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.

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 cm. 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.

Other types of cerebral aneurysm may involve widening (dilatation) of the entire circumference of the blood vessel in an area, or may appear as a ballooning out of part of a blood vessel. These types of aneurysms can occur in any blood vessel that supplies the brain. Trauma and infection, which can injure the blood vessel wall, can cause such aneurysms.

It is estimated that 5% of the population has some type of aneurysm in the brain. However, the incidence of ruptured aneurysm is approximately 10 out of 100,000 people per year. Multiple berry aneurysms are not unusual. About 10% of patients who have one aneurysm will have at least one more.

Risk factors include a family history of cerebral aneurysms, and some medical problems, including polycystic kidney disease and coarctation of the aorta.

Aneurysms usually cause no symptoms unless they rupture and cause bleeding into the brain. Often, an aneurysm is found when a CAT scan or MRI is performed for an unrelated reason. Occasionally, the aneurysm gets big enough to compress nearby structures and can cause symptoms such as:

• Double vision
• Loss of vision
• Headaches
• Eye and/or neck pain

Symptoms of an aneurysm that has bled may include:

• Sudden occurrence of a headache (severe or described as "the worst in patient's experience")
• 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
• Changes in mental status, the person may be confused, lethargic, sleepy, or stuporous
• Seizures
• Slow, sluggish, lethargic movement
• Speech impairment
• Sudden onset of irritability, impulsivity or poor temper control

There may be signs of increased pressure within the brain (raised intracranial pressure) including swelling of the optic nerve (papilledema) or tiny hemorrhages into the retina of the eye, which are evident on careful eye examination.

Other findings on exam (such as a third cranial nerve palsy) may help identify which blood vessel has the aneurysm.

A cerebral aneurysm is usually diagnosed by tests to determine the cause of bleeding within the brain:

• A CT scan of the head can identify bleeding and occasionally locate the aneurysm.
• A CSF (cerebrospinal fluid) examination (spinal tap) may confirm bleeding when CT scan is non-diagnostic.
• An MRI of the head may be an alternative to a CT scan, but is not as sensitive to bleeding within the brain (subarachnoid bleeding). Various MRI scans vary in their ability to detect smaller aneurysms.
• Conventional cerebral angiography or spiral CT scan angiography of the head are the most sensitive tools to pinpoint the location and size of the aneurysm(s).
• EEG (electroencephalogram) may be performed if seizures occur.

Because symptoms often do not appear until bleeding occurs, a ruptured cerebral aneurysm is an emergency condition when it is discovered. The goal of treatment is to control symptoms and prevent further bleeding. Lowering blood pressure can reduce the risk of further bleeding.

Neurosurgery is the primary treatment for cerebral aneurysm. The base of the aneurysm is closed off with clamps, sutures, or other methods that prevent blood flow through the aneurysm. In many cases, special coils can be placed into the aneurysm through the arteries.

This causes a clot to form in the aneurysm and prevents further bleeding. This is considered a less invasive approach than brain surgery, and in the appropriate circumstances, it is regarded as the best form of treatment.

If surgery is not feasible because of the location or size of the aneurysm or the condition of the person, medical treatment is similar to treatment for subarachnoid hemorrhage.

This may include restricting activity (often complete bedrest is advised), treating symptoms such as headache, controlling blood pressure, and prescribing preventive use of antiseizure medications.

Once the aneurysm is repaired, prevention of stroke due to blood vessel spasm may be necessary. This may include intravenous fluids, certain medications, and actually letting one's blood pressure run high.

The outcome varies. The best indicator for prognosis is the patient's status after the aneurysm ruptures. Patients who are deeply comatose after an aneurysm rupture generally don't do as well as those with minimal symptoms.

A cerebral aneurysm that does not rupture may not cause any symptoms. If one is discovered which has not ruptured, treatment must be considered carefully. The risks related to brain surgery are high and attempting to prevent the aneurysm from rupturing by operating on it may actually cause rupture and all the potential problems that can result.

However, about 25% of ruptured cerebral aneurysms are fatal within 24 hours. Approximately another 25% are fatal within about 3 months. Of the remaining people with ruptured cerebral aneurysm, more than 50% will have some sort of permanent disability.

• Subarachnoid hemorrhage
• Stroke
• Seizures, epilepsy
• Paralysis of any part of the body
• Permanent loss of sensation of any part of the face or body
• Other neurologic deficits (such as vision changes, loss of speech ability, cognitive decline)
• Communicating hydrocephalus

There is no known way to prevent the formation of a cerebral aneurysm. If discovered in time, unruptured aneurysms can be treated before causing problems.

The decision to repair an unruptured cerebral aneurysm is based on the size and location of the aneurysm, and the patient's age and general health. It must be carefully considered given the risks inherent both in operating and in watchful waiting.