Background The management of unruptured intracranial aneurysms requires kno
wledge of the natural history of these lesions and the risks of repairing t
hem.
Methods A total of 2621 patients at 53 participating centers in the United
States, Canada, and Europe were enrolled in the study, which had retrospect
ive and prospective components. In the retrospective component, we assessed
the natural history of unruptured intracranial aneurysms in 1449 patients
with 1937 such aneurysms; 727 of the patients had no history of subarachnoi
d hemorrhage from a different aneurysm (group 1), and 722 had a history of
subarachnoid hemorrhage from a different aneurysm that had been repaired su
ccessfully (group 2). In the prospective component, we assessed treatment-r
elated morbidity and mortality in 1172 patients with newly diagnosed unrupt
ured intracranial aneurysms.
Results In group 1, the cumulative rate of rupture of aneurysms that were l
ess than 10 mm in diameter at diagnosis was less than 0.05 percent per year
, and in group 2, the rate was approximately 11 times as high (0.5 percent
per year). The rupture rate of aneurysms that were 10 mm or more in diamete
r was less than 1 percent per year in both groups, but in group 1, the rate
was 6 percent the first year for giant aneurysms (greater than or equal to
25 mm in diameter). The size and location of the aneurysm were independent
predictors of rupture. The overall rate of surgery-related morbidity and m
ortality was 17.5 percent in group 1 and 13.6 percent in group 2 at 30 days
and was 15.7 percent and 13.1 percent, respectively, at 1 year. Age indepe
ndently predicted surgical outcome. Conclusions The likelihood of rupture o
f unruptured intracranial aneurysms that were less than 10 mm in diameter w
as exceedingly low among patients in group 1 and was substantially higher a
mong those in group 2. The risk of morbidity and mortality related to surge
ry greatly exceeded the 7.5-year risk of rupture among patients in group 1
with unruptured intracranial aneurysms smaller than 10 mm in diameter. (N E
ngl J Med 1998;339:1725-33.) (C) 1998, Massachusetts Medical Society.