Unruptured brain aneurysmOpen print version


An aneurysm is a balloon-like bulge of an artery wall. As an aneurysm grows it puts pressure on nearby structures and may eventually rupture. Most people find out they have an unruptured aneurysm by chance during a scan for some other problem. The risk of rupture varies depending on the aneurysm location and size. Treatment options include observation, clipping, coiling, flow diversion, or bypass.

Blood supply of the brain

To understand aneurysms, it is helpful to know how blood circulates to the brain (see Anatomy of the Brain). Blood is pumped from the heart and carried to the brain by two paired arteries, the internal carotid arteries and the vertebral arteries (Fig. 1). The internal carotid arteries supply the anterior (front) areas and the vertebral arteries supply the posterior (back) areas of the brain. After passing through the skull, the right and left vertebral arteries join together to form a single basilar artery. The basilar artery and the internal carotid arteries communicate with each other in a ring at the base of the brain called the Circle of Willis.

Blood supply of the brain
Figure 1. The internal carotid arteries form the anterior (green) circulation and the vertebral / basilar arteries supply the posterior (red) circulation of the brain. About 80% of aneurysms arise in the anterior system; 20% form in the posterior. Most aneurysms arise on the Circle of Willis.

Frequent aneurysm locations

Internal carotid artery
Middle cerebral artery
Anterior cerebral artery
Basilar artery
Vertebral basilar
Posterior communicating artery
Cavernous carotid artery



What is an unruptured brain aneurysm?

An aneurysm is a balloon-like bulge or weakening of an artery wall. (Similar to a balloon on the side of a garden hose.) As the bulge grows it becomes thinner and weaker. It can become so thin that the blood pressure within can cause it to leak or burst open — a life-threatening hemorrhage in the brain. Aneurysms usually occur on larger blood vessels at the fork where an artery branches off. Types of aneurysms include (Fig. 2):

Saccular - (most common, also called "berry") the aneurysm bulges from one side of the artery and has a distinct neck at its base.

Fusiform - the aneurysm bulges in all directions and has no distinct neck.

Dissecting -a tear in the inner wall of the artery allows blood to split the layers and pool; often caused by a traumatic injury.

aneurysm types
Figure 2. A saccular aneurysm arises at the weak point of the artery where it branches. Other aneurysm types include dissecting and fusiform.

Aneurysms are also classified by size:

  • Small = less than 7 millimeters in diameter
  • Medium = 7-12 millimeters
  • Large = 13-24 millimeters (size of a dime)
  • Giant = more than 25 millimeters (quarter size)

The risk of aneurysm rupture is about 1% but may be higher or lower depending on the size and location of the aneurysm. Generally, the larger the aneurysm (>12mm), the higher risk of rupture. Also, aneurysms in the posterior circulation (basilar, vertebral and posterior communicating arteries) have a higher risk of rupture.

Risk factors for rupture include smoking, high blood pressure, drug or alcohol abuse, atherosclerosis, and genetic factors (1, 2). Lifestyle changes can reduce your risk of rupture.

What are the symptoms?

Most aneurysms don't have symptoms until they rupture. Ruptured aneurysms release blood into the spaces around the brain called a subarachnoid hemorrhage (SAH). Unruptured aneurysms rarely show symptoms until they grow large or press on the brain or nerves. Rupture usually occurs while a person is active rather than asleep. If you experience the symptoms of a SAH, call 911 immediately!

Symptoms of an unruptured aneurysm:

  • Double vision
  • Dilated pupils
  • Pain above and behind the eye
  • Newly unexplained headaches (rare)

Symptoms of a ruptured aneurysm or subarachnoid hemorrhage (SAH):

  • sudden onset of a severe headache (often described as "the worst headache of my life")
  • nausea and vomiting
  • stiff neck
  • transient loss of vision or consciousness

What are the causes?

Studies have shown a strong link to family history (2). If an immediate family member has suffered an aneurysm, you are 4 times more likely to have one as well. The genetic link is not completely understood and studies are underway to determine if there is a pattern of inheritance. The most important inherited conditions associated with aneurysms include Ehlers-Danlos IV, Marfan’s syndrome, neurofibromatosis NF1, and polycystic kidney disease. For those with a strong family history, we recommend a screening test (CT or MR angiogram).

Who is affected?

About 3% of the population may have or develop an aneurysm; of those, 15% have multiple aneurysms. Unruptured aneurysms are more common than ruptured (1). However, 85% of aneurysms are not diagnosed until after they bleed. Aneurysms are usually diagnosed between ages 35 to 60 and are more common in women.

How is a diagnosis made?

Most people find out they have an unruptured aneurysm by chance (incidental) during a scan for some other medical problem. If you are experiencing symptoms and your primary care doctor suspects an aneurysm, you may be referred to a neurosurgeon. The surgeon will learn as much about your symptoms, current and previous medical problems, current medications, family history, and perform a physical exam. Diagnostic tests are used to help determine the aneurysm's location, size, type, and involvement with other structures.

  • Computed Tomography Angiography (CTA) scan is a noninvasive X-ray to review the anatomical structures within the brain to detect blood in or around the brain. A newer technology called CT angiography involves the injection of contrast into the blood stream to view the arteries of the brain. This type of test provides the best pictures of blood vessels through angiography and soft tissues through CT.

  • Angiogram is an invasive procedure, where a catheter is inserted into an artery and passed through the blood vessels to the brain. Once the catheter is in place, a contrast dye is injected into the bloodstream and the x-ray images are taken.

  • Magnetic resonance imaging (MRI) scan is a noninvasive test, which uses a magnetic field and radio-frequency waves to give a detailed view of the soft tissues of your brain. An MRA (Magnetic Resonance Angiogram) is the same non-invasive study, except it is also an angiogram, which means it also examines the blood vessels, as well as the structures of the brain.

Should the aneurysm be treated?

Deciding how, or even if, to treat an unruptured aneurysm involves weighing the risks of rupture versus the risks of treatment. The risk of aneurysm rupture is about 1% but may be higher or lower depending on the size and location of the aneurysm; however, when a rupture occurs there is a 50% risk of death. Risk factors for rupture include smoking, high blood pressure, alcohol, genetic factors (family inherited), atherosclerosis (hardening of the arteries), oral contraceptives, and lifestyle (3). Other factors such as the size and location of the aneurysm, overall health of the patient, and medical history must also be considered. Generally, the larger the aneurysm, the higher risk of rupture. Also, aneurysms in the posterior circulation (basilar, vertebral and posterior communicating arteries) have a higher risk of rupture. The neurosurgeon will discuss with you all the options and recommend a treatment that is best for your individual case.

What treatments are available?

Sometimes the best treatment may be to simply watch and reduce your risk of rupture (quit smoking, control high blood pressure). Aneurysms that are small, unruptured, and asymptomatic may be observed with imaging scans every year until the growth or symptoms necessitate surgery. Observation may be the best option for patients with other health conditions.

Surgical clipping
The most common treatment for an aneurysm is direct surgical clipping. Using general anesthesia, an opening is made in the skull, called a craniotomy. The brain is gently retracted so that the artery with the aneurysm may be located. A small clip is placed across the neck of the aneurysm to block the normal blood flow from entering the aneurysm (Fig. 3). The clip is made of titanium and remains on the artery permanently.

Figure 3: A titanium clip is placed across the neck of an aneurysm so that blood flows through the artery, but not the aneurysm.

Artery occlusion and bypass
If surgical clipping is not possible or the artery is too damaged, the surgeon may completely block (occlude) the artery that has the aneurysm. The blood flow is detoured (bypassed) around the occluded section of artery by inserting a vessel graft. The graft is a small artery, usually taken from your leg, that is connected above and below the blocked artery so that blood flow is rerouted (bypassed) through the graft.

A bypass can also be created by detaching a donor artery from its normal position on one end, redirecting it to the inside of the skull, and connecting it above the blocked artery. This is called a STA-MCA (superficial temporal artery to middle cerebral artery) bypass.

Endovascular coiling
In contrast to surgery, another form of treatment is endovascular coiling. This is performed in the angiography suites of the radiology department by a Neuro Interventionalist and sometimes requires general anesthesia. In a coiling procedure, a catheter is inserted into an artery in the groin and then passed through the blood vessels to the aneurysm. The doctor guides the catheter through the bloodstream while watching a fluoroscopy (a type of x-ray) monitor. Through the catheter, the aneurysm is packed with material, either platinum coils or balloons, that prevents blood flow into the aneurysm (Fig. 4). Since coiling is a relatively new procedure, follow-up angiograms are performed periodically to confirm the aneurysm is still occluded and not growing larger.

Figure 4. The aneurysm is packed with platinum coils. The coils induce clotting (embolization), which seals off the space and prevents blood from entering the aneurysm.

Endovascular flow diversion
If clipping or coiling would be difficult due to the shape, size, or wide neck of the aneurysm, a flow diversion stent may be used. A flow-diverter is a tightly woven mesh tube placed inside the parent artery across the aneurysm (Fig. 5). Because blood cannot easily get through the spaces of the tight mesh stent, the blood flows inside the flow-diverter and continues down the artery without going into the aneurysm. Without the pulsating blood flow, the aneurysm will eventually clot off and shrink. Recovery time typically is two to four days.

Figure 5. A flow-diverter mesh stent is placed inside the artery to reduce blood flow from entering the aneurysm. The aneurysm will eventually clot off and shrink.

Clinical trials

Clinical trials are research studies in which new treatments—drugs, diagnostics, procedures, and other therapies—are tested in people to see if they are safe and effective. Research is always being conducted to improve the standard of medical care. Information about current clinical trials, including eligibility, protocol, and locations, are found on the Web. Studies can be sponsored by the National Institutes of Health (see clinicaltrials.gov) as well as private industry and pharmaceutical companies (see www.centerwatch.com).

Recovery & prevention

Unruptured aneurysm patients recover from surgery or endovascular treatment much faster than those who suffer a SAH. The possibility of having a second bleed is 35% within the first 14 days after the first bleed. This is why neurosurgeons prefer to do direct surgical or endovascular treatment as soon as the aneurysm is diagnosed, so that the risk of a rebleed is lessened.

Aneurysm patients may suffer short-term and/or long-term deficits as a result of a treatment or rupture. Some of these deficits may disappear over time with healing and therapy.

Sources & links

If you have more questions, please contact Mayfield Brain & Spine at 800-325-7787 or 513-221-1100.


  1. Wiebers DO: Unruptured intracranial aneurysms – risk of rupture and risks of surgical intervention. N Engl J Med 339:1725-33, 1998.
  2. Leblanc R: Familial Cerebral Aneurysms. Canadian Journal of Neurological Sciences 24: 191-199, 1997.
  3. Juvela S, Porras M, Poussa K: Natural History of Unruptured Intracranial Aneurysms: Probability and Risk Factors for Aneurysm Rupture. Neurosurgical Focus 8: 2000.

National Brain Aneurysm Foundation
Tri-State Brain Aneurysm Support Group


aneurysm: a bulge or weakening of an arterial wall.

coiling: a procedure to insert platinum coils into an aneurysm; performed during an angiogram.

craniotomy: surgical opening in the skull.

Ehlers-Danlos IV: a genetic disorder of the connective tissue in the intestines, arteries, uterus, and other hollow organs may be unusually weak, leading to organ or blood vessel rupture.

embolization: inserting material, coil or glue, into an aneurysm so blood can no longer flow through it.

Marfan’s syndrome: a genetic disorder in which patients develop skeletal defects in long bones, chest abnormalities, curvature of the spine, and circulatory defects.

neurofibromatosis (NF1): a genetic disorder, also called von Recklinghausen disease, in which patients develop café-au-lait spots, freckling, and multiple soft tumors under the skin and throughout the nervous system.

polycystic kidney disease: a genetic disorder in which patients develop multiple cysts on the kidneys; associated with aneurysms of blood vessels in the brain.

subarachnoid hemorrhage (SAH): bleeding in the space between the brain and skull; may cause a stroke.

updated > 3.2018
reviewed by > Andrew Ringer, MD and Lincoln Jimenez, MD, Mayfield Clinic

Mayfield Certified Health InfoMayfield Certified Health Info materials are written and developed by the Mayfield Clinic. We comply with the HONcode standard for trustworthy health information. This information is not intended to replace the medical advice of your health care provider.

Mayfield services

Each year Mayfield Brain & Spine treats nearly 400 people with life-threatening unruptured brain aneurysms.

Through our affiliation with premier hospital systems in the Greater Cincinnati-Northern Kentucky region, Mayfield doctors care for people who have brain aneurysms, brain hemorrhages, blocked carotid arteries, moyamoya, vasospasm, and related diseases.

Our neurosurgeons work collaboratively with neurologists, neuroradiologists, and neurointensivists to bring you the very best care available.

To make an appointment call 513-221-1100.


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