PMSAH is rare with an incidence of 0.5 in 100 000 in adults.
PMSAH represents 5-10% of all subarachnoid haemorrhages and ~33% of all non-aneurysmal SAH.
The vast majority of perimesencephalic subarachnoid hemorrhage cases are reported as negative-finding etiologies. Recently, high-resolution images allowed us to overcome the previous difficulty of finding the source of bleeding, which underlies the concept of a “negative finding”.
A venous etiology, hidden behind the tip of the basilar artery; namely, the lateral pontine vein.
Hafez et al highlight this type of aneurysm as a candidate source of perimesencephalic hemorrhage. This case may change our way of dealing with what we have termed a negative finding of subarachnoid hemorrhage 1).
As per subarachnoid hemorrhage.
95% of cases have a normal cerebral angiography and the source of bleeding is not identified; the cause is thought to be a venous bleed. 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 tumours.
PMSAH has been defined as subarachnoid haemorrhage, which on CT within three days of symptom onset:
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 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.
Perimesencephalic subarachnoid hemorrhage is linked to a benign prognosis compared with aneurysmal SAH.
Elderly patients, and especially the subgroup with a Fisher Scale 3 bleeding pattern, had a high risk for an unfavorable outcome, whereas the subgroup of Non perimesencephalic subarachnoid hemorrhage NPM-SAH without a Fisher Grade 3 bleeding pattern had a favorable outcome, similar to perimesencephalic subarachnoid hemorrhage (PM)-SAH 2)