SCN4A and Myotonia: From the first week of life, the main sodium channel expressed in skeletal muscles is Nav1.4, which alpha-subunit is encoded by SCN4A. The spectrum of SCN4A-related myopathy spans from the complete loss of the channel (null mutations) to a gain of function (missense mutations) through various degrees of altered function, resulting in highly heterogeneous clinical presentation, mainly characterized by myotonia or PP, which represents a continuum in the clinical spectrum (Figure 1).