Trachea Anatomy

This discussion of tracheal anatomy covers the following aspects:

• Development of the human trachea - Highlights of the different periods of embryonic and fetal development
• Gross anatomy - The structure, dimensions, and anatomic relationships, as well as the neurovascular and lymphatic supply of the upper airway; differences between the child and adult tracheas
• Microscopic anatomy and a few ultrastructural points pertinent to the function of the cartilaginous framework of the trachea
• Clinical correlations related to tracheal anomalies, tracheal deviation, or shift as well as artificial trachea

Human development is divided into prenatal and postnatal periods. The prenatal period is further subdivided into two periods, the embryonic (see the following image) and fetal periods. The embryonic period (first 8 weeks after conception) is divided into 23 stages or "horizons" according to the Carnegie system. Each specific stage is defined by specific morphologic criteria and encompasses all internal and external morphogenetic changes occurring within the embryo. These stages provide a snapshot of the development status of all body systems within a specific timeframe. The stages up to stage 5 are concerned with the implantation of the blastocyst. Somite number is linked to early embryonic stages. When the somites become too numerous to count accurately, the development of the pharyngeal arches is used instead. External staging becomes clearer with the appearance of limb buds. ^[1]

Graph showing the embryonic period (first 6 wk of life) in relation to the fetal period. The crown-rump (C-R) length of the fetus is also illustrated.

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Understanding these developmental stages can provide critical insights for diagnosing and treating congenital tracheal anomalies. Before the embryonic development of the trachea, the primitive foregut forms through cranial and lateral folding of the endodermal germ layer around 3 weeks of embryonic development (Carnegie stage 9, 19-21 days). The formation of the tracheal lumen begins when the respiratory diverticulum develops in the ventromedial part of the foregut, posterior to the pharynx, at Carnegie stage 12 (26-30 days of development). ^[2]

The respiratory diverticulum (laryngotracheal diverticulum) appears between the fourth and sixth branchial arches. ^[3] There is no evidence of identifiable respiratory system development during the first eight stages of the Carnegie system. During stage 4 (day 20 of the embryonic period), the foregut begins its appearance and the respiratory diverticulum, from which the respiratory system develops, is identified in a medial position. Tracheal development begins at stage 12 as a ventral outgrowth from the endodermal foregut into the mesenchyme surrounding the sinus venosus and inflow tract of the heart. ^[1]

With the continuous growth of the respiratory diverticulum, at about 22 days (stage 10), the primitive pharynx is visible, among the branchial arches and pouches. Around 24 days (stage 11), the respiratory diverticulum divides into two lung buds. The respiratory diverticulum then migrates caudally into the mesenchyme ventral to the foregut at 26 days of embryonic life (stage 12) (see the image below). According to O'Rahilly and Tucker, this stage marks the period when the "respiratory tap" is turned on. ^{[4, 5]}

According to this theory, the lung buds (tracheal bifurcation) descend, whereas the tracheoesophageal separation remains fixed. During this important stage of human development, the respiratory and digestive tracts develop separately. The mesenchymal tissue separating the two systems is known as the tracheoesophageal septum. During stage 13 (embryo is 28 days old), the ventrally situated trachea separates from the dorsally situated esophagus and the respiratory diverticulum bifurcates into two primary lung buds. ^[2]  The trachea and lung buds become more evident as the esophagus, respiratory tract, and tracheoesophageal septum elongates. Longitudinal tracheal growth causes the region of the future carina to descend until it ultimately lies within the thorax. ^[1]

Tracheoesophageal septum at stage 12 embryonic development, separating the foregut into the esophagus and laryngotracheal tube.

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The tracheal rings, lung mesenchyme, and other elements of the airway wall are of mesenchymal origin, while the specialized respiratory epithelium lining the respiratory tree develops from the endoderm. ^{[1, 2]}  The following changes are seen during the development of the C-shaped cartilage tracheal rings (Table 1). ^[2]

Table 1. Carnegie Stages (Open Table in a new window)

At the beginning of the fetal period (30 mm crown-rump [C-R] stage), the chondrocytes are well identified within the incomplete rings of the trachea. The cartilage of the rings is hyaline. The paries membranaceus shows primordium of the circular trachealis muscle. The epiglottis, thyroid cartilage, and the tracheal wall, including the cartilaginous rings, are well defined. The circular shape of the trachealis muscle contains spindle-shaped myoblasts with elongated nuclei. Tracheal glands are not identifiable in the submucosa at this stage yet. During the 42-50 mm C-R stages, the circular trachealis muscle is well defined in the paries membranaceus and between the cartilages, but no glands are visible yet.

The circular muscle fibers of the trachealis muscle are attached to the inner surfaces of the cartilages at the 62 mm C-R stage. Some longitudinal muscle fibers are identified posterior to the circular layer. These fibers attach to the lower part of the posterior aspect of the cricoid, and caudally, they insert into the posterior surface of the carina.

At 100 mm C-R stage, both muscular layers are well indicated. However, elastic fibers, lymphocytes, and tracheal glands are not seen in the submucosa or lamina propria.

Table 2 summarizes the key developmental events during the fetal period. ^{[1, 2]}

Table 2. Trachea: Developmental Events (Open Table in a new window)

The structure, dimensions, and anatomic relations of the trachea as well as the neurovascular and lymphatic supply of the upper airway are described below (see the following images). ^{[6, 7, 8]} Some differences between the child and adult tracheas are also mentioned. ^{[9, 10, 11, 12, 13, 14, 15, 16]}

Anatomical Location

The trachea is located approximately in the midline sagittal plane, but its bifurcation point is usually a little to the right. In children under the age of 5 years, a chest radiograph might show the trachea bending sharply to the right, at or just above the superior thoracic aperture. This is a normal variation. The inferior most portion of the trachea, the bifurcation, is called the carina (usually at the level of the fourth or fifth thoracic vertebra). ^[1]

Cross-sectional shapes of the trachea. A: Juvenile circular trachea. B: Adult circular shapes. C: Female trachea. D: Common deformity related to proximity of the aorta. E: Saber-sheath trachea. F: Different shapes in chronic obstructive pulmonary disease (COPD). G: Tracheopathia osteoplastica. H: Trachea in tracheobronchomegaly (Mounier-Kuhn syndrome).

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Tracheal blood supply. Left anterior view.

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Tracheal blood supply. Right anterior view.

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Structure and dimensions

The trachea is nearly, but not quite, cylindrical and is flattened posteriorly. The adult trachea measures about 10-11 cm in length with cervical and thoracic parts. It is a cartilaginous and fibromuscular tube that extends from the inferior aspect of the cricoid cartilage, sixth (female) or seventh (male) cervical vertebra, to the main carina. ^[1]  The tracheal lumen narrows slightly as it progresses toward the carina where it terminates by bifurcating into two bronchi, at the level of fifth or sixth thoracic vertebra or the T5-T6 intervertebral disc, superior to the heart at the level of the sternal angle. ^[17]

In cross-section, it is D-shaped, with incomplete cartilaginous rings anteriorly and laterally and a straight membranous wall posteriorly. The cross-sectional shape of the trachea can be elliptical (larger transverse than anteroposterior [AP] diameter), C-shaped (equal transverse and AP diameters), or U-shaped. ^[17]

The trachea is a flexible and distensible structure that constantly changes its dimension, morphology, and muscular tone. Therefore, understanding variations in tracheal shape and diameter is important for providing better respiratory function and patient comfort during tracheal surgeries and in the management of respiratory conditions. ^[18]

Sex differences

The coronal and sagittal tracheal dimensions vary in males and females. The upper limits of the coronal and sagittal diameters in men are 25 and 27 mm, respectively. In women, they are 21 and 23 mm, respectively. The lower limits for both dimensions are 13 and 10 mm for adult males and females, respectively. The external transverse diameter is typically 2 cm in adult males and 1.5 cm in adult females, always greater in males than in females. ^[1]  The lumen is about 12 mm in diameter in live adults but increases due to the relaxation of the smooth muscle postmortem. ^[1]  In a fascinating anatomical autopsy study, Mehta and Myatt discovered that the U-shaped trachea is the most common variant in adult men, whereas the elliptical shape is the most prevalent in adult women. ^[18]

Age differences

In small children, body weight correlates better with tracheal growth than height and age. In the child, the trachea is smaller, more deeply placed, and more movable than in the adult. The bifurcation is also at a higher level until the age of 10-12 years. The subcarinal angle decreases gradually with age, and the right bronchial angle is reported usually to be smaller than the left. The images below depict the length and AP diameter of the trachea from birth to the age of 20 years.

Length of the trachea from birth to 20 years of age.

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Anteroposterior diameter of the trachea according to age.

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The video below depicts the trachea of a small child.

During the first postnatal year of life, tracheal diameter does not exceed 4 mm. The mean transverse diameter is greater than the AP diameter up to the age of 6 years, after which the two diameters are nearly equal. During later childhood, tracheal diameter in millimeters is approximately equal to age in years. ^[1]  In neonates, the trachea is funnel-shaped, where the upper end is wider than the lower end and gradually becomes cylindrical with increasing age. ^[1] Later, the discrepancy between the subcricoid area and the carina gradually diminishes, and the tracheal lumen changes from the cylindrical to the more adult-shaped ovoid form. The trachea stops growing in females at around the age of 14 years, but in males, it continues to enlarge in cross-section but not in length.

The shape of the adult trachea varies even without disease. Some remain circular rather than being ovoid. A triangular configuration is rather rare. The transverse shape of the lumen is variable, especially in the later decades of life, and it may be round, lunate, or flattened. ^[1]

The shape of the trachea also varies with changes in the intraluminal pressure alterations. This may be due to cough, respiration, ventilation, or Valsalva maneuver. Coughing narrows the lumen by causing the trachealis muscle of the posterior wall to pull the cartilaginous C-arms together. ^[19]  The cross-sectional configuration may change markedly with age, especially in the presence of chronic obstructive pulmonary disease (COPD).

Other changes related to disease are softening (malacia) of the tracheal cartilages. The "saber-sheath" trachea is flattened from side-to-side, with a narrow lateral diameter and increased AP diameter. These changes may lead to various degrees of obstruction on coughing and expiration. Other unusual forms occur with tracheal disease, such as tracheopathia, osteoplastica, and tracheobronchomegaly (Mounier-Kuhn syndrome [MKS]). The image below depicts different shapes of the trachea in health and disease.

Cross-sectional shapes of the trachea. A: Juvenile circular trachea. B: Adult circular shapes. C: Female trachea. D: Common deformity related to proximity of the aorta. E: Saber-sheath trachea. F: Different shapes in chronic obstructive pulmonary disease (COPD). G: Tracheopathia osteoplastica. H: Trachea in tracheobronchomegaly (Mounier-Kuhn syndrome).

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Cartilaginous rings and bands of fibrous and elastic tissue

The anterior and lateral surfaces of the trachea have superimposed incomplete 15-20 horseshoe-, C-, or U-shaped rings of hyaline cartilage and intervening anular ligaments formed from fibroblastic tissue. The C-shaped cartilage rings provide the lateral rigidity of the organ. Together with smooth muscles present between the cartilaginous rings, the tracheal length is regulated during inspiration or neck movement (extension). ^[1]

The posterior wall of the trachea lies adjacent to the anterior aspect of the esophagus. This area is flat membranous, fibromuscular structure containing smooth muscle known as trachealis that helps the trachea to contract and decrease its diameter. Trachealis plays a crucial role in coughing and getting rid of secretions, foreign particles, saliva, or food. ^{[1, 19]}  It also helps the trachea to narrow, especially while eating food, during which there is a concomitant expansion of the esophagus. The cartilaginous rings are deficient on the posterior aspect of the trachea. These rings mechanically hold the airway open but also give it flexibility. By preventing the collapse of the conducting pathway, respiration is not impeded.

The cartilaginous rings may become calcified and lose elasticity with advancing age. ^[1]  Each of the cartilages is enclosed in perichondrium; this is continuous within a sheet of dense, irregular connective tissue forming a fibrous membrane between adjacent rings of cartilage and at the posterior aspect of the trachea and extrapulmonary bronchi where the cartilage is incomplete.

The cartilages are placed horizontally above each other, separated by narrow intervals (two rings per cm of the trachea). ^[17]  They measure about 4 mm in depth and 1 mm in thickness. Their outer surfaces are flattened in a vertical direction, but the internal surfaces are convex. The cartilages are thicker in the middle than at the margins. Two or more of the cartilages often unite, partially or completely, and they are sometimes bifurcated at their extremities.

The first cartilage is broader than the rest and is often divided at one end; it is connected by the cricotracheal ligament to the lower border of the cricoid cartilage; it is sometimes blended with the cricoid cartilage or with the succeeding cartilage.

The last cartilage is thick and broad in the middle, in consequence of its lower border being prolonged into a triangular hook-shaped process, which curves downward and backward between the two bronchi. It ends on each side in an imperfect ring, which encloses the commencement of the bronchus. The tracheal bifurcation is marked by a cartilaginous spur, the carina. ^[1]  The cartilage above the last is somewhat broader than the others at its center. COPD can cause the softening of tracheal rings, leading to the narrowing of the AP diameter of the lumen, along with thickening of the posterior wall. This can result in luminal obstruction during expiration or coughing. ^[19]

Carina

At the bottom of the trachea, there is a keel-like partition called the carina (frequently membranous) separating the two bronchi. It is situated slightly to the right of the midline, ^[1]  at the level of the fourth or fifth thoracic vertebra posteriorly and sternomanubrial junction anteriorly. ^[17] The tracheal lumen narrows gradually as it approaches the carina. The angle between the two main stem bronchi varies among individuals and is typically larger in children than in adults. Additionally, the configuration of the cartilage at the carina shows considerable variability. ^[17] The right main stem bronchus appears to be a more direct continuation of the trachea than the left. During intubation, if the endotracheal tube is pushed beyond the carina, it will enter the right side. This tendency is facilitated by the larger diameter of the right tube which is shorter and more vertically oriented. ^[1]  In contrast, the left bronchus emerges at a more oblique angle and has a more horizontal alignment. ^[17] It also explains why a foreign body in the trachea falls more frequently into the right bronchus.

Mucous membrane

The mucous membrane is continuous with, and similar to, the larynx above and intrapulmonary bronchi below. It consists of a layer of pseudostratified ciliated columnar epithelium with numerous mucus-secreting goblet cells. External to this is a submucosa, also composed of connective tissue that contains airway smooth muscle, glands, cartilage plates, vessels, lymphoid tissue, and nerves. ^[1]  The ciliated columnar cells near the free or apical surface of the epithelium have thin, "hair-like" projections. They are the driving force of the mucociliary rejection current (escalator) in the tracheobronchial tree. ^[1]

Anatomic relationships

In the neck

The surface of the trachea is covered, from above downward, by the isthmus of the thyroid gland, the inferior thyroid veins, the arteria thyroidea ima (if it exists), the sternothyroid and sternohyoid muscles, the superficial and deep cervical fascia, and, more superficially, by the jugular venous arch between the anterior jugular veins; the thyroid isthmus is in front of the second and third tracheal cartilages. ^[1]  In children, the thymus, particularly the left lobe, can extend anteriorly to the trachea, reaching the level of the thyroid gland. Additionally, the brachiocephalic trunk crosses obliquely in front of the trachea at, or slightly above, the upper border of the manubrium. ^[1]

Laterally, in the neck, it is in relation with the common carotid arteries, the right and left lobes of the thyroid gland, the inferior thyroid arteries, and the recurrent laryngeal nerves. The paired lobes of the thyroid gland commonly descend to the level of the fifth or sixth tracheal cartilage. Immediately lateral to the cervical trachea lies the recurrent laryngeal nerves that ascend on each side, in or near the tracheoesophageal groove. Lateral to these structures lies the common carotid arteries, enclosed in a carotid sheath. ^[1]

Posteriorly, it is in contact with the cervical part of the esophagus, which separates it from the vertebral column and the prevertebral fascia in the neck and thorax. ^[1]

Zuckerkandl's tubercle (ZT) is a pyramidal extension of the thyroid gland, present at the most posterior side of each lobe. Won et al. published a study on the comparative anatomy of the thyroid gland and ZTs and their relation to the trachea of the same cadavers before and after fixation. ^[20] They found that ZT was at the posteromedial border or posterior surface of the thyroid lobe in both the fresh and fixed states, contrary to most previous reports.

In the thorax

The trachea lies in the superior mediastinum.

Anteriorly, it is covered by the manubrium sterni, the inferior attachments of the sternohyoid and sternothyroid muscles, the inferior thyroid veins, the remains of the thymus, the left brachiocephalic (innominate) vein, the aortic arch, the brachiocephalic trunk and left common carotid arteries, and the deep cardiac plexus. The brachiocephalic trunk and left common carotid artery lie on the right and left of the trachea, respectively, as they diverge superiorly into the neck. In children, the thymus lies anterior to the trachea. ^[1]

Laterally, on the right side, the trachea is in relation with the right lung and its pleura, right vagus, right brachiocephalic vein, superior vena cava, and azygos vein. ^[1]  Laterally, on its left side, the trachea is in relation with the aortic arch and the left common carotid and subclavian arteries. The left recurrent laryngeal nerve is initially situated between the trachea and the aortic arch and then lies within or just anterior to the tracheoesophageal groove. ^[1]

Posteriorly, the esophagus, with its venous and neural plexuses, separates trachea from the vertebral column. The close relation of these two tubular structures forms a tracheoesophageal groove on either side. ^[1]

Neurovascular supply of the trachea

Arterial supply

The arterial supply of the trachea divides it into the upper (cervical) and lower (thoracic) trachea. The arteries supplying the trachea approach the tracheal wall laterally and vascularize the trachea in a segmental fashion along its longitudinal access. Table 3 summarizes the arterial blood supply of the trachea. ^[19]

Table 3. Arteries Supplying the Trachea (Open Table in a new window)

The lateral parts of the trachea and esophagus are supplied via longitudinal vascular anastomoses of interconnected branches along the lateral surface of the trachea from the following arteries (see the images below): ^[16]

• Inferior thyroid artery
• Subclavian artery
• Superior intercostal artery
• Internal thoracic artery
• Brachiocephalic trunk
• Superior and middle bronchial arteries at the tracheal bifurcation

Tracheal blood supply. Left anterior view.

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Tracheal blood supply. Right anterior view.

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The lateral and anterior tracheal walls receive their blood supply through transverse segmental vessels that run in the soft tissues between the cartilages. These transverse vessels interconnect the aforementioned longitudinal anastomoses across the midline and feed the submucosal capillary network. This network is arborized richly beneath the endotracheal mucosa. The tracheal cartilages receive nourishment from the capillary bed located on their internal surface. The esophageal arteries and their subdivisions that supply the posterior membranous tracheal wall contribute almost nothing to the circulation of the cartilaginous walls.

Venous and lymphatic drainage

Small tracheal veins join the laryngeal vein or empty directly into the left inferior thyroid vein. The inferior thyroid veins arise as a venous plexus on the anterior surface of the isthmus of the thyroid gland. Left and right descending veins enter the respective brachiocephalic veins. The two veins may form a common trunk entering the superior vena cava or the left brachiocephalic vein.

The tracheal lymphatics drain to the pretracheal, paratracheal, and tracheobronchial groups of lymph nodes. The right lower paratracheal lymph nodes drain into the thoracic duct tributaries that course along the azygos vein. The left superior bronchial lymph nodes below the trachea drain directly into the mediastinal thoracic duct or to the arch of the duct via the left recurrent chain. An alternative pathway is to the aortic arch lymph nodes and up along the arch. Tracheobronchial lymph nodes drain through accessory ducts on both sides of the esophagus to the thoracic duct.

Nerve supply

The autonomic nervous system (either by direct axonal innervation or circulating neurotransmitters) controls many aspects of airway function, such as the regulation of tracheobronchial smooth muscle tone (bronchoconstriction or bronchodilation), mucus secretion from submucosal glands and surface epithelial goblet cells, and vascular permeability and blood flow. ^[1]

The trachea and bronchi are innervated by the branches of the vagus nerves, recurrent laryngeal nerves, and sympathetic trunks via the anterior and posterior pulmonary plexuses. ^[1] The primary contractile innervation of airway smooth muscle is parasympathetic cholinergic, while the main relaxant innervation is non-cholinergic (parasympathetic nerves containing nitric oxide synthase and vasoactive intestinal peptide). Afferent vagal fibers are also responsible for sneezing and cough reflex. ^[17] Circulating norepinephrine also induces bronchodilation via β-adrenergic receptors in the airways. ^[1]

The muscle fibers of the trachea are innervated by the right and left recurrent laryngeal nerves, which also carry sensory fibers from the mucous membrane. Sympathetic nerve fibers are derived mainly from the middle cervical ganglion and are responsible for bronchodilation and reduced secretions. They also have connections with the recurrent laryngeal nerves.

The tracheal cartilages are enclosed in an elastic fibrous membrane, which consists of two layers: (1) the thicker layer, passing over the outer surface of the ring and (2) the other layer over the inner surface. At the upper and lower margins of the cartilages, the two layers blend together and connect the rings. Between the ends of the rings, the membrane forms a single layer. ^{[21, 22, 23]}

The muscular tissue consists of two layers of nonstriated muscle fibers, longitudinal and transverse. The longitudinal fibers are external and consist of a few scattered bundles. The transverse fibers (trachealis muscle) are internal and form a thin layer, which extends transversely between the ends of the cartilages.

The mucous membrane is continuous with that of the larynx superiorly and the bronchi inferiorly. It consists of areolar and lymphoid tissue and presents a well-marked basement membrane, supporting a pseudostratified epithelium, the surface layer of which is columnar and ciliated, whereas the deeper layers are composed of oval or rounded cells. Beneath the basement membrane, there is a distinct layer of longitudinal elastic fibers with a small amount of intervening areolar tissue. The submucous layer is composed of a loose connective tissue, containing airway smooth muscle, cartilage plates, large blood vessels, nerves, and mucous glands; the ducts of the latter pierce the overlying layers and open on the surface. It has mixed compound tubuloacinar seromucous glands, composed largely of mucin and serous-secreting cells. They are an important source of mucus at the surface of the ciliated respiratory epithelium. ^[1]

When Roberts et al. used scanning electron microscopy, histochemistry, and equilibrium tensile testing to investigate the relationship between collagen organization and equilibrium tensile modulus within the structure of airway cartilage, they found that the surfaces of tracheal cartilage matrix are collagen rich and surround a proteoglycan-rich core. ^[23]

Collagen fibrils in the superficial zones are oriented in the plane of the cartilage surface. In deeper layers of the cartilage, collagen fibrils are oriented less regularly. Equilibrium tensile modulus of 100-micron-thick strips of cartilage was measured and was found to decrease with depth, from 13.6 ± 1.5 MPa (MPa = N/mm²) for the abluminal superficial zone to 4.6 ± 1.7 MPa in the middle zone. ^[23] Stress-strain curves were linear for strains up to 10% with minimal residual strain. ^[23] This is consistent with a model in which collagen fibers in the outer layers of the cartilage resist tensile forces and hydrated proteoglycans in the central zone resist compression forces as the cartilage crescent bends. ^[24]

Tracheal deviation and shift 

Causes of tracheal deviation and shift include the following: ^{[9, 10, 11, 22]}

• Pleural effusion
• Pneumothorax
• Pulmonary fibrosis
• Lung cancer
• Pulmonary collapse
• Surgical lobectomy
• Pulmonary atelectasis
• Mediastinal tumor
• Kyphoscoliosis
• Hiatal hernia
• Thoracic aortic aneurysm
• Tension pneumothorax
• Pulmonary tuberculosis
• Lung collapse
• Retrosternal thyroid

   

Congenital tracheal malformations

Congenital tracheal malformations may be intrinsic to the trachea itself or extrinsic. The primary presenting symptom is most commonly biphasic stridor. Other airway-related symptoms (e.g., wheezing, cough, pneumonia, and croup) may be present.

Tracheal agenesis and atresia

Tracheal agenesis and atresia are almost uniformly fatal; fortunately, these conditions are very rare. The trachea may be completely absent (agenesis) or it may be partially in place but considerably deformed (atresia). Communication, in either case, between the larynx and the alveoli of the lungs is lacking. Affected newborns survive only if an alternate pathway for ventilation exists. Surgical attempts yield poor results, making this an uncorrectable malformation.

Tracheal web

Tracheal web consists of a thin layer of tissue draped across the tracheal lumen. Although the thickness of the web may vary, no deformity or abnormality of the underlying cartilage framework exists (in contrast to tracheal stenosis). The web is not complete; the degree of ventilatory symptoms present is directly related to the size of the remaining tracheal lumen. Treatment consists of rupturing the web. This may be accomplished via rigid dilatation, through use of a laser, or with other ablative or cutting instruments. Only the most thickened webs require a more invasive open surgical approach; local resection is the treatment of choice.

Tracheomalacia

Abnormalities in cartilage development can lead to the affected airways being "floppy" or collapsible during breathing. These anomalies are termed tracheomalacia when the trachea is affected. It usually presents in early infancy with cough, tachypnoea, stridor, and wheezing, and in some cases, it may also be associated with cardiac or respiratory anomalies. ^[1]

Tracheal stenosis

Tracheal stenosis is a rather rare condition. The pathologic process continues to a much greater underlying tissue depth when compared with a tracheal web (see the following images). Congenital tracheal stenosis is often due to complete tracheal rings, where the posterior membranous trachea is replaced by circular cartilaginous rings, affecting either short or long segments of the trachea. ^[25] A single tracheal ring, multiple rings, or even the entire length of the trachea may be involved. Although affected patients may be symptomatic at birth, symptoms may be delayed several months until the airway lumen is compromised further by exacerbation of an upper respiratory tract infection.

Weblike stenosis of the trachea.

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Complex stenosis of the trachea.

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Pediatric tracheal stenosis can be classified broadly into congenital lesions and acquired lesions caused primarily by trauma. Commonly it presents as a congenital disorder due to complete tracheal rings, vascular compression, or tracheomalacia. Acquired tracheal stenosis typically results from iatrogenic endotracheal tube injury, direct trauma (e.g., clothesline injury), and, less frequently, tracheal tumors. The tracheal narrowing can also occur due to tracheostomy, leading to conditions such as A-frame deformity or suprastomal collapse. ^[26]  Symptoms often appear gradually and include respiratory distress, sleep-disordered breathing, or exercise intolerance. Diagnostic evaluation typically involves a combination of imaging and endoscopy. ^[26]

Severe tracheal stenosis in children is a rare, life-threatening condition that requires surgery. Several tracheal reconstruction techniques, such as tracheal resection with end-to-end anastomosis (for short-segment complete tracheal stenosis), patch tracheoplasty, slide tracheoplasty, and homograft and autograft augmentation repairs, have been developed to treat the condition. ^[25] Yong et al. (2013) reported on the use of slide tracheoplasty, with lower mortality and morbidity compared with patch tracheoplasty. ^[27] The 2-year survival after surgery was associated with an excellent outcome.

Symptoms of tracheal stenosis include dyspnea and biphasic stridor with a prolonged expiratory component. If an affected patient requires intubation, efforts should be made to extubate early to prevent development of further edema and acute additional airway narrowing.

Various other abnormalities are associated with tracheal stenosis, including vascular slings, tracheoesophageal fistulas, pulmonary hypoplasia, and trisomy 21. Short stenoses may be removed directly via a tracheal resection with primary end-to-end anastomosis. More severe lesions require tracheal reconstruction or tracheoplasty.

Tracheoesophageal fistulae

Tracheoesophageal fistulae affect about 1 in 3000 births. Five distinct types of tracheoesophageal fistulae can be identified. Prenatally, polyhydramnios may be a clinical indication, though it often isn't noticeable until the third trimester. In nearly all cases, the esophagus terminates in a blind pouch while the stomach is connected to the lower portion of the trachea. As a result, the abdomen rapidly inflates with air once the newborn starts breathing. In two of the five fistula types, there is no connection between the stomach and the upper digestive tract . It is commonly associated with other congenital anomalies, including cardiovascular defects (30%) and anorectal (15%) and genitourinary (15%) anomalies. ^[1]

Vascular anomalies

Vascular anomalies are associated with an essentially normal trachea. However, large vessels are extrinsically impinging upon it; the infant may hyperextend the neck in an effort to straighten the compressed airway, thereby stretching the trachea and pushing away the compressive extraluminal vessels. Fortunately, these episodes spontaneously resolve.

Brachiocephalic trunk compression

The most common vascular anomaly is tracheal compression from the brachiocephalic trunk (earlier known as the innominate artery compression syndrome [IACS]). The cause is likely multifactorial and not solely due to the artery's anomalous origin to the left of, and hence crossing, the trachea. It is suspected that IACS may also occur due to anterior mediastinum crowding or be linked to primary tracheomalacia. Recent studies have shown that the artery's origin shifts to the right in adulthood, explaining the rarity of IACS beyond infancy. ^[28]  Operative intervention is reserved for severe cases. Surgical correction (i.e., inominopexy or aortopexy) involves suspending the brachiocephalic or the aorta anteriorly to the sternum.

Complete vascular ring (double aortic arch)

"Vascular ring" encompasses all congenital aortic arch anomalies that compress the trachea, esophagus, or both. The second most common vascular anomaly is the complete vascular ring, also known as the double aortic arch (see the following image). This condition provides circumferential compression of both the trachea and esophagus. Infants with a double aortic arch usually exhibit severe symptoms early, which may include respiratory symptoms such as noisy breathing, stridor, cough, and recurrent upper respiratory infections and, occasionally, esophageal symptoms such as dysphagia and choking. ^[29]  Treatment consists of surgically dividing the ring, generally by ligating the smaller of the two arches.

Double aortic arch causing vascular ring.

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Aberrant left pulmonary artery

Pulmonary artery sling (PAS) occurs when the left pulmonary artery arises from the right pulmonary artery (instead of the main pulmonary artery) and runs posteriorly between the esophagus and trachea. Although PAS is not considered a "true complete" vascular ring, symptoms can be similar and require a similar diagnostic workup. ^[29]

Another vascular anomaly affecting the trachea is an aberrant left pulmonary artery. In these cases, the left pulmonary artery passes between the trachea and esophagus, resulting in distal tracheal and right bronchus compression. It is associated with the presence of complete tracheal rings. Treatment involves surgical rerouting of the aberrant vessel. Other vascular anomalies include a right aortic arch with a persistent ligamentum arteriosum, as well as an anomalous right subclavian artery compressing the airway if it courses immediately anterior or posterior to the trachea.

Complete tracheal rings and other anomalies 

Complete or deformed rings occur when a complete tracheal ring exists, so no posterior membranous portion of the trachea is present, leaving only a rigid ring around the airway. Extensive severe disease may present with neonatal respiratory distress; later presentations may include apparently steroid-resistant asthma. Single rings and short segments may be resected primarily with end-to-end anastomosis, whereas longer segments require tracheoplasty or tracheal reconstruction. Tracheal transplantation has been used to treat long-segment tracheal stenosis. ^[1]  In some cases, it may be associated with vascular anomalies such as PAS. ^[1]

Other tracheal anomalies include tracheoesophageal fistulas, tracheal cysts, trachiectasis (congenital tracheal enlargement), tracheal bronchus (incidence is estimated at 3%, with right side bronchus far outnumbering the left), tracheal clefts and congenitally short tracheas

Idiopathic subglottic stenosis

To assess the size of the trachea among female patients with idiopathic subglottic stenosis (SGS), Zaghi et al. used CT scans of the neck and chest from female patients with idiopathic SGS. ^[30]  The diameter of the trachea was measured at the level of the subglottis, mid-cervical level, and level of the mid-thoracic trachea. Patients with idiopathic SGS were found to have a significantly smaller cross-sectional area throughout the course of the cervical and thoracic trachea. Idiopathic SGS is a rare but distinct subclass of SGS, characterized by a smaller cross-sectional area throughout the course of the trachea.

Mounier-Kuhn syndrome 

MKS or congenital tracheobronchomegaly (TBM) is a rare congenital disease of unknown etiology characterized by abnormal dilatation of the tracheobronchial tree. It presents as respiratory infections that can progress to bronchitis, bronchiectasis, and pulmonary fibrosis. ^[31] Histologically, TBM is marked by severe atrophy of elastic fibers and longitudinal muscle and thinning of the muscularis mucosa. Immunohistochemical studies show a significant reduction of elastic fibers with diffuse inflammatory infiltrates, mainly comprising CD4 cells, and increased matrix metalloproteinases. ^[31]  The diagnosis of MKS is made using computed tomography scanning of the chest, on the basis of enlarged diameters of trachea and main bronchi. Kuwal et al. published a histologically confirmed case of MKS in which the diameter of the right main bronchus was below the minimum diameter (mean + 3 standard deviations) required for the diagnosis. ^[32]  They suggested that the diagnosis of MKS should not be solely based on the diameter of airways but on the basis of the overall clinical, pathologic, and radiologic profile.

Artificial trachea

The main indications for tracheal surgery include congenital or posttraumatic stenoses, inflammatory (generally postintubation) or degenerative lesions, and benign or malignant neoplasms. Although surgical resection has now become part of the surgical practice, other treatment modalities are approaching a new clinical application era, in particular tracheal transplantation and bioengineering.

Since the 1980s, tissue engineering has become one of the major areas of endeavor in medical research. Using this technology, various attempts have been made to create and apply tissue-engineered prosthetic trachea. One type of artificial trachea is a spiral stent composed of Marlex mesh made of polypropylene and covered with collagen sponge made from porcine skin.

Patients undergo a two-stage operation. In the first operation, after resection of the pathologic regions, the edge of the tracheal cartilage is sutured to the edge of the skin. The tracheal lumen is exposed, and a T-shaped cannula is inserted into the large tracheostoma. After a few weeks, the trachea and skin are separated. The trimmed artificial trachea with venous blood and basic fibroblast growth factor is then implanted into the cartilage defect. ^[11]

According to Hovatta (2012), ^[33] many groups have been active in clinical trials with mesenchymal stem cells in Europe. Successful transplantations of trachea using tissue-engineered stem cells have been made recently. Optimizing the stem cell type for these constructs is ongoing.