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Introduction/Aims: Mutations in the SCN4A gene encoding a voltage‐gated sodium channel (Nav1.4) cause hyperkalemic periodic paralysis (HyperPP) and hypokalemic periodic paralysis (HypoPP). Typically, both HyperPP and HypoPP are considered as monogenic disorders caused by a missense mutation with a large functional effect. However, a few cases with atypical periodic paralysis phenotype have been caused by multiple mutations in ion‐channel genes expressed in skeletal muscles. In this study we investigated the underlying pathogenic mechanisms in such cases. Methods: We clinically assessed two families: proband 1 with HyperPP and proband 2 with atypical periodic paralysis with hypokalemia. Genetic analyses were performed by next‐generation sequencing and conventional Sanger sequencing, followed by electrophysiological analyses of the mutant Nav1.4 channels expressed in human embryonic kidney 293T (HEK293T) cells using the whole‐cell patch‐clamp technique. Results: In proband 1, K880del was identified in the SCN4A gene. In proband 2, K880del and a novel mutation, R1639H, were identified in the same allele of the SCN4A gene. Functional analyses revealed that the K880del in SCN4A has a weak functional effect on hNav1.4, increasing the excitability of the sarcolemma, which could represent a potential pathogenic factor. Although R1639H alone did not reveal functional changes strong enough to be pathogenic, Nav1.4 with both K880del and R1639H showed enhanced activation compared with K880del alone, indicating that R1639H may modify the hNav1.4 channel function. Discussion: A cumulative effect of variants with small functional alterations may be considered as the underpinning oligogenic pathogenic mechanisms for the unusual phenotype of periodic paralysis. [ABSTRACT FROM AUTHOR]