What is myotonia congenita?

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 the symptoms can be mild. Most children are two or three 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.

This Answer is based on source information from the National Institute of Neurological Disorders and Stroke.