Discussion

Left main bronchus narrowing can occur due to intrinsic structural wall abnormalities resulting in bronchial collapse, or extrinsic compression of the bronchus, most often by adjacent vascular structures. Compared to right bronchus, left bronchus is more vulnerable for compression due to the longer course and smaller caliber compared to right. Any abnormality in the surrounding structures like pulmonary artery, descending aorta, vertebral body, and oesophagus can result in bronchial compression. They can manifest with stridor, wheeze, recurrent pneumonia, and apneic spells.

Abnormal pre-vertebral course of descending thoracic aorta resulting in left main bronchial compression is a rare entity, first described by Donnelly and his colleagues.^{Reference Donnelly, Bisset and McDermott2} They described the magnetic resonance images of two infants where the descending aorta, instead of coursing in the left paraspinal space, had an abnormal pre-vertebral course resulting in bronchial compression, caused by the pincer effect of aorta and right pulmonary artery. In our patient too, the descending thoracic aorta coursed abnormally, crossing from left to right and subsequently descended in the anterior right paraspinal space. At the level of bronchus, the aorta crossed the midline and was pre-vertebral, slightly onto the right. Goldman et al had reported associated CHDs in 50% of their patients.^{Reference Goldman, Rimell and Meza3} In their series of 18 patients, Arcieri et al reported associated malformations in 16 children, which included CHDs, oesophageal atresia, and trachea-oesophageal fistula.^{Reference Arcieri, Serio and Nenna4} Our patient had a structurally normal heart, but had associated pectus excavatum. This association was earlier reported by Speggiorin et al.^{Reference Speggiorin, Atamanyuk and Wallis5} Whether associated chest wall abnormality contributes to the abnormal course of the aorta is speculative.

A left aortic arch with right descent, most often has a retro-oesophageal course as it crosses the midline, and is described as circumflex aorta. This often forms a complete vascular ring and causes airway compression. In our patient, though the arch was left sided with right descent, the aortic arch was not retro-oesophageal, as evidenced on CT angiogram and barium swallow. It is important to differentiate abnormal pre-vertebral course of descending aorta from circumflex aorta, as the surgical approach is different. Posterior aortopexy gives good symptom relief in the former, while a more extensive aortic uncrossing procedure is required for the latter.

Our patient showed a dramatic and sustained symptom relief following posterior aortopexy. This has been described as an effective mode of management by most previous authors too.^{Reference Donnelly, Bisset and McDermott2,Reference Arcieri, Serio and Nenna4–Reference Hungate, Newman and Meza6} Speggiorin and his colleagues adopted a combined approach of Nuss procedure and aortopexy in two cases, where pectus excavatum was associated. However, we decided to do aortopexy alone initially, as there was no significant compression of the trachea and heart and had planned to consider Nuss procedure only if child continued to remain symptomatic. Symptoms may sometimes persist after aortopexy, due to secondary bronchomalacia, and may require bronchial stenting for symptom relief.^{Reference Arcieri, Serio and Nenna4} Arcieri et al reported residual bronchomalacia in 44% of their cases.^{Reference Arcieri, Serio and Nenna4} Repeat bronchoscopy at one-year follow-up revealed normal caliber of bronchus with no residual malacia in our patient.