Myotonia Congenita

What is myotonia congenita?

Myotonia congenita is a rare genetic condition that causes myotonia. This prevents your muscles from relaxing properly after they contract. For example, your child might not be able to let go of a toy right away after they grasp it. Myotonia congenita begins in childhood or adolescence. It can cause other symptoms, like muscle stiffness and muscle overgrowth (hypertrophy).

Healthcare providers classify it as a non-dystrophic myotonia disorder. In other words, the structure of your child’s muscles remains healthy, but the electrical process that regulates your child’s muscle contraction is abnormal. There are other disorders with myotonia, called dystrophic myotonias, in which the muscle structure is affected.

Any difference in how your child’s body works can cause worry and panic. But know that myotonia congenita is a condition that doesn’t typically get worse. Physical therapy and other accommodations can make the symptoms manageable.

Types of myotonia congenita

There are two types of myotonia congenita:

• Becker disease. Becker disease is the most common type of myotonia congenita. It causes attacks of muscle weakness, often in your child’s arms and legs. Becker disease symptoms usually appear around 4 to 12 years old. Becker disease is different from Becker muscular dystrophy.
• Thomsen disease. Thomsen disease is a rarer and milder form of myotonia congenita. Symptoms typically begin between the first few months of life to 2 to 3 years of age.

How common is myotonia congenita?

Myotonia congenita is rare. It affects about 1 out of every 100,000 people. People in Scandinavian countries, like Finland, Norway and Sweden, have a higher incidence of myotonia congenita. In those countries, the condition affects 1 in every 10,000 people.

What are the symptoms of myotonia congenita?

The main symptom of myotonia congenita is delayed relaxation of a muscle after it contracts — a condition called myotonia. Myotonia can be worse after a period of rest or in cold temperatures.

Other myotonia congenita symptoms you may notice in your child include:

• Feeding difficulties, choking, gagging and reflux — especially in early childhood
• Clumsiness and frequent falls (even after walking is established)
• Double vision or lazy eye
• Enlarged muscles (hypertrophy), leading to an athletic appearance
• Muscle cramps
• Muscle stiffness, especially in your child’s legs (the stiffness may get better with activity — warm-up phenomenon)
• Weakness, if your child has Becker disease
• Scoliosis, or abnormal curving of their spine

People assigned male at birth (AMAB) tend to have more severe symptoms than people assigned female at birth (AFAB). But symptoms can become worse during pregnancy and menstruation.

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

What causes myotonia congenita?

Myotonia congenita happens due to a change in the CLCN1 gene. It affects the chloride channels on your child’s muscle membrane, leading to issues with muscle relaxation after contraction.

People inherit Becker disease (BD) in an autosomal recessive pattern. This means that both biological parents carry the gene change and pass it on to their child. But the parents don’t have symptoms of the condition.

Thomsen disease (TD) is an autosomal dominant condition. This means that just one biological parent needs to have the genetic change to pass the condition on to their child.

How is myotonia congenita diagnosed?

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

• Asking your child to rapidly open and close their eyes
• Seeing if your child can quickly release their grip on an object or open and close their fist
• Tapping on your child’s muscles with a small hammer (percussor) to see if the muscles tense up

Your child’s provider may also recommend certain tests to confirm a myotonia congenita diagnosis or rule out conditions that cause similar symptoms. Tests may include:

• Creatine kinase (CK) blood test. Myotonia congenita can cause elevated CK levels.
• Electromyography (EMG). This test measures electrical impulses from your muscles. It can help differentiate between a muscle disorder and a nerve disorder. But the results of an EMG can’t distinguish between the two types of myotonia congenita.
• Genetic testing. This test looks for gene changes that cause myotonia congenita.
• Muscle biopsy. Myotonia congenita biopsies usually appear normal but can have mild abnormalities.

What is the treatment for myotonia congenita?

There’s no cure for myotonia congenita, and your child may not need specific treatment for it. Light physical activity after resting can help reduce muscle stiffness. Your child’s healthcare provider may also suggest physical therapy to help maintain muscle function.

In some cases, your child might benefit from medication. Their provider may prescribe mexiletine or ranolazine to help with muscle cramping and stiffness. Other medications may help as well, like lamotrigine and carbamazepine. But they often cause unpleasant side effects.

Other steps your child and family can take to help manage the symptoms and avoid triggers of myotonia congenita include:

• Avoiding cold temperatures
• Getting regular physical activity — your child’s symptoms may go away once their muscles warm up (the warm-up phenomenon)
• Managing stress
• Modifying their diet if needed by providing foods they can easily swallow to reduce their risk of choking

Can I prevent myotonia congenita?

Myotonia congenita happens due to genetic changes, so you can’t do anything to prevent it. If you have a biological family history of the condition or have it yourself and are planning to have biological children, consider meeting with a genetic counselor. They can go over your risk of passing on the condition.

What is the prognosis for someone with myotonia congenita?

Myotonia congenita usually doesn’t worsen over time — your child’s symptoms should stay mostly the same throughout their life. Physical therapy and avoiding triggers can help your child be active and go about regular activities. People with myotonia congenita can participate in sports and otherwise live typical lives. In Becker disease, your child may develop some minor weakness as they get older.

What is the life expectancy for someone with myotonia congenita?

People with myotonia congenita have a typical life expectancy. The condition doesn’t affect other body systems.

When should my child see their healthcare provider?

Myotonia congenita is a chronic (long-term) condition. While it doesn’t usually worsen or change, your child may need to see their healthcare provider regularly if they take medication for it. Physical therapy needs may also change as your child develops.

It’s important to note that myotonia congenita may affect your child’s reaction to anesthesia. You should contact their provider if they have a surgery planned, and notify the anesthesiologist who’ll be taking care of them during surgery.