What is Myotonia Congenita?

Myotonia congenita is a rare genetic, neuromuscular disorder characterized by the slow relaxation of the muscles. Symptoms may include muscle stiffness and hypertrophy (enlargement). The disorder is caused by a genetic mutation involving the chloride channel of the muscles. The muscle stiffness, which particularly occurs in the leg muscles, may be enhanced by cold and inactivity, and is often relieved by exercise.

Affected individuals have difficulty relaxing certain muscles after contracting them (myotonia), muscle stiffness (rigidity), and associated symptoms. Such symptoms tend to occur when attempting to move certain muscles after rest. In many cases, individuals with myotonia congenita also have abnormal enlargement of the muscles (hypertrophy), resulting in a “herculean” or “body-builder like” appearance.

Disorder Subdivisions

  • Thomsen Disease (THD) Myotonia Congenita, Autosomal Dominant (less severe form)
  • Becker Disease; Myotonia Congenita, Autosomal Recessive (MCR); Generalized (more severe form)
  • Myotonia Levior, Autosomal Dominant (less severe than Thomsen Disease)

Two main forms of myotonia congenita have been described: Thomsen disease and Becker disease. In individuals with Thomsen disease, symptoms and findings such as myotonia, associated muscle rigidity, and abnormal muscle enlargement may become apparent from infancy to approximately two to three years of age. In many cases, muscles of the eyelids, hands, and legs may be most affected. Thomsen disease is transmitted as an autosomal dominant trait.

In those with Becker disease, symptoms most commonly become apparent between the ages of four to 12 years. As in Thomsen type myotonia congenita, affected individuals develop myotonia, associated muscle rigidity, and abnormal muscle enlargement (hypertrophy). However, in contrast to Thomsen type, such symptoms are progressive and tend to be more severe. In addition, muscle hypertrophy may be particularly striking, and muscle weakness may be present. Becker disease is inherited as an autosomal recessive trait.

The third form; Myotonia Levior is a mild form of autosomal dominant myotonia. Affected individuals had isolated myotonia without muscle weakness, hypotrophy, or hypertrophy. They suggested that myotonia levior was a ‘low expressivity variant’ of Thomsen disease

Genetics and Inheritance

Myotonia congenita is present at birth and comes from a genetic mutation. The gene must be present in one parent or both parents for their child to have myotonia congenita.

Myotonia congenita can be either an autosomal dominant or autosomal recessive disease. With the autosomal dominant form, a person only needs to inherit the mutation from one parent to be affected. Inheriting the recessive form means a person must inherit the mutation from both parents to be affected. The gene that causes this condition resides on chromosome 7.

Disease inheritance is both autosomal dominant and autosomal recessive. Autosomal recessive inheritance requires that both the mother and father have at least one copy of an altered gene located on one of the 22 autosomes (that is, not the X or Y chromosomes). These parents may be carriers who show no signs of the disease (i.e., each has one copy of an altered gene), but they have a 25% chance of producing either a normal child or a child with the disease; they have a 50% chance of producing offspring who are carriers with no signs of disease. Two affected parents (i.e., each has two copies of an altered gene) usually produce children who are all affected. Males and females are at equal risk for being affected.

Autosomal dominant inheritance means that one affected parent (mother or father) has a 50% chance of passing the trait to a child; both sons and daughters are equally likely to be affected. Two affected people can have an unaffected child. Although it is likely that an affected person has an affected parent, there are exceptions: the affected person is the result of a new mutation; the parent transmitting the gene is an asymptomatic carrier; or the parent of the affected individual expressed the gene but in ways that were not readily recognized.

The cause of myotonia congenita is believed to be an abnormality in the chloride channels of muscle cells (chloride ions are required for a muscle to relax). The abnormal chloride channels also cause an accumulation of potassium outside the cells and an activation of sodium channels in the muscle cells (sodium ions trigger muscle contraction).

When the cells have more than enough sodium but not enough chloride, abnormal repetitive electrical discharges cause a stiffness called myotonia

How is Myotonia Congenita Diagnosed?

A family and patient history showing signs and symptoms of myotonia congenita is diagnostic: slow relaxation of muscles following voluntary relaxation, gagging, swallowing difficulties, difficulty initiating a movement but repetitive movement becomes easier. An EMG reveals myotonic potentials; muscle biopsy may reveal an absence of type 2B fibers.

Symptoms

The hallmark of this condition is the myotonia — the inability of the muscle to quickly relax after voluntary contraction. For example, after a handshake, the affected individual is only very slowly able to open and disengage his hand.

Early symptoms may include gagging and difficulty in swallowing, because oropharyngeal muscles are slow to relax. Initial movements may be stiff but improve with immediate repetition.

Children with myotonia congenita often appear to be muscular and well-developed. There may not be symptoms of myotonia congenita until the child is 2 or 3 years old.

Signs and tests

  • There is often a family history of myotonia congenita.
  • An EMG demonstrates myotonic potentials.
  • A muscle biopsy may show an absence of type 2B fibers.

Is there any treatment?

Most cases of myotonia congenita do not require treatment. Sometimes, however, symptoms of the disorder may be relieved with quinine, phenytoin and other anticonvulsant drugs. Physical therapy and other rehabilitative measures may be used to help muscle function. A person with myotonia congenita can lead a relatively normal life. Genetic counselling is available.

What is the prognosis?

Most cases of myotonia congenita are non-progressive and relatively non-limiting. A precise diagnosis allows physicians to distinguish mild myotonia congenital (also called Thomsen type) from the more severe form, called Becker type.

Complications

  • Frequent choking, gagging, or difficulty swallowing in an infant
  • Aspiration pneumonia, related to swallowing difficulties

What research is being done?

There is research being conducted world wide on genetic disorders such as myotonia congenita. The goals of these studies are to increase scientific understanding of these disorders and to find ways to prevent, treat, and cure them.


Additional Information

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