Myotonia congenita is a genetic muscle condition where your muscles contract normally but are slow to relax afterward. If you grip a doorknob and then can’t immediately let go, or your legs stiffen when you stand up from a chair, that delayed relaxation is myotonia. It affects roughly 1 in 100,000 people worldwide, though it’s about ten times more common in northern Scandinavia.
How Chloride Channels Cause Muscle Stiffness
Your skeletal muscles rely on a precise balance of electrical charge to contract and relax on command. A protein called ClC-1, found only in skeletal muscle, forms channels in muscle cell membranes that let chloride ions flow in. That influx of chloride stabilizes the cell’s electrical charge after a contraction, essentially telling the muscle fiber it can relax now.
In myotonia congenita, mutations in the CLCN1 gene alter these chloride channels so they don’t let enough chloride through. Without that stabilizing signal, the muscle fiber stays electrically excited and keeps firing. The muscle doesn’t lack strength. It simply can’t shut off quickly. This is why the hallmark experience is stiffness rather than weakness: your muscles work fine, they just won’t stop working when you want them to.
Thomsen Type vs. Becker Type
There are two forms of myotonia congenita, both caused by mutations in the same gene but inherited differently.
Thomsen disease is the milder, rarer form. It follows an autosomal dominant pattern, meaning a single copy of the mutated gene from one parent is enough to cause it. Symptoms typically appear in early childhood but tend to be less severe. In Thomsen disease, the mutation alters one of the two protein subunits that form the chloride channel, but the changed protein actively disrupts the function of both subunits.
Becker disease is the more common and more severe form. It follows an autosomal recessive pattern, so a person needs two copies of the mutated gene (one from each parent) to develop symptoms. Both subunits of the chloride channel are structurally changed, which greatly reduces chloride flow into muscle cells and produces more pronounced stiffness. People with Becker disease may also develop noticeable muscle enlargement over time, particularly in the legs, because the muscles are essentially getting a constant low-grade workout from sustained contractions.
What the Stiffness Feels Like
The core symptom is muscle stiffness that hits hardest at the start of movement. You sit down for a while, then stand up and your legs feel locked. You reach for something after resting your hand and your fingers won’t open smoothly. The stiffness is most obvious in the legs, hands, and eyelids, though it can affect any skeletal muscle.
One of the most distinctive features is the “warm-up phenomenon.” If you keep moving, the stiffness gradually fades. The first few steps might be rigid and awkward, but after a minute of walking, your muscles loosen up and move normally. This warm-up effect is a strong clue that separates myotonia congenita from other conditions that cause muscle stiffness.
Cold temperatures are the most reliable trigger. Going outside on a winter day, grabbing something from the freezer, or swimming in cold water can all make the myotonia significantly worse. Periods of rest followed by sudden movement are another common trigger. The pattern is predictable enough that most people learn to manage around it: warming up before activity, staying warm in cold environments, avoiding sudden explosive movements from a standstill.
How It’s Diagnosed
Diagnosis usually involves a combination of clinical history, physical exam, and a test called electromyography (EMG). During an EMG, a small needle electrode is inserted into the muscle to record its electrical activity. In myotonia congenita, the muscle fibers produce a very specific pattern: repetitive discharges that rise and fall in both speed and intensity. When played through a speaker, these discharges make a distinctive sound often compared to a dive bomber or a motorcycle engine revving up and slowing down. The pattern is so characteristic that an experienced clinician can recognize it immediately.
Sometimes the myotonic discharges are so abundant that they make it difficult to evaluate the muscle’s normal voluntary activity during the test. Genetic testing of the CLCN1 gene can confirm the diagnosis and identify whether the mutation pattern is consistent with Thomsen or Becker disease, which matters for understanding how the condition might be passed to children.
Treatment Options
Many people with mild myotonia congenita manage their symptoms through lifestyle adjustments alone: gradual warm-ups, avoiding cold exposure, and staying physically active. When stiffness is severe enough to interfere with daily life, medication can help.
The first-line treatment is mexiletine, a drug that works by calming the electrical excitability of muscle fibers. Adults typically take 400 to 600 milligrams per day, split into multiple doses. It doesn’t cure the underlying channel defect but reduces how easily the muscle fiber keeps firing after a contraction.
If mexiletine isn’t effective or isn’t tolerated, several alternatives exist, including lamotrigine, carbamazepine, flecainide, acetazolamide, and phenytoin. The evidence behind these is less robust than for mexiletine, but they can provide relief for some people.
Exercise and Physical Activity
Staying active is important. Inactivity leads to deconditioning, which compounds whatever muscle limitations the condition already causes. The key is choosing the right intensity.
Low-intensity strength training with higher repetitions (around 10 to 15 per set) is beneficial for people who have enough strength to move against gravity. Moderate endurance training, where you’re slightly out of breath but your muscles are still getting adequate oxygen, can improve cardiovascular fitness. Pool-based exercise and bodyweight-supported treadmill walking are good options that reduce joint stress while building endurance.
Heavy strength training and heavy eccentric exercises (where the muscle lengthens under load, like lowering a heavy weight slowly) are not recommended. There’s no evidence they provide additional benefit over moderate exercise, and they risk overloading muscles that are already prone to prolonged contractions.
Anesthesia Risks
If you have myotonia congenita and need surgery, your anesthesia team needs to know. Certain anesthetic agents pose specific risks. A muscle relaxant called succinylcholine, commonly used during intubation, can trigger severe myotonic contractions and should be avoided. Cooling during surgery, which can happen naturally under anesthesia, also worsens myotonia. There have been case reports of malignant hyperthermia, a dangerous rise in body temperature, in patients with myotonia congenita.
People with the condition also tend to be more sensitive to sedatives and pain medications, which can lead to prolonged periods of inadequate breathing after surgery. Standard non-depolarizing muscle relaxants generally work normally and are considered safer alternatives. The important thing is that your surgical team is aware of the diagnosis well before the procedure so they can plan accordingly.
Inheritance and Family Planning
The inheritance pattern depends on which type you have. With Thomsen disease (dominant), a parent who carries the mutation has a 50% chance of passing it to each child, and each child who inherits it will have the condition. With Becker disease (recessive), both parents must carry a copy of the mutation. If they do, each child has a 25% chance of developing the condition, a 50% chance of being a carrier without symptoms, and a 25% chance of neither carrying nor having it.
Genetic testing can clarify carrier status for family members, which is particularly useful for the recessive Becker form where carriers have no symptoms and may not know they carry the mutation.