Abstract
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.
Highlights
Muscle-specific tyrosine kinase myasthenia gravis (MuSKMG) is a type of myasthenia that is characterized by bulbar-predominant symptoms that tend to be refractory to pyridostigmine therapy
10–20% of MG patients are seronegative for Acetylcholine Receptor Antibody (AChRAb), and several studies have found that 20–40% of these AChR-Ab negative patients in the United States test positive for muscle-specific tyrosine kinase (MuSK) Ab [1,2,3]
Diagnostic testing was significant for markedly elevated serum MuSK antibody titers as well as 3 Hz Repetitive nerve stimulation (RNS) leading to significant (>10%) electrodecrement, leading to a diagnosis of MuSK-MG
Summary
Muscle-specific tyrosine kinase myasthenia gravis (MuSKMG) is a type of myasthenia that is characterized by bulbar-predominant symptoms that tend to be refractory to pyridostigmine therapy. 10–20% of MG patients are seronegative for Acetylcholine Receptor Antibody (AChRAb), and several studies have found that 20–40% of these AChR-Ab negative patients in the United States test positive for MuSK Ab [1,2,3]. The number of AChR-Ab negative patients who are MuSK Ab positive is estimated to be near 40–60% [1,2,3]. Atrophy of bulbar and proximal muscle groups has commonly been described [1, 4]. A myopathic pattern has been reported during needle EMG testing in MuSKMG patients, at times with muscle membrane irritability in the form of fibrillation potentials and positive sharp waves [4, 5]. We report two such cases and attempt to provide plausible explanations for its occurrence along with practical ramifications
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