SCN4A and Myotonia: Mutations in CLCN1 and SCN4A are associated with non-dystrophic myotonic myopathies (skeletal muscle channelopathies); however, studies of these genes in case series or case reports of patients with DM2 from Germany and Italy have suggested their possible role in the development or severity of myotonia and/or muscle pain in some patients with DM2 [88,89,90].