Cardiac conduction abnormalities in myotonic dystrophy type I: The ongoing value of the ECG

Lorca et al. present within the current issue of the IJC Heart & Vasculature a very interesting analysis of cardiac conduction abnormalities in patients with myotonic dystrophy type 1. Their work contributes valuable data on risk stratification, although it focuses less on the clinical management of these patients. The study reports the incidence of conduction abnormalities and identifies risk factors associated with a higher prevalence, particularly younger age at diagnosis, reflecting the phenomenon of genetic anticipation and its correlation with longer CTG repeat expansions.

Myotonic dystrophy type 1 is a multisystem genetic disorder characterized by RNA toxicity and widespread splicing defects [1]. These abnormalities affect multiple organs, including the heart, which is a major determinant of quality of life and survival. The extent of cardiac involvement varies among the congenital, classical, and mild or late-onset forms and is influenced by the number of CTG repeats. Lorca et al. retrospectively included 549 patients from 16 specialized inherited heart disease centers across Spain. Pediatric patients and those with missing data were excluded. Despite these limitations, the reported incidence of cardiac conduction abnormalities was 31.1 %. This is lower than previously reported rates, with some studies showing cardiac conduction abnormalities in up to 65.4 % of patients with myotonic dystrophy type 1 [2].

Typical cardiac involvement in myotonic dystrophy type 1 primarily affects the conduction system and may result in abnormal impulse formation and propagation, including sinus bradycardia, sick sinus syndrome, atrial tachycardia, atrial flutter or fibrillation (AF), premature ventricular contractions, and, more rarely, ventricular tachycardia (VT) or ventricular fibrillation (VF) [3,4]. Over time, these abnormalities may contribute to the development of heart failure [5]. Lorca et al. reported first-degree atrioventricular block (AVB-I) and QRS prolongation on electrocardiography (ECG) in 42.4 % and 31.0 % of patients, respectively. Notably, in younger patients (<40 years), who typically exhibit greater CTG expansion due to genetic anticipation, the risk of these abnormalities was 8.9-fold higher than in older group. These findings underscore that a simple, widely available tool such as the ECG can provide clinically meaningful information for detecting arrhythmias and potentially improving survival. Indeed, ECG changes such as PR-interval prolongation have been previously described in myotonic dystrophy type 1 [6].

The authors reported an 18.2 % prevalence of cardiovascular death in the overall cohort. However, sudden cardiac death in myotonic dystrophy type 1 is more commonly associated with high-degree atrioventricular block [7], while ventricular arrhythmias appear to be relatively uncommon [8]. In this context, programmed electrical stimulation may be useful to better define the true incidence of inducible VT in this population. Additionally, measurement of the His–ventricular interval may serve as a more sensitive predictor of major bradyarrhythmic events, complementing surface ECG parameters such as PR and QRS duration [9].

In this retrospective registry, higher rates of AF, implantable cardioverter-defibrillator (ICD) implantation, and mortality were observed in older patients. These outcomes appear to be driven more by age than by myotonic dystrophy type 1 itself. Interestingly, CTG repeat lengths ≥ 600 were not associated with a higher risk of AF [10]. Furthermore, patients from the third generation (1991–2015) were diagnosed earlier and treated more frequently than those from earlier generations, allowing for better documentation of primary endpoints. However, the background differences among generations, age at diagnosis, and CTG repeat groups were not clearly detailed and warrant further clarification.

In conclusion, simple diagnostic tools such as the ECG provide valuable information regarding cardiac conduction abnormalities in myotonic dystrophy type 1, particularly AVB-I and PR-interval prolongation. Nevertheless, systematic ambulatory rhythm monitoring, including the use of implantable loop recorders, is essential for early detection of clinically relevant electrical abnormalities. Electrophysiological studies may further refine risk assessment. Given that myotonic dystrophy type 1 is a multisystem disorder, a multimodal approach is required, and important gaps remain in our understanding of genotype–phenotype correlations.