Myotonia refers to delayed relaxation after muscle contraction, which can cause muscle stiffness.
There are two forms of myotonia congenita, Thomsen disease and Becker disease, both of which involve the same gene, CLCN1. Thomsen disease is autosomal dominant and manifests from infancy to 2 to 3 years of age. Becker disease is more common, is autosomal recessive, has a later onset from 4 to 12 years of age, and tends to be more severe than the autosomal dominant form.
Myotonia congenita should not be confused with congenital myotonic dystrophy, a separate disorder.
In children with myotonia congenita, there is delayed relaxation after muscle contraction, which can cause muscle stiffness. Parents describe weakness or clumsiness in their children, as well as stiffness. Myotonic symptoms lessen with age but do not disappear, and they are most noticeable after a period of rest. Patients typically have hypertrophy of the skeletal muscles due to the sustained muscle activity and the increased muscle bulk gives them an "athletic" appearance.
In Thomsen disease, onset is in infancy or early childhood and begins in the upper limbs and face, whereas in Becker disease it begins later in childhood in the lower limbs, and has more pronounced muscle hypertrophy. There is no weakness in Thomsen disease, but Becker disease is associated with transient weakness after prolonged rest and sometimes with progressive weakness.
The following are English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
Muscular Dystrophy Association: Information on research, treatment, technology, and support for patients living with myotonia congenita
National Organization for Rare Disorders: Comprehensive information regarding myotonia congenita, including standard and investigational therapies and links to related topics