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

The study team hypothesize that non-diabetic patients with Myotonic dystrophy type I (DM1) will improve their symptoms, especially their motor deficit which is the main feature of the disease, because of the splicing defect correction by metformin. The primary objective of the study is to evaluate the efficacy of metformin vs placebo, on the improvement of muscle function in patients with DM1 compared to its placebo. As the secondary objectives, the study aims: * To evaluate the safety of metformin on patient with DM1. * To evaluate the efficacy of metformin vs placebo on: 1. The hand-grip strength; 2. The thumb-index pinch strength; 3. The locomotor function; 4. The respiratory function; 5. The cardiac function; 6. The quality of life; 7. The daily and social activity.