Genetic Testing

Genetic testing (also referred to as DNA testing) is a definitive diagnostic of whether or not a person has myotonic dystrophy (DM). DNA, the genetic material in the nucleus of cells, is isolated from a sample of blood or other tissue, then analyzed to determine whether or not a specific alteration, also called a mutation, is present.

Individuals with myotonic dystrophy have one of two possible mutations: Patients with DM1 have a mutation that creates an abnormally expanded length of DNA in the dystrophia myotonica protein kinase (DMPK) gene on chromosome 19; Individuals with DM2 have an expanded form of the zinc finger 9 (Znf9) gene on chromosome 3.  The possible existence of other forms of myotonic dystrophy, caused by different mutations, are currently being investigated.

Who should be tested?

Typically, genetic testing is used to confirm a presumptive diagnosis of DM after clinical observation and medical tests suggest the presence of the disease. Also, myotonic dystrophy may be one of multiple disorders tested when an individual has myotonic dystrophy-like symptoms, but diagnosis is uncertain. In these cases, compelling medical reasons exist for testing:

  • A confirmed diagnosis allows surveillance of symptoms and enables treatment at the earliest stage. Knowledge can also reduce uncertainty and anxiety about existing symptoms. It can also prevent unnecessary treatments or further testing.
  • Patients can be warned of potential dangers of anesthesia, so precautions can be taken during surgery. See Anesthesia Guidelines for further information.
  • Appropriate family planning can be done. Please see the Family Planning section for further information on the options available to individuals with myotonic dystrophy who are considering having children.
  • Mothers with DM1 can have special monitoring during pregnancy and preparations can be made for immediate intervention at birth as children of affected mothers are at high risk for having congenital myotonic dystrophy. This surveillance is typically not required for women with DM2 as no congenital forms of the disease have been reported.

However, in many other instances, the decision whether or not to test is more complicated. Once an affected individual is diagnosed, should the person tell family members who show no symptoms? If he does share his positive diagnosis of myotonic dystrophy, his family must then decide whether or not to be tested. Because the mutation can be present with few or no symptoms, family members who are affected may not be aware they have myotonic dystrophy. There are three explanations as to why some individuals with myotonic dystrophy might not be symptomatic:

  • symptoms may be so mild these individuals do not realize they have the disorder,
  • they may have a late onset form of the disease and do not yet exhibit symptoms, or
  • they may carry a pre-mutation (a form of the mutation that is less extensive than that seen in patients who display symptoms). These individuals are not likely to develop disease symptoms, but their children are at risk of inheriting the mutation and having the disorder.

Testing of asymptomatic individuals does present some disadvantages, including the following:

  • difficulties in obtaining insurance, e.g. health, disability and life
  • prejudice in the workplace or elsewhere
  • impact of being diagnosed with a disorder when no cure or treatment capable of slowing the progression of myotonic dystrophy yet exists

There is no right answer to the question of whether or not to be tested. The decision is a personal one, and it is recommended that individuals speak with a genetic counselor to better understand their options and the consequences of testing.

Regardless of whether relatives decide to be tested or not, these individuals should inform their physicians that they have a family history of myotonic dystrophy.