Blood and nutrient circulation to and from the surface of the retina is mostly done through one vein and one artery. If these passages or any of the smaller branches connected to them are blocked, blood flow to the retina can become seriously disrupted. Blockage of one of these passages is known as occlusion and can result in sudden vision loss.

The retinal vein carries blood away from the body and may cause blood to build up and hemorrhage if it is blocked. Retinal vein occlusion, also known as venous stasis retinopathy, can also cause swelling, bleeding and growth of abnormal blood vessels which can in turn lead to other serious eye diseases.