BRANCH RETINAL VEIN OCCLUSION (BRVO)

A branch retinal vein occlusion (BRVO) is a blockage of one of the small blood vessels that drains blood from the retina. The retina is the light-sensitive nerve tissue lining the back of the eye.

Like film in a camera, the retina continually "takes pictures" of everything you look at. When part of the retinal vein becomes blocked, the retinal blood flow slows or stops. Suddenly and usually without warning, a patch of retina losses some of its "picture-taking" function and you may be aware that part of your field of vision has darkened.

Why Is Vision Lost?
Normally, the retina is nourished by oxygen-rich blood that is brought to it by arteries and drained away by veins. When a vein - a drainage channel - is blocked, blood backs up, leading to bleeding and swelling (edema) in the retina which hampers its nourishment.

The extent of damage and the visual symptoms produced depend on the size of the blocked vein and its exact location. If the blockage occurs toward the peripheral part of the retina, you may hardly notice it. But if it occurs in or near the macula - the central zone of the retina that is critical for sharp vision - and causes it to swell, vision is likely to be reduced or distorted.

A more serious threat to vision can develop later from a complication called neovascularization (neo = new, vascular = blood vessels). A month or more after the BRVO, new blood vessels may begin to appear in the retina, as if they were trying to renourish it. These are not normal blood vessels as they are very fragile and bleed easily. They are dangerous to the eye because they can lead to still other problems that damage vision.

What Causes a Vein Occlusion?
Several factors combine to bring on an occlusion. The usual situation is that something causes the blood flow in a retinal vein to slow down so much that it permits a clot to form there. The clot prevents blood from flowing freely.

The most common reason for slowed venous blood flow is, surprisingly, a hardened artery (arteriosclerosis). If a stiff artery happens to lie across a vein and compresses it, it can slow the flow of blood in that vein in the same way that a log across a stream can obstruct the flow of water. Because arteriosclerosis occurs so often in people who have hypertension it is considered to be a risk factor for the development of BRVO.

Other conditions that can lead to a BRVO are venous inflammation (vasculitis), which can plug the vein , and some rare blood conditions that produce a greater-than-normal tendency for blood to clot. Even estrogen medication (as in oral contraceptives) can introduce a slight risk of blood clotting.

Examination
You will have a complete eye examination and vision test. Your pupils will be dilated (enlarged) with eye drops so the insides of both eyes can be studied. An ophthalmoscope and slit lamp (clinical microscope) are instruments used for looking inside the eyes; they are especially useful for studying the retina and its blood supply.

Retinal photographs may be taken to help determine the extent of the problem. An angiogram (photographs of blood vessels) may also be made. For this test, an orange-colored dye (fluorescein) is injected into a vein in your arm and is immediately followed by a series of retinal photographs that track the dye, and time its flow, as it travels through the eye's blood vessels. The angiogram helps identify the exact site of the vein's blockage, the extent of damage to capillaries (the smallest retinal blood vessels), and whether or not neovascularization has developed.

Because BRVO can be associated with medical conditions that affect the rest of the body (high blood pressure, for example, which also increases the risk of a heart attack or stroke), you may be referred to an internist or family physician for a complete check-up after your eye examination.

Treatment
Once a BRVO has occurred, there is no simple way to speed the healing process along. Eventually, over several months, the blocked vein may re-open on its own, or some nearby blood vessels (called colaterals) may develop and reroute the blood flow around the site of blockage. Either of these may help restore at least part of the lost retinal function.

If neovascularization develops, a type of laser surgery called panretinal photocoagulation (PRP) can help reduce or even eliminate the abnormal blood vessels. PRP is not intended to improve vision directly. It reduces the risk of further vision loss from internal bleeding or, possibly, from a retinal detachment.

PRP is performed on an outpatient basis and is a painless treatment. It consists of making hundreds of tiny laser burns in and around the damaged part of the retina. If the neovascularization does not respond to this treatment by decreasing substantially within a month or so, additional laser PRP can be applied.

After a BRVO, the central retina (macula) sometimes remains swollen for months, reducing vision significantly. To help minimize the macular swelling, another type of laser treatment (called grid-pattern) can sometimes be used. Its risks and intended results, however , are distinctly different from the laser PRP technique used for treating neovascularization.

Prognosis
If you are under age 50 or so, a BRVO may not impair your vision much, if at all. Even if vision is reduced initially, it has some potential for returning, perhaps even to its previous level, over the next few months. If neovascularization occurs, it can be treated with laser PRP.

If you are older, good visual recovery is not as likely. But even if some reduced vision remains, the degree of impairment is not likely to be severe.