What is branch retinal vein occlusion?
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 video camera, the retina continually "takes pictures". When a retinal vein becomes blocked, part of the 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 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 and hampering 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, new blood vessel growth. 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, however, they are very fragile and bleed easily. They are dangerous to the eye because they can lead to still other problems that damage vision.
Neovascularization only occurs in about one in five BRVO patients. New blood vessel growth in your eye should be treated before causing harm. In most cases there are no warning symptoms; but occasionally there are, such as the sudden appearance of new floaters (translucent specks that move about in your field of vision) or a sudden decrease in vision. Often, your ophthalmologist will detect the neovascularization before you notice any symptoms.
What causes a vein occlusion?
Several factors combine to bring on an occlusion. The usual situation is that the blood flow in a retinal vein is slowed down and a clot forms. The clot prevents blood from flowing freely. The blood is back logged and spills into the retina.
The most common reason for slowed venous blood flow is a hardened artery (arteriosclerosis). If a stiff artery happens to lie across a vein and compresses the vein, the flow of blood in that vein stops, in the same way that a log across a stream can obstruct the flow of water.
Hypertension, high blood pressure, is the most common association with central vein occlusions.
Other conditions that can lead to a BRVO are glaucoma, diabetes mellitus, lupus, and some blood conditions. Medication such as oral contraceptives have been associated with branch retinal vein occlusions.
- 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 immediately followed by as series of retinal photographs that track the dye and time the flow of blood through the eye. 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.
How is branch retinal vein occlusion treated?
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. The laser reduces the risk of further vision loss from vitreous bleeding.
PRP is performed on an outpatient basis and is usually painless. 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, 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 mascular 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.
What is the prognosis?
Not all BRVO cause vision loss. If vision is reduced initially, it has some potential for returning, perhaps even to its previous level, over the next few months.
But even if some reduced vision remains, the degree of impairment may not be severe in most cases.
Regular follow-up examinations are important to protect your eyesight. Your eyes should be checked regularly for potential late complications, such as neovascularization or mascular edema, and for the development of a second vascular occlusion in the other eye.