Edwin M. Stone, M.D., Ph.D., Director of the Center for Macular Degeneration and the Molecular Ophthalmology Laboratory at the University of Iowa, provided this medical outline:
The term macular degeneration refers to a group of disorders that affect the central portion of the retina and, as a result, the center of the field of vision. The most common forms of this disease usually affect patients over the age of 65 and are collectively the most frequent cause of legal blindness in developed countries.
These late-onset forms are often called "age-related macular degeneration." Age-related macular degeneration is extremely prevalent, with as many as one in 10 patients over the age of 65 affected to some degree. Fortunately, severe visual loss is much less common, affecting perhaps one in 100 people over the age of 65.
What Is Macular Degeneration?
The inside of the eye is lined by three layers of tissue, each critical for normal vision. The innermost layer (the one first struck by the light that enters the eye) is known as the retina and consists of a complex network of nervous tissue. Some of the cells in this layer (the photoreceptors) convert light into an electrical signal that is then amplified and processed by other cells before being sent to the brain via the optic nerve.
Image: Edwin M. Stone
The central part of the retina--the macula--has a number of special structural features that allow images focused on it to be seen with very high resolution. The middle layer is a one-cell-thick sheet known as the retinal pigment epithelium, or RPE. The RPE provides metabolic support for the photoreceptor cells and also removes old bits of cellular debris from the tips of the photoreceptor cells as they renew themselves. The layer farthest from the incoming light is a rich network of blood vessels known as the choroid. These vessels supply oxygen and nutrients to the retinal pigment epithelium and photoreceptor cells and carry away waste products.
In macular degeneration, clumps of yellowish cellular debris--possibly of retinal origin--gradually accumulate within and beneath the retinal pigment epithelium. These deposits are visible to the clinician looking inside the eye as small yellow dots known as drusen (singular: druse). With the passage of time, patches of retinal pigment epithelial cells may die, resulting in bare spots known as geographic atrophy.
When the support functions of the RPE are lost, the photoreceptor cells overlying the areas of geographic atrophy cannot function and the vision from this patch of retina is lost. If these patches become large and involve the very center of the macula (the fovea), the individual's visual acuity can fall to the point that he or she is considered legally blind. This atrophic phase of macular degeneration is sometimes referred to as "dry" macular degeneration.
In approximately 10 percent of patients with macular degeneration, the injury to the retinal pigment epithelium stimulates the choroidal blood vessels to grow up into the RPE and retina--seemingly in an attempt to heal the defects in these layers. This reparative response is very similar to those that occur elsewhere in the body in response to injury--such as scar formation after a cut on the skin. Unfortunately, the retina is such a complex and highly ordered tissue that the in-growth of these new blood vessels causes more visual loss than the original degenerative process. In fact, although only 10 percent of patients develop new blood vessels, this complication is responsible for most of the legal blindness associated with macular degeneration. The vascular phase of macular degeneration is sometimes called "wet" macular degeneration.