SCN5A and myotonic dystrophy: Among the subgroup with structurally normal hearts (7 patients; 43% of the cohort), 3/7 were diagnosed with myotonic dystrophy, 2/7 with SCN5A mutations (both presenting with atrial flutter years before the ventricular arrhythmia), 1/7 following ajmaline administration, and 1/7 remained of undefined etiology.