A brain aneurysm is a weak ballooning area in a blood vessel in the brain. The standard treatments for brain aneurysm are clipping and coiling. Rarely, an aneurysm cannot be treated with either of these common methods; in such cases, doctors may perform a bypass.

Aneurysms form on blood vessels called arteries, which transport oxygen-rich blood under high pressure. The constant pressure of blood on arterial walls over time can cause a weaker area of the artery wall to bulge out. Eventually, the aneurysm may burst, releasing blood into the brain. It is estimated that about 20–50% of aneurysms burst.

The most common kind of aneurysm is a saccular aneurysm. This type, shaped like a small sac, is the type most likely to rupture and bleed.

A less common type is a fusiform aneurysm. In this type, the artery wall does not protrude in just one location. Instead it is swollen all the way around, like an inflating long balloon. Fusiform aneurysms usually cause no symptoms and are much less likely to rupture and bleed. They are sometimes found incidentally.

Brain aneurysms are very rare in children, but they do occur. For information specific to aneurysms in children, see our pediatric aneurysm page.



Unfortunately, aneurysms tend to cause no symptoms until they rupture. A ruptured brain aneurysm is an extreme medical emergency. It may cause intracerebral hemorrhage or subarachnoid hemorrhage. These are both types of hemorrhagic stroke. Both can be fatal.

Symptoms of a ruptured brain aneurysm include:

  • Sudden, severe headache (“the worst headache ever”)
  • Stiff neck
  • Double vision, eye movement problems, sensitivity to light
  • Nausea or vomiting
  • Loss of consciousness
  • Seizure


There are three main ways that brain aneurysms are diagnosed. All of them use similar imaging scans, described below.

  • When diagnosed—generally in the emergency department—after rupture, a brain aneurysm may stop bleeding on its own for a time. In these cases, the aneurysm can often be successfully treated. In other cases, the bleeding does not stop on its own, no drug or surgical procedure is effective, and the rupture is fatal.
  • The second method of diagnosis is the screening of people who have a family history of brain aneurysm. Familial screening may allow an aneurysm to be caught and treated before it ruptures. Treatment of an unruptured aneurysm is often successful.
  • The third way brain aneurysms are diagnosed is incidentally. With improved scanning techniques, incidental findings of aneurysm are increasingly common.

Doctors and patients must weigh the specific risks of treating each individual aneurysm against its individual risk of rupture.

The following imaging scans are used to diagnose ruptured or unruptured aneurysms:

  • Magnetic resonance scan (MR scan, or MRI): Uses radio waves and a large magnet to form images of the body’s structures.
  • Computed tomography scan (CT scan, or CAT scan): Uses X-rays and a computer to form images of the body’s structures.
  • Angiography: Uses a special dye injected into the bloodstream to make blood vessels visible on imaging scans. May be used alone or in combination with MRI or CT scans, called MRA and CTA scans.

Risk Factors

In adults, aneurysm formation is linked to a variety of risk factors, both modifiable and nonmodifiable.

Modifiable risk factors include:

  • Smoking
  • High blood pressure
  • Use of drugs like cocaine, crack, or amphetamines

Nonmodifiable risk factors include:

  • Age (increased risk over the age of 40)
  • Family history of aneurysms
  • Being female
  • Having conditions such as Marfan Syndrome, Ehlers-Danlos Syndrome (vascular type), polycystic kidney disease, or fibromuscular dysplasia
  • History of tumor, infection, or traumatic brain injury
  • Presence of arteriovenous malformation (AVM)

Every risk factor listed above is associated with the formation of an aneurysm. Once an aneurysm has formed, it may or may not rupture. An estimated 50–80% of aneurysms never rupture. Rupture of an aneurysm is associated with two risk factors: smoking and high blood pressure.

Of course, aneurysms may rupture in people who have never smoked, and in those with normal blood pressure, but the presence of each risk factor increases the risk for rupture. Patients at particular risk for an aneurysm, or those who have been diagnosed with an aneurysm, should speak with their neurosurgeons or primary care physicians to get any help they need in controlling their blood pressure or quitting the use of tobacco products.


Our doctors weigh all relevant factors when deciding on an individual treatment plan. They consider the size and shape of the aneurysm, its location and blood flow, the age and medical condition of the patient, the patient’s neurological condition, and whether there is a history of aneurysm rupture in the patient or the family.

There are more treatment options for brain aneurysms today than ever before.

Most frequently, our neurosurgeons treat aneurysms by clipping or coiling. Clipping an aneurysm means placing a small metal clip over its neck, so that blood cannot flow into it. This prevents the aneurysm from rupturing, or from rupturing again. 

Coiling means filling an aneurysm with tiny metal coils. Any blood inside the aneurysm clots, and the aneurysm is sealed off. Coiling is an endovascular procedure—one that is performed with specialized equipment from inside the blood vessels. Coiling may be performed along with other endovascular procedures such as stenting and flow diversion.

When an aneurysm cannot be treated with either of these common methods, our neurosurgeons sometimes perform a bypass procedure to reroute blood flow around the aneurysm, so that the section of artery with the aneurysm can safely be shut down.