A sodium channel myotonia due to a novel SCN4A mutation accompanied by acquired autoimmune myasthenia gravis

► Mutations of Na channel are known to be responsible for myotonia and myasthenia. ► A patient with non-dystrophic myotonia with acquired myasthenia was presented. ► A novel mutation G1292D was identified in the skeletal muscle Na channel. ► Consistently with myotonia, the mutant channel showed enhanced activation. ► Reduced use-dependent inactivation suggested opposing effect to myasthenia.