The retina is the light-sensitive layer at the back of the eye that is responsible for vision. Blood circulation to most of the retina's surface is through only one artery and only one vein. If either blood vessel or one of their smaller branches is blocked, blood circulation to the retina can be significantly disrupted. The blockage is called an occlusion. When this happens, the eye typically loses vision, often suddenly. The condition is painless.
Retinal Artery Occlusion
The retinal artery carries oxygen-rich blood to the retina. When a blockage occurs in the retina's main artery, or in one of its small branches, the retina's light-sensitive cells gradually begin to suffocate from lack of oxygen. Unless normal circulation to the retina can be restored promptly, these cells will die within a few minutes or hours depending on how completely the blood flow is obstructed. This can cause permanent and often substantial loss of vision.
In adults, there are two main reasons that the retina's artery would become blocked: a thrombus or an embolus.
A thrombus (blood clot) – Inside the retinal artery, a blood clot typically develops at a site where the artery's lining has already been damaged by a chronic illness, such as high blood pressure (hypertension), diabetes or atherosclerosis. Atherosclerosis is a common cardiovascular problem that produces cholesterol deposits called plaques along the walls of arteries, decreasing blood flow.
An embolus (floating blood clot or debris in the bloodstream) – In the retinal artery, an embolus is usually a tiny blood clot or a piece of atherosclerotic plaque that has been carried through the bloodstream from a damaged area of the heart, aorta or carotid artery (in the neck). For this reason, the embolus is often interpreted as a warning sign of cardiovascular disease elsewhere, especially in the carotid artery. Rarely, pieces of a tumor from elsewhere in the body can embolize.
Less often, a retinal artery occlusion may be caused by vasculitis (inflammation of the artery's wall), trauma, sickle cell disease, clotting disorders, oral contraceptives or damage from radiation treatments. Overall, retinal artery occlusion is a rare illness that is responsible for only one out of every 10,000 visits to ophthalmologists (physicians who specialize in eye problems) in the United States. The average person with the illness is between 50 and 70 years old and has a history of heart disease, high blood pressure or diabetes. In almost all cases, only one eye is affected.
In the past, retinal artery occlusion has not included diabetic retinopathy. However, in some respects, diabetic retinopathy (which is a common condition) can be thought of as an occlusive condition of the retinal blood vessels. In diabetic retinopathy, the blood vessels affected are much smaller than the arteries involved in classic retinal artery occlusion.
Retinal Vein Occlusion
The retinal vein carries blood away from the retina. When the vein is blocked, blood flow backs up and causes tiny hemorrhages, areas of swelling, and other pressure-related damage in portions of the retina that are located near the blocked blood vessels. This can cause minimal or substantial loss of vision, depending on the extent of this retinal damage. Some common risk factors for this form of retinal vessel occlusion include high blood pressure, diabetes, open-angle glaucoma, lymphoma, leukemia, multiple myeloma, syphilis and use of oral contraceptives.