Effectiveness of Mexiletine for Treating People With Non-Dystrophic Myotonia

Inclusion Criteria: * Clinical symptoms or signs suggestive of myotonic disorders * Presence of myotonic potentials on electromyography (EMG) * Participant in the Non-Dystrophic Natural History study (RDCRN 5303) or a new patient with confirmed non-dystrophic myotonia Exclusion Criteria: * Other neurological condition that might affect the assessment of the study measurements * Genetic confirmation of DM1 (more than 50 repeats of CTG) or DM2 * Existing cardiac conduction defects, as evidenced on EKG, including but not limited to the following conditions: malignant arrhythmia or cardiac conduction disturbances (e.g., second degree AV block, third degree AV block, or prolonged QT interval) * Existing permanent pacemaker * Current use of any of the following antiarrhythmic medications for a cardiac disorder: flecainide acetate, encainide, disopyramide, procainamide, quinidine, propafenone, or mexiletine * Use of medications for myotonia, such as phenytoin and flecainide acetate, within 5 days of study entry; carbamazepine and mexiletine within 3 days of study entry; or propafenone, procainamide, disopyramide, quinidine, and encainide within 2 days of study entry * Use of medications that produce myotonia, which may include fibrate acid derivatives, hydroxymethylglutaryl CoA reductase inhibitors, chloroquine, and colchicines * Kidney or liver disease * Heart failure * Seizure disorder * Pregnant or breastfeeding