Puzzling Bronchial Trifurcation

Discussion

Tracheal bronchus was first described by Sandifort as a right upper lobe bronchus originating from the trachea. Recently the scope of the tracheal bronchus has encompassed various bronchial anomalies arising either from main bronchi or the trachea and supplying the upper lobe territory.¹ A tracheal bronchus arising from the main bronchus is termed according to the position of its origin.¹ On the right side, an anomalous bronchus that originates proximal to the origin of an upper lobe bronchus is termed pre-eparterial, whereas an anomalous bronchus that arises distal to the origin of an upper lobe bronchus is termed post-eparterial. On the left side, an anomalous bronchus with an origin above the arch of the left pulmonary artery is termed an eparterial bronchus. For anomalous bronchi that lie below the arch of the left pulmonary artery, a pre-hyparterial bronchus arises proximal, whereas a post-hyparterial bronchus arises distal to the origin of the upper lobe bronchus.

Tracheal bronchus is further subdivided into supernumerary and displaced types.¹ The supernumerary type is where there is a normal branching of the upper lobe bronchus. In the absence of normal branching of the upper lobe bronchus, an anomalous bronchus that aerates one or more missing segments of the upper lobe is described as the displaced type.

In patient 1, bronchus A was a displaced right pre-eparterial bronchus because it represented an anomalous right apical segmental bronchus (B¹) that originated proximal to the origin of the upper lobe bronchus, whereas B¹ was missing from the right upper lobe bronchus.

In patient 2, bronchus A was a displaced left pre-hyparterial bronchus because it represented an anomalous left apicoposterior segmental bronchus (B¹⁺²) that originated above the origin of the left upper lobe bronchus and below the arch of the left pulmonary artery, whereas B¹⁺² was missing from the left upper lobe bronchus.

Normally, B¹⁺² and the anterior segmental bronchus of the left upper lobe (B³) coexist from the left upper divisional bronchus. Although trifurcation of the left main bronchus was previously described by Beder et al³ and Doshi and Mehta,⁴ they gave no additional detail on the segmental divisions of both bronchi that supplied the left upper lobe. However, in all cases having the displaced left pre-hyparterial bronchus, Ghaye et al¹ found that this bronchus supplied only the S¹⁺², while the B³ and lingular bronchus originated from a common trunk, as in the present case.

Accessory fissures of the left upper lobe are occasionally found on CT. The so-called left minor fissure, which separates S³ from S⁴, is the most common. The fissure that separates S¹⁺² from S³ (as in patient 1) was found only in 2 cases in the series reported by Berkmen et al.² That fissure ran convex laterally to the chest wall.

Because the lung portion supplied by each branch of the left main bronchial trifurcation was separated from each other by the accessory fissure and the left major fissure, it would be described as a lobe. In patient 2, according to the origin and supply, we prefer to name these 3 lobes the pre-hyparterial lobe, the hyparterial lobe, and the left lower lobe, to avoid confusion with the normal left lung. To our knowledge, these coexisting variations have been briefly described only by Desir and Ghaye.⁵ Nevertheless, we speculate that all such anomalies always coexist and are unique to the left main bronchial trifurcation. CT is helpful in studying these anomalies.

There are 3 theories for the development of anomalous bronchi: reduction, migration, and selection.⁶ In our cases, the aberrant bronchi were displaced bronchi directing to the right S¹ and the left S¹⁺² that missed from the upper lobe bronchi. Therefore, the migration theory would be appropriate to explain their origin. The subsidiary branches can migrate from their initial loci to new points of origin of a bronchus.¹ Like a normal right upper lobe bronchus that migrates from its hyparterial position to its eparterial location on the right main bronchus, the right B¹ might migrate from its position to the lateral aspect of the right main bronchus proximal to the origin of the right upper lobe bronchus. Likewise, the left B¹⁺² might migrate from its position to the posterior aspect of the distal left main bronchus close to the origin of the left upper lobe bronchus, which results in the trifurcation of the left main bronchus.