An Overview of Myotonia Congenita

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Myotonia congenita, also called congenita myotonia, is a rare, genetic disease that begins during childhood and causes muscle stiffness and cramping. If you or your child have myotonia congenita, you should know that while the condition causes bothersome symptoms, it can be managed with therapy and medication, and it is not dangerous or life-threatening.

Symptoms

Myotonia congenita is a genetic disease that is equally likely to occur in males and females, although the symptoms affect males more severely than females. For both males and females, the symptoms are generally considered mild to moderate and occur intermittently.

Common Symptoms

Skeletal muscle stiffness is the most common symptom of the disorder, and it tends to occur after a period of rest. For example, your muscles may feel stiff after you get up from a chair. You may actually fall down if you try to walk or run quickly after you have been still for a while. Some people have trouble with their voice when beginning to speak, due to previous inactivity of the speech muscles. 

A “warm-up effect” is often described with myotonia congenita. This is a decrease in muscle stiffness that occurs with repeated movements. 

Myotonia congenita only affects skeletal muscles. It does not affect muscles of the internal organs, such as the muscles that control the heart, breathing, or digestive system.

Types

There are two types of myotonia congenita: Thomsen disease and Becker disease. The symptoms begin during infancy or early childhood in Thomsen disease, and they begin in later childhood in Becker disease. The condition tends to remain stable throughout life, without worsening or improvement. 

A few specific features of each type of myotonia congenita include: 

  • Becker disease: You or your child will likely have cramps, which can affect any muscle throughout the body, but are most prominent in the leg muscles. This type of myotonia congenita can cause muscle weakness. You may develop an appearance that is described as a "bodybuilder appearance" due to hypertrophy (excess growth) of the muscles.
  • Thomsen disease: You may experience cramps, stiffness, and rigidity, which can affect the eyelids, hands, legs, face, and throat muscles. You can develop eye deviation, which looks like a lazy eye. You may have trouble swallowing, which can cause choking and coughing when eating. You should not expect to have muscle weakness with this type, and you may develop mild muscle hypertrophy.

Causes

Myotonia congenita is a genetic condition. A mutation, which is an abnormal gene code, has been identified in the CLCN1 gene, which is found on chromosome 7. The location of this gene is 7q34, which means that it is on the long arm of chromosome 7, in the position described as 34.

Pathophysiology 

The condition is caused by an abnormality on the chloride channels of the skeletal muscle cells. This type of disorder is called a channelopathy. The chloride channels help muscles relax.

The effect of the disorder is a diminished function of chloride in the muscles, which results in a delay in the initiation of muscle activity and a prolonged muscle contraction (action) after a muscle moves. This delayed relaxation and prolonged contraction is what causes the characteristic muscle stiffness of myotonia congenita.

It is not completely clear why males are more severely affected than females, but it has been suggested that testosterone and other androgens (male hormones) may have an effect on chloride channels.

Genetics 

The CLCN1 gene has a code that directs the body to produce a protein that allows the chloride channels of the skeletal muscle cells to function normally. Interestingly, there are a few different CLCN1 gene mutations that can have this effect. 

Thomsen disease is autosomal dominant, which means that you only need to have the mutation on one of your copies of chromosome 7 in order to have the condition. Becker disease is autosomal recessive, which means that you must have two abnormal copies of your CLCN1 gene to have the condition. 

This mutation is usually inherited as a hereditary condition from parents, but it can occur without a family history as a de novo mutation. A de novo mutation is a genetic change that occurs without having been part of the family lineage before. After a de novo mutation occurs, and it can be passed on to future generations.

How Genetic Disorders Are Inherited

Diagnosis

Myotonia congenita is a clinical diagnosis, which means that it is usually identified based on a combination of symptoms and supportive diagnostic tests. However, genetic testing can be done to confirm the disease. 

It is more common in Scandinavian populations, occurring at a rate of about one in every 10,000 people. Worldwide, it occurs at a rate of approximately one in every 100,000 people.

If you or your child has symptoms of myotonia congenita, you may need an electromyography (EMG) test, which differentiates between nerve disease and muscle disease. An EMG in myotonia congenita shows a pattern that is described as repetitive discharges when you contract your muscles. This result is supportive of the diagnosis if you have the symptoms. 

A muscle biopsy, which is a sample of muscle, is usually normal in myotonia congenita, generally helping differentiate it from myopathies (muscles diseases).

Differential Diagnosis

Other conditions that often needed to be ruled out during an evaluation of possible myotonia congenita include myopathy or muscular dystrophy, which can cause cramping of the muscles, usually producing more obvious weakness and changes in the physical appearance of the muscles than what is seen in myotonia congenita.

Treatment

Most of the time, treatment isn’t necessary and the symptoms are managed by warming up the muscles. In fact, you can participate in physical exercise and sports if you have myotonia congenita.

There are also some medical treatments that can help reduce the symptoms, but they are not formally indicated for the treatment of myotonia congenita, and instead, may be used off-label. 

Prescription medications include:

  • Mexitil (mexiletine): A sodium channel blocking medication usually used to treat arrhythmias (irregular heartbeats)
  • Tegretol (carbamazepine) or Dilantin (phenytoin): Anti-seizure medications, which may have an effect on ion channels
  • Diamox (acetazolamide): A medication used for fluid retention that also has an impact on the body’s electrolytes
  • Dantroline (dantrolene): A muscle relaxant
  • Alimemazine, trimeprazine: An antihistamine used for treatment of allergic reactions
  • Qualaquin (quinine): An antimalarial medication that was used more commonly for treatment of myotonia congenita in the past, but is not used much anymore due to side effects

A Word From Verywell

Myotonia congenita is a rare muscle disorder that causes noticeable symptoms that may interfere with your daily life. The disease is not fatal or life-threatening, and it doesn’t affect life expectancy. Nevertheless, it is important that you are aware of the factors that exacerbate and improve your symptoms so that you can optimize your daily function. Additionally, you have a number of medical treatment options that can alleviate your symptoms, should you need them.

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Article Sources
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