The natural history of unruptured cerebral aneurysms has not been clearly defined.
From January 2001 through April 2004, we enrolled patients with newly identified,
unruptured cerebral aneurysms in Japan. Information on the rupture of aneurysms, deaths,
and the results of periodic follow-up examinations were recorded. We included 5720
patients 20 years of age or older (mean age, 62.5 years; 68% women) who had saccular
aneurysms that were 3 mm or more in the largest dimension and who initially presented
with no more than a slight disability.
Of the 6697 aneurysms studied, 91% were discovered incidentally. Most aneurysms were
in the middle cerebral arteries (36%) and the internal carotid arteries (34%). The
mean (±SD) size of the aneurysms was 5.7±3.6 mm. During a follow-up period that included
11,660 aneurysm-years, ruptures were documented in 111 patients, with an annual rate
of rupture of 0.95% (95% confidence interval [CI], 0.79 to 1.15). The risk of rupture
increased with increasing size of the aneurysm. With aneurysms that were 3 to 4 mm
in size as the reference, the hazard ratios for size categories were as follows: 5
to 6 mm, 1.13 (95% CI, 0.58 to 2.22); 7 to 9 mm, 3.35 (95% CI, 1.87 to 6.00); 10 to
24 mm, 9.09 (95% CI, 5.25 to 15.74); and 25 mm or larger, 76.26 (95% CI, 32.76 to
177.54). As compared with aneurysms in the middle cerebral arteries, those in the
posterior and anterior communicating arteries were more likely to rupture (hazard
ratio, 1.90 [95% CI, 1.12 to 3.21] and 2.02 [95% CI, 1.13 to 3.58], respectively).
Aneurysms with a daughter sac (an irregular protrusion of the wall of the aneurysm)
were also more likely to rupture (hazard ratio, 1.63; 95% CI, 1.08 to 2.48).
This study showed that the natural course of unruptured cerebral aneurysms varies
according to the size, location, and shape of the aneurysm. (Funded by the Ministry
of Health, Labor, and Welfare in Japan and others; UCAS Japan UMIN-CTR number, C000000418.).