Geographic atrophy (GA) is caused by the gradual death of light-sensing cells and their support layer in the back of the eye, driven primarily by a malfunctioning immune process, oxidative damage, and genetic susceptibility. It’s the advanced “dry” form of age-related macular degeneration, and unlike the “wet” form, it progresses slowly without abnormal blood vessel growth, instead leaving expanding patches of dead tissue in the retina that cause permanent vision loss.
How Retinal Cells Break Down Over Time
The retina’s support layer, called the retinal pigment epithelium (RPE), works harder than almost any other tissue in the body. It constantly recycles waste from the light-sensing photoreceptor cells above it, processes nutrients, and absorbs stray light. That intense workload generates large amounts of reactive oxygen species, the same damaging molecules that contribute to aging throughout the body.
Over decades, the RPE accumulates two types of waste deposits. Lipofuscin builds up inside the cells themselves, a yellowish pigment that becomes increasingly toxic as it piles up. Drusen, small yellowish clumps of protein and fat, form beneath the RPE between it and its blood supply. These deposits aren’t just markers of aging. They actively interfere with the RPE’s ability to nourish photoreceptors and clear waste, creating a vicious cycle of declining function. Histological studies show that atrophy can begin directly over individual drusen, with a high proportion of the most advanced drusen (about 82%) containing calcified nodules.
The RPE cells’ own energy factories, their mitochondria, are central to this decline. In GA, the normal processes that keep mitochondria healthy become deeply disrupted. Damaged mitochondria fragment rather than fusing and repairing, and the cell’s cleanup system fails to remove the broken ones. The result is an energy crisis: RPE cells can’t produce enough fuel to sustain themselves or the photoreceptors they support. The damaged mitochondria also leak molecular debris that triggers inflammation, accelerating the damage further.
The Immune System Turns Against the Retina
The single most important driver of GA is a branch of the immune system called the complement system. This is a network of proteins that normally patrols the body for damaged cells and pathogens. It works through a chain reaction: one protein activates the next in a cascade. In healthy eyes, this cascade stays tightly controlled. In GA, it doesn’t.
The cascade converges on a central protein called C3. When C3 is cleaved, it produces fragments that tag cells for destruction and triggers the next step, activating another protein called C5. C5 activation is especially dangerous because it leads to assembly of the membrane attack complex, a ring-shaped structure that literally punches holes in cell membranes. In the retinas of people with AMD, elevated concentrations of C3, its breakdown products, and other complement proteins have been found in the tissue layer beneath the RPE and in the blood vessels that feed it. Blood tests show the same pattern: people with AMD have higher circulating levels of complement fragments like C3a and C5a compared to people without the disease.
The waste deposits beneath the RPE play a direct role in triggering this immune overreaction. Drusen and oxidative stress products activate the complement cascade and a separate inflammatory pathway called the NLRP3 inflammasome. When the body’s normal regulatory checks on these pathways are weakened, whether by genetics or aging, the inflammation becomes chronic. Instead of clearing a threat and calming down, the immune response sustains itself, steadily killing RPE cells and the photoreceptors that depend on them.
Genetic Variants That Raise Risk
Not everyone with drusen develops GA. Genetics play a major role in determining who crosses that threshold. Three gene variants have the strongest associations with GA susceptibility. A variant in the CFH gene, which normally produces a protein that puts the brakes on complement activation, is powerfully linked to disease risk. A variant in the ARMS2 gene, which appears to affect mitochondrial function and inflammation in the retina, shows an even stronger statistical association. A third variant in the C3 gene itself also increases risk, consistent with the central role of complement in driving the disease.
These genetic links are so strong that in studies of patients with bilateral GA (both eyes affected), all three variants reached high levels of statistical significance. However, these same variants don’t appear to control how fast the disease progresses once it starts. That means your genes largely determine whether you develop GA, but other factors influence the speed at which it worsens.
Smoking, Weight, and Earlier Onset
Among modifiable risk factors, smoking and excess body weight stand out. Current smokers develop advanced AMD roughly 3.5 years earlier than people who have never smoked. A BMI of 30 or higher accelerates onset by about 2 years. When smoking and obesity occur together, the effect compounds, pushing the age of progression even earlier. Combined with high-risk genetics, smoking and elevated BMI can lower the age of developing advanced disease by 7 to 11.5 years, meaning a significantly longer period of vision loss over a lifetime.
How GA Patches Grow
GA doesn’t appear all at once. It starts as one or more small patches of dead RPE and photoreceptor tissue, often near but not in the very center of the macula. These patches expand outward over time. Studies using advanced imaging measure the average growth rate at about 0.32 mm per year (square root transformed), though individual rates vary considerably. The expanding border of a GA lesion has a characteristic zone about 200 to 500 micrometers wide where the RPE is thickened and photoreceptor layers are already disrupted, even though the cells haven’t fully died yet. This junctional zone is where active degeneration is happening, and it’s the target area for current treatments.
As patches enlarge and merge, they eventually involve the fovea, the tiny central point of sharpest vision. That’s when people notice the most significant impact: difficulty reading, recognizing faces, and seeing fine detail. Peripheral vision typically remains intact, so GA rarely causes total blindness, but the central vision loss is permanent.
How New Treatments Target the Cause
Understanding the complement system’s role has led to the first FDA-approved treatments for GA. Both work by interrupting the complement cascade at different points. One targets C3, the central protein where all complement pathways converge, blocking its activation and preventing the downstream chain reaction. The other targets C5, stopping the final step that assembles the membrane attack complex. Both are delivered as injections into the eye and have been shown to slow the rate of lesion growth, though neither reverses damage that has already occurred.
These treatments represent a shift from managing symptoms to addressing one of the root biological causes. Because the complement system is only one piece of a complex process involving oxidative stress, mitochondrial failure, and waste accumulation, slowing the disease rather than stopping it entirely reflects the reality that GA has multiple interacting causes rather than a single trigger.