Size ratio can highly predict rupture risk in intracranial small (<5 mm) aneurysms.

BACKGROUND AND PURPOSE Management strategies for unruptured intracranial aneurysms (UIAs) are controversial. This study aimed to identify surrogate parameters that highly predict the rupture risk of small (<5 mm) UIAs. METHODS Radiological data were collected from 854 patients with aneurysmal subarachnoid hemorrhages who were enrolled in the Sapporo SAH Study. They had a total of 854 ruptured intracranial aneurysms and 180 UIAs. The size, aneurysm-to-vessel size ratio, and distribution were precisely compared between ruptured intracranial aneurysms and UIAs. RESULTS For all aneurysms, the size was significantly larger in ruptured intracranial aneurysms (7.0±1.3 mm) than in the UIAs (3.7±1.2 mm; P<0.001). Size ratio was also significantly higher in ruptured intracranial aneurysms (4.3±1.9) than in the UIAs (2.2±1.6; P<0.001). Multivariate logistic analysis showed that size and size ratio were correlated with aneurysm rupture. However, in small aneurysms, multivariate logistic regression revealed that only size ratio was associated with ruptured aneurysms (P=0.008; odds ratio, 9.1). There were no significant differences in size or aneurysm location. A receiver operating characteristic analysis was performed for size ratio in small aneurysms, and the threshold separating ruptured and unruptured groups was 3.12 and the area under the curve was 0.801. CONCLUSIONS This study revealed that the size ratio, and not the absolute size, may highly predict the risk of rupture in small UIAs. Size ratio measurements are very simple and provide useful information for determining treatment and follow-up strategies for patients with small UIAs.