Myotonia refers to delayed relaxation after muscle contraction, which can cause muscle stiffness. Myotonia congenita should not be confused with congenital myotonic dystrophy, a separate disorder.
Myotonia congenita has two main forms, Thomsen disease and Becker disease. Both of these forms are caused by defects in the same gene.
Thomsen disease is autosomal dominant, which means a defective gene from only one affected parent is needed to pass the trait on to offspring. Symptoms of Thomsen disease usually start in infancy to 2 to 3 years of age.
Becker disease is more common and is autosomal recessive, which means a defective gene from both parents is needed to pass on the trait. Symptoms of Becker disease start from 4 to 12 years of age, and it tends to be more severe than Thomsen disease.
In children with myotonia congenita, there is delayed relaxation after muscle contraction, which can cause muscle stiffness. Parents may notice weakness or clumsiness in their children, as well as stiffness. The hands, legs, and eyelids become very stiff because of an inability to relax the muscles. Symptoms of myotonia lessen with age but do not disappear, and they are most noticeable after a period of rest. Unlike some other muscle disorders, myotonia congenita does not cause the muscles to become weak and waste away (atrophy). Instead, affected children typically have enlarged muscles. The increased muscle bulk gives them an "athletic" appearance.
The symptoms of Thomsen disease begin in infancy or early childhood and first affect the upper limbs and face, whereas in Becker disease they begin later in childhood and first affect the lower limbs. Muscle enlargement is more noticeable in Becker disease. There is no weakness in Thomsen disease, but children with Becker disease sometimes have temporary weakness after a long period of rest and sometimes with weakness that gets worse over time.
The diagnosis of myotonia congenita is suspected from the child’s characteristic appearance, inability to relax the grip of the hand rapidly after closing the hand, and prolonged contraction after the doctor taps a muscle.
Electromyography (a test in which electrical impulses from muscles are recorded) is needed to confirm the diagnosis. A biopsy of a muscle is sometimes done.
Genetic testing can be done to identify mutations in the gene that causes both forms.
Treatment of myotonia congenita includes physical therapy, which may help maintain muscle function.
Doctors sometimes give drugs to relieve muscle stiffness and cramping. However, the drugs are of limited effectiveness and often cause undesirable side effects. Regular exercise may be beneficial.
People with myotonia congenita have a normal life expectancy.
The following is an English-language resource that may be useful. Please note that THE MANUAL is not responsible for the content of this resource.
Muscular Dystrophy Association: Information on research, treatment, technology, and support for people living with myotonia congenita