MuSK-related myasthenia prone to myotonic phenomena?

Myasthenia gravis with positive autoantibodies against the protein MuSK, a muscle-specific tyrosine kinase, is distinct from other forms of myasthenia gravis by the frequency of bulbar involvement. Electrophysiologically detectable myotonia could be a manifestation of this, as reported in two original observations:

  • the first one concerns a 53-year-old patient with classic myasthenic symptoms and signs with a decrement measured at 15%. Moreover, the needle examination clearly showed associated myotonic discharges.
  • the second is that of a 35-year-old woman initially admitted for a very probable inaugural myasthenic crisis and who also presented myotonic discharges on the electromyogram (EMG), in addition to a moderate decrement (11-15%) .

However, we must remain cautious about the causal link, as genetic causes of myotonia have not been studied in this case.

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Myotonia in MuSK Myasthenia Gravis: A Unique Electrodiagnostic Finding.Skolka MP, Lamb C, Klein CJ, et al. J Clin Neuromuscul Dis. 2022 Mar 1;23(3):165-166.