Introduction: Myotonic dystrophy (DM) type 1 is a genetic disorder, caused by a trinucleotide CTG repeat expansion in the DMPK gene on chromosome 19. The skeletal muscle is the most severely affected organ, patients clinically presenting weakness and myotonia. Additionally, it may affect other organs, transforming it into a multisystemic disease. The pathophysiological mechanism involves RNA toxicity resulting from the expanded repeat in the mutant DM alleles. Phenotypes of DM1 may be classified as congenital, juvenile, classical or late onset. Case Report: We present the case of a 36-year-old male diagnosed with chronic hepatitis B virus infection 4 years ago, admitted to the Neurology Department for progressive weakness of the distal lower limbs and multiple falls without loss of consciousness due to muscle weakness. Considering the clinical evolution, neurological examination and paraclinical investigations, the patient was diagnosed with myotonic dystrophy type 1 and mild sensitive axonal polyneuropathy of the lower limbs. Conclusion: The diagnosis of DM can be established clinically if the patient has typical manifestations and positive family history. Genetic testing for CTG repeat expansion in the DMPK gene is the gold standard in diagnosing DM1. There is no disease-modifying therapy available for DM yet and management is only symptomatic.