Hereditary Myopathies

The diagnosis and treatment of myotonic disorders

“…Various medications are also reported to cause electrical myotonia without clinical myotonia. These include HMG‐CoA reductase inhibitors, colchicine, clofibrate, propranolol, fenoterol, terbutaline, penicillamine, diazocholesterol, monocarboxylic acids, cyclosporine, anthracene‐9‐carboxylic acid, and 2,4‐dichlorophenoxyacetic acid . Although it is important to recognize the myotonic potential of these agents, an appropriate diagnostic evaluation may still be required in a symptomatic patient with electrical myotonia who takes one of these medications.…”

“…Electrodiagnostic testing discloses widespread myotonic discharges on needle EMG examination, which increases in intensity with cold exposure . Other electrodiagnostic features include increased jitter and blocking during single‐fiber needle EMG studies, a decrement in compound muscle action potential (CMAP) amplitude with 3‐H Z repetitive nerve stimulation or after exercise, and characteristic findings on short exercise testing . The most helpful test is repetitive short exercise testing at room temperature and with muscle cooling.…”

“…Several other electrodiagnostic findings have been reported in paramyotonia congenita. Repetitive stimulation at 5 H Z can show a CMAP amplitude decrement, single‐fiber EMG may show increased jitter, fiber density, and blocking, and the short and long exercise tests show characteristic amplitude drops . Specifically, the short exercise test shows a dramatic postexercise drop in CMAP amplitude or area over 1 minute of rest that is worsened by repetition of the procedure or muscle cooling (Fournier pattern I) …”

“…Electromyography in myotonia fluctuans demonstrates generalized myotonic discharges and fibrillation potentials. Creatine kinase levels are elevated, and muscle pathology shows increased central nuclei, fiber size variability, and sarcolemmal vacuoles . To date, 3 point mutations in the SCN4A gene have been associated with this clinical phenotype …”

“…Generalized myotonia is triggered by potassium ingestion, fasting, and, occasionally, cold temperatures; it is relieved with acetazolamide use . Myotonic symptoms begin before the age of 10 years in proximal, extraocular, and mastication muscles . Myotonia in this disorder frequently presents with muscle pain; it is affected minimally by exercise and is not accompanied by episodic weakness .…”