A Fasciculoventricular Accessory Pathway Featuring Functional Decremental Conduction and QRS Variability.

Fasciculoventricular accessory pathways (FVAPs), once considered rare variants of pre-excitation syndrome, are now recognised as ubiquitous in both humans and murine. Nonetheless, most FVAPs are likely electrically silent. However, they can become evident, as reported for some glycogen storage diseases (such as Danon disease and PRKAG2) and during high-voltage septal pacing. Typically, FVAPs only exhibit antegrade and non-decremental conducting properties. A block at the FVAP results in a normal His-to-ventricle interval and a narrow QRS complex without signs of pre-excitation. Decremental conduction over FVAP was once reported in a setting of PRKAG2 mutation. However, in the present case, incremental atrial pacing revealed varying His-to-ventricle intervals, with functional decremental conduction that was accompanied by varied QRS morphologies. These findings underscore the diagnostic challenges posed by FVAPs, and highlight the need for meticulous electrophysiological assessment to accurately distinguish them from other pre-excitation syndromes. This case exemplifies the nuanced behaviour of FVAPs, emphasising their clinical and diagnostic complexity in electrophysiological practice.