This is an open-label extension study evaluating the long-term efficacy and safety of mexiletine in paediatric patients with myotonic disorders who have completed the initial parent paediatric study with mexiletine (Protocol No. MEX-NM-301 (PIP Study 4) for children and adolescents aged 6 to < 18 years and who continue to meet the eligibility criteria.~Patients who meet the eligibility criteria and provide consent for this study will be enrolled sequentially by decreasing age groups. Patients aged 12 to < 18 years will enter first as this is the first cohort expected to complete the parent study PIP Study 4 based on top down recruiting. Once initial pharmacokinetics (PK), safety and efficacy are confirmed in this population, patients aged 6 to <12 years will be first enrolled in PIP Study 4 and subsequently this study (PIP Study 7).~Enrolled patients will receive mexiletine at a dose determined in the parent study. Dosing is determined according to body weight and tolerability.~The study includes 9 clinic visits - V1 (baseline), and V2 to V9 every 3 months, approximately, thereafter.~The total duration of study will be 24 months per patient. End-of-treatment (EOT) visit will occur at 24 months or in accordance with the availability of product. The overall study duration would be approximately 5 years.
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.