Age-related macular degeneration: review of pathogenesis

Age-related macular degeneration is a condition (a) characterized by accumu lation of membranous debris on both sides of the retinal pigment epithelium (RPE) basement membrane. Clinical manifestations of drusen, atrophy of the RPE/choriocapillaris, RPE detachment, and choroidal new vessel (CNV) forma tion occur after age 50 years. A hypothetical pathogenic sequence of events consistent with known data is: 1) RPE dysfunction (e.g., precipitated by a n inherited susceptibility and/or environmental exposure); 2) accumulation of intracellular material in the RPE (e.g., accumulation of normal substrat e material that is not enzymatically degraded properly vs. abnormal substra te material); 3) abnormal accumulation of extracellular material (basal lam inar and basal linear deposit); 4) change in Bruch's membrane composition ( e.g., increased lipid deposition and protein crosslinking); 5) change in Br uch's membrane parmeability to nutrients (e.g., impaired diffusion of water soluble plasma constituents across Bruch's membrane); and 6) response of t he RPE to metabolic distress (i.e., atrophy vs. CNV growth). Histopathologi cal and clinical studies indicate that areas of choroidal ischemia often ar e seen near CNVs in AMD patients. In response to decreased oxygen delivery/ metabolic "distress", the RPE may elaborate substances leading to CNV growt h. Perhaps RPE atrophy, followed by choriocapillaris and photoreceptor atro phy, is a response to decreased nutrients/increasing metabolic abnormalitie s in areas of excessive accumulation of extracellular debris. Unanswered qu estions regarding AMD include: 1) is AMD an ocular manifestation of a syste mic disease or purely an ocular disease ?; 2) what determines whether CN Vs vs. a trophy of the RPE-choriocapillaris-photoreceptors develops?; and 3) what induces the maturation of CNVs into an inactive scar, and what limits the growth of most CNVs to the area centralis?