Perimesencephalic subarachnoid hemorrhage
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Perimesencephalic subarachnoid hemorrhage is rare with an incidence of 0.5 in 100 000 in adults 4. PMSAH represents 5-10% of all subarachnoid hemorrhages and ~33% of all non-aneurysmal SAH 3,4.
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.
Perimesencephalic subarachnoid hemorrhage has been defined as subarachnoid hemorrhage on CT within three days of symptom onset 4:
- is centered anteriorly to the pons and midbrain
- may extend into the basal and suprasellar cisterns and into the proximal/basal Sylvian fissure and interhemispheric fissure
- may settle as sediment in the occipital horns of the lateral ventricles but there is no overt intraventricular hemorrhage
There are specific criteria for non-aneurysmal perimesencephalic SAH, which, some have argued, negate the need for DSA if fulfilled with a negative CTA 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 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 1,2,4.