Key result
The Met1592Val mutation of SCN4A was identified in a descendent of the original normokalemic periodic paralysis family, confirming it is actually a variant of hyperkalemic periodic paralysis.
Case Report
Normokalemic periodic paralysis is not a distinct disorder but a variant of hyperkalemic periodic paralysis caused by the Met1592Val mutation of SCN4A.
Normokalemic periodic paralysis may represent a hyperkalemic variant; confirms Met1592Val in the index family but leaves reclassification open pending additional cases.
Normokalemic periodic paralysis (normoKPP) is well established in the literature, but there are doubts as to whether it exists as a discrete entity. Retrospective clinical and molecular analysis has confirmed suspicions that most normoKPP families actually have a variant of hyperkalemic periodic paralysis (hyperKPP) due to a mutation of the muscle-specific sodium channel gene (SCN4A). However, the original normoKPP family described by Poskanzer and Kerr (Poskanzer DC, Kerr DNS. A third type of periodic paralysis, with normokalemia and favourable response to sodium chloride. Am J Med 1961;31:328-342) has remained unchallenged. We identified the Met1592Val mutation of SCN4A in an affected descendent of this original normoKPP family. This is the final piece in the puzzle: normoKPP is actually a variant of hyperKPP and is not a distinct disorder.
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Chinnery et al. (2002) conducted a case report in Normokalemic periodic paralysis. Met1592Val mutation of SCN4A was evaluated. The Met1592Val mutation of SCN4A was identified in a descendent of the original normokalemic periodic paralysis family, confirming it is actually a variant of hyperkalemic periodic paralysis.
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