To determine the surgical indication of small unruptured intracranial aneurysm, a prospective study (SUAVe Study, Japan) is in progress at 16 national hospitals. These hospitals are observing the natural history of small unruptured intracranial aneurysms (under 5 mm diameter) without surgical treatment.
During 1 1/2 years from October 2000, 287 aneurysms (251 cases) with 49 aneurysms excluded by film judgment committees were registered. After registration, 153 aneurysms were followed 6 months and 75 were followed 12 months. These aneurysms were classified into the following 4 types. Type 1: The aneurysm ruptures within a few days to a few months after formation. Type 2: The aneurysm builds up slowly for several years after formation and ruptures in this process. Type 3: The formed aneurysm keeps growing slowly for many years without rupturing. Type 4: The aneurysm grows to a certain size, probably under 5 mm in diameter, and remains unchanged thereafter. The results were none in Type 1, 2 aneurysms (0.9%) in Type 2, 8 aneurysms (3.9%) in Type 3 and 218 aneurysms (95.2%) in Type 4. The annual rupture rate was 1.8%. Of the 2 ruptured aneurysms, both were multiple. Seven aneurysms were enlarged, 8 were multiple and 1 aneurysm was single. These findings show that the growth rate in the case of multiple aneurysms may be faster than in single aneurysms, and multiple aneurysms may more easily rupture than single aneurysms.