Truncus arteriosus malformation: a spectrum including fourth and sixth aortic arch interruptions

Previous explanations of the pathogenesis of truncus arteriosus malformation have emphasized absence of conotruncal ridges, absence of pulmonary conus, or fusion of ventricular outflow streams. These concepts explain the persistence of the single semilunar valve and outflow vessel, but have not elucidated the significance of many associated anatomic lesions. We studied the 19 patients with truncus arteriosus malformation listed in the autopsy files of The Johns Hopkins Hospital whose hearts were available for review. The patients ranged in age from one day to 30 years, and included 12 males. All hearts showed a single semilunar valve with three or four cusps, and a high ventricular septal defect. In 13 patients, the aorta was larger in diameter than the pulmonary artery, and no remnant of the ductus arteriosus was present (interruption of the embryonic sixth arch). In one patient, the aorta was larger than the pulmonary artery, and a small, patent ductus arteriosus was present. In two patients, the pulmonary artery was larger than the aorta, and the aortic arch was interrupted. The remaining three cases could not be fully evaluated for the status of the ductus arteriosus and size of the great arteries. We tabulated the number of associated anatomic lesions which were unexplained by each of the three pathogenetic hypotheses. The fused outflow stream hypothesis, which explains truncus arteriosus malformation as the result of a maldirection of ventricular outflow streams so that the separation of two semilunar valves is prevented, was superior at the 0.05 level of significance. This concept explains the spectrum of great vessel patterns in which flow is dominant into either the sixth or fourth embryonic aortic arch, permitting early disappearance of unperfused segments of the aortic arches, and is commonly seen at autopsy as either an interrupted aorta or an absent ductus arteriosus.