Subarachnoid hemorrhage caused by ruptured posterior inferior cerebellar artery aneurysm arising from the extracranial portion of an anomalous vertebral artery.

A 42-year-old woman presented with subarachnoid hemorrhage caused by a ruptured posterior inferior cerebellar artery aneurysm arising from the extracranial portion of an anomalous vertebral artery. We speculated that the aneurysm formed by dissection of the media because of its shape and location. Therefore, the aneurysm was isolated by trapping and excised, with occipital artery to posterior inferior cerebellar artery bypass. Histological examination showed a typical true aneurysm without evidence of dissection. This case suggests that an aneurysm of an anomalous artery is caused by hemodynamic stress and is a true aneurysm, not a dissection.