Discussion

Anomalous left pulmonary artery from the descending aorta is exceedingly rare. The three cases reported so far share common characteristics: left-sided aortic arch, anomalous origin of the left pulmonary artery origin from the ipsilateral wall of the descending aorta at a significant distance from the left subclavian artery, patent arterial duct connecting the pulmonary trunk to the under surface of the left aortic arch in two cases and no associated intra-cardiac lesions. ^{Reference Gnanappa, Laohachai, Orr and Ayer3,Reference Rajanbabu and Chigullapally5} Our case is very similar, with a patent arterial duct, and in addition, a left superior caval vein to coronary sinus. These cases are very different from the more frequent, albeit rare, anomalous origin of the left pulmonary artery from the ascending aorta. Various embryologic hypotheses have been reported about the constitution of anomalous origin of branch pulmonary arteries from either right or left aortic arch. ^{Reference Aru, English, Gaymes and Heath2,Reference Prifti, Bonacchi and Murzi4}

Developmentally, central pulmonary arteries have a dual origin: the pulmonary trunk originates from the embryonic outflow tract, while the right and left pulmonary arteries originate from the ventral pharyngeal mesenchyme (splanchnic plexus) connecting within the aortic sac to the pulmonary arch, ^{Reference Waldo and Kirby7} formerly named sixth aortic arch. ^{Reference Graham, Poopalasundaram, Shone and Kiecker8} Intra-pulmonary arteries develop within the lung buds and connect to the dorsal aorta through the intersegmental arteries, which disappear when the connection with the central pulmonary arteries occurs. On the left side, the left pulmonary artery derives from the proximal part of the left pulmonary arch, while the distal part gives rise to the arterial duct. Various hypotheses have been raised for anomalous origin of one pulmonary artery from the ascending aorta, the most prevalent being an asymmetric development of the aortic sac. This hypothesis is not applicable for the anomalous origin of left pulmonary artery from descending aorta. This rare anomaly could be due to a lack of development of the left pulmonary arch, total when no duct is present, ^{Reference Prifti, Bonacchi and Murzi4} or partial, involving only the proximal part of the arch, like in our case and others. ^{Reference Gnanappa, Laohachai, Orr and Ayer3,Reference Rajanbabu and Chigullapally5} Another subject of interrogation is the nature of the vessel connecting the descending aorta and the intra-parenchymal left pulmonary artery. If this anomaly results from lack of development of the left pulmonary arch, this vessel should be of another origin, like in tetralogy of Fallot with pulmonary atresia. In this setting, major aortopulmonary collateral arteries originating from the descending aorta are considered persistent intersegmental arteries. In our case like in other cases described, the argument against this anomalous vessel being a major aortopulmonary collateral artery is that it does supply the entire left pulmonary arterial tree, while major aortopulmonary collateral arteries, if they also enter in the hilum, usually supply a lobe or a segment and not the entire lung. ^{Reference Gnanappa, Laohachai, Orr and Ayer3,Reference Rajanbabu and Chigullapally5} However, some cases with tetralogy of Fallot with pulmonary atresia with one lung supplied by a single collateral artery arising from the descending aorta have been published. ^{Reference Jefferson, Rees and Somerville9}

The anatomical differences regarding associated anomalies in anomalous origin of the left pulmonary artery from the ascending aorta and anomalous origin of the left pulmonary artery from the descending aorta may reflect a different embryological background. Anomalous origin of the left pulmonary artery from the ascending aorta is commonly associated with tetralogy of Fallot and right aortic arch, which could be related to disturbances in neural crest cells migration. Indeed, anomalous origin of the left pulmonary artery from the ascending aorta was described in patients with DiGeorge syndrome and Waardenburg syndrome. ^{Reference Dodo, Alejos and Perloff10} On the contrary, the rare cases reported of anomalous origin of the left pulmonary artery from the descending aorta display no associated intra-cardiac lesions and have a left aortic arch, despite a similar alleged mechanism, namely the absence of proximal pulmonary arch.

The diagnosis of heterotaxy in our case was made according to Lin’s criteria, based on the association between: abdominal situs abnormality (midline liver), spleen abnormality (asplenia), left bronchial isomerism, bilateral superior vena cava and intestinal malrotation, despite the absence of intra-cardiac defects. ^{Reference Lin, Krikov and Riehle-Colarusso6} The definition of heterotaxy and the very term “heterotaxy” itself are still subject to many controversies. According to the IPCCC ICD-11 nomenclature, heterotaxy is defined as “a congenital malformation in which the internal thoraco-abdominal organs demonstrate abnormal arrangement across the left-right axis of the body”, right and left isomerisms are defined as variants of heterotaxy, and “heterotaxy does not include normal and mirror-imaged arrangements of the internal organs (situs solitus or inversus)”. ^{Reference Franklin, Béland and Colan11} According to Van Praagh, heterotaxy is defined as “an abnormal symmetry of certain viscera and veins (lungs, liver, caval veins) and situs discordance between various organ systems and between the various segments of the heart”. ^{Reference Van Praagh12} Although some anatomical studies claimed that atrial appendages were uniformly isomeric in the setting of heterotaxy, ^{Reference Uemura, Ho and Devine13,Reference Tremblay, Loomba and Frommelt14} several other reports demonstrate that this is not always the case. ^{Reference Loomba, Pelech, Shah and Anderson15,Reference Yim, Nagata and Lam16} We cannot affirm in our case that there was isomerism of the pectinate muscles, even if their external appearance looked similar on the CT scan, but the presence of lateralised atrial appendages would not preclude heterotaxy anyway. ^{Reference Loomba, Pelech, Shah and Anderson15,Reference Yim, Nagata and Lam16} Moreover, the absence of intra-cardiac defects does not rule out the diagnosis of heterotaxy, as it is now recognised that 10–16% of heterotaxy patients have no CHD. ^{Reference Lin, Krikov and Riehle-Colarusso6,Reference Cohen, Anderson and Cohen17} The fact that the electrocardiogram was normal with a normal P wave axis does not rule out heterotaxy neither, as assessed by Wren et al. ^{Reference Wren, Macartney and Deanfield18} Some authors have defined a CHD in the setting of heterotaxy as “any intracardiac lesion or an abnormality of venous returns to the heart”. ^{Reference Loomba, Pelech, Shah and Anderson15} For all these reasons, the diagnosis of heterotaxy, made on the association of left bronchial isomerism, abdominal organs laterality defects and bilateral superior caval vein, is unequivocal in our patient. Disharmony between the arrangement of the bronchi and the abdominal organs is relatively frequent (20% in the series by Yim ^{Reference Yim, Nagata and Lam16} ), but the association of left bronchial isomerism and asplenia is rare. All these considerations are in favour of an analytic approach based on accurate description of bronchi, abdominal organs and cardiac features independently, including the venous and arterial connections. ^{Reference Loomba, Pelech, Shah and Anderson15,Reference Sanders and Geva19} We have no embryologic explanation to propose for the unusual association between anomalous origin of the left pulmonary artery from the descending aorta and heterotaxy syndrome found in our patient.

Complete diagnosis and anomalous origin of the left pulmonary artery origin analysis are not always easy by echocardiography alone. CT angiogram and three-dimensional reconstruction were very helpful in our case to detail the lesions and guide the surgeon.

Overall, the surgical repair strategies for the re-implantation of the anomalous origin of the left pulmonary artery to the pulmonary trunk show good long-term results despite the late high risk of post-operative restenosis across the anastomotic site. When the distance between anomalous origin of the left pulmonary artery and pulmonary trunk is large, cardiopulmonary bypass is required during the procedure. Although early repair avoids pulmonary hypertension and irreversible pulmonary vascular disease, outcome of our patient was complicated by ventilation issues related to post-operative oedema compressing the left bronchus and severe bronchiectasis.

In conclusion, we present a rare case of anomalous origin of the left pulmonary artery from the descending aorta, associated with a particular form of heterotaxy, with disharmonious patterns of bronchi and abdominal organs, bilateral superior caval veins and no intra-cardiac lesions. This complex and unusual case reinforces the growing consensus that all the still pending controversies about heterotaxy should be resolved by the accurate description of each thoracic and abdominal organ independently, including the segments of the heart and the venous and arterial connections. ^{Reference Loomba, Pelech, Shah and Anderson15,Reference Sanders and Geva19,Reference Loomba, Tretter and Anderson20}