September 25 (HealthDay News) -- The risk of a brain aneurysm rupturing over time depends on the location and size of the aneurysm, according to a new Mayo Clinic study.

The international study found that people with aneurysms in the back of the brain had a slightly higher risk of rupture than those people with aneurysms in the front of the brain. Those with aneurysms greater than 13 millimeters in diameter were at least twice as likely to have them rupture, compared to those whose aneurysms were 7 to 12 millimeters in diameter.

The study was to be presented Sept. 25 at the World Stroke Congress meeting in Vienna, Austria.

"When one compares the risk of rupture determined in this study to the risk of treatment for an aneurysm, it appears that these risks are similar for small aneurysms less than 10 to 12 millimeters in size. It is unclear whether these aneurysms need to be treated in all patients, and this will be clarified with further research," study lead investigator Dr. Robert Brown, a Mayo Clinic neurologist, said in a news release issued by the clinic.

The findings are based on a long-term study of more than 4,000 North American and European patients with an unruptured brain aneurysm, an abnormal sac or tiny balloon on a blood vessel to the brain. Aneurysms can rupture and bleed into the area between the brain and the surrounding membrane, leading to stroke and death. An estimated 6 million people, or 2 percent of all Americans, have brain aneurysms, and about 25,000 ruptures occur every year, according to the Mayo Clinic.

SOURCE: Mayo Clinic, news release, Sept. 25, 2008