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. 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.