BACKGROUNDNonaneurysmal subarachnoid hemorrhage (SAH) has been historically associated with a benign clinical course. However, recent studies have suggested that nonaneurysmal SAH can present with different hemorrhage patterns that may be associated with differential rates of morbidity. Herein, we analyze a retrospective consecutive cohort of patients with nonaneurysmal SAH to determine outcomes. We also seek to evaluate a validated radiographic grading scale to determine its utility in predicting vasospasm in the setting of different hemorrhage patterns.METHODSAfter institutional review board approval, the records of 563 consecutive patients admitted with spontaneous SAH between January 2007 and 2014 were retrospectively reviewed. A total of 138 of these patients had no identifiable source of hemorrhage and were further divided into 2 groups depending on their pattern of hemorrhage: perimesencephalic or diffuse. Clinical characteristics and outcomes were assessed.RESULTSIn nonaneurysmal SAH, 70 patients (50.7%) had a perimesencephalic pattern of hemorrhage, whereas 68 (49.3%) experienced diffuse SAH. Radiographic vasospasm developed in 6 patients (8.6%) with perimesencephalic SAH and in 14 patients (20.6%) with a diffuse SAH pattern. When comparing historical rates of vasospasm based on the Barrow Neurological Institute (BNI) scale and rates in the nonaneurysmal diffuse pattern in this series, there was no significant difference in distribution (chi-square; P = .149), compared with a difference seen with the perimesencephalic group (P CONCLUSIONSNonaneurysmal SAH is associated with the potential for vasospasm, with higher rates in the diffuse versus perimesencephalic SAH patterns. The BNI grading scale for aneurysmal SAH can be used to predict the risk of vasospasm in diffuse, nonaneurysmal SAH.