Mutations in CLCN1 and SCN4A genes are associated with non-dystrophic myotonic disorders (skeletal muscle channelopathies); double-troubled cases of patients with DM2 from Germany and Italy suggest a possible modifying role in the development or severity of myotonia and/or muscle pain by enhancing the myotonia and/or muscle pain in some patients with DM2 [71–73]. Here, SCN4A is linked to Myotonia.