The retina is the nerve layer that lines the back of the eye. Like every other tissue in the body, the retina is nourished by a system of blood vessels. The retinal arteries and veins originate in the optic nerve, and spread across the entire retina. Sometimes a clot, or an obstruction to blood flow, can occur in a retinal vein. This clot or obstruction can affect the entire retina if it occurs at the central vein, where the optic nerve is -- or it may affect a portion of the retina if it occurs further out from the center. This condition is called Retinal Vein Occlusion. Typically, a Retinal Vein Occlusion will cause fluid or blood to build up in the affected parts of the retina, including the part of the retina that provides sharp vision, called the macula. Retinal Vein Occlusion can cause sudden and severe blurring, or dimming of vision. Over time, these acute symptoms may turn into permanent visual loss. Retinal vein occlusion needs prompt attention by an eye doctor, because in many cases, treatment can be provided to reduce swelling of the macula, and to prevent other complications.