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A Wide Complex Tachycardia in a Woman with Myotonic Dystrophy: What Is the Tachycardia Mechanism?

A 29-year-old woman with myotonic dystrophy and developmental delay presented to the emergency room with failure to thrive. A 12-lead ECG revealed a regular wide complex tachycardia with a rate of approximately 255 bpm (Fig. 1). She was hemodynamically stable, but was treated with electrical cardioversion, which restored sinus rhythm. After cardioversion, a 12-lead ECG revealed normal sinus rhythm with first degree atrioventricular (AV) block (PR interval = 230 msec) and incomplete left bundle branch block (LBBB). She was subsequently found to have a structurally normal heart. Several years earlier, her mother, who also has myotonic dystrophy, presented with a regular wide complex tachycar-