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Science News: Electrodiagnostic Characteristics Suggestive of Muscle-Specific Kinase Myasthenia Gravis

Submitted by: Niranjan Singh, MD
Edited by: Rebecca O’Bryan, MD

Skolka MP, Lamb CJ, Rubin DI, Klein CJ, Laughlin RS. Electrodiagnostic characteristics suggestive of muscle-specific kinase myasthenia gravis. Neurol Clin Pract. 2022; 12(3):211-217.
Summary: Muscle specific kinase (MuSk) antibody positive myasthenia gravis (MG) is a form of MG with bulbar predominant symptoms resistant to conventional treatment. They may have a distinct EDX profile compared to AchRAb +MG. The authors analyzed 22 MuSk+MG, 18 female, average symptom duration at presentation was shorter -4.7 years compared to AchR+ 10.9 years. Myopathic appearing motor unit potentials were seen in 41% (MuSk+MG) versus 30% (AchRAb) and presence of myotonic discharges were seen in bulbar and proximal muscles. Clinically, MuSK had higher percentage of bulbar muscle weakness, which has been previously established. Both groups have similar age symptom onset, age at diagnosis, proportion of ocular and generalized weakness, similar degree of RNS changes and abnormal EMG / single fiber EMG. Interruption of AChR clustering via MuSK antibodies may theoretically lead to muscle membrane instability due to compromise of synaptic architecture and preserved, but asynchronous, sodium conductance of fewer AChR channels. Ultimately this membrane instability may result in myotonic discharges and earlier myopathic appearing motor unit potentials in milder disease.
Comments: The authors highlighted potentially distinguishing electrodiagnostic features in the patient with MuSK+MG, consisting of myotonic discharges and myopathic appearing motor unit potentials, which may help to expedite the workup. The presence of these changes may be correlated to alteration of sodium channels.     
Article of Similar Interest:
Rodolico C, Bonanno C, Toscano A, Vita G. MuSK-Associated Myasthenia Gravis: Clinical Features and Management. Front Neurol. 2020;11:660. doi:10.3389/fneur.2020.00660

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