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Myotonia Congenita

By

Michael Rubin

, MDCM, New York Presbyterian Hospital-Cornell Medical Center

Reviewed/Revised Jan 2024
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Myotonia congenita is an inherited disorder that causes muscle stiffness and muscle enlargement. It affects what most people think of as muscle, the type that can be contracted to move the various parts of the body.

Myotonia refers to delayed relaxation after muscle contraction, which can cause muscle stiffness.

Myotonia congenita has 2 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 or code for functional RNA molecules.... 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 or code for functional RNA molecules.... 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

  • Electromyography (EMG)

  • Muscle biopsy

  • Genetic testing

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.

Treatment of Myotonia Congenita

  • Physical therapy

  • Sometimes medications to relieve muscle stiffness

Doctors sometimes give medications to relieve muscle stiffness and cramping. Of the medications used, mexiletine has the most evidence of being helpful, but other medications include lamotrigine, carbamazepine, and phenytoin. However, these medications are of limited effectiveness and often cause undesirable side effects.

Regular exercise may be beneficial.

People with myotonia congenita have a normal life expectancy.

More Information

The following English-language resource may be useful. Please note that THE MANUAL is not responsible for the content of this resource.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Mexitil
Lamictal, Lamictal CD, Lamictal ODT, Lamictal XR, Subvenite
Carbatrol, Epitol , Equetro, Tegretol, Tegretol -XR
Dilantin, Dilantin Infatabs, Dilantin-125, Phenytek
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