Structure and development of the upper respiratory system





Pearls





  • The structures of the upper airway undergo extensive changes from infancy through young adulthood. An understanding of the numerous variations, congenital anomalies, and resulting special vulnerabilities of the developing airway and underlying illness should result in improved health outcomes and a lower morbidity rate in children with airway disease.



  • The upper and lower respiratory tracts develop separately. Thus, there are few coincident congenital anomalies between these two contiguous areas.



  • An understanding of possible congenital anomalies of the airway will improve diagnosis and treatments.



The respiratory tract can be divided into the upper or conducting airways and the lower or gaseous exchange airways. For the purposes of this chapter, the trachea down to the carina is included as part of the upper or conducting airways. The upper airway shares its development with that of the upper digestive tract; the lower airway shares its development with the cardiovascular system.


Developmental anatomy of the upper airway


The embryologic development of the nasal cavity, mouth, nasopharynx, and hypopharynx occurs in a separate environment from that of the larynx, trachea, bronchi, and lung parenchyma. Because the upper airway and lower respiratory tract develop separately, there are few coincident congenital anomalies between these two contiguous, but developmentally distinct, areas.


The branchial arches derived from the neural crest cells begin to appear during the fourth week of embryogenesis. The branchial arches give rise to the formation of the face, neck, nasal cavities, mouth, larynx, pharynx, and striated muscles in the head and neck that are involved in breathing and swallowing. The development of these structures is usually complete by week 14.


The respiratory system—including parts of the larynx, trachea, and lungs—also begins to appear during week 4, when the laryngotracheal groove develops into a diverticulum that subsequently separates from the pharynx. In the fourth and fifth weeks, the longitudinal tracheoesophageal folds fuse, forming the tracheoesophageal septum and dividing the foregut into ventral and dorsal portions. The ventral portion becomes the larynx, trachea, bronchi, and lungs; the dorsal portion becomes the esophagus. Abnormalities in the development of the esophagus and trachea can lead to tracheoesophageal fistula. A rare but significant tracheoesophageal fistula is the H type that could present with recurrent lower airway disease ( Fig. 40.1 ).




• Fig. 40.1


Bronchoscopy with trachea above with tracheoesophageal fistula below.


The embryogenesis of the larynx is complex. The cartilages and muscles are derived from the fourth and sixth branchial arches. The epithelium is derived from the endoderm of the laryngotracheal tube. As this epithelium proliferates rapidly, the larynx is temporarily occluded until the 10th week, when recanalization occurs. Failure to recanalize can result in laryngeal webs, stenosis, or, rarely, atresia ( Fig. 40.2 ). The epiglottis forms by mesenchymal proliferation of the third and fourth branchial arches ( Fig. 40.3 ).




• Fig. 40.2


Laryngeal web.



• Fig. 40.3


Embryologic development of the larynx.

(Modified from Arvedson JC, Brodsky L, Lefton-Greif MA . Pediatric Swallowing and Feeding: Assessment and Management . 3rd ed. San Diego: Plural Publishing; 2020.)


Another developmental abnormality is a laryngeal cleft that presents as a defect in the posterior arytenoid muscles and, sometimes, tracheal cartilage, leading to aspiration. The cleft can vary from mild to severe, depending on the extent of the defect that may extend from the top of the cricoid cartilage into the thoracic trachea ( Fig. 40.4 ). Findings on videofluoroscopic swallow studies are suspicious for laryngeal cleft when liquid is seen to move into the trachea posteriorly and lower than typically seen with simple delayed airway closure. Those findings typically lead to further workup to include operative endoscopy with palpation of the larynx, which is the definitive diagnostic procedure.




• Fig. 40.4


Laryngeal cleft during repair.


The tracheobronchial tree also has several embryonic origins. Its epithelium is derived from the laryngotracheal tube. The connective tissue, cartilages, and muscles are derived from the surrounding splanchnic mesenchyme. All cartilages of the trachea are C-shaped with a membranous posterior tracheal wall, giving the airway flexibility to expand, except for the cricoid cartilage immediately below the true vocal folds. The cricoid cartilage is considered to be anatomically part of the larynx. It is the only cartilage in the airway to form a complete ring. Because of its inflexibility, edema from intubation and inflammation can result in serious injury that should be avoidable in most cases.


Laryngeal web or subglottic stenosis can present with stridor, likely biphasic, and respiratory distress in a newborn infant ( Fig. 40.5 ). While medical treatment—such as steroids, racemic epinephrine, heliox, or intubation—may manage these infants in the acute setting, many infants with congenital webs or stenosis will need surgical intervention. Laryngeal atresia that is not diagnosed prenatally will lead to often fatal respiratory distress at birth ( Fig. 40.6 ). The infants’ mothers will likely present with severe polyhydramnios and fetal hyperinflation of the lungs. Fetal MRI assists in the diagnosis. A tracheostomy in the newborn period is necessary, in coordination with an ex utero intrapartum treatment procedure for airway management.




• Fig. 40.5


Subglottic stenosis.



• Fig. 40.6


Laryngeal atresia.


Abnormalities of the lower airway can lead to critical airway distress. Although the only normal complete tracheal ring is the cricoid cartilage, complete tracheal rings or tracheal stenosis can lead to difficult intubations and respiratory distress. Depending on the severity of distress and tracheal stenosis, surgical management is needed in the majority of cases ( Figs. 40.7 and 40.8 ). Children with Down syndrome can present with multilevel airway problems as a result of midface hypoplasia, a small nasopharynx, a relatively large tongue, and a congenitally narrowed subglottic space. If a child with Down syndrome requires intubation, a smaller endotracheal tube that is one-half size to one full size lower than that used for a child with a normal airway should be employed.




• Fig. 40.7


Bronchoscopy demonstrating distal complete tracheal rings.



• Fig. 40.8


Congenital tracheal stenosis.


Anatomy and physiology of the upper airway


Nasal passages


The upper airway begins at the tip of the nose and the vermilion border of the lips. Both the nasal and oral passages allow air to stream from the environment through the larynx into the lungs, where oxygen and carbon dioxide are exchanged in alveolar-capillary units. Oral passages (oral cavity, oropharynx, and hypopharynx) are conduits for the ingestion of the food and liquid needed to support growth and development.


The structures of the upper airways of the infant ( Fig. 40.9 ) gradually change in the first few years of life to assume their adult configuration ( Fig. 40.10 ). Preferential nasal breathing is present in typical term neonates and persists until 6 months of age because of the high position of the larynx in the neck with the soft palate and valleculae in close anatomic approximation. Nasal breathing is a necessary underpinning for nipple feeding at the breast or bottle in order for infants to coordinate sucking, swallowing, and breathing sequencing.


Jun 26, 2021 | Posted by in CRITICAL CARE | Comments Off on Structure and development of the upper respiratory system
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