The vast majority of brain arteriovenous malformations are not inherited. Roughly 95% of brain AVMs are sporadic, meaning they develop on their own without being passed down from a parent. About 5% of cases are considered familial, occurring in people who carry a genetic syndrome or have multiple affected relatives. So while a hereditary link exists in a small minority of cases, most people diagnosed with a brain AVM have no family history of the condition.
Why Most Brain AVMs Are Not Inherited
A brain AVM is a tangle of abnormal blood vessels where arteries connect directly to veins, bypassing the normal capillary network. For decades, researchers assumed these were random developmental errors. A landmark 2018 study confirmed that most sporadic brain AVMs are driven by somatic mutations, meaning genetic changes that happen spontaneously in the cells lining brain blood vessels after conception. These mutations are not present in sperm or egg cells and cannot be passed to children.
The specific mutation found in most sporadic cases involves a gene called KRAS, which plays a broad role in cell growth. Activating mutations in KRAS cause the endothelial cells of brain blood vessels to behave abnormally, forming the tangled vessel clusters that define an AVM. Studies across multiple large patient groups have confirmed that KRAS mutations appear in an estimated 50 to 70% of sporadic brain AVMs. Because these mutations arise randomly in a localized group of cells rather than being carried in every cell of the body, they are not heritable.
The Hereditary Syndromes Linked to Brain AVMs
The clearest hereditary connection to brain AVMs is a condition called hereditary hemorrhagic telangiectasia, or HHT. HHT is an inherited vascular disorder that affects roughly 1 in 5,000 to 10,000 people. It causes fragile, abnormal blood vessels throughout the body, including the brain, lungs, and liver. About 10.4% of people with HHT develop a brain AVM.
HHT follows an autosomal dominant inheritance pattern, meaning a child needs only one copy of the mutated gene (from one parent) to be affected. Three genes are primarily responsible. Mutations in the ENG gene cause HHT Type 1, which carries the highest brain AVM risk at about 13.4%. Mutations in the ACVRL1 gene cause HHT Type 2, where the brain AVM rate drops to about 2.4%. A third gene, SMAD4, causes a rarer form that overlaps with a gastrointestinal condition. All three genes are involved in the same signaling pathway that helps blood vessels form and maintain their structure.
A second, less common hereditary condition called capillary malformation-AVM syndrome (CM-AVM) is caused by mutations in the RASA1 gene. People with CM-AVM typically have visible reddish skin lesions and can develop AVMs in the brain and other areas. This condition is also autosomal dominant, so each child of an affected parent has a 50% chance of inheriting the mutation.
How Hereditary and Sporadic AVMs Differ
Brain AVMs that develop as part of HHT tend to be smaller and lower-grade than sporadic ones. Nearly 90% of HHT-related brain AVMs score a 2 or less on the Spetzler-Martin grading scale, which rates complexity from 1 to 5. Despite being smaller, about 55% of these AVMs are symptomatic, meaning they cause noticeable problems like headaches, seizures, or bleeding.
Sporadic AVMs, by contrast, can range widely in size and complexity. They are almost always single lesions, while people with hereditary syndromes sometimes have multiple AVMs. The presence of multiple brain AVMs, or AVMs combined with visible vascular skin lesions, is a signal that a hereditary syndrome may be involved.
Risk to Family Members
If your brain AVM is sporadic, the risk to your siblings, children, or other relatives is extremely low. Brain AVMs in the general population are rare, with an overall prevalence estimated at about 18 per 100,000 people. A systematic review identified only 25 families worldwide where multiple members had brain AVMs without an underlying syndrome like HHT. That number is so small it could represent coincidence rather than a true genetic pattern.
That said, some research suggests genetics may play a subtle background role even in non-syndromic cases. One study from a Japanese population with an unusually high AVM prevalence found that about 16% of AVM patients had an affected sibling, which would represent a dramatically elevated risk compared to the general population. However, this came from a very specific, small population, and the findings have not been broadly replicated. For the average person with a sporadic brain AVM, routine screening of family members is not standard practice.
The picture changes entirely if HHT or CM-AVM is involved. Because these are autosomal dominant conditions, each first-degree relative has a 50% chance of carrying the same mutation. Family members of someone with confirmed HHT should be evaluated, even if they have no symptoms, since the signs of HHT can be subtle or absent in younger people.
When Genetic Testing Makes Sense
Genetic testing is not recommended for everyone with a brain AVM. It becomes relevant in specific situations: if you have a brain AVM along with frequent nosebleeds, small red spots on your skin or lips, or AVMs in other organs like the lungs or liver. These are hallmarks of HHT, which doctors evaluate using the Curaçao diagnostic criteria, a checklist of four clinical features. Meeting three or more of these criteria is highly predictive of an underlying genetic cause. One study found that when clinicians applied these criteria strictly, 96% of patients tested positive for a mutation in ENG or ACVRL1.
For people under 40, clinical features of HHT may not yet be obvious. Current guidelines recommend that at-risk family members in this age group get a brain MRI (with and without contrast) even before symptoms appear. If the initial scan is done in childhood, it should be repeated after puberty, since brain AVMs can develop or change during the first two decades of life.
If a specific mutation has already been identified in your family, genetic testing for that known variant is the most straightforward path. If the family mutation is unknown and HHT cannot be ruled out, at-risk relatives are typically monitored with the same imaging and screening schedule used for confirmed HHT patients.
The Genetic Overlap Between Hereditary and Sporadic Cases
One of the more interesting findings in recent research is that hereditary and sporadic brain AVMs may share a deeper biological connection than their separate categories suggest. In hereditary cases, a person is born with a germline mutation (in ENG, ACVRL1, or SMAD4) that weakens blood vessel signaling throughout the body. But AVMs only form in specific spots, which has led researchers to propose a “two-hit” model: the inherited mutation creates vulnerability, and a second, somatic mutation in a localized group of cells triggers the actual AVM to form.
In sporadic cases, the somatic KRAS mutation alone appears sufficient to drive AVM formation without any inherited vulnerability. Both pathways converge on endothelial cell dysfunction, where the cells lining blood vessels grow and connect abnormally. This shared mechanism is opening new avenues for targeted treatments that could eventually apply to both hereditary and sporadic AVMs, though such therapies remain in early stages and are currently limited to select research centers.