Arteriovenous malformations (AVMs) are almost always something you’re born with. They form during fetal development when blood vessels fail to develop properly, creating direct connections between arteries and veins that bypass the normal network of tiny capillaries in between. The estimated prevalence of brain AVMs is 0.01% to 0.5% of the general population, and most people don’t know they have one until symptoms appear, typically around age 20 to 40.
What Goes Wrong During Development
In a healthy circulatory system, arteries carry high-pressure blood away from the heart, and capillaries slow that blood down so oxygen and nutrients can reach surrounding tissue. Veins then carry the low-pressure blood back. In an AVM, the capillary step is missing entirely. Arteries feed directly into veins through a tangled knot of abnormal vessels called a nidus.
This matters because veins aren’t built to handle the force of arterial blood. Over time, the walls of those veins stretch and weaken under pressure they were never designed for. That’s what creates the risk of rupture and bleeding. Scientists believe this structural error originates in the womb, when the body’s chemical signals that guide blood vessel formation go off track. New vessels are constantly forming and disappearing during fetal development, and disruptions in that process can leave behind these abnormal connections.
Genetic Links and Hereditary Conditions
Most AVMs occur randomly, without a clear inherited pattern. But in some cases, they’re linked to a genetic condition called hereditary hemorrhagic telangiectasia (HHT), an inherited disorder that causes abnormal connections between arteries and veins in the skin, mucous membranes, and internal organs. HHT comes in several forms, each tied to mutations in different genes that help regulate how blood vessels grow and maintain their structure.
There are also rare congenital syndromes where AVMs are a defining feature. Wyburn-Mason syndrome, for example, involves multiple AVMs affecting the eye, brain, and sometimes the skin. These syndromes are not inherited in the traditional sense but rather result from spontaneous developmental errors. If you have a family history of recurrent nosebleeds, visible red spots on the skin, or known vascular abnormalities, your doctor may screen for HHT or related conditions.
Can You Develop an AVM Later in Life?
In rare cases, AVMs appear to develop after a traumatic injury. This has been documented most often in the scalp, where roughly 10% to 20% of superficial temporal artery AVMs are associated with prior trauma. There’s ongoing debate about whether the injury truly creates a new AVM or simply triggers the breakdown of abnormal vessel connections that were already present from birth but dormant.
Two mechanisms have been proposed for trauma-related AVMs. The first is that an injury directly damages an artery and a nearby vein, creating a new connection between them. The second is that the healing process itself stimulates the growth of small bridging vessels that form an abnormal link. Either way, acquired AVMs remain uncommon compared to the congenital form.
Hormonal Changes Can Worsen Existing AVMs
Hormones don’t cause AVMs, but they can accelerate growth in ones that already exist. Estrogen and progesterone, which surge during puberty and pregnancy, appear to fuel the expansion of congenital vascular malformations. Multiple case reports describe women whose AVMs remained stable for years, then grew rapidly or began bleeding during pregnancy.
In one documented case, a woman born with an AVM in her hand first experienced pain during puberty. Her symptoms then worsened with each of three pregnancies, eventually requiring amputation. Other reports describe AVMs of the forehead expanding dramatically during pregnancy, and in some cases symptoms improved significantly after delivery. These patterns suggest that AVMs are sensitive to hormonal shifts, which is one reason they tend to cause the most problems in women of reproductive age.
Where AVMs Can Form
Brain AVMs get the most attention because of their potential for life-threatening bleeding, but AVMs can occur almost anywhere in the body. They’ve been found in the spinal cord, lungs, liver, skin, and extremities. The location determines the symptoms: a brain AVM might cause seizures or headaches, a spinal AVM can lead to weakness or numbness in the limbs, and a lung AVM may cause shortness of breath or low oxygen levels because blood passes through without picking up enough oxygen.
Regardless of location, the underlying problem is the same: high-pressure arterial blood flowing directly into fragile veins without the buffer of capillaries.
How AVMs Are Found
Many AVMs are discovered incidentally, during brain imaging for an unrelated issue. When doctors suspect an AVM, the gold standard for diagnosis is catheter angiography, an imaging procedure where dye is injected into the blood vessels to map them in detail. This allows doctors to see the nidus, identify which arteries are feeding it, and trace the veins draining it.
MRI and specialized MR angiography techniques are also used, particularly for monitoring AVMs over time or after treatment. Doctors look specifically for the presence of abnormal vascular structures and early-draining veins, which are veins that fill with blood faster than normal because they’re receiving high-pressure arterial flow. When those early-draining veins disappear on follow-up imaging, it’s a sign the AVM has been successfully treated.
The Risk of Bleeding
For someone with a known brain AVM, the overall risk of hemorrhage is about 2% to 4% per year. That number varies depending on the AVM’s characteristics. Unruptured AVMs carry a lower annual risk, around 2%. AVMs that have already bled once carry a higher risk of about 4.5% per year, with the first year after an initial hemorrhage being the most dangerous period.
These percentages may sound small on a yearly basis, but they accumulate over a lifetime. A 2% annual risk translates to roughly a 40% chance of bleeding over 25 years for a young person diagnosed in their twenties. That cumulative risk is a major factor in deciding whether to treat an AVM or monitor it conservatively.