Myotonia Congenita

What is myotonia congenita?

Myotonia congenita is a rare disease in which your muscles aren’t able to relax immediately after they contract. This condition can lead to muscle stiffness that affects your ability to move. Myotonia congenita can also increase the size of your muscles and cause muscle cramping.

Myotonia congenita affects the chloride channels on your muscle membrane. This disease is classified as a non-dystrophic myotonia disorder — the structure of your muscle remains intact, but the electrical process that regulates your muscle contraction is abnormal.

Some people have mild muscle stiffness. Others have serious stiffness that impacts their ability to:

• Climb stairs.
• Get up from a seated position.
• Open their eyes completely after closing them, such as after crying or sneezing.
• Release their grip after shaking hands.
• Walk or run.

Defects in a gene cause myotonia congenita, an inherited disorder passed down through families. You have myotonia congenita from birth, but people usually don’t notice symptoms until they’re older.

There are two types of myotonia congenita:

• Becker disease: Becker disease is the most common type of myotonia congenita. This condition causes attacks of muscle weakness, often in your arms and legs. Becker disease symptoms usually appear around 4 to 12 years old.
• Thomsen disease: Thomsen disease is a rarer and milder form of myotonia congenita. Symptoms of this condition begin between the first few months of life to 2 to 3 years of age.

Who might get myotonia congenita?

Only individuals with the abnormal gene will have the disorder. People designated male at birth (DMAB) tend to have more severe symptoms.

How common is myotonia congenita?

One out of every 100,000 people has myotonia congenita. People in Scandinavian countries, such as Finland, Norway and Sweden, have a higher incidence of myotonia congenita. In those countries, the condition affects 1 in every 10,000 people.

What causes myotonia congenita?

A change (mutation) in a gene causes myotonia congenita. Children inherit these gene mutations from one or both of their parents. A parent may not realize they have the gene until their child develops symptoms.

Defective genes that come from both parents cause Becker disease. A defective gene from one parent causes Thomsen disease.

What are the symptoms of myotonia congenita?

The main symptom of myotonia congenita is an inability to relax your muscle quickly after it contracts, a condition called myotonia. Myotonia can be worse after a period of rest or in cold temperatures.

Other myotonia congenita symptoms include:

• Choking, gagging and reflux, especially in young children.
• Clumsiness and frequent falls.
• Difficulty chewing, swallowing (dysphagia) or talking.
• Double vision or a lazy eye.
• Enlarged muscles (hypertrophy), leading to an athletic appearance. Becker disease usually results in larger muscles than Thomsen disease.
• Muscle cramps.
• Stiffness, especially in your eyelids, face, hands, legs and tongue.
• Weakness, if you have Becker disease.

Men tend to have worse symptoms than women. But women’s symptoms can become worse during pregnancy and menstruation.

Thomsen disease symptoms tend to start earlier and first impact your face and arms. Becker disease symptoms often start later and first affect your legs.

Myotonia congenita may affect your reaction to anesthesia. You should contact your healthcare provider if you have a surgery planned and notify the anesthesiologist who’ll be taking care of you during surgery.

How is myotonia congenita diagnosed?

Healthcare providers usually diagnose myotonia congenita in childhood. Your healthcare provider will ask about symptoms and family health history. They’ll do an exam to test for muscle stiffness, including:

• Asking you to rapidly open and close your eyes.
• Seeing if you can quickly release your grip on an object or open and close your fist.
• Tapping on your muscles with a small hammer (percussor) to test your reflexes.

Other tests used to help diagnose myotonia congenita include:

• Blood test: Your healthcare provider may test for the enzyme creatine kinase (CK). Muscle damage can elevate CK levels.
• Electromyography (EMG): An electromyogram measures electrical impulses from your muscles. It can help differentiate between a muscle disorder and a nerve disorder.
• Genetic testing: This test looks for gene changes that cause myotonia congenita.
• Muscle biopsy: Your healthcare provider takes a small tissue sample from your muscle for testing under a microscope. Myotonia congenita biopsies usually appear normal but can have mild abnormalities.

How is myotonia congenita treated?

You may not need specific treatment for myotonia congenita. Light exercise after resting can help reduce muscle stiffness. Your healthcare provider may also suggest physical therapy to help you maintain your muscle function.

Your healthcare provider may prescribe a medication that blocks the sodium channel on your muscle membrane. This can help with muscle cramps and stiffness. Mexiletine is the primary medication used in this condition.

How can I manage symptoms of myotonia congenita?

To help relieve myotonia congenita symptoms, you can:

• Avoid cold temperatures.
• Exercise regularly. Your symptoms may go away once your muscles warm up (the warm-up phenomenon).
• Limit or manage stress.
• Modify your diet if needed. Eat foods you can easily swallow to reduce your risk of choking.
• Avoid certain types of medications that may worsen symptoms.

How can I reduce my risk of myotonia congenita?

Since myotonia congenita is a genetic condition, you can’t reduce your risk.

What can I expect if I have myotonia congenita?

Myotonia congenita usually doesn’t worsen over time. Most people with this condition can expect to live a full life.

If you become or want to become pregnant, see a specialist. They’ll ensure you know how to manage your condition during and after your pregnancy.

Your healthcare provider may send you to a genetic counselor to discuss how this disease might be passed on to your children.

How do I take care of myself?

You can make small adjustments to your home, school and work environments to decrease your risk of falls. Taking these steps can improve your safety and quality of life.

A note from Cleveland Clinic

Myotonia congenita is a genetic disorder that causes muscle stiffness and muscle growth. Myotonia congenita may be one of two types, Becker disease or Thomsen disease. Symptoms may include difficulty swallowing, enlarged muscles and weakness. You can control myotonia congenita with regular exercise, physical therapy, avoidance of triggers and medications. Most people with this disorder lead full and active lives.