The terminal crest: morphological features relevant to electrophysiology

Abstract

Objective: To investigate the detailed anatomy of the terminal crest (crista terminalis) and its junctional regions with the pectinate muscles and intercaval area to provide the yardstick for structural normality.

Design: 97 human necropsy hearts were studied from patients who were not known to have medical histories of atrial arrhythmias. The dimensions of the terminal crest were measured in width and thickness from epicardium to endocardium, at the four points known to be chosen as sites of ablation.

Results: The pectinate muscles originating from the crest and extending along the wall of the appendage towards the vestibule of the tricuspid valve had a non-uniform trabecular pattern in 80% of hearts. Fine structure of the terminal crest studied using light and scanning electron microscopy consisted of much thicker and more numerous fibrous sheaths of endomysium with increasing age of the patient. 36 specimens of 45 (80%) specimens studied by electron microscopy had a predominantly uniform longitudinal arrangement of myocardial fibres within the terminal crest. In contrast, in all specimens, the junctional areas of the terminal crest with the pectinate muscles and with the intercaval area had crossing and non-uniform architecture of myofibres.

Conclusions: The normal anatomy of the muscle fibres and connective tissue in the junctional area of the terminal crest/pectinate muscles and terminal crest/intercaval bundle favours non-uniform anisotropic properties.

Figure 1

(A) Opened right atrium in simulated right anterior oblique view to show the horseshoe shaped terminal crest (double dotted line). The crest arches anterior to the orifice of the superior vena cava (SCV) and extends to the area of the anterior interatrial groove. Inferiorly, the crest turns in beneath the orifice of the inferior vena cava (ICV), breaking up into a series of trabeculations in the area of the atrial wall known as the inferior isthmus (brace). Numbers 1 to 4 mark the points at which the thickness and width were measured. (B) The right atrium opened along the incisions marked by blue asterisks and the lateral wall deflected so as to display the crest (TC) on the endocardial surface separating the smooth walled venous component from the trabeculated appendage (RAA). The white asterisks are the cut surfaces of the crest. Note the ramifications in the inferior isthmus (brace). (C) Anterior aspect between the superior vena cava (SCV) and right appendage. The “septum spurium” (SS) is the most prominent anterior pectinate muscle arising from the crest (white asterisk). OF, oval fossa; TV, tricuspid valve; CS, coronary sinus.

Figure 2

(A–C) Endocardial aspects of the lateral wall of the right atrium opened and displayed in the same orientation as fig 1B. Transillumination shows the variability (70%, 22%, and 8%) in branching of the terminal crest in the inferior isthmus. (D) Endocardial surface of the lateral wall has a criss-crossing trabecular architecture within the tip of the appendage whereas in the remaining wall the pectinate muscles have a uniform parallel alignment almost without crossovers between them. In contrast, the pectinate muscles in (E) have a non-uniform arrangement with abundant interlacing trabeculations between them. (F) Scanning electron micrograph of non-macerated specimen confirmed the irregular alignment (arrows) of the muscular myofibrils within the pectinate muscles. Original magnification (F) ×75. CS, coronary sinus; OF, oval fossa; PM, pectinate muscle; SS, septum spurium; TC, terminal crest; V, Vestibule of the right atrium.

Figure 3

(A) Scanning electron micrograph of a cross section through the terminal crest at point 3 after digestion with sodium hydroxide that removed the myocytes, leaving the fibrous matrix. At low power magnification, the presence of collagenous sheaths gives the terminal crest a honeycomb appearance, interrupted only by the presence of thick collagenous septums of the perimysium (arrows) and vascular spaces (stars). Original magnification ×120. (B) Scanning electron micrograph of non-macerated specimen shows the perimysial septums (arrows) arising from the endocardium (E). Original magnification ×65. Scanning electron microscopic images at high power magnification after digestion are shown in (C) and (D). (C) Collagen sheaths of the endomysium (arrowheads), which are oval or round, support and connect individual cells. Coiled perimysial fibres or tendon-like collagen bundles (CPF) course along the perimysial septum. Original magnification ×1600. (D) Endomysial sheaths are connected to adjacent myocytes by lateral struts (arrows) of variable width and length. Original magnification ×1500.

Figure 4

(A) Scanning electron micrograph of the sinus node and nodal artery (NA) of a cross section through the terminal crest at point 2, after digestion of the nodal cells. The collagenous sheaths of endomysial formed a complex network in the gaps between the nodal cells, whereas occasional perimysial septums were seen between them. Original magnification ×65. (B) Scanning electron micrograph of non-macerated sinus node cells (N), which are encased in dense and coarse endomysial sheaths (arrows) in a specimen from a 75 year old. Original magnification ×600. (C) Scanning electron micrograph of a cross section through the crest, after digestion, from a specimen of 70 years shows a diffuse notable excess of endomysial sheaths (arrows) indicating focal interstitial reactive fibrosis. Vascular space (V) corresponds to a coronary vein. Original magnification ×360. (D) Scanning electron micrograph of non-macerated cross section through the body of the terminal crest shows mainly longitudinal fibres (H) with intermingling oblique (O) or lateral (L) fibres. Original magnification ×160.

Figure 5

(A) Cross section through the terminal crest (TC) at its origin (point 1 of fig 1A) showing its relation to the musculature of Bachmann's bundle (BB) in a 46 year old man. Note the change between the predominantly transverse muscle fibres of the terminal crest and the longitudinal fibres of Bachmann's bundle. (B) This section from a 76 year old woman taken at point 4 (fig 1A) shows a fibrotic area at the border between the terminal crest and the pectinate muscle (PM). Masson's trichrome stain. Endo, endocardium; Epi, epicardium.