What is Myotonia?
Myotonia is a condition where the muscles can’t relax immediately after they’ve been voluntarily contracted or electrically stimulated. Various causes can lead to myotonia, which can be categorized into two types: dystrophic and non-dystrophic myotonias. Dystrophic myotonias are a type of muscular dystrophy, a group of diseases that cause progressive weakness and loss of muscle mass. Non-dystrophic myotonias, on the other hand, are less common and often misdiagnosed due to their rarity.
What Causes Myotonia?
Myotonia, or involuntary tensing of muscles, can occur due to many different causes. The most frequent disorder causing this condition is called myotonic dystrophy type 1. This happens due to a repeated sequence of three molecules (a trinucleotide repeat) on a specific gene. The specific gene is the dystrophia myotonica protein kinase (DMPK for short). The effect of this repeat sequence on body proteins may vary based on its length.
Other common myotonia-causing disorders are known as myotonic channelopathies. The most common of these is myotonia congenita, which happens because of defects in the channels that control the movement of electrolytes in cells. These defects make the skeletal muscles overly excited.
The types of myotonia fall into three main categories:
1. Dystrophic Myotonias:
* Myotonic dystrophy type 1 is caused by a repeated sequence of three molecules (trinucleotide repeat) on the DMPK gene.
* Myotonic dystrophy type 2 happens due to a sequence of four molecules repeating (tetranucleotide repeat) on the CNBP gene.
2. Non-dystrophic Myotonias:
* Myotonia congenita is due to a disorder of the CLCN1 gene; it affects the chloride channels in cells.
* Paramyotonia congenita and Sodium channel myotonias are both caused by disorders in the SCN4A gene, affecting the sodium channels in cells.
3. Periodic Paralyses:
* Hypokalemic periodic paralysis type 1 is due to a disorder in the CACNA1S gene, leading to problems with the calcium channels in cells.
* Hypokalemic periodic paralysis type 2 and Hyperkalemic periodic paralysis both come from issues with the SCN4A gene, which can disrupt the sodium channels in cells.
* Andersen-Tawil syndrome is caused by a disorder in the KCNJ2 gene, leading to problems with the potassium channels in cells.
Risk Factors and Frequency for Myotonia
- Myotonic dystrophy type 1 is the most common dystrophic myotonia, affecting 1 in 8,000 people.
- Myotonic dystrophy type 2 is a rare disease with its occurrence estimated between 1 to 9 in 100,000 people.
- Among the non-dystrophic myotonias, Myotonia congenita is the most common, affecting between 0.2 to 7.3 per 100,000 people.
- Paramyotonia congenita has a prevalence of roughly 1 in 250,000.
- Hypokalemic periodic paralysis affects approximately 13 in 100,000 people.
- Hyperkalemic periodic paralysis affects around 1 in 200,000 people.
- Andersen-Tawil syndrome is extremely rare with a prevalence of about 1 in 1,000,000 people.
Signs and Symptoms of Myotonia
Myotonia is a condition where a person has trouble relaxing muscles after they contract. It can result in symptoms like muscle pain, weakness, and fatigue, often because the muscle remains contracted for more extended periods. Depending on the specific type, myotonia can also alter muscle appearance, causing them to develop abnormally or appear larger than usual. People with myotonia might experience periods of muscle weakness and muscle stiffness that improves as they use the muscles, known as a ‘warm-up’ phenomenon.
Dystrophic Myotonias, a variant of the disorder, often show other system-wide symptoms. These symptoms can range from blurred vision caused by cataracts, difficulty in controlling blood sugar due to insulin resistance, or specific changes in facial features, including a distinctive “fish-shaped” mouth. If a person shows these broader symptoms alongside classic myotonia symptoms but has normal electrolyte levels, this might suggest Dystrophic Myotonia.
In contrast, Hyperkalemic Periodic Paralysis, typically seen in young children, involves episodes of muscle paralysis lasting from 1 to 4 hours. These episodes can be triggered by rest after exercise, consuming foods high in potassium, fasting, stress, or fatigue. In severe instances, the muscles involved in breathing might also be affected. Between episodes, patients usually regain their strength entirely but might develop weakness as they age.
People with Hypokalemic Periodic Paralysis typically experience episodes of muscle weakness, usually at night or early in the morning. These episodes, ranging from mild weakness to severe paralysis, can last from hours to days. Triggers can include foods high in carbohydrates, alcohol, stress, exercise, menstruation, and certain medications like insulin and corticosteroids. In profound cases of low potassium, eye muscles, neck muscles, and respiratory muscles could be affected.
Testing for Myotonia
If your doctor suspects you might have myotonia, a condition that causes muscle stiffness and enlargement, the first step would be to rule out other more common conditions that have similar symptoms. Depending on your symptoms and medical history, the doctor may test your thyroid function, given that thyroid problems can mimic the symptoms of myotonia. The doctors might also consider whether conditions like inflammation or toxins might be affecting your muscles. To definitely diagnose myotonia, your doctor might order a needle electromyogram (EMG), a test that measures the electrical activity produced by your muscles. But the most reliable test, known as the ‘gold standard,’ is genetic testing, the results of which can identify more than 130 gene mutations associated with non-dystrophia myotonia, one type of the condition.
If your doctor thinks you might have hypokalemic or hyperkalemic periodic paralysis, conditions that cause muscle weakness or paralysis due to abnormal levels of potassium in the blood, the first step would be to test your blood for potassium levels. As with myotonia, the doctor would also rule out other, more common causes of low or high potassium levels. This could include checking for kidney problems, medication side effects, losses from your digestive tract, and basic hormonal function. For a definitive diagnosis of periodic paralysis, your doctor might also order a needle electromyogram (EMG) or genetic testing. Once again, genetic testing is considered the ‘gold standard’ because it is the most reliable test.
Treatment Options for Myotonia
The treatment of myotonia, a symptom that makes it hard to relax muscles after use, caused by the dystrophic and non-dystrophic myotonias, types of muscular disorders, isn’t fully established. However, a medication called mexiletine has been shown to help lessen myotonia in people with myotonic dystrophy type I and certain non-dystrophic myotonias. Other potential drugs for treating myotonia include imipramine, taurine, and clomipramine, however, there isn’t enough evidence to strongly recommend these at this point.
For individuals with non-dystrophic myotonias, avoiding certain things that trigger symptoms, such as cold exposure and intense physical activity, can provide significant relief. For those with dystrophic myotonias, specific treatments aimed at lessening the overall symptoms of the disease can notably make discomfort more bearable. Since these overall symptoms can greatly vary from patient to patient, the chosen treatment will vary too, depending on the specific symptoms present.
People with periodic paralysis should focus on avoiding known triggers and maintaining balanced levels of potassium in their blood. Reducing stressful situations such as working night shifts, irregular eating schedules, abnormal sleep patterns, high-carbohydrate meals, and lengthy rest after exercise can help. Those with hyperkalemic periodic paralysis, characterized by high levels of potassium in the blood, should stick to a diet low in potassium-rich foods and consider the use of diuretics. On the other hand, those with hypokalemic periodic paralysis, characterized by low blood potassium levels, should increase their intake of potassium-rich foods. If these measures don’t work, both forms of periodic paralysis can potentially be treated with medications such as dichlorphenamide or acetazolamide.
What else can Myotonia be?
When trying to diagnose Myotonia, doctors have to make sure it’s not actually another condition that presents similarly. One of those conditions could be Hypothyroidism. Generally, patients with Hypothyroidism wouldn’t describe experiencing muscle weakness that comes and goes. But, it’s important to run routine thyroid function tests on anyone suspected of having Myotonia to rule out the chance of an unusual presentation of Hypothyroidism.
Also, abnormal electrolyte levels could temporarily present in a way that’s similar to Myotonia – especially related to conditions that involve periods of either high or low potassium levels. However, a person would likely not have these symptoms come and go over a long period of time. Also, more detailed testing might help find the root cause of the electrolyte disturbance.
What to expect with Myotonia
Non-dystrophic myotonias, a group of muscle disorders, are generally considered to have a mild impact on one’s health. This is however, based on limited information due to the rarity of these conditions. The severity of these illnesses can be best guessed by the patient’s own report of pain and tiredness.
On the other hand, in dystrophic myotonias, which are a different group of muscle disorders, the severity largely depends on the intensity of the systemic symptoms, basically the symptoms that affect the whole body.
Myotonic dystrophy type 1 and 2, specific types of dystrophic myotonias, often shows several systemic symptoms, and this implies that the patient’s condition can be quite severe. But, the muscle weakness, or myopathy, is thought to be most severe in the periodic paralyses, another muscle disorder. This muscle weakness can worsen with age and result in permanent severe muscle weakness.
Possible Complications When Diagnosed with Myotonia
The myriad types of myotonias can eventually worsen or lead to a more severe muscle disease over time, particularly in the case of periodic paralysis. Additionally, dystrophic myotonias, a specific type of these conditions, could lead to significant body-wide symptoms. These symptoms might include cataracts, resistance to insulin, and changes in facial structure. It’s crucial to note that treatments for myotonia can also carry certain risks, as many of the medications used carry significant side effects.
Common Side Effects:
- Worsening or severe muscle disease
- Periodic paralysis
- Cataracts
- Resistance to insulin
- Changes in facial structure
- Significant side effects from medications used to treat myotonia
Preventing Myotonia
It’s important for patients to understand that myotonias are inherited health conditions, meaning they’re not brought on by anything the patient did. These disorders can be treated and it’s usually best to have regular check-ups with a specialist to manage them. It’s important not to lose hope, as individuals with myotonia can mostly live normal lives. This can be achieved through avoiding triggers that aggravate the condition, making certain changes to the diet, and taking prescribed medications.
