Aneurysms develop when the wall of a blood vessel weakens and balloons outward, and the causes range from everyday risk factors like high blood pressure to rare genetic conditions. About 2% of adults carry an unruptured brain aneurysm without knowing it. Understanding what drives that vessel wall to weaken in the first place can help you recognize whether you’re at higher risk.
How a Blood Vessel Wall Breaks Down
A healthy artery wall contains layers of elastic fibers and collagen that keep it strong and flexible. In people who develop aneurysms, the body produces an excess of enzymes that digest these structural proteins faster than they can be repaired. Normally, cells in the artery wall produce these enzymes at low levels for routine tissue maintenance, and the body keeps them in check with natural inhibitors. When that balance tips, the enzymes chew through elastin and collagen, thinning the wall.
Inflammation accelerates the process. Immune cells called macrophages infiltrate the artery wall and release their own destructive enzymes on top of what the vessel’s own cells are already producing. At the same time, the smooth muscle cells that give arteries their structural backbone begin to die off through a process driven by oxidative stress and chronic inflammation. As these cells disappear, the wall loses its ability to hold shape under pressure, and the weakened section begins to bulge.
High Blood Pressure
Hypertension is one of the most common contributors to aneurysm formation. Every heartbeat pushes blood against the artery wall, and chronically elevated pressure puts extra mechanical stress on vulnerable spots. Over years, that constant force stretches weakened areas and promotes further degradation of the wall’s structural fibers. In studies of brain aneurysms, hypertension independently raises the odds of an aneurysm rupturing by roughly 50% compared to people with normal blood pressure.
The damage is cumulative. Even mild hypertension sustained over decades can gradually reshape an artery. This is why many aneurysms, particularly in the aorta, tend to appear in people over 60 who have lived with poorly controlled blood pressure for years.
Smoking
Cigarette smoking is the single most modifiable risk factor for aneurysms, especially abdominal aortic aneurysms. Chemicals in tobacco smoke directly damage the cells lining blood vessels and trigger the inflammatory cascade that weakens the arterial wall. Smoking also raises blood pressure and stiffens arteries, compounding the mechanical stress on already vulnerable tissue.
The numbers are striking. Smokers face about 57% higher odds of a brain aneurysm rupturing compared to nonsmokers. When smoking and hypertension occur together, the combined risk more than doubles (an odds ratio of 2.28), which is greater than what you’d expect from simply adding the two risks together. For screening purposes, the U.S. Preventive Services Task Force defines “ever smoker” as someone who has smoked 100 or more cigarettes in their lifetime, and recommends a one-time ultrasound screening for abdominal aortic aneurysms in men aged 65 to 75 who meet that threshold.
Genetic and Connective Tissue Disorders
Some people are born with a predisposition to aneurysms because of inherited conditions that weaken connective tissue throughout the body. Three of the most significant are Marfan syndrome, vascular Ehlers-Danlos syndrome, and Loeys-Dietz syndrome. Each affects different proteins that give blood vessel walls their strength and elasticity.
Marfan syndrome involves a defect in fibrillin, a protein that acts like scaffolding for elastic fibers. People with this condition tend to develop aneurysms in the thoracic aorta, the large vessel leaving the heart. Vascular Ehlers-Danlos syndrome affects collagen production and carries a rate of about 3 arterial events per 1,000 person-years, with the highest risk occurring in the 20 to 29 age range. Loeys-Dietz syndrome disrupts growth factor signaling in the vessel wall and can produce particularly aggressive aortic disease, with one subtype showing a rate of nearly 42 aortic events per 1,000 person-years in people in their 30s.
Beyond these named syndromes, a family history of aneurysms on its own increases your risk. If a first-degree relative (parent or sibling) had a brain or aortic aneurysm, the chance that you carry one is meaningfully higher than the general population’s baseline of 2%.
Structural Heart Defects
A bicuspid aortic valve, where the valve between the heart’s main pumping chamber and the aorta has two flaps instead of the usual three, is one of the most common congenital heart abnormalities. People born with this condition are significantly more likely to develop a thoracic aortic aneurysm. The valve’s abnormal shape alters blood flow patterns leaving the heart, creating turbulence and uneven stress on the aortic wall that promotes dilation over time. Many people with a bicuspid valve don’t know they have one until an aneurysm or other complication is detected.
Polycystic Kidney Disease
Autosomal dominant polycystic kidney disease (ADPKD) is a genetic condition best known for causing cysts in the kidneys, but it also affects blood vessels. An estimated 10% of people with ADPKD will develop a brain aneurysm. The genetic defect weakens connective tissue in arterial walls, making the vessels inside the skull particularly vulnerable. People diagnosed with ADPKD are often offered screening brain scans, especially if they have a family history of aneurysm rupture.
Infections
Infected, or “mycotic,” aneurysms are uncommon but serious. They form when bacteria or fungi settle in the wall of a blood vessel, usually after traveling through the bloodstream from an infection elsewhere in the body. The microorganisms trigger intense local inflammation that rapidly destroys the vessel wall. Common culprits include staphylococci, streptococci, and enterococci. In the abdomen, infections involving E. coli and anaerobic bacteria can damage grafts or native vessels. Infections with drug-resistant organisms like MRSA tend to produce worse outcomes and are harder to treat.
These aneurysms can develop quickly, over days to weeks, compared to the years it takes for most other types to form. They most often arise as a complication of heart valve infections (endocarditis), where clumps of bacteria break off and lodge in distant arteries.
Drug Use
Cocaine and amphetamines pose a direct threat to blood vessel integrity. Cocaine blocks the reabsorption of stress hormones like norepinephrine at nerve endings, causing sudden, severe spikes in blood pressure and intense constriction of blood vessels. This combination can both create new aneurysms and rupture existing ones. A systematic review linked cocaine use to a spectrum of vascular damage in the brain, including vessel constriction, aneurysm formation, aneurysm rupture, and increased risk of re-rupture after an initial bleed.
The risk is not limited to long-term users. Even a single use can trigger a blood pressure spike high enough to rupture a pre-existing weak spot in an artery wall.
Physical Trauma
A forceful blow to the body can damage an artery without fully tearing it, creating what’s called a pseudoaneurysm. Unlike a true aneurysm, where all layers of the vessel wall stretch outward, a pseudoaneurysm involves a partial tear that allows blood to collect in a pocket just outside the vessel, held in place only by surrounding tissue. Blunt force injury accounts for the majority of these cases (about 57%), though penetrating injuries like stab wounds or catheter procedures can also be responsible. Pseudoaneurysms are most common in the limbs and groin but can occur wherever a vessel sustains damage.
Age and Sex
Aging itself weakens arteries. Elastic fibers degrade naturally over decades, and the cumulative effects of blood pressure, cholesterol, and oxidative stress thin the vessel wall. This is why most aortic aneurysms are diagnosed in people over 60. Men are significantly more likely to develop abdominal aortic aneurysms than women, which is reflected in screening guidelines that focus primarily on men aged 65 to 75. Women who have ever smoked or who have a family history of aneurysms may also benefit from screening, though the evidence is less definitive. Brain aneurysms, by contrast, are slightly more common in women, particularly after menopause, suggesting that hormonal changes play a role in cerebral vessel health.
Atherosclerosis
The buildup of fatty plaques inside arteries does more than narrow them. Plaques trigger chronic, low-grade inflammation in the vessel wall that activates the same destructive enzymes responsible for breaking down elastin and collagen. Over time, this weakens the wall beneath the plaque. Atherosclerosis is especially relevant to abdominal aortic aneurysms, where years of plaque accumulation and inflammation combine with the mechanical stress of blood flow to create conditions ripe for dilation. The same lifestyle factors that drive heart disease, including a high-fat diet, inactivity, and diabetes, also feed this process.