Discussion

Airway development, and therefore related anomalies, occur early in gestation starting from 24 days to approximately 17 weeks.^{Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3} The anomalous airways can be either supernumerary or displaced bronchi.^{Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3} A supernumerary bronchus co-exists with the normal patent bronchus and may either supply pneumatised lung or be blind-ended, whereas a displaced bronchus replaces a normal anatomical airway.^{Reference Chaddha, Chang and Lee2,Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3}

Several basic principles of airway anatomy exist. In general, the trachea is a 10–12-cm tube with incomplete cartilaginous rings bifurcating at the T4–T5 thoracic vertebral level.^{Reference Chaddha, Chang and Lee2,Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3} The mainstem bronchi are wider and shorter.^{Reference Chaddha, Chang and Lee2} Normal situs bronchial anatomy includes a right eparterial bronchus and left hyparterial bronchus.^{Reference Chaddha, Chang and Lee2–Reference Subha, Gail, Alla Godelman and Spindola-Franco4}

Some of the common congenital airway anomalies are tracheal and accessory cardiac bronchi, both of which are widely discussed in literature. However, in this case report, we are presenting a much rarer entity: a type 2 bridging bronchus supplying the entire right lung with an associated left pulmonary artery sling in an symptomatic adult male patient.

A bridging bronchus is diagnosed when all or part of the right lung is aerated by a displaced bronchus arising from the left mainstem bronchus.^{Reference Chaddha, Chang and Lee2,Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3,Reference Westin and Kapalczynski5} There are two types of bridging bronchi.^{Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3} In type 1, the right upper lobe bronchus arises from the trachea, whereas the right bronchus intermedius arises from the medial wall of the left mainstem bronchus as a bridging bronchus.^{Reference Chaddha, Chang and Lee2,Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3} In type 2, the right mainstem bronchus is a blind-ended congenital tracheal diverticulum at the carina and the right lung is entirely aerated by a bridging bronchus arising from the medial wall of the left mainstem bronchus.^{Reference Chaddha, Chang and Lee2,Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3} The origin of the bridging bronchus is at the level of T6–T7 (lower than the normal level of the carina at T4) and termed the pseudo-carina.^{Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3}

Bridging bronchus anomalies are very commonly associated with a left pulmonary artery sling, in up to 27% of type 1 cases and in up to 57% of type 2 cases.^{Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3,Reference Westin and Kapalczynski5} An alternative classification scheme exists that describes the arterial sling as the main anomaly classified into types 1 and 2 depending on the presence or absence of a bridging bronchus.^{Reference Westin and Kapalczynski5} The presence of complete tracheal rings is an important finding in patients with type 2 pulmonary slings, termed the “ring-sling” complex.^{Reference Lee, Kim and Lee6} The tracheal rings with associated stenosis drive the patients’ ongoing shortness of breath with variable severity and can be fatal in early infancy.^{Reference Lee, Kim and Lee6} The gold standard for diagnosis is bronchoscopy, where a stenotic trachea and absence of the posterior membrane are seen.^{Reference Westin and Kapalczynski5} In the presence of complete cartilaginous tracheal rings, treatment options may expand to include tracheal resection or autograft rather than isolated bronchoscopic dilation depending on severity.^{Reference Westin and Kapalczynski5,Reference Lee, Kim and Lee6} In our case, the patient was found to have complete bronchial cartilage rings on bronchoscopy, but the thoracic surgeon decided to attempt bronchoscopic dilatation prior to surgery in the hopes that this would provide symptomatic relief.

Patients with bridging bronchi can present with wheezing, respiratory distress, or poor feeding, often in infancy.^{Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3,Reference Westin and Kapalczynski5} Both cross-sectional assessment and bronchoscopy are usually warranted prior to treatment.^{Reference Chassagnon, Morel, Carpentier, Le Pointe and Sirinelli3}