Perimesencephalic subarachnoid hemorrhage (PMSAH) is defined as presence of extra axial blood in the basal cisterns anterior to the midbrain. It may also extend to the prepontine cistern, and then it is called pretruncal subarachnoid hemorrhage.

PMSAH is quite rare, representing about 5% of all SAH with incidence of 0.5 cases per 100,000 person’s age.  

The majority of PMSAH are non-aneurysmal with only small portion caused by aneurysm rupture. Patients with PMSAH tend to be younger and less hypertensive as compared to those with aneurysmal SAH.

The exact etiology of the non-aneurysmal type is not well known, however, some studies suggest venous or capillary rupture at the level of the tentorial hiatus as a source for the bleed.

The clinical presentation of non-aneurysmal PMSAH is that of aneurysmal SAH, with sudden onset of headache, meningeal irritation, photophobia, nausea, and vomiting.

The outcome of non-aneurysmal PMSAH is very good compared to the aneurysmal causes of SAH with minimal risk of rebleed. So, once aneurysms are excluded by CT angiography, no need to repeat CTA angiography later.