Age-related macular degeneration (AMD) has two forms – wet and dry – and is labeled according to the way in which the macula is damaged. Wet AMD is known as advanced AMD and does not have stages like dry AMD.
Wet AMD occurs when abnormal blood vessels behind the retina start to grow under the macula. These new blood vessels tend to be very fragile and often leak blood and fluid. The blood and fluid raise the macula from its normal place at the back of the eye, causing damage to the macula to occur rapidly.
Wet AMD also damages the retina in one of two ways: retinal pigment epithelial detachment or choroidal neovascularization.
Retinal Pigment Epithelial Detachment:
The retinal pigment epithelium (RPE) is a thin layer of cells that lies just below the rod cells of the retina. The RPE digests waste that is shed from these rod cells. However, when fluid from leaking choroid vessels accumulates between the choroid and the RPE, the epithelial layer can detach.
No blood vessel migration into the retina is apparent and your vision may be stable for months. Eventually, though, this fluid buildup distorts vision by elevating the macula from its normal position, the way the skin on your fingertips puckers when you stay in a bathtub or swimming pool too long. And without warning, this form of wet AMD often progresses to the choroidal neovascularization form.
The retina is nourished by a layer of blood vessels just beneath it called the choroid. The choroid and retina are separated by a barrier membrane (Read: How the Eye Works). In wet AMD, the blood vessels of the choroid migrate beneath and into the retina as this barrier breaks down. This process is called neovascularization.
These tiny, fragile vessels often leak blood and other fluid into the macula – that's where we get the term wet AMD. The bleeding inflames the macula, eventually leaving a scar. We call this a disciform scar because of its round shape.