Object: Knowing the rate of rupture associated with unruptured cerebral aneurysms (UCAs) can help surgeons determine a case management strategy in patients harboring these lesions. According to large-scale cohort studies involving populations in North America and Europe, small unruptured aneurysms carry a very low risk of rupture. In Japan, however, there have been sporadic reports of higher rates of rupture. To identify the rupture risk associated with UCAs in the Japanese population, the authors systematically reviewed retrospective studies of the natural course of these lesions.
Methods: The authors searched Medline and the Japan Medical Abstract Society Index for reports of UCAs in Japan. Two of the authors verified the eligibility of the reports and extracted data independently. Additional information was directly obtained from the authors of the original reports. Thirteen reports covering a total of 3801 patient-years fulfilled the criteria for our study. Subsequent rupture was documented in 104 patients and the annual rupture rate was 2.7% (95% confidence interval 2.2-3.3%). Large, posterior-circulation, and symptomatic aneurysms were associated with significantly higher rates of rupture (relative risks 6.4, 2.3, and 2.1, respectively). The risk of rupture determined by the authors' review was significantly higher than that reported by investigators from international cohort studies.
Conclusions: Although a selection bias of patients may be the cause of the higher rupture risk, untreated UCAs that have been followed in Japanese institutions have a considerably high rate of rupture. The natural course of UCAs should be carefully estimated in countries not included in the international studies.