[Unusual distribution of blood in a ruptured aneurysm--case report].

Computed tomography (CT) is an important tool for the evaluation of subarachnoid hemorrhages caused by ruptured aneurysms. However, an unusual radiologic presentation of blood can result in the misdiagnosis of the rupture site. We report a case of ruptured right internal carotid-posterior communicating (IC-PC) artery aneurysm with an unusual radiologic presentation of hemorrhage. The CT scans images of this patient were indicative of blood clots in the cisterna magna and the ventricles with no blood collection in the suprasellar cistern or sylvian fissure. The intraventricular clots were mainly distributed in the fourth ventricle, with few clots in the lateral ventricles. On the basis of these findings, we suspected the origin of the hemorrhage was an unknown ruptured posterior fossa aneurysm. However, angiography revealed a right internal carotid-posterior communicating (IC-PC) artery aneurysm. Twenty-three days after onset, a repeat CT confirmed that the origin of the hemorrhage was the right IC-PC aneurysm. Possible factors involved in this case were: (1) formation of adhesions in the subarachnoid cisterns due to an earlier minor leakage, (2) the orientation of the aneurysm (posteroinferior direction), and (3) early wash out of the blood clot.