Walsh & Hoyt: Myotonia

Myotonia is a phenomenon in which muscle fibers have a pathologically persistent activity after a strong contraction or are continuously active when they should be relaxed. Myotonia is identified physiologically as a delay in muscle relaxation after percussion or electric stimulation of a muscle, or after a voluntary contraction. The phenomenon is caused by mutations affecting chloride, sodium, and calcium ion channels in surface membranes. It persists after blockage of either the peripheral nerve or the neuromuscular junction. In patients with myotonia, the spontaneous action potentials as recorded in the EMG are high-frequency discharges of single muscle fibers that wax and wane in both amplitude and frequency. When translated into sound, these discharges produce a noise resembling that of a dive-bomber or a motorcycle engine. Clinical myotonia is a common feature of a number of different genetically determined, primary muscle diseases, including autosomal-dominant myotonia congenita, autosomal-recessive myotonia, paramyotonia congenital, the familial periodic paralyses, myotonic muscular dystrophy, and chondrodystrophic myotonia.