Perimesencephalic subarachnoid hemorrhage

Case contributed by Ahmed Abdelrahman


Severe headache, which he described as "thunderclap headache".

Patient Data

Age: 35 years
Gender: Male

Non-contrast study was done which revealed a small amount of hyperdense smearing of the CSF space anterior to the mid brain (perimesencephalic) which represents perimesencephalic subarachnoid hemorrhage (PMSAH). No intra-axial hemorrhage. No hydrocephalus or mass effect.

CT angiography was done at the same day to rule out any underlying vascular cause of this SAH. It was negative for any aneurysm or vascular malformations

The follow-up non-contrast CT study was done after one month, which showed complete resolution of the perimesencephalic SAH after conservative treatment and observation.

Case Discussion

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.

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