Myotonia congenita is an inherited neuromuscular disorder characterized by the inability of muscles to quickly relax after a voluntary contraction. The condition is present from early childhood, but symptoms can be mild. Most children will be 2 or 3 years old when parents first notice their muscle stiffness, particularly in the legs, often provoked by sudden activity after rest. The disease doesn’t cause muscle wasting; in fact, it may cause muscle enlargement.
Muscle strength is increased. There are two forms of the disorder: Becker-type, which is the most common form; and Thomsen’s disease, which is a rare and milder form. The disorder is cause by mutations in a gene responsible for shutting off electrical excitation in the muscles.
Most individuals with myotonia congenita lead long, productive lives. Although muscle stiffness may interfere with walking, grasping, chewing, and swallowing, it is usually relieved with exercise.
Most people with myotonia congenita don’t require special treatments. Stiff muscles usually resolve with exercise, or light movement, especially after resting. For individuals whose symptoms are more limiting, doctors have had some success with medications such as quinine, or anticonvulsant drugs such as phenytoin. Physical therapy and other rehabilitative therapies are also sometimes used to improve muscle function.