Clinical Reasoning: A 6 year old boy with muscle twitching.

A 6-year-old biracial (Black and White) boy presented with worsening muscle twitching and stiffness. He had normal birth and development. His family noticed muscle twitching involving his thighs when he was 3 years old. Over the next 3 years, twitching spread to involve shoulders, chest, lower back, arms, and lower legs. The patient would feel muscle twitching underneath his skin and at times, these could be visible. His symptoms were worse with cold exposure. Recently, he started experiencing difficulties with fine motor activities such as writing, holding pencils, tying shoelaces, and buttoning. The patient denied myalgia, weakness, rhabdomyolysis, and paresthesias. Medical history includes well-controlled asthma, allergic rhinitis, right hydrocelectomy, and a small bowel intussusception. Family history was unremarkable. Neurologic examination revealed increased appendicular muscle tone, fasciculations involving upper and lower extremities, anterior chest, and paraspinal muscles, mild difficulty releasing hand grip, nasal dysarthria, and bilateral tight heel cords. Sensory examination, reflexes, and gait including heel and toe walking were normal.