title: Seasonal variation in stroke and stroke-associated mortality in patients with a hospital diagnosis of nonvalvular atrial fibrillation or flutter. A population-based study in Denmark.
abstract: OBJECTIVE
  There are few data on seasonal variation in stroke and seasonal variation in mortality after stroke in patients with atrial fibrillation. We examined the seasonal pattern in stroke occurrence and the effect of the season on mortality after stroke in patients with a history of nonvalvular atrial fibrillation.
  
  
  METHODS
  We identified all individuals, aged 40-89 years, with an incident diagnosis of stroke of any nature (ischemic or hemorrhagic) in the 1980-2002 period and no history of heart valve disease and a previous or concomitant diagnosis of atrial fibrillation or flutter in the Danish National Registry of Patients. Subjects were followed in the Danish Civil Registration System for emigration and vital status. We used periodic regression models to estimate the peak-trough ratio stratified by sex, age and comorbid medical conditions. Seasonal effect on mortality after stroke was analyzed in a Cox proportional hazards model.
  
  
  RESULTS
  The relative incidence of stroke estimated as the ratio of the incidence in the month of the peak (January) to the incidence in the month of the trough (July) was 1.11 (95% confidence interval: 1.07-1.15). The relative incidence of stroke was similar for men and women, did not differ by age (stratified by age 75 years) and was essentially similar for comorbid conditions considered. There was no seasonal effect on mortality after stroke.
  
  
  CONCLUSIONS
  The occurrence of stroke in patients with atrial fibrillation is modestly higher during the winter. Stroke-associated mortality does not vary by season.
authors.affiliation: Department of Cardiology A, Aarhus University Hospital, Denmark. Lars.Frost@as.aaa.dk
authors.forename: Lars
authors.initials: L
authors.lastname: Frost
authors.forename: Ljubica
authors.initials: L
authors.lastname: Vukelic Andersen
authors.forename: Leif Spange
authors.initials: LS
authors.lastname: Mortensen
authors.forename: Claus
authors.initials: C
authors.lastname: Dethlefsen
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001281:Atrial Fibrillation / Q000401:mortality*; D001282:Atrial Flutter / Q000401:mortality*; D002423:Cause of Death; D015897:Comorbidity; D003430:Cross-Sectional Studies; D003718:Denmark; D005260:Female; D006760:Hospitalization / Q000706:statistics & numerical data; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D011159:Population Surveillance; D016016:Proportional Hazards Models; D012621:Seasons; D020521:Stroke / Q000401:mortality*