[A case of muscle sodium channelopathy with markedly high value of serum creatine kinase and mild eyelid myotonia].

We report on a Japanese 13-year-old male without a family history of muscle disease admitted to our hospital due to an elevated serum creatine kinase. From the age of 3 he was complaining of muscle stiffness during and after exercise. At the age of 7 he experienced muscle stiffness and weakness during long-distance running, which would continue till the next day, disappearing only after resting for a day. Upon examination, we noted that repeated eyelid contractions induced myotonia that increased in the cold. Electromyography revealed myotonic discharge in the tongue muscle. Genetic analysis revealed a mutation of Nav1.4, M1592V. Although this mutation had originally been reported in families with Hyperkalemic periodic paralysis (Hyper PP), we diagnosed as paramyotonia congenita due to the symptoms of exercise and cold-induced myotonia without an attack of generalized weakness. This case suggest that sodium channelopathy is very rare, but should be considered in the differential diagnosis of an elevation of serum creatine kinase even if coexisting myotonia is only mild.