Distinct dopaminergic abnormalities in traumatic brain injury and Parkinson's disease

Objective Traumatic brain injury (TBI) and rapid eye movement sleep behavioural disorder (RBD) are risk factors for Parkinson's disease (PD). Dopaminergic abnormalities are often seen after TBI, but patients usually lack parkinsonian features. We test whether TBI, PD and RBD have distinct striatal dopamine abnormalities using dopamine transporter (DaT) imaging. Methods I-123-ioflupane single-photon emission CT scans were used in a cross-sectional study to measure DaT levels in moderate/severe TBI, healthy controls, patients with early PD and RBD. Caudate and putamen DaT, putamen to caudate ratios and left-right symmetry of DaT were compared. Results 108 participants (43 TBI, 26 PD, 8 RBD, 31 controls) were assessed. Patients with early PD scored significantly higher on the Unified Parkinson's Disease Rating Scale motor subscale than other groups. Patients with TBI and PD had reduced DaT levels in the caudate (12.2% and 18.7%, respectively) and putamen (9.0% and 42.6%, respectively) compared with controls. Patients with RBD had reduced DaT levels in the putamen (12.8%) but not in the caudate compared with controls. Patients with PD and TBI showed distinct patterns of DaT reduction, with patients with PD showing a lower putamen to caudate ratio. DaT asymmetry was greater in the PD group than other groups. Conclusions The results show that patients with early PD and TBI have distinct patterns of striatal dopamine abnormalities. Patients with early PD and moderate/severe TBI showed similar reductions in caudate DaT binding, but patients with PD showed a greater reduction in putamen DaT and a lower putamen to caudate ratio. The results suggest that parkinsonian motor signs are absent in these patients with TBI because of relatively intact putaminal dopamine levels.