An arteriovenous malformation, or AVM, is a tangle of abnormal blood vessels where arteries connect directly to veins, bypassing the tiny capillaries that normally sit between them. This skipped connection means blood flows at high pressure from arteries straight into veins that aren’t built to handle it, creating a risk of rupture and bleeding. AVMs can form anywhere in the body, but the ones that cause the most concern develop in the brain or spinal cord.
How an AVM Differs From Normal Blood Vessels
In a healthy vascular system, arteries carry oxygen-rich blood to your tissues, then branch into progressively smaller vessels until they become capillaries. These capillaries are thin enough to let oxygen and nutrients pass into surrounding tissue before the blood collects into veins and returns to the heart. The capillary bed acts as a buffer, slowing blood flow and reducing pressure before it reaches the veins.
An AVM eliminates that buffer entirely. The core of the malformation, called the nidus, is a dense knot of arteries and veins fused together through one or more direct connections. Without capillaries to slow things down, high-pressure arterial blood slams directly into thin-walled veins. Over time, those veins can stretch, weaken, and eventually rupture. The surrounding brain tissue also misses out on the oxygen and nutrients that capillaries would normally deliver.
Who Gets AVMs and Why
Most brain AVMs are present from birth, forming during fetal development when blood vessels are first taking shape. They aren’t caused by anything a person did or didn’t do. The majority occur sporadically, meaning they appear without a family history or known trigger.
There is one important genetic connection. A hereditary condition called hereditary hemorrhagic telangiectasia (HHT) significantly raises the likelihood of developing AVMs. HHT is caused by mutations in genes that help regulate blood vessel formation, most commonly the endoglin gene or the ALK1 gene, which together account for roughly 90% of cases. People with HHT can develop AVMs in the brain (2 to 20% of patients), lungs (15 to 50%), liver (47 to 74%), and digestive tract (13 to 30%). If you have a family history of frequent nosebleeds, visible red spots on the skin, or unexplained anemia, HHT screening may be worth discussing.
Symptoms and Warning Signs
Many people with a brain AVM have no idea it’s there. AVMs can remain silent for decades, and some are discovered only incidentally when imaging is done for an unrelated reason. When symptoms do appear, they most commonly show up between ages 20 and 40.
The most dramatic presentation is a hemorrhage, a bleed in or around the brain that causes a sudden, severe headache, sometimes with nausea, vomiting, or loss of consciousness. In one study of pediatric patients, about half presented with headache as their primary symptom, often tied to a bleed. Seizures are the next most common symptom, accounting for roughly 27% of cases in that same study, and they’re particularly common in AVMs that haven’t ruptured. Focal neurological deficits, such as weakness on one side of the body, vision changes, or difficulty speaking, appeared in about 14% of cases. A small percentage of patients, around 3%, had no symptoms at all when diagnosed.
The specific symptoms depend heavily on where in the brain the AVM sits. One located near the motor cortex might cause arm or leg weakness. One near the visual processing area could affect sight. Headaches alone aren’t a reliable indicator, since they’re extremely common in the general population, but a new, unusually severe headache or a seizure with no prior history warrants urgent evaluation.
The Risk of Bleeding
For an unruptured brain AVM, the annual risk of hemorrhage is approximately 2%. That number sounds small in any single year, but it compounds over a lifetime. For a 30-year-old with an unruptured AVM, the cumulative risk over several decades becomes substantial.
Certain features raise the bleeding risk above that 2% baseline. The strongest predictor is a previous hemorrhage. If an AVM has already bled once, the chance of it bleeding again is significantly higher, at least in the first few years after the initial event. Other factors that increase risk include deep location within the brain, drainage through deep veins rather than surface veins, and location in the back of the brain (the area near the brainstem and cerebellum). Tiny silent bleeds that don’t cause obvious symptoms have also been proposed as an early warning sign of a larger hemorrhage to come.
How AVMs Are Diagnosed
Brain AVMs are typically first spotted on an MRI or CT scan, often after a patient presents with a seizure, hemorrhage, or unexplained neurological symptoms. These scans can reveal the tangle of abnormal vessels and any associated bleeding.
The gold standard for definitive diagnosis is digital subtraction angiography, a specialized imaging technique where contrast dye is injected through a catheter threaded into the blood vessels of the brain. This provides a detailed, real-time map of the AVM’s feeding arteries, the nidus itself, and the draining veins. It’s more invasive than a standard MRI, carrying a small risk of complications like blood vessel injury, but it gives surgeons the precise architectural detail they need to plan treatment. Angiography is also used after treatment to confirm the AVM has been fully eliminated.
How Doctors Assess Severity
Once diagnosed, brain AVMs are graded using the Spetzler-Martin scale, a scoring system developed in 1986 that remains the most widely used classification tool. It evaluates three features: the size of the nidus, whether the AVM sits in a critical (eloquent) area of the brain that controls important functions like speech or movement, and the pattern of venous drainage (surface veins carry lower risk than deep veins).
The scale runs from Grade I (small, in a non-critical area, draining through surface veins) to Grade V (large, in a critical area, draining through deep veins). Lower grades generally mean safer surgical outcomes, while higher grades carry more risk from intervention. This grading helps medical teams weigh the risks of treatment against the risks of leaving the AVM alone.
Treatment Options
There are three primary approaches to treating a brain AVM, and they can be used alone or in combination depending on the size, location, and complexity of the malformation.
- Surgical removal (microsurgery): The most direct option. A neurosurgeon opens the skull and physically removes the AVM. When successful, this is considered a cure. Recovery typically involves a hospital stay of at least a day, restricted activity for four to six weeks, and a full recovery timeline of two to six months. Surgery works best for smaller, accessible AVMs in non-critical brain areas.
- Embolization: A catheter-based procedure where a material is injected into the AVM’s feeding arteries to block blood flow and shrink the malformation. This is often used to reduce the size of an AVM before surgery or radiosurgery rather than as a standalone treatment.
- Stereotactic radiosurgery: Focused beams of radiation are aimed precisely at the nidus, causing the abnormal vessels to gradually thicken and close over months to years. This is a well-established approach for AVMs that are small (generally under 10 to 15 milliliters in volume) or located in areas too dangerous for open surgery, such as the brainstem or deep brain structures. Small AVMs in non-critical locations treated with high-dose radiosurgery achieve complete closure rates above 90%. Larger AVMs may require the radiation to be delivered in multiple sessions.
The decision of whether to treat at all is not always straightforward. A major clinical trial called ARUBA, along with a large Scottish prospective study, found that for unruptured brain AVMs, observation alone produced better short-term outcomes than intervention. This challenged the long-standing assumption that treatment is always preferable and shifted the conversation toward more careful risk stratification. Today, treatment decisions weigh the AVM’s specific features, the patient’s age, the cumulative lifetime bleeding risk, and the potential complications of each treatment approach.
AVMs Outside the Brain
While brain AVMs get the most attention, arteriovenous malformations can also develop in the lungs, liver, spine, and other organs. Pulmonary AVMs allow blood to pass through the lungs without picking up oxygen, which can cause low oxygen levels, shortness of breath, and an increased risk of stroke or brain abscess because the lung’s filtering function is bypassed. Liver AVMs can lead to heart strain, portal hypertension, or bile duct damage depending on how they alter blood flow through the organ. Spinal AVMs, though rare, can cause progressive weakness or numbness in the limbs. Each type requires its own diagnostic workup and treatment strategy, but the underlying problem is the same: arteries and veins connected without the capillary network that should sit between them.