title: Head injury as a risk factor for bipolar affective disorder.
abstract: BACKGROUND
  Case reports have associated head injury with psychoses including affective disorders, but little is known regarding head injury as a risk factor for the onset of bipolar affective disorder.
  
  
  METHODS
  The Danish Psychiatric Case Register and the Danish National Patients Register were linked together with the Danish Population Register, thus identifying 10,242 patients with bipolar affective disorder, and 102,420 matched controls. History regarding head injury was recorded from the National Patients Register data. Data were analysed using conditional logistic regression.
  
  
  RESULTS
  Bipolar affective disorder was associated with an increased risk of a history of head injury (IRR=1.55; 95% CI 1.36-1.77). The increased risk was confined to head injury occurring less than 5 years before the first psychiatric admission. The finding could not be ascribed to increased accident proneness (as evaluated through the occurrence of other fractures not involving the skull).
  
  
  CONCLUSIONS
  In studies based on clinical diagnoses only and limited to patients who were hospitalised for psychiatric disorder, exposure was limited to injuries leading to admission to hospital.
  
  
  CONCLUSIONS
  Head injury may be a contributing factor to the onset of bipolar affective illness. However, this factor is probably only relevant to a relatively small minority of cases.
authors.affiliation: National Centre for Register-based Research, Aarhus University, Taasingegade 1 DK-8000 Aarhus C, Denmark. pbm@ncrr.au.dk
authors.forename: P B
authors.initials: PB
authors.lastname: Mortensen
authors.forename: O
authors.initials: O
authors.lastname: Mors
authors.forename: M
authors.initials: M
authors.lastname: Frydenberg
authors.forename: H
authors.initials: H
authors.lastname: Ewald
mesh_terms: D000059:Accidents; D001714:Bipolar Disorder / Q000209:etiology* / Q000503:physiopathology; D016022:Case-Control Studies; D006259:Craniocerebral Trauma / Q000150:complications*; D003718:Denmark; D005260:Female; D006801:Humans; D008297:Male; D012042:Registries / Q000706:statistics & numerical data*; D012044:Regression Analysis; D012307:Risk Factors