Clinical Reasoning: A child with muscle stiffness

A 10-year-old boy presented with muscle stiffness of all extremities. His symptoms typically began during the second or third period of his hockey games and were intermittent, resolving within minutes of cessation of activity. Otherwise, he was asymptomatic. He was able to play baseball and soccer without similar symptoms, and he and his family denied clear triggers such as cold, heat, or meals with high carbohydrates or potassium. He denied vision problems, facial weakness, dysphagia, dysarthria, dyspnea, orthopnea, cramping, and fasciculations. He also denied sensory changes and bladder or bowel dysfunction. His family history was notable for myotonia congenita in his father, who was genetically heterozygous in 2007 for the pathogenic c.1437_1450del variant in the CLCN1 gene.1 His father reported similar symptoms including stiffness exacerbated by physical exertion. His parents were specifically concerned that the patient had inherited the mutation from his father. On neurologic examination, the patient had eyelid myotonia and trace percussion myotonia in the right thenar eminence. Strength was 5/5 throughout, and deep tendon reflexes were 2+ and symmetrical in all 4 extremities. Muscle bulk and tone were normal, sensory examination was normal, and coordination was intact. The patient's father had grip and eyelid myotonia and muscle hypertrophy.