Discussion

Anomalous origin of a branch of the left pulmonary artery from the right pulmonary artery in the setting of normal bifurcation of the pulmonary trunk is extremely rare. Our search of the literature revealed only four similar cases. In the case reported by Bamman et al,Reference Bamman, Ward and Woodrum ² the anomalous left pulmonary artery circled over the bronchus to the right lower lobe and passed to the left hilum between the trachea and the oesophagus, forming a partial pulmonary arterial sling. In the case described by Erickson et al,Reference Erickson, Cocalis and George ³ the aberrant artery to the left lower lobe ran anterior to the trachea and caudal to the left lower lobe bronchus. As there was no encirclement of the trachea, this case cannot be considered to represent an arterial sling. In the example reported by Collins et al,Reference Collins, Weinberg, Goldmuntz and Harris ⁴ the anomalous artery coursed anterior to the left bronchus, producing no compression, and therefore again is not a sling. In the case of Ge et al,Reference Ge, De Groff, Knudson, Strain and Chan ⁵ the anomalous left lower pulmonary artery ran posterior to the left main bronchus. The case described by Marins et alReference Marins, Anderson and Aiello ¹ is also of interest, as in their patient the entire left pulmonary artery had an extrapericardial origin, but again without producing a sling. Our two cases are most comparable to the arrangement described by Ge et al.Reference Ge, De Groff, Knudson, Strain and Chan ⁵

Such anomalous origin of the left lower lobe pulmonary artery from the right pulmonary artery has previously been described as a partial pulmonary arterial sling. Our review shows, however, that only the patient described by Bamman et alReference Bamman, Ward and Woodrum ² had the anomalous artery coursing between the trachea and oesophagus, thus forming the so-called sling. In all other cases, including our own, the anomalous branch caused neither tracheal nor oesophageal compression. Therefore, all the cases reviewed, including the two from this report, except the one reported by Bamman and associates, are more accurately described as partial anomalous origin of the left pulmonary artery.

When first formed, the right and left pulmonary arteries arise within the pharyngeal mesenchyme and take their origin from the base of the aortic sac.Reference Anderson, Chaudhry and Mohun ⁶ When seeking to explain the development of the pulmonary arterial sling, Jue et alReference Jue, Raghib, Amplatz, Adams and Edwards ⁷ postulate that when the primitive lung tissue is still undivided into the left and right lungs, the pulmonary supply from the left sixth aortic arch either fails to form or, having formed, gets obliterated. Consequent to a lack of arterial supply, the right sixth aortic arch sends a collateral branch into the part of the primitive pulmonary mass, which will later become the left lung. This explanation, however, fails to stand rigorous examination, as the lung buds themselves arise separately from the tracheal groove, and the pulmonary arteries arise from the base of the aortic sac, rather than from the sixth pharyngeal arch arteries. Therefore, it remains difficult to provide a rational explanation for either the pulmonary arterial sling, or partial anomalous origin of the left pulmonary artery from the right pulmonary artery.

With regard to clinical implications, the partial anomalous origin of the left pulmonary artery would have affected flows of blood to the left lung had our first patient been selected for further surgery, and may ultimately have impacted on the suitability for a cavopulmonary circulation. The abnormal branching pattern had no clinical manifestations in our second case. Awareness of this rare variation is important, nonetheless, for formulating an appropriate therapeutic plan, as the finding of normal bifurcation of the pulmonary trunk does not exclude additional abnormal origin of part of the left pulmonary artery.