An aneurysm forms when a section of an artery wall weakens and bulges outward, like a balloon inflating at a thin spot. The underlying cause is always structural: the layers of the artery lose their strength and can no longer hold their shape against the pressure of blood flowing through them. What drives that weakening varies depending on where the aneurysm develops, your genetics, and a handful of controllable risk factors like smoking and high blood pressure.
How Artery Walls Break Down
Healthy arteries have layered walls built from proteins like collagen and elastin, which give them both flexibility and strength. An aneurysm develops when these structural proteins degrade faster than the body can repair them. Several overlapping processes drive this breakdown: enzymes called metalloproteinases chew through the supportive matrix, immune cells infiltrate the wall and sustain chronic inflammation, and smooth muscle cells that normally maintain the artery begin dying off or changing their behavior. Oxidative stress compounds the damage.
Think of it like wood rot in a load-bearing beam. The beam doesn’t snap all at once. It softens gradually, losing its ability to resist the constant force pressing on it. In an artery, that force is blood pressure, and the weakened section slowly stretches outward into a bulge.
Brain Aneurysms: Blood Flow and Geometry
Brain aneurysms, sometimes called berry aneurysms because of their shape, typically form at branching points in the arteries at the base of the brain, a ring-shaped structure called the circle of Willis. These branch points are where blood flow hits the vessel wall hardest. Research published by the American Heart Association found that the shear stress at these locations can exceed 30 pascals, enough to damage the inner lining of the artery and eventually trigger aneurysm formation.
The geometry of each branching point matters. Arteries that split at wider angles or have branches of unequal size create more turbulence and higher stress on the wall. This is why certain locations along the circle of Willis develop aneurysms far more often than others. The anterior communicating artery, which connects the two sides of the brain’s blood supply, is one of the most common sites.
Most brain aneurysms are small and cause no symptoms. They’re often discovered incidentally during imaging for something else. Size plays a significant role in rupture risk: aneurysms 7 mm or larger carry roughly three times the rupture risk of smaller ones. But location changes the math considerably. A 4 mm aneurysm at the anterior communicating artery may be more dangerous than a larger one in other locations, because the average size of ruptured aneurysms there is only about 5.5 mm. Aneurysms under 12 mm in the cavernous segment of the internal carotid artery, by contrast, carry almost no rupture risk.
Aortic Aneurysms: Inflammation and Plaque
The aorta, your body’s largest artery, is the other common site. Abdominal aortic aneurysms (AAAs) develop in the section running through the belly, while thoracic aortic aneurysms occur in the chest. For decades, doctors called these “atherosclerotic aneurysms” because they so often appeared alongside plaque buildup. The relationship turns out to be more complicated than simple cause and effect.
One leading theory holds that when plaque narrows the aorta, the artery tries to compensate by remodeling and expanding to maintain normal blood flow. This remodeling weakens the wall’s structural matrix. Inflammatory cells within the plaque release chemicals that break down elastin, the protein that gives the aorta its stretch. Once elastin breaks start accumulating, a cycle of inflammation and further degradation takes hold.
An alternative view is that atherosclerosis and aortic aneurysms don’t directly cause each other but share the same underlying risk factors: smoking, high blood pressure, high cholesterol, and aging. In practice, both processes often develop in the same patients, and each can accelerate the other.
Smoking and High Blood Pressure
Smoking is the single most important modifiable risk factor for aneurysm formation. A meta-analysis in the Journal of Neurosurgery found that smokers had a 34% higher chance of having a detectable brain aneurysm compared to nonsmokers, with some individual studies showing odds as high as five to six times greater. Smoking damages artery walls directly through oxidative stress and chronic inflammation, and it also raises blood pressure, compounding the effect.
High blood pressure forces arteries to endure greater mechanical stress with every heartbeat. Over years, this constant pounding accelerates the breakdown of collagen and elastin in the vessel wall. Together, smoking and hypertension account for a large share of preventable aneurysms. Quitting smoking and managing blood pressure won’t reverse an existing aneurysm, but both reduce the risk of new ones forming and may slow the growth of those already present.
Genetic and Hereditary Causes
Some people are born with connective tissue that is structurally vulnerable. Several inherited conditions directly predispose to aneurysm formation by disrupting the assembly of collagen or elastin proteins. According to Johns Hopkins University researchers, the most common of these are Marfan syndrome, the vascular type of Ehlers-Danlos syndrome (known as vEDS), Loeys-Dietz syndrome, and familial thoracic aortic aneurysm and dissection.
These conditions don’t just increase risk slightly. They create a fundamental weakness in blood vessel walls from birth, which means aneurysms can develop at younger ages and in locations where they’d otherwise be rare. People with a first-degree relative who had an aneurysm also face elevated risk even without a named genetic syndrome, suggesting that more subtle inherited variations in connective tissue strength play a role.
What Happens if an Aneurysm Ruptures
Most aneurysms never rupture. They grow slowly, sometimes stabilize, and many people live their entire lives unaware they have one. When a rupture does occur, the consequences depend heavily on location. A ruptured brain aneurysm is fatal in about 50% of cases. Roughly 15% of people die before they even reach a hospital. Among survivors, about two-thirds are left with some permanent neurological impairment.
A ruptured aortic aneurysm is similarly dangerous, with survival depending on how quickly emergency surgery can be performed. The high stakes of rupture are the reason screening programs exist for people at elevated risk.
Who Should Be Screened
The U.S. Preventive Services Task Force recommends a one-time ultrasound screening for abdominal aortic aneurysm in men aged 65 to 75 who have ever smoked. “Ever smoked” is defined as having smoked 100 or more cigarettes in a lifetime, a threshold many former smokers meet. Men in the same age range who have never smoked may still benefit from screening on a case-by-case basis. For women who have never smoked and have no family history, routine screening is not recommended. For women aged 65 to 75 who have smoked or have a family history, the evidence is currently inconclusive.
Brain aneurysm screening is not part of routine care for the general population but is sometimes recommended for people with a strong family history or a known connective tissue disorder. If you have two or more first-degree relatives who have had brain aneurysms, imaging to check for them is a reasonable conversation to have with your doctor.