Exercise-Induced Alternating Fascicular Block: Unmasking Lenègre-Lev Disease in a Patient With a Rosenbaum Pattern

Exercise-induced conduction failure may reveal advanced His-Purkinje system disease despite apparently stable resting electrocardiographic findings. This case underscores the value of functional stress testing in unmasking clinically high-risk infranodal conduction disorders.

A patient with recurrent syncope demonstrated complete right bundle branch block, left anterior fascicular block, and first-degree atrioventricular block on baseline electrocardiography. Exercise stress testing unmasked rate-dependent fascicular alternans with progression to advanced atrioventricular block and a 7-second episode of asystole. Findings were consistent with degenerative conduction system disease (Lenègre-Lev disease), prompting implantation of a permanent dual-chamber pacemaker.

Chronotropic stress exposed latent infranodal conduction failure due to diffuse His-Purkinje fibrosis. Exclusion of reversible etiologies supported permanent pacing, resulting in complete clinical resolution.

Normal resting studies do not exclude life-threatening conduction disease; exercise stress testing is essential in the evaluation of unexplained syncope.