The study enrolled 2,621 patients at 53 centers throughout the U.S., Canada, and Europe, and is the first large, multicenter study to use retrospective and prospective data to determine the natural course of UIAs in patients without a history of subarachnoid hemorrhage (group I) and patients with a history of subarachnoid hemorrhage (group II). Advances in imaging technology have made it possible to detect intracranial aneurysms before they rupture. Subjects in the NEJM study were diagnosed with UIAs through arteriography, an imaging technique that uses X-rays to photograph the arteries of the head made visible by a dye injected into the blood flowing through the artery. CT scans and MRI scans also can detect a brain aneurysm.

Dr. Wiebers and his colleagues found that for patients who have not had a previous subarachnoid hemorrhage, the risk of rupture for aneurysms smaller than 10 millimeters (mm) or about 1/3 of an inch in diameter was very low, less than 1/20 of 1 percent per year. For patients with a history of subarachnoid hemorrhage, the risk of rupture for the same sized aneurysm is about 11 times higher, about 1/2 of 1 percent per year. For aneurysms between 10 and 25 mm, the risk of rupture is still quite low, slightly less than 1 percent per year for both groups. The risk of rupture for giant aneurysms (greater than 25 mm in diameter) was about 6 percent for the first year for both groups. The exact position of the aneurysm in the brain was also a predictor of rupture in both groups.

Of course, the risk of rupture for an aneurysm must be compared to the overall risk of surgery to repair an aneurysm. For patients without a history of subarachnoid hemorrhage (
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Contact: Marcia Vital
301-496-5751
NIH/National Institute of Neurological Disorders and Stroke
9-Dec-1998