Normokalemic periodic paralysis is not a distinct disease

Introduction: Recent molecular studies of the original cases of normokalemic periodic paralysis (normoKPP) have raised suspicions that these families actually had hyperkalemic periodic paralysis (hyperKPP) due to mutations in the skeletal muscle sodium channel gene SCN4A. However, there is still a debate about the existence of normoKPP. Methods: We screened 230 individuals with primary periodic paralysis for mutations in the SCN4A, CACNA1S, and KCNJ2 genes. All patients had either a hyperKPP or a hypoKPP phenotype, and none had a normoKPP phenotype. Results: In 4 hyperKPP patients from 2 families, molecular analyses revealed Arg675Gly and Arg675Gln mutations of SCN4A, which were previously reported to cause normoKPP. Each patient exhibited the characteristic clinical and laboratory features (including hyperkalemia during spontaneous attacks) of hyperKPP. Conclusion: Our findings support the notion that normoKPP is not a distinct disease.