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

• Transient muscle weakness, a frequent symptom of nondystrophic myotonias, can be measured by the transient compound muscle action potential depression.
• Mexiletine, the currently preferred antimyotonic drug, efficiently reduces the transient compound muscle action potential depression.
• The 3 Hz prolonged low-rate repetitive nerve stimulation might be considered a helpful tool to objectively assess the anti-myotonic effect of drugs in myotonic patients.

ObjectiveWe 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.MethodsWe 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 (3 Hz-PLRS) before and after the beginning of treatment.ResultsWe 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).ConclusionsThe 3 Hz-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.SignificanceThis neurophysiological test might be considered a helpful tool to assess the effect of anti-myotonic drugs, as mexiletine, in recessive myotonia congenita.