Branch Retinal Vein Occlusion
Hypertension is considered to be a risk factor for BRVO. A branch retinal vein occlusion (BRVO) is a blockage of one of the small blood vessels that drains blood from the retina (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 a retinal vein becomes blocked part of the retinal blood flow slows or stops. Suddenly and usually without warning a patch of retina loses some of its picture-taking function and part of your field of vision may become 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 in the retina.
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 outer part of the retina you may hardly notice it. But if it occurs in or near the macula (the retina’s central zone that provides sharp vision) and causes it to swell vision is likely to be reduced or distorted.
An additional 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 re-nourish it. These blood vessels are not normal; they are very fragile and bleed easily. They are dangerous to the eye because they can lead to more problems that damage vision.
About one in five BRVO patients develop some neovascularization. In most cases there are no warning symptoms; but if bleeding occurs you may have a sudden appearance of new floaters (translucent specks that move about in your field of vision) or a sudden decrease in vision.
What Causes a Vein Occlusion?
Several factors combine to bring on a vein occlusion. The usual situation is that something causes the blood flow in the vein to slow down so much that a clot forms there. The clot prevents blood from flowing at all.
The most common cause of slowed venous blood flow is surprisingly a hardened artery (arteriosclerosis). A stiffened artery lying across a vein can compress it slowing the flow of blood in the same way that a log across a stream obstructs the flow of water. Because arteriosclerosis occurs so often in people who have hypertension (high blood pressure) hypertension is considered to be a risk factor for 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. Estrogen medication as in oral contraceptives and hormone replacement therapy can also introduce a slight risk of blood clotting.
You will have a complete eye exam and vision test. Your pupils will be dilated (enlarged) with eyedrops then an ophthalmoscope and slit lamp will be used for looking inside the eyes. These instruments 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 required. For this test an orange-colored dye (fluorescein) is injected into a vein in your arm and immediately followed by a series of retinal photographs that track the dye and time its flows it travels through the eye’s blood vessels. The fluorescein angiogram helps identify the exact site of the vein’s blockage the extent of damage to the capillaries and whether or not neovascularization has developed.
Because BRVO can be associated with medical conditions that affect the rest of the body you may be referred to an internist or family physician for a complete check-up after your eye examination.
There is no simple way to speed the healing process. Eventually over several months the blocked vein may re-open on its own or some nearby blood vessels 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 laser treatment is the only way to try to stop the fragile neovascular blood vessels from causing harm. A type of laser surgery called panretinal photocoagulation (PRP) can help reduce or even eliminate the abnormal blood vessels with hundreds of tiny laser burns made in and around the damaged part of the retina.
PRP is not intended to improve vision directly. It reduces the risk of further vision loss from internal bledding or possibly from a retinal detachment. If the neovascularization does not decrease substantially within a month or so additional PRP can be applied. PRP is performed on an outpatient basis and is painless.
If the macula is swollen it sometimes remains that way for months reducing vision significantly. To help minimize the swelling another type of laser treatment (called grid-pattern) can sometimes be used. Its risks and intended results however are distinctly different from the PRP technique used for treating neovascularization.
A BRVO may not impair your vision much if at all. Even if vision is reduced initially it may improve over the next few months perhaps even to its previous level. For those over 50 or so good visual recovery is not as likely. But even if you are left with reduced vision the degree of impairment will probably not be severe.
Regular follow-up examination s are important to protect your eyesight. Your eyes should be checked regularly for potential late complications such as neovascularization or macular edema and for the development of a second vascular occlusion in either eye.