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

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.