Myotonia congenita and paramyotonia congenita are nonlethal and generally nonprogressive diseases of voluntary muscle. The principal feature of these disorders is myotonia, a condition in which muscles are slow to relax after contracting and are consequently difficult to move. This myotonia may be severe enough to interfere with normal activity. Myotonia congenita and paramyotonia congenita differ from each other in the age of onset, muscles affected and the form of myotonia present.