Head injury and Parkinson's disease risk in twins

Objective: Head injury is an inconsistently reported risk factor for Parkinson's disease (PD). Many related variables might confound this association, such as differences in childhood and adolescent lifestyles or genetically determined risk-taking behaviors. Twin studies circumvent some of these problems, because twins are genetically and environmentally much more similar than typical cases and control subjects. Methods: We conducted a case-control study in 93 twin pairs discordant for PD ascertained from the National Academy of Sciences/National Research Council World War II Veteran Twins Cohort. Results: A prior head injury with amnesia or loss of consciousness was associated with an increased risk for PD (odds ratio, 3.8; 95% confidence interval, 1.3-11; p = 0.014). Truncating observations 10 years before PD onset enhanced the association. Though less precise, the association was somewhat stronger in monozygotic than in dizygotic pairs. Risk increased further with a subsequent head injury (p trend = 0.022) and with head injuries requiring hospitalization. Duration of unconsciousness was not associated. In a subanalysis of 18 pairs concordant for PD, the twin with younger onset PD was more likely to have sustained a head injury, although numbers were small. Interpretation: Our results suggest that mild-to-moderate closed head injury may increase PD risk decades later.