High frequency somatosensory stimulation in dystonia: Evidence for defective inhibitory plasticity

Background: Apart from motor symptoms, multiple deficits of sensory processing have been demonstrated in dystonia. The most consistent behavioural measure of this is abnormal somatosensory temporal discrimination threshold, which has recently been associated with physiological measures of reduced inhibition within the primary somatosensory area. High-frequency repetitive sensory stimulation is a patterned electric stimulation applied to the skin through surface electrodes that has been recently reported to shorten somatosensory temporal discrimination in healthy subjects and to increase the resting level of excitability in several different types of inhibitory interaction in the somatosensory and even motor areas. Objectives: We tested whether high-frequency repetitive sensory stimulation could augment cortical inhibition and, in turn, ameliorate somatosensory temporal discrimination in cervical dystonia. Methods: Somatosensory temporal discrimination and a number of electrophysiological measures of sensorimotor inhibition and facilitation were measured before and after 45 minutes of high-frequency repetitive sensory stimulation. Results: As compared with a group of healthy volunteers of similar age, in whom high-frequency repetitive sensory stimulation increased inhibition and shortened somatosensory temporal discrimination, patients with cervical dystonia showed a consistent, paradoxical response: they had reduced suppression of paired-pulse somatosensory evoked potentials, as well as reduced high-frequency oscillations, lateral inhibition, and short interval intracortical inhibition. Somatosensory temporal discrimination deteriorated after the stimulation protocol, and correlated with reduced measures of inhibition within the primary somatosensory cortex. Conclusions: We suggest that patients with dystonia have abnormal homeostatic inhibitory plasticity within the sensorimotor cortex and that this is responsible for their paradoxical response to high-frequency repetitive sensory stimulation.