Retinal vein occlusion is a blockage of the small veins that carry blood away from the retina. The retina is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. A blockage in the retina’s main vein is referred to as a central retinal vein occlusion (CRVO), while a blockage in a smaller vein is called a branch retinal vein occlusion (BRVO)