Which of the following are infectious causes of third-degree AV block?

• A. Lyme disease
• B. Congenital lupus
• C. Diphtheria
• D. All of the above

Answer: (D)

Complete heart block happens when the electrical signal from the atria cannot reach the ventricles because the conduction system is inflamed or damaged. Several infectious or immune-mediated processes can injure the AV node or His-Purkinje network and lead to a third-degree block. Lyme disease is known to involve the heart during disseminated infection and can cause AV block that may be reversible with appropriate antibiotics. Diphtheria can cause myocarditis that involves the conduction system, producing AV block as a complication. Congenital lupus, while not an infection itself, causes fetal or neonatal complete heart block through maternal autoantibodies that cross the placenta and injure the fetal conduction tissue. Because each of these conditions can produce complete AV block, all of the above are relevant causes. In practice, treatment varies by cause: antibiotics and sometimes temporary pacing for Lyme carditis, antitoxin and supportive care for diphtheritic myocarditis, and pacing or long-term management considerations for neonatal block due to congenital lupus, with attention to maternal autoantibody status.