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).