What happens inside an artery to create an aneurysm

A brain aneurysm forms when the wall of an artery in your brain weakens and bulges outward, like a thin spot developing in a tire. The artery wall has three layers—an inner lining, a muscular middle layer, and an outer layer. When the middle muscular layer thins or the inner lining develops a tear, pressure from blood flowing through can push the weakened section into a balloon-like pouch. This pouch is the aneurysm itself.

The bulge doesn't happen overnight. Most aneurysms develop slowly over years or even decades as the artery wall gradually deteriorates. Blood pressure constantly pushes against the weakened spot, and over time the pouch grows larger. Some people have aneurysms that never rupture and never cause symptoms—they live with them undetected. Others develop aneurysms that grow quickly and become dangerous within months.

The location matters. Aneurysms most often form where arteries branch or curve, because blood flow is turbulent at those points and puts more stress on the vessel wall. The most common site is where the anterior communicating artery branches off in the front of the brain, followed by the junction where the middle cerebral artery splits.

Key Takeaways

  • Brain aneurysms form when the muscular middle layer of an artery wall weakens, allowing blood pressure to push the vessel into a bulging pouch over months or years.
  • High blood pressure, smoking, and family history of aneurysms all increase the risk that an artery wall will weaken in the first place.
  • Most aneurysms form at points where arteries branch or curve, where blood flow creates more stress on the vessel wall.
  • A ruptured aneurysm causes sudden, severe bleeding in the brain; an unruptured one may cause no symptoms at all or mild headaches and vision changes.
  • Imaging tests like CT angiography or MRI can detect aneurysms before they rupture, which is why screening matters for people at high risk.

Why some people develop weak artery walls

Certain conditions and habits damage the inner lining of arteries and weaken the muscular layer underneath. High blood pressure is the single largest risk factor—it forces blood to push harder against vessel walls, and over time this constant strain causes the muscle to thin and lose elasticity. People with untreated or poorly controlled high blood pressure are significantly more likely to develop aneurysms than those with normal pressure.

Smoking damages the inner lining of arteries directly and also raises blood pressure. Smokers develop aneurysms at younger ages than non-smokers with the same blood pressure. Heavy alcohol use raises blood pressure and may weaken vessel walls through direct toxic effects. Cocaine use causes sudden spikes in blood pressure that can damage arteries acutely.

Some people inherit a genetic tendency toward weak connective tissue. Conditions like polycystic kidney disease, Ehlers-Danlos syndrome, and Marfan syndrome all increase aneurysm risk because they affect how the body builds and maintains the proteins that give artery walls their strength. If a parent or sibling had a brain aneurysm, your own risk is higher than the general population, though most people with a family history never develop one.

Age also plays a role. Aneurysms are rare in children and young adults but become more common after age 40 as arteries accumulate years of wear. Women are slightly more likely to have aneurysms than men, though men who do have them are more likely to experience rupture.

How rupture happens and what triggers it

Rupture occurs when the aneurysm wall becomes so thin that it can no longer contain the pressure of blood flowing through it. The exact moment of rupture is often unpredictable, but certain situations increase the immediate risk. A sudden spike in blood pressure—from straining during bowel movements, heavy lifting, intense exercise, or sudden emotional stress—can push an already-weakened aneurysm past its breaking point.

Some ruptures happen during sleep or rest, with no obvious trigger. Others occur during activities that raise blood pressure acutely. Coughing, sneezing, or straining can be enough to rupture an aneurysm that is close to the breaking point. This is why people with known unruptured aneurysms are sometimes advised to avoid sudden pressure increases and to manage their blood pressure carefully.

Once rupture occurs, blood spills into the space surrounding the brain (the subarachnoid space) or into the brain tissue itself. This bleeding causes immediate, severe symptoms: the worst headache of the person's life, neck stiffness, nausea, sensitivity to light, and sometimes loss of consciousness. A ruptured aneurysm is a medical emergency requiring immediate hospital care.

The difference between unruptured and ruptured aneurysms

An unruptured aneurysm is one that has not yet burst. Many people have them without knowing, because they cause no symptoms at all. Some unruptured aneurysms do cause symptoms—persistent headaches, vision changes, facial pain, or a drooping eyelid—if they are large enough to press on nearby nerves or brain tissue. These symptoms develop gradually over weeks or months.

Unruptured aneurysms are usually found by accident when someone gets a brain scan for another reason, or through screening in people with high risk (family history, certain genetic conditions, or high blood pressure). Once found, the decision about treatment depends on the size, location, growth rate, and the person's age and overall health. Some unruptured aneurysms are monitored with repeat imaging rather than treated immediately.

A ruptured aneurysm causes sudden, catastrophic bleeding. Symptoms appear instantly and are severe: sudden explosive headache, stiff neck, light sensitivity, nausea and vomiting, confusion, seizures, or coma. A ruptured aneurysm requires emergency surgery or an interventional procedure to stop the bleeding and prevent re-rupture. Survival and recovery depend on how quickly treatment begins and how much damage the bleeding causes.

How size and location affect rupture risk

Larger aneurysms are more likely to rupture than smaller ones. An aneurysm smaller than 5 millimeters has a very low rupture risk in a given year—less than 1 percent. As size increases, so does risk. An aneurysm between 5 and 10 millimeters has a rupture risk of roughly 1 to 2 percent per year. Aneurysms larger than 10 millimeters carry significantly higher risk, and those over 25 millimeters are considered high-risk and usually treated regardless of whether they are causing symptoms.

Location also determines risk. Aneurysms in the posterior circulation (the back of the brain) rupture more often than those in the anterior circulation (the front). An aneurysm at the tip of the basilar artery or in the posterior communicating artery is considered higher-risk than one at the anterior communicating artery, even if they are the same size.

Growth rate is another factor. An aneurysm that is enlarging on repeat imaging is at higher risk than one that remains stable. If imaging shows an aneurysm has grown by more than 1 millimeter in a year, or if it has grown significantly since the first scan, treatment is usually recommended even if the aneurysm is still relatively small.

What imaging shows about aneurysm formation

CT angiography and MR angiography can visualize the exact shape, size, and location of an aneurysm. These images show whether the aneurysm is saccular (a round pouch on one side of the artery, the most common type) or fusiform (a spindle-shaped bulge that involves the entire circumference of the artery). The images also reveal whether the aneurysm has a neck (a narrow connection to the parent artery) or a broad base, which affects treatment options.

Serial imaging—scans repeated over time—shows whether an aneurysm is stable or growing. An aneurysm that does not change in size over years may carry lower rupture risk than one that enlarges. This is why people with unruptured aneurysms often have follow-up scans scheduled at intervals ranging from months to years, depending on initial size and risk factors.

Imaging cannot predict exactly when or if an aneurysm will rupture. It shows the anatomy and can measure growth, but rupture remains partly unpredictable. This uncertainty is why the decision to treat an unruptured aneurysm involves weighing the risk of rupture against the risks of the treatment itself.

Frequently Asked Questions

Can you feel an aneurysm before it ruptures?

Most unruptured aneurysms cause no symptoms at all. Some larger ones cause persistent headaches, vision changes, or facial pain if they press on nearby structures. These symptoms develop gradually. A ruptured aneurysm causes sudden, severe symptoms—the worst headache of your life, neck stiffness, and sensitivity to light—which require emergency care immediately.

Does high blood pressure always lead to an aneurysm?

No. Many people have high blood pressure their entire lives without developing an aneurysm. However, high blood pressure significantly increases the risk, especially over decades. The combination of high blood pressure plus smoking, heavy alcohol use, or a family history raises risk much higher than any single factor alone.

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

No. Having a family member with an aneurysm increases your risk compared to the general population, but most people with a family history never develop one. If you have a close relative with an aneurysm, talk with your doctor about whether screening is appropriate for you, especially if you also have high blood pressure or smoke.

Can an aneurysm shrink on its own?

Aneurysms do not shrink on their own. They remain stable, grow slowly, or grow quickly. Once formed, an aneurysm will not disappear without treatment. Managing blood pressure and avoiding smoking can slow growth and reduce rupture risk, but these steps do not eliminate the aneurysm itself.

What happens if an aneurysm is found but not treated?

This depends on size, location, and growth rate. Small, stable aneurysms in low-risk locations may be monitored with repeat imaging every one to two years rather than treated immediately. Larger aneurysms, those in high-risk locations, or those that are growing are usually treated to prevent rupture. Your doctor will discuss the specific risks and benefits for your situation.