What is Myotonic Muscular Dystrophy?

Myotonic muscular dystrophy (MMD), also known as dystrophia myotonica (DM), is a type of muscular dystrophy wherein myotonia is the cardinal symptom. Myotonia is a sustained involuntary contraction of a group of muscles, which means there is an inability to relax muscles voluntarily. For instance, there is difficulty in releasing grip on a handle or after a handshake. MMD is also characterized by muscle stiffness after use, progressive muscle wasting, and muscle weakness. The two common types of myotonic muscular dystrophy with distinct genetic defects are myotonic dystrophy type 1 (MMD1 or DM1) and myotonic dystrophy type 2 (MMD2 or DM2).

DM1 is the classic muscular dystrophy that is also known as Steinert disease. It is due to a mutation in the dystrophia myotonica protein kinase (DMPK) gene located at the long arm of chromosome 19. DM2, also called proximal myotonic myopathy, is due to a mutation in the zinc finger protein 9 (ZNF9) gene located at the long arm of chromosome 3. Both types of myotonic muscular dystrophy are autosomal dominant disorders. This means that one copy of a mutated gene in either parent is enough to cause the disorder in all of their children.

Myotonic muscular dystrophy often presents in adulthood and less commonly in late childhood. Distal limb muscles and neck muscles are involved early in the course, resulting in abnormalities in gait, foot drop, and swallowing difficulty. A person with myotonic muscular dystrophy may also experience weakness of facial muscles leading to the typical hatchet-faced appearance; weakness of muscles in wrists, fingers, and hands leading to impairment of normal daily function; and weakness of respiratory muscles affecting breathing and lung function. Constipation or diarrhea may occur because of weakness of the digestive tract muscles, and fainting or dizziness may occur because of weakness of heart muscles. Other abnormalities found in affected individuals are speech and voice problems, frontal balding, infertility, intellectual impairment, eye lenses clouding, insulin resistance, and excessive daytime sleepiness.

Heart disturbances, such as heart block and mitral valve prolapse, more commonly occur in DM1 than in DM2. A more severe form of DM1 occurs in approximately 25% of infants of affected mothers. This is known as congenital myotonic dystrophy and is characterized by severe muscle weakness, mental retardation, and difficulty in breathing, sucking, and swallowing. DM2 affects mainly the muscles near the center of the body. Heart disturbances, hatchet face, and frontal balding occurrences are uncommon.

Diagnosis of myotonic muscular dystrophy is usually established through history and physical examination. The doctor might request for electromyogram and muscle biopsy. Electromyogram involves measurement of the electrical activity of muscles. Muscle biopsy involves the surgical removal of a small piece of muscle for microscopic examination.

Treatment goals are to minimize disability as well as relieve symptoms and discomfort. Phenytoin and mexiletene are the preferred antimyotonia drugs. Cardiac pacemakers and cataract surgery should be considered for patients with heart disturbances and cloudy lens of the eyes, respectively. Use of walking assistive devices such as canes, walkers, and molded ankle-foot orthoses may help in mobility and gait problems. Sleep studies may be beneficial to prevent excessive daytime sleepiness.