A brain aneurysm (AN-yoo-riz-um) — also known as a cerebral aneurysm or intracranial aneurysm — is a bulge or ballooning in a blood vessel in the brain. An aneurysm often looks like a berry hanging on a stem.

Experts think brain aneurysms form and grow because blood flowing through the blood vessel puts pressure on a weak area of the vessel wall. This can increase the size of the brain aneurysm. If the brain aneurysm leaks or ruptures, it causes bleeding in the brain, known as a hemorrhagic stroke.

Most often, a ruptured brain aneurysm occurs in the space between the brain and the thin tissues covering the brain. This type of hemorrhagic stroke is called a subarachnoid hemorrhage.

Brain aneurysms are common. But most brain aneurysms aren't serious, especially if they're small. Most brain aneurysms don't rupture. They usually don't cause symptoms or cause health problems. In many cases, brain aneurysms are found during tests for other conditions.

However, a ruptured aneurysm quickly becomes life-threatening and requires medical treatment right away.

If a brain aneurysm hasn't ruptured, treatment may be appropriate in some cases. Treatment of an unruptured brain aneurysm may prevent a rupture in the future. Talk with your health care provider to make sure you understand the best options for your specific needs.


  • Saccular aneurysm, also known as a berry aneurysm. This type of aneurysm looks like a berry hanging from a vine. It's a round, blood-filled sac that protrudes from the main artery or one of its branches. It usually forms on arteries at the base of the brain. A berry aneurysm is the most common type of aneurysm.
  • Fusiform aneurysm. This type of aneurysm causes bulging on all sides of the artery.
  • Mycotic aneurysm. This type of aneurysm is caused by an infection. When an infection affects the arteries in the brain, it can weaken the artery wall. This can cause an aneurysm to form.


Most brain aneurysms that haven't ruptured don't cause symptoms. This is especially true if they're small. Brain aneurysms may be found during imaging tests that are done for other conditions.

However, a ruptured aneurysm is a very serious condition, typically causing a severe headache. And if an unruptured aneurysm presses against brain tissue or nerves, it may cause pain and other symptoms.

Ruptured aneurysm

A sudden, severe headache is the key symptom of a ruptured aneurysm. This headache is often described by people as the worst headache they've ever experienced.

In addition to a severe headache, symptoms of a ruptured aneurysm can include:

  • Nausea and vomiting
  • Stiff neck
  • Blurred or double vision
  • Sensitivity to light
  • Seizure
  • A drooping eyelid
  • Loss of consciousness
  • Confusion

'Leaking' aneurysm

In some cases, an aneurysm may leak a slight amount of blood. When this happens, a more severe rupture often follows. Leaks may happen days or weeks before a rupture.

Leaking brain aneurysm symptoms may include:

  • A sudden, extremely severe headache that may last several days and up to two weeks.

Unruptured aneurysm

An unruptured brain aneurysm may not have any symptoms, especially if it's small. However, a larger unruptured aneurysm may press on brain tissues and nerves.

Symptoms of an unruptured brain aneurysm may include:

  • Pain above and behind one eye.
  • A dilated pupil.
  • A change in vision or double vision.
  • Numbness of one side of the face.

When to see a doctor

Seek immediate medical attention if you develop a:

  • Sudden, extremely severe headache

If you're with someone who complains of a sudden, severe headache or who loses consciousness or has a seizure, call 911 or your local emergency number.


Brain aneurysms are caused by thinning artery walls. Aneurysms often form at forks or branches in arteries because those areas of the vessels are weaker. Although aneurysms can appear anywhere in the brain, they're most common in arteries at the base of the brain.

Risk factors

Several factors can contribute to weakness in an artery wall. These factors may increase the risk of a brain aneurysm or aneurysm rupture.

Some of these risk factors develop over time. But some conditions present at birth can increase the risk of developing a brain aneurysm.

Risk factors include:

  • Older age. Brain aneurysms can occur at any age. However, they're more common in adults between ages 30 and 60.
  • Being female. Brain aneurysms are more common in women than in men.
  • Cigarette smoking. Smoking is a risk factor for brain aneurysms to form and for brain aneurysms to rupture.
  • High blood pressure. This condition can weaken arteries. Aneurysms are more likely to form and to rupture in weakened arteries.
  • Drug use, particularly using cocaine. Drug use raises blood pressure. If illicit drugs are used intravenously, it can lead to an infection. An infection can cause a mycotic aneurysm.
  • Heavy alcohol use. This also can increase blood pressure.
  • Inherited connective tissue disorders, such as Ehlers-Danlos syndrome. These disorders weaken blood vessels.
  • Polycystic kidney disease. This inherited disorder results in fluid-filled sacs in the kidneys. It also may increase blood pressure.
  • A narrow aorta, known as coarctation of the aorta. The aorta is the large blood vessel that delivers oxygen-rich blood from the heart to the body.
  • Brain arteriovenous malformation, known as AVM. In this condition, arteries and veins in the brain are tangled. This affects blood flow.
  • A family history of brain aneurysm. Your risk is higher if you have family members who have had a brain aneurysm. This is particularly true if two or more first-degree relatives — such as a parent, brother, sister or child — has had a brain aneurysm. If you have a family history, you can ask your health care provider about getting screened for a brain aneurysm.

Some types of aneurysms may occur after a head injury or from certain blood infections.

Risk factors for a ruptured aneurysm

There are some factors that make it more likely an aneurysm will rupture. They include:

  • Having a large aneurysm.
  • Having aneurysms in certain locations.
  • Smoking cigarettes.
  • Having untreated high blood pressure.


When a brain aneurysm ruptures, the bleeding usually lasts only a few seconds. However, the blood can cause direct damage to surrounding cells and can kill brain cells. It also increases pressure inside the skull.

If the pressure becomes too high, it may disrupt the blood and oxygen supply to the brain. Loss of consciousness or even death may occur.

Complications that can develop after the rupture of an aneurysm include:

  • Re-bleeding. An aneurysm that has ruptured or has leaked is at risk of bleeding again. Re-bleeding can cause further damage to brain cells.
  • Narrowed blood vessels in the brain. After a brain aneurysm ruptures, blood vessels in the brain may contract and narrow. This is known as vasospasm. Vasospasm can cause an ischemic stroke, in which there's limited blood flow to brain cells. This may cause additional cell damage and loss.
  • A buildup of fluid within the brain, known as hydrocephalus. Most often, a ruptured brain aneurysm occurs in the space between the brain and the thin tissues covering the brain. The blood can block the movement of fluid that surrounds the brain and spinal cord. As a result, an excess of fluid puts pressure on the brain and can damage tissues.
  • Change in sodium level. Bleeding in the brain can disrupt the balance of sodium in the blood. This may occur from damage to the hypothalamus, an area near the base of the brain. A drop in blood sodium levels can lead to swelling of brain cells and permanent damage.


In many cases, brain aneurysms can't be prevented. But there are some changes you can make to lower your risk. They include quitting smoking if you smoke. Also work with your health care provider to lower your blood pressure if it's high. Don't drink large amounts of alcohol or use drugs such as cocaine.