Myotonia congenita is an inherited neuromuscular disorder characterized by the inability of muscles to quickly relax after a voluntary contraction. The condition is present since early childhood, but symptoms can be mild. The disorder is cause by mutations in a gene responsible for shutting off electrical excitation in the muscles.
Some form of myotonia congenita is estimated to affect 1 in 100,000 people worldwide. 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.
Early symptoms in a child may include:
Possible complications may include:
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.
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.
Becker disease usually appears later in childhood than Thomsen disease and causes more severe myotonia, muscle stiffness and pain. People with Becker disease often experience temporary attacks of muscle weakness, particularly in the arms and hands, brought on by movement after periods of rest. They may also develop mild, permanent muscle weakness over time.
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.