Central Retinal Vein Occlusion is a complicated problem that may cause a significant loss of vision. In order to understand this problem, it is necessary to understand some of the anatomy of the eye.
The retina is the nerve layer in the back of the eye. The term macula refers to the small central area of the retina which is the part most important for vision. The macula is the part of the retina that is responsible for seeing straight ahead, for seeing color, and for performing fine visual tasks such as reading. The retina has its own blood supply which comes from behind the eye and enters the retina through the optic nerve. A single artery (the central retinal artery) brings the blood into the retina and a single vein (the central retinal vein) drains the blood out of the eye as it makes its way back to the heart.
Central Vein Occlusion
A central retinal vein occlusion (CRVO) occurs when the blood flow out of the eye through the central vein becomes sluggish or completely obstructed. No one understands what causes the blood flow to slow. It is thought that somehow the central vein becomes clogged up inside or perhaps pinched by some structure from the outside. Whatever the cause, the blood cannot get out of the eye, and it begins to back up, much like a stopped-up drain in a sink or a dam on a river. The blood vessels behind the obstruction, in this case the smaller veins feeding into the central vein, begin to swell with the extra blood. Soon the pressure in the veins begins to build and blood begins to seep out of the veins and into the retina. This appears as bleeding (hemorrhages) in all areas of the retina.
If the obstruction is severe enough, the pressure will drive more and more blood and fluid into the retina. The retina begins to swell like a sponge. Much of the swelling occurs in the center of the retina, the macula. This swelling may severely impair vision and is known as macular edema.
Generally, a central retinal vein occlusion is classified by its severity into one of two types. These two types are called by many different terms but can most easily be thought of as mild or severe (the mild form may also be called impending or non-ischemic vein occlusion and the severe may be called complete or ischemic vein occlusion). The mild form of central retinal vein occlusion is generally due to sluggish blood flow without complete obstruction of the vessels involved. There are fewer hemorrhages and less swelling seen in the retina. In the more severe form, the blood flow is completely obstructed. The blood may even back up so much that fresh blood cannot enter the retina through the arteries. The cells in the retina become starved for oxygen that is normally brought in by the blood. This condition, lack of oxygen, is called ischemia. An ischemic vein occlusion can cause severe damage to the retina and, thus, to the vision.
If the ischemia (or lack of oxygen) is very severe, the eye may attempt to grow new blood vessels. Although this may seem like a helpful response, but in fact, the process of new blood vessel growth (called neovascularization) can be very destructive to an eye. If these blood vessels grow out of the retina, they can lead to a loss of vision from severe bleeding or retinal detachment. If they grow onto the iris, they may cause an unusual form of glaucoma to develop. Glaucoma is a condition in which the pressure inside the eye rises to intolerable levels. Known as neovascular glaucoma, this disease is almost impossible to treat with drops or medication and can lead to total blindness or loss of the eye.
Causes of Central Retinal Vein Occlusion
Although the process of central retinal vein occlusion has been extensively studied, no one yet understands exactly what causes this problem. It is known that it usually occurs in adults over 55 years old, but it may occur in young adults as well. It is also known that patients with central retinal vein occlusion generally have a higher rate of high blood pressure than that seen in the general population, but the occurrence of a vein occlusion is not related to episodes of uncontrolled blood pressure. Patients with vein occlusions tend to have other associated vascular problems as well, such as arteriosclerosis. These findings do not explain why this disease also occurs in young people. Your physician may want to do other tests on you in order to find out if you have any other, associated medical problems. The degree of suspicion for these conditions depends on other symptoms and findings. Generally speaking, these lab tests are not usually helpful in treating a vein occlusion, but under some circumstances they may be useful in discovering other, treatable conditions.
Treatment of Central Retinal Vein Occlusion
The most frustrating aspect of this condition is that there is no treatment for a central retinal vein occlusion. Many investigators have studied this problem and attempted treatments without success. Periodically there are claims or success made for one medication or another, but no treatment has been proved effective under scientific testing conditions. Scientists have tried to thin the blood and improve the flow with various blood thinners (anticoagulants) such as aspirin, Coumadin and heparin. They have tried using the same "clot busting" agents that are used successfully in heart attacks and strokes, such as TPA and streptokinase. They have tried to lower the pressure in the eye to dilate blood vessels and promote better flow. They have surgically tried to create alternative blood flow channels. None of these approaches has had any real success. One simply has to wait for the body to reopen the vein on its own. It will do so, but it may take anywhere from a few months to a few years for the vein occlusion to completely resolve.
Several complications can occur that may make the central vein occlusion more troublesome.
Macular edema was mentioned earlier. This condition is caused by fluid leaking from damaged blood vessels into the central retina. For many years it was thought that laser treatment played a useful role in treating macular edema. However, in recent years, a major scientific study demonstrated conclusively that laser was of no benefit in resoling or improving macular edema.
Neovascular glaucoma is caused by abnormal blood vessels growing on the iris and causing the pressure to build up inside the eye. If this problem is detected early, then it can be successfully treated with laser therapy. Interestingly, studies have shown that the success for laser is higher if the treatment is not given preventatively. That is, it seems best to wait until the problem has actually begun, rather than to treat ahead of time in anticipation of problems. Practically speaking that means a patient with a central retinal vein occlusion will need to be examined carefully about every 4 - 6 weeks during the first six months after diagnosis in order to catch the problem on time. Your physician may also diagnose this using a test called a fluorescein angiogram. This involves injecting a dye into an arm vein and taking photographs of the retina as the dye circulates through it. This test will give your ophthalmologist information about the health of the circulation inside your eye.
Retinal neovascularization is the growth of abnormal new blood vessels onto the retina in response to poor circulation. These abnormal vessels can bleed or cause severe scarring of the retina. The can also be treated with heavy laser treatment. Laser treatment should be started if and when the problem is first noted.
Unfortunately, central retinal vein occlusion is a very frustrating problem and final vision is often not very good. The visual outcome is not improved by any treatment since there is no way to alter the natural progression of the vein occlusion itself. It is important for your physician to monitor the situation carefully and to administer appropriate laser treatment if complications arise. However, the vision may or may not return on its own. We do have some predictors of outcome. In particular, the vision the patient has when first seeing the eye doctor is a good predictor of the eventual visual acuity. The good news is that is relatively rare to get a central retinal vein occlusion in the fellow eye.
Although a central retinal vein occlusion can cause permanently poor vision in the affected eye, it is important to follow your ophthalmologist’s advice and keep the follow-up appointments. The complications of this disease are treatable and, left untreated, they can lead to even worse vision. It is also important to keep any appointments your ophthalmologist may make with your primary care physician. Treating an associated condition can often be very important.