Open Label Study in Adolescents and Children With Myotonic Disorders

Inclusion Criteria:

1. Male or female patients aged ≥ 6 and < 18 years who are able to comply with thestudy requirements

2. A genetically confirmed diagnosis of NDM or DM (DM1or DM2)

3. Presence of clinical symptoms of myotonia (hand grip myotonia, myotonia in the legmuscles, any other myotonia symptoms)

4. No significant cardiac abnormalities as determined by a cardiologist's assessment ofthe ECG and echocardiogram performed within 3 months prior to enrolment in thestudy. (If not done within 3 months before trial, electrocardiogram (ECG) andechocardiogram assessments will be performed at screening)

5. No history of any significant liver disorder

6. Patients receiving mexiletine treatment agree to stop treatment at least 7 daysprior to initiation of treatment with Namuscla

7. Patients receiving other antimyotonic treatment agree to stop treatment for at least 7 times the half-life of respective drug

8. Laboratory investigations for haematology, biochemistry, and urinalysis at screeningare within the normal range, or showing no clinically relevant abnormal values, asjudged by the Investigator.

9. Female patients of childbearing potential must be using an acceptable form of birthcontrol as determined by the Investigator (e.g., oral contraception, implantable,injectable/transdermal hormonal contraception, intrauterine device (IUD), barriermethods), tubal ligation or are practicing abstinence.

10. Patients able to provide assent to study participation and a parent or legalguardian to sign the written informed consent prior to study entry.

Exclusion Criteria:

1. Any contra-indication to mexiletine as listed in the Namuscla Summary of ProductCharacteristics (SmPC):

2. Hypersensitivity to the active substance, or to any of the excipients

3. Hypersensitivity to any local anaesthetic

4. Ventricular tachyarrhythmia

5. Complete heart block (i.e., third-degree atrioventricular block) or any heartblock susceptible to evolve to complete heart block (first-degreeatrioventricular block with markedly prolonged PR interval (≥ 200 ms) and/orwide QRS complex (≥ 120 ms), second-degree atrioventricular block, bundlebranch block, bifascicular and trifascicular block),

6. QT interval > 450ms

7. Myocardial infarction (acute or past), or abnormal Q-waves

8. Symptomatic coronary artery disease

9. Heart failure with ejection fraction <50%

10. Atrial tachyarrhythmia, fibrillation or flutter

11. Sinus node dysfunction (including sinus rate < 50 bpm) • Co-administration with medicinal products inducing torsades de pointes (classIa, Ic, III antiarrhythmics): Co-administration of mexiletine andantiarrhythmics inducing torsades de pointesclass Ia: quinidine, procainamide,disopyramide, ajmaline; class Ic: encainide, flecainide, propafenone,moricizine; class III: amiodarone, sotalol, ibutilide, dofetilide, dronedarone,vernakalant) increases the risk of potentially lethal torsades de pointes.

12. Co-administration with medicinal products with narrow therapeutic index

13. Any other neurological or psychiatric condition that might affect the studyassessments

14. Any clinically significant illness, laboratory findings, ECG, or other clinicalsymptoms, which in the opinion of the Investigator could affect the patient'soptimal participation in the study

15. Strong inducer or inhibitor of CYP2D6 or CYP1A2 within 7 days prior to study drugadministration

16. Any concurrent illness, or medications which could affect the muscle function

17. Seizure disorder, diabetes mellitus requiring treatment by insulin

18. Pregnant or breastfeeding

19. Concurrent participation in any other clinical trial.