About Cerebral Aneurysms

What is a cerebral aneurysm?

An aneurysm is a weak area in a blood vessel that usually enlarges. It’s often described as a “ballooning” of the blood vessel. Watch an animation (opens in new window).

How common are aneurysms?

About 1.5 to 5 percent of the general population has or will develop a cerebral aneurysm. About 3 to 5 million people in the United States have cerebral aneurysms, but most are not producing any symptoms. Between 0.5 and 3 percent of people with a brain aneurysm may suffer from bleeding.

How do aneurysms form? Are people born with an aneurysm?

People usually aren’t born with aneurysms. Most develop after age 40. Aneurysms usually develop at branching points of arteries and are caused by constant pressure from blood flow. They often enlarge slowly and become weaker as they grow, just as a balloon becomes weaker as it stretches. Aneurysms may be associated with other types of blood vessel disorders, such as fibromuscular dysplasia, cerebral arteritis or arterial dissection, but these are very unusual. They may run in families, but people are rarely born with a predisposition for aneurysms. Some aneurysms are due to infections, drugs such as amphetamines and cocaine that damage the brain’s blood vessels, or direct brain trauma from an accident.

Are all aneurysms the same?

Brain aneurysms are all different. They vary in size, shape and location.


  • Small aneurysms are less than 5 mm (1/4 inch).
  • Medium aneurysms are 6–15 mm (1/4 to 3/4 inch).
  • Large aneurysms are 16–25 mm (3/4 to 1 1/4 inch).
  • Giant aneurysms are larger than 25 mm (1 1/4 inch).

Aneurysms can be:

  • Saccular (sac-like) with a well-defined neck
  • Saccular with a wide neck
  • Fusiform (spindle shaped) without a distinct neck

An aneurysm is usually located along the major arteries deep within brain structures. When approaching an aneurysm during surgery, normal brain tissue must be carefully spread apart to expose it. Aneurysms can occur in the front part of the brain (anterior circulation) or the back part of the brain (posterior circulation).

How is an aneurysm diagnosed?

Special imaging tests can detect a brain aneurysm. Two noninvasive tests show the blood vessels in the brain. In the first, called CTA (computed tomographic angiography), patients are placed on a table that slides into a CT scanner. A special contrast material (dye) is injected into a vein, and images are taken of the blood vessels to look for abnormalities such as an aneurysm. In the second test, called MRA (magnetic resonance angiography), patients are placed on a table that slides into a magnetic resonance scanner, and the blood vessels are imaged to detect a cerebral aneurysm. Both of these screening tests detect most cerebral aneurysms larger than 3–5 mm (about 3/16 inch).

The most reliable test is called a diagnostic cerebral angiogram. In this test, the patient lies on an X-ray table. A small tube (catheter) is inserted through a blood vessel in the leg (groin) and guided into each of the blood vessels in the neck that go to the brain. Contrast is then injected, and pictures are taken of all the blood vessels in the brain. This test is slightly more invasive and less comfortable, but it is the most reliable way to detect all types and sizes of cerebral aneurysms.

Before any treatment is considered, a diagnostic cerebral angiogram is usually performed to fully map a plan for therapy.

If one aneurysm forms, will others form?

Having one aneurysm means there’s a 15 to 20 percent chance of having one or more other aneurysms.

What are the symptoms of an unruptured aneurysm?

Smaller aneurysms usually don’t have symptoms. But as an aneurysm enlarges, it can produce headaches or localized pain. If an aneurysm gets very large, it may produce pressure on the normal brain tissue or adjacent nerves. This pressure can cause difficulty with vision, numbness or weakness of an arm or leg, difficulty with memory or speech, or seizures.

What causes an aneurysm to bleed?

We usually don’t know why an aneurysm bleeds or exactly when it will bleed. We do know what increases the chance for bleeding:

  • High blood pressure is the leading cause of subarachnoid hemorrhage. Heavy lifting or straining can cause pressure to rise in the brain and may lead to an aneurysm rupture.
  • Strong emotions, such as being upset or angry, can raise blood pressure and can subsequently cause aneurysms to rupture.
  • Blood “thinners” (such as warfarin), some medications and prescription drugs (including diet pills that act as stimulants such as ephedrine and amphetamines), and harmful drugs like cocaine can cause aneurysms to rupture and bleed.

What are the chances that an unruptured aneurysm may bleed?

Many factors determine whether an aneurysm is likely to bleed. These include the size, shape and location of the aneurysm and symptoms that it causes. Smaller aneurysms that are uniform in size may be less likely to bleed than larger, irregularly shaped ones. Once an aneurysm has bled, there’s a very high chance of re-bleeding. That’s why we recommend treatment as soon as possible.

What happens if an aneurysm bleeds?

If an aneurysm ruptures, it leaks blood into the space around the brain. This is called a “subarachnoid hemorrhage.” Depending on the amount of blood, it can produce:

  • a sudden severe headache that can last from several hours to days
  • nausea and vomiting
  • drowsiness and/or coma

The hemorrhage may also damage the brain directly, usually from bleeding into the brain itself. This is called a “hemorrhagic stroke.” This can lead to:

  • weakness or paralysis of an arm or leg
  • trouble speaking or understanding language
  • vision problems
  • seizures

What is the usual damage to the brain after an aneurysm bleeds?

Once an aneurysm bleeds, the chance of death is 30 to 40 percent and the chance of moderate to severe brain damage is 20 to 35 percent, even if the aneurysm is treated. Fifteen to 30 percent of patients have only mild difficulties or almost none. If the aneurysm isn’t treated quickly enough, another bleed may occur from the already ruptured aneurysm.

In 15 to 20 percent of patients, vasospasm (irritation by the leaked blood causing narrowing of the blood vessels) may occur. This can lead to further brain damage. Other problems may include hydrocephalus (enlargement of the spaces within the brain that produce cerebrospinal fluid), difficulty breathing that requires a mechanical ventilator, and infection. Heart and lung problems may result due to extensive brain damage that can affect the body’s normal functions.

Why is the damage so extensive after bleeding?

After blood enters the brain and the space around it, direct damage to the brain tissue and brain function results. The amount of damage is usually related to the amount of blood. Damage is due to the increased pressure and swelling from bleeding directly into the brain tissue, or from local cellular damage to brain tissue from irritation of blood in the space between the brain and the skull.

Blood can also irritate and damage the normal blood vessels and cause vasospasm (constriction). This can interrupt normal blood flow to the healthy brain tissue and can cause even more brain damage. This is called an “ischemic stroke.”

Will treating a ruptured aneurysm reverse or improve brain damage?

Once an aneurysm bleeds and brain damage occurs, treating the aneurysm will not reverse the damage. Treatment helps prevent more bleeding, which can cause more damage to the brain and, consequently, to the body’s functions. If bleeding has already caused brain damage, patients may benefit from rehabilitation therapy after the aneurysm has been treated.

How is a treatment method for an aneurysm chosen?

Each patient and each aneurysm is different. Doctors must evaluate the risk factors that favor treatment vs. non-treatment and decide which technique may be best.

It’s important to consult with experts in this field. This should include a discussion with a cerebrovascular neurosurgeon who specializes in surgically clipping aneurysms, a neurosurgeon with endovascular expertise and training, a neurointerventionalist (a neurologist with endovascular training) or a neuroradiologist who specializes in the less invasive treatment of cerebral aneurysms by coiling. These types of medical specialists have different expertise and training backgrounds. We recommend you consult with several types of physicians.

How should an aneurysm be treated?

The best treatment depends on many things, including whether the aneurysm has ruptured or not. A ruptured aneurysm usually requires treatment right away, because the re-bleeding rate remains quite high. However, the treatment time and options for treatment depend on the size, location and shape of the aneurysm, as well as the patient’s overall medical condition. If an aneurysm hasn’t ruptured, the treatment decision depends on its size, location and shape, and the patient’s symptoms. Each factor is important and requires consultation with a neurosurgeon and a neurointerventionalist or a neuroradiologist who has special skills and training in treating these types of aneurysms.

What treatments are available?

Medical therapy. Small, unruptured aneurysms that aren’t creating any symptoms may not need treatment unless they grow, trigger symptoms or rupture. It’s very important to have annual check-ups to monitor blood pressure, cholesterol and other medical conditions. Small, unruptured aneurysms require regular imaging examinations to make sure that they have not grown or changed significantly.

Neurosurgery. Depending on a person’s risk factors, open surgery may be recommended. Patients are placed under general anesthesia, an opening is made in the skull, the brain tissue is spread apart, and the aneurysm is surgically exposed. Then the neurosurgeon places a surgical clip around its base. The clip seals off the aneurysm so blood can’t enter. For an uncomplicated surgical clipping procedure, the hospital stay is usually four to six days. Full recovery usually takes several weeks to months.

Neurointerventionalist/neuroradiologist. Depending on the aneurysm’s size, location and shape, it may be treatable from inside the blood vessel. This minimally invasive procedure is similar to the cerebral angiogram. However, in addition to taking pictures, a catheter is directed through the blood vessels into the aneurysm itself. Then, using X-ray guidance, the endovascular surgeon carefully places soft platinum micro-coils into the aneurysm and detaches them. The coils stay within the aneurysm and act as a mechanical barrier to blood flow, thus sealing it off. For an uncomplicated procedure, the hospital stay is usually one to two days. Recovery after the operation usually takes five to seven days. For a complicated surgery or endovascular treatment, or if an aneurysm has bled into the brain, hospitalization may last from one to four weeks, depending on the patient’s medical condition and any complications caused by the hemorrhage.

What are the potential complications of aneurysm treatment?

Until the aneurysm is safely and completely treated, there’s always the risk it may re-bleed and cause more brain damage. If normal blood vessels are damaged, it could also result in more brain damage. This could cause weakness or paralysis of the arm or leg, difficulty with speech or understanding, vision loss, confusion, loss of memory and/or seizures. There is also the risk of the anesthesia, infection, bleeding, damage to the kidneys from the X-ray dye and other potential problems. All these risks need to be carefully considered when deciding on a course of treatment. If the aneurysm isn’t treated, it may bleed or grow in size.

What follow-up is required after aneurysm treatment?

Depending on the type of treatment, the two follow-up procedures are:

  • Surgical clipping. After this type of surgery, a post-operative angiogram is usually performed during the hospital stay to make sure the surgical clip has completely treated the aneurysm.
  • Neurointerventionalist/neuroradiologist. After coiling an aneurysm, a routine follow-up angiogram is usually performed six to 12 months after the procedure to make sure the aneurysm remains blocked off. In some cases, particularly with larger aneurysms, further coil treatment may be needed to make sure the aneurysm is no longer at risk.