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.

Anatomy

The retina is the nerve layer in the back of the eye (see anatomy). 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 come 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 it 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.

Find out More:
	— Retinal Vein Occlusion:   Causes, Treatment, Complications, Visual Outcome — Retina Services