Beyond the muscular involvement in non-dystrophic myotonias: The emerging role of neuromodulation

Background: The central nervous system involvement, in terms of a maladaptive sensory-motor plasticity, is well known in patients with dystrophic myotonias (DMs). To date, there are no data suggesting a central nervous system involvement in non-dystrophic myotonias (NDMs). Objective: To investigate sensory-motor plasticity in patients with Myotonia Congenita (MC) and Paramyotonia Congenita (PMC) with or without mexiletine. Methods: Twelve patients with a clinical, genetic, and electromyographic evidence of MC, fifteen with PMC, and 25 healthy controls (HC) were included in the study. TMS on both primary motor cortices (M1) and a rapid paired associative stimulation (rPAS) paradigm were carried out to assess M1 excitability and sensory-motor plasticity. Results: Patients showed a higher cortical excitability and a deterioration of the topographic specificity of rPAS aftereffects, as compared to HCs. Therewas no correlation among neurophysiological and clinical-demographic characteristics. Noteworthy, the patients who were under mexiletine showed a minor impairment of the topographic specificity of rPAS aftereffects as compared to those who did not take the drug. Conclusion: Our findings could suggest the deterioration of cortical sensory-motor plasticity in patients with NDMs as a trait of the disease.