Mexiletine and Non Dystrophic Myotonias

Inclusion criteria : * Genetically definite MC and PC. * Male and female participants, age between 18 and 65 who are able to comply with the study conditions. * Participants who experience myotonic symptoms severe enough to justify treatment. The severity will be evaluated on: * Clinical criteria: myotonia is considered as severe if it involves at least two segments (upper limb, lower limb or face) * Disabling criteria: myotonia is considered severe if patients notice impacts on at least 3 of the 7 daily activities listed in the disabling section of the clinical myotonia scale (Annex 2). Thus, patients who experience myotonic symptoms severe enough to justify treatment are those with myotonia that involves at least two segments and that have an impact on at least 3 daily activities. * Participants who are drug naive or those who receiving mexiletine at Effective dosage and agreeing to stop treatment at least four days before inclusion . * Pregnancy: Women: non-childbearing potential (i.e., postmenopausal or Surgically sterile) or must use a medically accepted contraceptive regimen; a pregnancy test will ensure that they are not pregnant. * Normal cardiac exam performed by a cardiologist including EKG, and Cardiac ultrasound (if not done within 3 months before trial). Exclusion criteria : * Intercurrent event which could interfere with the muscle function (infection,trauma, fracture, …) * Coincidental renal, hepatic, respiratory, thyroid, other neuromuscular disease or …