Myotonia congenita is a rare disease that causes muscle stiffness and sometimes weakness. Stiff muscles can make it hard to walk, eat, and talk.

A faulty gene causes myotonia congenita. You can inherit this condition if one or both of your parents has it.

There are two forms of myotonia congenita:

  • Becker disease is the most common form. The symptoms start between ages 4 and 12. Becker causes attacks of weak muscles, especially in the arms and hands.
  • Thomsen disease is rare. The symptoms often start within the first few months of life, and usually appear by age 2 or 3. Muscles in the legs, hands, and eyelids are most likely to be affected.

The main symptom of myotonia congenita is stiff muscles. When you try to move after being inactive, your muscles spasm and become rigid. Your leg muscles are most likely to be affected, but the muscles of your face, hands, and other parts of your body can also get stiff. Some people have only mild stiffness. Others are so stiff that they have a lot of trouble moving.

Myotonia congenita often affects muscles of the:

  • legs
  • arms
  • eyes and eyelids
  • face
  • tongue

The stiffness is usually worst when you first start to move your muscles after they’ve been at rest. You might have difficulty:

  • getting up out of a chair or bed
  • climbing stairs
  • starting to walk
  • opening your eyelids after closing them

Once you have been moving for a while, your muscles will start to relax. This is called “warming up.”

Often, people with myotonia congenita have unusually large muscles, called hypertrophy. These big muscles can give you the appearance of a bodybuilder, even if you don’t lift weights.

If you have Becker disease, your muscles will also feel weak. This weakness can be permanent. If you have Thomson disease, cold weather might worsen your muscle stiffness.

Other symptoms include:

  • double vision
  • trouble chewing, swallowing, or talking if your throat muscles are affected

A change, called a mutation, to the CLCN1 gene causes myotonia congenita. This gene instructs your body to make a protein that helps your muscle cells tighten and relax. The mutation causes your muscles to contract too often.

Myotonia congenita is known as a chloride channel disease. It affects the flow of ions across muscle cell membranes. Ions are negatively or positively charged particles. CLCN1 helps maintain the normal function of chloride channels in muscle cell membranes. These channels help control the way your muscles respond to stimuli. When the CLCN1 gene is mutated, these channels don’t work as well as they should, so your muscles can’t relax normally.

You inherit each type of myotonia congenita in a different way:

Thomsen diseaseautosomal dominant patternonly need to inherit the faulty gene from one parent your children have a 50 percent chance of inheriting the gene
Becker diseaseautosomal recessive patternneed to inherit gene from both parentsyour children have a 25 percent chance of inheriting the gene if both parents are carriers, or a 50 percent chance if one parent is affected and one is a carrier

Additionally, with Becker disease, if you only inherit one copy of the gene, you’ll be a carrier. You won’t have symptoms, but you can pass myotonia congenita to your children.

Sometimes the mutation that causes myotonia congenita happens on its own in people who don’t have a family history of the disease.

Myotonia congenita affects 1 out of every 100,000 people. It’s more common in Scandinavian countries, which include Norway, Sweden, and Finland, where it affects 1 in 10,000 people.

Myotonia congenita is usually diagnosed in childhood. Your child’s doctor will do an exam to look for muscle stiffness, and ask you about your family’s medical history and your child’s health history.

During the exam, the doctor might:

  • have your child grip an object, and then try to release it
  • tap on your child’s muscles with a small, hammer-like device called a percussor, to test your child’s reflexes
  • use a test called electromyography (EMG) to check the electrical activity in the muscles while they tense and relax
  • do a blood test to check for high levels of the enzyme creatine kinase, which is a sign of muscle damage
  • remove a small sample of muscle tissue to look at under a microscope, which is called a muscle biopsy

The doctor can also do a blood test to look for the CLCN1 gene. This can confirm that you or your child has the disease.

To treat myotonia congenita, your child might see a team of doctors that includes a:

  • pediatrician
  • orthopedist, a specialist who treats problems with the joints, muscles, and bones
  • physical therapist
  • geneticist or genetic counselor
  • neurologist, a doctor who treats conditions of the nervous system

Treatment for myotonia congenita will address your child’s specific symptoms. It may include medicine and exercises to ease tight muscles. Most of the medicines that are prescribed for the symptoms of myotonia congenita are used experimentally, and are therefore off-label.

To relieve severe muscle stiffness, doctors may prescribe medication, such as:

  • sodium channel blocking drugs like mexiletine
  • antiseizure drugs such as carbamazepine (Tegretol), phenytoin (Dilantin, Phenytek), and acetazolamide (Diamox)
  • muscle relaxants such as dantrolene (Dantrium)
  • the antimalarial drug quinine (Qualaquin)
  • antihistamines, such as trimeprazine (Temaril)

If myotonia congenita runs in your family, you might want to consider seeing a genetic counselor. The counselor can look at your family history, do blood tests to check for the CLCN1 gene, and figure out your risk of having a child with this condition.

Complications from myotonia congenita include:

  • trouble swallowing or speaking
  • muscle weakness

People with myotonia congenita are more likely to react to anesthesia. If you or your child has this condition, talk to your doctor before having surgery.

Though myotonia congenita starts in childhood, it usually doesn’t get worse over time. You or your child should be able to lead a normal, active life with this condition. The muscle stiffness can affect movements like walking, chewing, and swallowing, but exercise and medicine can help.