Brain AVMs occur in less than 1 percent of the general population. It’s estimated that about one in 200–500 people may have an AVM. AVMs are more common in males than in females.

We don’t know why AVMs occur. Brain AVMs are usually congenital, meaning someone is born with one. But they’re usually not hereditary. People probably don’t inherit an AVM from their parents, and they probably won’t pass one on to their children.

Brain AVMs can occur anywhere within the brain or on its covering. This includes the four major lobes of the front part of the brain (frontal, parietal, temporal, occipital), the back part of the brain (cerebellum), the brainstem, or the ventricles (deep spaces within the brain that produce and circulate the cerebrospinal fluid).

Most AVMs don’t grow or change much, although the vessels involved may dilate (widen). Some AVMs may shrink due to clots in part of the AVM. Some may enlarge to redirect blood in adjacent vessels toward an AVM.

A brain AVM contains abnormal and, therefore, “weakened” blood vessels that direct blood away from normal brain tissue. These abnormal and weak blood vessels dilate over time. Eventually they may burst from the high pressure of blood flow from the arteries, causing bleeding into the brain.

The chance of a brain AVM bleeding is 1 percent to 3 percent per year. Over 15 years, the total chance of an AVM bleeding into the brain — causing brain damage and stroke — is 25 percent.

The risk of recurrent intracranial bleeding is slightly higher for a short time after the first bleed. In two studies, the risk during the first year after initial bleeding was 6 percent and then dropped to the baseline rate. In another study, the risk of recurrence during the first year was 17.9 percent. The risk of recurrent bleeding may be even higher in the first year after the second bleed and has been reported to be 25 percent during that year. People who are between 11 to 35 years old and who have an AVM are at a slightly higher risk of bleeding.

The risk of death related to each bleed is 10 percent to 15 percent. The chance of permanent brain damage is 20 percent to 30 percent. Each time blood leaks into the brain, normal brain tissue is damaged. This results in loss of normal function, which may be temporary or permanent. Some possible symptoms include arm or leg weakness/paralysis, or difficulty with speech, vision or memory. The amount of brain damage depends on how much blood has leaked from the AVM.

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