Chapter 12 The Trachea
The trachea is a cartilaginous and membranous tube that extends from the cricoids cartilage to the carina, and it is approximately 11 cm long. The trachea is almost cylindrical, with slight flattening posteriorly. Its diameter from side to side is approximately 2 to 2.5 cm. The trachea is divided in to a shorter extrathoracic and a longer intrathoracic segment at the upper border of manubrium. The trachea and extrapulmonary bronchi are composed of hyaline cartilage, fibrous tissue, muscular fibers, mucous membrane, and glands. The tracheal cartilages form incomplete C-shaped rings that occupy the anterior two thirds of the trachea. The bronchial cartilages are shorter and narrower than those of the trachea, but they have the same shape and arrangement. Calcification of cartilage is seen in older people, more so in women. Posteriorly, the membranous wall of the trachea and main bronchi is completed by fibrous tissue and nonstriated muscular fibers.
Several radiographic studies are used to evaluate the trachea and main bronchi (Table 12-1). The plain chest radiograph in the posteroanterior and lateral projections is the most frequently used screening study. A high-kilovoltage (140-kVp) technique is preferred because it reduces the visibility of the bony thorax and improves imaging of the various mediastinal interfaces. However, it is easy to miss a tracheal or main bronchial lesion on standard posteroanterior and lateral chest radiographs because there is considerable overlap of the trachea with the mediastinum and bony thorax. Bilateral, oblique chest radiographs improve visibility of the trachea and main bronchi by rotating the spine so that it is not superimposed on the central airways (Fig. 12-1). In most cases, additional imaging is usually indicated.
|Chest radiographs (posteroanterior, lateral, oblique projections)||Screening study|
|Conventional tomography (anteroposterior, lateral, 55-degree posterior oblique projections)||Postintubation tracheal stenosis|
Preoperative assessment of length of lesion
Postoperative assessment of bronchial anastomosis
|Computed tomography||Tracheobronchial tumor location and extent|
Vascularity of tumor
Compression of airway by mediastinal mass or vessel
|Magnetic resonance imaging||Multiplanar imaging|
Mediastinal invasion by airway, neoplasm
Airway obstruction by vascular rings
Air trapping due to bronchial obstruction
Figure 12-1 Tracheal tumor. A, Posteroanterior view of the chest reveals a focal area of increased opacity in the midtrachea projected over the spine (arrow). B, A left anterior oblique projection shows the trachea to the right of the spine. A focal, well-defined, intraluminal mass (arrow), which is better characterized in the midtrachea, represents an adenoid cystic carcinoma.
Computed tomography (CT) has become the imaging modality of choice for most tracheobronchial lesions. Because of the clarity of anatomic detail on cross-sectional imaging, there is a direct display of tracheobronchial anatomy. The superior contrast resolution compared with conventional radiography permits evaluation of adjacent mediastinal soft tissues. This is particularly important in cases of tracheal neoplasms, which may invade the adjacent mediastinum, or in cases of mediastinal masses such as goiters or vascular rings, which may compress the trachea. CT can identify calcific and fatty densities and vascular enhancement of tumors and aneurysms after intravenous contrast administration. Paired inspiratory and expiratory CT scans can identify abnormal collapsibility of the trachea and main bronchi in cases of tracheobronchomalacia. High-resolution computed tomography (HRCT) can be obtained with the use of thin (1- to 1.5-mm) sections and a bone reconstruction algorithm. The major disadvantage of conventional CT is that craniocaudally oriented trachea and bronchi are not imaged in the long axis. Although images may be reconstructed in sagittal and coronal planes, the resolution is limited by the inherent scan thickness on conventional CT scans.
Multidetector CT (MDCT) has significantly improved the way the trachea is imaged. MDCT increases the capacity to register data and significantly shortens the scan time. The entire trachea and major bronchi can be imaged in a single breath hold of a few seconds. The isometric data set obtained with MDCT has equal resolution in all the imaging planes, and artifacts seen with multiplanar reformats in conventional CT can be avoided. High-quality, multiplanar images are possible if thin sections (1 to 2.5 mm) and an overlapping image reconstruction algorithm are used (Fig. 12-2). The image reconstruction thickness can be selected prospectively or retrospectively after raw data set is acquired. The same data can be transferred to a dedicated workstation to obtain three-dimensional images. The two commonly used three-dimensional techniques are external volume rendering of the airways (Fig. 12-3A) and internal rendering or virtual bronchoscopic views (see Fig. 12-3B). These imaging techniques supplement but do not replace conventional axial images. There are several advantages of using these techniques. The craniocaudal extent and shape of the lesion is better displayed, complex and subtle lesions can be better demonstrated (Fig. 12-4), and simulated bronchoscopic views can be used as a guide in planning interventional procedures.
Figure 12-2 Tracheoesophageal fistula. High-resolution, multiplanar reconstruction with multidetector CT in the sagittal plane demonstrates communication between the lower trachea (T) and the esophagus (E).
Figure 12-3 Three-dimensional imaging techniques. A, The technique of external volume rendering shows subtle tracheal stenosis (arrow) due to a tracheostomy injury. B, Virtual bronchoscopic view at the level of carina shows the normal carina and main bronchi.
Because of high temporal resolution, MDCT allows dynamic CT imaging during expiration and coughing. These techniques are more sensitive in demonstrating malacia than paired inspiratory and expiratory images.
Historically, conventional tomography in the anteroposterior, lateral, and oblique projections was routinely used to evaluate the trachea and central bronchi. With refinements in CT scanning, conventional airway tomography is no longer routinely employed. However, it does permit an accurate assessment of patency or the degree of obstruction in the central airways and provides a direct image of the airways in the long axis. Conventional tomograms can determine accurately the length of tracheal lesions relative to the larynx or carina. The major disadvantage of conventional tomography is the inability to visualize adjacent mediastinal structures.
The role of magnetic resonance imaging (MRI) in the evaluation of the trachea and bronchi is limited. The trachea, main bronchi, and lobar bronchi are well demonstrated on spin echo images, but the spatial resolution of MRI permits observation of only an occasional segmental bronchus. MRI offers the advantages of multiplanar imaging, high-contrast resolution without the use of intravenous contrast agents, and absence of ionizing radiation. MRI is particularly useful in patients with vascular rings or tracheal compression by vascular anomalies and other mediastinal masses (Fig. 12-5).
Figure 12-5 Tracheal compression by a right aortic arch. A, The anteroposterior tomogram demonstrates extrinsic compression and smooth narrowing of the midtrachea by a right paratracheal mass. B, Axial, T1-weighted MRI shows a slightly dilated ascending aorta (A), which extrinsically compresses and narrows the tracheal lumen (T).
Tracheal diverticula are outpouchings from the tracheal wall. They are seen in up to 1% of population. They can be congenital or acquired. The congenital diverticula are single, located 4 to 5 cm below the vocal cords or just above the carina, and contain all the layers of tracheal wall. The acquired diverticula are outpouchings from a weak posterior wall and are lined by respiratory epithelium. They lack other layers of the tracheal wall, such as smooth muscle and cartilage. The acquired diverticula may be single or multiple and are thought to be associated with increased intraluminal pressure and chronic cough.
Most tracheal diverticula are asymptomatic and are discovered incidentally. They can manifest with recurrent episodes of airway infection and chronic cough because the diverticula may act as reservoirs of infection.
On standard radiographs, small diverticula are often missed. Thin-section CT with multiplanar and three-dimensional reconstructions establishes the relation to the trachea, and further imaging is usually not required (Fig. 12-6; see Fig. 12-4).
The differential diagnosis of a paratracheal air collection includes laryngocele, pharyngocele, Zenker’s diverticula, and apical paraseptal blebs or bullae. These entities can be easily distinguished by CT and barium swallow occasionally is required.
Diffuse tracheal narrowing is seen in several conditions, including chronic obstructive pulmonary disease (COPD); after tracheal trauma; as the result of viral, fungal, tuberculous, or bacterial infections; and in other diseases, such as sarcoidosis, Wegener’s granulomatosis, relapsing polychondritis, amyloidosis, and tracheobronchopathia osteochondroplastica. The narrowing may be idiopathic and may involve the larynx (Table 12-2).
|Causative Conditions||Tracheobronchial Findings||Other Findings|
|Idiopathic||Smooth, tapered, irregular, lobulated, or eccentric morphology 2-4 cm long, usually subglottic||±Laryngeal involvement|
|Postintubation cuff injury||Smooth narrowing with hourglass configuration|
|Posttraumatic||Smooth narrowing with hourglass configuration||±Upper rib and sternal fractures|
|Saber-sheath trachea||Smooth narrowing of intrathoracic trachea|
Coronal diameter ≤ one half of sagittal diameter
|Tracheopathia osteochondroplastica||Submucosal nodularity of anterolateral walls of trachea and main bronchi with ossification|
Membranous wall is spared
|Relapsing polychondritis||Diffusely thickened tracheobronchial walls with diffuse narrowing of trachea and main bronchi|
±Calcification of wall
|Auricular and nasal chondritis, arthritis|
|Wegener’s granulomatosis||Focal or diffuse tracheobronchial wall thickening and narrowing|
±Enlarged calcified cartilages
|Amyloidosis||Diffuse tracheobronchial wall-thickening and narrowing|
Focal nodular masses
Contrast enhancement of masses on CT or MRI
|±Lymphadenopathy, may calcify|
|Sarcoidosis||Smooth, irregular, or nodular stenosis|
Tracheobronchial wall thickening
Bronchial compression by lymph node
±Reticular/nodular interstitial lung disease
|Tracheobronchial papillomatosis||Diffuse nodules or masses in trachea and bronchi||Laryngeal involvement|
±Multiple pulmonary nodules, may cavitate
Complicated by squamous cell carcinoma
|Rhinosclerosis||Nodular masses or diffuse, symmetric narrowing of trachea and bronchi|
|Hyperplastic stage||Irregular tracheobronchial wall thickening and narrowing||Hilar/mediastinal lymphadenopathy|
Parenchymal cavitation and consolidation
|Fibrostenotic stage||Smooth tracheobronchial narrowing||Atelectasis and scarring|
Calcified lymph nodes
Idiopathic laryngotracheal stenosis is an uncommon cause of narrowing in the larynx and subglottic trachea. It typically affects middle-aged women who have no history of intubation, trauma, infection, or other underlying systemic disease. Clinically, patients experience progressive shortness of breath accompanied by wheezing, stridor, or hoarseness. The average duration of symptoms is approximately 2 years.
The radiologic appearance of idiopathic laryngotracheal stenosis varies, including lesions that may be smooth and tapered (Fig. 12-7) or irregular, lobulated, and eccentric (Fig. 12-8). The stenosis is 2 to 4 cm long, with severe compromise of the lumen measuring no more than 5 mm at the narrowest point.
Histologically, the stenotic areas show dense keloid fibrosis involving the adventitia and the lamina propria and sparing the mucosa, muscularis propria, and the cartilage. Small areas of spindle cell proliferation are similar to fibrosing mediastinitis or retroperitoneal fibrosis. The mucosa may show squamous metaplasia without dysplastic changes. The lesions may be treated surgically or conservatively with dilation, intubation, stenting, steroid injection, cryotherapy, or electrocoagulation.
Most long-term complications of intubation are related to cuff injury. Cuffed endotracheal tubes became commonplace only after the introduction of intermittent positive-pressure breathing for the treatment of respiratory failure during the poliomyelitis epidemic of 1952. As patients survived for longer periods with respiratory assistance, new long-term complications arose, including tracheal stenosis, tracheoesophageal fistula, and tracheoinnominate artery fistula (Box 12-1).
Box 12-1 Postintubation Injury
A stenosis may occur at the level of the tracheostomy stoma or rarely where the tip of the tube impinges on the tracheal mucosa. However, pressure necrosis at the cuff site is responsible for most long-term postintubation complications. If the cuff pressure exceeds capillary pressure, blood supply to the mucosa is compromised, leading to ischemic necrosis. Initially, the mucosa becomes inflamed, followed by ulceration in the mucosa overlying the cartilaginous rings. As the ulcers enlarge, there is increasing exposure of the cartilage, which becomes colonized with bacteria. With further pressure on the wall, necrosis develops, and softening and dissolution of the cartilage lead to tracheomalacia or to scarring and stenosis formation. Radiographically, the stenosis has a smooth, gradual narrowing with an hourglass configuration (Fig. 12-9). Typically, symptoms of stenosis develop in 2 to 6 weeks after extubation and, in some patients, months later. Most stenoses are appropriately treated with resection of the damaged segment and end-to-end anastomosis. In the past 20 years, most tracheostomy and endotracheal tubes have been designed with large-volume, low-pressure cuffs, which has dramatically reduced the incidence of tracheal stricture. Postintubation tracheal strictures continue to occur but at a much reduced rate.