Low-frequency repetitive transcranial magnetic stimulation of the motor cortex in writer's cramp

To study the short-term effects of slow repetitive transcranial magnetic stimulation (rTMS) of the motor cortex on cortical excitability and handwriting in patients with writer's cramp.

Cortical excitability of the primary motor cortex is abnormally enhanced in patients with writer's cramp. Therefore, reducing cortical excitability by low-frequency rTMS of the motor cortex might result in beneficial effects on handwriting in writer's cramp.

We studied the effects of subthreshold 1-Hz rTMS on motor threshold and cortico-cortical excitability using the paired-pulse technique in seven patients and seven controls. In another 16 patients and 11 age-matched controls we evaluated changes in cortical excitability by measuring the stimulus-response curve and the postexcitatory silent period before and after subthreshold 1-Hz rTMS. In addition, we analyzed the handwriting before and 20 minutes after 1-Hz rTMS.

In the first experiment, low-frequency rTMS resulted in a normalization of the deficient cortico-cortical inhibition in the patients without affecting motor threshold. In the second experiment, 1-Hz rTMS resulted in a significant prolongation of the postexcitatory silent period without affecting the stimulus-response curve in the patient group. Moreover, the dystonic patients showed a significant reduction of mean writing pressure after subthreshold 1-Hz rTMS that was associated with clear but transient improvement in six patients.

In some patients 1-Hz rTMS can reinforce deficient intracortical inhibition and may improve handwriting temporarily. Our data support the notion that reduced intracortical inhibition plays a part in the pathophysiology of focal dystonia.