Teaching Video NeuroImage: Carbamazepine Improves Gait Initiation in Autosomal Recessive Myotonia Congenita

A 60-year-old man presented with muscle stiffness. He had tended to fall since childhood. His parents were first cousins, and his relatives had no symptoms. Examination showed myotonia in ocular, hand, and limb muscles, followed by improvement with repeated activity (warm-up phenomenon; Video 1). Cold exposure did not aggravate myotonia. His lower legs showed muscle hypertrophy. Genetic tests showed a homozygous p.M560T mutation in the CLCN1 gene, which encodes skeletal muscle chloride channel 1, consistent with autosomal recessive myotonia congenita (Becker disease).1 Although treatment with mexiletine or phenytoin did not improve his symptoms, carbamazepine (350 mg/d) did, especially the walking disturbance (Video 2). Although the warm-up phenomenon is sometimes also observed in sodium channel myotonia,2 it is characteristic of myotonia congenita and an important clue for the diagnosis.