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).
Limitations: 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.