Brain aneurysms form when a weak spot in an artery wall bulges outward under the force of blood flow. About 1 in 50 people in the U.S. are living with one right now, most without knowing it. The causes are a mix of structural vulnerabilities in artery walls, genetic predisposition, and lifestyle factors that accelerate damage over time.
What Happens Inside the Artery Wall
Arteries in the brain are built in layers: a smooth inner lining, an elastic middle layer of muscle cells, and a protective outer sheath. A brain aneurysm develops when these layers break down in a specific spot. The elastic inner layer erodes, smooth muscle cells die off, and inflammatory cells move in. What’s left is a thinned, weakened patch of artery that can no longer resist the constant pressure of blood pumping through it. That patch slowly balloons outward.
The process is driven in part by hemodynamic stress, the physical force of blood pushing against artery walls. This is especially intense at branch points where arteries split, because blood flow hits these junctions at sharper angles. In response to that stress, cells in the artery wall trigger an inflammatory signaling loop that, over time, weakens the tissue further rather than repairing it. This is why aneurysms tend to form in predictable locations: roughly 85% develop in the front portion of a ring of arteries at the base of the brain called the Circle of Willis, right where the major branches divide.
Genetics and Inherited Conditions
Some people are born with a higher risk because of the way their connective tissue is built. Several genetic conditions are strongly linked to brain aneurysms:
- Autosomal dominant polycystic kidney disease (ADPKD): About 10% of people with this condition have an unruptured brain aneurysm.
- Vascular Ehlers-Danlos syndrome (type IV): Around 12% of screened patients have been found to have one. This condition weakens the collagen that gives artery walls their structure.
- Marfan syndrome: Roughly 14% of screened patients had unruptured aneurysms in one study.
- Loeys-Dietz syndrome: The highest screening rate among connective tissue disorders, with 28% of patients found to have an unruptured aneurysm.
Even without one of these named conditions, family history matters significantly. If you have first-degree relatives (a parent or sibling) who had a brain aneurysm, your own risk is elevated enough that Mayo Clinic recommends regular brain imaging to catch any developing aneurysm early. The genetics aren’t always tied to a single identifiable disorder. Sometimes the inherited vulnerability is subtler, involving variations across multiple genes that collectively weaken arterial walls.
Hypertension, Smoking, and Drug Use
High blood pressure is one of the most significant modifiable risk factors. Chronically elevated pressure accelerates the wear on artery walls, particularly at those vulnerable branch points. In studies of aneurysm rupture, hypertension alone increased the odds by about 50%.
Smoking carries a similar independent risk, raising the odds of rupture by roughly 57%. Tobacco damages blood vessel walls directly through chemical exposure and also raises blood pressure. When the two factors combine, the danger compounds: people who both smoke and have high blood pressure face more than double the odds of aneurysm rupture compared to people with neither risk factor.
Stimulant drugs, particularly cocaine, pose an acute danger. Cocaine causes sudden, extreme spikes in blood pressure by flooding the nervous system with stress signals. Among people who already have a brain aneurysm, cocaine use is associated with nearly three times the rate of re-rupture compared to non-users. The drug also triggers spasms in brain arteries through multiple mechanisms, further stressing already weakened walls.
Who Gets Brain Aneurysms
Women are affected about twice as often as men. In a large pooled analysis of nearly 10,000 patients with unruptured brain aneurysms, 66% were women. The average age at diagnosis is around 61, and women tend to be diagnosed slightly later than men (62 versus 59.5 years on average). Hormonal changes after menopause likely play a role, as the drop in estrogen appears to reduce the protective maintenance of artery walls.
Age itself is a risk factor. The longer arteries endure hemodynamic stress, the more cumulative damage builds up. Heavy alcohol use and a personal history of previous aneurysms also increase the likelihood of developing new ones.
Why Some Aneurysms Rupture and Others Don’t
Most brain aneurysms never rupture. The risk depends on a combination of factors that clinicians now assess using a scoring system called PHASES, which accounts for the patient’s age, blood pressure, personal history, and the aneurysm’s size and location.
Size is one of the strongest predictors. Small aneurysms under 7 millimeters in a lower-risk location carry a 5-year rupture risk as low as 0.25% in some populations. Giant aneurysms over 20 millimeters in the back of the brain, especially in older patients with hypertension and a history of previous bleeding, can carry a 5-year risk above 15%. Location matters because arteries in the posterior circulation (toward the back of the brain) tend to have thinner walls and handle blood flow differently than those in the front.
This wide range is why not every discovered aneurysm requires surgery. Many small, stable aneurysms are monitored with periodic imaging while the patient focuses on controlling blood pressure and quitting smoking, the two most impactful things you can do to lower rupture risk if you’re living with an unruptured aneurysm.
Why Aneurysms Form Where They Do
The Circle of Willis is a loop of arteries at the brain’s base that distributes blood to every region of the brain. Its design means blood is constantly being redirected, split, and merged at junction points. These forks experience turbulent flow patterns that put extra mechanical stress on the artery wall, particularly on the outer curve of each branch. Over decades, that stress chips away at the wall’s structural integrity, especially if the person also has high blood pressure, a genetic vulnerability, or smokes.
Some people also have natural variations in the Circle of Willis, with certain segments missing or underdeveloped. These anatomical differences force more blood through fewer arteries, increasing the stress on whatever paths remain open and raising the chance of an aneurysm forming at those overworked junctions.