At the last documented follow-up visit, the majority of patients presented myotonia (NDM-CLCN1 83.3%, NDM-SCN4A 72.2%, p = 0.31) and up to 25% of patients presented muscle weakness at their final neurological examination (NDM-CLCN1 23.4%, NDM-SCN4A 5.2%, p = 0.15). Here, CLCN1 is linked to Myotonia.