Long-term evolution in Myotonic Dystrophy Type 1: Predictors of cardiac involvement

Background

Myotonic Dystrophy Type 1 (DM1) is the most common inherited neuromuscular disorder in adults. Cardiac involvement is frequent and often underdiagnosed because it is frequently asymptomatic. Prognosis remains difficult to predict.

Purpose

This study aimed to describe neuromuscular and cardiac involvement in DM1 and to identify clinical predictors of cardiac implantable electronic device (CIED) implantation and other adverse outcomes during long-term follow-up in a cohort of DM1 patients.

Methods

We conducted a retrospective observational study including patients with genetically confirmed DM1 evaluated at a tertiary referral centre between July 2005 and June 2025. Neuromuscular severity was assessed using the Muscular Impairment Rating Scale (MIRS), MIRS ≥2 defined neuromuscular involvement and MIRS ≥4 severe impairment. Cardiac involvement was defined as the presence of conduction disturbances, arrhythmias, systolic dysfunction, CIED implantation, or cerebrovascular events during follow-up. Conduction delay was defined as PR ≥200 ms and/or QRS ≥100 ms and considered advanced when PR ≥200 ms and QRS ≥120 ms were both present. The composite endpoint included all-cause mortality, cerebrovascular events (stroke or transient ischaemic attack), and all-cause hospitalisation.

Results

A total of 158 patients were included, of whom 89 (56.3%) were male. Mean age at first specialised evaluation was 38 ± 15 years, with a mean follow-up of 10 ± 6 years. Myotonia was the most frequent neurological finding (91.8%), and 27 patients (17.1%) developed severe neuromuscular impairment (MIRS ≥4). Overall, 103 patients (65.2%) developed cardiac involvement during follow-up, most of whom were asymptomatic (64.1%).

Conduction delay was present in 33.5% at diagnosis and increased to 61.4% during follow-up. Arrhythmias occurred in 79 patients (50%), with atrial fibrillation/flutter (AF/AFL) being the most common (22.8%). Cardiac involvement was more frequent among patients with neuromuscular involvement (69.4% with MIRS ≥2 vs 51.4% with MIRS =1, p 0.050) and in those with breathing disorders (73.5% vs 45.8%, p 0.002) but was not associated with severe neuromuscular impairment.

23 patients (14.6%) required CIED implantation (16 Pacemakers, 7 Implantable Cardioverter Defibrillators, 8 Cardiac Resynchronization Therapy Devices). Advanced conduction delay at baseline independently predicted CIED implantation (OR 10.08, p 0.008). The composite endpoint occurred in 66 patients (41.8%). The development of AF/AFL was a significant predictor of the composite endpoint (OR 2.84, p 0.014).

Conclusions

Cardiac involvement is common in DM1, frequently asymptomatic and more prevalent in patients with neuromuscular involvement and breathing disorders. Early identification of advanced conduction delay and AF/AFL is particularly relevant, as they predict CIED implantation and adverse clinical outcomes, respectively.Neuromuscular and Cardiac involvementCardiac involvement