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The eye has a very complex and crowded network of vessels that circulate blood to the retina. Arteries carry blood to the retina and veins drain blood away from the retina. The arteries, although very small (about 1/10th of millimeter), have thicker, firmer walls compared to the veins, which are therefore more collapsible than the artery.

In this crowded space, the firmer artery can sometimes press against either the neighboring weaker central vein or at a point where an artery crosses a branch retinal vein. In either case, the squeezing of the blood flow in the vein can lead to a more turbulent flow which can cause clotting and eventual vein blockage.

Where this occurs at the central vein the condition is called CRVO, or central retinal vein occlusion. BRVO, or branch retinal vein occlusion, is the name applied when the blockage threatens a branch of the central vein.
Typically, someone undergoing either of these occlusions would experience a painless loss of vision in one eye. Both CRVO and BRVO are more likely to occur in people over the age of fifty, but will also occur in younger persons.

Treatment can be by laser, medications, or surgery or a combination of these treatments.