Parkinson's disease, parkinsonism, and traumatic brain injury.

We review the relationship between traumatic brain injury (TBI) and the development of idiopathic Parkinson's disease (PD) or secondary parkinsonism. Limited by methodological issues such as recall bias and confounding risk factors, epidemiological studies on the association between TBI and idiopathic PD have so far yielded mixed results. While clinical reports describe parkinsonism - often with lesions in the substantia nigra - secondary to traumatic brain injury, these do not represent cases of idiopathic PD. In laboratory studies, animal models of traumatic brain injury demonstrate neuronal loss in the substantia nigra, altered dopaminergic metabolism, or altered synuclein pathology. While parkinsonism does occur secondary to TBI, the relationship between TBI and subsequent idiopathic PD remains controversial.