Embryology

During early embryonic development a series of symmetric pharyngeal arches develop, connecting the ventral truncus arteriosus with the dorsal aorta.[1] An asymmetric involution of portions of some or all of these individual arches then occurs, with the residua of left sixth arch persisting as the ductus arteriosus, connecting the left pulmonary artery (also a sixth arch derivative) with the left-sided dorsal aorta. This differential sixth arch evolution explains why the recurrent laryngeal nerves have different anatomic locations – around the 6^th arch on the left and the 4^th arch on the right (after involution of the right 5^th and 6^th arch). If the involution of the arches is reversed, with persistence of the right dorsal aorta, then the aortic arch is on the right side, typically with so called mirror-image aortic arch branching. A patent ductus arteriosus in this arrangement may be on the right side and connect the proximal descending aorta to the right pulmonary artery if the proximal descending aorta is on the right. If the descending aorta is on the left, the patent ductus may connect the proximal descending aorta to the left pulmonary artery. Alternatively, it may arise anteriorly, connecting the base of the mirror image left-sided innominate artery to the left pulmonary artery. Because of the variety of possible variations, it is crucial that assessment of the patient with a patent ductus include delineation of the laterality of the aortic arch, its branching pattern, and the origin and insertion of the ductus.