(Radiographics. 2001;21:105-119.)
© RSNA, 2001
Congenital Bronchial Abnormalities Revisited1
Benoit Ghaye, MD,
David Szapiro, MD,
Jean-Marc Fanchamps, MD and
Robert F. Dondelinger, MD
1 From the Department of Medical Imaging, University Hospital Sart Tilman, Domaine Universitaire du Sart Tilman, Batiment B35, B-4000 Liège 1, Belgium. Presented as a scientific exhibit at the 1999 RSNA scientific assembly. Received April 12, 2000; revision requested May 19 and received July 10; accepted July 10. Address correspondence to R.F.D. (e-mail: rdondelinger@chu.ulg.ac.be).
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Abstract
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Bronchial anatomy is adequately demonstrated with the appropriate spiral computed tomographic technique on cross-sectional images, multiplanar reconstruction images, and three-dimensional reconstruction images. Contrary to the numerous variations of lobar or segmental bronchial subdivisions, abnormal bronchi originating from the trachea or main bronchi are rare. Major bronchial abnormalities include accessory cardiac bronchus (ACB) and "tracheal" bronchus. An ACB is a supernumerary bronchus from the inner wall of the right main bronchus or intermediate bronchus that progresses toward the pericardium. Fourteen ACBs were found in 17,500 consecutive patients (frequency, 0.08%). The term tracheal bronchus encompasses a variety of bronchial anomalies originating from the trachea or main bronchus and directed to the upper lobe. In a series of 35 tracheal bronchi, only eight originated from the trachea, three originated from the carina, and 24 originated from the bronchi. Displaced tracheal bronchi (27 of 35) are more frequent than supernumerary tracheal bronchi (eight of 35). Minor bronchial abnormalities include variants of tracheal bronchus, displaced segmental bronchi, and bronchial agenesis. The main embryogenic hypotheses for congenital bronchial abnormalities are the reduction, migration, and selection theories. Knowledge and understanding of congenital bronchial abnormalities may have important implications for diagnosis, bronchoscopy, surgery, brachytherapy, and intubation.
Index Terms: Bronchi, abnormalities, 671.1499 • Bronchi, anatomy, 671.92 • Bronchi, CT, 671.12115
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Introduction
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In the past, 1%–12% of patients who underwent bronchography or bronchoscopy demonstrated some form of congenital tracheobronchial variant or anomaly (1–4). Anomalies of the lung and bronchial tree are diagnosed with increasing frequency as a result of refinements in modern imaging and classification. Spiral computed tomography (CT) has broadened the potential of imaging of lung anatomy by offering various reformation techniques, which demonstrate the anatomy down to the subsegmental level. These reformation techniques include multiplanar reconstruction, shaded-surface display (SSD), minimum-intensity projection, maximum-intensity projection, sliding thin slab imaging, volume rendering, and virtual bronchoscopy (5). However, confusion has been created due to a nonuniform nomenclature in the literature.
In this article, we review the basic bronchial anatomy and the most common variants, report our experience with congenital bronchial abnormalities in asymptomatic adult patients, and discuss hypotheses about the embryology of bronchial abnormalities. The dysmorphic lung, including the agenesis-aplasia complex and congenital pulmonary venolobar syndrome, is occasionally seen in adults but will not be discussed, since it represents a true pulmonary malformation (6,7).
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Bronchial Anatomy and Most Common Variants
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A comprehensive bronchial nomenclature, modified from Boyden (8) and Yamashita (9) and adapted to cross-sectional imaging, is given in Figures 1 and 2. Lungs that are made up wholly of the prevailing pattern are seldom encountered, since variation in even one zone necessarily modifies the development of adjacent segments. For illustration, the prevailing variations of bronchial bifurcation observed in 30 consecutive asymptomatic and healthy patients, who were referred due to clinical suspicion of pulmonary embolism, are presented in Table 1. The study was performed with a spiral CT scanner (Picker International, Cleveland, Ohio) from 2 cm above the top of the aortic arch to the diaphragm. The imaging parameters were 2- or 3-mm section thickness, 1- or 2-mm reconstruction increment, pitch of 1.5–2, a high-spatial-resolution algorithm for lung windows, and a soft-tissue algorithm for mediastinal windows. The breath hold during acquisition varied from 20 to 40 seconds.
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Figure 1. Bronchi of the right lung. Three-dimensional schematic (lateral view) shows the right upper lobe bronchus (RULB), intermediate bronchus (IB), middle lobe bronchus (MLB), right lower lobe bronchus (RLLB), and basilar truncus (BT); the corresponding apical (B1), anterior (B2), posterior (B3), lateral (B4), medial (B5), superior (apical) (B6), subsuperior (B*), medial basilar (paracardiac) (B7), anterior basilar (B8), lateral basilar (B9), and posterior basilar (B10) segments; and the corresponding posterior or apical (B1a), anterior (B1b), lateral (B2a), anterior (B2b), lateral (B3a), posterior or apical (B3b), lateral or posterior (B4a), medial or anterior (B4b), superior (B5a), inferior (B5b), medial (B6a), superior (B6b), lateral (B6c), lateral or anterior (B7a), medial or posterior (B7b), lateral (B8a), basilar (B8b), lateral (B9a), basilar (B9b), laterobasal (B10a), and mediobasal (B10b) subsegments. ext = external, inf = inferior, int = internal, sup = superior.
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Figure 2. Bronchi of the left lung. Three-dimensional schematic (lateral view) shows the left upper lobe bronchus (LULB), culminal bronchus (CB), lingular bronchus (LB), left lower lobe bronchus (LLLB), and basilar truncus (BT); the corresponding apicoposterior (B1+3), anterior (B2), superior (B4), inferior (B5), superior (apical) (B6), subsuperior (B*), anteromedial basilar (B7+8), lateral basilar (B9), and posterior basilar (B10) segments; and the corresponding apical (B1+3a), posterior (B1+3b), lateral (B1+3c), lateral (B2a), anterior or medial (B2b), superior (B2c), posterior or lateral (B4a), anterior (B4b), superior (B5a), inferior (B5b), medial (B6a), superior (B6b), lateral (B6c), anterior (B7a), medial (B7b), lateral (B8a), basilar (B8b), lateral (B9a), basilar (B9b), laterobasal (B10a), and mediobasal (B10b) subsegments.
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The results were similar to those of other major anatomic studies (8–13). Most of the variations were seen in the upper lobes. In the right upper lobe, the typical trifurcation of segmental bronchi was seen in only 30% of cases; various types of bifurcation were seen in the other 70%. The apical bronchus (B1) of the right upper lobe was missing in 22%. In 23%, the middle lobe bronchus had a superior-inferior division similar to that of the lingula; in 25%, the lingula had a lateromedial division similar to that of the middle lobe. In the lower lobes, the predominant pattern of division of the superior segmental bronchus (B6) was B6a+b and B6c on the right side and B6a and B6b+c son the left side. A subsuperior bronchus (B*) was seen in the right lung in 56% and in the left lung in 26%. A left medial basilar bronchus (B7), as occurs in the right lung, was seen in only 14%.
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Congenital Bronchial Abnormalities
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Contrary to the numerous variations of lobar or segmental bronchial subdivisions, abnormal bronchi originating from the trachea or main bronchi are rare. They were first reported by anatomists and pathologists and later by surgeons, bronchoscopists, and radiologists at bronchography. All congenital bronchial abnormalities presented in this article were incidentally discovered during regular reading of film images by radiologists with a special interest in this topic. Reconstructions were performed on a separate workstation (Voxel Q; Picker International). The region-growing segmentation technique was used for three-dimensional representation of the bronchial tree.
Accessory Cardiac Bronchus
An accessory cardiac bronchus (ACB) was defined by Brock (13) in 1946 as a "supernumerary bronchus arising from the inner wall of the right main bronchus or intermediate bronchus opposite to the origin of the right upper lobe bronchus." The bronchus progresses conically for 1–5 cm in a caudal direction toward the pericardium, paralleling the intermediate bronchus. It is lined by normal bronchial mucosa and has cartilage within its wall, which distinguishes it from a diverticulum or acquired fistula. Most ACBs have a blind extremity, but imaging and anatomic studies have demonstrated that some develop into a series of small bronchioles, which may end in vestigial or rudimentary bronchiolar parenchymal tissue, cystic degeneration, or a ventilated lobulus (14–17). In normal anatomy, the only bronchus arising from the medial wall of the right main bronchus or a right lobar bronchus is the medial basilar segmental bronchus (B7). An ACB is different from the medial basilar segmental bronchus and does not correspond to proximal migration of this structure, which arises from the right lower lobe bronchus.
We have found 14 ACBs in 17,500 consecutive patients (frequency, 0.08%). The mean largest diameter was 8.7 mm (range, 4.0–13.8 mm), and the mean length was 12.0 mm (range, 4.2–23.4 mm). Ten bronchi had a blind extremity (Fig 3), whereas four demonstrated a ventilated lobulus with a mean diameter of 36.1 mm (range, 18.6–62.0 mm), which was located in the azygoesophageal recess and demarcated from the right lower lobe by an anomalous fissure (Fig 4). A small soft-tissue mass around the extremity of the ACB was found in five cases; according to published CT-anatomic correlations, this mass may correspond to collapsed vestigial parenchyma (16), with no clear differentiation from normal mediastinal lymph nodes in our study. The ACB originated from the intermediate bronchus in 12 cases and from the right main bronchus in two cases. The abnormal bronchus was demarcated by a spur at its origin in 12 patients (86%). An abnormal pulmonary artery was observed in one case (7%) (Fig 5). All ACBs were asymptomatic and incidentally discovered, but cough or hemoptysis can result from superinfection, aspergilloma, or tumor (15–17). Some associated anomalies are reported in the literature, including right or left tracheal bronchus, coexistence of two ACBs, and bronchiectasis (13,15,16,18).
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Figure 3a. Blind ACB. (a) Three-millimeter-thick spiral CT section shows an anomalous bronchus (arrow) arising from the medial aspect of the intermediate bronchus. (b) Contrast material-enhanced CT scan obtained 3 mm distal to a shows a soft-tissue mass (arrowhead) below the ACB and medial to the intermediate bronchus; this mass may correspond to collapsed vestigial lung parenchyma. (c) Coronal SSD image shows a large-mouthed outpouching (large arrow) originating from the medial wall of the intermediate bronchus. Note the medial basilar segmental bronchus (B7) (small arrow) originating from the right lower lobe bronchus. (Reprinted, with permission, from reference 17.)
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Figure 3b. Blind ACB. (a) Three-millimeter-thick spiral CT section shows an anomalous bronchus (arrow) arising from the medial aspect of the intermediate bronchus. (b) Contrast material-enhanced CT scan obtained 3 mm distal to a shows a soft-tissue mass (arrowhead) below the ACB and medial to the intermediate bronchus; this mass may correspond to collapsed vestigial lung parenchyma. (c) Coronal SSD image shows a large-mouthed outpouching (large arrow) originating from the medial wall of the intermediate bronchus. Note the medial basilar segmental bronchus (B7) (small arrow) originating from the right lower lobe bronchus. (Reprinted, with permission, from reference 17.)
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Figure 3c. Blind ACB. (a) Three-millimeter-thick spiral CT section shows an anomalous bronchus (arrow) arising from the medial aspect of the intermediate bronchus. (b) Contrast material-enhanced CT scan obtained 3 mm distal to a shows a soft-tissue mass (arrowhead) below the ACB and medial to the intermediate bronchus; this mass may correspond to collapsed vestigial lung parenchyma. (c) Coronal SSD image shows a large-mouthed outpouching (large arrow) originating from the medial wall of the intermediate bronchus. Note the medial basilar segmental bronchus (B7) (small arrow) originating from the right lower lobe bronchus. (Reprinted, with permission, from reference 17.)
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Figure 4a. ACB with a ventilated lobulus in a patient who underwent right upper lobectomy and radiation therapy. (a) Five-millimeter-thick spiral CT section shows an anomalous bronchus (thin arrow) originating from the intermediate bronchus. Aerated lung (thick arrow) is seen between this bronchus and the left main bronchus. (b) CT scan obtained 15 mm caudad to a shows dystrophic aerated parenchyma (large arrow) demarcated by an anomalous fissure (arrowheads) from the normal parenchyma of the right lower lobe. Note that the origin of the medial basilar segmental bronchus (B7) (small arrow) is in a normal position. (c) Coronal SSD image shows the ACB (arrow) arising from the intermediate bronchus and ventilating a lobulus with a diameter of 31.9 mm. The spatial rearrangement of the middle lobe bronchus and right lower lobe bronchus is the result of postoperative and postirradiation changes. Arrowhead = resected right upper lobe bronchus. (d) Virtual endoscopic view shows the ACB (right side of image) separated by a spur (arrow) from the middle part of the intermediate bronchus (left side of image), an appearance that mimics a sharply defined second carina. (Reprinted, with permission, from reference 17.)
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Figure 4b. ACB with a ventilated lobulus in a patient who underwent right upper lobectomy and radiation therapy. (a) Five-millimeter-thick spiral CT section shows an anomalous bronchus (thin arrow) originating from the intermediate bronchus. Aerated lung (thick arrow) is seen between this bronchus and the left main bronchus. (b) CT scan obtained 15 mm caudad to a shows dystrophic aerated parenchyma (large arrow) demarcated by an anomalous fissure (arrowheads) from the normal parenchyma of the right lower lobe. Note that the origin of the medial basilar segmental bronchus (B7) (small arrow) is in a normal position. (c) Coronal SSD image shows the ACB (arrow) arising from the intermediate bronchus and ventilating a lobulus with a diameter of 31.9 mm. The spatial rearrangement of the middle lobe bronchus and right lower lobe bronchus is the result of postoperative and postirradiation changes. Arrowhead = resected right upper lobe bronchus. (d) Virtual endoscopic view shows the ACB (right side of image) separated by a spur (arrow) from the middle part of the intermediate bronchus (left side of image), an appearance that mimics a sharply defined second carina. (Reprinted, with permission, from reference 17.)
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Figure 4c. ACB with a ventilated lobulus in a patient who underwent right upper lobectomy and radiation therapy. (a) Five-millimeter-thick spiral CT section shows an anomalous bronchus (thin arrow) originating from the intermediate bronchus. Aerated lung (thick arrow) is seen between this bronchus and the left main bronchus. (b) CT scan obtained 15 mm caudad to a shows dystrophic aerated parenchyma (large arrow) demarcated by an anomalous fissure (arrowheads) from the normal parenchyma of the right lower lobe. Note that the origin of the medial basilar segmental bronchus (B7) (small arrow) is in a normal position. (c) Coronal SSD image shows the ACB (arrow) arising from the intermediate bronchus and ventilating a lobulus with a diameter of 31.9 mm. The spatial rearrangement of the middle lobe bronchus and right lower lobe bronchus is the result of postoperative and postirradiation changes. Arrowhead = resected right upper lobe bronchus. (d) Virtual endoscopic view shows the ACB (right side of image) separated by a spur (arrow) from the middle part of the intermediate bronchus (left side of image), an appearance that mimics a sharply defined second carina. (Reprinted, with permission, from reference 17.)
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Figure 4d. ACB with a ventilated lobulus in a patient who underwent right upper lobectomy and radiation therapy. (a) Five-millimeter-thick spiral CT section shows an anomalous bronchus (thin arrow) originating from the intermediate bronchus. Aerated lung (thick arrow) is seen between this bronchus and the left main bronchus. (b) CT scan obtained 15 mm caudad to a shows dystrophic aerated parenchyma (large arrow) demarcated by an anomalous fissure (arrowheads) from the normal parenchyma of the right lower lobe. Note that the origin of the medial basilar segmental bronchus (B7) (small arrow) is in a normal position. (c) Coronal SSD image shows the ACB (arrow) arising from the intermediate bronchus and ventilating a lobulus with a diameter of 31.9 mm. The spatial rearrangement of the middle lobe bronchus and right lower lobe bronchus is the result of postoperative and postirradiation changes. Arrowhead = resected right upper lobe bronchus. (d) Virtual endoscopic view shows the ACB (right side of image) separated by a spur (arrow) from the middle part of the intermediate bronchus (left side of image), an appearance that mimics a sharply defined second carina. (Reprinted, with permission, from reference 17.)
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Figure 5a. ACB with a ventilated lobulus and an abnormal pulmonary artery. (a) Two-millimeter-thick spiral CT section shows the origin of an anomalous bronchus (arrow), which is separated by a spur (arrowhead) from the proximal part of the intermediate bronchus. (b) CT scan obtained 20 mm distal to a shows a ventilated accessory cardiac lobulus (arrow) demarcated by an anomalous fissure (arrowheads) from the right lower lobe bronchus. (c) Axial sliding thin slab maximum-intensity projection image shows an anomalous artery (arrowheads) originating from the posterior wall of the right pulmonary artery and vascularizing the territory of the ACB (arrow). (d) Coronal SSD image shows the ACB (thin arrow) arising from the intermediate bronchus and ventilating a lobulus 6.2 cm in diameter (thick arrow). (Reprinted, with permission, from reference 17.)
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Figure 5b. ACB with a ventilated lobulus and an abnormal pulmonary artery. (a) Two-millimeter-thick spiral CT section shows the origin of an anomalous bronchus (arrow), which is separated by a spur (arrowhead) from the proximal part of the intermediate bronchus. (b) CT scan obtained 20 mm distal to a shows a ventilated accessory cardiac lobulus (arrow) demarcated by an anomalous fissure (arrowheads) from the right lower lobe bronchus. (c) Axial sliding thin slab maximum-intensity projection image shows an anomalous artery (arrowheads) originating from the posterior wall of the right pulmonary artery and vascularizing the territory of the ACB (arrow). (d) Coronal SSD image shows the ACB (thin arrow) arising from the intermediate bronchus and ventilating a lobulus 6.2 cm in diameter (thick arrow). (Reprinted, with permission, from reference 17.)
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Figure 5c. ACB with a ventilated lobulus and an abnormal pulmonary artery. (a) Two-millimeter-thick spiral CT section shows the origin of an anomalous bronchus (arrow), which is separated by a spur (arrowhead) from the proximal part of the intermediate bronchus. (b) CT scan obtained 20 mm distal to a shows a ventilated accessory cardiac lobulus (arrow) demarcated by an anomalous fissure (arrowheads) from the right lower lobe bronchus. (c) Axial sliding thin slab maximum-intensity projection image shows an anomalous artery (arrowheads) originating from the posterior wall of the right pulmonary artery and vascularizing the territory of the ACB (arrow). (d) Coronal SSD image shows the ACB (thin arrow) arising from the intermediate bronchus and ventilating a lobulus 6.2 cm in diameter (thick arrow). (Reprinted, with permission, from reference 17.)
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Figure 5d. ACB with a ventilated lobulus and an abnormal pulmonary artery. (a) Two-millimeter-thick spiral CT section shows the origin of an anomalous bronchus (arrow), which is separated by a spur (arrowhead) from the proximal part of the intermediate bronchus. (b) CT scan obtained 20 mm distal to a shows a ventilated accessory cardiac lobulus (arrow) demarcated by an anomalous fissure (arrowheads) from the right lower lobe bronchus. (c) Axial sliding thin slab maximum-intensity projection image shows an anomalous artery (arrowheads) originating from the posterior wall of the right pulmonary artery and vascularizing the territory of the ACB (arrow). (d) Coronal SSD image shows the ACB (thin arrow) arising from the intermediate bronchus and ventilating a lobulus 6.2 cm in diameter (thick arrow). (Reprinted, with permission, from reference 17.)
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Spectrum of "Tracheal" Bronchus
A tracheal bronchus was described by Sandifort in 1785 as a right upper lobe bronchus originating from the trachea (4). In the recent literature, the term tracheal bronchus encompasses a variety of bronchial anomalies originating from the trachea or main bronchus and directed to the upper lobe territory (19). A prevalence of 0.1%–2% for right tracheal bronchus and 0.3%–1% for left tracheal bronchus has been found in bronchographic and bronchoscopic studies (19,20). Demonstration of a tracheal bronchus at CT has been occasionally reported as isolated case reports (21–25).
In our series of 35 tracheal bronchi, only eight originated from the trachea, three originated from the carina, and 24 originated from more distal bronchi. Therefore, we suggest use of a modified nomenclature from Boyden (8) and Kubik and Müntener (4) to clarify the classification of aberrant bronchi directed to the upper lobes (Fig 6). The normal right upper lobe bronchus is described as eparterial because it arises above the right pulmonary artery. The normal left upper lobe bronchus is described as hyparterial because it arises below the left pulmonary artery. An anomalous bronchus arising proximal to the origin of the upper lobe bronchus is called preeparterial on the right side (Fig 7) and eparterial or prehyparterial on the left side (Fig 8). An anomalous bronchus arising distal to the origin of the upper lobe bronchus is called posteparterial on the right side and posthyparterial on the left side.
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Figure 6. Aberrant bronchi to the upper lobes. Schematic shows prearterial (true right tracheal) (1), preeparterial (right "tracheal") (2), posteparterial (3), eparterial (true left tracheal) (4), eparterial (left "tracheal") (5), prehyparterial (6), and posthyparterial (7) bronchi. LULB = left upper lobe bronchus, PA = pulmonary artery, RULB = right upper lobe bronchus.
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Figure 7a. Types of origins of displaced right preeparterial bronchus. (a) Frontal multiplanar reconstruction image shows partial splitting of B1 from the right upper lobe bronchus (RULB). The two-dimensional axial sections failed to demonstrate the origin of B1, which is from the innermost part of the right upper lobe bronchus (6% of patients [Table 1]). This anomaly should not be confused with a true preeparterial bronchus. (b) Frontal multiplanar reconstruction image shows complete splitting of B1 from the right upper lobe bronchus (RULB) arising from the right main bronchus. (c) Frontal multiplanar reconstruction image shows complete splitting of B1 from the right upper lobe bronchus (RULB) arising from the junction between the trachea and carina.
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Figure 7b. Types of origins of displaced right preeparterial bronchus. (a) Frontal multiplanar reconstruction image shows partial splitting of B1 from the right upper lobe bronchus (RULB). The two-dimensional axial sections failed to demonstrate the origin of B1, which is from the innermost part of the right upper lobe bronchus (6% of patients [Table 1]). This anomaly should not be confused with a true preeparterial bronchus. (b) Frontal multiplanar reconstruction image shows complete splitting of B1 from the right upper lobe bronchus (RULB) arising from the right main bronchus. (c) Frontal multiplanar reconstruction image shows complete splitting of B1 from the right upper lobe bronchus (RULB) arising from the junction between the trachea and carina.
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Figure 7c. Types of origins of displaced right preeparterial bronchus. (a) Frontal multiplanar reconstruction image shows partial splitting of B1 from the right upper lobe bronchus (RULB). The two-dimensional axial sections failed to demonstrate the origin of B1, which is from the innermost part of the right upper lobe bronchus (6% of patients [Table 1]). This anomaly should not be confused with a true preeparterial bronchus. (b) Frontal multiplanar reconstruction image shows complete splitting of B1 from the right upper lobe bronchus (RULB) arising from the right main bronchus. (c) Frontal multiplanar reconstruction image shows complete splitting of B1 from the right upper lobe bronchus (RULB) arising from the junction between the trachea and carina.
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Figure 8a. Most common types of left aberrant bronchus directed to the left upper lobe. (a) Axial minimum-intensity projection image shows a left prehyparterial bronchus (arrow) (6 in Fig 6), which arises from the anterosuperior aspect of the left main bronchus (LMB) proximal to the origin of the left upper lobe bronchus (LULB) and below the arch of the left pulmonary artery (arrowheads). (b) Axial minimum-intensity projection image shows a left eparterial bronchus (arrow) (5 in Fig 6), which arises from the posterosuperior aspect of the left main bronchus (LMB) proximal to the origin of the left upper lobe bronchus (not shown because it is located at a more caudal level) and posterior to the arch of the left pulmonary artery (arrowheads).
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Figure 8b. Most common types of left aberrant bronchus directed to the left upper lobe. (a) Axial minimum-intensity projection image shows a left prehyparterial bronchus (arrow) (6 in Fig 6), which arises from the anterosuperior aspect of the left main bronchus (LMB) proximal to the origin of the left upper lobe bronchus (LULB) and below the arch of the left pulmonary artery (arrowheads). (b) Axial minimum-intensity projection image shows a left eparterial bronchus (arrow) (5 in Fig 6), which arises from the posterosuperior aspect of the left main bronchus (LMB) proximal to the origin of the left upper lobe bronchus (not shown because it is located at a more caudal level) and posterior to the arch of the left pulmonary artery (arrowheads).
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These bronchi are described as supernumerary when they coexist with a normal type of branching of the upper lobe bronchus. They are described as displaced when, in addition to the aberrant bronchus, one branch of the upper lobe bronchus is missing (Fig 9). The displaced type is more frequent than the supernumerary type (4). This fact is well demonstrated with high-resolution spiral CT, which can demonstrate that the aberrant bronchus may correspond to a segmental, subsegmental, or subsubsegmental bronchus. As Foster-Carter (26) stated, "unless there is a clear evidence that all the normal bronchi are present, as well as the abnormal bronchus, it is always possible that the latter is merely a normal branch arising in an abnormal position (displaced) rather than a true supernumerary."
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Figure 9a. Types of displaced and supernumerary tracheal bronchus. (a) Schematics show a displaced segmental bronchus (left image), a displaced subsegmental bronchus (middle image), and a displaced subsubsegmental bronchus (right image). (b) Schematics show a supernumerary bronchus ventilating normal lung (left image), a supernumerary bronchus with a communicating cyst (middle image), and a supernumerary bronchus with a noncommunicating cyst (right image).
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Figure 9b. Types of displaced and supernumerary tracheal bronchus. (a) Schematics show a displaced segmental bronchus (left image), a displaced subsegmental bronchus (middle image), and a displaced subsubsegmental bronchus (right image). (b) Schematics show a supernumerary bronchus ventilating normal lung (left image), a supernumerary bronchus with a communicating cyst (middle image), and a supernumerary bronchus with a noncommunicating cyst (right image).
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A true tracheal bronchus is any bronchus originating from the trachea, usually within 2 cm of the carina and up to 6 cm from the carina (27–28). When the entire right upper lobe bronchus is displaced on the trachea, it is also called a "pig bronchus" and has a reported frequency of 0.2% (Fig 10) (1,29). All bronchial anomalies affecting the upper lobe are seven times more frequent on the right side (4). Vascularization is usually normal for the territory ventilated by the anomalous bronchus. In our series of 35 tracheal bronchi, three cases of bilateral aberrant bronchi were found (Fig 11); 23% of the tracheal bronchi were supernumerary and 77% were displaced (Table 2). A displaced right preeparterial bronchus was the most common type (57%). Displaced bronchi ventilated predominantly the apical segment on the right side and the apicoposterior segment on the left side (Table 3).
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Figure 10. True right tracheal bronchus (pig bronchus). Frontal SSD image shows the right upper lobe bronchus (RULB) displaced on the trachea (arrow) 2 cm proximal to the carina. Although this appearance is the most dramatic form of tracheal bronchus, it is less common than other forms. Note the splitting of B1 with B1b arising from B2 and B1a from B3 (Table 1). MLB = middle lobe bronchus.
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Figure 11. Bilateral tracheal bronchus. Anteroposterior volume-rendered bronchographic view shows two supernumerary tracheal bronchi (arrows). The right anomalous bronchus is preeparterial and the left anomalous bronchus is prehyparterial, since it lies below the arch of the left pulmonary artery (see also Figs 8a and 13). LULB = left upper lobe bronchus, RULB = right upper lobe bronchus.
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All anomalous bronchi were asymptomatic and were discovered incidentally at chest CT. Associated anomalies included two cases of azygos lobe ventilated by a displaced right aberrant bronchus; three cases of partial anomalous pulmonary venous return, including one draining the abnormal territory; four cases of displaced accompanying segmental arteries (Fig 12); and two cases of a supernumerary lobe completely separated from adjacent lung by an accessory fissure (Fig 13) (Table 2). Although these aberrant bronchi are usually asymptomatic, respiratory distress may occur if drainage is impaired or in association with other abnormalities. In the literature, these aberrant bronchi have manifested as recurrent local infections, persistent cough, stridor, acute respiratory distress (especially in children), and hemoptysis (19,24,28–30). Bronchiectasis, atelectasis, focal emphysema (especially of the left upper lobe), and cystic lung malformations may coexist (19,31).
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Figure 12a. Abnormal artery associated with a tracheal bronchus. (a) Two-millimeter-thick spiral CT section shows an anomalous artery (arrowhead) arising from the posterosuperior aspect of the right pulmonary artery and directed to a displaced right preeparterial apical bronchus (arrow) (2 in Fig 6). (b) Left posterior oblique SSD image of the central pulmonary arteries shows the anomalous artery (arrowhead). Note the absence of a single anterior trunk, which is replaced by multiple ascending arterial branches (arrows) from the right interlobar artery.
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Figure 12b. Abnormal artery associated with a tracheal bronchus. (a) Two-millimeter-thick spiral CT section shows an anomalous artery (arrowhead) arising from the posterosuperior aspect of the right pulmonary artery and directed to a displaced right preeparterial apical bronchus (arrow) (2 in Fig 6). (b) Left posterior oblique SSD image of the central pulmonary arteries shows the anomalous artery (arrowhead). Note the absence of a single anterior trunk, which is replaced by multiple ascending arterial branches (arrows) from the right interlobar artery.
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Figure 13a. Accessory left tracheal lobe. (a) Two-millimeter-thick spiral CT section shows a prehyparterial bronchus (arrow) (6 in Fig 6) arising from the left main bronchus (see also Figs 8a and 11). (b) CT scan obtained cephalad to a shows an accessory lobe (N), which is completely separated from the left upper lobe by an accessory fissure (arrowheads).
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Figure 13b. Accessory left tracheal lobe. (a) Two-millimeter-thick spiral CT section shows a prehyparterial bronchus (arrow) (6 in Fig 6) arising from the left main bronchus (see also Figs 8a and 11). (b) CT scan obtained cephalad to a shows an accessory lobe (N), which is completely separated from the left upper lobe by an accessory fissure (arrowheads).
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Other Displaced Bronchi
Approximately 10% of individuals demonstrate discrete forms of proximal or distal displacement of segmental or subsegmental bronchi in the same lobe (29), which are easily recognized at CT. More rarely, a segmental or subsegmental bronchus originates from an adjacent lobe. Such anomalies predominantly involve the upper lobes, particularly the right side (Fig 14). The territory ventilated by the displaced bronchus remains normal, but CT often demonstrates an incomplete fissure between both territories (Fig 15). Occasional displacement or fusion of lobar bronchi is also reported (eg, displacement and fusion of the middle lobe bronchus on the right upper lobe bronchus, mimicking a left isomerism) (8,32). Other displaced bronchi (in the pediatric age group) include the bridging bronchus, which is a displaced bronchus arising from the left main bronchus and crossing through the mediastinum to supply the right lower lobe, and the esophageal bronchus, which arises from the esophagus and is directed toward a lower lobe (31,33,34).
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Figure 14a. Origin of B3 from the posterior part of the intermediate bronchus. (a) Two-millimeter-thick spiral CT section shows B3 originating from the intermediate bronchus (arrow) (posteparterial bronchus [3 in Fig 6]). Arrowheads indicate the upper part of the right major fissure, which is posteriorly displaced. S3 is anterior to the fissure, S6 posterior to the fissure. (b) Right anterior oblique SSD image shows B3 originating from the intermediate bronchus (IB).
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Figure 14b. Origin of B3 from the posterior part of the intermediate bronchus. (a) Two-millimeter-thick spiral CT section shows B3 originating from the intermediate bronchus (arrow) (posteparterial bronchus [3 in Fig 6]). Arrowheads indicate the upper part of the right major fissure, which is posteriorly displaced. S3 is anterior to the fissure, S6 posterior to the fissure. (b) Right anterior oblique SSD image shows B3 originating from the intermediate bronchus (IB).
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Figure 15a. Segmental B7 displacement on the middle lobe bronchus. (a) Two-millimeter-thick spiral CT section shows the basilar truncus posteriorly (arrow) and B7 originating from the middle lobe bronchus. (b) CT scan obtained caudad to a shows bifurcation of the basilar truncus into B8 anteriorly and B9+10 posteriorly. Note that the internal part of the right major fissure is incomplete (arrowheads in a and b); this finding often occurs when B5 or B7 shows a pattern of variation of branching of its segmental or subsegmental bronchi. Such a displaced bronchus should not be confused with an ACB. (c) Medial SSD image shows the displaced bronchus.
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Figure 15b. Segmental B7 displacement on the middle lobe bronchus. (a) Two-millimeter-thick spiral CT section shows the basilar truncus posteriorly (arrow) and B7 originating from the middle lobe bronchus. (b) CT scan obtained caudad to a shows bifurcation of the basilar truncus into B8 anteriorly and B9+10 posteriorly. Note that the internal part of the right major fissure is incomplete (arrowheads in a and b); this finding often occurs when B5 or B7 shows a pattern of variation of branching of its segmental or subsegmental bronchi. Such a displaced bronchus should not be confused with an ACB. (c) Medial SSD image shows the displaced bronchus.
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Figure 15c. Segmental B7 displacement on the middle lobe bronchus. (a) Two-millimeter-thick spiral CT section shows the basilar truncus posteriorly (arrow) and B7 originating from the middle lobe bronchus. (b) CT scan obtained caudad to a shows bifurcation of the basilar truncus into B8 anteriorly and B9+10 posteriorly. Note that the internal part of the right major fissure is incomplete (arrowheads in a and b); this finding often occurs when B5 or B7 shows a pattern of variation of branching of its segmental or subsegmental bronchi. Such a displaced bronchus should not be confused with an ACB. (c) Medial SSD image shows the displaced bronchus.
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Segmental Bronchial Agenesis
The uncommon agenesis-hypoplasia complex corresponds to arrested development of one lung at different stages: agenesis (absence of bronchus and lung), aplasia (absence of lung with bronchus present), and hypoplasia (bronchus and rudimentary lung present) (7). Diagnosis is easy when an entire lung or lobe is involved (hypogenetic lung syndrome) but can be more difficult when segmental bronchi are involved. Segmental bronchial agenesis predominates in the right upper lobe (Fig 16). Partial anomalous pulmonary venous return may coexist (congenital pulmonary venolobar syndrome) (6).
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Figure 16a. Segmental bronchial agenesis of B2 and B3. (a) Two-millimeter-thick spiral CT section shows a small-caliber right upper lobe bronchus (arrow) giving off only B1. (b, c) Two-millimeter-thick spiral CT sections show anteroposterior bifurcation of the subsegmental bronchi (b) and lateromedial subdivision of these bronchi (c), proving that the single segmental bronchus is indeed B1. Note that the right major fissure (arrowheads), which appears as slight shadowing, is displaced anteriorly due to moderate hypoplasia of the right upper lobe without abnormality of the parenchyma.
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Figure 16b. Segmental bronchial agenesis of B2 and B3. (a) Two-millimeter-thick spiral CT section shows a small-caliber right upper lobe bronchus (arrow) giving off only B1. (b, c) Two-millimeter-thick spiral CT sections show anteroposterior bifurcation of the subsegmental bronchi (b) and lateromedial subdivision of these bronchi (c), proving that the single segmental bronchus is indeed B1. Note that the right major fissure (arrowheads), which appears as slight shadowing, is displaced anteriorly due to moderate hypoplasia of the right upper lobe without abnormality of the parenchyma.
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Figure 16c. Segmental bronchial agenesis of B2 and B3. (a) Two-millimeter-thick spiral CT section shows a small-caliber right upper lobe bronchus (arrow) giving off only B1. (b, c) Two-millimeter-thick spiral CT sections show anteroposterior bifurcation of the subsegmental bronchi (b) and lateromedial subdivision of these bronchi (c), proving that the single segmental bronchus is indeed B1. Note that the right major fissure (arrowheads), which appears as slight shadowing, is displaced anteriorly due to moderate hypoplasia of the right upper lobe without abnormality of the parenchyma.
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Embryology
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The pathogenesis of anomalous bronchial development remains somewhat controversial. Normal tracheobronchial development is 24–26 days as a median bulge of the ventral wall of the pharynx that develops at the caudal end of the laryngotracheal groove. At 26–28 days, this bulge gives rise to right and left lung buds. As the lung buds elongate, the trachea is separated from the esophagus by lateral ingrowth of the mesoderm, forming the tracheoesophageal septum. At 28–30 days, the lung buds elongate into primary bronchi; at 30–32 days, the five lobar bronchi appear as a monopodial outgrowth of the primary bronchi. The lobar bronchi elongate at 32–34 days and then rapidly branch to form all segmental bronchi by 36 days. Over the same period, the vascular supply to this tissue shifts from branches of the splenic plexus to definite pulmonary arteries, as fusion of the lung bud plexus with the sixth branchial arches occurs (31). Three major hypotheses have been formulated to explain anomalous bronchi: the reduction, migration, and selection theories (27).
The reduction theory postulates a basic ground plan or primitive pattern encompassing all of the components of the extant mammalian bronchial distribution, including bilateral symmetry and bilateral eparterial bronchi. The final anatomy is the result of shrinkage and finally suppression of a portion or portions of the original distribution (Fig 17) (35). The migration theory, also called the extension theory, postulates a basic bilaterally symmetric hyparterial bronchial plan with a fixed number of derivative branches. The subsidiary outgrowths have the potential to move or migrate from their initial loci to new points of origin of a bronchus or the trachea (Fig 18). The selection theory suggests that bronchial anomalies result from local disturbances of morphogenesis. Normally, the bronchi continue to bud under the inducing influence of bronchial mesenchyme. For example, budding can be induced in the trachea if active bronchial mesenchyme is grafted onto tracheal epithelium (Fig 19) (36). It is likely that defects involving supernumerary tracheal bronchi occur early in development, at about 29–30 days, as the lobar bronchi start to differentiate. Conversely, displaced bronchi are more likely to occur after 32 days, as the bronchi elongate and branch further (31).
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Figure 17. Reduction theory of congenital bronchial abnormalities. Schematic shows the usual human bronchial pattern (gray areas) in relation to the pulmonary arteries (red areas). All major potential abnormal bronchial patterns are represented in green. Normally, the latter bronchi shrink completely during embryologic development. A variable degree of persistence of such bronchi results in a diverticulum, a blind bronchus associated or not associated with vestigial parenchyma, or a bronchus ventilating parenchyma that is demarcated or not demarcated by an anomalous fissure. CB = culminal bronchus, LB = lingular bronchus, LLLB = left lower lobe bronchus, LTB = left tracheal bronchus, LULB = left upper lobe bronchus, MLB = middle lobe bronchus, RLLB = right lower lobe bronchus, RTB = right tracheal bronchus, RULB = right upper lobe bronchus.
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Figure 18. Migration theory of congenital bronchial abnormalities. Schematic shows that the right upper lobe bronchus (RULB) (green area) has migrated from its hyparterial position to its definitive eparterial location on the right main bronchus. An aberrant bronchus or a branch of this bronchus may migrate as far as the trachea. CB = culminal bronchus, LB = lingular bronchus, LLLB = left lower lobe bronchus, LULB = left upper lobe bronchus, MLB = middle lobe bronchus, RLLB = right lower lobe bronchus.
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Figure 19. Selection theory of congenital bronchial abnormalities. The development of the lungs is based on the interaction between bronchial epithelium and bronchial mesenchyme (green areas). The tracheal epithelium is protected by tracheal mesenchyme (yellow line), which is unable to induce epithelial budding, in contradistinction to the bronchial mesenchyme. Weak areas of tracheal mesenchyme can favor epitheliomesenchymal interaction and abnormal organogenesis. A right tracheal bronchus (RTB) is shown as an example.
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Conclusions
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Our adult patients with bronchial abnormalities were all asymptomatic, a fact that contradicts the hypothesis that anomalous bronchi are more frequently pathogenic. Nevertheless, symptomatic anomalous bronchi have been reported in the pediatric literature, especially the left tracheal bronchus (19). Precise knowledge of bronchial anatomy by the radiologist is important in reporting CT findings in the chest and has numerous clinical implications. Knowledge of bronchial variants and anomalies is necessary for the pneumonologist (for fiberoptic bronchoscopy, bronchoalveolar lavage, biopsy, and endobronchial treatment such as laser therapy, stent therapy, or brachytherapy); the chest surgeon (for lung resection [29,30] and transplantation [37]); and the anesthesiologist (for placement of endotracheal tubes [38], especially the double-lumen type [39]). Therefore, a uniform nomenclature is necessary as a guideline for recognition of bronchial variants and anomalies.
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Footnotes
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Abbreviations: ACB = accessory cardiac bronchus,
SSD = shaded-surface display
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References
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Abstract
Introduction
