Retinal vein occlusion occurs when the circulation of a retinal vein becomes obstructed by an adjacent blood vessel. This results in the stoppage of blood flow, causing hemorrhages in the retina. The retinal veins are the small "pipes" in the retina that drain blood out of the retina, back to the heart. The veins drain the blood out of the eye, while the retinal arteries are the small pipes that deliver the blood (from the heart) to the retina.
Retinal vein occlusion is second only to diabetic retinopathy as a cause of visual loss due to retinal vascular disease. There are two forms of retinal vein occlusion, branch retinal vein occlusion (BRVO) and central retinal vein occlusion (CRVO). While there are similarities in the pathogenesis and clinical nature of these two events, each has unique etiologies, differential diagnosis, management and prognosis.
A branch retinal vein occlusion is essentially a blockage of the portion of the circulation that drains the retina of blood. The arteries deliver blood to the retina. The red blood cells and plasma then course through the capillaries and eventually into the venous system, beginning with small veins and ending with larger ones, and eventually reaching the central retinal vein. With blockage of any vein, there is back-up pressure in the capillaries, which leads to hemorrhages and also to leakage of fluid and other constituents of blood. Usually, the occlusion occurs at a site where an artery and vein cross. The occlusion site determines the extent or distribution of the hemorrhage, ranging from a small vein branch to a quadrantic occlusion involving one fourth of the retina to a hemispheric (hemi-retinal) occlusion involving one half of the retina to an occlusion of the central retinal vein, which involves the entire retina (when the central vein is involved, this is called a central retinal vein occlusion which is discussed below).
Branch retinal vein occlusions are by far the most common cause of retinal vascular occlusive disease. Males and females are affected equally. Most occlusions occur after age 50, although younger patients are sometimes seen with this disorder (in this age group it is often called papillophlebitis). The highest rate of occurrence is in individuals in their 60's and 70's. The risk factors for this disorder are similar to those for vascular occlusive disease elsewhere in the body such as stroke and coronary artery disease. Specifically, aging, high blood pressure, diabetes, and smoking are all risk factors. Glaucoma has also been identified as a risk factor in some studies.
