Mechanisms of fluid accumulation in retinal edema

This paper reviews the anatomic and physiologic conditions which predispose to fluid accumulation within the retina. Retinal edema has its inception i n disease that causes a breakdown of the blood-retinal barrier in retinal c apillaries and/or the retinal pigment epithelium (RPE). Edema develops not only because protein and fluid enter the extracellular space, but because t he external limiting membrane and the convoluted extracellular pathway with in the retina limit the clearance of albumin and other large osmotically-ac tive molecules. These molecules bind water to cause edema. Recognition of e dema clinically is complicated by the facts that angiographic markers (fluo rescein and ICG) do not match albumin in size, and that clinical leakage do es not always correlate closely with tissue swelling or functional loss. Ac tive water transport across the RPE is efficient at removing subretinal wat er, but the flow resistance of the retina limits RPE access to the water of retinal edema. Consideration of the pathophysiology of retinal edema may a id in the development of better strategies for managing retinal edema.