FIG 48.6 Images showing the course of the vessel found in a solitary human embryo, at Carnegie stage 14, equivalent to embryonic day 10.5 in the mouse, in which there is evidence of an artery of the fifth pharyngeal arch (A) Oblique view of the channel from the left side, with the pharyngeal endoderm reconstructed in green (B) Left lateral view showing how the vessel is contained within a discrete segment of pharyngeal mesenchyme It joins the sixth arch artery to gain its attachment to the dorsal aorta Candidates for Persistent Fifth Aortic Arch Arteries Pediatric cardiologists have interpreted multiple channels found in the extrapericardial arterial pathways as persistent arteries of the fifth pharyngeal pouch.13–73,78–106 Following a systematic review of the cases existing at that time, Freedom suggested that the candidates could be classified as providing systemic-to-systemic channels, or systemic-to-pulmonary pathways, pulmonaryto-systemic entities, or they could persist bilaterally.57 The initial channel categorized in this way was a systemic-to-systemic channel that produced a double-barreled aorta.13 The authors of the initial description highlighted the origin of the proposed fifth arch from the ascending aorta proximal to the brachiocephalic artery Such was the impact of their initial observation13 that many subsequent investigators labeled any vessel arising from the distal part of the ascending aorta as persistence of the fifth arch artery We now know that, during embryogenesis, there is differential growth of medial and lateral surfaces of the aorta This, in turn, creates the potential for marked variation in the position of the brachiocephalic artery relative to the channel presumed to be the fifth arch artery There is still further potential variation in the relationship of the proximal origin of the arterial duct to the origin of the brachiocephalic artery This anatomic variability further complicates the assessment of the threedimensional orientation of the aortic arch and its branches, creating particular difficulty in demonstrating clinically that the origin of any extra vessel is unequivocally proximal to the origin of the brachiocephalic artery Although this may be possible when using computed tomography or magnetic resonance imaging, it is extremely difficult if it is based solely on echocardiographic interrogation, as has been the case in many reports.41,45,47,65,69,102 It is also unfortunate that alleged persistence of the artery of the fifth arch has been cited to explain the morphogenesis of several congenital malformations without any attention to the scant support thus far available from developmental studies.108 A reassessment of many, if not most, of these reports, including those describing communications with the pulmonary arteries, suggests that they may better be considered as representing persisting collateral channels or else explained on the basis of remodeling of the aortic sac and the third, fourth, and sixth arch arteries.68,70,108 Double-Barreled Aorta or Double-Lumen Aortic Arch Subsequent to the initial interpretation offered by Van Praagh and Van Praagh,13 which was endorsed enthusiastically by Gerlis and colleagues,21,23 at least 44 cases of double-barreled aorta have been reported as representing fifth arch arteries (Table 48.1) More and more cases are now labeled in this fashion simply because of the presence of the double-lumen aorta However, many of the investigators making this diagnosis fail to assess the origin of the second channel relative to that of the brachiocephalic artery As we have emphasized, this relationship is subject to error if not assessed carefully.41,45,47,65,69,102 In one of our own cases with double-barreled aorta, which we studied echocardiographically, the origin of arterial duct proximal to the second channel suggests that the findings are better explained on the basis of persistence of collateral channels combined with remodeling of the lesser curvature of the aortic arch (Fig 48.7) It is difficult to account for the proximal location of arterial duct in this patient if it is presumed that the second channel is a persistent artery of the fifth pharyngeal arch Thus far, furthermore, most clinical investigators interpreting double-barreled aorta on the basis of persistence of the artery of the fifth arch have failed to consider the possibility that the second channel is no more than persistence of one of the collateral channels formed between the dorsal insertions of the fourth and sixth arch arteries As already discussed, such collateral channels were found in up to half of our developing mice and are also to be found in developing human embryos.3 Table 48.1 All the Cases Published in the World Literature That Have Implicated Persistence of the Artery of the Fifth Arch Producing Double-Barreled Aorta With or Without Persistent Patency of the Arterial Duct or Coarctation of the Aorta Year 1969 1973 1973 Author Van Praagh Izukawa Izukawa Case 1 Anatomy of the Proposed Fifth Arch Double-barreled aorta Double-barreled aorta with patent arterial duct Double-barreled aorta with coarctation Reference 13 14 14 ...Candidates for Persistent Fifth Aortic Arch Arteries Pediatric cardiologists have interpreted multiple channels found in the extrapericardial arterial pathways as persistent arteries of the fifth pharyngeal