A5 Age-Related Macular Degeneration (AMD)

Topic

Located at the center of the retina—the light-sensitive layer that receives the image focused by the eye—is the macula; within the macula, at its very center, lies the fovea. This area contains the highest concentration of light receptors and is responsible for central vision—the part of the image seen with the greatest sharpness and detail—as opposed to peripheral vision, which surrounds central vision but never achieves the same level of clarity. Age-related macular degeneration specifically damages the macula and the fovea it contains, leaving the peripheral retina (which supports peripheral vision) relatively intact; this contrasts with glaucoma, where damage begins in the peripheral vision.

Just behind the retina, and separated from it by a thin membrane, lies the choroid—a layer of blood vessels that nourishes the retina from the outside without directly invading it. In the "wet" form of age-related macular degeneration, macular cells release excessive amounts of vascular endothelial growth factor, a chemical signal that triggers the growth of new blood vessels. Driven by this signal, new vessels from the choroid breach the membrane that normally separates it from the retina and grow into the macula—a process known as choroidal neovascularization.

The new blood vessels created by choroidal neovascularization lack the structural integrity of normal choroidal vessels; their walls allow fluid, lipids, and sometimes blood to leak into and beneath the macula—a process called exudation. This accumulating exudate causes edema, a swelling that separates and distorts the organized layers of the macula (including the fovea) in the very area responsible for central vision. Since exudation and edema are concentrated specifically in the macula—and the fovea contained within it—rather than in the peripheral retina, choroidal neovascularization specifically degrades central vision while leaving peripheral vision relatively intact, even in advanced stages. This deterioration of central vision can progress much more rapidly than that associated with other eye diseases because exudation and edema—unlike damage that accumulates cell by cell over time—can appear and expand within days or weeks if the choroidal neovascularization remains active.

Because choroidal neovascularization relies on the continued excessive release of vascular endothelial growth factor (VEGF) by macular cells, the primary treatment involves the direct intraocular injection of a medication—such as ranibizumab, aflibercept, or bevacizumab—that blocks this factor; this halts the stimulus driving the neovascularization and allows the exudation and edema surrounding the macula to subside. To monitor whether the choroidal neovascularization remains active and if the exudation and edema are resolving, optical coherence tomography (OCT) generates detailed images of the macular and foveal layers, directly revealing any residual fluid accumulation between them; meanwhile, the Amsler grid allows individuals to detect—by observing whether the grid's straight lines appear distorted—any new flare-up of exudation or edema affecting their central vision at home, prior to their next OCT follow-up.

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