Importance of prospective studies for deciding on a therapeutic guideline for unruptured cerebral aneurysm

(1) In a town of 100,000 population, a 2% incidence rate of unruptured cerebral aneurysm means 2,000 patients. (2) In a town of 100,000 population, the annual occurrence of subarachnoid hemorrhage is 20. (3) The turnover of patients with unruptured cerebral aneurysm is for the 50-year cycle of patients at age from 30 to 80. On the basis of these data, in a town of 100,000 population the number of unruptured cerebral aneurysm cases and of subarachnoid hemorrhage cases occurring in 50 years are 2,000 and 1,000, respectively. In total 3,000 patients with unruptured and ruptured cerebral aneurysm, the distribution of these patients for different sizes of aneurysm can be estimated. We examined the rupture rate for each size of aneurysm. And we found that some of the aneurysm smaller than 10 mm in size rupture soon after their formation and that some of aneurysm of size remain unchanged.