Study to Investigate the Efficacy and Safety of Mexiletine in Patients With Myotonic Dystrophy Type 1 and Type 2

Inclusion Criteria: 1. DM1 or DM2 diagnosis confirmed genetically; 2. Ability to provide informed consent; 3. Ability to understand the study requirements including intention to stay in the study until the end-of-study visit at 26 weeks of treatment; 4. Male or non-pregnant female ≥18 years of age; 5. Female patients of childbearing potential must be using an acceptable form of birth control as determined by the investigator (e.g., oral contraception, implantable, injectable/transdermal hormonal contraception, intrauterine device (IUD), barrier methods), tubal ligation, have a vasectomized partner, or are practicing abstinence; 6. No significant cardiac abnormalities as determined by a cardiologist's assessment of the electrocardiogram (ECG) and echocardiogram; 7. Capable of swallowing capsules; 8. Have sufficient finger flexor strength to grasp the handle of the dynamometer used to measure myotonia; 9. Presence of clinical handgrip myotonia (delayed relaxation of grip of ≥ 3 seconds after maximum voluntary contraction) at screening; 10. Have a Day 1 (pre-dose) handgrip dynamometer mean relaxation time of ≥1.5 seconds for the force to decline from 90% of maximum voluntary contraction force to 5%; 11. Be able to walk independently 10 meters (cane, walker, orthoses allowed); 12. DM1 patients only - Muscular impairment rating scale (MIRS) score of 2, 3, or 4. Exclusion Criteria: 1. Are pregnant or lactating; 2. Have any one of the following medical conditions: uncontrolled diabetes mellitus, cancer other than skin cancer less than five years previously (e.g., basal-cell carcinoma (BCC) and squamous-cell carcinoma (SCC) of skin allowed), multiple sclerosis, seizure disorders, or other serious medical illness; 3. Severe renal impairment (glomerular filtration rate (GFR) \< 30 mL/min); 4. Medical conditions which could interfere with muscle function such as infections, trauma, fractures, or planned surgery; 5. Medical conditions that could affect hand functioning including but not limited to rheumatoid arthritis, Dupuytren's contracture, hand deformity, etc.; 6. Severe arthritis or other medical condition (besides DM1/DM2) that would significantly impact ambulation; 7. High incidence of falls or fall-associated fractures (\>5 falls during the past 12 months); 8. Preexisting elevated liver function tests \> 3 times the upper limit of normal (ULN) at screening (alanine transaminase (ALT)/aspartate transaminase (AST), gamma-glutamyl transferase (GGT)) and/or any abnormal chemistry, hematology or urine lab considered clinically significant by the investigator; 9. Treatment with mexiletine within 4 weeks prior to baseline (Day 1); 10. Intake of any anti-myotonic treatment within 4 weeks prior to baseline (Day 1) such as propafenone, flecainide, lamotrigine, carbamazepine or any other channel-blocker/ anticonvulsive drugs; 11. Use of any concomitant medications that could increase the cardiac risk; 12. Known allergy to mexiletine or any local anesthetics; 13. Participation in another interventional clinical study during the last 3 months; 14. Wheelchair-bound or bed-ridden; 15. Any cardiac safety-associated condition including any of the following criteria detected by screening cardiac evaluations including 24-hour Holter monitoring, ECG, echocardiogram and clinical evaluations: * PR interval ≥240 ms or QRS duration ≥120 ms on resting ECG * Personal history of 3rd degree or 2nd degree type 2 atrioventricular block or sinus node dysfunction with pauses ≥3 seconds * Personal history of sustained atrial fibrillation, flutter or tachycardia (duration \>30 seconds) * Personal history of non-sustained (ventricular triplets or more) or sustained ventricular tachycardia * Myocardial infarction (acute or past) or coronary artery stenosis \>50% * New York Heart Association (NYHA) Class II to IV heart failure * Left ventricular systolic dysfunction with ejection fraction \<50% * Sinus node dysfunction (including ECG sinus rate \<50 beats per minute (BPM)) * Co-administration of mexiletine and antiarrhythmics inducing torsades de pointes (class Ia: quinidine, procainamide, disopyramide, ajmaline; class Ic: encainide, flecainide, propafenone, moricizine; class III: amiodarone, sotalol, ibutilide, dofetilide, dronedarone, vernakalant) * Patients with implantable cardioverter defibrillators (ICDs) and pacemakers are excluded