To diagnose myotonic dystrophy, a doctor will take a family history, perform a physical exam, and order lab tests. These tests may include a blood test, a muscle biopsy, and an electromyogram (EMG). A blood test can look for the specific genetic defect that causes DM1 or DM2. A muscle biopsy can tell if muscle weakness is caused by myotonic dystrophy, which is inherited, or something else, such as inflammation or exposure to a toxic chemical. An EMG measures electrical activity in a muscle as it relaxes and contracts.