1. Definition: an electrophysiological label with an anatomical meaning

A para-Hisian accessory pathway is an atrioventricular myocardial connection whose course or insertion lies close to the atrioventricular bundle and the site at which a His potential is recorded. The term should indicate demonstrated proximity to the normal conduction axis, not serve as a synonym for every pathway in the traditional anteroseptal region. In attitudinal terms, these pathways occupy the superior paraseptal atrioventricular junction. They are beside the septal structures and the His bundle; they do not necessarily traverse a true cardiac septum [1–4].

Electrophysiological definitions have historically used the amplitude of the local His potential and the response to para-Hisian pacing or catheter pressure. These observations remain useful, but the recorded His signal may be near-field or far-field and its amplitude depends on catheter orientation and tissue contact. The label is therefore most informative when activation mapping, pathway behavior, and three-dimensional anatomy converge. It should also be separated from fasciculoventricular connections, which link the conduction axis to ventricular myocardium and do not constitute an atrioventricular bypass tract [2,5].

2. The central fibrous body, membranous septum, and His bundle

The compact atrioventricular node is located at the apex of the inferior pyramidal space. It continues as the non-branching His bundle, which becomes insulated as it penetrates the central fibrous body. The membranous septum forms part of this fibrous complex, with atrioventricular and interventricular components whose proportions vary. The penetrating bundle is thus protected within fibrous tissue, whereas a para-Hisian accessory connection is composed of working myocardium crossing an adjacent zone of discontinuity in the insulating plane [3,4,6].

From the right atrium, the relevant territory is near the superior apex of the triangle of Koch and the hinge of the septal leaflet of the tricuspid valve. Deep to this surface are the central fibrous body and the crest of the muscular ventricular septum. The superior aspect of the supraventricular crest lies more anteriorly, while the non-coronary and right coronary aortic sinuses occupy adjacent leftward and superior relations. The apparent proximity of several endocardial surfaces explains why the same pathway may generate early signals from the right atrium, ventricular septum, or an aortic valvar sinus [2–4].

3. Para-Hisian versus superior paraseptal or crestal connections

True para-Hisian pathways run close and approximately parallel to the His bundle, with atrial and ventricular insertions near the normal conduction axis. Their subendocardial location can make them susceptible to transient mechanical block from catheter contact without simultaneous His-bundle block. Other superior paraseptal pathways may have an atrial insertion near the His recording zone but a ventricular insertion several millimetres away at the superior supraventricular crest. These have been described as crestal pathways. Conflating the two patterns can overestimate the danger at one end of the connection or misdirect mapping at the other [2].

The ventricular insertion deserves explicit definition. A pathway may be oblique, so the site of earliest atrial activation during orthodromic tachycardia need not coincide with the site of earliest ventricular activation during pre-excitation. Mapping only one direction of conduction can therefore present an incomplete picture. A carefully reconstructed atrial-to-ventricular course is especially important in this region, where a few millimetres can separate an effective target from the penetrating atrioventricular bundle [2,5].

4. Multiple surfaces and procedural safety

The superior paraseptal junction is a three-dimensional neighbourhood rather than a single annular point. Mapping may include the right atrial aspect of the tricuspid junction, the adjacent right ventricular septal surface, the left ventricular subaortic region, and selected aortic valvar sinuses. The non-coronary sinus can lie close to the atrial component of the superior paraseptal region, whereas the right coronary sinus relates to the muscular septal crest. Success from an aortic sinus does not imply that the pathway arises from a valve leaflet; it indicates that the sinus provides a favourable anatomical approach to an adjacent insertion [3,4].

Preservation of normal atrioventricular conduction is the central constraint. A stable His electrogram, atrial and ventricular signal ratios, pathway potentials when present, and responses to pacing should be interpreted together. Energy delivery should never be based on the earliest signal alone. Catheter stability, the anticipated depth of the connection, and the distance to the conduction axis must guide the choice of surface and energy modality. Cryothermal energy can offer reversibility during initial tissue cooling in selected cases, while radiofrequency energy may be appropriate when a safer site is demonstrated; neither eliminates the need for anatomical discipline [5,7].

The operator must also distinguish pathway block from injury to the atrioventricular node or His bundle. Continuous monitoring of atrioventricular conduction, prompt cessation when conduction changes occur, and reassessment after mechanical suppression are essential. The safest target may be remote from the largest His potential if an oblique connection can be reached at its ventricular insertion or from an adjacent chamber. The anatomical objective is to interrupt the accessory myocardial strand while leaving the insulated conduction axis intact.