Brain aneurysms form when a weakened spot in an artery wall bulges outward under the pressure of blood flow. About 5.4% of the general population is living with an unruptured brain aneurysm right now, most without knowing it. The causes range from the constant mechanical stress of blood pushing against artery walls to genetic conditions, lifestyle factors, hormonal changes, and infections. Usually, it’s not one single cause but a combination of forces working together over time.
How an Aneurysm Forms in an Artery Wall
Your brain’s arteries are built in layers, each with a specific structural role. The innermost layer lines the channel where blood flows. A middle layer of muscle cells provides strength and flexibility. An outer layer holds everything together. An aneurysm develops when these layers break down faster than the body can repair them.
The process typically starts where blood flow hits the artery wall hardest, especially at branching points where arteries split. This constant mechanical stress damages the inner lining, creating tiny gaps between cells. Once that lining is compromised, white blood cells migrate into the artery wall and trigger an inflammatory response. These immune cells release enzymes that chew through the structural proteins (collagen, elastin, and others) holding the wall together. One of the earliest visible changes is the disappearance of the internal elastic layer, the structure that gives the artery its ability to stretch and snap back. Once that layer is gone, the outermost layer of the artery is the only thing resisting blood pressure from the inside.
The muscle cells in the artery wall also change behavior during this process. Instead of maintaining the wall’s structure, they start producing inflammatory signals that accelerate the damage. The body does attempt repairs, laying down new tissue, but chronic inflammation and ongoing blood flow stress overwhelm these efforts. The wall thins, weakens, and eventually balloons outward.
Where Aneurysms Most Often Develop
Nearly all brain aneurysms form along the Circle of Willis, a ring of arteries at the base of the brain where major blood vessels connect. The most common location is the anterior communicating artery, which accounts for roughly 40% of cases. The middle cerebral artery is second at about 22%, followed by the junction of the internal carotid and posterior communicating arteries at around 12%. These are all branching points where blood flow changes direction and hits the wall at sharper angles, creating the mechanical stress that initiates wall damage.
Blood Pressure, Smoking, and Other Lifestyle Factors
High blood pressure is one of the most significant modifiable risk factors. Chronically elevated pressure increases the mechanical load on artery walls, accelerating the breakdown of structural proteins. Smoking is equally dangerous, and for overlapping reasons: chemicals in tobacco smoke promote inflammation in blood vessel walls and impair the body’s ability to maintain healthy arterial tissue. Each factor independently raises the risk of an existing aneurysm rupturing by roughly 50 to 75%. Together, smoking and hypertension more than double the rupture risk, producing a combined effect greater than either alone.
Cocaine use is another established cause. It spikes blood pressure sharply and can damage artery walls directly. When injected, it also carries the risk of introducing infections into the bloodstream, which can weaken arteries through a separate mechanism. Heavy alcohol consumption contributes as well, primarily through its effects on blood pressure and vascular inflammation.
Genetic Conditions That Weaken Artery Walls
Some people are born with connective tissue disorders that make their arteries structurally vulnerable from the start. Several specific conditions carry a measurably higher risk:
- Autosomal dominant polycystic kidney disease (ADPKD): About 10% of people with this condition have an unruptured brain aneurysm, roughly double the general population rate.
- Loeys-Dietz syndrome: Screening studies have found aneurysms in 28% of patients.
- Marfan syndrome: Around 14% of screened patients had unruptured aneurysms.
- Vascular Ehlers-Danlos syndrome (type IV): About 12% of screened patients had aneurysms. This condition affects collagen production throughout the body, leaving blood vessels prone to tears and bulging.
- Microcephalic osteodysplastic primordial dwarfism type II: Up to 50% of patients develop brain aneurysms, the highest rate of any known genetic condition.
These conditions all involve defects in the proteins that give artery walls their strength and elasticity. When collagen or elastin is abnormal from birth, arteries are less able to withstand normal blood pressure, let alone the turbulent flow at branching points.
Family History Without a Genetic Syndrome
Even without a diagnosed connective tissue disorder, having close relatives with brain aneurysms increases your risk. The genetics here are more complex, likely involving multiple genes that each contribute a small amount of vulnerability. Current screening guidelines suggest that imaging is not routinely beneficial for people who have just one or two first-degree relatives with a ruptured aneurysm, because the chance of finding a dangerous aneurysm is low and treatment itself carries risk. Screening tends to be considered only when the family history is unusually strong or when other risk factors are also present.
Why Women Are at Higher Risk
Brain aneurysms are about 60% more common in women than in men overall. After menopause, that gap widens further: women over 50 face roughly double the risk compared to men of the same age. The peak in female aneurysm prevalence occurs in the sixth decade of life, and the shift becomes statistically significant after age 55.
The reason centers on estrogen. Throughout a woman’s reproductive years, estrogen protects artery walls in several ways. It supports the inner lining of blood vessels, limits the inflammatory response that drives wall breakdown, and helps maintain collagen content in arterial tissue. When estrogen levels decline during and after menopause, those protective effects disappear. The artery wall undergoes changes similar to what happens in skin and bones after menopause: collagen and water content decrease, the wall thins, and elasticity drops. Animal studies confirm this pattern. Female rodents develop aneurysms more often than males, and surgically removing the ovaries to eliminate estrogen further increases both aneurysm formation and rupture rates.
Estrogen also suppresses a key inflammatory signaling pathway that promotes the enzyme activity responsible for breaking down artery wall proteins. Without estrogen’s restraining effect, this cycle of inflammation and structural damage amplifies itself, creating conditions favorable for aneurysm growth.
The Role of Inflammation
Chronic inflammation in the artery wall is not just a byproduct of aneurysm formation. It actively drives the process forward. Once blood flow stress damages the inner lining of an artery, immune cells (particularly white blood cells called macrophages) infiltrate the wall. These cells release enzymes that break down collagen and elastin, along with inflammatory signaling molecules that recruit even more immune cells to the site.
This creates a self-reinforcing loop. Damage to the lining allows blood proteins to seep into the wall, activating muscle cells that then produce their own inflammatory signals. The inflammatory cascade also promotes the formation of small blood clots within the aneurysm wall, which further destabilizes the structure. The enzymes most responsible for this destruction are found at significantly higher levels in aneurysms that have ruptured compared to those that remain stable, suggesting that the intensity of inflammation may be what separates a dormant aneurysm from a dangerous one.
Infections, Trauma, and Other Acquired Causes
Not all aneurysms develop from the gradual wear and tear described above. Infections that reach the bloodstream can directly attack artery walls, creating what are called mycotic aneurysms. Bacterial endocarditis (an infection of the heart valves) is the most common source, as infected material can travel through the bloodstream and lodge in brain arteries. These aneurysms tend to form in unusual locations, away from the typical branching points.
Head trauma can also damage brain arteries, creating weak spots that balloon over time. Traumatic aneurysms are rare but can appear days to weeks after an injury. In children and teenagers, aneurysms are uncommon overall, and when they do occur, they are more often linked to infections, trauma, or congenital vessel wall defects rather than the lifestyle-related causes that dominate in adults.
Age and How Risk Accumulates
The prevalence of unruptured aneurysms roughly doubles with age. Among people under 45, the rate is about 3.2%. For those over 45, it rises to 5.3%. This reflects the cumulative nature of the process: decades of blood flow stress, gradual loss of arterial elasticity, and longer exposure to risk factors like high blood pressure all add up. Age also brings changes in how effectively the body repairs damaged tissue, meaning small injuries to artery walls that might have healed in a younger person can instead progress toward aneurysm formation.