Inadequate Modulation of Excitability With Voluntary Dorsiflexion in Parkinson's Disease

Abstract

Freezing phenomenon at onset of movement causes gait disturbance in Parkinson's disease (PD), but the pathophysiology is unclear. We studied motor property at onset of dorsiflexion in PD. In 9 patients with PD and 8 normal subjects, motor evoked potential was recorded from the tibialis anterior muscle under 3 conditions: at rest, during tonic contraction, and at onset of contraction. Motor threshold, size of motor evoked potential and the relationship between the intensity of transcranial magnetic stimulation, and the size of motor evoked potentials (recruitment gain) were examined. Motor threshold decreased with voluntary contraction in both PD and normal subjects, but the threshold at rest and during tonic contraction was lower in Parkinson's disease. The size of motor evoked potential with maximal stimulus intensity increased with voluntary contraction in both groups; this tendency was more pronounced in normal subjects. The recruitment gain during contraction was steeper than at rest in normal subjects. However, there was no such increase in PD. There was no increase in recruitment gain with voluntary contraction in PD, which was obvious in normal subjects, especially at onset of voluntary contraction. Modulation of motor excitability at onset of voluntary contraction was impaired in PD.