A brain arteriovenous malformation (AVM) is a tangle of blood vessels that connects arteries and veins in the brain.
The arteries take oxygen-rich blood from the heart to the brain. Veins carry the oxygen-depleted blood back to the lungs and heart. A brain AVM disrupts this vital process.
An arteriovenous malformation can develop anywhere in the body but common locations include the brain and spinal cord — though overall, brain AVMs are rare.
The cause of brain AVMs isn't clear. Most people are born with them, but they can form later in life. Rarely, they can be passed down among families.
Some people with a brain AVM experience signs and symptoms, such as headaches or seizures. An AVM is often found after a brain scan for another health issue or after the blood vessels rupture and bleed (hemorrhage).
Once diagnosed, a brain AVM can be treated to prevent complications, such as brain damage or stroke.
A brain arteriovenous malformation may not cause any signs or symptoms until the AVM ruptures, resulting in hemorrhage. In about half of all brain AVMs, hemorrhage is the first sign.
But some people with brain AVM may experience signs and symptoms other than bleeding, such as:
- Headache or pain in one area of the head
- Muscle weakness or numbness in one part of the body
Some people may experience more-serious neurological signs and symptoms, depending on the location of the AVM, including:
- Severe headache
- Weakness, numbness or paralysis
- Vision loss
- Difficulty speaking
- Confusion or inability to understand others
- Severe unsteadiness
Signs and symptoms of a brain AVM may begin at any age but usually emerge between ages 10 and 40. Brain AVMs can damage brain tissue over time. The effects slowly build and often cause symptoms in early adulthood.
By middle age, however, brain AVMs tend to remain stable and are less likely to cause symptoms.
Research hasn't found that pregnancy increases the risk of hemorrhage for people with a brain AVM. However, more research needs to be done.
One severe type of brain AVM involves the vein of Galen. It causes signs and symptoms that emerge soon or immediately after birth. The major blood vessel involved in this type of brain AVM can cause fluid to build up in the brain and the head to swell. It can also cause swollen veins that are visible on the scalp, seizures, failure to thrive and congestive heart failure.
When to see a doctor
Seek immediate medical attention if you notice any signs or symptoms of a brain AVM, such as seizures, headaches or other symptoms. A bleeding brain AVM is life-threatening and requires emergency medical attention.
The cause of brain AVM is unknown. Researchers believe most brain AVMs are present at birth and form during fetal development, but brain AVMs can develop later in life, as well. Brain AVMs are seen in some people who have hereditary hemorrhagic telangiectasia (HHT), also known as Osler-Weber-Rendu syndrome. HHT affects the way blood vessels form in several areas of the body, including the brain.
Typically, the heart sends oxygen-rich blood to the brain through arteries. The arteries slow blood flow by passing it through a series of progressively smaller networks of blood vessels, ending with the smallest blood vessels (capillaries). The capillaries slowly deliver oxygen through their thin, porous walls to the surrounding brain tissue.
The oxygen-depleted blood passes into small blood vessels and then into larger veins that return the blood to the heart and lungs to get more oxygen.
The arteries and veins in an AVM lack this supporting network of smaller blood vessels and capillaries. Instead, blood flows quickly and directly from the arteries to the veins, bypassing the surrounding tissues.
Anyone can be born with a brain AVM, but these factors may raise the risk:
- Being male. Brain AVMs are more common in males.
- Having a family history. In rare cases, brain AVMs have been reported to occur in families, but it's unclear if there's a certain genetic factor or if the cases are only coincidental. It's also possible to inherit other medical conditions that increase the risk of brain AVMs, such as hereditary hemorrhagic telangiectasia (HHT).
Complications of a brain AVM include:
Bleeding in the brain. An AVM puts extreme pressure on the walls of the affected arteries and veins, causing them to become thin or weak. This may result in the AVM rupturing and bleeding into the brain.
This risk of a brain AVM bleeding ranges from around 2% to 3% each year. The risk of bleeding may be higher for certain types of AVMs or if there's been a previous AVM rupture.
Some hemorrhages associated with AVMs go undetected because they cause no major brain damage or signs or symptoms. However, potentially life-threatening bleeding episodes may occur.
Brain AVMs account for about 2% of all hemorrhagic strokes each year. They're often the cause of hemorrhage in children and young adults who experience brain hemorrhage.
Reduced oxygen to brain tissue. With a brain AVM, blood bypasses the network of capillaries and flows directly from arteries to veins. Blood rushes quickly through the altered path because it isn't slowed by channels of smaller blood vessels.
Surrounding brain tissue can't easily absorb oxygen from the fast-flowing blood. Without enough oxygen, brain tissues weaken or may die off completely. This results in stroke-like symptoms, such as difficulty speaking, weakness, numbness, vision loss or severe unsteadiness.
- Thin or weak blood vessels. An AVM puts extreme pressure on the thin and weak walls of the blood vessels. A bulge in a blood vessel wall (aneurysm) may develop and become susceptible to rupture.
Brain damage. The body may recruit more arteries to supply blood to the fast-flowing brain AVM. As a result, some AVMs may get bigger and displace or compress portions of the brain. This may prevent protective fluids from flowing freely around the hemispheres of the brain.
If fluid builds up, it can push brain tissue up against the skull.