An 18-year-old male runner was referred to the authors' clinic with a 1-year history of cramping left foot pain. His symptoms developed insidiously when he increased his training, with an onset of severe left foot pain and tightness that would develop after about 20 minutes of exercise. The more he continued to run, the more the symptoms were aggravated and evolved to the toes curling with intrinsic muscle spasm. This symptom was easily provoked when he was exposed to cold temperature. A family history of similar symptoms was revealed in his sister and uncle. Physical examination including neurologic examination was normal. Diagnostic workup revealed generalized myotonia. According to the history, physical examination, and diagnostic workup, his diagnosis was considered to be most compatible to paramyotonia congenita. He was given phenytoin, which lessened his symptoms and allowed him to continue running with minimal symptoms, but he stopped running because he was not able to maintain mileage high enough to compete successfully.
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