Myotonia refers to delayed relaxation after muscle contraction, which can cause muscle stiffness. Myotonia congenita should not be confused with congenital myotonic dystrophy Congenital Myopathies Congenital myopathies are rare, inherited disorders of the muscles that cause reduced muscle tone and weakness. These disorders are present at birth or show up during infancy. There are several... read more , 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 Dominant disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are made of a very long strand... read more , 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 Recessive disorders Genes are segments of deoxyribonucleic acid (DNA) that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are made of a very long strand... read more , 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.
Symptoms of Myotonia Congenita
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.
Diagnosis of Myotonia Congenita
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 Electromyography and Nerve Conduction Studies Diagnostic procedures may be needed to confirm a diagnosis suggested by the medical history and neurologic examination. Electroencephalography (EEG) is a simple, painless procedure in which... read more (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 Genetic Screening Before Pregnancy Genetic screening is used to determine whether a couple is at increased risk of having a baby with a hereditary genetic disorder. Hereditary genetic disorders are disorders of chromosomes or... read more can be done to identify mutations in the gene that causes both forms.
Treatment of Myotonia Congenita
Sometimes drugs to relieve muscle stiffness
Treatment of myotonia congenita includes physical therapy Physical Therapy (PT) Physical therapy, a component of rehabilitation, involves exercising and manipulating the body with an emphasis on the back, upper arms, and legs. It can improve joint and muscle function, helping... read more , which may help maintain muscle function.
Doctors sometimes give drugs to relieve muscle stiffness and cramping. Of the drugs used, mexiletine has the most evidence of being helpful, but other drugs include lamotrigine, carbamazepine, and phenytoin. However, these 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