A nationwide population study of severe head injury and Parkinson's disease.

The aim was to analyze prospectively the association between severe head injury and Parkinson's disease. All people in Denmark who were at least 20 years old as on 1 January 1981 were followed for hospitalisation due to previous head trauma during 1981-1993 and for hospital contacts due to PD during 1995-2004. We observed 107 cases of PD among people at hypothetical risk due to previous head injury. The expected number was 112.1, which yielded a standardised morbidity ratio of 0.954 (95% CI: 0.782-1.15). The study provides no support for severe head injury among adults being a risk factor for Parkinson's disease.