This clinical trial is focused on studying the long-term safety and effectiveness of a medication called Mexiletine in children with certain muscle disorders known as myotonic disorders. These disorders include nondystrophic myotonias and myotonic dystrophies, which can cause muscle stiffness and difficulty relaxing muscles after use. The study involves children who have already participated in previous studies and tolerated the medication well.
The treatment being tested includes different doses of Mexiletine in the form of hard capsules, specifically Mexiletine 62 mg, Mexiletine 83 mg, and Namuscla 167 mg. The purpose of the study is to gather more information about how safe and effective Mexiletine is for treating symptoms of muscle stiffness in children over a longer period. Participants will take the medication by mouth and will be monitored regularly to check for any side effects and to see how well the medication is working in reducing muscle stiffness.
Throughout the study, the children will have their muscle stiffness assessed using simple scales and tests. These assessments will help determine any changes in their condition over time. The study will also look at the overall quality of life of the participants and any changes in their health. The goal is to ensure that Mexiletine is a safe and effective treatment option for children with these muscle disorders.



France