LARYNX
The larynx connects the pharynx to the trachea, and its anterior upper limit is represented, at the level of the base of the tongue, by the hyoid bone, which is a mobile, crescent-shaped bone. Notably, the hyoid bone is the first bone to ossify (4). The larynx is conventionally divided into three regions relative to the level of the vocal cords: i) the supraglottic region , which includes the epiglottis, arytenoids, aryepiglottic folds, and false vocal folds; ii) theglottic region , which includes the vocal folds and the region immediately below them; and iii) the subglottic region , which refers to the region starting 1 cm below the vocal folds, ending in the tracheal opening and partly corresponding to the cricoid cartilage, which is the only complete cartilage ring in the respiratory tract (4). During childhood, the larynx is located in a more cephalic and anterior position than it is during adulthood: in newborns and children up to 2 years of age, the lower limit of the cricoid cartilage lies at the level of the fourth cervical vertebra; at 6 years of age, it lies at the level of the fifth cervical vertebra, and in adulthood, it lies at the level of the sixth cervical vertebra (Figure 1 - 2) (4, 9, 10). Due to increased acute angulation on the lower levels, visualization of the vocal cords by laryngoscopy is particularly difficult in infants and toddlers (4, 9). The laryngoscopic view in younger children is also hampered by the epiglottis, which appears larger, longer and omega (Ω)-shaped. Moreover, the epiglottis lies more horizontally than it does in adults (45-degree vs 20-degree angle from the anterior pharyngeal wall). Due to the higher location of the larynx, the tip of the epiglottis almost reaches the soft palate (11): this anatomical conformation allows the infant to breathe and suckle simultaneously without aspirating due to the intermittent pression of the tongue on the soft palate (the so-called “veloglossal sphincter ”) (12, 13) and explains why infants have long been considered “obligate nose breathers ” up to age 2 - 6 months, when they improve their ability to breath by mouth apart from crying (13-15). Actually, it is not possible to infer from the literature how many infants are able to switch to oral breathing and at what age, so the term ”preferential nasal breather ” should be preferred, being more accurate (16, 17). In children, vocal cords extend posteriorly and superiorly so that they are not angled at almost 90 degrees to the trachea as they are in adults, further hampering the laryngoscopic view. Additionally, they are not linear but concave because the vocal process of the arytenoid cartilage is inclined inferiorly and medially. Unsurprisingly, articulated speech becomes possible when the larynx starts to descend (approximately around the second year of life) (12). The description of the shape and diameters of the developing larynx is one of the most debated subjects in the literature (18, 19): the paediatric larynx has been considered funnel-shaped for a long time, with the narrowest point being at the cricoid ring until 6-8 years of age, as opposed to the adult larynx, which is cylindrical and has the narrowest point localized at the glottis opening. However, it should be emphasized that this description came from anatomical measurements of the glottic and subglottic areas in cadaveric studies, particularly from the work by Bayeux (20) and Eckenhoff (11), so the data might have been biased due to post-mortem tissue muscle tonicity loss in the portions of the larynx not supported by full circumferential cartilage. As a matter of fact, this hypothesis has recently been reconsidered in some in vivo studies: Littman and Dalal, using MR and video-bronchoscopic imaging, respectively, have shown that the paediatric larynx is conical in the transverse dimension (with the apex of the cone at the vocal cords, which is the narrowest point) and cylindrical in the anteroposterior dimension (21, 22). However, since CT is considered the gold standard for measuring the inner portions of the larynx and trachea due its spatial resolution and discrimination of the air/tissue interface (23, 24), this technique has been used in subsequent studies by Wani et al., which specified that the narrowest part of the paediatric larynx is located in the subglottic area (19). Specifically, these studies have clarified that the narrowest portion of the larynx in the transverse dimension is localized at the subglottic level, and in the anteroposterior dimension, it is localized at the cricoid level. However, since the subglottic area is relatively elastic and distensible, unlike the cricoid ring, which is the only circumferential complete non-distensible structure in the airways, the latter still functionally represents the narrowest portion of the paediatric larynx (23-25). More recently, Luscan et al. performed 23 measurements in the airways, from the larynx to the main bronchi, by using HRCT on 192 healthy children aged from 1 day to 14 years, providing new data suggesting that the paediatric upper airways and the main bronchi show a rapid growth phase during the first 3 years of life, followed by a slow growth phase and a second rapid growth phase during adolescence (26), similar to what has been described for height development (27). These data question the long-believed hypothesis on linear airway growth. The geometry of the cross-section of the larynx is another particularly important topic: since the studies by Littman and Dalal (21, 22) demonstrated that the subglottic region is more elliptical than circular in shape, as previously suggested by Bayeux, with a greater anterior-posterior dimension than transverse dimension, many physicians are shifting from the routine use of non-cuffed endotracheal tubes, previously preferred to avoid the risk of ischaemic damage to the mucosa at the cricoid level (28, 29), to increased use of cuffed endotracheal tubes, even in infants and younger children (30). Nonetheless, in the study by Luscan et al., the cricoid was described as having a round shape regardless of the child’s age, with a diameter smaller than the anteroposterior diameter of the glottic area (26). As far as consistency, the paediatric larynx, except for the hyoid bone, is composed of non-ossified cartilaginous structures; hence, the whole structure is softer and more pliable than it is in adults, increasing the risk of airway obstruction. Complete calcification of the components of the larynx and trachea occurs in the teenage years (9).