What are the causes of complete heart block?

Causes of Complete Heart Block

Complete heart block arises from either congenital or acquired etiologies, with congenital cases primarily linked to maternal autoantibodies (anti-Ro/SSA and anti-La/SSB) or structural cardiac malformations, while acquired cases result from myocardial infarction, degenerative conduction system disease, cardiac surgery, infections, medications, or infiltrative diseases.

Congenital Complete Heart Block

Autoimmune-Mediated (Most Common)

• Maternal autoantibody passage causes the majority of congenital complete heart block (CCHB), occurring in 2-5% of pregnancies with positive anti-Ro/SSA and/or anti-La/SSB antibodies 1
• The highest-risk period for fetal AV node destruction is between 16 and 28 weeks' gestational age 1
• Nearly every mother with an affected child has circulating antibodies, though many are asymptomatic carriers and less than one-third have a preexisting diagnosis of rheumatological disease (lupus erythematosus or other connective tissue disorders) 2, 3
• The recurrence rate in subsequent pregnancies is 12-25% 1
• Transplacental antibodies attack and destroy the AV node through immune-mediated injury and fibrosis of the conduction system 4, 3

Structural Heart Disease

• Congenital complete block occurs in the context of complex congenital heart malformations 2
• Anomalous development of conduction tissue accompanies certain cardiac structural defects 3
• The incidence of congenital AV block is between 1 in 15,000 and 1 in 20,000 live births 2, 3

Genetic/Non-Immune Isolated Block

• Isolated nonimmune early- or late-onset heart block detected later in childhood may be associated with specific genetic markers or other pathogenic mechanisms 1
• Recent genetic discoveries have highlighted underlying mechanisms beyond autoimmune causes 5

Acquired Complete Heart Block in Adults

Post-Myocardial Infarction

• AV block after myocardial infarction relates primarily to the extent of myocardial injury and intraventricular conduction defects 2
• Persistent advanced second-degree or complete heart block after MI requires permanent pacing 6
• The long-term prognosis is related more to myocardial damage than to the AV block itself 2

Degenerative Conduction System Disease

• Progressive fibrosis and degeneration of the conduction system (Lev's disease, Lenègre's disease) causes acquired complete heart block in adults 2
• Bifascicular and trifascicular block can progress to complete heart block, particularly when associated with syncope and increased risk of sudden death 2

Iatrogenic Causes

• Post-AV junction ablation is a recognized cause requiring permanent pacing 2
• Cardiac surgery, particularly when advanced second-degree AV block persists 10-14 days postoperatively 6
• Medications that depress escape heart rates or aggravate AV block, including negative chronotropic agents 2

Neuromuscular Disease

• Myotonic dystrophy is specifically associated with complete heart block requiring pacemaker implantation 2

Acquired Complete Heart Block in Neonates/Children

Infectious Etiologies

• Viral myocarditis is the primary infectious cause of acquired complete AV block in neonates 2
• HIV infection can cause acquired complete heart block 2
• Myocarditis and endocarditis manifest with intraventricular conduction abnormalities 2

Neoplastic

• Tumors (cardiac or infiltrative) can cause acquired complete AV block in neonates 2

Metabolic/Medication-Related

• Long QT syndrome is occasionally complicated by 2:1 AV block, with infra-Hisian block location at the His-Purkinje level 2
• Cisapride and other QT-prolonging agents (diphemanil) cause heart block associated with prolonged QT interval in neonates and premature infants 2

Anatomic Classification

Complete heart block is anatomically defined as occurring at three levels 2:

• Supra-His (AV node level): Generally has stable junctional escape rhythm and better prognosis 2
• Intra-His: Within the His bundle
• Infra-His (distal conduction system): Associated with Mobitz type II pattern, wide QRS escape rhythm, and higher risk of sudden death requiring urgent pacing 2

Critical Clinical Pitfalls

Mortality is highest in utero and during the first 3 months of life, reaching 15-30% in congenital cases 2, 1. Even asymptomatic patients with CCHB should receive pacemakers when first diagnosed, as data indicate all patients eventually require pacing and early intervention prevents left ventricular dysfunction 1. Traditional right ventricular apical pacing may produce pacing-induced cardiomyopathy, necessitating consideration of alternate pacing sites 1, 7.