MExiletine Versus Lamotrigine in Non-Dystrophic Myotonias

This is a clinical trial to determine if Lamotrigine is non-inferior to Mexiletine for the treatment of myotonia in patients with Non-Dystrophic Myotonia. Non-dystrophic Myotonia is a genetic condition for which there is no cure. It affects patients for the duration of their life and impacts work, leisure and can lead to significant morbidity. The study is a cross-over design - participants will be randomized to either lamotrigine or mexiletine first for 8 weeks and then swap over after a week wash-out to the other medication for a further 8 weeks. Participants and investigators will be blinded to the treatment schedule. 60 participants will recruited through the clinical service, national registry and national liaison.