What is myotonic dystrophy?

Myotonic dystrophy (DM) is a muti-systemic inherited disease that affects at least 1 in 8,000 people or 40,000 individuals in the US alone. Although often viewed as a muscle disease, individuals affected by DM may have skeletal muscle problems, heart function abnormalities, breathing difficulties, cataracts, issues with speech and swallowing (dysarthria and dysphagia), cognitive impairment, excessive daytime sleepiness, or diabetic symptoms. Any single individual is unlikely to have all or even most of these symptoms.

Myotonic dystrophy is one of the most variable and complicated disorders known. The systems affected, the severity of symptoms, and the age of onset of those symptoms vary greatly between individuals, even in the same family. In general, the younger an individual is when symptoms first appear, the more severe symptoms are likely to be. However, prognosis is as variable as the symptoms of this disease.

What are other names for myotonic dystrophy?

  • Myotonic muscular dystrophy - often abbreviated as MMD
  • Myotonia atrophica - a Latin name, not commonly used
  • Dystrophia myotonica - a Latin name used by many doctors; often abbreviated as DM. The different types of DM are typically referred to as DM1 or DM2.
  • DM1 is also known as Steinert's Disease, named for the German doctor who first identified this disorder in 1909.
  • DM2 is also known as Proximal myotonic myopathy or PROMM.

Myotonic dystrophy should not be confused with other disorders with similar names (e.g.myotonia congenita [Thomsen’s disease] and congenital muscular dystrophy).

What is the difference between myotonic dystrophy and muscular dystrophy?

Muscular dystrophy (MD) refers to a group of nine genetic diseases that cause progressive weakness and degeneration of muscles used during voluntary movement.  Myotonic dystrophy (DM) is one of the muscular dystrophies. It is the most common form seen in adults and is suspected to be among the most common forms overall.

What are the types of myotonic dystrophy?

There are two well-defined types of the disease (DM1 and DM2) which have distinct but overlapping symptoms. Both DM1 and DM2 are characterized by muscle weakness and myotonia, heart abnormalities, cataracts and insulin resistance. In general, DM2 is less severe than DM1: fewer systems are affected, patients develop the disease only as adults, and the disorder’s impact on everyday life is relatively less disruptive

In contrast, DM1 can occur from birth to old age. Symptoms vary greatly among patients, from minor muscle pain to serious respiratory and cardiac issues. The congenital form of DM1 is the most severe version and has distinct symptoms that can be life-threatening.

How do people get myotonic dystrophy?

Myotonic dystrophy is an inherited disease where a change, called a mutation, has occurred in a gene required for normal muscle function. The mutation prevents the gene from carrying out its function properly. The change is an autosomal dominant mutation, which means one copy of the altered gene is sufficient to cause the disorder. As a result, affected individuals have a 50% chance of passing on the mutated gene to their children. A child is equally likely to have inherited the mutated gene from either parent. If both parents do not have the disease, their children cannot inherit it. The congenital form of DM1 is inherited differently from the other types of myotonic dystrophy. Children with congenital myotonic dystrophy almost always inherit the disease from an affected mother.

How is myotonic dystrophy diagnosed?

A complete diagnostic evaluation, which includes family history, physical examination, and medical tests, is typically required for a presumptive diagnosis of myotonic dystrophy. The presence of the disorder can then be confirmed by genetic testing. The genetic test requires a sample of blood from the patient. The DNA is then extracted from the blood and analyzed to see if that person has the mutation that causes myotonic dystrophy. Prenatal testing, where the DNA of the fetus is checked for the presence of the myotonic dystrophy mutation, is also available.

Diagnosis of myotonic dystrophy is not difficult once the disorder is suspected. However, delays in diagnosis are common. More common diseases with symptoms that mimic myotonic dystrophy must typically first be ruled out before this disorder is considered. The symptoms are complex. Physicians may see only one or two patients with DM in their entire practice and may not be familiar with the range of ways this disease can present.