Understanding Myotonic Dystrophy : Causes, Types, Symptoms, and Treatment

Myotonic dystrophy is a form of muscular dystrophy, which is a category of genetic disorders of which Myotonic dystrophy is the commonest type of muscular dystrophy seen in adults.⁽¹⁾ The symptoms of this condition develop around the age of 20s or 30s; however, they can occur at any age also. Myotonic dystrophy is characterized by weakness and muscle wasting, which worsens as the time goes on.⁽³⁾ This makes it hard for the patient to relax their muscles after they have held something.

The two primary types of myotonic dystrophy are:

Myotonic Dystrophy Type-I: The cause of this type is mutations occurring in the DMPK gene. According to experts, the protein present in the DMPK gene is responsible for affecting the transmission in the cells. This is considered to perform a role in the functioning of the skeletal muscles, heart and brain; all of which are important for movement of the body.

There are 2 subtypes of myotonic dystrophy type-I. The first is a mild form often occurring in mid to late adulthood. The second subtype is a congenital form, which is present at birth itself. In case of mild myotonic dystrophy type-I, the intensity of the symptoms is less.

In the congenital subtype, patient can suffer from breathing problems, weak muscle tone, clubfoot, delayed development or intellectual disability. Some of these symptoms can be life threatening.

Myotonic Dystrophy Type-II: The cause of this second type is mutations in the CNBP gene. The protein derived from this gene is present in the skeletal muscles and heart; and it helps in managing the action of the other genes within the body.

Any type of structural changes in any of these two genes causes Myotonic Dystrophy where an area of the DNA of the genes multiplies excessively resulting in an unbalanced piece. The gene containing this part then produces an exceptionally long messenger RNA, which causes problems with other proteins.⁽²⁾ As time goes on, this can halt the normal functioning of the tissues and muscles.

In case of myotonic dystrophy type I, the DMPK gene is affected and in myotonic dystrophy type II, the CNBP gene in affected.

Some of the common symptoms of this condition are slurred speech, myotonia, cataracts, temporary locking of the jaw, diabetes and defects in the cardiac conduction.^{(1, 2)}

Weakness of the muscle in type I of myotonic dystrophy can affect the muscles, which are situated farthest from the center of the body and these are known as distal muscles and consist of face, hands, neck and lower legs. Myotonic dystrophy type I can also cause problems with swallowing, gallstones or constipation. Myotonic Dystrophy can also cause abnormal contraction of the uterine muscles.

Myotonic dystrophy type II can cause muscle weakness often in the muscles, which are located closer to the center of the body and are referred to as the proximal muscles. There may also be weakness in your neck, shoulders, elbows and hips.

A physical exam conducted by doctors specializing in neuromuscular disorders is able to diagnose this condition. For diagnosis, the doctor looks for patterns of weakness, muscle wasting and myotonia.

Lab tests are also done to confirm the diagnosis and include:⁽²⁾

Electromyography (EMG) is a test where a small needle is inserted into the muscle and its electrical activity is observed.

A genetic test is done to look for certain cells in the blood and see if there are any changes in the DMPK or CNBP gene.

There is no specific treatment or cure for Myotonic type I and II.⁽²⁾ At present, treatment consists of management of symptoms and disability control.⁽²⁾ Treatment of some of the symptoms, such as below, is done by:⁽²⁾

If Pain is present, then over-the-counter pain medications are prescribed or medications which specifically can address the pain of this condition, such as gabapentin and nonsteroidal anti-inflammatory drugs, mexiletine, tricyclic antidepressants, and low-dose glucocorticoids, such as oral prednisone are prescribed. Other measures, such as massages, warm baths and heating pads can also be utilized for pain relief.

In case of Myotonia, mexiletine is prescribed.

For treating Muscle Weakness, the use of assistive devices, such as scooters, walking aids or wheelchairs are recommended.

Cataracts can be surgically removed if they are interfering with daily life.

For treating Breathing and Coughing Muscle Weakness, air is pumped into the lungs at night using a BiPAP device. Assisted cough techniques or cough assistance machine can also be used to alleviate this symptom.

Gastrointestinal dysfunction can be treated with laxatives, suppositories or enemas.

Daytime fatigue can be combated with medications such as modafinil and methylphenidate.

Behavior and Cognitive problems can be addressed by neuropsychologists, counseling and special education assistance. Medications if needed can also be given to increase attention span and alertness.

Cardiac problems, such as abnormal heart rhythm caused by myotonic dystrophy can be treated with pacemaker, medication or an implantable defibrillator.

Insulin resistance is treated with medication to lower the levels of blood sugar.

If surgery is done, then it is imperative to be extremely cautious with anesthesia in patients having myotonic dystrophy, as there is an increased rate of complications from general anesthesia and even death for patients having type I myotonic dystrophy.

Prognosis differs from patient to patient and often there is gradual worsening of the symptoms.⁽²⁾ The life expectancy is less for patients suffering from congenital myotonic dystrophy type I and adult-onset myotonic dystrophy type I.

However, if myotonic dystrophy type I starts before adolescence, then the disease pattern can be different. After a child survives the congenital-onset myotonic dystrophy type I then, often there is improvement in the symptoms; however, the patient can continue to suffer from cognitive problems, trouble eating and drinking, delayed speech and other development problems.

The prognosis of myotonic dystrophy type II is better when compared to type I, with less severity and slow worsening of the symptoms.⁽⁴⁾

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