What Causes a Brain Aneurysm?

A brain aneurysm — also called a cerebral aneurysm — is a weakened area in the wall of an artery in the brain that bulges outward, often compared to a small balloon or berry-shaped sac forming at a branch point in the vessel. Despite the alarming name, most aneurysms are small, never bleed, and never cause symptoms. They are frequently discovered incidentally, when a person has a brain scan for an unrelated reason such as headache, dizziness or a head injury.

Aneurysms form where two things meet: weakness in an arterial wall and the constant force of blood flow pushing against that spot. Every heartbeat delivers pressure to the vessel wall, and over years that force can gradually push a weakened area outward. As the bulge grows, the wall becomes thinner — which is what makes a larger or irregularly shaped aneurysm more likely to leak or rupture.

Several factors make that wall weakness more likely. Established aneurysm risk factors include:

Importantly, having none of these does not make someone immune. A brain aneurysm can develop without any identifiable risk factor at all, which is one reason aneurysms are so often found by chance rather than because of symptoms.

Brain aneurysms are diagnosed more often in women than in men. One reason may be biology: hormonal changes appear to influence the structure and elasticity of blood vessel walls, and the risk of an aneurysm forming or rupturing rises after menopause. That said, the relationship between menopause and aneurysm formation is not fully established — researchers have not confirmed hormones as a direct cause, and the difference is a matter of increased likelihood, not certainty. Both high blood pressure and tobacco use remain the most important modifiable risks for everyone.

Most unruptured aneurysms are completely asymptomatic. When symptoms do occur, they usually come from a larger aneurysm pressing on a nearby nerve or brain structure. Possible brain aneurysm symptoms include a headache in a consistent location, double vision or other changes in vision, a drooping eyelid, a dilated pupil on one side, numbness or weakness, or pain above and behind one eye. Symptoms like these deserve medical evaluation — they may point to something that needs imaging, and the same signs can come from many other conditions, including migraine, cluster headache or other causes of headache.

When an aneurysm bursts, blood leaks into the space around the brain. This is called a subarachnoid hemorrhage, and it is a life-threatening emergency. The sudden rise in pressure and the toxic effect of blood on brain tissue can cause brain injury, stroke, seizures, hydrocephalus (fluid buildup), vasospasm (narrowing of arteries days later), coma or death. A ruptured aneurysm is one cause of hemorrhagic stroke. Warning features often include a thunderclap headache, nausea and vomiting, a stiff neck, sensitivity to light, confusion, fainting or a seizure. Someone who survives the initial bleed needs intensive care, because the risk of rebleeding and delayed complications is significant.

Aneurysms are found with blood-vessel imaging: CTA (CT angiography) uses a contrast dye with a CT scan, MRA (MR angiography) uses an MRI sequence that highlights blood vessels without radiation, and cerebral angiography — a catheter-based study threading a thin tube from the groin up to the brain arteries — remains the most detailed test and is often used when treatment is being planned.

Screening healthy people without symptoms is not routinely recommended. It is generally reserved for selected higher-risk individuals — for example, someone with two or more first-degree relatives who had a brain aneurysm or aneurysmal bleeding, or a person with certain inherited conditions such as polycystic kidney disease. A neurologist weighs the chance of finding a meaningful aneurysm against the risks of the test and of treating something that might never have caused a problem. Related vascular testing, such as a carotid ultrasound, looks at a different set of arteries and a different stroke mechanism.

No. Many small, unruptured aneurysms are managed with observation — periodic imaging to watch for growth, along with aggressive control of blood pressure and tobacco cessation. The decision to treat rather than monitor depends on several factors working together: aneurysm size, location, whether it shows growth over time, its morphology (shape, including an irregular or daughter-sac outline), and patient-specific risk factors such as age, overall health, smoking status and family history.

When treatment is recommended, the two main approaches are surgical clipping — a craniotomy in which a small clip is placed across the aneurysm neck — and endovascular treatment, in which coils or a flow-diverting device are delivered through a catheter to seal the aneurysm off from circulation. Neither approach fits every aneurysm or every patient, so the choice is made individually, ideally with a neurosurgical and neurovascular team.

Some risk is not modifiable — age, sex and family history cannot be changed — but the two most influential contributors can be. Stopping smoking removes a major source of ongoing arterial wall injury, and managing blood pressure lowers the force acting on a weakened area. Avoiding cocaine and other illicit stimulants matters too, because these substances cause abrupt, dramatic spikes in blood pressure.

Honest limits apply: risk reduction cannot prevent every aneurysm. Someone who does everything right can still develop one, which is why new or unusual symptoms and sudden severe headache should never be ignored.