Open-label Extension Study in Paediatric Patients Who Have Completed the MEX-NM-301 Study.

Open-label Extension Study to Evaluate the Long-term Safety and Efficacy of Mexiletine in Paediatric Patients with Myotonic Disorders Who Have Completed the MEX-NM-301 study.

Inclusion Criteria: 1. Patients previously completed the parent study PIP study 4 (MEX-NM-301) and tolerated the Mexiletine in the study. 2. Able and willing to provide assent to study participation and a parent or legal guardian willing to sign written informed consent prior to study entry. 3. Patients continue to meet inclusion criteria of parent study (MEX-NM-301): * No significant cardiac abnormalities as determined by a cardiologist's assessment of the ECG and Echocardiogram * No history or evidence of any significant liver disorder Laboratory investigations for haematology, biochemistry, and urinalysis at screening are within normal range, or showing no clinically relevant abnormal values, as judged by the Investigator * 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 or have a vasectomized partner or are practicing abstinence Exclusion Criteria: 1. Clinically significant laboratory abnormality, ECG or other clinical findings on physical examination indicative of a clinically significant exclusionary disease as determined by the investigator 2. Any contra-indication to mexiletine (as described in the Namuscla Summary of Product Characteristics \[SmPC\]) * Hypersensitivity to the active substance, or to any of the excipients * Hypersensitivity to any local anaesthetic * Ventricular tachyarrhythmia * Complete heart block (i.e., third-degree atrioventricular block) or any heart block susceptible to evolve to complete heart block (first-degree atrioventricular block with markedly prolonged PR interval (≥ 200 ms) and/or wide QRS complex (≥ 120 ms), second-degree atrioventricular block, bundle branch block, bifascicular and trifascicular block), * QT interval \> 450ms * Myocardial infarction (acute or past), or abnormal Q-waves * Symptomatic coronary artery disease * Heart failure with ejection fraction \