Beyond limb stiffness: dysarthria in a Persian patient with becker myotonia and a homozygous CLCN1 (p.Arg377Ter) variant-a case report and literature review

Background Myotonia congenita is a rare inherited skeletal muscle channelopathy caused by pathogenic variants in the CLCN1 gene, typically characterized by muscle stiffness, delayed relaxation, and a warm-up phenomenon. Although the clinical and genetic spectrum of CLCN1-related myotonia congenita is expanding, dysarthria is not a well-recognized presenting feature. We report a Persian patient with Becker-type myotonia congenita carrying a homozygous pathogenic CLCN1 variant, in whom dysarthria was the chief presenting complaint. Case presentation A 13-year-old Persian boy presented with dysarthria and several years of progressive proximal muscle weakness, predominantly affecting the lower limbs. He also reported muscle stiffness most prominent after rest and improved with repeated movements, consistent with a warm-up phenomenon, and exacerbated by cold exposure. Neurological examination revealed generalized true muscle hypertrophy, particularly of the deltoid and back muscles, along with generalized myotonia involving the tongue. Proximal muscle strength was reduced, while sensory examination and deep tendon reflexes were normal. EMG demonstrated myogenic abnormalities with prominent myotonic discharges and no evidence of neuropathy. Whole-exome sequencing identified a homozygous pathogenic nonsense variant, NM_000083.3(CLCN1):c.1129 C > T (p.Arg377Ter), in exon 10 of CLCN1, confirming Becker-type myotonia congenita. Treatment with mexiletine and a rehabilitation program including aerobic exercise, stretching, and physical therapy was initiated. Conclusion This case describes a Persian patient with genetically confirmed Becker-type myotonia congenita carrying the previously reported homozygous CLCN1 c.1129 C > T (p.Arg377Ter) variant. Dysarthria was the chief presenting complaint and co-occurred with clinically evident tongue myotonia. This report highlights the importance of assessing speech difficulties in patients with myotonia congenita.