title: Seasonal differences and circadian variation in stroke occurrence and stroke subtypes.
abstract: BACKGROUND
  India is a subtropical country with clear seasonal variations in weather conditions. Seasonal and circadian variation in occurrence of subtypes of cerebrovascular disease has been of interest in several studies from different countries and climate zones, but discrepant results have made the conclusions unclear. The aim of the present study was to observe the seasonal and circadian variation in the occurrence of stroke and its subtypes among our population.
  
  
  METHODS
  This was a cross-sectional observational study based on new cases and past cases of stroke on follow-up, conducted between January 2011 and December 2012 in the Department of Neurology, at the All India Institute of Medical Sciences, New Delhi, India. The date and time of onset of the stroke was recorded. The categorization of months into season was in accordance with the Indian Meteorological Department guidelines. The time of onset was distributed into 6 hourly intervals. Statistical calculations were performed using Stata version 12.1 and SPSS version 20.
  
  
  RESULTS
  A total of 583 patients were included for the study. The rate of occurrence of stroke was highest in the late morning 0600-1159 hours (P value <.001) compared with other times of the day, regardless of gender or age for both ischemic and hemorrhagic strokes. It was lowest in late evening (1800-2359 hours) quadrant compared with other quadrants. Although there was no significant difference found by dichotomizing the groups into two 6-month periods, there was an increasing trend in number of patients with stroke during the months November-February. There was no difference in stroke occurrence between the types of stroke or within each type among different seasons with different temperatures. Trial of Org 10172 in Acute Stroke Treatment (TOAST) classification of ischemic strokes also did not show any association with season or circadian rhythm.
  
  
  CONCLUSIONS
  There is a significant increase in occurrence of strokes between 0600 and 1159 hours and lowest between 1800-2359 hours. No significant variation in stroke occurrence or subtype for any of the seasons was observed.
authors.affiliation: Department of Neurology, All India Institute of Medical Sciences, New Delhi, India.
authors.forename: Kishan
authors.initials: K
authors.lastname: Raj
authors.affiliation: Department of Neurology, All India Institute of Medical Sciences, New Delhi, India. Electronic address: rohitbhatia71@yahoo.com.
authors.forename: Rohit
authors.initials: R
authors.lastname: Bhatia
authors.affiliation: Department of Neurology, All India Institute of Medical Sciences, New Delhi, India.
authors.forename: Kameshwar
authors.initials: K
authors.lastname: Prasad
authors.affiliation: Department of Neurology, All India Institute of Medical Sciences, New Delhi, India.
authors.forename: Madakasira Vasantha Padma
authors.initials: MV
authors.lastname: Srivastava
authors.affiliation: Department of Biostatistics, All India Institute of Medical Sciences, New Delhi, India.
authors.forename: Sreenivas
authors.initials: S
authors.lastname: Vishnubhatla
authors.affiliation: Department of Neurology, All India Institute of Medical Sciences, New Delhi, India.
authors.forename: Mamta Bhushan
authors.initials: MB
authors.lastname: Singh
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002940:Circadian Rhythm / Q000502:physiology*; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D015994:Incidence; D007194:India; D008297:Male; D008875:Middle Aged; D012621:Seasons; D020521:Stroke / Q000145:classification* / Q000453:epidemiology*
keywords: ICH; Stroke; circadian rhythm; ischemic Stroke; seasonal variation