A young patient presenting with atrioventricular block diagnosed as myotonic dystrophy

We encountered a 42-year-old woman with a history of diabetes mellitus and cataracts presenting with repeated syncope whose electrocardiogram showed advanced atrioventricular block. On admission, we excluded major potential differential diagnoses as causes of an atrioventricular block but did not suspect myotonic dystrophy, which was eventually diagnosed by chance based on a suspected weakness of the respiratory muscles followed by a detailed neurological physical examination. Myotonic dystrophy should be suspected as a differential diagnosis when relatively young patients present with conductance disorder.