====== Perimesencephalic Subarachnoid Hemorrhage Etiology ======

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95% of cases of [[perimesencephalic subarachnoid hemorrhage]] have a normal [[cerebral angiography]] 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 subarachnoid hemorrhage]].

Rare causes include [[arteriovenous malformation]], [[dural arteriovenous fistula]], trauma and vascular tumours.

The causes of [[Perimesencephalic Subarachnoid Hemorrhage]] suggest a venous or capillary rupture at the level of the [[tentorial notch]]
((Schwartz TH, Yoon SS, Cutruzzola FW, Goodman RR. Third ventriculostomy: Post-operative ventricular size and outcome. Minim Invasive Neurosurg. 1996;39:122–9.))
((van Gijn J, van Dongen KJ, Vermeulen M, Hijdra A. Perimesencephalic hemorrhage: A nonaneurysmal and benign form of subarachnoid hemorrhage. Neurology. 1985;35:493–7.))
((Rinkel GJ, Wijdicks EF, Vermeulen M, Hageman LM, Tans JT, van Gijn J. Outcome in perimesencephalic (nonaneurysmal) subarachnoid hemorrhage: A follow-up study in 37 patients. Neurology. 1990;40:1130–2.))

The other 5% of cases are due to a vertebrobasilar aneurysm and the prognosis is worse 
((van der Schaaf IC, Velthuis BK, Gouw A, Rinkel GJ. Venous drainage in perimesencephalic hemorrhage. Stroke. 2004 Jul;35(7):1614-8. doi: 10.1161/01.STR.0000131657.08655.ce. Epub 2004 May 27. PMID: 15166390.))
((Velthuis B, Rinkel G, Ramos L et-al. Stroke. 1999;30 (5): . doi:10.1161/01.STR.30.5.1103))
Rare causes include arteriovenous malformation, dural arteriovenous fistula, trauma and vascular tumors 
((Marder CP, Narla V, Fink JR, Tozer Fink KR. Subarachnoid hemorrhage: beyond aneurysms. AJR Am J Roentgenol. 2014 Jan;202(1):25-37. doi: 10.2214/AJR.12.9749. PMID: 24370126.)).

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
((Hafez A, Numminen J, Rahul R, Järveläinen J, Niemelä M. Perimesencephalic
subarachnoid hemorrhage with a positive angiographic finding: case report and
review of the literature. Acta Neurochir (Wien). 2016 Jun;158(6):1045-9. doi:
10.1007/s00701-016-2801-9. Epub 2016 Apr 22. PubMed PMID: 27106848.
)).