Introduction Perimesencephalic subarachnoid hemorrhage (PMSAH) is characterized by bleeding centered in the basal cisterns anterior to the midbrain and pons without intraparenchymal or overt intraventricular extension. The term ‘benign’ is often attached, because typically no source of bleeding is identified on high-resolution vascular imaging, and recovery is often uncomplicated. Rarely however, PMSAH can be secondary to ruptured vertebrobasilar aneurysms and outcomes in these patients is underreported.

Methods Retrospective analysis of patients with PMSAH to determine the rate of underlying ruptured aneurysm or vascular abnormality, associated complications, and outcomes. Age, sex, vascular risk factors, presenting symptoms, Hunt and Hess grade, modified Fisher grade, rate of underlying ruptured aneurysm, vasospasm, re-bleed, hydrocephalus, venous drainage pattern, and modified Rankin scale (mRS) were collected. Primary outcome was good functional status at discharge (mRS 0-2), reported as odds ratio (OR) with 95% confidence interval (CI).

Results A total of 81 patients with PMSAH from 2007-2022 were identified. Mean age was 55.5 ± 10.3 years and 62% were male. Hypertension and smoking were reported in 55% and 35% of patients, respectively. The most common presenting symptom was thunderclap headache in 89% of patients. Median (IQR) of Hunt and Hess grade was 2 (1-2), and modified Fisher grade was 3 (1-3). An underlying ruptured aneurysm was found in 5 patients (6.2%); four of which were in the vertebrobasilar system and one in the posterior communicating artery. Most common complications in this cohort was vasospasm in 30%, followed by hydrocephalus in 10%. Among patients with aneurysmal bleed, vasospasm and hydrocephalus occurred in 20% and 40% of patients respectively, compared to 30% and 8%, respectively in patients with non-aneurysmal PMSAH. Re-bleeding occurred only in one patient (1.2%); which occurred in a patient with non-aneurysmal source of hemorrhage. A total of 88% of patients in our cohort had a favorable functional outcome (mRS 0-2) at discharge. An underlying ruptured aneurysm and acute hydrocephalus were associated with poor functional status (OR= 14.7, [2.1-104], P=0.007), and OR= 22.6, [4.2-123.5], P <0.001), respectively. However, vasospasm was noted to be asymptomatic in most cases (90%) and was not associated with unfavorable outcomes (OR= 0.55, [0.11-2.8], P=0.48).

Conclusion ‘Benign’ PMSAH pattern was associated with a ruptured aneurysm in 6.2% of patients in our cohort. An underlying aneurysm and acute hydrocephalus were associated with poor outcomes. However, vasospasm was not associated with unfavorable outcomes in patients with PMSAH.

Disclosures A. Alrohimi: None. M. Davison: None. A. Pandhi: None. A. Abdulrazzak: None. M. Bain: None. N. Moore: None. D. Wadden: None. J. Tsai: None. P. Rasmussen: None. M. Hussain: None. G. Toth: None.