The prolonged cortical silent period in patients with Huntington's disease

Objectives: In a group of patients with Huntington's disease and age-matched controls, we studied the cortical silent period (SP) elicited by single transcranial magnetic stimulation (TMS) pulses. Methods: We measured the area of the pre-stimulus electromyographic (EMG) activity, the area of the motor evoked potentials (MEPs) and the duration of the SP induced by stimuli delivered at an intensity of 150% of motor threshold with a round coil placed over the vertex. We determined the cortical SP by sampling only the 5 traces containing the shortest SPs and by collecting 10 consecutive unselected traces without selecting trials. Results: Patients and controls had normal EMG background areas, and MEP latencies and areas. Whereas data measured from selected trials gave a normal duration of the SP (patients, 154 ± 58 ms; controls, 166 ± 22 ms), data from unselected trials yielded a significantly longer SP duration in patients than in controls (356 ± 251 vs. 159 ± 44 ms) and also a larger variance and range. Conclusions: We conclude that in Huntington's disease, an abnormal cortical SP is best sought by collecting unselected consecutive traces. We suggest that the prolonged SP in HD originates from a dysfunction of the mechanisms controlling the restart of voluntary movement after TMS.