Perimesencephalic subarachnoid hemorrhage (PMSAH) is a distinct pattern of subarachnoid hemorrhage (SAH), which is centered on the basal cisterns around the midbrain.
There are two main categories based on the distribution of blood on the initial CT:
Blood is confined to the cisterns around the midbrain and pons, especially:
Interpeduncular cistern
Prepontine cistern
Ambient cistern
Minimal or no extension into lateral sylvian fissures or interhemispheric fissure
No intraventricular or intraparenchymal hemorrhage
No aneurysm on CTA/DSA
Blood starts in the perimesencephalic region but extends into:
Sylvian fissures
Interhemispheric fissure
Other cortical cisterns
May raise suspicion for aneurysm → usually leads to repeat angiography to exclude a missed aneurysm
Still, if angiography remains negative, often behaves like classic PNSAH
see Non-aneurysmal perimesencephalic subarachnoid hemorrhage.
van Gijn et al. studied 28 patients with subarachnoid hemorrhage and normal angiograms. On early CT (within 5 days) in 13 cases, blood was seen mainly or only in the cisterns around the midbrain. This pattern of hemorrhage was found in only 1 of 92 patients with a ruptured aneurysm. None of the unexplained perimesencephalic hemorrhages was associated with intracerebral hematoma or intraventricular hemorrhage. The clinical features also differed from those of aneurysmal hemorrhage; loss of consciousness was rare, and after 3 months, all 13 patients had returned to normal life. The cause of this benign disorder remains elusive, but a venous or capillary source seems likely 1)