Filling of all perimesencephalic cisterns with blood is a necessary factor for the development of acute hydrocephalus 1).

The patients with PM SAH experienced a benign course, whereas those with n-PM SAH showed a higher risk of vasospasm and hydrocephalus as well as worse exit scores 2).

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 3)

1)
Rinkel GJ, Wijdicks EF, Vermeulen M, Tans JT, Hasan D, van Gijn J. Acute hydrocephalus in nonaneurysmal perimesencephalic hemorrhage: evidence of CSF block at the tentorial hiatus. Neurology. 1992 Sep;42(9):1805-7. doi: 10.1212/wnl.42.9.1805. PMID: 1513471.
2)
Arslan A, Acik V, Bilgin E, Afser KA, Cavus G, Gezercan Y, Okten AI. Clinical comparison of patients with angiography: Negative perimesencephalic and nonperimesencephalic subarachnoid hemorrhages. Niger J Clin Pract. 2021 Apr;24(4):595-599. doi: 10.4103/njcp.njcp_647_18. PMID: 33851683.
3)
Konczalla J, Kashefiolasl S, Brawanski N, Senft C, Seifert V, Platz J. Increasing numbers of nonaneurysmal subarachnoid hemorrhage in the last 15 years: antithrombotic medication as reason and prognostic factor? J Neurosurg. 2015 Nov 13:1-7. [Epub ahead of print] PubMed PMID: 26566212.