Excitability of the lower-limb area of the motor cortex in Parkinson's disease

Abstract

Summary objective: The excitability of the lower-limb area of the motor cortex was investigated in patients with Parkinson's disease (PD) and in control subjects. Our results were compared to literature data assessing upper-limb cortical area. We analysed the effect of dopaminergic substitution therapy (DST).

Methods: Motor evoked potential (MEP) were assessed with transcranial magnetic stimulation (TMS) in 24 PD patients with (ON) and without (OFF) DST, and nine age-matched controls.

Results: Resting motor threshold (RMT), active motor threshold (AMT), cortical silent period (CSP), MEP amplitude and area did not differ significantly between groups and medication states. A paired-pulse TMS study revealed normal short-interval intracortical inhibition (SICI) but impaired intracortical facilitation (ICF) in PD OFF, partially normalized under DST. Post-hoc analysis uncovered two opposite effects of DST on MEP amplitude, separating the population in two groups. The paired-pulse study confirmed this division, showing that both groups exhibited distinct intracortical functioning, which was differently influenced by DST.

Conclusions: The lower-limb motor cortical areas of PD patients essentially exhibited an ICF reduction whereas in upper-limb areas, literature data demonstrated impairment of both SICI and ICF. Our data revealed two groups of patients showing different excitability states and opposite responses to DST.

Significance: The defective ICF in lower-limb areas could play a key role in the pathophysiology of gait disorders in PD. The fact that two cortical excitability states are inversely influenced by DST may reflect different conditions of denervation and compensatory mechanisms progression.