1. Why “paraseptal” is usually the correct term

Accessory atrioventricular pathways are strands of working myocardium that cross the normally insulating plane of an atrioventricular junction. Their atrial and ventricular insertions may be narrow or broad, aligned or displaced, and endocardial or epicardially related. These variables influence activation patterns and the site from which a pathway can be interrupted. The traditional labels “anteroseptal,” “midseptal,” and “posteroseptal” were created from surgical and fluoroscopic viewpoints that did not preserve the heart’s position within the thorax. In attitudinal orientation, the former anteroseptal region is superior, whereas the former posteroseptal region is inferior [1–4].

The adjective septal should be reserved for tissue that directly separates right-sided from left-sided cardiac cavities. Much of the territory in which so-called septal pathways are mapped is instead composed of atrial vestibular myocardium, fibro-fatty tissue of the atrioventricular grooves or inferior pyramidal space, and the crest of the muscular ventricular septum. A connection may therefore be close to the septum without traversing a true septal partition. “Paraseptal” conveys this relationship more accurately. Traditional terms can remain in parentheses for communication with established electrocardiographic algorithms, but should not define the anatomy [2–4].

2. Superior and mid-paraseptal regions

The superior paraseptal region corresponds broadly to the territory traditionally called anteroseptal. It lies beside the central fibrous body and the membranous septum and is closely related to the penetrating atrioventricular bundle. Not all pathways in this region are para-Hisian. Some have insertions close to the His bundle, whereas others extend toward the superior aspect of the supraventricular crest and are better considered superior paraseptal or crestal connections. This distinction matters because the distance between the target and the normal conduction axis is not predicted reliably by a legacy regional name [2,4].

The traditional midseptal territory is also heterogeneous. Right mid-paraseptal pathways may be approached from the atrial surface of the triangle of Koch, crossing fibro-fatty tissue toward the crest of the muscular ventricular septum. Left mid-paraseptal pathways may connect left atrial myocardium to the left-sided aspect of the septal crest and can be approached from the immediately subaortic left ventricular region. These locations are anatomically related but not interchangeable. Electrograms must establish the relevant atrial and ventricular insertions rather than assuming that a single “midseptal” annular point identifies the entire connection [2].

3. Inferior paraseptal region and the inferior pyramidal space

Inferior paraseptal pathways were traditionally grouped as posteroseptal. In reality, the region is situated inferiorly at the cardiac crux. The right and left atrial vestibules diverge here, enclosing the fibro-fatty inferior pyramidal space. Its base is continuous with the inferior atrioventricular grooves and the coronary sinus, while its apex points toward the central fibrous body and the compact atrioventricular node. The right atrial wall, the left ventricular inferoseptal surface, and the buttress of the atrial septum form important boundaries. The atrioventricular nodal artery crosses this complex territory on its course toward the compact node [3,5].

Three clinically useful categories can be distinguished. A right inferior paraseptal pathway is interrupted from the right side outside the coronary sinus. A left inferior paraseptal pathway is approached from the left-sided junction. An inferior paraseptal pathway associated with the coronary venous system may be mapped within the coronary sinus or a tributary, most often the middle cardiac vein. Myocardial sleeves around the coronary sinus and its tributaries, and occasionally a coronary sinus diverticulum, can provide an epicardial atrioventricular connection. The earliest endocardial signal may therefore represent only the closest surface to a deeper or venous pathway [2,3].

4. Mapping logic and structures at risk

Mapping should be anatomical and comparative. Superiorly, the right and left paraseptal aspects, the region of the His recording, and—when indicated—the adjacent aortic root are spatially close. In the middle sector, the triangle of Koch and the subaortic left ventricular aspect of the septal crest represent different surfaces of a three-dimensional region. Inferiorly, right-sided, left-sided, coronary sinus, and middle cardiac venous recordings may all be required to define the shortest route to the pathway. A coronary sinus catheter is a useful landmark, but the venous channel is variably separated from the hinge of the mitral valve and cannot be treated as a precise electrical annulus [2,3].

The safety profile changes along the paraseptal continuum. The atrioventricular node and penetrating bundle dominate risk in the superior and mid-paraseptal sectors. In the inferior pyramidal space, attention shifts additionally to the atrioventricular nodal artery, the coronary sinus and tributaries, and the adjacent right or circumflex coronary arterial branches. Coronary venous ablation requires particular caution because a successful venous recording does not establish a safe distance from an artery. The aim is not merely to find the earliest activation, but to understand which anatomical surface records the pathway and what structures lie between that surface and the connection [2–5].