Electrocardiogram vs Electrophysiological Study and Major Conduction Delays in Myotonic Dystrophy Type 1

Importance For the prevention of sudden cardiac death in myotonic dystrophy type 1 ( dystrophia myotonica ; DM1), professional practice guidelines recommend pacemaker implantation in asymptomatic patients with a PR interval greater than or equal to 240 milliseconds and/or QRS duration greater than or equal to 120 milliseconds on electrocardiogram (ECG), or a His-ventricular (HV) interval greater than or equal to 70 milliseconds during electrophysiological study (EPS), as class IIa indications. Objective To determine which of these strategies—ECG or EPS based—is more effective in predicting major bradyarrhythmic events (MBAEs). Design, Setting, and Participants This was a cohort analysis of retrospectively collected longitudinal data from the DM1 Heart Registry. The setting included cardiology and neurology departments of 6 French university hospitals. Study participants were selected from individuals enrolled in the DM1 Heart Registry between 2000 and 2020. The DM1 Heart Registry includes adults with genetically confirmed DM1. Included patients had a history of first EPS after 1999 and no personal history of advanced atrioventricular block or sustained ventricular tachycardia. Study data were analyzed from January to July 2025. Exposures ECG- and EPS-based strategies. Main Outcomes and Measures The primary outcome was MBAEs, defined as sudden cardiac death, resuscitated cardiac arrest, or second-degree type II or complete atrioventricular block. Results Of 1778 adults with genetically confirmed DM1 enrolled in the DM1 Heart Registry, a total of 706 patients (mean [SD] age, 42 [13] years; 359 male [51%]) were included in this study. At baseline, 273 patients (38%) had an HV interval greater than or equal to 70 milliseconds, and 232 (32%) met ECG criteria. Over a median (IQR) follow-up of 5.9 (2.3-9.7) years, 99 patients (14%) experienced an MBAE. In multivariable Cox and joint models incorporating baseline and time-varying values of PR and QRS durations, the HV interval was the only variable significantly associated with the incidence of MBAEs (hazard ratio [HR], 1.77; 95% CI, 1.46-2.16; P < .001 and HR, 1.78; 95% CI, 1.40-2.22; P = .001, respectively). Compared with ECG-based criteria, the EPS criterion proved to be a more reliable (HR, 2.89; 95% CI, 1.92-4.34 vs HR, 1.95; 95% CI, 1.31-2.89) and more sensitive (performance index [SE], 68.35% [6.24%] vs 34.76% [6.47%]) predictor of MBAE and accurately reclassified 28.8% of patients with an MBAE. Lowering the threshold to HV greater than or equal to 65 milliseconds further improved sensitivity (performance index [SE], 90.18% [3.85%]) and net reclassification improvement (33.7%; 95% CI, 19.6%-48.2%) for MBAE prediction. Conclusions and Relevance In this cohort of patients with DM1, the HV interval outperformed ECG criteria in predicting MBAEs. An HV threshold greater than or equal to 65 milliseconds may enhance risk stratification for prophylactic pacing.