Parkinson's disease progression is shaped by longitudinal changes in cerebral compensation

Parkinson's disease (PD) is a common and debilitating neurodegenerative disorder characterized by motor slowing (bradykinesia), which is thought to arise mainly due to nigro-striatal dopaminergic cell loss. Paradoxically, longitudinal changes in striatal dopamine relate poorly to the progression of bradykinesia, indicating that other pathophysiological mechanisms play a role. In line with this, cross-sectional studies have shown that more benign motor phenotypes of PD are characterized by increased activity in the parieto-premotor cortex, indicative of cerebral compensation. However, the role of cerebral compensation in disease progression remains unclear. Here, we used a longitudinal design to test the hypothesis that the clinical progression of bradykinesia in PD is related to a decline in compensatory parieto-premotor function, over and above worsening nigro-striatal cell loss. We used a validated action selection task in combination with functional MRI to measure motor- and selection-related brain activity in a large sample of 351 PD patients (≤5 years disease duration) and 60 healthy controls. In addition, we used diffusion-weighted MRI to obtain structural indices of substantia nigra and cerebral cortex integrity. These measurements were acquired at baseline and at two-year follow-up, enabling us to compare longitudinal changes in brain metrics between patients and controls, and to investigate their relationships with clinical metrics of bradykinesia progression. Consistent with our hypothesis, we observed that bradykinesia progression was inversely related to longitudinal changes in selection-related dorsal premotor cortex activity, suggesting that faster loss of cortical compensation contributes to faster symptom worsening. Importantly, this relationship remained after adjusting for longitudinal changes in the functional and structural integrity of the nigro-striatal system, indicating that bradykinesia progression is uniquely determined by loss of cortical compensation. In group comparisons of longitudinal change, PD patients showed an overall reduction in putamen activity, which did not decrease further over time, in combination with an acceleration of structural decline in the substantia nigra and the premotor cortex. Despite showing expected patterns of PD pathology, neither of these metrics correlated with bradykinesia progression. We conclude that the progression of bradykinesia in PD is determined mainly by longitudinal changes in compensatory premotor cortex function, rather than by deterioration of the nigro-striatal dopaminergic system. This presents opportunities to develop new progression-slowing interventions that focus on preserving and enhancing cortical compensation.