An arteriovenous malformation (AVM) is an abnormal tangle of blood vessels that can occur anywhere in the body. In an AVM, blood does not follow its normal path, but instead flows directly from arteries into veins, without passing through the tiny capillaries that normally separate the two. Veins, designed to accept blood from minuscule capillaries, become stressed and damaged by the higher flow rate and pressure of blood flowing in from arteries.
An AVM can give no signs at all — indeed, some people may have AVMs and never know it — but AVMs can also be life-threatening depending on their size and location, and whether they rupture and bleed. They can also lead to the development of an aneurysm in either the arteries or veins.
When it occurs in the brain, an arteriovenous malformation is known as a cerebral AVM or a brain AVM. It can occur deep in the brain (in the thalamus, basal ganglia, or brainstem), or on the brain’s surface. When an AVM occurs in the dura mater — the thick, leathery covering that surrounds the brain — it is known as a dural AVM, or dural arteriovenous fistula (AVF). A cerebral or dural AVM can cause headaches, seizures, and other symptoms, but it can also be completely silent.
An AVM may also occur in the spinal cord, where it can cause back pain, sensory loss, and weakness in the extremities. Ten to 20 percent of spinal AVMs cause a sudden onset of symptoms rather than a slow progression. In these cases an abnormal vessel may have burst, causing bleeding (a hemorrhage) and resulting in weakness, numbness, and other neurological deficits. (See Symptoms of an AVM.)
Risks
The greatest risk of an AVM is rupture and bleeding — a ruptured AVM (either a subarachnoid, subdural, or intraparenchymal hemorrhage) can cause a stroke or produce other temporary or permanent brain or spinal cord damage or even death. After aneurysms, cerebral AVMs are the next leading cause of subarachnoid hemorrhage. Even if the AVM does not rupture, it can cause pain and damage if it grows large and creates pressure against the brain tissue surrounding it. That pressure can also lead to seizures, and can interfere with the free movement of cerebrospinal fluid (CSF), causing hydrocephalus. Since an AVM can interfere with the delivery of oxygen to brain tissue, it can damage otherwise healthy brain cells nearby.
What Causes an AVM?
Cerebral and spinal AVMs are thought to be genetic in origin, so they cannot be prevented. Dural AVMs, on the other hand, are usually not genetic — they appear to be the result of venous thrombosis (clotting of the veins), often due to trauma, infection, or surgery. They often occur when a portion of the circulatory system in the brain becomes obstructed with a thrombus (blood clot) and the body grows new blood vessels to bypass the clot. This process, known as angiogenesis, can lead to the development of a dural AVM.
Although a genetic AVM is present from birth, it may not cause any symptoms until early adulthood or middle age, after years of damage to the brain tissue. Pregnancy may cause symptoms to appear due to changes in blood flow and blood volume. After middle age, an AVM becomes stable and less likely to cause symptoms.
Men are more likely to develop AVMs than women, and those with a family history of vascular malformations are also at higher risk.
Use our online form to request an appointment with one of our cerebrovascular neurosurgeons, who have advanced expertise in treating AVMs and other vascular conditions of the brain and spine.
Reviewed by: Justin Schwarz, MD
Last reviewed/last updated: March 2024
Illustration by Thom Graves, CMI
