|
Understanding Dry AMD
By Richard Spaide, MD
With age, changes may occur in the macula, a small area in the back of the eye used for detailed vision. These changes are common, fairly specific and associated with a potential for deterioration of vision. Therefore, they're called age-related macular degeneration or AMD.
When talking to patients about these changes, retinal specialists use terms to describe various AMD findings. There are two broad categories: dry and wet. Wet AMD refers to an invasion of blood vessels and other cells that can lead to severe, permanent loss of vision. The term dry AMD covers many different manifestations in the eye, including drusen, pigmentary changes and geographic atrophy.
Dry AMD precedes wet AMD. Both wet AMD and geographic atrophy, a component of dry AMD, can lead to severe vision loss. Wet AMD and geographic atrophy are sometimes lumped together in a term called late AMD -- also referred to as late age-related maculopathy or late ARM. Here, I will focus on important aspects of dry macular degeneration that are risk factors for progression to late AMD.
Drusen
Drusen are accumulations of material that build up under the retina and are commonly found in the macular region. These appear as small yellow-white dots or mounds. Contained within drusen are lipids and proteins, some of which have suffered oxidative damage. Also found in drusen are proteins related to the immune system, particularly those from what is called the complement system.
We know that a particular type of drusen -- soft drusen -- carries a higher risk of progressing to wet macular degeneration than other types. The more soft drusen present, the greater the risk for progression to wet AMD.
Pigmentation
Pigmentation in the back of the eye can change with age as well. We are all familiar with spots and splotches of pigment on our skin that appear with age. In the eye, there can be minute accumulations of pigment under the macula. Curiously, these accumulations also carry a risk for progression to late forms of macular degeneration, including the development of geographic atrophy and wet AMD.
Geographic Atrophy
As people age, it's common to see the loss or withering of some cells in the body. Everyone loses some hair with age -- some people more than others. Just like a person becoming bald, some people lose retinal pigment epithelial (RPE) cells in the macular region. This loss occurs in ovoid shapes with irregular margins that look like countries on a map. For that reason, the areas are called geographic atrophy.
As RPE cells grow older, they contain a waste material called lipofuscin. Most of the material in lipofuscin is related to abnormal products formed from vitamin A-related molecules. Ordinarily, these molecules are used by the eye to sense light. However, light can lead to damage of these molecules and the cells in the eye have a difficult time metabolizing these damaged molecules. Instead, waste material is stored in bags distributed throughout the cell. An interesting feature of lipofuscin is that it can be made to glow with the application of certain wavelengths of light. Using this feature, we have developed a method that photographically records areas of geographic atrophy. By eliciting this glow and photographing the result, we can create accurate maps that reflect the health of the cells.
What's Ahead
On average, people in their 70s develop wet macular degeneration, while the average age for those who develop geographic atrophy is usually in their 80s. With our growing aging population, the number of people with geographic atrophy is on the rise. We have many treatments for wet AMD, but currently we have no treatment for geographic atrophy. Many patients with geographic atrophy have a difficult time reading and seeing fine details. Low vision devices, although sometimes frustrating to use, are usually helpful for patients with geographic atrophy.
Unfortunately, geographic atrophy is a "tough nut to crack." When cells want to die, it's difficult to prevent that from happening. While sunlight exposure has not been shown to be definitively associated with wet AMD, there are theoretical reasons to think that light exposure, particularly blue light exposure, may activate lipofuscin in retinal pigment epithelial cells. Activated lipofuscin may then cause oxidative and other forms of damage in the cells, ultimately leading to cell death.
Promising research interventions target the processing of vitamin A, reducing the amount of vitamin A available to the cells to use in vision. This strategy may result in less vitamin A waste material, which is toxic to cells, but may also adversely affect night vision. Other strategies are examining cell survival factors -- chemical signals that may prolong the life of cells.
Richard Spaide, MD, is an ophthalmologist with Vitreous, Retina, Macula Consultants of New York, and is affiliated with the Manhattan Eye, Ear and Throat Hospital
This article originally appeared in the Fall/Winter 2007 edition of Sharing Solutions. To download the entire newsletter, click here