How brain aneurysms develop

A brain aneurysm forms when the wall of a blood vessel in your brain weakens and bulges outward, like a thin spot developing in an inner tube. The bulge happens gradually over time as the vessel wall loses its strength. Most people with a brain aneurysm never know they have one—it causes no symptoms and never ruptures. But understanding how they form can help you recognize risk factors that apply to you or your family.

The exact reason a particular person develops an aneurysm is not fully understood. Doctors know that certain conditions and habits make aneurysms more likely, but they cannot predict which people will develop one. Some people with multiple risk factors never get an aneurysm, while others with few risk factors do.

Key Takeaways

  • Brain aneurysms form when a blood vessel wall weakens and bulges, usually over many years, and most never cause symptoms or rupture.
  • High blood pressure is the strongest known risk factor, followed by smoking, family history, and certain connective tissue disorders.
  • Age, sex, and ethnicity play a role—aneurysms are more common in people over 40 and in women, and some populations have higher rates.
  • A ruptured aneurysm is a medical emergency, but unruptured aneurysms found by imaging are often monitored rather than treated immediately.

High blood pressure and smoking as primary risk factors

High blood pressure is the strongest known risk factor for developing a brain aneurysm. When blood pushes harder against vessel walls over years or decades, the walls weaken and can begin to bulge. People with untreated or poorly controlled high blood pressure face significantly higher risk than those whose blood pressure is managed with medication.

Smoking is the second major risk factor. Tobacco damages blood vessel walls directly and also raises blood pressure. People who smoke are more likely to develop aneurysms at a younger age than non-smokers with the same genetic risk. The risk drops after quitting, though it does not return to the level of someone who never smoked.

Having both high blood pressure and smoking together multiplies the risk more than either factor alone. This is one reason doctors strongly recommend blood pressure control and smoking cessation for people with known aneurysms or a family history of them.

Family history and inherited conditions

If a close relative—a parent, sibling, or child—has had a brain aneurysm, your risk is higher than the general population. The increased risk appears to run in families, though no single gene has been identified as the cause. People with a parent or sibling who had an aneurysm are sometimes offered screening with imaging, depending on their age and other factors.

Certain inherited connective tissue disorders also raise aneurysm risk substantially. Autosomal dominant polycystic kidney disease (ADPKD) is one of the strongest genetic links—people with this kidney condition have a notably higher rate of brain aneurysms. Other rare inherited conditions, such as Ehlers-Danlos syndrome and Marfan syndrome, also increase risk because they affect how the body builds and maintains blood vessel walls.

If you have one of these conditions or know of a family history of aneurysms, discuss screening options with your doctor. Screening does not prevent aneurysms but can detect them before they rupture.

Age, sex, and ethnic background

Brain aneurysms are more common in people over age 40, though they can occur at any age. The longer a person lives, the more time blood vessel walls have to weaken. Aneurysms found in younger people are often smaller and less likely to rupture, but they still require monitoring.

Women are about 1.5 times more likely than men to have a brain aneurysm. The reason is not fully clear, but hormonal factors may play a role. Women also tend to have aneurysms rupture at slightly younger ages than men.

Certain ethnic groups have higher rates of brain aneurysms. People of Finnish, Japanese, and Hispanic descent have higher prevalence rates than other populations. This may reflect a combination of genetic and environmental factors, though research is ongoing.

Other medical conditions and lifestyle factors

Several other health conditions increase aneurysm risk. People with arteriovenous malformations (AVMs)—abnormal tangles of blood vessels in the brain—have a higher rate of aneurysms. Chronic high cholesterol, diabetes, and obesity also appear to raise risk, though the connection is not as strong as with high blood pressure and smoking.

Heavy alcohol use may increase risk, particularly when combined with high blood pressure. Cocaine use is also associated with aneurysm formation and rupture, especially in younger people. Some medications and hormonal factors, such as estrogen use in certain contexts, may influence risk, though the evidence is mixed.

Head trauma can sometimes cause an aneurysm to form, though this is rare. More commonly, trauma can cause a pre-existing aneurysm to rupture.

Why some aneurysms rupture and others do not

Not all aneurysms rupture. In fact, most unruptured aneurysms never do. The size of the aneurysm is one factor—larger aneurysms are more likely to rupture than smaller ones. Location matters too; aneurysms in certain parts of the brain are at higher rupture risk than others.

How fast an aneurysm grows also affects rupture risk. Aneurysms that grow quickly are more likely to rupture than those that remain stable. This is why doctors monitor aneurysms with repeat imaging over time—to track whether they are growing, shrinking, or staying the same size.

High blood pressure, smoking, and heavy alcohol use all increase the risk that an existing aneurysm will rupture. This is why managing these factors is important for people who know they have an unruptured aneurysm.

What happens after an aneurysm is found

Many brain aneurysms are discovered by accident—during imaging done for another reason, like a head injury or headache investigation. When an unruptured aneurysm is found, your doctor will discuss monitoring versus treatment based on the size, location, and your personal risk factors.

Small, stable aneurysms in low-risk locations are often monitored with repeat MRI or CT scans every 6 to 12 months, then less frequently if they do not change. Larger aneurysms or those in high-risk locations may be treated with surgery or a minimally invasive procedure to prevent rupture. Your doctor will explain the risks and benefits of each approach for your specific situation.

A ruptured aneurysm is a medical emergency requiring immediate hospital care. If you experience a sudden, severe headache unlike any you have had before—especially with neck stiffness, vision changes, or loss of consciousness—call 911 immediately.

Frequently Asked Questions

Can stress cause a brain aneurysm?

Stress does not directly cause an aneurysm to form, but sudden stress or intense emotion can temporarily raise blood pressure, which may increase the risk of rupture in someone who already has an aneurysm. Long-term stress management is part of overall health, but it is not a prevention strategy for aneurysm formation.

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

No. Having a family history increases your risk above the general population, but most people with a family history of aneurysms never develop one. Your doctor can discuss whether screening is right for you based on your age and other factors.

Can you feel a brain aneurysm before it ruptures?

Most unruptured aneurysms cause no symptoms at all. Some people with larger aneurysms report headaches or vision problems, but these are not common. A ruptured aneurysm causes a sudden, severe headache that people often describe as "the worst headache of my life."

Does high cholesterol cause brain aneurysms?

High cholesterol appears to raise aneurysm risk, but the connection is weaker than with high blood pressure and smoking. Managing cholesterol is part of overall cardiovascular health, which benefits people with or at risk for aneurysms.

What should I do if aneurysms run in my family?

Talk to your doctor about your family history. They may recommend screening with MRI or CT if you have close relatives who had aneurysms, especially if they ruptured at a young age. In the meantime, controlling blood pressure, not smoking, and managing other health conditions are the most important steps you can take.