Evaluation of the Efficacy and Safety of Metformin in the Myotonic Dystrophy Type 1 (Steinert's Disease)

Inclusion Criteria: * DM1 disease confirmed by genetic analysis * Men and women between 18 and 70 years of age. * Preserved walking abilities (stick assistance possible) * MIRS score 3 or 4 * Women of childbearing potential under efficient contraception during treatment * Patient able to consent * All patients who have completed and signed the specific information and informed consent form * Affiliation to a social security system Exclusion Criteria: * Pregnant or breast-feeding women * Men with an intention to conceive a child during the time of the study * Contraindications to Metformin (hypersensitivity to metformin or to one of the excipients) * Respiratory: * Patient requiring tracheotomy or * Patient requiring non-invasive-ventilation: - more than 12 hours per day; - insufficiently ventilated * Creatinine clearance inferior to 50 ml/min * Cardiac: * Left ventricular ejection fraction below 35% * Conduction system disease on the electrocardiogram with PR interval \>200 ms or QRS duration \>110 ms without a pacemaker or an implantable defibrillator or cardiac electrophysiological study performed over the past 5 years * Third-degree or Second degree type II atrioventricular block without a pacemaker or an implantable defibrillator * Sustained ventricular tachycardia * Acute disease that may lead to tissue hypoxia