Myotonia is a neuromuscular disorder characterized by the slow relaxation of the muscles after voluntary contraction or electrical stimulation. Generally, repeated effort is needed to relax the muscles, and the condition improves after the muscles have warmed-up. Individuals with the disorder may have trouble releasing their grip on objects or may have difficulty rising from a sitting position and a stiff, awkward gait. The disorder can affect all muscle groups. It may be acquired or inherited, and is caused by an abnormality in the muscle membrane. Myotonia is a symptom commonly seen in patients with myotonic muscular dystrophy and in a group of disorders called channelopathies (hereditary diseases that are caused by mutations in the chloride sodium or potassium channels that affect the muscle membrane). Myotonia also may be triggered by exposure to cold.
Treatment for myotonia may include quinine, phenytoin, mexiletine, and other anticonvulsant drugs. Physical therapy and other rehabilitative measures may help muscle function.
Myotonia can be a static or a progressive disorder. Symptoms may improve later in life.
Prepared by the National Institutes of Health