Electrophysiological study and catheter ablation of a Mahaim fibre located at the mitral annulus-aorta junction.

Accessory pathways with slow and anterograde decremental conduction (Mahaim fibres) are responsible for a minority of atrioventricular reentrant tachycardias. While usually located along the tricuspid annulus, left-sided Mahaim fibres have been occasionally reported. We here report on a unique case of radiofrequency catheter ablation of a Mahaim pathway located at the supero-septal aspect of the mitral annulus, in a region known as mitral annulus-aorta junction, between the right and left fibrous trigons. Electrophysiological properties and embryological implications of this unusual accessory pathway are discussed.