Slow rTMS to the somatosensory cortex attenuates afferent inhibition in patients with writer's cramp

Previous studies have shown a number of functional abnormalities in the somatosensory system including a distorted functional organisation of the somatosensory cortex (S1) in patients with writer's cramp. Here we tested the hypothesis that these functional alterations render S1 more susceptible to the „inhibitory“ effects of low-frequency repetitive transcranial magnetic stimulation (rTMS). To this end, we explored the effects of subthreshold 1Hz rTMS (1200 stimuli) given to S1 on clinical symptoms (writing speed) and short latency afferent inhibition (SAI), a measure of sensorimotor integration, in seven writer's cramp patients and eight healthy subjects. In a control experiment, patients also received rTMS to the motor cortex hand area (M1). SAI was tested in the relaxed first dorsal interosseous muscle (FDI) of the dominant hand using conditioning electrical stimulation of the index finger and TMS pulses given over the contralateral M1 at interstimulus intervals (ISI) of 25, 30 and 40 ms. Baseline SAI did not differ between patients and healthy subjects. S1 but not M1 rTMS reduced SAI in patients with significant changes occurring at an ISI of 25 ms. There were no clinical changes. rTMS had no effects on SAI in healthy subjects.

Because SAI is predominantly mediated at a cortical level in S1 we conclude that 1Hz rTMS produces abnormal disinhibition in S1 in writer's cramp which may be of relevance for the pathophysiology of focal task-specific dystonia. We propose that a bias towards disinhibition may favour maladaptive plasticity in the sensorimotor system which may ultimately lead to dystonia.