Morphogenesis of bulboventricular malformations. II. Observations on malformed hearts.

Abstract

The results are described of an investigation of specimens with bulboventricular malformations. The hearts examined had been previously classified as Fallot's tetralogy, double outlet right ventricle, or variousforms of transposition. After re-examination of the specimens, it was concluded that they represented a spectrum of malformations. While some were typical of the 'classical' description of known anomalies, others hadfeatures intermediate between two 'classical' anomalies. Yet others were considered to be examples ofrecently described anomalies such as 'posterior' transposition or double outlet left ventricle. We believe that the spectrum of anomalies can be directly interpreted in terms of variation in position of the embryological components of the bulboventricular loop. These components were described in detail in the first part of this study which was devoted to development in the normal heart, and are defined as the conoventricular flange, the conus septum, the trabecula septomarginalis, and the anterior and posterior segnients of the muscular ventricular septum. Our findings support the recent hypothesis that the spectrum can be explained in terms of conal rotation and conal absorption, but additionally suggest that reorientation of the anterior part of the muscular septum is a vitalpart of the process. The conclusions are discussed with regard to controversies concerning bulboventricular morphogenesis and nomenclature of supraventricular muscle bands of the right ventricle.