SCN5A and Myotonia: The mixture of loss- and gain-of-function in other voltage-gated sodium channel isoforms is known to cause overlapping syndromes with characteristics of multiple disease states; these include mixed Brugada and long QT syndrome type 3 variants in NaV1.5 (Bezzina et al., 1999; Abdelsayed et al., 2015) as well as mixed periodic paralysis and myotonia variants in NaV1.4 (Webb and Cannon, 2008; Ghovanloo et al., 2018).