In the early stages, toxic metabolic waste products, especially metabolites of the retinal photopigments, accumulate in the retinal pigment epithelial cells immediately beneath the retinal photoreceptors. Accumulations of photoreceptor debris overwhelm the cellular transport mechanisms and form extracellular deposits known as “drusen” between the retinal pigment epithelium and the underlying basement membrane. While small, isolated drusen are essentially innocuous, larger, coalescent drusen indicate a high risk of atrophic breakdown of the retinal pigment epithelium and the underlying capillary bed, the choriocapillaris. This often takes the form of sharply demarcated atrophic patches, referred to as “geographic atrophy” or “dry” macular degeneration. The overlying retinal photoreceptors cease to function, resulting in central scotomas and loss of visual acuity. In some cases, the underlying vascular bed reacts to the atrophic process by the elaboration of vascular membranes which proliferate under the retinal pigment epithelium, or between the retinal pigment epithelium and the retina. These neovascular membranes are prone to extravasation into the extracellular space, causing macular edema, as well as subretinal hemorrhage, again leading to loss of central vision. This “wet” or “exudative” macular degeneration is much less frequent than “dry” macular degeneration, but accounts for most cases of severe visual loss from macular disease.