BACKGROUND: Patients with myotonic dystrophy type 1 (DM1) have a three-fold higher risk of sudden cardiac death (SCD) than age-matched healthy controls. Despite numerous attempts to define the cardiac phenotype and natural history, existing literature suffers from low power, selection-bias and lack of controls. Thus, the optimal strategy for assessing cardiac involvement in DM1 is unclear.

METHOD: In this large single-centre study, we evaluated 129 unselected DM1 patients (49.6% men), mean (SD) age 44 (14.7) years with family history, physical examination, electrocardiogram (ECG), echocardiography, Holter-monitoring and muscle strength testing.

RESULTS: Cardiac involvement was found in 71 patients (55%) and included: 1) Conduction abnormalities: atrio-ventricular block grade I (AVB grade I) (23.6%), AVB grade II (5.6%), right/left bundle branch block (5.5/3.2%) and prolonged QTc (7.2%); 2) arrhythmias: atrial fibrillation/flutter (4.1%), other supraventricular tachyarrhythmia (7.3%) and non-sustained ventricular tachycardia (4.1%); and 3) structural abnormalities: left ventricular systolic dysfunction (20.6%) and reduced global longitudinal strain (21.7%). A normal ECG was not significantly associated with normal findings on Holter-monitoring or echocardiography. Patients with abnormal cardiac findings had weaker muscle strength than those with normal cardiac findings: ankle dorsal flexion (median (range) 4.5 (0-5) vs. 5.0 (2.5-5), p=0.004) and handgrip (median 4.0 (0-5) vs. 4.50 (2-5), p=0.02).

CONCLUSION: The cardiac phenotype of DM1 includes a high prevalence of conduction disorders, arrhythmias and risk factors of SCD. Systematic cardiac screening with ECG, Holter-monitoring and echocardiography is needed in order to make a proper characterization of cardiac involvement in DM1.