The larynx is a complex, funnel-shaped organ ( ▶ Fig. 21.1). It comprises a cartilaginous framework that supports membranes and fascial layers to allow the attachment of a number of intrinsic and extrinsic muscles and has a luminal surface lined with epithelium and an intricate neurovascular supply. Negus was the first to prioritize laryngeal functions in evolutionary order as follows 1:
Protection of the airway.
Voluntary closure of the glottis occurs when straining to increase intra-abdominal pressure. The trachea and bronchi consist of a series of incomplete cartilaginous rings, completed posteriorly by a layer of muscle with a continuous internal mucosal lining of columnar epithelium. The tracheobronchial tree transmits respiratory gases to and from the lung parenchyma with minimal resistance (see ▶ 20) while also facilitating toilet of the terminal airways and alveoli.
These diverse and highly specialized functional requirements of the larynx and tracheobronchial airway are reflected in their complex embryology, anatomy, and physiology, anomalies of which can lead to life-threatening clinical sequelae.
Fig. 21.1 Midsagittal section of the laryngeal cavity viewed from the left side. (Reproduced from Gilroy and MacPherson, Atlas of Anatomy, 3rd edition, © 2016, Thieme Publishers, New York. Illustration by Markus Voll.)
Development of the larynx, trachea, and bronchi begins in the fourth week of gestation. The respiratory (laryngotracheal) diverticulum arises from the ventral surface of the foregut, just distal to the fourth pharyngeal arch ( ▶ Fig. 21.2; see also ▶ 24). Its endodermal lining forms the pseudostratified columnar epithelium that lines the definitive airway. The enlarging diverticulum is then invaginated by splanchnic mesenchyme, the precursor of the cartilaginous and muscular structures of the lower respiratory organs. The caudal end of the diverticulum finally gives rise to primordial bronchopulmonary buds by the end of the fourth week of gestation, and the bronchi and primordial pleural cavities develop during the fifth week. In parallel with this, the esophagus develops as an outpouching of the ventral foregut—a ventrocaudal elongation of the respiratory diverticulum—reaching its full length by the seventh week.
The laryngeal cartilages and intrinsic muscles arise from fourth and sixth pharyngeal arch mesenchymal derivatives ( ▶ Fig. 21.2). The fourth arch gives rise to the thyroid and epiglottic cartilages and the cricothyroid muscle, receiving sensory and motor innervations from the superior laryngeal branch of the vagus nerve. The sixth arch produces the cricoid, arytenoid, and corniculate cartilages and all other intrinsic laryngeal muscles, innervated by the inferior (recurrent) laryngeal branch of the vagus nerve. The glottis itself forms from primordia in the floor of the primitive pharynx, at the origin of the respiratory diverticulum.
Importantly, in the context of congenital anomalies, the mesenchyme of the developing larynx proliferates very rapidly from the sixth week of gestation, which eventually leads to obliteration of the primordial pharyngolaryngeal lumen. Recanalization occurs in a dorsoventral direction from 10 weeks onward, with eventual formation of contiguous supra- and subglottic lumina and laryngeal ventricles on either side. Failure of this recanalization process is believed to be responsible for congenital laryngeal atresias.
Fig. 21.2 Embryology of the larynx and trachea. (Reproduced from Probst R, Grevers G, Iro H. Basic Otorhinolaryngology: A Step-by-Step Learning Guide. Stuttgart/New York: Thieme; 2006, with permission.)
21.4 Clinical Anatomy
The laryngeal framework consists of the thyroid and cricoid cartilages, which articulate at the cricothyroid joint and are connected anteriorly by the cricothyroid membrane ( ▶ Fig. 21.3). The thyroid cartilage comprises two laminae that meet in the midline, but are separate superiorly (the thyroid notch) and posteriorly. Superior and inferior cornua, and the oblique line of each lamina serve as sites of muscular and fascial attachment, allowing suspension from the hyoid superiorly and the sternum inferiorly.
The cricoid cartilage is the only complete ring of cartilage in the airway and is shaped like a signet ring, with short anterior and longer posterior laminae.
Fig. 21.3 Midsagittal section of the laryngeal cavity viewed from the left side. (Reproduced from Gilroy and MacPherson, Atlas of Anatomy, 3rd edition, © 2016, Thieme Publishers, New York. Illustration by Markus Voll.)
The epiglottis (epiglottic cartilage and loosely adherent squamous mucosal coverings) is attached to the thyroid cartilage, folding down passively over the laryngeal inlet during food bolus ingestion. The arytenoid cartilages articulate with the superior border of the posterior cricoid lamina through synovial joints to allow rotation in the vertical axis and side-to-side gliding movements during phonation. Their vocal and muscular processes attach to the vocal folds and intrinsic laryngeal muscles, respectively.
Various fibroelastic membranes are suspended from the cartilaginous framework. The quadrangular membrane hangs down from the thyroid, epiglottic, and arytenoid cartilages. The upper free edges form the aryepiglottic folds at the laryngeal inlet, within which the corniculate and cuneiform cartilages are suspended. The lower free edges are the vestibular folds (false cords). The cricothyroid membrane completes the lateral gap between the cricoid and thyroid cartilages, and the upper free edges of this form the vocal ligaments within the vocal folds. The laryngeal ventricles are pouches on either side between the true and false cords. Each ventricle has a further outpouching beyond the quadrangular membrane (the saccule), lined with mucinous glands to lubricate the glottis.
The vocal folds are the structures between the vocal processes of the arytenoids and the anterior commissure ( ▶ Fig. 21.4). They are covered by nonkeratinizing stratified squamous epithelium (in contrast to the pseudostratified columnar epithelium elsewhere), with three deep layers of lamina propria to this. The intermediate and deep layers of lamina propria are also termed the vocal ligaments, overlying the thyroarytenoid muscles, which form the body of each fold. The lower and deeper fibers of each thyroarytenoid muscle are also referred to as vocalis. The vocal cords comprise the vocal folds anteriorly and the bodies and vocal processes of the arytenoids posteriorly.
Fig. 21.4 Superior view of the structure of the larynx. (Reproduced from Gilroy and MacPherson, Atlas of Anatomy, 3rd edition, © 2016, Thieme Publishers, New York. Illustration by Markus Voll.)
This distinction has functional significance: the narrow anterior, membranous portion of the glottis (glottis vocalis) is involved primarily in phonation and airway protection, whereas the wider posterior cartilaginous portion (glottis respiratoria) is more relevant to respiration. The larynx is divided arbitrarily into the supraglottis (epiglottis, aryepiglottic folds, arytenoids, false cords, and ventricles), glottis (vocal cords), and subglottis (undersurface of vocal cords to lower border of cricoid).
In contrast to the extrinsic laryngeal muscles of the laryngohyoid complex that raise, lower, or stabilize the larynx, the intrinsic laryngeal muscles are restricted to the larynx itself ( ▶ Fig. 21.5). The cricothyroids are the largest of these, located externally and innervated by the superior laryngeal nerve. They tilt the thyroid cartilage forward, tensing the vocal folds to raise pitch. Fibers run in two bellies: the pars recta (running vertically) and pars obliqua (running obliquely). These muscles form a useful surgical landmark for the cricoid cartilage during pediatric laryngeal surgery, particularly when the area has undergone scarring as a result of pathology or previous procedures. The remainder of the intrinsic muscles are located posteriorly and/or internally, all supplied by the recurrent laryngeal nerve. The posterior cricoarytenoids are the sole abductors of the cords. The lateral cricoarytenoids are their main antagonists, adducting, elongating, and lowering the cords. The unpaired interarytenoid muscle acts to bring the arytenoid bodies together, thereby closing the posterior glottis (glottis respiratoria). Further extensions into the aryepiglottic folds on each side (aryepiglotticus) assist in closure of the laryngeal inlet during swallowing.
Fig. 21.5 Laryngeal muscles. (Reproduced from Gilroy and MacPherson, Atlas of Anatomy, 3rd edition, © 2016, Thieme Publishers, New York. Illustration by Markus Voll.)
In line with its embryological origins, the supraglottis receives sensory supply from the superior laryngeal nerve; the glottis, the subglottis, and the proximal trachea are supplied by the recurrent laryngeal nerve.
Pediatric upper airways differ from those of adults in several ways:
The infantile larynx is about a third the size of the adult equivalent, with vocal cords 6- to 8-mm-long and subglottic dimensions of 4.5 by 7 mm.
In contrast to adults, where the glottis is the narrowest section of the upper airway, the subglottis is the narrowest point in children, hence the propensity of acquired pathology to affect this area. This explains the importance of routine assessment of the subglottic dimensions during microlaryngoscopy.
The larynx is also situated higher (vertebral level C2–C4) and more anteriorly in the neck than in an adult (C4–C6), tucked right up to the hyoid. This is reflected in the obligate nasal breathing of infants, in whom the nasal passages account for 50% of total airways resistance.
The epiglottis is rather more tightly Ω-shaped and softer than in the adult, tending to flop down over the laryngeal inlet more readily during swallowing.
These geometrical and anatomical differences adapt the infant well to a wholly liquid diet, facilitating nasal ventilation during milk feeds and reducing the risks of aspiration. Such a configuration is also seen in ruminant herbivores for the same reasons. A young child has a relatively large head, prominent occiput, short neck, and large tongue.
Together with the high anterior position of the larynx, these factors mean that the infant is most appropriately mask-ventilated in a neutral rather than the neck-flexed, head-extended position used for adults.
The trachea is a fibroelastic and muscular tube ( ▶ Fig. 21.6), comprising 16 to 20 C-shaped cartilaginous rings open posteriorly, spanned by smooth muscle, the trachealis. This runs from the lower border of the cricoid (level of C4 in the child) to its bifurcation at the carina (level of T3/T4 in the child). Importantly, variable tone of the trachealis allows its caliber to alter with the phase and level of respiration, adjusting the physiological dead space. Additionally, apart from being distensible, the trachea has a significantly larger cross-sectional area than the cricoid ring (subglottis) in infants.
An age-appropriate tracheostomy tube will therefore require a larger external diameter than the age-equivalent endotracheal tube to enable ventilation with minimal leak.
Innervation is primarily from the recurrent laryngeal nerves. Important anatomical relations include the esophagus posteriorly, and the left brachiocephalic (innominate) vein, aortic arch, and left brachiocephalic (innominate) artery anteriorly. These structures are important to consider during surgical approaches to the trachea, and abnormalities of them may consequently lead to tracheal anomalies and pathology.
The bronchi are similarly structured, with the right wider, slightly shorter, and more vertically oriented than the left, hence the propensity for inhaled foreign bodies to lodge on the right side ( ▶ Fig. 21.6). They divide into upper, middle, and lower lobe bronchioles on the right, and upper and lower lobe bronchioles on the left. Overall, the airway has 23 divisions from the larynx proximally to the alveoli terminally.
Fig. 21.6 The trachea and bronchial tree. (Reproduced from Schuenke, Schulte, and Schumacher, Atlas of Anatomy: Inner Organs [Volume II], © 2016, Thieme Publishers, New York. Illustration by Markus Voll.)
21.5 Clinical Manifestations of Airway Pathology
The precise presenting features of airway pathology, congenital or acquired, result from derangements of normal functions and vary with the subsite affected. Typical laryngeal sequelae are total or partial respiratory obstruction, inspiratory stridor, a weak or abnormal cry, dyspnea, tachypnea, aspiration, and eventually death from asphyxiation. Tracheobronchial pathology on the other hand tends to produce biphasic or expiratory stridor (wheeze), but many of the aforementioned features may also be seen. Laryngeal and tracheobronchial pathology not infrequently coexist (see ▶ 20).
The congenital anomalies affecting these areas, and their features and management options will be discussed according to anatomical level rather than in order of incidence. It is important from the outset to be mindful that such anomalies, while by definition present at the time of birth, may not be manifested in early life and should also be considered in older children presenting with suggestive symptoms and signs. Complete endoscopic examination of the airway (microlaryngobronchoscopy [MLB]) complemented by other modalities as appropriate remains the gold standard of investigation.
Laryngomalacia (congenital flaccid larynx) is a clinical entity characterized by collapse of the supraglottic structures during inspiration. Although it is by far the commonest congenital laryngeal anomaly, 2 its pathophysiology remains uncertain. The particular structures and mechanisms involved are likely to vary on an individual basis. Dynamic endoscopic assessment will demonstrate any of a number of possible findings: a tightly curled, Ω-shaped epiglottis ( ▶ Fig. 21.7) often particularly soft and retroverted; tall and bulky aryepiglottic folds, which are tightly tethered to the epiglottis; and redundant arytenoid and supra-arytenoid mucosa, often with anteromedial prolapse of the mucosa/corniculate complex and the arytenoids themselves into the posterior glottis during inspiration.
Most cases of laryngomalacia present within the first few weeks of life; the majority resolve spontaneously within infancy, and almost all by 2 years of age. Importantly, other airway pathologies may coexist in a small proportion of cases.
Inspiratory stridor is the characteristic feature. This is often harsh in quality and tends to be more pronounced during periods of excitement, distress, or feeding, accentuated in the supine position, and improved when prone or when asleep. Most cases are mild, or even subclinical, but some children will experience significant respiratory distress, tracheal tug, sternal recession, and, very occasionally, cyanosis. 3 Feeding may be disrupted. 4 Gastroesophageal reflux often coexists, although no causative relationship has been confirmed. Long-term sequelae in more severe cases include sternal deformity (pectus excavatum) and failure to thrive due to disruption to feeding, coexisting reflux, and increased work of breathing (noting that respiratory effort accounts for 15% of the healthy infant’s oxygen consumption and energy utilization).
Fig. 21.7 Laryngomalacia. Tight aryepiglottic folds resulting in a Ω-shaped epiglottis.
Management of Laryngomalacia
Mild cases are often managed in the primary care setting or by pediatricians, frequently without further investigations. 5 Regular reviews are required to ensure nonprogression and eventual resolution of symptoms, with careful monitoring of weight and height. Antireflux therapy (ranitidine or omeprazole plus domperidone) may or may not be indicated, and it is our preference to restrict medical treatment to those cases with obvious signs and symptoms of reflux.
Cases with sufficient symptoms to warrant otolaryngological referral should undergo flexible nasendoscopic assessment in clinic in the first instance. This allows an excellent dynamic view in most cases, demonstrating the aforementioned features. However, the subglottis and distal airway will not be seen in this way.
Severe cases, and certainly those with any suspicion of coexisting pathology, should undergo complete MLB under general anesthesia. 6 A full structural and dynamic assessment is always advisable, as milder forms of laryngomalacia are easily missed, particularly if the epiglottis is pulled forward strongly by the blade of the laryngoscope and/or if the child is too heavily anesthetized for adequate self-ventilation. It is usually possible to anesthetize children to allows spontaneous ventilation of sufficient intensity for a dynamic assessment, although this may be challenging in young infants, in whom awake flexible endoscopy may be more informative.
The cause of laryngomalacia is not certain. 7 Histological analysis has demonstrated edematous, redundant arytenoid mucosa, little or no aryepiglottic musculature, and loose mucosal lining of the epiglottis, findings that correlate well with macroscopic clinical assessment. Neuromotor immaturity leading to discoordinated activity of the vagal nuclei has also been suggested as a cause, accounting for arytenoid prolapse and the association with gastroesophageal reflux. It is certainly recognized that some children with underlying neurologic problems will also have problematic laryngomalacia that may persist for several years. Apart from the prolonged duration of symptoms, it should be borne in mind that these children generally have poorer outcomes with surgical treatment, highlighting the mixed structural and functional pathophysiology of this condition. 8
Once coexisting pathology has been excluded, monitoring as mentioned previously, is sufficient for the great majority of children, typically under the care of pediatricians. Reflux may be treated medically, if required. Parents should be reassured that the condition will resolve spontaneously with time, albeit more slowly in neurologic cases, in whom surgery is typically best avoided. 8
Surgery should be reserved for the most severe cases, and then only after serial clinical and/or endoscopic assessments have shown persistent problems, rather than at the first MLB.
Supraglottoplasty may take various forms and should be tailored to the individual findings. 9
Division of short aryepiglottic folds (aryepiglottoplasty; ▶ Fig. 21.8) and/or trimming of excessive arytenoid mucosa are typical options, although more unusual techniques have been described, including epiglottopexy.
We prefer cold steel dissection rather than power-assisted or hot techniques, where lateral thermal injury and scarring may occur.
Fig. 21.8 Same patient as in ▶ Fig. 21.7 following division of the aryepiglottic folds (aryepiglottoplasty).
Parents should be fully aware that the child’s stridor may persist despite surgery. Feeding may be disrupted, necessitating temporary nasogastric feeding and prolonged admission, and rarer complications such as bleeding, aspiration, 10 and even supraglottic stenosis may occasionally occur. Tracheostomy is virtually never required in cases of isolated laryngomalacia. Severe symptoms should alert the clinician to possible coexisting pathology.
Laryngoceles and Saccular Cysts
Both these anomalies are the results of dilatation of the saccule of the laryngeal ventricle. Congenital forms, which present in infancy, are much rarer than their acquired counterparts more typically seen in adults such as glass blowers who produce high intraluminal airway pressures. In the natural world, congenital saccular dilatation is most dramatically seen in howler monkeys, the loudest of all land animals. Laryngoceles communicate with the airway and typically have an air-filled lumen, whereas saccular cysts do not communicate with the airway and are fluid-filled.
Laryngoceles and saccular cysts may be difficult to diagnose and indeed to distinguish clinically, endoscopically, and even radiologically, particularly as laryngoceles may themselves become inflamed and fill with fluid or pus (laryngopyocele). Both may present with a rather variable history, most often with dysphonia and/or respiratory distress, made worse when crying, as a result of raised intraluminal pressure. The individual presentation will depend upon the size, site, and distensibility of the lesion.
Laryngoceles are classified as follows:
Internal (completely within the laryngeal cartilage).
External (piercing the thyrohyoid membrane).
Combined (with internal and external components).
Internal lesions are usually visible endoscopically, although they may be missed after collapse under general anesthesia. External lesions are usually suggested by parental history, but are not obvious endoscopically. Computed tomography (CT) may be helpful, but may also miss a collapsed laryngocele or confuse a fluid-filled one with a saccular cyst. 11
Saccular cysts ( ▶ Fig. 21.9) are thought to result from loss of patency between the laryngeal ventricle and saccule, resulting in a walled-off saccule that fills with fluid. They are classed as anterior (extending medially and posteriorly and protruding into the laryngeal airway between the true and false cords) or lateral (the commoner form in infants, which protrudes into the false cord and the aryepiglottic fold).
Treatment of these anomalies should be tailored to individual presentation and symptoms. Conservative management is certainly reasonable for external laryngoceles, although the propensity of internal laryngoceles and saccular cysts to become infected, increase in size, and cause airway embarrassment usually necessitates preemptive surgery.
Both types of lesion may be tackled endoscopically, particularly saccular cysts that can be widely marsupialized.
An open approach (midline, with full laryngofissure or lateral cervical with a “window” through the thyrohyoid membrane and thyroid ala) is an alternative for more extensive or recurrent lesions and allows complete excision of all secretory epithelium.
Fig. 21.9 Saccular cyst.
This is a rare cause of neonatal respiratory distress, which may or may not have been appreciated on antenatal scans (congenital high airway obstruction syndrome). It is seldom an isolated problem ( ▶ Fig. 21.10). Many cases will have associated glottic atresia and subglottic stenosis (SGS), as well as other congenital anomalies including esophageal atresia (OA), tracheoesophageal fistula (TOF), genitourinary, and skeletal anomalies.
Where the diagnosis is made antenatally, the baby may be best delivered by elective caesarean section, allowing ex utero, intrapartum treatment (EXIT) in the form of a tracheostomy. Where the diagnosis is not made antenatally, the baby may tolerate initial mask ventilation as a result of airflow down the esophagus and through a TOF ( see ▶ 28). 12 This affords some extra time to secure the airway, but it also offers a false sense of reassurance to the unwary, with little possibility of endotracheal intubation once the child inevitably deteriorates. A high index of suspicion and prompt tracheostomy are essential in these rare cases.
Fig. 21.10 Supraglottic atresia. This neonate was intubated with a size 2.5 tube. Note the absence of typical supraglottic anatomy. After initial tracheostomy, definitive reconstruction with laryngotracheal reconstruction and stents was undertaken at 6 months.
These are rare and are considered as part of the laryngeal atresia spectrum. They may be associated with systemic anomalies in approximately 10% of cases. 7 They may also be seen in the context of other laryngeal abnormalities, but sometimes occur in isolation (as opposed to the far commoner congenital glottic webs, which almost always involve concomitant SGS).
Management is tailored to individual circumstances. Tracheostomy may be required, but division of the web plus balloon dilatation is often effective. This technique uses an expandable balloon to radially dilate the web following an endoscopic incision. This is in contrast to glottic webs, where formal laryngotracheal reconstruction (LTR) is the surgical treatment of choice, allowing separation of the cords and simultaneous expansion of the subglottis.
Epiglottic Anomalies: Absent or Bifid Epiglottis
These anomalies are also very rare. The epiglottis develops from the third and fourth branchial arches, therefore epiglottic agenesis is almost always associated with other major laryngeal abnormalities including glottic stenosis. Neonatal respiratory distress is likely and urgent tracheostomy is usually required.
A bifid epiglottis ( ▶ Fig. 21.11) occurs as a consequence of failure of fusion in the midline, resulting in a cleft that extends to just above the epiglottic tubercle. Although few isolated cases are reported in the literature, it has been noted in Pallister–Hall’s syndrome (polydactyly, hypothalamic hamartoma, laryngeal cleft, renal dysgenesis, imperforate anus). Presentation may be mistaken for that of laryngomalacia, with stridor and respiratory distress during feeds, although cyanotic spells and aspiration are also often seen, which should alert the clinician to an alternative diagnosis. Treatment will depend upon symptom severity. Tracheostomy may be necessary in the first instance. Amputation of the epiglottis may subsequently improve the airway, although chronic aspiration may result.
Fig. 21.11 Bifid epiglottis.