Abnormal facilitation of the response to transcranial magnetic stimulation in patients with Parkinson's disease.

We studied the facilitation of the motor evoked potential (MEP) elicited with transcranial magnetic stimulation by increasing the stimulus intensity and the degree of voluntary activation of the target muscle in patients with Parkinson's disease (PD) and in normal volunteers. The threshold intensity for eliciting MEPs with the muscle at rest did not differ in PD patients and normal subjects. At rest, stimuli of similar intensity, related to the individual's threshold, elicited MEPs with amplitudes consistently larger in patients than in normal subjects, although when we compared the averaged MEP amplitude across all stimulus intensities, the differences reached only borderline statistical significance. Voluntary muscle activation elicited a smaller increase in the MEP area in PD patients than in normal subjects. Increasing the degree of voluntary muscle activation at fixed stimulus intensities elicited a smaller increase of MEP amplitude, duration, and area in PD patients than in normal subjects. These results suggest that control of the excitability of the motor system is abnormal in PD patients, with enhancement of excitability at rest and weak energization during voluntary muscle activation.