What is geographic atrophy? What treatments are coming?
Reported by Elaine A. Richman, Ph.D.
Geographic atrophy is the advanced (late) form of dry AMD. Here, atrophy refers to the degeneration of the deepest cells of the retina. These are cells of the retinal pigment epithelium (RPE). This RPE normally helps maintain the health of the next deepest layer, the photoreceptor cells known as rods and cones. These photoreceptor cells are triggered by light to set off a series of electrical and chemical reactions that result in the brain interpreting what is in the visual field.
Degeneration of RPE cells leads to the death of rods and cones. What an eye doctor sees in a dilated eye exam in a patient with GA is what Dr. Sunness describes as a “continent” of atrophy surrounded by a “sea” of retina. The continent appears different from the surrounding retina because of the loss of the pigmented RPE cells. Areas of GA look whiter than the surrounding retina.
GA tends to progress slowly. Progression is currently studied using a technique called autofluorescence (AF) imaging to define the areas of GA. A newer technique called high density optical coherence tomography (OCT) allows the doctor to visualize the different layers of the retina, and to determine when cells are becoming thinned or destroyed. Doctors use the term “scotoma” to refer to the blind spot in a person’s vision caused by the absence of cells in the part of the macula that has degenerated. By measuring enlargement of the area of atrophy, a doctor can estimate the loss in visual function. (Low vision specialists train patients to use healthy portions of the retina to view the otherwise missing visual field.)
Dr. Sunness estimates that 3.5% of the U.S. population age 75 and older has GA. In people older than 90, the percentage is 22%.
Geographic atrophy is more common in whites, in people with a light colored iris, in cigarette smokers, in older age, and in people with a greater retinal area containing drusen. GA also appears to have a genetic component.
Researchers believe that a gene called toll-like receptor 3 (TLR3) is likely involved in GA.
People with macular degeneration often complain that their reading speed is not what it used to be. A drop in the speed at which a person with GA reads can indicate an increase in the size of the geographic atrophy.
Another event in GA is something called low luminance deficit (LLD) (difficulty reading in dim light). “This is one of my patients’ greatest complaints,” says Dr. Sunness. “They complain, for example, of not being able to read menus in dimly lit restaurants.”
“Forty percent of people with GA require print to be twice as large after two years of worsening,” says Dr. Sunness. She adds, “Educating patients with GA about obtaining their best possible vision is extremely helpful for them and rewarding for me. There are three basic lessons for them to learn. One, they need things bigger, which magnification provides. Two, they need things bolder, meaning contrast and lighting have to be good. Three, they need to learn to look around the boulder that is their scotoma. Helping people with scotoma awareness helps them function better.” Hoover Low Vision Services and other low vision centers teach about applying this information.
Clinical trials underway for geographic atrophy:
Ciliary Neurotrophic Factor (CNTF) by Neurotech delivered via a unique encapsulated cell technology (ECT) slowed vision loss.
Fenretinide is under development by Sirion Therapeutics.
Clinical trials of two other types of treatment for GA recently showed promising results. One involves implanting cells in the eye that then secrete a growth factor for sustaining the health of RPE and photoreceptor cells. Another is a drug that is taken by mouth and appears to work by preventing the build up of potentially harmful byproducts of metabolism. These are experimental treatments and, if they continue to show good results, could possibly be commercially available in about 5 years.
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