An aneurysm is an abnormal widening or ballooning of a section of a blood vessel. When an aneurysm occurs in the brain, it is called a cerebral aneurysm.

Aneurysms in the brain occur when there is a weakened area in the wall of a blood vessel. They may occur as a congenital (present from before birth) defect or may develop later in life. It is estimated that 5% of the population has some type of aneurysm in the brain.

A saccular aneurysm (berry aneurysm) can vary in size from a few millimeters to over a centimeter. Giant berry aneurysms can reach well over 2 centimeters. The aneurysm resembles a sack of blood attached to one side of the blood vessel by a narrow neck. These are more common in adults. Multiple berry aneurysms are not unusual.

Aneurysms usually cause no symptoms unless they rupture and cause bleeding into the brain. Often, an aneurysm is found when a CT scan or MRI is performed for another reason. If the aneurysm gets big enough to push on nearby structures, it can cause the following symptoms:

• Double vision
• Loss of vision
• Headaches
• Eye pain
• Neck pain

Symptoms of an aneurysm that have broken open (ruptured) may include:

• Sudden occurrence of a severe headache (often described as "the worst headache of your life")
• Headaches with nausea or vomiting
• Stiff neck (occasionally)
• Muscle weakness, difficulty moving any part of the body
• Numbness or decreased sensation in any part of the body
• Vision changes (double vision, loss of vision)
• Eyelid drooping
• Confusion, lethargic, sleepiness, or stupor
• Seizures
• Slow, sluggish, lethargic movement
• Speech impairment
• Sudden onset of irritability, impulsivity, or poor temper control

NOTE: A ruptured aneurysm is a medical emergency. Seek immediate medical help.

An eye exam may show increased pressure within the brain (raised intracranial pressure), including swelling of the optic nerve (papilledema) or bleeding into the retina of the eye.

The following tests may be used to diagnose cerebral aneurysm and determine the cause of bleeding within the brain:

• CT scan of the head can identify bleeding and occasionally locate the aneurysm.
• CSF (cerebrospinal fluid) examination (spinal tap) may confirm bleeding.
• MRI of the head may be an alternative to a CT scan, but is not as good at showing bleeding in the brain.
• Cerebral angiography or spiral CT scan angiography of the head is used to pinpoint the location and size of the aneurysm.
• EEG (electroencephalogram) may be performed if seizures occur.