Brain aneurysms form when a weak spot in an artery wall bulges outward under the pressure of blood flow. About 2.8% of adults worldwide are living with an unruptured brain aneurysm right now, most without knowing it. The causes range from the mechanical forces of blood flowing through your arteries to genetic conditions, lifestyle habits, infections, and hormonal changes.
How an Aneurysm Forms in the Artery Wall
Brain arteries have layered walls, including a tough elastic lining on the inside and a muscular middle layer that keeps the vessel strong. An aneurysm develops when these layers break down. The elastic lining fragments, the muscular layer thins, and the wall loses its structural integrity. Most brain aneurysms are saccular, meaning they balloon out like a small pouch from one side of the artery. In these aneurysms, the muscular layer is thin or completely absent, and the elastic lining is severely fragmented.
This breakdown doesn’t happen overnight. Blood flow constantly pushes against artery walls, and the force is especially intense at points where arteries branch apart. At these branching points, the combination of high pressure and rapid changes in flow direction acts like a slow-grinding force on the vessel lining. Over time, the cells that maintain and repair the artery wall become less active. The elastic fibers degrade, the muscle cells thin out, and the wall gradually weakens. Turbulent blood flow at these spots generates vibrations that accelerate structural fatigue, much like bending a paperclip back and forth until it snaps.
Research from the American Heart Association has shown what this looks like at the tissue level. In the zones of highest stress at arterial branch points, the vessel wall develops a visible groove where the lining erodes, muscle cells die off, and the remaining tissue is mostly collagen with fewer living cells to maintain it. The body’s own inflammatory response, involving immune cells that infiltrate the weakened wall, further promotes the breakdown. This remodeling process is what turns a healthy artery into an aneurysm over months or years.
Genetic Conditions That Raise Risk
Some people are born with a significantly higher chance of developing a brain aneurysm because of inherited conditions that affect connective tissue or blood vessel structure.
- Autosomal dominant polycystic kidney disease (ADPKD) carries roughly an 11% prevalence rate for brain aneurysms, far above the general population’s 2.8%.
- Loeys-Dietz syndrome, a connective tissue disorder, is associated with brain aneurysms in 10% to 28% of affected individuals.
- Ehlers-Danlos syndrome (vascular type) causes pronounced blood vessel fragility, raising the risk of both aneurysm formation and rupture.
- Neurofibromatosis type 1 has been linked to incidental brain aneurysms in up to 9% of patients who undergo brain imaging.
Family history matters even without a named genetic syndrome. If two or more first-degree relatives (a parent or sibling) have had a brain aneurysm, your own risk is elevated enough that Mayo Clinic recommends regular MRI screening. The underlying reason is likely a combination of shared genes affecting vessel wall strength and shared environmental exposures.
High Blood Pressure and Smoking
Hypertension and cigarette smoking are the two most significant modifiable risk factors for brain aneurysms, and they’re especially dangerous together. High blood pressure increases the mechanical force on artery walls with every heartbeat, accelerating the breakdown of the elastic lining and muscular layer. Smoking damages blood vessels through chemical injury and chronic inflammation, weakening the wall from the inside out.
A multivariate analysis found that hypertension independently raised the odds of aneurysm rupture by about 51%, and smoking raised it by about 57%. But the combination was worse than either alone. People with both high blood pressure and a smoking habit had a 128% increase in rupture odds compared to those with neither risk factor. The two don’t just add up; they amplify each other, likely because smoking impairs the artery’s ability to repair the damage that high blood pressure causes.
Why Women Are More Affected
Women develop brain aneurysms nearly three times as often as men. The peak in this gender gap occurs between ages 50 and 59, which aligns closely with menopause. This isn’t a coincidence. Estrogen plays a direct role in maintaining blood vessel walls. It regulates inflammatory processes in artery tissue, supports collagen production, and helps preserve the elastic lining. When estrogen levels drop after menopause, the same kind of collagen loss seen in bone and skin also occurs in cerebral arteries.
Animal studies have confirmed the connection. When researchers removed the ovaries of female mice (eliminating estrogen production), the mice developed brain aneurysms that grew and ruptured. Restoring estrogen activation through specific receptors in the artery wall prevented this progression. The protective effect works partly by maintaining the production of nitric oxide, a molecule that keeps blood vessels flexible and healthy. For women, the postmenopausal period represents a window of increasing vulnerability as this protective mechanism fades.
Infections and Trauma
A small but important category of brain aneurysms is caused by infection rather than gradual wear. These are called mycotic aneurysms, though the name is misleading because bacteria cause them more often than fungi. They form when pathogens reach the artery wall and trigger inflammation that weakens it from within.
The most common pathway is infective endocarditis, an infection of the heart valves. Infected material breaks off from the valve and travels through the bloodstream to the brain, lodging in small arteries and seeding infection in the vessel wall. Brain aneurysms develop in 2% to 10% of infective endocarditis cases. Intravenous drug use is another route, as it can introduce bacteria directly into the bloodstream. Trauma to the head can also damage artery walls enough to create a weak point where an aneurysm forms, though this is uncommon.
What Determines Whether an Aneurysm Ruptures
Having an aneurysm doesn’t mean it will burst. Most never do. Doctors assess rupture risk using a combination of factors that have been validated in large studies. The key predictors include the size of the aneurysm (larger ones are more dangerous), its location within the brain’s arteries, the patient’s age, whether they have high blood pressure, and whether they’ve had a previous brain hemorrhage. Geography also plays a role, with higher rupture rates observed in Japanese and Finnish populations for reasons that aren’t fully understood but likely involve both genetic and environmental factors.
Aneurysms smaller than 7 millimeters in certain locations carry a relatively low annual rupture risk, while larger aneurysms in high-risk locations may warrant treatment even without symptoms. The practical takeaway is that size alone doesn’t tell the whole story. A small aneurysm in a person who smokes and has uncontrolled blood pressure may be more concerning than a slightly larger one in someone without those risk factors.
Reducing Your Risk
You can’t change your genetics or your sex, but the two biggest controllable risk factors, blood pressure and smoking, are well within reach. Keeping blood pressure in a healthy range reduces the constant mechanical stress on artery walls. Quitting smoking removes a major source of chemical damage and inflammation. Together, addressing both eliminates the amplified combined risk that makes the pair so dangerous.
If you have a connective tissue disorder like ADPKD, Loeys-Dietz syndrome, or vascular Ehlers-Danlos syndrome, or if multiple close family members have had brain aneurysms, screening with MRI can detect aneurysms before they cause problems. Many aneurysms found through screening are small and stable, requiring only periodic monitoring rather than immediate treatment.