title: Head injury is not a risk factor for multiple sclerosis: a prospective cohort study.
abstract: BACKGROUND
  The idea of physical trauma being involved in the causation of multiple sclerosis (MS) had been discussed since the earliest description of the illness. Despite the ongoing debate, the proposed association between physical and especially head trauma and MS failed to be proved or to be refuted conclusively.
  
  
  OBJECTIVE
  To determine whether head trauma is associated with an increased risk of developing MS.
  
  
  METHODS
  A cohort of 150,868 subjects, 95,111 men, and 55,757 women registered in the National Danish Patient Registry with hospital admission for cerebral concussion, contusion, or skull fracture between 1977 and 1992, aged under 55, was selected. This trauma cohort was linked with the Danish MS Registry and followed up to the end of 1999 to retrieve subjects who had onset of MS after the year of the head trauma. We calculated the expected number of subjects, who, under a null-hypothesis, would subsequently develop MS, by using population age-, year-, and sex-specific MS-incidence densities from the Danish MS Registry.
  
  
  RESULTS
  For men and women combined, the observed to expected number of MS cases (possible cases included) with onset after the head injury was 182/193.6 (standardized incidence ratio [SIR], 0.94; 95% CI, 0.81-1.09) and for possible MS excluded, 171/164.7 (SIR, 1.04; 95% CI, 0.89-1.21). In an analysis of a sub-cohort of 16,425 subjects with severe trauma (contusion, traumatic cerebral hemorrhage, and base or skull fracture), the observed to expected numbers, including possible MS, were 15/15.3 (SIR, 0.98; 95% CI, 0.55-1.62) and with possible MS excluded, 13/12.9 (SIR, 1.01; 95% CI, 0.53-1.73). As for the total group and for any of the subgroups and for men and women separately, none of the SIRs differed statistically significantly from unity. Neither were there any trends, which could be missed by type II errors.
  
  
  CONCLUSIONS
  Head injury of any severity does not affect the risk of acquiring MS later in life.
authors.affiliation: Department of Neurology, Arhus University Hospital, Aalborg, Denmark. ccp@rn.dk
authors.forename: C C H
authors.initials: CC
authors.lastname: Pfleger
authors.forename: N
authors.initials: N
authors.lastname: Koch-Henriksen
authors.forename: E
authors.initials: E
authors.lastname: Stenager
authors.forename: E M
authors.initials: EM
authors.lastname: Flachs
authors.forename: C
authors.initials: C
authors.lastname: Johansen
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D015331:Cohort Studies; D006259:Craniocerebral Trauma / Q000453:epidemiology*; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009103:Multiple Sclerosis / Q000453:epidemiology*; D011446:Prospective Studies; D012307:Risk Factors; D055815:Young Adult