What Is a Brain Aneurysm?

A brain aneurysm occurs when a weak or thin area in the wall of a cerebral artery—the type of blood vessel that carries oxygen-rich blood to the brain—becomes enlarged from the pressure of circulating blood. A sac or bulge forms, putting pressure on surrounding nerves and tissue.

If a brain aneurysm leaks or ruptures (bursts), it becomes a medical emergency that can lead to stroke, brain damage, coma, or death.

A brain aneurysm is also known as a cerebral aneurysm or intracranial aneurysm.

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Locations and Types

Brain aneurysms can occur anywhere in the brain, but they are most commonly found in areas where major arteries divide into branches along the base of the skull. This includes the circle of Willis, a group of blood vessels in the bottom central portion of the brain.

Common locations of aneurysms include:

  • Anterior communicating artery
  • Posterior communicating artery
  • Middle cerebral artery

There are three types of brain aneurysms:

  • Saccular aneurysm (berry aneurysm): This type is a spherical sac filled with blood that resembles a berry on a vine. It is attached to a main artery or one of its branches, and is the most common type of brain aneurysm.
  • Fusiform aneurysm: A fusiform aneurysm balloons or bulges out on all sides of the artery and is often associated with atherosclerosis (plaque build-up inside arteries).
  • Mycotic aneurysm: This type forms after an infection that weakens the artery, causing a bulge.

Aneurysms are also classified by their size in width:

  • Small: Less than 11 millimeters (mm) in diameter; equivalent to a large pencil eraser
  • Large: 11 to 25 mm in diameter; about the width of a dime
  • Giant: More than 25 mm in diameter; a width larger than a quarter

Symptoms of a Brain Aneurysm

Small aneurysms often do not cause any symptoms. Sometimes, however, a small brain aneurysm might push against nearby blood vessels or other structures in the brain as it grows and lead to mild symptoms, such as headaches or pain around the eyes.

Seek medical attention if you experience any of the following symptoms of a growing aneurysm that is pressing on tissue or nerves:

  • Blurred or double vision
  • Drooping eyelid
  • Dilated pupil
  • Pain above and behind one eye
  • Weakness and/or numbness
  • Paralysis on one side of the face

Often, these symptoms serve as a warning that prompts diagnosis. Effective treatment can then be started before any more serious symptoms occur.

Signs of Rupture

If a brain aneurysm ruptures, the most common symptom is a so-called thunderclap headache, which many people describe as the “worst headache of their lives.”

Seek urgent medical attention if you experience any of the following symptoms of a ruptured aneurysm or its complications:

  • Sudden onset of a severe headache
  • Double vision
  • Nausea
  • Vomiting
  • Numbing or tingling sensation
  • Stiff neck
  • Sensitivity to light
  • Seizures
  • Loss of consciousness (this can be brief or prolonged)
  • Suddenly collapse
  • Gasping for breath

Hemorrhage and Stroke

When a brain aneurysm ruptures, it causes hemorrhage (bleeding, which can lead to a hemorrhagic stroke.

There are three types of intracerebral hemorrhage:

  • Subarachnoid hemorrhage: Bleeding under the arachnoid layer of the meninges.
  • Intraparenchymal (bleeding within the brain tissues themselves)
  • Intraventricular (bleeding into the ventricle)

In these instances, the region of the brain that normally receives blood supply from the bleeding artery may not receive enough blood flow, which can also lead to an ischemic stroke.


Anyone can have a brain aneurysm at any age, but they are most common in adults ages 30 to 60. They are also more common in women than men.

Vascular changes or inflammation may contribute to brain aneurysms, and there are factors that can increase your risk of developing them. They include:

In some cases, brain aneurysms are congenital (there at birth) due to an abnormality in the artery walls.

There are also some inherited risk factors for brain aneurysms, which include:

  • Inherited connective tissue disorders that can weaken artery walls
  • Polycystic kidney disease (multiple cysts form in the kidneys)
  • Brain arteriovenous malformations (AVMs, tangles of blood vessels in the brain that disrupt blood flow)
  • Family history of aneurysm, especially in a first-degree family member

Risk of Bleeding and Rupture

The risk of rupture and bleeding is greatest for people with large or giant aneurysms, especially those with multiple aneurysms who have already suffered a previous rupture.

Women, especially those over age 55, are about 1.5 times more likely to have a rupture than men. Those with a family history of brain aneurysm ruptures may also have an increased risk of experiencing one themselves.

Aneurysms may also bleed during situations when blood pressure is excessively elevated. Episodes of markedly high blood pressure can be triggered by a number of causes, including the use of illicit drugs (cocaine, amphetamines) or major fluctuations in heart, kidney, or liver function.

It is estimated that approximately 2% of people in the United States (about 1 in 50 people) have at least one brain aneurysm. It is also estimated that between 50% and 80% of brain aneurysms will never rupture.


If you have any symptoms that indicate a possible brain aneurysm, or if you have a family history of brain aneurysms or inherited conditions that increase your risk of them, your physician may order imaging tests to get a view of the brain. These tests may include:

  • Computed tomography (CT scan): Specialized X-rays are able to produces three-dimensional images.
  • CT angiography (CTA): A dye (known as contrast) is injected to be able to observe blood vessels and blood flow in the brain during a CT scan.
  • Magnetic resonance imaging (MRI): A magnetic field and radio waves are used to get two- or three-dimensional images of the brain.
  • MR angiography (MRA): A dye is injected to get a more detailed view of blood vessels and blood flow in the brain during an MRI.
  • Cerebral angiogram: A catheter attached to a camera is inserted through an artery and threaded to the brain; a dye is injected to get detailed X-ray images.

Some aneurysms may be visible with MRI or CT scans. But in many cases, a cerebral angiography (CTA or MRA) is needed to find an aneurysm or to determine if there's a rupture and bleeding. Cerebral angiography can offer more detailed information than other types of imaging, but it is also more invasive and has serious risks, which is why it is typically a follow-up test if other testing is inconclusive.

If you experience symptoms of rupture, your physician may also order a spinal tap (lumbar puncture). A needle is inserted into the lumbar spine to collect a sample of cerebrospinal fluid, which is tested for signs of subarachnoid hemorrhage.


Some aneurysms can be repaired surgically or with neuro-interventional procedures to reduce the risk of bleeding.

The procedures used to treat brain aneurysms and prevent bleeds include:

  • Microvascular clipping: This involves stopping blood flow to the aneurysm with a clipping device and requires open brain surgery.
  • Platinum coil embolization: A catheter is inserted into an artery, usually in the groin, and threaded through the body to the brain aneurysm. A wire with detachable platinum coils is then put into the catheter tube. The coils are released to block the aneurysm and reduce blood flowing into it.
  • Flow diversion devices: These are used to treat very large aneurysms and those that cannot be treated with the above options. It involves placing a small stent (flexible mesh tube) in the artery to reduce blood flow into the aneurysm. The stent is also threaded through the body via a catheter.

While effective, each of these procedures carry serious risks, such as damage to other blood vessels or stroke.

Because of this, those with small brain aneurysms may only require monitoring, which can include periodic imaging tests, such as MRA or CTA, to check for any growth.

Whether or not others may be candidates for a brain aneurysm repair depends on the location and size of the aneurysm, as well overall health and ability to safely tolerate a procedure.

After a brain aneurysm bleeds, surgery may be needed to remove the blood. This depends on the amount of blood and the location of bleeding in the brain. Often, however, the blood slowly dissolves on its own and surgery is not necessary.


Aneurysms that don't rupture often go unnoticed and have no consequences.

The prognosis after an aneurysm rupture varies and depends on the size of the bleed, its location, and treatment received, but about half of people with ruptured aneurysms do not survive. About 25% of people with a ruptured aneurysm die within the first 24 hours, and another 25% die from complications within six months.

A subarachnoid hemorrhage can lead to brain damage, and those who experience this type of brain bleed often need physical, speech, and occupational therapy to regain lost function and to learn to manage any permanent disabilities. 

A Word From Verywell

If you or your loved one has been diagnosed with a brain aneurysm, treat the condition with the attention it deserves. But keep in mind that most cases do not result in bleeding in the brain and there are effective ways to prevent this from occurring.

Even after a brain aneurysm ruptures, many people recover and continue to improve over time with close medical care and rehabilitation that is very similar to stroke rehabilitation.

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. National Institute of Neurological Disorders and Stroke. Cerebral aneurysms fact sheet.

  2. Brain Aneurysm Foundation. Warning signs/symptoms.

  3. Brain Aneurysm Foundation. Statistics and facts.

  4. Williams LN, Brown RD. Management of unruptured intracranial aneurysms. Neurol Clin Pract. 2013;3(2):99-108. doi:10.1212/CPJ.0b013e31828d9f6b

  5. Cleveland Clinic. Brain aneurysm: diagnosis and tests.

By Jose Vega MD, PhD
Jose Vega MD, PhD, is a board-certified neurologist and published researcher specializing in stroke.