Myotonia is a condition where muscles cannot relax quickly after they contract, leading to stiffness and difficulty releasing grip or performing certain movements. This inherited disorder, while rare, can significantly impact daily activities and requires careful medical management throughout life.

Understanding Myotonia

Myotonia refers to the delayed relaxation of skeletal muscles after voluntary contraction or electrical stimulation. When someone with myotonia uses a muscle, it contracts normally, but then the muscle has trouble relaxing back to its resting state. This creates a distinctive stiffness that can be both uncomfortable and limiting in everyday life. For example, a person might shake someone’s hand and then find they cannot quickly open their hand to let go. The muscle remains tight and contracted for longer than it should, sometimes for several seconds or even longer.^[1]

The underlying cause of myotonia lies in problems with ion channels, which are tiny gateways in muscle cell membranes that control the movement of electrically charged particles. These channels are responsible for regulating muscle activity. When these channels don’t work properly due to genetic changes, abnormal electrical signals build up in the muscles, causing them to stay contracted when they should be relaxing. This creates the characteristic stiffness and rigidity that people with myotonia experience.^[2]

Types of Myotonic Disorders

Medical professionals divide myotonic disorders into two main categories: dystrophic myotonias and non-dystrophic myotonias. The word “dystrophic” refers to conditions that also cause muscle wasting and deterioration over time, while non-dystrophic myotonias primarily affect muscle relaxation without causing the same degree of muscle tissue breakdown. Understanding which type of myotonia someone has is important because it affects both the symptoms they might experience and how their condition should be managed.^[1]

Dystrophic myotonias include myotonic dystrophy type 1 and myotonic dystrophy type 2. These conditions don’t just affect the muscles—they can impact many body systems including the heart, eyes, digestive system, and brain. People with myotonic dystrophy often experience progressive muscle weakness along with myotonia, and they may develop cataracts, heart rhythm problems, and other complications. The severity and combination of symptoms can vary greatly from person to person, even within the same family.^[5]

Non-dystrophic myotonias are generally less severe and primarily affect muscle relaxation without causing significant muscle wasting. The most common form is myotonia congenita, which comes in two varieties: Thomsen disease and Becker disease. Other non-dystrophic myotonias include paramyotonia congenita and sodium channel myotonias. There is also a related group of conditions called periodic paralyses, which include hyperkalemic and hypokalemic periodic paralysis, as well as Andersen-Tawil syndrome. These conditions can cause episodes of muscle weakness or temporary paralysis in addition to myotonia.^[1]

How Common Is Myotonia?

Myotonic dystrophy type 1 is the most frequently occurring myotonic disorder worldwide, affecting approximately 1 in every 8,000 people. This makes it the most common form of muscular dystrophy that begins in adulthood. Myotonic dystrophy type 2 is considerably rarer, with estimates suggesting it affects between 1 to 9 in every 100,000 people. However, some research indicates that in certain populations, particularly in Germany and Finland, type 2 may be almost as common as type 1.^[1]

Among the non-dystrophic myotonias, myotonia congenita is the most prevalent, affecting between 0.2 to 7.3 people per 100,000, though this varies considerably depending on geographic location and ethnic background. The condition occurs more frequently in northern Scandinavia than in other regions. Paramyotonia congenita is even rarer, affecting roughly 1 in 250,000 people. The periodic paralyses show varied frequencies, with hypokalemic periodic paralysis affecting about 13 in 100,000 people, while hyperkalemic periodic paralysis affects approximately 1 in 200,000. Andersen-Tawil syndrome is extremely rare, with a prevalence of about 1 in 1,000,000 people.^[1]

What Causes Myotonia?

All forms of myotonia are caused by changes in specific genes that are passed down through families. These genetic changes affect how muscle cells function at the most basic level. Myotonic dystrophy type 1 results from a trinucleotide repeat on the DMPK gene, which means a sequence of three DNA building blocks is repeated too many times. The more repetitions there are, the more severe the condition tends to be. Myotonic dystrophy type 2 is caused by a similar problem, but with a tetranucleotide repeat on the CNBP gene—a sequence of four DNA building blocks that repeats abnormally.^[1]

These abnormal gene repeats cause the cells to produce unusually long messenger RNA molecules. These molecules form clumps inside cells that interfere with the production of many important proteins. When this happens in muscle cells and other tissues, it prevents them from functioning normally, leading to the various symptoms of myotonic dystrophy. The problems extend beyond just the muscles because these genes are active in many different body tissues.^[5]

Non-dystrophic myotonias are caused by defects in genes that control specific ion channels in muscle cell membranes. Myotonia congenita results from problems with the CLCN1 gene, which controls chloride channels. Paramyotonia congenita and sodium channel myotonias are caused by defects in the SCN4A gene, affecting sodium channels. The periodic paralyses involve different genes: hypokalemic periodic paralysis type 1 stems from issues with the CACNA1S gene affecting calcium channels, while type 2 involves the SCN4A gene. Andersen-Tawil syndrome is caused by problems with the KCNJ2 gene, which affects potassium channels. Each of these channel defects makes skeletal muscles overly excitable, causing the delayed relaxation characteristic of myotonia.^[1]

Who Is at Risk?

Because myotonia is an inherited condition, the primary risk factor is having a family history of the disorder. If one or both biological parents carry the genetic change that causes myotonia, their children may inherit it. The inheritance pattern varies depending on the specific type of myotonia. Some forms, like Thomsen disease, follow an autosomal dominant pattern, meaning only one copy of the changed gene from one parent is enough to cause the condition. Other forms, like Becker disease, are autosomal recessive, requiring changed genes from both parents for the condition to develop.^[7]

In dystrophic myotonias, both myotonic dystrophy type 1 and type 2 are inherited in an autosomal dominant manner. This means that if a parent has the condition, each of their children has a 50% chance of inheriting the genetic change. However, there’s an unusual feature of myotonic dystrophy type 1 called anticipation—the condition tends to become more severe and appear at earlier ages in successive generations. A grandparent might have mild symptoms appearing in their 50s, while their child might develop symptoms in their 30s, and their grandchild might be born with congenital myotonic dystrophy.^[5]

Recognizing the Symptoms

The hallmark symptom of myotonia is the inability to quickly relax muscles after they contract. This creates noticeable stiffness, particularly when starting a movement after a period of rest. Someone with myotonia might have difficulty releasing their grip on a doorknob, opening their eyes after squeezing them shut, or letting go of someone’s hand after a handshake. Many people with myotonia experience what’s called the “warm-up phenomenon”—their muscle stiffness improves after they repeat the movement several times. This is especially characteristic of myotonia congenita.^[3]

Beyond the primary stiffness, people with myotonia may experience muscle pain, fatigue, and weakness. The constant tension in muscles can be exhausting and uncomfortable. Depending on the specific type of myotonia, muscles may appear larger than normal due to sustained activity, giving some people an unusually muscular or athletic appearance. Children with myotonia congenita often look particularly well-developed muscularly, even though they may struggle with muscle stiffness and coordination.^[4]

In dystrophic myotonias, symptoms extend well beyond muscle stiffness. People with myotonic dystrophy type 1 often develop a characteristic facial appearance with a long, thin face and drooping eyelids. They may experience excessive daytime sleepiness that interferes with work, school, and social activities. Vision problems from cataracts are common, as are heart rhythm abnormalities that can be serious. Digestive issues, including difficulty swallowing, abdominal pain, and constipation, frequently occur. Men may experience early baldness, and both men and women can have difficulty with blood sugar regulation similar to diabetes.^[6]

Myotonic dystrophy type 2 tends to cause somewhat different symptoms than type 1. The muscle weakness in type 2 primarily affects proximal muscles—those closer to the center of the body, such as the hip, shoulder, and neck muscles. People with type 2 often experience significant muscle pain, which can sometimes be the most prominent and disabling feature early in the disease. While myotonia is present, it may be less noticeable than in type 1. Facial muscles and distal muscles (those farther from the body’s center, like hands and feet) are usually less affected in type 2.^[8]

Some types of myotonia have specific triggers that worsen symptoms. Cold temperatures commonly trigger myotonia in paramyotonia congenita, causing muscle stiffness to increase with exposure to cold. In the periodic paralyses, people may experience episodes of temporary muscle weakness or paralysis. With hyperkalemic periodic paralysis, these episodes typically last one to four hours and may be triggered by rest following exercise. Hypokalemic periodic paralysis can cause weakness that lasts from hours to days, often appearing at night or early morning. These episodes can be frightening and significantly disrupt daily life.^[4]

Preventing Myotonia

Because myotonia is an inherited genetic condition, there is no way to prevent someone from being born with it if they inherit the genetic change. However, genetic counseling can help prospective parents understand their risk of having a child with myotonia and make informed family planning decisions. Genetic testing can identify whether someone carries the genetic changes associated with myotonic disorders, even before symptoms appear or before having children.^[7]

For people who already have myotonia, certain preventive strategies can help minimize symptoms and complications. Understanding and avoiding specific triggers is important. Many people with myotonia find that cold temperatures worsen their muscle stiffness, so staying warm and avoiding sudden exposure to cold can help. In paramyotonia congenita especially, keeping muscles warm during rest periods and avoiding cold environments is crucial.^[20]

Regular, appropriate exercise can help maintain muscle function and overall health. Moderate to low-intensity aerobic activities like walking, cycling, and swimming are generally beneficial. Warming up gradually before exercise and warming down afterward with stretching can help reduce myotonia episodes. However, sudden forceful contractions should be avoided, and people should work with their healthcare team to develop an exercise plan that’s safe and appropriate for their specific type of myotonia.^[20]

For people with dystrophic myotonias, regular monitoring and preventive healthcare are essential. Regular heart evaluations can detect rhythm problems early before they become dangerous. Eye examinations can identify cataracts so they can be treated when needed. Monitoring lung function can help prevent respiratory complications. People with myotonia should also be vigilant about preventive dental care, as weakness of facial and jaw muscles can affect oral health.^[12]

How Myotonia Affects the Body

The fundamental problem in myotonia involves the electrical activity that controls muscle contraction and relaxation. Normally, when a nerve signals a muscle to contract, electrically charged particles called ions flow in and out of muscle cells through specialized channels. This flow of ions creates electrical signals that tell the muscle first to contract, then to relax. In myotonia, defects in these ion channels disrupt this process, causing excessive or prolonged electrical activity. The muscle receives the signal to contract but doesn’t get a clear signal to stop, so it remains contracted longer than it should.^[2]

On an electrical recording called an electromyography or EMG, myotonia shows a very distinctive pattern. The abnormal electrical signals wax and wane in frequency and amplitude, creating a sound that some describe as resembling a dive bomber or a motorcycle revving. This characteristic pattern helps doctors diagnose myotonia and distinguish it from other muscle conditions. The electrical activity continues spontaneously even after the person stops trying to contract the muscle, reflecting the inability of the muscle to properly turn off its activity.^[2]

In dystrophic myotonias, the problems extend beyond just the ion channels. The abnormal repeating sequences in the DMPK or CNBP genes create messenger RNA that forms clumps inside cells. These clumps trap proteins that are needed to regulate many other genes. As a result, the production of numerous proteins throughout the body becomes disrupted. This explains why myotonic dystrophy affects so many different organ systems beyond just skeletal muscles. The heart, brain, eyes, and digestive system all rely on proper protein production to function, and when this is disrupted, widespread problems can develop.^[5]

In the heart, myotonic dystrophy can affect the electrical conducting system that coordinates heartbeats. This can lead to heart block, where electrical signals don’t pass properly from the upper to the lower chambers of the heart, or to dangerous arrhythmias. In the brain, changes in certain regions may contribute to excessive sleepiness, cognitive difficulties, and personality changes that some people with myotonic dystrophy experience. In the eyes, clouding of the lens creates cataracts. In the digestive system, weakness of smooth muscles can cause swallowing difficulties, reflux, and constipation.^[6]

The muscle weakness that develops in dystrophic myotonias results from actual damage to muscle tissue structure over time. The continuous abnormal electrical activity and the disruption of normal cellular processes gradually wear down the muscle fibers. This is different from the myotonia itself, which is primarily an electrical problem. The combination of both myotonia (difficulty relaxing) and progressive weakness (difficulty contracting with full strength) creates particular challenges for people with dystrophic myotonias.^[9]

Breathing can be affected in several ways in dystrophic myotonias. The muscles that control breathing can become weak, reducing the ability to take deep breaths and cough effectively. The brain’s drive to breathe may also be affected, particularly during sleep. Many people with myotonic dystrophy develop sleep apnea, where breathing repeatedly stops and starts during the night. This contributes to daytime sleepiness and can affect overall health. Regular monitoring of breathing function is an important part of managing dystrophic myotonias.^[12]