Overview of Myotonic Muscular Dystrophy

In This Article

Table of Contents

Myotonic muscular dystrophy, which is sometimes called myotonic dystrophy, is a type of muscular dystrophy. It is estimated that the condition affects about one in 8,000 people worldwide. There are two types of myotonic muscular dystrophy, described as type 1 (DM 1) and type 2 (DM 2). DM 1 is also called Steinert’s Disease.

Myotonic muscular dystrophy causes weakness of the skeletal muscles and the internal organs including the heart, the muscles that power breathing, and muscles of the digestive system. Myotonic muscular dystrophy is typically also characterized by delayed muscle relaxation.

Myotonic muscular dystrophy symptoms
Illustration by Emily Roberts, Verywell

Symptoms 

The symptoms of myotonic muscular dystrophy can begin at any age between infancy and the age of 40. If you or your child has DM 1 or DM 2, you may experience some of the following.

Skeletal Muscle Weakness

Skeletal muscles are the voluntary muscles attached to your bones. They move your arms, legs, head, neck, and torso. They can be mild to moderately weak with DM 1 or DM 2, but these muscles do not become completely paralyzed.

An example of this would be if you tried to squeeze your hand shut, but the action takes longer than you want it to.

Muscle Atrophy

Atrophy is the loss of muscle and it causes further weakness, as well as produces an appearance of thinning muscles. 

Myotonia

The increased muscle tone of myotonic muscular dystrophy is described as myotonia, and it manifests as a prolonged contraction and slowed relaxation of the muscles. This means that once a muscle moves, it takes a few seconds longer than usual to return to its relaxed state.

Myotonia can affect skeletal muscles as well as the muscles of the internal organs. This can be a very subtle symptom when it affects the skeletal muscles, but it can cause significant symptoms in the body’s internal organs. For instance, it can cause the heart to beat slowly or slow digestive function.

Digestive Problems

Weak gastrointestinal muscles and prolonged contraction can cause stomach pain, constipation, and gallbladder problems.

Heart Problems

The heart muscle weakness of this condition can cause decreased power of each heart contraction, which may manifest as fatigue. Myotonic dystrophy can also impact the heart's electrical system, potentially producing bradycardia (slow heart rate which can cause weakness, fatigue, lightheadedness or syncope), or ventricular tachycardia, which can cause sudden death. Heart problems are more common with DM 1 than with DM 2.

Cataracts

A progressive (gradually worsening) eye problem characterized by cloudy vision, cataracts (very common with aging) tend to occur more frequently and at a younger age with myotonic muscular dystrophy. 

Insulin Resistance

A medical condition characterized by high blood glucose, insulin resistance is commonly associated with both types of myotonic muscular dystrophy. 

Early Male Pattern Baldness

Men are more likely than women to experience early baldness, but women who have DM 1 or DM 2 can have hair loss as well. 

Characteristic Facial Appearance

This is often what suggests to doctors that myotonic dystrophy may be a diagnosis to consider. This characteristic appearance often includes ptosis (drooping eyelids), sagging jaw, a narrow face. These symptoms are caused by weakening of the facial musculature.

DM 1

DM 1 usually begins during infancy, but may begin at any time during a person’s life. It has been described as congenital onset, juvenile onset, and adult onset, based on the age at which the symptoms begin. Symptoms include skeletal muscle weakness, atrophy, and myotonia, which progressively worsen over time. The skeletal muscles that are most commonly affected include the facial muscles, the hands, the feet, and the neck.

Heart problems are common with DM 1, and muscle weakness can also interfere with breathing, especially during sleep. The most common digestive problem is constipation, but diarrhea can occur as well. Women may have problems during labor and delivery due to weakness and prolonged contractions of the uterine muscles. Some people with DM 1 may experience lifelong learning problems. 

DM 2

DM 2 begins in adulthood, typically between the ages of 20 to 40. Symptoms include skeletal muscle weakness, atrophy, myotonia, and enlargement of the calves. The muscles most commonly affected include the muscles of the thighs, upper arms, and trunk. Heart involvement, digestive problems, and pregnancy issues are not common.

Overall, the symptoms and progression of DM 2 are not as severe as those of DM 1, and it is not associated with cognitive problems. 

Causes 

Myotonic muscular dystrophy is a hereditary condition. Each of the two types is caused by a different genetic error that results in defective muscle function.

Genetics of DM 1 is caused by a defect in a protein that normally helps skeletal and heart muscles function efficiently. It is a hereditary autosomal dominant condition, which means that a person who inherits the genetic defect from one parent will develop the condition. Parents who have the condition will pass on the genetic defect to half of their children. The affected gene is called the myotonic dystrophy protein kinase gene, and it is located on chromosome 19.

Genetics of DM 2 is caused by a defect in a muscle protein called cellular nucleic acid binding protein (CNBP). This protein is present throughout the body and is more abundant in skeletal and heart muscle. Like DM 1, DM 2 is also autosomal dominant. It is caused by a defect in the CNBP gene, which is located on chromosome three. 

The genetic defects of DM 1 and DM 2 are both described as expansion mutations, which are mutations (alterations) in a genetic code characterized by abnormally elongated strands of DNA, which result in defective protein formation.

It is not completely clear why some associated problems that do not directly affect muscles (insulin resistance, baldness, and cataracts) develop with DM1 and DM2.

The genetic defect of DM 1 is associated with a problem called anticipation, which is an earlier onset of symptoms with each generation. This problem occurs due to biological, not psychological, reasons. With each generation, the DNA expansion can elongate, resulting in a more obvious effect of protein deficiency. Anticipation is not a characteristic of DM 2. 

Diagnosis 

Myotonic muscular dystrophy is one of the types of muscular dystrophy, and the symptoms often resemble those of the other muscular dystrophies.

Overall, there are nine types of muscular dystrophy and while they all cause muscle weakness, their symptoms differ slightly from each other, and they are each caused by different genetic defects.

Myotonic muscular dystrophy is diagnosed based on the symptoms, physical examination, and diagnostic tests. These methods do not confirm the diagnosis with absolute certainty, and DM 1 and DM 2 can both be confirmed by genetic testing. 

Physical Examination 

If you or your child has symptoms of myotonic muscular dystrophy, your doctor will begin by evaluating you with a thorough physical examination. Some of your muscles are expected to be slightly weak, and you may also demonstrate signs of myotonia, with a visible delay as you relax your muscles. This condition is also characterized by percussion myotonia, which is a sustained muscle contraction that occurs after your doctor places mild pressure on your muscles. 

Your doctor may also order diagnostic tests if you have symptoms and signs of myotonic muscular dystrophy, including the following.

Electromyography (EMG)

An EMG is an electrical examination of the muscles. It is very useful, and while it is slightly uncomfortable, it is not painful. During the test, your doctor places a tiny needle in your muscles which allows your muscle and nerve activity to be measured into a computer.

Myotonia produces a specific pattern on an EMG test, which is a range in the frequency of muscle contraction (how fast the muscles contract) between 20 and 80 Hertz and a variation of the amplitude (the size of the muscle contractions). 

The most defining feature of myotonic muscular dystrophy on EMG is a sound that is described as resembling an acceleration and deceleration of an engine, often described as similar to the sound of a dive-bomber. Someone trained in performing and interpreting this test would be familiar with this sound. 

Muscle Biopsy

A muscle biopsy is not diagnostic of myotonic muscular dystrophy because it is expected to show muscle fiber degeneration, which is common with many types of myopathies (muscle diseases) and muscular dystrophies. Nevertheless, you may have a muscle biopsy if your doctor needs it to rule out another condition.

A muscle biopsy is a sample of muscle tissue, usually taken from an affected muscle, which is then examined under a microscope. This is a minor surgical procedure that requires injection of local pain medication. After the procedure, you will need stitches for your wound, and you will need to avoid excess movement or strain of the biopsy area for about a week until it heals. 

Treatment 

There is no cure for either DM 1 or DM 2. The treatment is focused on relief of symptoms and prevention of complications.

Anesthesia

Anesthesia care is one of the most important aspects of myotonic muscular dystrophy treatment. If you have this condition, routine general anesthesia can be especially dangerous because the muscles that control your heart and your respiratory (breathing) muscles can relax more than usual or for longer than usual in response to the medications used for anesthesia.

To avoid complications, you need to have an experienced anesthesiologist closely monitoring your condition during any surgical procedures that require general anesthesia. 

Heart Problems

Regularly scheduled assessment of heart function is part of the management of myotonic muscular dystrophy. Treatment for heart rhythm abnormalities and heart failure will be initiated if and when any heart problems are identified. If you have bradycardia that does not improve with medication, you may need to have a pacemaker surgically implanted to regulate your heart rhythm. 

Respiratory Function

As with your heart function, your breathing function will be regularly assessed. If you develop breathing problems, you will be treated with oxygen or you may be prescribed a mechanical breathing device. This type of assistive breathing device is usually needed for sleep and is rarely required during waking hours. 

Insulin Resistance

Insulin resistance causes elevated blood sugar, which can cause major health complications. Insulin resistance can be easily diagnosed with a blood test, and it can be well managed with diet and medication to prevent complications. Because of the risk of insulin resistance, it is important that you have your blood sugar checked at regular intervals if you have DM 1 or DM 2.

Digestive Problems

If you have digestive problems due to your myotonic muscular dystrophy, your doctor may send you to a digestive specialist, and you will be given dietary guidance and possibly medications to manage your symptoms. 

Obstetrical Problems

If you have DM 1, a maternal-fetal medicine obstetrician who is familiar with high-risk conditions should manage your prenatal care and delivery. Your doctors will closely monitor your pregnancy and delivery and adapt to unexpected complications as needed. Due to the muscle problems that may occur, you may need a cesarean section, but this type of decision depends on your specific situation. 

Muscular Dystrophy Doctor Discussion Guide

Get our printable guide for your next doctor's appointment to help you ask the right questions.

Doctor Discussion Guide Child

Coping

The fatigue, learning difficulties, and muscle weakness that accompany myotonic muscular dystrophy are challenges that require individualized strategies. These problems are not treatable with medication or physical therapy, but rather with lifestyle adjustments. 

It is important to get enough rest when you feel sleepy or physically exhausted when you have myotonic muscular dystrophy.

If you think that learning difficulties could be a problem, it is best to have a formal evaluation as soon as possible and to take steps to ensure that you or your child enrolls in an appropriate educational program.

If your muscle weakness is associated with choking on food, it is best to have a speech and swallow evaluation and to adopt strategies for safe eating, such as chewing and swallowing your food carefully or eating soft foods. 

A Word From Verywell

If you or your child has been diagnosed with myotonic muscular dystrophy, it is hard to predict how severely the condition will affect you because it can affect each person with a different degree of severity. Be sure to be attentive to your symptoms and to discuss any changes with your doctor. Since this is an autosomal dominant condition, you may have a child with the condition if you or your spouse has it. Rest assured that with good medical care, any medical problems associated with myotonic muscular dystrophy can usually be greatly diminished.

Was this page helpful?

Article Sources