Myotonia congenita.

There are two types of MC, autosomal dominant and autosomal recessive (also called recessive generalized myotonia), both with the predominant clinical feature of diffuse myotonia. Recessive MC patients have more weakness than patients with dominant MC. MC patients of both types have a normal life span. Ongoing genetic studies have not as yet identified any chromosomal linkage. Electrophysiologically and pathophysiologically, there is no difference between these two types. The major pathophysiologic abnormality is decreased membrane chloride conductance. Treatment of myotonia is not always necessary, but when it is, the most effective medications are those that stabilize the muscle membrane. Phenytoin is frequently the first choice because it has more benign side effect profile than other drugs and a reasonable response rate.