The Retina and AMD: Dry and Wet Types
The retina is a complicated network of nerve cells that changes light into nerve impulses that travel to the brain where they are interpreted as visual images. The central part of the retina is called the macula and is responsible for vision that is needed for reading and other detailed work. Damage to the macula results in poor vision. The most common disease process that affects the macula is age-related macular degeneration (AMD). In patients with AMD, retinal cells in the macula die over the course of several years. The cell death and gradual visual loss usually do not begin until age 60 or older and hence the name age-related macular degeneration. It is not known why the cells die, but two clues suggest that genetic defects called mutations may be involved. One clue is that AMD runs in families and another clue is that AMD shares some features with other retinal degenerations known to occur from mutations.
AMD: Dry Type
Patients who are affected have gradual loss of central vision due to the death of photoreceptor cells and their close associates; retinal pigmented epithelial (RPE) cells. Photoreceptors, the cells in the retina that actually "see" light, are essential for vision. RPE cells are like the nursemaids for photoreceptor cells and are necessary for photoreceptor survival and functioning. Death of either of these cells types leads to death of the other. The cell death occurs in the macula. This is unfortunate because the macula is the center portion of the retina that is responsible for reading and other complex visual tasks. Therefore, patients with AMD gradually lose their central vision, and with it the ability to drive and read. As bad as this may seem, it is a gradual process and is compatible with reasonable functioning for many years.
AMD: Wet Type
However, there is another aspect of AMD that is even more devastating. As the photoreceptor and RPE cells slowly degenerate, there is a tendency for blood vessels to grow from their normal location in the choroid into an abnormal location beneath the retina.
This abnormal new blood vessel growth is called choroidal neovascularization (CNV). The abnormal blood vessels leak and bleed, resulting in sudden and severe loss of central vision. Depending on the location, laser treatment can sometimes be given to destroy the blood vessels. Only 15% of the cases of wet AMD are eligible to have laser treatment because the blood vessels can not be located too close to the center part of the macula. The laser is a beam of light that is absorbed by the RPE cells and changes to heat energy that cauterizes the abnormal blood vessels. Often times the blood vessels come back and result in severe loss of vision. In fact, most of the patients with AMD, who have very poor vision, have it as the result of abnormal blood vessel growth. Currently, there is no treatment available that prevents the cell death or abnormal blood vessel growth that occurs in AMD.
Only 10% of patients with AMD have the wet type, but it is responsible for 90% of all blindness resulting from AMD.