Perimesencephalic Subarachnoid Hemorrhage: Incidence, Risk Factors, and Outcome

Background: Nonaneurysmal perimesencephalic subarachnoid hemorrhage (PMSAH) appears to have an origin and natural history distinct from aneurysm rupture. Referral-based studies suggest that 15% of patients with subarachnoid hemorrhage (SAH) have no discernable cause of bleeding, but the incidence of PMSAH is unknown. We describe the first population-based study of PMSAH and place it in the context of all nontraumatic SAH, with presentation of incidence rates, patient demographics, and clinical outcomes. Methods: All patients age 18 years or older hospitalized with first-ever, nontraumatic SAH in the greater Cincinnati/northern Kentucky area were identified from May 1998 to July 2001 and August 2002 to April 2004. PMSAH was defined as hemorrhage restricted to the cisterns surrounding the brainstem and suprasellar cistern and a negative cerebral angiogram. Incidence rates were age, race, and sex adjusted to the 2000 US population. Results: There were 431 SAHs identified. Cases in Asian Americans (two) were excluded, leaving 429 SAHs for analysis. Of these patients, 77 did not have angiograms. Among remaining patients, 285 had aneurysm rupture, 43 had nonaneurysmal hemorrhage not of the PMSAH pattern, and 24 had PMSAH. The overall annual incidence rates for SAH and PMSAH were 8.7 (95% confidence interval 7.9-9.5) and 0.5 (95% confidence interval 0.3-0.7) per 100,000 persons age 18 years or older. Patients with PMSAH were younger (P = .018) and less likely to be female (P = .020) or hypertensive (P = .005) than other patients with SAH. There was one death among patients with PMSAH during 14 months mean follow-up. Conclusions: PMSAH represents approximately 5% of all SAH. Its risk factors and outcome differ from other forms of SAH.