Macular Degeneration—Ag... Health Article

Definition

Age-related macular degeneration (AMD) is one of the most common causes of vision loss among adults over age 55 living in developed countries. It is caused by the breakdown of the macula, the central part of the retina located in the back of the eye. The macula allows people to see objects directly in front of them (called central vision), as well as fine visual details. People with AMD usually have blurred central vision, difficulty seeing details and colors, and they may notice distortion of straight lines.

Description

The normal function of the macula and AMD is best understood accompanying a description of normal eye function. The eye is made up of many layers of different types of cells that all work together to send images from the environment to the brain, similar to the way a camera records images. When light enters the eye, it passes through the lens and lands on the retina, a very thin tissue, which lines the inside of the eye. The retina is made up of 10 different layers of specialized cells, which allow it to function similarly to film in a camera, by recording images. The macula is a small, yellow-pigmented area located in the center of the back of the eye on the retina. The macula contains blood vessels and nerve fibers. The retina contains many specialized cells called photoreceptors that sense light coming into the eye, convert light into electrical messages, and send them to the brain through the optic nerve. They allow the brain to perceive the environment.

The retina contains two types of photoreceptor cells: rod and cones. The rods are located primarily outside of the macula and they allow for peripheral (side) and night vision. Most of the photoreceptor cells inside the macula are the cone cells, which are responsible for perceiving color and viewing objects directly in front of the eye (central vision). If the macula is diseased, as in AMD, color and central vision are altered. There are two different types of AMD: dry AMD and wet AMD.

Dry AMD

Approximately 90% of individuals with AMD have the dry form. This condition is sometimes referred to as nonexudative, atrophic, or drusenoid macular degeneration. In dry AMD, some of the layers of retinal cells (called retinal pigment epithelium, or RPE cells) near the macula begin to degenerate. The RPE is the insulating layer between the retinal and choroid layer, which contains blood vessels. The RPE acts as a protective shield against damaging chemicals and a filter for the nutrients that reach the retina from the choroid blood vessels. The RPE cells normally help remove waste products from the rods and cones. When the RPE cells are no longer able to provide this function, fatty deposits called drusen begin to accumulate, enlarge, and increase in number underneath the macula. The drusen formation can disrupt the cones and rods in the macula, causing them to degenerate, or atrophy (die). This usually leads to central and color vision defects for people with dry AMD. However, some people with drusen deposits have minimal or no vision loss, and require regular eye examinations to check for AMD. Dry AMD is sometimes called nonexudative, because even though fatty drusen deposits form in the eye, people do not have leakage of blood or other fluid (often called exudate) in the eye. Dry AMD symptoms remain stable or worsen slowly from early stages to intermediate or advanced stages of dry AMD. Advanced stages of AMD may result in vision loss. In addition, approximately 10% of people with dry AMD eventually develop wet AMD, the advanced stage of AMD.

Wet AMD

Approximately 10% of patients with AMD have wet AMD that progressed from some stage of the dry form. This form of AMD is also called subretinal neovascularization, choroidal neovascularization, exudative form, or disciform degeneration. Wet AMD is caused by leakage of fluid and the formation of abnormal blood vessels (called neovascularization) in the choroid layer of the eye. The choroid is located underneath the retina and the macula, and it normally supplies them with nutrients and oxygen. When new, delicate blood vessels form, blood and fluid can leak from them. The formation of abnormal blood vessels underneath the macula leaks enough fluid to raise the macula up and away from the back of the eye and damages it. This causes central vision loss and distortion as the macula is pushed away from nearby retinal cells. Eventually a scar (called a disciform scar) can develop underneath the macula, resulting in severe and irreversible vision loss. Wet AMD does not have early or intermediate stages. It is considered advanced AMD and is more severe than dry AMD.