Aneurysms form when a section of an artery wall weakens and bulges outward, and the causes range from chronic conditions like high blood pressure to inherited genetic disorders. At a cellular level, the process involves breakdown of the structural proteins that keep artery walls strong and flexible, combined with the death of smooth muscle cells that normally maintain those walls. Several forces can set this process in motion, and most people who develop an aneurysm have more than one risk factor working against them.
How an Artery Wall Weakens
Healthy arteries have three layers, and each plays a role in handling the constant pressure of blood flow. An aneurysm develops when the middle layer, which contains elastic fibers and smooth muscle cells, begins to degrade. The key mechanisms include breakdown of the structural scaffold between cells, chronic inflammation that attracts immune cells into the artery wall, death of the smooth muscle cells that normally repair and maintain the vessel, and oxidative stress that damages cells faster than they can recover.
When oxygen supply to the artery wall itself drops, a condition called hypoxia, it triggers even more inflammation and accelerates the production of enzymes that chew through the vessel’s structural proteins. This creates a feedback loop: damage leads to inflammation, inflammation leads to more damage, and the wall gradually thins until it can no longer resist the outward push of blood pressure. The result is a balloon-like bulge that can grow slowly over years or, in some cases, expand quickly.
High Blood Pressure
Chronically elevated blood pressure is one of the most significant contributors to aneurysm formation. Every heartbeat pushes blood against the artery walls, and when that force is persistently too high, it accelerates the mechanical wear on the vessel. Over time, the elastic fibers in the artery stretch and fragment, and the smooth muscle cells that would normally compensate begin to die off or change their behavior in ways that weaken the wall further.
This is particularly relevant for the aorta, the body’s largest artery, which absorbs the full force of each heartbeat. But high blood pressure also stresses smaller arteries in the brain, where aneurysms can form at branch points where blood flow creates turbulence. Managing blood pressure is one of the most effective ways to reduce aneurysm risk precisely because it addresses the constant mechanical force that drives wall degradation.
Smoking
Tobacco use is the single strongest modifiable risk factor for aneurysms, particularly in the abdomen and brain. Current smokers have roughly 71% higher odds of a brain aneurysm rupturing compared to nonsmokers. Heavy smokers face even steeper risk: those with a long smoking history of more than 20 pack-years are about 61% more likely to develop multiple brain aneurysms and 40% more likely to develop large ones.
Smoking damages artery walls through several pathways. It promotes inflammation, accelerates the breakdown of elastin (the protein that gives arteries their stretch), and impairs the ability of smooth muscle cells to repair damage. These effects are cumulative, meaning the longer and more heavily someone smokes, the greater the toll on their blood vessels. The U.S. Preventive Services Task Force considers anyone who has smoked 100 or more cigarettes in their lifetime an “ever smoker” for the purpose of aneurysm screening recommendations.
Genetic and Inherited Conditions
Some people are born with connective tissue that is structurally prone to aneurysms. In these conditions, the body produces defective collagen or elastin, the two main proteins that give blood vessels their strength and flexibility. This creates an inherent predisposition to vessel wall degeneration, loss of structural integrity, and eventual aneurysm formation or spontaneous rupture.
The most common inherited conditions linked to aneurysms include:
- Marfan syndrome: affects the body’s connective tissue throughout, with aortic aneurysms being a major cardiovascular concern
- Vascular Ehlers-Danlos syndrome: causes fragile blood vessels that are prone to tearing and aneurysm formation, often at younger ages
- Loeys-Dietz syndrome: involves widespread arterial aneurysms and a high risk of dissection (tearing of the artery wall)
- Familial thoracic aortic aneurysm and dissection: a genetic predisposition to aneurysms in the upper portion of the aorta, often running through multiple generations
Even without a named syndrome, having a first-degree relative (parent, sibling, or child) with an aneurysm increases your own risk. Family history matters enough that it factors into screening decisions.
Atherosclerosis and Chronic Inflammation
Atherosclerosis, the gradual buildup of fatty deposits inside arteries, shares several risk factors with aneurysms: aging, high blood pressure, smoking, and obesity. But the two conditions damage arteries in distinct ways. While atherosclerosis narrows vessels by building plaque on the inner wall, aneurysms involve destruction of the middle layer of the artery, with prominent smooth muscle cell death and breakdown of elastic tissue.
Chronic inflammation plays a central role in both. When immune cells infiltrate the artery wall in response to damage or plaque buildup, they release enzymes that degrade the structural proteins holding the vessel together. The smooth muscle cells, which would normally maintain and repair the wall, instead begin to behave abnormally. They proliferate in unhelpful ways, migrate from their normal positions, and eventually undergo programmed cell death. The net result is a wall that progressively loses its ability to withstand blood pressure.
Age and Sex
Aneurysm risk rises significantly with age, particularly after 50. In a large screening study, abdominal aortic aneurysms were found in 4.8% of people screened, with a stark sex difference: 8.2% of men versus 1.3% of women. Among men aged 50 to 64, the prevalence was already 5.4%.
This disparity is why screening guidelines focus heavily on older men. The USPSTF recommends a one-time ultrasound screening for abdominal aortic aneurysms in men aged 65 to 75 who have ever smoked. For men in that age range who have never smoked, screening is offered selectively based on other risk factors. For women who have never smoked and have no family history, routine screening is not recommended because the condition is uncommon enough that the harms of screening (false positives, unnecessary anxiety, follow-up procedures) outweigh the benefits.
Cocaine and Stimulant Drugs
Cocaine use is a well-documented trigger for aneurysm rupture, particularly brain aneurysms. The drug causes sudden, severe spikes in blood pressure by flooding the nervous system with stress hormones. This transient but profound hypertension can push an existing aneurysm past its breaking point.
Beyond the blood pressure spike, cocaine also causes blood vessels to constrict through multiple mechanisms, including blocking the reuptake of certain chemical signals at nerve endings and increasing calcium flow into the smooth muscle of vessel walls. This combination of extreme pressure inside the vessel and constriction of the vessel itself is particularly dangerous. Someone who already has a small, undiagnosed aneurysm faces a real risk of rupture with each use.
Infections
Though less common, infections can directly attack artery walls and cause what are called mycotic aneurysms. Despite the name (coined in the 1800s because the infected tissue resembled fungal growths), most mycotic aneurysms are caused by bacteria, not fungi. They can develop when bacteria from an infection elsewhere in the body, such as a heart valve infection, travel through the bloodstream and lodge in an artery wall. Historically, syphilis was a well-known cause of ascending aortic aneurysms, though this is now rare in countries with widespread antibiotic access.
Physical Trauma and Medical Procedures
Injury to an artery, whether from a car accident, a fall, or a medical procedure, can cause a pseudoaneurysm. Unlike a “true” aneurysm where all three layers of the artery wall bulge outward, a pseudoaneurysm is essentially contained bleeding from a hole in the artery. Blood pools outside the vessel but is held in place by surrounding tissue.
The most common cause is a complication from catheter-based procedures, where a thin tube is inserted into an artery (typically in the groin) for diagnostic imaging or treatment. Blunt trauma to the chest can cause pseudoaneurysms of the aorta, and pancreatitis can damage arteries supplying the abdominal organs. In rare cases, a heart attack can weaken the heart muscle enough to cause a pseudoaneurysm of the heart itself. These differ from the slow-developing aneurysms caused by high blood pressure or smoking, but they require the same urgent attention because rupture is a serious risk.