title: Perimesencephalic subarachnoid hemorrhage: incidence, risk factors, and outcome.
abstract: BACKGROUND
  Nonaneurysmal perimesencephalic subarachnoid hemorrhage (PMSAH) appears to have an etiology and natural history distinct from aneurysm rupture. Referral-based studies suggest that 15% of SAH patients 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 non-traumatic SAH, with presentation of incidence rates, patient demographics, and clinical outcomes.
  
  
  METHODS
  All patients age >/= 18 hospitalized with first-ever, non-traumatic SAH in the Greater Cincinnati area were identified from 5/98-7/01 and 8/02-4/04. 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 (2) were excluded, leaving 429 SAHs for analysis. Of these patients, 77 did not have angiograms. Among remaining cases, 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% CI 7.9-9.5) and 0.5 (95% CI 0.3-0.7) per 100,000 persons age >/= 18. Patients with PMSAH were younger (p = 0.018) and less likely to be female (p = 0.020) or hypertensive (p = 0.005) than other SAH patients. There was one death among PMSAH patients 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.
authors.forename: Matthew L
authors.initials: ML
authors.lastname: Flaherty
authors.forename: Mary
authors.initials: M
authors.lastname: Haverbusch
authors.forename: Brett
authors.initials: B
authors.lastname: Kissela
authors.forename: Dawn
authors.initials: D
authors.lastname: Kleindorfer
authors.forename: Alexander
authors.initials: A
authors.lastname: Schneider
authors.forename: Padmini
authors.initials: P
authors.lastname: Sekar
authors.forename: Charles J
authors.initials: CJ
authors.lastname: Moomaw
authors.forename: Laura
authors.initials: L
authors.lastname: Sauerbeck
authors.forename: Joseph P
authors.initials: JP
authors.lastname: Broderick
authors.forename: Daniel
authors.initials: D
authors.lastname: Woo