What is the management approach for cardiac involvement in patients with myotonic dystrophy?

Management of Cardiac Involvement in Myotonic Dystrophy

All patients with myotonic dystrophy type 1 require regular cardiac surveillance even when asymptomatic, with follow-up intervals no greater than 6 months, and should follow standard ICD indications for non-ischemic cardiomyopathy when cardiac involvement develops. 1, 2

Surveillance Strategy

Initial Cardiac Assessment

• Perform baseline electrocardiogram (ECG), 24-hour Holter monitoring, and echocardiography in all patients with myotonic dystrophy type 1, regardless of symptoms 1
• Include cardiovascular magnetic resonance (CMR) imaging when available, as it detects structural and functional abnormalities in 44% of patients, including myocardial fibrosis in 12.5% 3
• A normal ECG does not exclude significant cardiac pathology—32% of patients with normal ECG show arrhythmias or conduction abnormalities on Holter monitoring 4, 5

Ongoing Monitoring Schedule

• Repeat Holter monitoring every 6 months or less, as high-grade conduction abnormalities requiring pacemaker implantation can develop rapidly and unpredictably between surveillance intervals 2
• Perform annual echocardiography to assess for left ventricular systolic dysfunction (present in 20-21% of patients) and structural changes 5, 3
• Monitor QRS duration annually, as it increases by approximately 0.54 ms per year and correlates with PR interval prolongation 4

Cardiac Phenotype Recognition

Conduction System Disease (Most Common)

• Expect progressive conduction abnormalities including: first-degree AV block (24%), second-degree AV block (6%), bundle branch blocks (9%), and prolonged QTc (7%) 5
• Maintain high suspicion for bundle-branch reentrant tachycardia in patients presenting with wide QRS complex tachycardia or tachycardia-related symptoms 1
• Sudden cardiac death can occur from either ventricular arrhythmias OR bradyarrhythmias due to rapid conduction system degeneration 1

Myocardial Involvement

• Left ventricular systolic dysfunction occurs in approximately 20% of patients, with reduced global longitudinal strain in 22% 5, 3
• Left ventricular dilatation (9%) and hypertrophy (8%) are less common but clinically significant 3
• Patients typically have low left ventricular mass indexes overall 3
• Right ventricular involvement is uncommon and only occurs with concurrent left ventricular abnormalities 3

Arrhythmias

• Atrial fibrillation/flutter occurs in 4%, other supraventricular tachyarrhythmias in 7%, and non-sustained ventricular tachycardia in 4% 5

Device Therapy Indications

ICD Recommendations

• Apply standard ICD indications for non-ischemic cardiomyopathy (NICM) to all myotonic dystrophy patients for both primary and secondary prevention when meaningful survival >1 year is expected 1
• When a permanent pacemaker is indicated, consider upgrading to an ICD to minimize risk of sudden cardiac arrest from ventricular tachycardia (Class IIb recommendation) 1
• This ICD-over-pacemaker consideration is particularly relevant given the dual risk of bradyarrhythmias and ventricular arrhythmias 1

Pacemaker Indications

• Follow standard ACC/AHA/HRS pacemaker guidelines for bradyarrhythmias and high-grade conduction blocks 1

Risk Stratification

Predictors of Cardiac Involvement

• Male gender strongly predicts cardiac abnormalities (p<0.001) 5, 3
• Age correlates with cardiac involvement (p=0.04) 5, 3
• Abnormal ECG associates with functional or structural cardiac disease (p<0.001) 5, 3
• Weaker muscle strength (ankle dorsiflexion and handgrip) correlates with abnormal cardiac findings 5

Poor Predictors

• CTG repeat length does NOT predict cardiac involvement severity—genetic testing cannot determine individual cardiac risk 4, 3
• Disease duration and NT-proBNP levels do not reliably predict myocardial abnormalities 3
• Severity of neuromuscular symptoms does not correlate with cardiac disease 3

Critical Pitfalls to Avoid

• Never rely on ECG alone for cardiac screening—significant arrhythmias and conduction abnormalities are frequently missed without Holter monitoring 4, 5
• Do not extend surveillance intervals beyond 6 months—conduction disease progresses unpredictably and rapidly, with pacemaker-requiring abnormalities developing between longer surveillance periods 2
• Do not assume preserved left ventricular function excludes risk—conduction abnormalities and sudden death can occur with normal systolic function 1
• Maintain high clinical suspicion in patients with arrhythmia symptoms—these patients warrant immediate comprehensive evaluation 1