Myotonia occurs with abnormalities of the muscle membrane, and there have been reports of various channel abnormalities including Cl, Na, and Ca channels. Historically in 1966, Lipicky and Bryant found the abnormality of the muscle Cl channel in
Thomsen's disease and hereditary
myotonia of goat. Thereafter, Na channel abnormality was found in
myotonia associated with
familial hyperkalemic periodic paralysis and
paramyotonia congenita by using patch clamp technique.
Myotonic dystrophy is the most common
myotonic disorder and genetic studies disclosed the presence of abnormal triplet repeat in the chromosome 19q13.1 and the gene product is known as
myotonin protein kinase. However, pathophysiological mechanism of
myotonia in
myotonic dystrophy is not yet clear except that there are mild decrease of Cl conductance in 5 out of 8 muscle fibers and late opening of Na channels also occurs. As for the treatment of
myotonia, medications which work as Na channel blockers have been used and they are effective to reduce
myotonia, but they do not improve
muscle weakness. Recently
dehydroepiandrosterone sulfate which is a male
hormone secreted from the adrenal cortex has been used for
myotonic dystrophy and the
drug reduces
myotonia and improves
activities of daily living. The author confirmed that
myotonia can be induced in the muscle preparations of mice by either
anthracene-9-carboxylic acid (Cl channel blocker), or by anemone toxin (blocker of inactivation of Na channel), and
dehydroepiandrosterone sulfate can reduce myotonic bursts in both of these muscle preparations by intracellular recordings.