Pathophysiology of Macular Edema: Results from Basic Research

Macular edema is the final common pathway of numerous retinal diseases, and ocular disorders associated with this condition are a major cause of blindness. Fluid and molecular exchanges between the blood and retina are tightly regulated and restricted by the blood-retinal barrier (BRB), which is mainly composed of intercellular junctions both at the levels of capillary endothelial cells (inner BRB) and at the retinal pigment epithelium (outer BRB). When a break occurs in the BRB, whether caused by pathological changes in the intervening cells, by imbalance in hydrostatic/oncotic pressure, or by other mechanical forces, fluid and macromolecules enter the retinal intercellular space, causing retinal edema. When this barrier break occurs at or near the macula, fluid accumulation induces formation of macular edema and vision loss ensues. Common diseases associated with macular edema include diabetic retinopathy, age-related macular edema, retinal venous occlusion, retinopathy of prematurity, intraocular inflammation (uveitis, postsurgical, traumatic), anatomical disturbances of retinal vasculature (macular telangiectasias, macroaneurysms), drug-induced break, and tumors. In this chapter, we review and discuss the cellular and molecular mechanisms of BRB integrity and breakdown.