Nav1.4 related channelopathies that affect skeletal muscle excitability (Vicart et al., 2005; Jurkat-Rott et al., 2010; Nicole and Fontaine, 2015) are dominant diseases classified in two opposite groups as defined by the prevalent clinical symptoms: muscle stiffness and hypertonia (myotonia) episodes [non dystrophic myotonias (NDM)], and muscle weakness resulting in paralysis episodes (periodic paralyses; PP). This evidence concerns the gene SCN4A and Myotonia.