The diagnosis Myotonic Dystrophy ususally is made by a neurologist, based on typical symptoms, such as myotonia and muscle weakness. The best test to confirm the diagnosis is by DNA testing. For this, a blood sample is needed. You may also be referred to a clinical geneticist (genetics expert).
A clinical geneticist can make the diagnosis, and can provide information about the possible risk for relatives. The clinical geneticist can also provide counselling about hereditary aspects of the disease in case of the desire to become parents.