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 - Northwestern Memorial Hospital - Chicago

Cerebral Aneurysm

Cerebral (brain) aneurysms are much more common in adults than in children however, they can occur in anyone and at any age.

A cerebral aneurysm (also known as an intracranial or intracerebral aneurysm) is a weak or thin spot on a blood vessel in the brain that balloons out and fills with blood. Cerebral aneurysms can occur anywhere in the brain; very often they are found on the large arteries at the base of the brain at the points where arteries branch off.

Some cerebral aneurysms, particularly those that are very small, do not bleed or cause other problems. Small aneurysms are less than 11 millimeters in diameter (about the size of a standard pencil eraser), larger aneurysms are 11 to 25 millimeters (about the size of a dime), and giant aneurysms are greater than 25 millimeters in diameter (more than the size of a quarter).

Incidence of Cerebral Aneurysms

  • It is estimated that about 2 percent of Americans (6 million people) have brain aneurysms

  • Slightly more common in women than in men

  • People with certain inherited disorders are also at higher risk

  • The vast majority of aneurysms do not rupture

  • Incidence of reported ruptured aneurysm is about 10 in every 100,000 persons per year

  • About 27,000 patients per year in the US, most commonly in people between ages 30 and 60 years

Causes of Cerebral Aneurysms


Cerebral aneurysms can be congenital (present at birth), resulting from an inborn abnormality in an artery wall.


They also are more common in people with certain genetic diseases, such as connective tissue disorders or polycystic kidney disease. They can be associated with other brain abnormalities such as an arteriovenous malformation (snarled tangles of arteries and veins in the brain that disrupt blood flow). This occurs about 10 percent of the time.

Head Injury/Infection/Cancer

In children, especially, brain aneurysms may also result from a head injury, an infection in an arterial wall or may be associated with cancerous tumors of the head and neck. Aneurysms of these types tend to present most often as subarachnoid hemorrhages (bleeding between the brain and its surrounding membrane, the arachnoid).

Adult Risk Factors

Possible risk factors for rupture in adults include hypertension, alcohol abuse, drug abuse (particularly cocaine) and smoking.

Dangers of Cerebral Aneurysms

Bleeding in the Brain

Aneurysms may burst and bleed into the brain, causing serious complications including:

  • Hemorrhagic stroke 
  • Permanent nerve damage
  • Death

Mortality from a ruptured aneurysm may approach 40 percent. 


A delayed but serious complication of subarachnoid hemorrhage is hydrocephalus, in which the excessive buildup of cerebrospinal fluid in the skull dilates fluid pathways called ventricles that can swell and press on the brain tissue.


Another delayed complication after a rupture is vasospasm, in which other blood vessels in the brain, in response to bleeding around the base of the brain, contract and limit blood flow to vital areas of the brain. This reduced blood flow can cause stroke or tissue damage.


Most cerebral aneurysms do not show symptoms until they either become very large or burst. Small, unchanging aneurysms generally do not produce symptoms, whereas a larger aneurysm that is steadily growing may press on tissues and nerves.

Symptoms may include the following:

  • Headache
  • Pain above and behind the eye
  • Numbness, weakness, or paralysis on one side of the face
  • Dilated pupils and vision changes (double or loss of vision)

When an aneurysm ruptures, an individual may experience the following:

  • Sudden and extremely severe headache (“the worst headache ever”)
  • Double vision
  • Nausea
  • Vomiting
  • Stiff neck and/or loss of consciousness.
Last UpdateJuly 10, 2012