The larynx is divided into supraglottis glottic, and subglottic parts, whereas the trachea is composed of the cervical extrathoracic trachea and the mid and lower intrathoracic trachea.1,2 The supraglottic portion of the larynx is constituted by the epiglottis, aryepiglottic folds, arytenoids, and false cords. The glottic portion of the larynx is made up of the laryngeal ventricles and both vocal cords. The crescent-shaped laryngeal ventricles are situated between the false and true cords as lateral invaginations of mucosa. The subglottic space extends from the undersurface of the vocal cords to the inferior margin of the cricoid cartilage, which is also the lower boundary of the larynx. This subglottic area is oval in shape and measures about 1.5 to 2 cm in length (Figures 4-1 through 4-4).3-5
The cervical trachea extends from the inferior margin of the cricoid cartilage to the thoracic inlet, and the intrathoracic trachea extends from the thoracic inlet to the carina where it divides into the right and left main bronchi.6,7 There are 14 to 22 (mean 17) C-shaped hyaline tracheal cartilage rings that support the anterior and lateral tracheal walls. The cartilaginous portion of each tracheal ring forms a "C" with the membranous portion found posteriorly, which is unsupported by cartilage. The first ring is partly recessed into the broader ring of the cricoid cartilage. The cartilaginous rings are usually semicircular or horseshoe-shaped and are the chief determinant of cross-sectional shape. Tracheal diameter grows from 3 to 4 mm in infancy to about 20 mm in adulthood.8,9 The cervical trachea is subject to atmospheric pressure and the intrathoracic trachea is subject to intrathoracic pressure that is equivalent to pleural pressure during quiet breathing. During forced expiration or coughing, the intrathoracic pressure becomes greater than atmospheric pressure, increasing the compressive transmural pressure that narrows the intrathoracic trachea.
figure 4-1 Normal laryngeal anatomy. Normal lateral view of the neck demonstrates the epiglottis, aryepiglottic folds (AEF), false vocal cords (FC), true cords (VC), and subglottic space (asterisk^.
figure 4-2 Normal laryngeal anatomy. Anteroposterior high-kilovoltage view of the larynx and trachea demonstrates the aryepiglottic folds (AEF), false cords (FC), vocal cords (VC), and subglottic space (lower asterisk^. Upper asterisk = laryngeal ventricle; plus sign = pyriform sinus.
figure 4-3 Normal laryngeal anatomy. A, Axial computed tomography (CT) scan, at the upper third of larynx level, outlines the epiglottis and aryepiglottic folds (AEF). B, Axial CT scan, at the vocal cord (VC) level, defines the arytenoids, cricoid cartilage, and thyroid cartilage.
figure 4-4 Normal laryngeal anatomy. A, Normal lateral T1-weighted image of the larynx and cervical trachea illustrates the epiglottis, preepiglottic fat space (asterisk), and subglottic space (plus sign). B, Coronal T1-weighted image of the larynx and cervical trachea depicts the vocal cord (VC) and subglottic space (asterisk).
Coronal narrowing occurs due to inward bending of the cartilaginous rings, and sagittal narrowing occurs as a result of invagination of the posterior membranous wall. Narrowing of the tracheal diameter up to 50% may be considered normal. Accentuated tracheal collapse may occur if there is abnormal flaccidity as occurs in tracheomalacia.
Normal reflections of the trachea can be identified on chest radiographs.10,11 The right paratracheal stripe (RPS) is a thin stripe formed by the reflection of the right upper lobe with the right tracheal wall, normally 1 to 4 mm in thickness. Widening of the RPS is indicative of inherent tracheal wall disease or widening of the paratracheal soft tissues or right mediastinal pleural reflection. Normally, the left tracheal wall is not discernible because it has no contact with the left lung. The tracheoesophageal stripe (TES) is formed by the posterior tracheal wall, the anterior wall of the esophagus, and interposed fat and connective tissues, and is seen only when the esophagus contains air. The posterior tracheal band (PTB) is comprised of the posterior membranous wall of the trachea, which is formed by the interface of air in the tracheal lumen and the aerated retrotracheal recess of the right upper lobe. It is present only from the thoracic inlet to the carina, whereas the TES can be discerned from the cervical and intrathoracic regions. The PTB and TES are seen on the lateral chest radiographs, in 80 to 90% of patients. They have a uniform width of 3 to 5 mm. A left-sided aortic arch will make an impression on the left lower tracheal wall. A right-sided aortic arch will make a similar impression on the right lower tracheal wall. The tracheal bifurcation is at the level of the fifth thoracic vertebra. The intrathoracic esophagus lies slightly to the left and behind the trachea. The azygous vein may be seen as a horizontal soft tissue band, which crosses posterior to the trachea toward the right tracheobronchial angle.
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