抄録
Based on many reported studies on patients with subarachnoid hemorrhage and unruptured cerebral aneurysm, I tried to calculate the incidence rate of cerebral aneurysm rupture. (1) Unruptured cerebral aneurysm exists in about 2% of the total population. (2) The annual occurrence of subarachnoid hemorrhage is 20 cases in a town of 100,000 population. (3) The turnover of patients with unruptured cerebral aneurysm is for the 50-year cycle of patients with cerebral aneurysm in ages from 30 to 80. (4) The size distribution of unruptured and ruptured cerebral aneurysm is already known.
In a town of 100,000 population, the number of unruptured cerebral aneurysm cases and that of subarachnoid hemorrhage cases occurring in 50 years are 2,000 and 1,000, respectively. This finding shows that the rupture rate is 0.67% per year.
The distribution of patients for different sizes of aneurysm can be estimated for a total of 3,000 patients with unruptured and ruptured cerebral aneurysm.
On the other hand, the growth process of cerebral aneurysm from its occurrence may be classified into 4 patterns as the follows. Type I : The aneurysm ruptures within a span of time as short as a few weeks to a few months after formation. Type II : The aneurysm builds up slowly for a few years after formation and ruptures in this process. Type III : The aneurysm keeps growing slowly for a long time without rupture. Type IV : The aneurysm grows to a certain size, within 10 mm in diameter, and remains unchanged thereafter.
Aneurysms larger than 10 mm mostly grow slowly. Among patients in this category, the aneurysm ruptures in 48.9% of 10-14 mm size group, 21.7% of 15-25 mm, and in 9.1% of 25 mm or larger size. Some aneurysms smaller than 10 mm rupture soon after formation (Type I), and the others remain unchanged (Type IV). Particularly, a considerable proportion of 5-9 mm aneurysms rupture soon or immediately after formation, while aneurysms smaller than 5 mm mostly remain unchanged.
