positive results.
Higher beta-carotene intake was associated with an increased risk of AMD.
Neovascular or exudative AMD, the “wet” form of advanced AMD, causes vision loss due to abnormal blood vessel growth (choroidal neovascularization) in the choriocapillaris, through Bruch’s membrane, ultimately leading to blood and protein leakage below the macula. Bleeding, leaking, and scarring from these blood vessels eventually cause irreversible damage to the photoreceptors and rapid vision loss if left untreated.
Only about 10% of patients suffering from macular degeneration have the wet type.
Macular degeneration is not painful, which may allow it to go unnoticed for some time.
SIGNS AND SYMPTOMPS
Signs and symptoms of macular degeneration include:
Macular degeneration by itself will not lead to total blindness. For that matter, only a very small number of people with visual impairment are totally blind. In almost all cases, some vision remains. Other complicating conditions may possibly lead to such an acute condition (severe stroke or trauma, untreated glaucoma, etc.), but few macular degeneration patients experience total visual loss. The area of the macula comprises only about 2.1% of the retina, and the remaining 97.9% (the peripheral field) remains unaffected by the disease. Interestingly, even though the macula provides such a small fraction of the visual field, almost half of the visual cortex is devoted to processing macular information.
The loss of central vision profoundly affects visual functioning. It is quite difficult, for example, to read without central vision. Pictures that attempt to depict the central visual loss of macular degeneration with a black spot do not really do justice to the devastating nature of the visual loss. This can be demonstrated by printing letters six inches high on a piece of paper and attempting to identify them while looking straight ahead and holding the paper slightly to the side. Most people find this difficult to do.
There is a loss of contrast sensitivity, so that contours, shadows, and color vision are less vivid. The loss in contrast sensitivity can be quickly and easily measured by a contrast sensitivity test performed either at home or by an eye specialist.
Similar symptoms with a very different etiology and different treatment can be caused by epiretinal membrane or macular pucker or leaking blood vessels in the eye.