In a brain AVM, blood passes directly from your arteries to your veins via abnormal vessels. This disrupts the normal process of how blood circulates through your brain.
Normal and abnormal blood vessels
In a brain AVM, blood passes directly from arteries to veins via a tangle of abnormal blood vessels.
A brain arteriovenous malformation (AVM) is a tangle of abnormal blood vessels connecting arteries and veins in the brain.
The arteries are responsible for taking 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 your body but occurs most often in the brain or spine. Even so, brain AVMs are rare and affect less than 1 percent of the population.
The cause of AVMs is not clear. Most people are born with them, but they can occasionally form later in life. They are rarely passed down among families genetically.
Some people with brain AVMs experience signs and symptoms, such as headache or seizures. AVMs are commonly found after a brain scan for another health issue or after the blood vessels rupture and cause bleeding in the brain (hemorrhage).
Once diagnosed, a brain AVM can often be treated successfully to prevent complications, such as brain damage or stroke.
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A brain arteriovenous malformation may not cause any signs or symptoms until the AVM ruptures, resulting in bleeding in the brain (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 related to the AVM.
In people without hemorrhage, signs and symptoms of a brain AVM may include:
- 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
Symptoms 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 up and often cause symptoms in early adulthood.
Once you reach middle age, however, brain AVMs tend to remain stable and are less likely to cause symptoms.
Some pregnant women may have worsened symptoms due to changes in blood volume and blood pressure.
One severe type of brain AVM, called a vein of Galen defect, 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. Signs and symptoms include 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.
AVM blood flow
In an arteriovenous malformation (AVM), blood passes quickly from the artery to vein, disrupting the normal blood flow and depriving the surrounding tissues of oxygen.
The cause of brain AVM is unknown, but researchers believe most brain AVMs emerge during fetal development.
Normally, your heart sends oxygen-rich blood to your 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 then passes into small blood vessels and then into larger veins that drain the blood from your brain, returning it to your 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, the abnormal connection causes blood to flow quickly and directly from your arteries to your veins, bypassing the surrounding tissues.
Anyone can be born with a brain AVM, but these factors may be a risk:
- Being male. AVMs are more common in males.
- Having a family history. Cases of AVMs in families have been reported, 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 predispose you to having vascular malformations such as AVMs.
A brain AVM may cause bleeding in the brain (hemorrhage), which can damage the surrounding brain tissue, as shown by this CT scan (left) and illustration (right) of an intracerebral hemorrhage.
Complications of a brain AVM include:
Bleeding in the brain (hemorrhage). 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 (a hemorrhage).
This risk of a brain AVM bleeding ranges around 2 percent each year. The risk of hemorrhage may be higher for certain types of AVMs, or if you have experienced previous AVM ruptures.
Some hemorrhages associated with AVMs go undetected because they cause no major brain damage or symptoms, but potentially life-threatening bleeding episodes may occur.
Brain AVMs account for about 2 percent of all hemorrhagic strokes each year and are often the cause of hemorrhage in children and young adults who experience brain hemorrhage.
Reduced oxygen to brain tissue. With an 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 down by channels of smaller blood vessels.
Surrounding brain tissues 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. As you grow, your body may recruit more arteries to supply blood to the fast-flowing 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 (hydrocephalus).