Cardiac Conduction Disorders Due to Acquired or Genetic Causes in Young Adults

Authors, Journal, Affiliations, Type, DOI

• Authors: Cristina Balla, Luca Canovi, Marco Zuin, Luca Di Lenno, Maria L. Berloni, Beatrice de Carolis, Assunta Di Domenico, Elisabetta Tonet, Francesco Vitali, Michele Malagu, Giuseppe Boriani, Matteo Bertini
• Affiliations: Cardiology Unit, Azienda Ospedaliero-Universitaria S.Anna, Ferrara; Cardiology Unit, Policlinico S.Orsola, Bologna; Cardiology Unit, Policlinico di Modena — all Italy
• Journal: Journal of the American Heart Association, 2025;14:e040274
• Type: Contemporary Review
• DOI: https://doi.org/10.1161/JAHA.124.040274

Overview

This contemporary review provides a comprehensive update on the causes, diagnosis, and treatment of cardiac conduction disorders (CCDs) in young adults (age <50 years). CCDs in this age group represent ~13% of all conduction disorders, with the underlying cause remaining unknown in approximately half. The review covers acquired causes (myocarditis, Lyme, Chagas, autoimmune, sarcoidosis, iatrogenic) and familial/genetic causes (isolated ion channel mutations, cardiomyopathy-associated, metabolic/neuromuscular, and congenital heart disease-related). Key messages include: genetic testing is underused despite European guideline recommendations for patients <50 with progressive CCD; conduction system pacing is now preferred over conventional RV pacing; LMNA-related disease warrants ICD over pacemaker; and patients receiving their first pacemaker before age 50 face 3–4-fold increased risk of adverse outcomes within the first 5 years.

Keywords

Atrioventricular block; cardiac conduction disorders; genetic; myocardial disease; young adults

Key Takeaways

Epidemiology

• CCDs in young adults (age <50) represent approximately 13% of all conduction disorders, though this estimate likely underestimates true prevalence due to underrecognition.
• Danish nationwide study leading causes of AVB in young adults: post-cardiac surgery (15.3%), congenital AVB (9.0%), cardioinhibitory reflex (5.0%), congenital heart disease (4.2%), post-RF ablation (3.4%), cardiomyopathies (3.0%). Ischemic heart disease accounts for only 1.4% — strikingly less common than in older adults.
• The underlying cause remains unknown in approximately 50% of young adults with CCD — a major clinical gap.
• Cardiac amyloidosis and infiltrative disorders (notably cardiac sarcoidosis) are relevant but currently underestimated causes of CCD in young adults.

Acquired Causes

Myocarditis

• Advanced AVB can represent the initial and sole symptom of viral myocarditis.
• European Study of Epidemiology and Treatment of Inflammatory Heart Disease: 18% of suspected acute myocarditis exhibited arrhythmic symptoms including complete AVB.
• Nationwide Inpatient Sample Database (n=31,760): non-advanced and advanced AVB during acute myocarditis up to 1.7% and 1.1%, respectively; in fulminant myocarditis, advanced AVB present in ~7% of patients.

Lyme Carditis

• Advanced AVB is the primary cardiac manifestation of Lyme disease in 80–90% of patients with cardiac involvement; due to spirochete affinity for cardiac tissue and autoimmune-mediated inflammation.
• AVB is typically transient; pacemaker requirement estimated at only 0.03 per 100,000 population.

Chagas Disease

• Chronic Trypanosoma cruzi infection; endemic Latin America but rising in non-endemic regions due to migration.
• Common ECG findings: RBBB and left anterior hemiblock (>50% of patients); various degrees of AVB affecting the AV node.
• LBBB and left posterior fascicle hemiblock less frequent.

Autoimmune Disease

• Rheumatoid arthritis: AV node infiltration → RBBB in ~35% of patients; complete AVB rare but typical when it occurs.
• Systemic lupus erythematosus: small vessel vasculitis + fibrous infiltration → sinus/AV node dysfunction in up to 70% of patients.
• Congenital heart block: Maternal anti-SSA antibodies (transplacental via neonatal FcRn) → fetal bradycardia (peak onset 18–24 weeks' gestation); mortality 17.5%; 93% of affected newborns have complete AVB; two-thirds require pacemaker before adulthood; 10% develop cardiomyopathy-related HF. Transplacental corticosteroid + IV gamma-globulin reverses 2nd-degree block in 25% of cases.

Cardiac Sarcoidosis

• AVB of different degrees is the most common manifestation of cardiac sarcoidosis.
• ~30% of patients aged <60 with unexplained AVB have undiagnosed cardiac sarcoidosis — a major diagnostic opportunity.
• Treatment with corticosteroids may reverse AVB; however, due to unpredictable reversibility, permanent pacing should be considered even for transient block.

Iatrogenic CCDs

• Immune checkpoint inhibitors: increasing reports of bradyarrhythmias and AVB as immune-related adverse events.
• Permanent pacemaker implantation for bradyarrhythmia/conduction defects: 1–3% after CABG, 5% after mitral valve replacement, up to 12% after aortic valve replacement.
• Post-surgical congenital heart disease: ASD closure — low risk; VSD closure — complete AVB in 3% at long-term follow-up; ToF repair — universal RBBB from surgical fibrosis of the infundibulum.

Familial/Genetic Causes

Isolated Ion Channel Mutations

• SCN5A (Nav1.5): the most implicated gene in familial CCD, detected in 76% of familial CCD cases. Loss-of-function → familial CCD, Brugada syndrome, sick sinus syndrome; gain-of-function → LQT3. The same mutation can produce different phenotypes within a family.
• TRPM4 (Ca²⁺-activated channel in pacemaker cells, Purkinje fibres, atrial and ventricular tissue): linked to sick sinus syndrome and AVB.

CCDs Associated With Cardiomyopathies

• LMNA (Lamin A/C): Found in ~6% of DCM patients, up to 10% in familial DCM, and up to 33% in patients with both DCM and CCDs. In young adults, early conduction defects and arrhythmias typically precede LV dysfunction and dilatation, usually manifesting in the 3rd or 4th decade of life.
• HCM: LBBB and RBBB are frequent; AVB may progress to complete block in up to 15% of patients. Septal reduction therapies (surgical myectomy, ASA) carry additional conduction risk due to proximity of bundle branches.
• ACM (Arrhythmogenic Cardiomyopathy): Intraventricular conduction delays are the primary conduction manifestation reflecting affected ventricle's conduction system. AVB has been described in 10% of ACM patients.

CCDs Associated With Metabolic and Neuromuscular Disorders

• Anderson–Fabry disease (GLA): Short PR interval is an early finding; progressive AVB occurs later with conduction tissue fibrosis. In a study of young Fabry patients, 6% required devices for bradyarrhythmias.
• PRKAG2 glycogen storage cardiomyopathy: Pseudohypertrophy of the ventricles + progressive conduction impairment, often requiring pacemaker.
• Myotonic dystrophy type 1 (DMPK): ~80% experience cardiac issues; life-threatening complications include IVCD and AVB; type 2 has milder cardiac involvement (10–20%, typically 1st-degree AVB and BBB).
• Emery–Dreifuss muscular dystrophy (EMD): Progressive conduction abnormalities → AVB and SCD.
• Kearns–Sayre syndrome (mtDNA/RRM2B): Mitochondrial myopathy affecting ~50% with cardiac complications, most commonly conduction disease that may progress to complete AVB and SCD.

CCDs Associated With Congenital Heart Disease

• Holt–Oram syndrome (TBX5): Autosomal dominant; prevalence 0.7–1 per 100,000; ASD/VSD + forelimb defects. Rhythm disturbances in 30% of cases; arrhythmias more frequent with missense variants; spectrum includes sinus bradycardia, various degrees of AVB, early-onset AF/flutter even without overt structural heart disease.
• NKX2-5: Homeobox transcription factor on chromosome 5; first gene associated with human congenital heart disease. Signature phenotype: ASD + AVB (characteristically not related to atrial defect or surgery). AF/flutter in ~1/3 of patients; SCD risk associated with cardiomyopathy. Screening familial ASD for NKX2-5 mutation recommended to evaluate life-threatening arrhythmia risk and ICD indication.
• Congenitally corrected transposition of great arteries (CC-TGA): AVB in ~10% initially; 30–38% develop complete AVB during follow-up. Conduction system pacing preferred over univentricular pacing due to systemic RV dyssynchrony risk.

Diagnostic Workup

First-Line Investigations

• 12-lead ECG + 24-hour Holter monitoring is the cornerstone; ECG signs may be pathognomonic (e.g., RBBB in sarcoidosis, short PR in Fabry disease, sinus node dysfunction in LMNA disease).
• Echocardiography as first-line structural imaging; CMR for suspected cardiomyopathy (including all young patients with BBBs).
• LMNA-specific CMR findings: mid-myocardial fibrosis even in asymptomatic carriers; LGE linked to VA risk.
• Fabry disease CMR: low native T1 values (glycosphingolipid overload) + midwall/subepicardial LGE.
• Cardiac sarcoidosis CMR: IVS LGE with active edema on T2-weighted imaging during acute phase.

Electrophysiology Study

• Not routinely used; reserved for: borderline AVB with ambiguous or unclear symptoms (supra/intra/infra-Hisian discrimination), and intraventricular conduction delays with unexplained syncope.

Genetic Testing

• Current European guidelines recommend genetic testing for all patients <50 years with progressive CCD.
• Familial CCD without structural heart disease: predominantly ion channel gene mutations (SCN5A, TRPM4).
• Structural cardiomyopathy-associated CCD: predominantly transcription factors, enzymes, or structural proteins (LMNA, desmin, etc.).
• A comprehensive panel including both ion channel and structural genes is recommended given phenotypic overlap.
• Mutations in SCN5A, DES, and LMNA are associated with highest SCD risk.
• Genetic testing remains underused in practice due to limited awareness, resource constraints, and cost.
• If an index case mutation is identified: genetic counselling + cascade testing of first-degree relatives; asymptomatic children may defer testing; regular monitoring of asymptomatic carriers is essential.

Treatment

Device Therapy

• Conduction system pacing (His-bundle pacing / left bundle branch area pacing) is now the preferred modality for young adults requiring pacing — to maintain physiological LV activation and avoid pacing-induced cardiomyopathy.
• AV hysteresis programming and AAI-DDD mode switching recommended to minimise unnecessary ventricular pacing.
• LMNA: ESC 2021 recommends ICD over pacemaker due to high concomitant VT/VA risk; even when pacing is the primary indication.
• Kearns–Sayre syndrome: Permanent pacing (not ICD) recommended due to complete AVB risk.
• Anderson–Fabry disease and Danon disease: ICD or CRT-D should be considered for symptomatic bradycardia patients with LVEF ≤35% or those meeting SCD primary prevention criteria.
• HCM: Pacemaker for advanced AVB, or ICD based on HCM-specific risk score.
• ACM: Pacemaker for advanced AVB, or ICD based on ACM-specific risk score.

Disease-Specific Therapies

• Cardiac sarcoidosis: corticosteroids may reverse AVB; pacing still indicated given unpredictable reversibility.
• Lyme carditis: antibiotic treatment (usually sufficient; pacemaker rarely required).
• Autoimmune disease: watchful waiting + management of primary disorder → pacemaker for advanced AVB.

Long-Term Outcomes

• Patients receiving their first pacemaker before age 50 have a 3–4-fold increased risk of death, HF hospitalisation, ventricular tachyarrhythmia, or cardiac arrest — risk highest in the first 5 years post-implantation, particularly for persistent AVB.
• Myocardial biopsy or CMR reveals cardiac sarcoidosis in 25–34% of young and middle-aged patients with unexplained AVB — reinforcing the need for systematic sarcoidosis workup.
• Currently, no standardized diagnostic protocol exists; cardiologists often operate in "no guidelines land" for long-term follow-up.
• Need for large multicenter prospective registries to define epidemiological scope, disease progression, and long-term outcome predictors.

Limitations of the Document

• Narrative review without systematic search protocol or quality assessment of included studies — selection bias inherent.
• Epidemiological data predominantly from European (Danish) and Italian cohorts — limited generalisability to other populations, including regions where Chagas disease or other infectious causes are more prevalent.
• Most prevalence/incidence estimates for familial CCD are derived from retrospective, often small, single-centre studies.
• Genetic testing yield and phenotypic prevalence figures cited (e.g., 76% for SCN5A in familial CCD) refer to specific familial cohorts, not unselected young adult CCD populations — true prevalence in unselected populations is unknown.
• No prospective outcome data comparing conduction system pacing vs. conventional pacing in this specific young adult CCD population.
• Long-term follow-up recommendations are not guideline-supported; the review acknowledges "no guidelines land."

Key Concepts Mentioned

• concepts/Conduction-Disorders-in-Young-Adults — central topic of this review
• concepts/Conduction-System-Pacing — preferred pacing modality in young adults
• concepts/Genetic-Testing-in-Cardiomyopathy — genetic testing recommendations for CCD <50 years
• concepts/Mitochondrial-Cardiomyopathy — Kearns–Sayre conduction disease
• concepts/Fabry-Cardiomyopathy — conduction manifestations of Anderson–Fabry disease
• concepts/Arrhythmogenic-Cardiomyopathy — AVB in 10% of ACM
• concepts/Late-Gadolinium-Enhancement — CMR characterisation across acquired and genetic CCD
• concepts/Cascade-Family-Screening — cascade testing strategy for familial CCD

Key Entities Mentioned

• entities/LMNA — up to 33% of DCM+CCD; ICD preferred over PM; conduction before LV dysfunction in 3rd/4th decade
• entities/SCN5A — most common familial isolated CCD gene; 76% prevalence; LOF → CCD/Brugada; GOF → LQT3
• entities/Anderson-Fabry-Disease — short PR as early conduction sign; 6% require bradyarrhythmia devices
• entities/HCM — AVB in up to 15%; additional risk from septal reduction therapy
• entities/ARVC — AVB in 10% of ACM patients
• entities/DCM — CCD common, especially in LMNA-DCM (up to 33%)

Wiki Pages Updated

• Created: wiki/sources/conduction-disorders-jaha-2025.md (this file)
• Created: wiki/concepts/Conduction-Disorders-in-Young-Adults.md
• Updated: wiki/sourceindex.md
• Updated: wiki/wikiindex.md
• Updated: wiki/entities/LMNA.md
• Updated: wiki/entities/SCN5A.md
• Updated: wiki/entities/Anderson-Fabry-Disease.md