DEAR MAYO CLINIC: What causes a brain aneurysm? Are there any warning signs? How is a brain aneurysm treated?

ANSWER: A brain aneurysm is a small, round outpouching of an artery in the brain that develops due to thinning of the blood vessel wall. A brain aneurysm that bursts, or ruptures, can lead to serious medical problems. However, many aneurysms do not rupture, and some unruptured aneurysms do not require treatment.

Brain aneurysms are common. About 2 percent of the population has an unruptured brain aneurysm. That equals about 6 million people in the United States. But only about 25,000 brain aneurysms rupture each year.

Cigarette smoking and high blood pressure can increase a person’s risk for having a brain aneurysm. Genetics also seems to play a role. People who have two or more members of their family with a history of a brain aneurysm have an increased risk of developing a brain aneurysm.

Most unruptured brain aneurysms don’t cause symptoms. Rarely, an aneurysm will push on brain tissue or a nerve, causing pain, double vision, loss of vision, facial weakness or numbness. Typically, though, an unruptured brain aneurysm is discovered on a computerized tomography (CT) or a magnetic resonance imaging (MRI) brain scan that is done for some unrelated reason. A specialized brain CT called a CT angiogram (CTA) or a specialized brain MRI called an MR angiogram (MRA) can be used to look at the aneurysm in more detail.

When an unruptured aneurysm is found, the risk of the aneurysm rupturing without treatment should be weighed against the risks of treatment.

A brain aneurysm that ruptures leads to a serious type of stroke called a subarachnoid hemorrhage, which involves bleeding over the surface of the brain and into deeper parts of the brain. These strokes can quickly become life threatening if they’re not treated promptly.

When determining how to deal with an unruptured brain aneurysm, find a health care team that includes a neurologist, neuroradiologist and neurosurgeon who have expertise in aneurysms and their treatment. With the help of these experts, patients can carefully consider their options and decide what is best for them.

Robert D. Brown Jr., M.D., Neurology, Mayo Clinic, Rochester, Minn.