Effect of Mexiletine on transitory depression of compound motor action potential in recessive myotonia congenita

Objective: we aim to demonstrate the effect of mexiletine on the compound muscle action potential (CMAP) amplitude transitory depression (TD) in a cohort of patients with recessive myotonia congenita. Methods: we evaluated 21 patients with recessive myotonia congenita referred to our Institute from 1990 to 2013 and treated with mexiletine chlorhydrate. All patients underwent prolonged 3 Hz repetitive nerve stimulation (3Hz-PLRS) before and after the beginning of treatment. Results: we observed in all subjects a reduction of CMAP amplitude TD after the beginning of treatment. The mean value of the TD nadir before starting mexiletine treatment was -62.0% and reduced to -28.8% after the therapy was started (51.6 % reduction, p<0.001). Conclusions: the 3Hz-PLRS is configured as a neurophysiological test able to indirectly detect and quantify, through the measurement of TD, the clinical phenomenon of the transitory weakness that occurs in myotonic syndromes due to CLCN1 mutations. Significance: this neurophysiological test might be considered a helpful tool to assess the effect of anti-myotonic drugs, as mexiletine, in recessive myotonia congenita.