What happens inside the artery wall

A brain aneurysm forms when a section of an artery wall in the brain weakens and balloons outward, like a bulge in an inner tube. The artery wall has three layers: a tough outer layer, a muscular middle layer, and a smooth inner lining. When the middle muscular layer thins or develops a defect, the inner lining pushes through under the pressure of blood flow, creating a pouch. This pouch fills with blood and grows larger over time.

The weakening usually happens at branch points where arteries split, because blood flow creates more turbulence and stress at those junctions. Most brain aneurysms occur in the circle of Willis, a ring of arteries at the base of the brain where several major vessels meet. The constant pulsing pressure of blood against a weakened section is what causes the bulge to expand.

Key Takeaways

  • A brain aneurysm develops when the muscular middle layer of an artery wall weakens, allowing the inner lining to balloon outward under blood pressure.
  • Most aneurysms form at artery branch points where blood flow creates more turbulence, particularly in the circle of Willis at the brain's base.
  • High blood pressure, smoking, and certain genetic conditions increase the risk that an artery wall will weaken, but many people with risk factors never develop an aneurysm.
  • An aneurysm may stay stable for years without growing, or it may enlarge slowly; rupture risk depends on size, location, and whether it has already bled.
  • Imaging tests like CT angiography or MRI can detect aneurysms before they rupture, which is why screening is sometimes recommended for people with family history.

Why artery walls weaken in the first place

The exact reason an artery wall develops a weak spot is not fully understood, but several factors increase the likelihood. High blood pressure is the strongest known risk factor—it puts constant extra stress on artery walls, and over years this can damage the muscular layer. Smoking damages the elastic fibers in artery walls and reduces the wall's ability to repair itself. People who smoke have roughly three times the risk of developing an aneurysm compared to people who do not.

Certain genetic conditions make aneurysm formation more likely. Autosomal dominant polycystic kidney disease (ADPKD), Ehlers-Danlos syndrome, and Marfan syndrome all affect connective tissue throughout the body, including artery walls. People with these conditions have a higher rate of aneurysms and often develop them at younger ages. Family history of aneurysm itself also increases risk—if a parent or sibling had an aneurysm, your own risk is higher, though most relatives of affected people never develop one.

Age plays a role as well. Aneurysms are rare before age 30 and become more common with each decade after 40. This reflects the cumulative damage to artery walls over time. Chronic heavy alcohol use and cocaine use have also been linked to aneurysm formation, likely because both raise blood pressure acutely and damage blood vessel walls.

How an aneurysm grows or stays stable

Once an aneurysm forms, it does not necessarily grow. Some remain the same size for years or even a lifetime without causing problems. Others enlarge slowly, and a few grow rapidly. The rate of growth depends on the size at discovery, the location in the brain, and individual factors like blood pressure control and smoking status.

Larger aneurysms tend to grow faster than smaller ones. An aneurysm smaller than 5 millimeters has a very low rupture risk in the first year, while one larger than 10 millimeters carries higher risk. Location matters too—aneurysms in certain positions, such as at the tip of the basilar artery, rupture more often than those in other locations. If an aneurysm has already bled once, the risk of a second rupture is much higher in the weeks and months following the first bleed.

What happens when an aneurysm ruptures

Rupture occurs when the weakened wall finally tears and blood spills into the space surrounding the brain. This causes a subarachnoid hemorrhage—bleeding in the subarachnoid space, the area between the brain and the membrane that covers it. The sudden bleeding raises pressure inside the skull, deprives brain tissue of oxygen, and triggers a severe inflammatory response.

A ruptured aneurysm is a medical emergency. The person typically experiences a sudden, severe headache—often described as "the worst headache of my life"—along with neck stiffness, nausea, sensitivity to light, and sometimes loss of consciousness. Roughly one-third of people who experience a rupture die before reaching a hospital. Of those who survive the initial bleed, many face long-term complications including rebleeding, vasospasm (narrowing of blood vessels), hydrocephalus (fluid buildup in the brain), and seizures.

Difference between unruptured and ruptured aneurysms

An unruptured aneurysm is one that has not yet torn. Most are found by accident during imaging done for other reasons—a CT scan or MRI for a headache, dizziness, or another concern. An unruptured aneurysm may never rupture. The decision about whether to treat an unruptured aneurysm depends on its size, location, growth rate, and your personal risk factors. A small, stable aneurysm in a low-risk location may be monitored with periodic imaging rather than treated immediately.

A ruptured aneurysm requires emergency treatment. The goal is to stop the bleeding and prevent rebleeding. This is done either through surgery to clip the aneurysm or through an endovascular procedure in which a catheter is threaded to the aneurysm and coils or other devices are placed inside to block blood flow into the pouch. The choice between these approaches depends on the aneurysm's location, shape, and the patient's overall health.

Screening and detection before rupture

Screening for unruptured aneurysms is not routine for the general population, but it may be recommended for people with certain risk factors. The American Heart Association suggests considering screening for people with a family history of aneurysm, particularly if more than one relative was affected. Screening is also sometimes offered to people with ADPKD or other genetic connective tissue disorders.

The most common screening test is CT angiography, which uses a CT scanner and injected contrast dye to create detailed images of blood vessels in the brain. MR angiography is another option that does not use radiation. These tests can detect aneurysms as small as 3 to 5 millimeters. If an aneurysm is found, follow-up imaging at set intervals—often every 6 months to a year—tracks whether it is growing. The frequency depends on the size and location of the aneurysm.

Reducing your risk of aneurysm formation

While you cannot change your genes or family history, several modifiable factors lower the risk that an artery wall will weaken. Controlling blood pressure is the most important step. This means taking prescribed blood pressure medications as directed, limiting salt intake, exercising regularly, and managing stress. People whose blood pressure is well controlled have a lower rate of aneurysm formation and slower growth of existing aneurysms.

Quitting smoking is the second major factor. Smoking cessation reduces artery wall damage and allows some repair of existing damage. The benefit begins within weeks of quitting. Limiting alcohol to moderate amounts—no more than one drink per day for women and two for men—also helps protect blood vessel walls. Avoiding cocaine and other stimulant drugs is important as well, since these raise blood pressure acutely and damage vessel walls.

Regular physical activity, a diet rich in fruits and vegetables, and maintaining a healthy weight all support overall cardiovascular health and may reduce aneurysm risk. If you have a family history of aneurysm or a genetic condition that increases risk, discuss screening options with your doctor rather than waiting for symptoms to appear.

Frequently Asked Questions

Can stress or sudden exertion cause an aneurysm to rupture?

Sudden spikes in blood pressure from intense exertion, straining, or emotional stress can trigger rupture in an aneurysm that is already at high risk, but stress alone does not cause aneurysms to form. Most ruptures happen without warning during normal activity or even sleep. If you have a known aneurysm, your doctor will advise you on activity restrictions based on its size and location.

If my parent had an aneurysm, will I definitely get one?

No. Having a parent with an aneurysm increases your risk compared to the general population, but most people with a family history never develop one. Your actual risk depends on how many relatives were affected, at what age they were affected, and whether you have other risk factors like high blood pressure or smoking. A conversation with your doctor about screening can help you understand your individual risk.

What is the difference between an aneurysm and a stroke?

A stroke occurs when blood flow to the brain is blocked (ischemic stroke) or when a blood vessel ruptures and bleeds into brain tissue (hemorrhagic stroke). A ruptured aneurysm causes a specific type of hemorrhagic stroke called subarachnoid hemorrhage. An unruptured aneurysm does not cause a stroke unless it ruptures.

Can an aneurysm heal on its own?

An aneurysm cannot heal or shrink on its own. Once the artery wall has weakened and ballooned, the pouch remains. However, an unruptured aneurysm may stay stable in size for years without growing or causing problems. Treatment is recommended based on rupture risk, which depends on size, location, and growth rate.

How often should I have imaging if I have an unruptured aneurysm?

The interval depends on the aneurysm's size and location. Small aneurysms (less than 5 millimeters) in low-risk locations may be monitored every 1 to 2 years or even less frequently. Larger aneurysms or those in higher-risk locations may need imaging every 6 months. Your doctor will recommend a schedule based on your specific aneurysm and risk factors.