Whats special in a child's larynx?

What's special in a child's larynx? Many of us know only a few specialties of the pediatric larynx, but there are much more features, which are unique and often not highlighted. To understand the pediatric larynx, we have reviewed the development, the functions in-utero and new born period and peculiarities.

There are certain modifications, which are seen in the pediatric larynx and like the brain this remarkable organ undergoes extensive modification in the structure as well as the function to attain the attire of an adult larynx. These unique modifications in the functional anatomy will not be clear unless we study the development from utero and also compare it with the adult larynx. This approach gives us a wholesome knowledge about the different aspects of development. Knowledge about these variations in age group is extremely important in clinical aspects like intubation, direct, indirect laryngosopy, tracheostomy, interpreting radiological images based on anatomical landmarks and also fewer congenital anomalies [Figure 1].[12]

Figure 1

Cross section of both the adult and paediatric larynx

Development of Larynx

The development of the larynx was first described by the pioneering work of Soulie and Bardier,[3] Lisser,[4] Hast.[5] The development takes origin from the median pharyngeal groove. The differentiation of development takes place at about 25 days from the ventromedial diverticulum of the forgut called as tracheobronchial groove. This leads to the fact that the larynx is also lined with the lining as the gut. Lateral furrows develop on either side of the venteromedial diverticulum which later joins to form the tracheo esophageal septum.[6] At about 33 days, laryngeal primordial appear. The laryngeal aditus depends on the relation with the primordium of the epiglottis anteriorly and the precursors of the arytenoids cartilages on either side. At 5–6 weeks, the tracheooesophageal septum reaches the first tracheal cartilage. At 7th week the criciod is a complete ring, hyoid is seen below the epiglottis. In the later stages, there is only fine tuning in this structure and the acquirement of neurogenic reflexes.[6] In 3rd month, the thyroid laminae fuse, cartilaginous process of the arytenoids are seen, along with the ventricle and saccule. Fetal breathing can be seen in ultrasonogram by the 3rd month.[6] In the 4th month goblet cells are seen in the sub mucosa. During 5th–6th month the cuneiform and corniculate cartilages develop along with the development of a mature laryngeal coordination[6] with swallowing, breathing. During the third trimester, it gets ready to perform the uphill tasks which are laid for it after birth.

Functions In-utero

Primitive swallowing ability develops by 11 weeks, and by 18–20 weeks sucking movements appear. When full-term gestation (37 completed weeks) is reached, the fetus can swallow and circulate nearly 500 mL of amniotic fluid per day. Thus, swallow-induced peristaltic activity begins in fetal life, and glottal airway and pharyngoesophageal functions are arranged in series. Thus, the fetal larynx prevents aspiration under normal conditions.[7]

At the First Cry

In-utero, the main function of the larynx was just to maintain respiration, but soon after its first cry it has to perform three contradictory tasks of maintain respiration, protect the lower airway from contamination and produce the first cry.[6] To perform these functions, there are certain unique modifications. The pharynx is vertically short, and it lies in the cervical area. The inferior margin of the cricoids cartilage lies at C4 vertebra level, and the tip of the epiglottis lies at C1 level. The opposition of the epiglottis and soft palate helps to perform the dual function of respiration and sucking, which helps in obligate nasal breathing of the child. The framework of the larynx is more vertical when compared to the adult larynx. The thyroid cartilage is within the hyoid arch and inferior to it. The vocal cords are more transverse when compared to the adults, the epiglottis is short and the aryepiglottic folds are thick and bulky along with comparatively large arytenoids. The larynx is well supported by soft tissue which helps in prolonged intubation and prevents sub-glottic injury.[6] The trachea of new born tends to collapse as its compliance is 3 times that of a 1-year-old and 6 times of an adult. The resistance to prevent collapse is partially due its cartilages and smooth muscle of the posterior wall.[8]

Reflexes Learnt for Life

There are some vital reflexes which are acquired by the pediatric laryngeal complex which are:

Vocal cord closure (laryngeal adductor reflex)

Glottic dynamics encompasses three tiers of laryngeal closure during the act of swallowing.

Tire 1: Closure of the vocal cords is the initial action and anatomically the most caudal portion of the tiers, and the duration of vocal cords closure has been reported to be ~1.7 s[9]

Tire 2: Laryngeal elevation follows in the majority of instances with an upward and forward movement; the vocal cords start to close during this time in a minority of instances

Tire 3: It consists of the sphincteric function of the glottis, which occurs due to laryngeal muscle activation and completes laryngeal closure.[7]

Esophago-glottal closure reflex

The esophago-glottal closure reflex (EGCR) is most likely the primary airway protective mechanism during retrograde transit of gastric contents associated with belching, regurgitation, vomiting, and acid reflux events. When a large volume of gatric contents reflux into the esophagus, the esophagus becomes distended at various levels that in turn stimulates the EGCR that causes reflex adduction of the vocal cords to prevent the entry into the trachea.[7]

The response time for esophageal reflexes was 3.8 ± 1.8 s.[10]

Pharyngoglottal closure reflex

Injection of small amounts of water into the pharynx induces brief vocal cord closure. Slow introduction of graded amounts of fluid into the pharynx causes partial adduction of the vocal cords while rapid injection of water causes complete closure of the vocal cords.[1011] Frequency, response latency, and duration of glottal closure with spontaneous swallows were 100%, 0.27 ± 0.1 s, and 1 ± 0.22 s, respectively.

The frequency, response latency, and duration of glottal closure with pharyngoglottal closure reflex were 100%, 0.56 ± 0.13 s, and 0.52 ± 0.1 s, respectively.[12]

Whats the Radiolosgists View

A study where computed tomography, magnetic resonance imaging of the pediatric larynx concluded that pediatric larynx differs from the adult larynx in four aspects: Consistency, size, position, and shape. Except for the hyoid bone, the laryngeal cartilaginous structures are not ossified in the pediatric larynx and appear featureless. This may mislead an inexperienced radiologist to think that it is a tumor. The airway is narrowest at the infraglottic portion. The supraglottic airway is triangular and oval, the glottic level has a teardrop shape, and the sub-glottic level is oval. The pediatric larynx is positioned higher in the neck, at the C2–C3 level, and is seen behind the mandible.[13]

Whats the Histologists Contribution

In new born and children, the membranous portion of the vocal fold makes less contribution to the total vocal fold length. The vocal fold mucosa is thinner in new born and young children, the ratio of mucosa to the length of the membranous portion of vocal fold is higher when compared to the adults. The different layers in the structure are not clearly seen in new born and young children. There is no ligamentous structure seen in the pediatric vocal fold and the first appearance is seen at 1–4 years after which it is clearly visible. It is also seen that there is a greater percentage of collagen in the pediatric vocal fold muscle and it is deficient in the anterior and posterior macula flava fibers which in turn leads to less anchoring strength to the laryngeal architecture[14] [Figures 2 and 3].[2]

Figure 2

Posterior view of both adult and pediatric larynx

Figure 3

Anterior view of both adult and pediatric larynx

Features Seen in Laryngoscopy

The larynx is displaced anteriorly, highly positioned, and the arytenoids are prominent and the membranous portion of the vocal fold is short. It lies in the first and third cervical level and continues to descend throughout childhood till adult level of sixth and seventh cervical vertebrae.[2] Epiglottis is omega shaped in 50% of the population.[15] New born glottis is 7 mm in anteroposterior diameter and 4 mm in transverse diameter.[6] The subglottis is the narrowest part of the airway that is around 4–5 mm when compared to the glottis that is the narrowest part in adults.[6] There is no vertical laryngeal prominence (Adam's apple) seen in the thyroid cartilage and does not occur till substantial changes happen in vocal fold at around 10–14 years.[16]

Infantile Reflex

After birth closure reflex is seen, this prevents aspiration of food contents into the lungs. The infantile glottis also closes in response to tactile, thermal, chemical stimulation of the laryngeal inlet, trachea or distal esophageal afferents. The larynx must open after the stimulus is withdrawn if not it is called as laryngospasm.[17] This has been postulated in the origin of sudden infant death syndrome.[1819]

Evolution during Childhood

In new born, the length of the vocal cord is 2.5–3.0 mm, and it grows throughout childhood. The difference in vocal cord length is seen around 10–14 years of age when it reaches adult female 11–15 mm and male 17–21 mm.[14] The position, structure and function of the larynx changes throughout childhood. By two years the lower border of the cricoids has descended to the neck at about the C5 vertebra, and it reaches C6 by 5 years of age. The final position of C6–C7 is reached by 15 years.[20] The overlapping thyroid cartilages separate. The pattern of growth is

The various aspects of the laryngeal growth is linear, that is, same in all aspects. Thus the configuration of the larynx is maintained to a great extent. Later the epiglottis increases in curvature till 3 years then flattens. About 60–75% of vocal fold length is due to the vocal process of the arytenoids at birth. At 3 years, the membranous part of vocal fold is the dominant size.[21] All through child hood the level of true vocal folds is halfway between the thyroid notch and the lower border of the thyroid cartilage. The upper border of the cricoid maintains an angle of 30 degree with the true vocal cords[22] [Figure 4].[4]

Figure 4

Shape of both adult and pediatric larynx

Changes in Vocal Spectrum

At birth, the pitch of the new born cry is 500 Hz and gradually as the child grows there is a gradual drop in the pitch and reaches 286 Hz by 7 years of age. Till the puberty, the vocal features are similar to both boys and girls only during puberty the change in voice depends on the sex of the child[6] [Figure 5].

Figure 5

Vocal frequency as the child grows

Clinical Application

The trachea, pharyngeal and laryngeal inlet must be aligned. The back of the head of the infant should be slightly elevated to help in this alignment. Too much flexion or improper elevation may cause difficulty in passing the scope.[2] For laryngoscopy, the flexible scope must be around 2.7–3.0 mm. Other important procedures, which involve adequate care and knowledge, are foreign body removal and tracheostomy in children.

Conclusion

We have reviewed the various aspects from the development of the larynx, morphology at birth to the changes it acquires during child hood. The clinical applications which are based on these concepts are very remunerating for many departments such as the neonatology, pediatrics, emergency medicine, anesthesiology and ENT. If proper knowledge of these aspects is not there, we have many chances of iatrogenic injury of many structures and cause long term morbidity. It is also recommended to stress about the pediatric anatomy of the neck along with simulation classes for training to prevent many iatrogenic complications.