Progressive High-Grade Atrioventricular Block in a Young Adult: Distinguishing Reversible Causes From Lev-Lenègre Disease.

High-grade atrioventricular (AV) block in young adults is uncommon and poses a diagnostic challenge when distinguishing reversible causes from intrinsic conduction system disease. We present a 27-year-old previously healthy male who presented with dizziness and was found to have a complete heart block with a wide QRS escape rhythm requiring temporary pacing. Initial evaluation revealed metabolic abnormalities and positive Coxsackie serology; however, correction of these factors did not result in improvement of conduction. Echocardiography demonstrated a normal cardiac structure. Given the possibility of a reversible etiology, permanent pacing was initially deferred. Two months later, the patient re-presented with near-syncope and progression to bifascicular block with first-degree AV block, consistent with progressive His-Purkinje system disease. He subsequently underwent successful dual-chamber leadless pacemaker implantation with resolution of symptoms. This case highlights the importance of recognizing progressive conduction system disease, such as Lev-Lenègre disease, even in young patients with apparent reversible factors, and emphasizes the role of serial electrocardiographic evaluation in guiding timely management.