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.
Myotonia congenita, also called Thomsen's disease, appears from infancy and affects the arms, the legs and the face, including the eyelids. Muscle wasting is not characteristic of the disease. Instead, pronounced muscle enlargement, particularly of the thighs, shoulders and forearms, may be present. Cold, fatigue, emotional stress or long periods of rest, such as a night's sleep, can bring on myotonia, or stiffness. The myotonia is generally not painful and can often be "worked off" with repeated movement. One form of myotonia congenita does not become more severe with time, while another form does, progressing until the early adult years, by which time the disorder may have led to some muscle weakness.
Paramyotonia congenita, also called Eulenberg's disease, is present at birth. It causes a type of myotonia in which exposure to cold brings on prolonged muscle contraction in the face, forearms and hands. It gives the face a stiff expression and makes the hands clumsy. The condition is seldom progressive and, like most forms of myotonia congenita, is not associated with muscle wasting. However, it differs from myotonia congenita in being more readily triggered by cold and in that the myotonia in paramyotonia congenita becomes worse rather than better with repetitive activity. Also, the contractions in paramyotonia congenita last longer and weakness may continue for up to half an hour.