Monograph: Myotonia: Recognition, Evaluation, and Differential Diagnosis-Article

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This monograph reviews how to recognize, evaluate, and differentiate myotonia, a state of muscle membrane hyperexcitability caused by ion channel dysfunction. Myotonia can be seen clinically as delayed muscle relaxation and electrically on EMG as myotonic discharges with waxing and waning frequency and amplitude. The article distinguishes true clinical myotonia from mimics such as neuromyotonia, Brody disease, and myoedema, emphasizing that EMG is often decisive.<br /><br />The major diagnostic split is between myotonic dystrophies and nondystrophic myotonias. Myotonic dystrophy types 1 and 2 are systemic splicing disorders with progressive weakness and extracmuscular features such as cataracts, diabetes, cardiac disease, respiratory involvement, and cognitive/behavioral changes. DM1 typically causes distal weakness and facial involvement, while DM2 more often causes proximal weakness and pain. Nondystrophic myotonias are channelopathies, mainly due to CLCN1 or SCN4A variants, and usually present with stiffness, exercise- or cold-related symptoms, and little fixed weakness.<br /><br />The paper details specialized electrodiagnostic testing beyond routine needle EMG, including repetitive nerve stimulation, short exercise testing, long exercise testing, and cold-provocation testing. These studies help distinguish myotonia congenita, paramyotonia congenita, sodium-channel myotonias, hyperkalemic periodic paralysis, and hypokalemic periodic paralysis. It also notes that ultrasound techniques may have emerging value in assessing myotonia.<br /><br />The review highlights that electrical myotonia is not specific to classic myotonic disorders and may also occur in Pompe disease, necrotizing autoimmune myopathy, congenital myopathies, and some toxic myopathies. Finally, it summarizes symptomatic treatment, especially mexiletine and lamotrigine, and stresses that careful clinical examination plus targeted electrophysiology and genetic testing remain central to diagnosis.

Keywords

myotonia

muscle hyperexcitability

ion channel dysfunction

electromyography

EMG

myotonic discharges

myotonic dystrophy

nondystrophic myotonia

CLCN1

SCN4A

mexiletine