Polarized Light Imaging of the Myoarchitecture in Tetralogy of Fallot in the Perinatal Period

Abstract

Background: The pathognomonic feature of tetralogy of Fallot (ToF) is the antero-cephalad deviation of the outlet septum in combination with an abnormal arrangement of the septoparietal trabeculations. Aims: The aim of this article was to study perinatal hearts using Polarized Light Imaging (PLI) in order to investigate the deep alignment of cardiomyocytes that bond the different components of the ventricular outflow tracts both together and to the rest of the ventricular mass, thus furthering the classic description of ToF. Methods and Materials: 10 perinatal hearts with ToF and 10 perinatal hearts with no detectable cardiac anomalies (control) were studied using PLI. The orientation of the myocardial cells was extracted and studied at high resolution. Virtual dissections in multiple section planes were used to explore each ventricular structure. Results and Conclusions: Contrary to the specimens of the control group, for all ToF specimens studied, the deep latitudinal alignment of the cardiomyocytes bonds together the left part of the Outlet septum (OS) S to the anterior wall of the left ventricle. In addition, the right end of the muscular OS bonds directly on the right ventricular wall (RVW) superior to the attachment of the ventriculo infundibular fold (VIF). Thus, the OS is a bridge between the lateral RVW and the anterior left ventricular wall. The VIF, RVW, and OS define an "inverted U" that roofs the cone between the interventricular communication and the overriding aorta. The opening angle and the length of the branches of this "inverted U" depend however on three components: the size of the OS, the size of the VIF, and the distance between the points of insertion of the OS and VIF into the RVW. The variation of these three components accounts for a significant part of the diversity observed in the anatomical presentations of ToF in the perinatal period.

Keywords: 3D architecture myocardial cells; outlet septum; polarized light imaging; tetralogy fallot; ventriculo infundibular fold.

Figure 1

Illustration of the different maps obtained by PLI. (A) Short-axis section (500 μm thickness) made after resin embedding and viewed in transmitted light. (B) False color azimuth map of the section with superimposition of LIC texture (0–180° range). (C) False color elevation map with superimposition of LIC texture (−90 to +90° range). (D) Orientation map limited to “streamlines” representation in LIC texture. The virtual dissection plan is false-colored in orange-brown, with the surface of the basal ventricular mass in brown.

Figure 2

Morphometric measurements: left column-control, right column-ToF. (A,B) Long axis view, the three dotted lines show the different levels along the ventricular mass that are imaged in (C–F): equatorial (green), aortic (yellow), and pulmonary valves (blue). (C,D) Short axis view equatorial level: IVS-eq angle (green dotted lines, average angle in degrees +/– standard deviation). (E,F) short axis view atrio-ventricular level: IVS-AV angle (blue dotted lines), Ao-PA angle (yellow dotted lines). The yellow dotted circle in (E) represents the projection of the aortic valve plan at the level of the pulmonary plan. At the apical level, no difference is measured between the angles in the control and ToF groups (p > 0.05). At the basal level, a statistically significant difference was measured between the control and ToF group for the IVS-AV angle and the Ao-PA angle (p < 0.01).

Figure 3

Virtual dissection of the ventricular mass LIC maps: left column-control, right column-ToF. Upper row, equatorial level (A,B); middle row, basal level (C,D); lower row (E,F) oblique sections. From the equatorial level to the apex (A,B), no marked difference in the 3D architecture of the myocardial cells was found between the two groups. At the basal level, in the control hearts, the deep muscular continuity of the VIF and the IVS is verified (C). In short axis section, the deep alignment of the cardiomyocytes of the VIF connect to the RVW while diverging as a Japanese fan. At its other end, the cardiomyocytes of the VIF connect to the IVS while diverging in two parts, one upper toward the OS and one lower toward the inferior part of the IVS. An oblique view (E) sharpens the description of the VIF and the IVS and their relationship with the septal trabeculations. At the basal level, in the ToF hearts, the muscular attachment of the VIF to the RVW remains. The other end of the VIF points to the lower part of the IVS positioned below the interventricular communication (D). Concerning the OS, the deep latitudinal alignment of the cardiomyocytes bonds together the left part of the OS to the anterior wall of the left ventricle. The right end of the muscular OS bonds directly to the RVW superior to the attachment of the VIF. The right side of the OS makes a new muscular attachment directly to the RVW superior to the VIF (E).

Figure 4

Virtual dissection of the ventricular mass to highlight the variability of the VIF. (A) Control heart–normal pattern, (blue dashed lines), (B) regular ToF, (C) ToF with severe hypoplasia of the outflow tract and (D) ToF with atretic pulmonary valve. The protrusion of the VIF in the right ventricular cavity is also variable. These three structures: the VIF, the RVW, and the OS define a muscular arch overriding the interventricular communication and the outflow tract of the left ventricle as an “inverted U,” well-seen in the short axis section (D,B). This “inverted U” muscular arch (green dashed lines) is present in all observed ToF of this study, irrespective of the size of the different constitutive parts of the RVOT.