Adult-Onset Hypokalemic Periodic Paralysis With the p.Arg672Cys Variant in the SCN4A Gene: A Case Report.

Hypokalemic periodic paralysis (HypoKPP) is a rare channelopathy that typically presents in adolescence with recurrent episodes of muscle weakness triggered by factors that lower serum potassium. We report a 37-year-old Japanese male with genetically confirmed HypoKPP caused by the rare SCN4A p.Arg672Cys variant. Despite late onset (early 30s) and infrequent attacks, the patient presented with classic flaccid paralysis and severe hypokalemia (1.8 mEq/L) triggered by exercise, alcohol consumption, and carbohydrate intake. Genetic testing confirmed the diagnosis after the condition was initially misattributed to insulin-related hypokalemia. This case highlights the importance of considering HypoKPP in adults with new-onset weakness and unexplained hypokalemia, even in atypical presentations characterized by late onset and infrequent attacks.