Myotonia congenita is a genetic disorder that affects the skeletal muscles. It is a type of channelopathy, meaning it is a disease caused by a problem with the tiny ion channels in our cell membranes. In this case, the problem lies specifically with the chloride ion channels in the muscle cells.

To understand this, it is essential to know how a muscle normally contracts and relaxes. The process is controlled by the flow of electrically charged ions, like sodium and chloride, across the muscle cell membrane.

• To make a muscle contract, the brain sends a signal that causes sodium channels to open, allowing positively charged sodium ions to rush into the muscle cell. This creates an electrical charge that makes the muscle “turn on” and contract.
• To relax the muscle, it must immediately “reset” this electrical charge. A healthy muscle cell does this by opening thousands of tiny chloride channels. This allows negatively charged chloride ions to flood into the cell, instantly neutralizing the positive charge and allowing the muscle to “turn off” and relax.

A helpful analogy is to think of each muscle fiber as a room powered by electricity.

• To turn the lights on (contract), you flip a switch that sends a jolt of positive electrical charge into the room’s wiring.
• To turn the lights off (relax), the room needs an efficient “grounding system” or an “exhaust port” to instantly discharge that electricity. Chloride channels are these essential exhaust ports.
• In myotonia congenita, these chloride channel exhaust ports are faulty or there are not enough of them. After the muscle contracts, the negative charge cannot rush in quickly enough to shut it off. The muscle remains electrically excited and “stuck” in the “on” position for several seconds before it can finally, slowly relax.

This delayed relaxation after a voluntary contraction is the phenomenon known as myotonia.

In my experience, patients often describe feeling “stuck” after attempting movement, especially after rest, it’s one of the hallmark signs that points me toward myotonia congenita.

The sole cause of myotonia congenita is a mutation in the CLCN1 gene. This gene contains the precise genetic instructions for building the primary chloride ion channel found in skeletal muscle, a channel known as ClC-1.

A mutation in the CLCN1 gene results in the production of chloride channels that are either non-functional or have a severely reduced ability to allow chloride ions to pass through them. With the “exhaust ports” effectively blocked, the muscle cell cannot reset its electrical charge efficiently, leading to the prolonged muscle contractions and stiffness that characterize the disease.

Clinically, I often diagnose it based on family history and classic presentation, confirmed with genetic testing and electromyography (EMG) that shows myotonic discharges.

Myotonia congenita is an inherited genetic disorder. It is not contagious and cannot be acquired. It is passed down through families in two different inheritance patterns, which define the two main types of the disease.

1. Thomsen’s Disease (Autosomal Dominant)

• This form is inherited in an autosomal dominant pattern. This means an individual only needs to inherit one copy of the mutated CLCN1 gene from one parent to have the condition.
• An affected parent has a 50% chance of passing the gene on to each of their children.
• Thomsen’s disease is generally considered the milder form of myotonia congenita, with symptoms often appearing in infancy or early childhood.

2. Becker’s Disease (Generalized Myotonia, Autosomal Recessive)

• This form is inherited in an autosomal recessive pattern. A child must inherit a mutated CLCN1 gene from both of their parents to be affected.
• The parents are typically unaffected carriers, meaning they each have one normal gene and one mutated gene.
• Because both parents must carry the same rare faulty gene, the chances of having a child with an autosomal recessive condition like Becker’s disease are higher in populations where marriage between close relatives is a common cultural practice.
• Becker’s disease often has a later onset, appearing in childhood or adolescence, and the symptoms of stiffness and myotonia are typically more severe than in Thomsen’s disease.

In my experience, most patients are born with the condition, though symptoms may not become apparent until early childhood especially with cold exposure or sudden movement.

The primary symptom across all forms of the disease is myotonia, which is muscle stiffness caused by delayed relaxation.

• Action Myotonia: The stiffness is most apparent after a period of rest and is triggered by a sudden, voluntary movement. For example:
  • The legs may feel stiff and frozen when trying to quickly stand up from a chair or start running.
  • The hand may be unable to let go of an object immediately after gripping it tightly.
  • The jaw may feel stiff after a big yawn.
  • The eyelids may temporarily get stuck closed after a hard sneeze.
• The “Warm-Up” Phenomenon: This is a classic and hallmark feature of myotonia congenita. Muscle stiffness improves with repeated movement or exercise. After taking a few stiff, awkward steps, a person’s gait will become more fluid and normal as their muscles “warm up.” This is a key feature that helps distinguish it from other conditions.

Other common signs and symptoms include:

• Muscle Hypertrophy: Individuals with myotonia congenita often have a very well-developed, athletic, or “muscle-bound” appearance, even without significant weight training. This is due to the constant state of activity in their muscles.
• A “Startle” Response: A sudden surprise or a loud noise can cause a person’s entire body to stiffen, which can sometimes lead to a fall without any loss of consciousness.
• Muscle Pain (Myalgia): Some people may experience aching or cramping in their muscles.

An Important Distinction: Myotonia Congenita vs. Myotonic Dystrophy

It is absolutely crucial to understand that myotonia congenita is NOT the same as myotonic dystrophy. While both cause myotonia, they are completely different disorders.

• Myotonia Congenita: Affects only the skeletal muscles, is non-progressive (it does not get worse over time), and does not cause muscle weakness. Life expectancy is normal.
• Myotonic Dystrophy: This is a much more severe, multi-system, and progressive disease. It causes not only myotonia but also significant muscle weakness and wasting (atrophy), as well as problems with the heart, lungs, eyes (cataracts), and endocrine system.

Clinically, I look for signs like difficulty releasing a grip, delayed eye opening after blinking, or muscle hypertrophy, some patients appear more muscular than expected.

The diagnosis of myotonia congenita is typically made by a neurologist based on the characteristic clinical symptoms and is confirmed with specialized tests.

• Clinical Examination: A doctor will suspect the diagnosis in a person who describes muscle stiffness that improves with exercise. The doctor can often elicit myotonia during the exam by having the patient grip their hand tightly and then try to release it quickly, or by gently tapping on a muscle (like in the thumb or tongue) with a reflex hammer, which will cause a sustained contraction (percussion myotonia).
• Electromyography (EMG): This is a key confirmatory test. An EMG measures the electrical activity of the muscles. A small needle electrode is placed in a muscle. In a person with myotonia, when the muscle is activated, the EMG will record a characteristic pattern of continuous, high-frequency electrical activity that does not stop immediately when the person tries to relax. Through the EMG speaker, this sounds like a “dive-bomber” or a motorcycle revving its engine. This sound is diagnostic of myotonia.
• Genetic Testing: A definitive diagnosis is made with a molecular genetic test. A blood sample is sent to a lab to sequence the CLCN1 gene. Identifying a disease-causing mutation confirms the diagnosis and can also distinguish between the dominant (Thomsen’s) and recessive (Becker’s) forms of the disease.

In my experience, physical exam clues like percussion myotonia (prolonged contraction after tapping a muscle) can be subtle but very telling when evaluating stiffness.

There is no cure for the underlying genetic defect in myotonia congenita. Because the condition is not progressive and does not cause muscle weakness, treatment is focused entirely on managing the symptoms of muscle stiffness and improving quality of life. Many people with mild myotonia require no medical treatment at all.

1. Non-Pharmacological Management

For individuals with mild symptoms, education and lifestyle adjustments are the mainstays of management.

• Understanding and Trigger Avoidance: Learning what makes the stiffness worse is key. This often includes avoiding prolonged periods of inactivity or exposure to cold temperatures, which can exacerbate myotonia.
• Regular, Moderate Exercise: A program of regular, gentle exercise can help to keep the muscles “warmed up” and reduce the severity of stiffness when starting an activity. A physical therapist can help to design a safe and effective program.
• Patient Education: Educating teachers, coaches, and employers about the condition can be very helpful. For example, a child with myotonia congenita may be slow to get out of their chair when a fire alarm rings, not because they are disobedient, but because their muscles have stiffened.

2. Medications

Medications are reserved for individuals whose myotonia is severe enough to interfere with their daily activities or to pose a safety risk (e.g., from falls due to the startle response).

• Mexiletine: This is the only medication specifically approved by the FDA for the treatment of myotonia. It is a type of sodium channel blocker that helps to reduce the electrical hyperexcitability of the muscle membrane, thereby lessening the stiffness.
• Other “Off-Label” Medications: Before the approval of mexiletine, other drugs were used and are still sometimes prescribed. These include other anti-seizure medications like lamotrigine and carbamazepine, or a diuretic called acetazolamide.

Safety Considerations

It is important for individuals with myotonia congenita to inform their anesthesiologist before any surgery, as certain anesthetic agents can cause dangerous complications.

Clinically, I’ve seen patients improve significantly with low-dose medications, though many learn to manage their symptoms effectively without pharmacologic treatment.

Myotonia congenita is a rare genetic muscle disorder that presents the unique and often perplexing symptom of muscles that cannot relax immediately after use. While the stiffness, cramping, and startle response can be challenging and socially awkward, it is a condition that is non-progressive and does not impact muscle strength or lifespan. It is not to be confused with the more severe and debilitating myotonic dystrophies. Living with myotonia congenita is a journey of understanding your body’s unique “operating system.” For many, no medical treatment is needed beyond education and a commitment to regular exercise. For those with more significant symptoms, effective medications are available to reduce the stiffness and improve function. In my experience, while myotonia congenita can cause functional challenges, it is often manageable with education, reassurance, and tailored physical activity.

National Organization for Rare Disorders (NORD). (2023). Myotonia Congenita. Retrieved from https://rarediseases.org/rare-diseases/myotonia-congenita/

National Institutes of Health, Genetic and Rare Diseases Information Center (GARD). (2021). Myotonia congenita. Retrieved from https://rarediseases.info.nih.gov/diseases/7074/myotonia-congenita

Muscular Dystrophy Association (MDA). (n.d.). Myotonia Congenita. Retrieved from https://www.mda.org/disease/myotonia-congenita