Purpose: Presenting the case of unusual onset hypokalemic periodic paralysis (HypoPP) where myopathy had developed two years before paralysis occurred.
Material and methods: A Polish three-generation family with HypoPP and mutation in CACNA1S (R1239G) has been investigated. Clinical presentation with unusual onset of the disease, biopsy results and genetic research in one family member were described.
Conclusion: HypoPP is a rare disease it needs to be taken into consideration not only in cases of paroxysmal weakness but also when there is myopathy of unknown origin.