The International Study of Unruptured Intracranial Aneurysm (ISUIA) was led by David O. Wiebers, M.D., a neurologist with the Mayo Clinic in Rochester, Minnesota, and included more than 4,000 patients at 61 sites in the United States, Canada, and Europe. Results of the study, which was sponsored by the National Institute of Neurological Disorders and Stroke (NINDS), appear in the July 12, 2003, edition of The Lancet¹.

"These results provide information to help in decision-making about how to treat unruptured brain aneurysms," says Dr. Wiebers. "Epidemiological data suggest that most aneurysms don't rupture, but we need to understand better which ones do and which ones don't. We now have more reliable risk information than we've had before about the risks of treatment and non-treatment options."

A cerebral aneurysm is a weak or thin spot on a vein or artery in the brain that balloons out under high arterial pressure and fills with blood. Researchers estimate that about 10 to 15 million people in the United States will develop an intracranial aneurysm during their lifetimes. When cerebral aneurysms rupture, they hemorrhage (bleed) into the spaces of the brain, causing strokes that often lead to death or permanent neurological damage. Many small brain aneurysms are discovered accidentally, during routine medical imaging or imaging following trauma. Treatment options for unruptured intracranial aneurysm include surgery to insert a metal clip that seals the aneurysm from the artery, or endovascular coiling, in which soft wire coils are implanted into the aneurysm to reduce its blood supply and induce formation of a blood clot. However, the relative risks and benefits of these procedures are still unclear.

The findings extend the results of an earlier study by Dr. Wiebers and colleagues that was published in 1998. That study, which looked retrospectively at patients who had been treated between 1970 and 1991, found that the size and location of an aneurysm in the brain and the patient's medical history are important predictors of future rupture.

Patients in the new study were identified by study coordinators at the participating centers after their aneurysms were diagnosed and treatment decisions were made. Among patients studied, 1,917 had surgery to repair the aneurysm, 451 underwent endovascular coiling, and 1,962 patients did not undergo surgical treatment. Patients were studied for up to 9 years, with an average of 4 to 5 years.

Results showed that aneurysm size, location within the brain, and whether the patient has had rupture of a previous aneurysm are significant factors in determining the risk that a newly diagnosed aneurysm will rupture. Patients with brain aneurysms smaller than 7 millimeters and with no history of a previous hemorrhage were at very low risk of rupture - only about one in a thousand of these aneurysms ruptured each year. Aneurysm size and location also affected the outcome of surgery: larger aneurysms and those located toward the back of the brain presented the highest surgical risk. Patients under age 50 had the best surgical outcome, while the risk of a poor surgical outcome increased significantly at age 50 and was greatest after age 60.

Patients with aneurysms should consult their physicians to decide what approach is best for their specific situation, Dr. Wiebers says. "There is no cookbook approach because there are many extenuating circumstances and individual factors to consider. There is no substitute for clinical judgment."

"These data are important and will allow us to better understand brain aneurysms and to identify treatment options for patients. While the study results provide much more information about aneurysms, we still need to know more about how patients in general will benefit from treatment, and what other factors make a difference," says John R. Marler, M.D., NINDS associate director for clinical trials.

The ISUIA investigators are continuing to study the same group of patients in order to obtain more information about the long-term results in treated and untreated patients.

The NINDS is a component of the National Institutes of Health within the Department of Health and Human Services and is the nation's primary supporter of biomedical research on the brain and nervous system.

A complete list of ISUIA study sites and investigators is available at http://mayoresearch.mayo.edu/mayo/research/ISUIA/.

¹International Study of Unruptured Intracranial Aneurysms (ISUIA) Investigators. Results of the International Study of Unruptured Intracranial Aneurysms. The Lancet, July 12, 2003, Vol. 362, No. 9378, pp. 103-110.