Abstract: Myotonic dystrophy type 1, also known as Steinert’s disease, is an autosomal dominant disorder with multisystemic clinical features affecting the skeletal and cardiac muscles, the eyes, and the endocrine system. Thiamine (vitamin B1) is a cofactor of fundamental enzymes involved in the energetic cell metabolism; recent studies described its role in oxidative stress, protein processing, peroxisomal function, and gene expression. Thiamine defciency is critical mainly in the central and peripheral nervous system, as well as in the muscular cells. Our aim was to investigate the potential therapeutical effects of long-term treatment with thiamine in myotonic dystrophy type 1 in an observational open-label pilot study. We described two patients with myotonic dystrophy type 1 treated with intramuscular thiamine 100 mg twice a week for 12 or 11 months. We evaluated the patients using the grading of muscle strength according to Medical Research Council (MRC), the Muscular Impairment Rating Scale (MIRS), and the Modifed Barthel index. High-dose thiamine treatment was well tolerated and effective in improving the motor symptomatology, particularly the muscle strength evaluated with the MRC scale, and the patients’ activities of daily living using the Modifed Barthel Index. At the end of treatment, the MRC score was 5 in the proximal muscles and 2–4 in the distal muscles (the MRC score before the treatment was 3–4 and 1–3, respectively). The MIRS grade improved by 25% compared to baseline for both patients. In patient #1, the Modifed Barthel Index improved by 44%, and in patient #2 by 29%. These fndings suggest that clinical outcomes are improved by long-term thiamine treatment.

Key words: nerve regeneration, myotonic dystrophy type 1, thiamine, Steinert’s disease, muscular strength, activity of daily living, neural regeneration