Although DM is inherited as a classic autosomal dominant disease, incomplete penetrance, possible anticipation, and extreme variation in clinical expression make genetic counseling a critical component of DM diagnosis and case management.

Genetic counseling encompasses the clinical, technical and psychosocial components of genetic testing and disease. The genetic counseling process is an opportunity for patients to explore their questions and concerns about genetic disease and testing, as well as a means for medical professionals to develop comprehensive clinical histories.

The following components may be part of a genetic counseling session:

Understanding the role of anticipation in DM is especially important when affected individuals contemplate reproduction. A mother with a 50 to 150 CTG repeat expansion in the DMPK gene (mild phenotype) can have a child with a 1000 to 2000 CTG repeat expansion (CMyD phenotype) as a result of anticipation.

In the event of pregnancy in a DM affected woman, a genetic counselor discusses the options for prenatal diagnostic genetic tests. DM1 and DM2 are both autosomal dominant disorders that have variable penetrance and extreme variation in clinical expression between multiple members of each pedigree. A dramatic example of variation in clinical expression is a mother with the mild form of DM1 transmitting the severest form of DM, CMyD, to her child.

The cause of variability in clinical expression between generations is anticipation. In DM1, the CTG trinucleotide repeat expansions can amplify in successive generations, resulting in a decrease in the age of onset and increase in the severity of symptoms in successive generations. CMyD can be inherited by either sex, but typically is through a mother affected with DM1, not an affected father. Anticipation has not been identified as a characteristic of DM2.