Perimesencephalic subarachnoid haemorrhage

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


PMSAH is rare with an incidence of 0.5 in 100 000 in adults 4. PMSAH represents 5-10% of all subarachnoid haemorrhages and ~33% of all non-aneurysmal SAH 3, 4

Clinical presentation

As per subarachnoid haemorrhage



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. 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 1,2. Rare causes include arteriovenous malformation, dural arteriovenous fistula, trauma and vascular tumours 4

Radiographic features


PMSAH has been defined as subarachnoid haemorrhage, which on CT within three days of symptom onset 4:

  • is centred anteriorly to the pons and midbrain
  • may extend into the basal and suprasellar cisterns and into the proximal/basal Sylvian and interhemispheric fissures
  • may settle as sediment in the occipital horns of the lateral ventricles but there is no overt intraventricular haemorrhage

Specifically, there are criteria for non-aneurysmal perimesencephalic SAH, which if fulfilled, in the presence of a negative CTA negates the need for DSA 5:

  • 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 haemorrhage
  • no intraparenchymal haemorrhage

Treatment and prognosis

CTA is recommended for PMSAH to investigate for possible aneurysmal cause. Overall, PMSAH has an excellent prognosis with better outcomes compared to aneurysmal SAH 1, 2, 4

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