Myotonia: Recognition, Evaluation, and Differential Diagnosis

Myotonia is a non-neurotypical muscle physiology of sarcolemmal hyperexcitability due to alterations in the structure and/or function of ion channels in the muscle cell membrane. This hyperexcitability can be observed electrically as spontaneous myotonic discharges during needle electromyography. Myotonic discharges consist of continuous sequential fibrillation or positive sharp wave morphology potentials which exhibit unstable, gradually changing, firing frequencies and amplitudes. In myotonic disorders, a clinical correlate of muscle stiffness is often present. Myotonic disorders are broadly divided into myotonic dystrophies (Types 1 and 2) and non-dystrophic myotonias (myotonia congenita, paramyotonia congenita, and sodium channel myotonias). The myotonic dystrophies are systemic disorders of dysregulated RNA splicing clinically exhibiting fixed weakness, cataracts, diabetes and cardiac disease. Non-dystrophic myotonic disorders are due to specific sarcolemmal ion channel genetic variants and generally clinically limited to muscle stiffness (myotonia) sometimes with muscle weakness which can be fixed or periodic. Although electrical myotonia is a distinctive feature of myotonic disorders, it is nonspecific and may occur in other neuromuscular conditions. Widespread myotonic discharges strongly suggest a myotonic disorder or a handful of other conditions including necrotizing autoimmune myopathy, some toxic myopathies, Pompe disease and several congenital myopathies. This monograph reviews clinical myotonia and mimics, electrical myotonic discharges, electrodiagnostic testing in myotonic disorders and clinical features of myotonic disorders and other myopathies with myotonia.