Myotonic dystrophy type 1 (DM1) is an inherited genetic disorder characterized by progressive muscle weakness, myotonia (prolonged muscle contractions), and various systemic features. In DM1, an abnormal DNA expansion is observed in the DMPK (dystrophia myotonica protein kinase) gene on chromosome 19q 13.3. Genetic testing for DM1 involves analyzing the number of CTG repeats in the DMPK gene. CTG repeat length exceeding 34 repeats is considered abnormal.

A positive result of DM1 test indicates the presence of the expanded CTG repeats in the DMPK gene, confirming a diagnosis of DM1. The result can also provide information about the severity and age of onset of the disease. A negative result means that the expanded CTG repeats were not detected in the tested region of the DMPK gene. However, in some cases, the number of repeats may fall within the “intermediate” range, where symptoms are not evident but the risk of having affected offspring is present. In such cases, genetic counseling is essential.