Age-Related Macular Degeneration Types
There are two types of age-related macular degeneration: non-neovascular and neovascular. It's possible for an individual to have both forms at the same time, and macular degeneration may occur in one or both eyes. It is also not uncommon for a person with one form to develop the other form later.
The onset and progression of either type of AMD do not follow any particular pattern, which can make AMD difficult to diagnose in its earliest stages.
Also known as nonexudative, atrophic, or dry AMD, non-neovascular AMD is the most common: 90 percent of people with AMD have this type. Non-neovascular AMD is usually characterized by shrinkage of tissues in the retina and formation of drusen. Drusen are small accumulations of debris underneath the retina and deep within the retinal pigment epithelium (RPE).
Although this form of AMD cannot be prevented or reversed, it may progress slowly and may stabilize for intermittent periods. In many individuals with non-neovascular AMD, vision is not seriously impaired. However, if the light-sensitive rods and cones disappear over time along with the RPE (a condition called geographic atrophy), then severe vision loss can develop. Use of vitamin supplements can decrease the likelihood of this outcome in some people.
In people with non-neovascular AMD, neovascular AMD may develop at any time. The chance of this happening is greater in those who have more or larger drusen and more changes in the pigment epithelial layer in the macula.
This form of AMD (also called exudative or wet AMD) is the more serious form of the disorder and is the primary cause of AMD-related vision loss. Loss of vision occurs because of neovascularization, the growth of new blood vessels from the choroid layer of the eye, the layer between the retina and sclera.
These new blood vessels tend to leak fluid under the retina and lead to scar tissue. The scar tissue replaces the light-sensitive cells and the RPE. Individuals who have neovascular AMD in one eye may have up to a 50 percent chance that the other eye will become affected within five years.
Neovascular AMD is further classified by the position of the new blood vessels and the pattern of leakage identified on a diagnostic test called fluorescein angiography. The location of the neovascularization is described in terms of its proximity to the fovea. New vessels farthest from the fovea are termed extrafoveal, those at the fovea itself are called subfoveal, and those in between are juxtafoveal.
The pattern of leakage is described as classic (in which the fluorescein leakage is rapid and intense) or occult (in which fluorescein leakage is slower and less intense). The extent of the leakage tends to be defined more clearly with classic new blood vessels than with occult new blood vessels.