[Ruptured aneurysm of distal posterior inferior cerebellar artery located at caudal loop].

We report an unusual case of subarachnoid hemorrhage (SAH) due to ruptured aneurysm originating at the caudal loop of posterior inferior cerebellar artery (PICA). The patient was a 77-year-old female presented with a sudden onset of headache and vomiting. Initial CT scan demonstrated a SAH with thick hematoma mainly in the cisterna magna. Vertebral angiogram revealed a saccular aneurysm arising from the tonsillomedullary segment of the left PICA, and communicating artery with supplying a territory of contralateral vermis as an anastmotic vessel. At surgery, no vessel branches were confirmed in the vicinity of the aneurysm, and this aneurysm was successfully clipped. In the fetus, numerous basilar and vertebral arteries are organized in plexiform formations around the brain stem. It has been suggested that the pathogenesis of such aneurysm or communicating artery could be related with a remnant of a primitive vertebrobasilar anastomosis. Based on these considerations, congenital vessel-wall weakness and hemodynamic stress associated with communicating artery may contribute to the development of distal PICA aneurysms.