What is Age-Related Macular Degeneration (AMD)?
Tonight's the night. You've been looking forward to this performance for months. You make your way down the aisle and locate your seat in the front parquet only to discover that your view of the piano on center stage is blotted out by the head of the tall guy in front of you. The rest of the evening, you have the pleasure of watching the cellist on the right, the violinist on the left, and the back of Mr. Big's head.
Now imagine if your sight were compromised this way all the time--with a blank spot or blur right smack in the middle of your visual field. That's what it's like to live with age-related macular degeneration (AMD), a disease that destroys sharp central vision.
What Is Age-Related Macular Degeneration?
Age-related macular degeneration destroys the macula (pronounced MAK-you-luh), a cluster of light-sensitive cells in the central part of the retina, the back of the eye. The macula gives you crisp central vision and allows you to perceive fine detail.
The retina is responsible for converting images into electrical signals and transmitting them to the brain via the optic nerve (How the Eye Works). Although the macula makes up only about 5% of the retina, the macula's specialized cells are about a hundred times more sensitive to detail than the rest of the retina.
Macular degeneration develops gradually at first, but it can progress abruptly to the advanced stage, requiring immediate consultation with your vision care provider. Early detection and treatment are important in preventing AMD--related vision loss, and learning about the disease is a great place to start.
Forms and Stages
AMD is classified as early, intermediate, or advanced and labeled "dry" or "wet" according to the way in which the macula is damaged.
Dry AMD
If the light-sensitive cells and supportive tissues of the macula break down over time, central vision gradually deteriorates. We call this dry AMD (Dry Age-Related Macular Degeneration).
In dry AMD, no bleeding or fluid leakage occurs. The occurrence of AMD in one eye dramatically increases your risk of developing it in the other eye.
People with advanced dry AMD see a blur or a blank spot in the center of the visual field where the tissue has thinned and lost pigment. This round, well-defined, nearly transparent spot in the center of the macula is called an area of geographic atrophy.
In one of every seven people with dry AMD, the disease evolves into wet AMD.
Wet AMD
Wet AMD damages the retina in one of two ways: choroidal neovascularization or retinal pigment epithelial detachment. Let's take those terms one at a time.
Choroidal neovascularization.
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 (How the Eye Works). In advanced 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.
Retinal pigment epithelial detachment.
Now we're ready to tackle the other way in which wet AMD can damage the retina. The process is called 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's 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 neovascular form.
Drusen
Using a hand-held or head-mounted viewing instrument called an ophthalmoscope, your vision care provider can see signs of AMD on your retina. One sign strongly associated with the disease is the accumulation of lipid deposits on the retina called drusen. An increase in the size or number of drusen is associated with a higher risk of developing AMD (Symptoms of Age-Related Macular Degeneration).
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