Perimesencephalic subarachnoid hemorrhage

Perimesencephalic subarachnoid hemorrhage (PMSAH) is a distinct pattern of subarachnoid hemorrhage (SAH), which is centered on the basal cisterns around the midbrain.


Perimesencephalic subarachnoid hemorrhage is rare with an incidence of 0.5 in 100 000 in adults . PMSAH represents 5-10% of all subarachnoid hemorrhages and ~33% of all non-aneurysmal SAH .

Clinical presentation

As per subarachnoid hemorrhage.



95% of cases of perimesencephalic subarachnoid hemorrhage have a normal cerebral angiogram and the source of bleeding is not identified; the cause is thought to be a venous bleed. This is referred to as non-aneurysmal perimesencephalic SAH.

The other 5% of cases are due to a vertebrobasilar aneurysm and the prognosis is worse . Rare causes include arteriovenous malformation, dural arteriovenous fistula, trauma and vascular tumors .

Radiographic features


Perimesencephalic subarachnoid hemorrhage has been defined as subarachnoid hemorrhage, which on CT within three days of symptom onset :

Specifically, there are criteria for non-aneurysmal perimesencephalic SAH, which if fulfilled, in the presence of a negative CTA negates the need for DSA :

  • SAH in perimesencephalic cisterns anterior to midbrain
  • if SAH extension into the anterior interhemispheric fissure, not extending into all of the fissure
  • if SAH extension into the medial Sylvian fissures, not extending into the lateral fissure
  • if layering interventricular extension, no frank intraventricular hemorrhage
  • no intraparenchymal hemorrhage

Treatment and prognosis

CTA is recommended for perimesencephalic subarachnoid hemorrhage to investigate for possible aneurysmal cause. Overall, PMSAH has an excellent prognosis with better outcomes compared to aneurysmal SAH .

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