SCN4A and Myotonia: Mutations of SCN4A are consequently associated with a range of neuromuscular phenotypes without systemic involvement, including autosomal‐dominant myotonia, and/or periodic paralysis, and autosomal recessive congenital myasthenia and congenital myopathy.1, 2, 3 Respiratory and laryngeal muscle compromise is common in affected infants and children and can cause life‐threatening respiratory impairment including recurrent apneas.4 The autosomal‐dominant SCN4A disorders such as myotonia are episodic.