[True "PICA communicating artery" aneurysm: a case report].

We present an unusual case of an aneurysm of the distal posterior inferior cerebellar artery (PICA). A 51-year-old female presented a subarachnoid hemorrhage with mild consciousness disturbance on August 6, 1992. Computed tomography (CT) on admission showed subarachnoid hemorrhage with thick hematoma in the cisterna magna and intraventricular hematoma in the 4th, 3rd and both lateral ventricles. The angiogram on admission revealed no definite vascular anomalies. Repeated angiograms on the 11th day after onset showed an aneurysm on anastomotic branch between the bilateral distal PICAs. The aneurysm was clipped successfully through a suboccipital craniectomy 14 days after the onset. In the literature reviewed, only one such aneurysm, located at an anastomotic vessel of the bilateral PICAs, has been reported by Hlavin et al in 1991. They reported that the aneurysm was associated with a unilateral PICA that supplied both cerebellar hemispheres and arose from an anastomotic vessel in the contralateral circulation. They called the aneurysm as "a PICA communicating artery" aneurysm. We assume that this "PICA communicating artery" is a remnant of a primitive lateral vertebrobasilar anastomosis, which appears in the embryo at the 9 mm stage. It is suggested that the pathogenesis may be not only the hemodynamic factor but also a congenital anomaly.