Sodium channel blockers can reduce the frequency and severity of myotonia in patients with gain-of-function NaV1.4 variants,44 including those with severe infantile myotonia who have life-threatening respiratory compromise (appendix pp 1–4).8, 9 Furthermore, acetazolamide, which may be effective in loss-of-function SCN4A channelopathies,12, 45 has been reported to abolish attacks of respiratory and bulbar weakness in a patient with myasthenic syndrome associated with the Val1442Glu loss-of-function variant.12 Here, SCN4A is linked to channelopathy.