Anatomy of conotruncal anomalies

Abstract

In this chapter, we provide an extensive description of the anatomic features of the various anomalies affecting the outflow tract of the heart (conotruncal defects or outflow tract defects) including tetralogy of Fallot (TOF) and its variants, common arterial trunk (CAT), interrupted aortic arch (IAA), various anatomic forms of double-outlet right ventricle (DORV), transposition of the great arteries (TGA) and congenitally corrected transposition of the great arteries (cc-TGA). These anomalies can be divided in two groups according to their embryologic origin, those due primarily to a lack of migration of the cardiac neural crest cells, and those due primarily to a perturbation of the laterality genes. The embryologic basis of this grouping is comforted by the anatomic differences between the two groups regarding the ventricular septal defect (VSD) and the coronary arteries pattern, as well as the course of the proximal great vessels. The VSD is always present and always of the outlet type in neural crest defects, although there are differences regarding its postero-inferior and superior rims. These differences suggest an anatomic continuum from TOF to IAA type B rather than distinct physiological phenotypes, related to various degrees of abnormal rotation of the outflow tract during heart development, incomplete in TOF, TOF with pulmonary atresia and DORV, absent in CAT, excessive in IAA type B. On the other hand, TGA is not always associated with a VSD, and when present, this VSD can be of any type (outlet, inlet, muscular, central membranous). The position of the coronary orifices on the aortic or truncal circumference depends on the rotation of the aorta, but also on the spatial relationship between the two great vessels, which accounts for the high frequency of coronary abnormalities in conotruncal defects. The great vessels emerge from the heart in a spiraling fashion in cardiac neural crest defects, but are parallel in TGA and cc-TGA. Congenitally corrected transposition of the great arteries includes as part of its phenotype a malformation of the outflow tract (ventriculo-arterial discordance). However, the major determinant of this laterality defect is the atrioventricular discordance, which puts it in a class of its own. In this complex malformation, the VSD is not constant and usually opens both in the outlet and in the inlet of the left ventricle, which, among other characteristics such as the abnormal location of the conduction tissue and the frequency of right ventricle and tricuspid valve anomalies, underlines its differences with the conotruncal defects mentioned above.