title: Circadian and seasonal occurrence of subarachnoid and intracerebral hemorrhage.
abstract: OBJECTIVE
  To determine whether the time of onset of subarachnoid hemorrhage (SAH) or intracerebral hemorrhage (ICH) is associated with a time of day or season of year.
  
  
  BACKGROUND
  Prior studies have suggested that there may be a circadian and seasonal pattern of ischemic stroke occurrence, but this is less certain for hemorrhagic stroke. Population-based data have been unavailable.
  
  
  METHODS
  All incident ICH and SAH among residents of Rochester, MN, were ascertained. The medical records of patients were reviewed to determine the time of onset and date of occurrence. The day was divided into 8-hour periods, and the year into seasonal quartiles. Each patient was assigned a period based on the time of onset of symptoms. The data were analyzed by chi(2) analysis to determine whether there was a trend toward increased occurrence based on time period or seasonal quartile of onset.
  
  
  RESULTS
  From 1960 to 1989, there were 155 cases (48 men, 107 women) of incident SAH. From 1975 to 1989, there were 137 cases (57 men, 80 women) of incident ICH. There was a significant increase in the time of onset for ICH and SAH in the 8 AM to 4 PM period (p = 0.005 and p = 0.03, respectively). The concomitant occurrence of hypertension, gender, and age did not affect the time of day of occurrence. In the analysis of seasonal variation, there was a significant increase in events during December, January, and February in the combined SAH and ICH group (p = 0.032) and a trend for SAH alone (p = 0.07) but not for ICH (p = 0.34). Hypertension and age had no impact on the association between season and the occurrence of SAH and ICH.
  
  
  CONCLUSIONS
  The occurrence of SAH and ICH is increased from 8 AM to 4 PM. The occurrence of hemorrhage is increased during the winter months, but this is likely limited only to SAH.
authors.affiliation: Department of Neurology, Mayo Clinic, Rochester, MN, USA.
authors.forename: P A
authors.initials: PA
authors.lastname: Nyquist
authors.forename: R D
authors.initials: RD
authors.lastname: Brown
authors.forename: D O
authors.initials: DO
authors.lastname: Wiebers
authors.forename: C S
authors.initials: CS
authors.lastname: Crowson
authors.forename: W M
authors.initials: WM
authors.lastname: O'Fallon
mesh_terms: D002543:Cerebral Hemorrhage / Q000503:physiopathology*; D002940:Circadian Rhythm / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D012621:Seasons; D013345:Subarachnoid Hemorrhage / Q000503:physiopathology*