Table 1 Inclusion and exclusion criteria

1. At least 18 years of age

1. Inability or willingness to approved to provide informed consent

2. Genetically confirmed diagnosis of NDMs

2. Other neurological conditions that might affect the assessment of the study measurement

3. Genetically confirmed DM1 (CTG > repeats), or DM2

4. Existing cardiac conduction defects, evidenced on ECG including but not limited to the following condition: malignant arrhythmia or cardiac conduction disturbance (such as second degree AV block, third degree AV block, or prolonged QT interval >500 ms or QRS duration > 150 msec)

5. Current use of the following antiarrhythmic medication for a cardiac disorder: flecainide acetate, encainide, disopyramide, procainamide, quinidine, propafenone or mexiletine

6. Women who are pregnant or lactating

7. Currently on medication for myotonia such as phenytoin and flecainide acetate within 5 days of enrollment, carbamazepine and mexiletine within 3 days of enrollment, or propafenone, procainamide, disopyramide, quinidine and encainide within 2 days of enrollment

8. Renal or hepatic disease, heart failure, history of myocardial infarction, or seizure disorders