title: Subarachnoid hemorrhage incidence in the United States does not vary with season or temperature.
abstract: OBJECTIVE
  Previous studies have suggested seasonal variations in rates of spontaneous rupture of intracranial aneurysms, leading to potentially devastating SAH. In an effort to identify a seasonal effect, variation in SAH incidence and in-hospital mortality rates were examined as they relate to admission month, temperature, and climate using HCUP's Nationwide Inpatient Sample.
  
  
  METHODS
  Cases of nontraumatic SAH and subsequent in-hospital mortality were extracted from the 2001-2008 NIS and associated with month of occurrence, local average monthly temperatures, and USDA climate zone. Multivariate regression analysis was used to study how admission month, temperature, and climate affected SAH admission and mortality rates.
  
  
  RESULTS
  Among 57,663,486 hospital admissions from the 2001-2008 NIS, 52,379 cases of spontaneous SAH (ICD-9-CM 430) and 13,272 cases of subsequent in-hospital mortality were identified. SAH incidence and in-hospital mortality rates were not significantly correlated with a monthly/seasonal effect (incidence, χ(2) = 2.94, P = .99; mortality, χ(2) = 6.91, P = .81). However, SAH incidence significantly varied with climate (P < .0001, zones 11 and 7) but not with temperature (P = .1453), whereas average monthly temperature and climate had no significant correlation with in-hospital mortality (temperature, P = .3005; climate, P = .0863).
  
  
  CONCLUSIONS
  We identified no significant monthly or temperature-related effect in the incidence of SAH. Our data suggest that certain climate zones within the United States may be associated with significantly different SAH incidence, but the origins of these differences remain unclear and are probably unrelated to meteorologic variables.
authors.affiliation: Department of Radiology, Mayo Clinic, 200 1st St SW, Rochester, MN 55905, USA. mcdonald.robert@mayo.edu
authors.forename: R J
authors.initials: RJ
authors.lastname: McDonald
authors.forename: J S
authors.initials: JS
authors.lastname: McDonald
authors.forename: J P
authors.initials: JP
authors.lastname: Bida
authors.forename: D F
authors.initials: DF
authors.lastname: Kallmes
authors.forename: H J
authors.initials: HJ
authors.lastname: Cloft
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D012307:Risk Factors; D012621:Seasons; D013345:Subarachnoid Hemorrhage / Q000401:mortality*; D016019:Survival Analysis; D015996:Survival Rate; D013696:Temperature; D014481:United States / Q000453:epidemiology; D055815:Young Adult