Tracheal Bronchus

Etiology

The precise origins of tracheal bronchus remain controversial. Tracheobronchial development begins early in embryogenesis, with lobar differentiation of the lungs commencing around days 29 to 30. Supranumerary bronchial anomalies are believed to occur during this timeframe, whereas bronchial displacement anomalies are more likely to arise after day 32, as the distal branching continues to develop.[4][6][4]

There are 3 theories regarding the origins of the tracheal bronchus:

• Reduction theory: This theory postulates that a basic, primitive pattern of tracheobronchial branching develops first in embryogenesis, and the final branching architecture is formed through regression, shrinkage, or suppression of portions of this original primitive distribution. Any aberrance in this process can result in bronchial branching anomalies, including tracheal bronchi.[2][7][2]

• Migration theory (extension theory): According to this theory, a bilateral, symmetric bronchial branching pattern with a fixed number of derivative branches exists first, and subsidiary branches of these fixed derivatives can migrate from their initial location to other locations within the tracheobronchial tree.[2][8]

• Selection theory: This theory suggests that disturbances in local morphogenesis lead to bronchial abnormalities. Bronchi normally continue to bud under the influence of mesenchyme; thus, tracheal bronchus may arise if the bronchial mesenchyme comes in contact with tracheal epithelium.[2][9]

Epidemiology

The overall incidence of tracheal bronchus is 0.9% to 5%, with the majority of cases arising from the right lateral wall of the trachea.[10] The prevalence of tracheal bronchus is 0.1% to 2% on the right side and 0.3% to 1% on the left side.[3] The tracheal bronchus is associated with other congenital abnormalities, such as Down syndrome, VATER (vertebral, anal, tracheoesophageal, and renal defects), tracheoesophageal fistula, esophageal atresia, laryngeal and duodenal webs, spinal fusion defects, cardiac congenital defects, and hypoplastic lung. In the pediatric population, tracheal bronchus detected by flexible bronchoscopy had an incidence of 1.9% at a median age of 15 months (age range 1 month to 13 years), with no gender differences. There are concurrent congenital pathologies reported in 92.3% of cases, with heart disease accounting for 69%, chromosomal abnormality for 35%, and spinal fusion defect for 11%.[11][12]

Pathophysiology

Although the majority of tracheal bronchi are asymptomatic and may not be discovered until adulthood, symptoms can occur in up to 25% of patients in some series.[13] Any obstruction of the tracheal bronchus can lead to atelectasis, decreased ventilation, and infection. Hence, in children, recurrent upper lobe pneumonia may be the diagnostic presentation of a tracheal bronchus.[11] Abnormal ventilation mechanics may contribute to patient symptoms, with additional presentations including wheezing and hypoxia, which could be mistaken for asthma.[14] Cases have been reported of tracheal bronchus associated with tuberculosis, leiomyoma, cancers, and as a source of bleeding leading to death. There is insufficient evidence to suggest any association between the tracheal bronchus and its predisposition to any malignancy.[15][16][17][18]

One classification system categorizes the location of the tracheal bronchus in relation to the carina:

• Type I: The tracheal bronchus is more than 2 cm from the carina, and there is a narrowing of the distal trachea.
• Type II: The tracheal bronchus is more than 2 cm from the carina, but there is no narrowing of the distal trachea.
• Type III: The tracheal bronchus is less than 2 cm above the carina.[19]

Another classification system describes tracheal bronchi relative to the upper lobe bronchus and pulmonary arteries:

• A normal right upper lobe bronchus is termed eparterial, as it arises cranial to the right pulmonary artery.
• A normal left upper lobe bronchus is called hyparterial, as it arises caudal to the normal left pulmonary artery.
• An aberrant bronchus arising proximal to the upper lobe bronchus is called pre-eparterial on the right side and eparterial or pre-hyparterial on the left side.
• An aberrant bronchus arising distal to the upper lobe bronchus is called post-eparterial on the right side and post-hyparterial on the left side.[2][4]

Tracheal bronchus may also be classified according to hypothetical embryologic origin as either displaced or supernumerary:

• If the anatomic upper lobe bronchus is missing a single branch, the tracheal bronchus is defined as displaced.
• If the right upper lobe bronchus has a normal trifurcation into apical, posterior, and anterior segmental bronchi, the tracheal bronchus is defined as supernumerary.

The supernumerary bronchi may end blindly; in such cases, they are also called tracheal diverticula. If they terminate in aerated or bronchiectatic lung tissue, it is termed either an apical accessory lung or a tracheal lobe.[20]

Incidental intubation of the tracheal bronchus can cause obstruction, pneumothorax, post-obstructive pneumonia, and respiratory failure.[21]

History and Physical

Children may remain asymptomatic; however, symptomatic cases often manifest early in life with wheezing, stridor, refractory cough, pneumonia, or atelectasis.[12] Tracheal bronchus should be suspected in cases of persistent upper lobe pneumonia, atelectasis or air trapping, and chronic bronchitis. This condition can present with chronic symptoms, such as productive cough over multiple years, or it may also present as recurrent pneumonia with computed tomography (CT) findings of bronchiectasis.[22] Important aspects of the history include any seasonal or environmental variations, association with eating or swallowing, any positional factors, association with exercise, concern for foreign body aspirations, and any smokers in the home. As the most common presenting symptoms are vague, these historical factors aid in identifying other conditions that may mimic the presentation of a symptomatic tracheal bronchus, such as asthma, allergies, laryngomalacia, tracheobronchial foreign body, or acid reflux.[23][24]

Physical examination of a patient with a suspected airway abnormality should begin with a comprehensive upper respiratory examination.[25] Vital signs must be recorded, and a thorough assessment of the individual's overall general appearance should be conducted. Attention should be given to the respiratory rate, and any breathing-associated noises, such as stridor or stertor, should be reported.[26] If present, the clinician should determine whether these noises occur with mouth breathing, nasal breathing, or both. Any vascular lesions of the head or neck should be noted. These lesions should also be characterized as inspiratory, expiratory, or biphasic. The use of accessory muscles during respiration should be noted. In infants and children, it is important to note whether these symptoms worsen when crying and whether there is any positional component, such as whether symptoms are worse when supine or improved when placed prone.[27] The quality of the voice and the nature of the cough should also be assessed. Hoarseness may suggest laryngeal pathology, whereas a bovine cough may indicate vocal cord paralysis. The examination should then proceed with a thorough cardiopulmonary examination. All lung fields should be auscultated and percussed, with additional signs to elucidate lung consolidation, such as tactile fremitus, or 'e' to 'a' changes, assessed.[28] The final component of the in-office examination is flexible laryngoscopy. This procedure should evaluate the nasal airway bilaterally for conditions such as pyriform aperture stenosis or choanal atresia and evaluate the larynx.  Vocal cord function and appearance should be assessed to identify neoplastic or infectious lesions. The subglottis and proximal trachea are often visible, but the lower airways are not reliably visualized through this technique.[29]

Evaluation

The definitive diagnosis of tracheal bronchus is established through direct visualization on bronchoscopy. This procedure allows for a comprehensive examination of the tracheobronchial tree while also assessing for coexisting pathologies such as tracheomalacia, bronchomalacia, tracheoesophageal fistula, vascular rings or vascular lesions, neoplasms, or foreign bodies.[30] In cases where tracheal bronchus is found incidentally, such as during intubation and in the perioperative setting, special attention should be paid. Extra care should be taken while intubating patients with tracheal bronchus as the tube can potentially obstruct or even migrate into the bronchus itself, resulting in atelectasis, hypoventilation, or both.[31]

Chest x-rays can occasionally reveal a tracheal bronchus; however, it is more commonly identified incidentally on chest CT scans performed for other reasons.[32] Multi-detector CT (MDCT) with three-dimensional (3D) image reconstruction is the gold-standard test for detecting congenital tracheobronchial anomalies. In addition, 3D-CT is a valuable tool for diagnosing the relationship between the tracheal and arterial tree. Reconstruction images have overcome the limitations of axial CT images, which included failure to identify subtle tracheal stenosis, a craniocaudal extension of the airway disease, and disease limited to airways obliquely located to axial planes. Imaging findings in symptomatic patients include bronchiectasis, focal emphysema, and focal cystic malformations.[33][34][35] 

A retrospective cross-sectional analysis of 20 individuals with different tracheobronchial anomalies, such as stenosis, tracheomalacia, tracheal bronchus, and agenesis, compared the diagnostic benefit of magnetic resonance imaging (MRI), MDCT, and 3D imaging confirmed by bronchoscopy or surgery. The study found that MDCT was superior to MRI in diagnosing anomalies. In 12 patients who underwent both MDCT and 3D imaging, researchers determined that reconstruction provided specific information in 7 cases; in 3 cases, it played a crucial role in attempting surgical interventions, whereas in another 4 cases, it provided specific information for surgical planning.[36]

Treatment / Management

Currently, the primary approach to managing tracheal bronchus is symptomatic treatment. Asymptomatic cases require only expectant observation, whereas symptomatic tracheal bronchus is treated based on the severity of symptoms. Given the broad range of symptoms that tracheal bronchus can present, medical management includes bronchodilators, inhaled corticosteroids, muscarinic antagonists, and antibiotics. However, if the symptoms are refractory to medication, then surgical treatment is indicated. Segmentectomy and lobectomy have been performed for recurrent pneumonia caused by the tracheal bronchus and have been shown to be curative.[37]

When surgery is considered, careful coordination between the anesthetist and the surgical team is paramount. Based on normal anatomy, the left main bronchus is longer and divides into upper and lower secondary bronchi. The right main bronchus is shorter and divides into apical, anterior, and posterior secondary bronchi. The left-sided double-lumen tube is preferred in most cases. The right-sided tube is used in selective cases where the right lung needs to be isolated. In right-sided tracheal bronchus, a right-sided tube is contraindicated, given it can easily block the tracheal bronchus from its tracheal cuff. Based on the location of the tracheal bronchus and its origin, one-lung ventilation for procedures such as thoracotomy, lobectomy, and segmentectomy can be achieved using a double-lumen tube on the opposite side. The position of the endobronchial tube should be confirmed using flexible tracheobronchoscopy. Lung isolation is achieved by clamping either the tracheal tube or the endobronchial tube, depending on which side the procedure is performed.

As a tracheal bronchus can arise at any point from the trachea, there is no standard safe distance point from the incisor mark to prevent its obstruction either by insufflation of the cuff or by endotracheal tube entrance itself. Therefore, follow-up chest x-rays, direct visualization, or other confirmatory measures are critical in such cases.[11] One-lung ventilation can be complicated by the presence of a tracheal bronchus. In such cases, based on the site of the lesion, certain measures can assist anesthesiologists. Flexible bronchoscopy should be performed before to find the location of the tracheal bronchus and its distance from the carina. Then, a double-lumen tube, an univent tube with a bronchial blocker, or a regular tracheal tube with the bronchial blocker is used.[38](B3)

Similarly, an univent tube, which has 2 lumens within a single tube and a tracheal and endobronchial cuff in the form of an in-built bronchial blocker, can be used. Another technique involves using bronchial blockers for the tracheal bronchus and a standard endotracheal tube for the rest of the lung.[39]

Some reports have preferred the use of a left-sided double-lumen tube over the use of bronchial blockers for tracheal bronchus.[40] Cases have reported the use of a Fogarty catheter along with an univent tube to be effective in providing successful lung isolation in the presence of tracheal bronchus. Other potential solutions include using a Fogarty catheter in the tracheal bronchus with a bronchial blocker located in the right main bronchus.[41] (B3)

One-lung ventilation, using a bronchial blocker in cases of unilateral thoracotomy or a double-lumen endotracheal tube, is an option for intubation in these patients. Small-caliber tubes are preferred, and the endotracheal tube is typically positioned slightly above the carina to avoid obstruction of the tracheal bronchus. Fiber optic bronchoscopy is then used to confirm the correct placement and ensure that there is no obstruction of the tracheal bronchus.[42][43](B3)

Differential Diagnosis

The following conditions should be considered in the differential diagnosis as they can present with similar symptoms and must be ruled out:

• Pneumonia
• Recurrent bronchitis
• Tracheal stenosis
• Subglottic stenosis
• Vocal fold paralysis
• Hemangioma
• Tracheal neoplasm
• Bronchial neoplasm
• Tracheomalacia
• Bronchomalacia
• Congenital heart, esophageal, and laryngeal abnormalities and syndromes, such as Down syndrome and VACTERAL syndrome (vertebral defects, anal atresia, cardiac defects, tracheoesophageal fistula, renal anomalies, and limb abnormalities)
• Accessory or supernumerary bronchus
• Asthma
• Gastroesophageal reflux disease or laryngopharyngeal reflux

Prognosis

Overall, the prognosis for patients with tracheal bronchus is quite good. Most individuals with an isolated tracheal bronchus lead normal lives without significant morbidity. However, for patients with a tracheal bronchus associated with an additional syndrome, the overall prognosis depends on the underlying syndrome. There are no comparative studies for observational versus surgical management of tracheal bronchus; however, an observational pediatric surgical case series reported positive outcomes at 2 to 10 years, supporting the efficacy of surgical treatment in selected patients.[44]

To date, only 2 cases of thoracoscopic upper lobe lobectomy or segmentectomy in adults with tracheal bronchus have been reported, both with successful outcomes.[45][46]

Complications

The potential complications of tracheal bronchus include the following:

• Recurrent pneumonia
• Atelectasis
• Acute respiratory failure
• Aspiration pneumonia if there is communication with the esophagus
• Congestive heart failure if coexisting congenital heart anomaly