Masquerading bundle branch block: An electrocardiographic harbinger of advanced conduction system disease

Masquerading bundle branch block (MBBB) is a rare and underrecognized electrocardiographic pattern characterized by features of both right and left bundle branch block on the same tracing. Unlike typical bifascicular block, MBBB is increasingly recognized as a marker of advanced and often bilateral His–Purkinje system disease, with potential progression to high-grade atrioventricular block. We report the case of an 84-year-old woman with a history of atrial fibrillation, heart failure with preserved ejection fraction, chronic kidney disease, and aortic stenosis who presented with generalized weakness and severe hyponatremia. Electrocardiography (EKG) demonstrated atrial fibrillation with a wide QRS complex and right bundle branch block morphology in the precordial leads. Notably, there was absence of the expected terminal S wave in leads I and aVL with left axis deviation, consistent with masquerading bundle branch block. Comparison with prior electrocardiography revealed progression from bifascicular block to MBBB. During hospitalization, telemetry showed episodes of bradycardia with heart rates as low as 30 beats per minute, raising concern for advanced conduction system disease. The patient was managed conservatively and discharged with ambulatory rhythm monitoring and electrophysiology follow-up. MBBB represents a complex conduction disturbance often reflecting diffuse His–Purkinje system involvement rather than isolated right-sided conduction delay. It is frequently misclassified as right bundle branch block with left anterior fascicular block, leading to underrecognition of its clinical significance. Accumulating evidence suggests that MBBB is associated with a higher risk of progression to symptomatic bradyarrhythmias and advanced atrioventricular block. Recognition of its characteristic electrocardiographic features is essential for appropriate risk stratification and management, including consideration of prolonged monitoring or early electrophysiologic evaluation. Masquerading bundle branch block is an important but underrecognized electrocardiographic marker of advanced conduction system disease. Early identification may facilitate timely monitoring and intervention to prevent adverse outcomes related to conduction system failure.