Perimesencephalic hemorrhage is a distinct type of subarachnoid hemorrhage confined to the basal cisterns around the midbrain (typically the interpeduncular and prepontine cisterns). 95% of cases have a normal cerebral angiogram and the source of bleeding is not identified; the cause is thought to be a venous bleed. In these cases rebleeding and ischemia are rare and the prognosis is excellent. The other 5% of cases are due to a vertebrobasilar aneurysm and the prognosis is worse.

Credit: Dr Donna D'Souza.