Motor cortical excitability studied with repetitive transcranial magnetic stimulation in patients with Huntingtonʹs disease

Objective TMS techniques have provided controversial information on motor cortical function in Huntingtonʹs disease (HD). We investigated the excitability of motor cortex in patients with HD using repetitive transcranial magnetic stimulation (rTMS). Methods Eleven patients with HD, and 11 age-matched healthy subjects participated in the study. The clinical features of patients with HD were evaluated with the United Huntingtonʹs Disease Rating Scale (UHDRS). rTMS was delivered with a Magstim Repetitive Magnetic Stimulator through a figure-of-8 coil placed over the motor area of the first dorsal interosseus (FDI) muscle. Trains of 10 stimuli were delivered at 5 Hz frequency and suprathreshold intensity (120% resting motor threshold) with the subjects at rest and during voluntary contraction of the target muscle. Results In healthy subjects at rest, rTMS produced motor evoked potentials (MEPs) that increased in amplitude over the course of the trains. Conversely in patients, rTMS left the MEP size almost unchanged. In both groups, during voluntary contraction rTMS increased the silent period (SP) duration. Conclusions Because rTMS modulates motor cortical excitability by activating cortical excitatory and inhibitory interneurons these findings suggest that in patients with HD the excitability of facilitatory intracortical interneurones is decreased. Significance We suggest that depressed excitability of the motor cortex in patients with HD reflects a disease-related weakening of cortical facilitatory mechanisms.