3. Electrophysiologic diagnostics of myotonia

Abstract

Myotonia derives from uncontrolled repetitive action potentials of the sarcolemma following an initial voluntary muscle contraction. It is clinically manifested by inability of muscle relaxation after voluntary contraction and is registered as a myotonic discharge during muscle relaxation on electromyography. Electrical myotonia consists of repeated muscle fiber action potentials with frequency of 20–80 Hz which wax and wane in amplitude and frequency. Myotonia ameliorates with exercise, while paramyotonia aggravates with exercise and cooling. Myotonia is present in myotonic dystrophy type 1 and 2, myotonia congenita, paramyotonia congenita, sodium channel myotonia and hyperkalemic periodic paralysis. Electrophysiological diagnostics of myotonia consists of the measurement of the CMAP amplitude after short and long exercise and electromyography for determining the presence of electrical myotonia. Recessive form of myotonia congenita is characterized by early decrement in CMAP amplitude with rapid recovery and Fournier pattern II, dominant form of myotonia congenita is characterized by similar findings but only after cooling and Fournier pattern II or III, paramyotonia congenita is characterized by the gradual and persistent reduction of CMAP amplitude enhanced by repetition and cooling and Fournier pattern I, while sodium channel myotonia is characterized by the absence of CMAP amplitude reduction and Fournier pattern III.