Risk of Parkinson's disease after hospital contact for head injury: population based case-control study.

Objective: To determine whether a hospital contact for a head injury increases the risk of subsequently developing Parkinson's disease.

Design: Population based case-control study.

Setting: Denmark.

Participants: 13 695 patients with a primary diagnosis of Parkinson's disease in the Danish national hospital register during 1986-2006, each matched on age and sex to five population controls selected at random from inhabitants in Denmark alive at the date of the patient's diagnosis (n=68 445).

Main Outcome Measures: Hospital contacts for head injuries ascertained from hospital register; frequency of history of head injury.

Results: An overall 50% increase in prevalence of hospital contacts for head injury was seen before the first registration of Parkinson's disease in this population (odds ratio 1.5, 95% confidence interval 1.4 to 1.7). The observed association was, however, due almost entirely to injuries that occurred during the three months before the first record of Parkinson's disease (odds ratio 8.0, 5.6 to 11.6), and no association was found between the two events when they occurred 10 or more years apart (1.1, 0.9 to 1.3).

Conclusions: The steeply increased frequency of hospital contacts for a head injury during the months preceding the date at which Parkinson's disease was first recorded is a consequence of the evolving movement disorder rather than its cause.