Paradoxical Cardiac Conduction during Exercise Stress Testing in Myotonic Dystrophy Type 1: A Case Report

Background Exercise stress testing identifies functional abnormalities that may manifest only during physiologic stress to the heart. This may have significant prognostic value in identifying latent conduction abnormalities in asymptomatic patients with Myotonic dystrophy type 1, who may benefit from prophylactic PPM implantation. Case report We report the case of a patient with Myotonic dystrophy type 1 with a 5-month history of atypical left sided chest pain. Her baseline ECG showed sinus rhythm and variable PR interval prolongation (206 to 220 ms) without symptoms of cardiac conduction disease. Routine blood tests and cardiac investigations including a 24-hour ECG monitoring, echocardiogram and a cardiac MRI scan, revealed no abnormalities. To investigate her chest pain and to determine the need for prophylactic permanent pacemaker implantation, exercise stress testing and an electrophysiological study were performed. Exercise testing revealed minimal PR shortening (PR = 200ms) at peak exercise and paradoxical PR prolongation (PR = 280ms) during the early recovery period. A prophylactic DDDR permanent pacemaker was implanted following an electrophysiological study that revealed a prolonged HV interval of 84 ms. Discussion and Conclusion The current use of annual ECG and 24 Holter monitoring may not adequately detect abnormal cardiac conduction in asymptomatic patients with Myotonic dystrophy type 1. The invasive nature of electrophysiology studies limits its use as a screening tool for conduction abnormalities in asymptomatic patients. Thus, exercise stress testing could be used to identify underlying conduction abnormalities in Myotonic dystrophy type 1 patients without any specific symptoms of bradycardia, which warrant further invasive electrophysiological studies.