The Myotonic Plot Thickens: Electrical Myotonia in Antimuscle-Specific Kinase Myasthenia Gravis

Electrical myotonia is known to occur in a number of inherited and acquired disorders including myotonic dystrophies, channelopathies, and metabolic, toxic, and inflammatory myopathies. Yet, electrical myotonia in myasthenia gravis associated with antibodies against muscle-specific tyrosine kinase (MuSK) has not been previously reported. We describe two such patients, both of whom had a typical presentation of proximal muscle weakness with respiratory failure in the context of a significant electrodecrement in repetitive nerve stimulation. In both cases, concentric needle examination revealed electrical myotonia combined with myopathic motor unit morphology and early recruitment. These findings suggest that MuSK myasthenia should be included within the differential diagnosis of disorders with electrical myotonia.