Anatomy of Direct Laryngoscopy

James Snyder

Steve Orebaugh

ORIENTATION

X-rays in supine and sniffing positions with corresponding illustrations help convey anatomy dynamics as well as details (Fig. 1-1). Direct laryngoscopy (DL) requires displacement above a line of sight (LOS) between the upper teeth to the glottis, of the hyoid bone, tongue, and epiglottis. Hyoid bone movement forward depends on adequate slack in the stylohyoid ligament (SHL). The tongue when flaccid acts likes a viscous mass. Control of the epiglottis depends on the type of blade used.

ANATOMY OF THE MOUTH AND TONGUE

The most obvious oral impediments are the teeth and tongue (Fig. 1-2). The tonsillar pillars and fauces are visible on either side as they course from the soft palate to the base of the tongue, in effect creating a tubular inlet to the oropharynx.

Laryngoscopy from the right corner of the mouth can bypass the bulk of the tongue, shorten the distance to the larynx, and lower (improve) the angle of approach to the glottis, and has been successful in many cases where alternatives failed. Practice is required to maintain orientation.

The Dorsal Tongue

The posterior third of the dorsum of the tongue looks backward and contains numerous submucous adenoid collections and lymph follicles called the lingual tonsil. Hypertrophy of the lingual tonsil has been described as a common and important cause of unpredicted difficult intubation where the epiglottis cannot be lifted from a dorsal approach over the tongue (Fig. 1-3).¹ Hypertrophic lingual tonsils impair tongue displacement and are prone to bleed with minimal trauma. The potential for hemorrhage should encourage early consideration of paraglottic straight blade technique.²

The Tongue is a Dome of Muscle

The tongue is rooted in and rises up from two roughly concentric U-shaped bones, the mandible and hyoid (Fig. 1-4). The bulk of the tongue is the intrinsic genioglossus muscle, which arises from the mandible anteriorly and extends fibers to the hyoid bone posterior (Fig. 1-4A). Laterally the tongue is secured to the hyoid by a vertical sheet of muscle, the hyoglossus (Fig. 1-4B). The floor of the mouth is predominantly formed by the mylohyoid muscle, which slopes from attachments around the mandible to form a midline raphe anteriorly and to the hyoid bone posterior (Fig. 1-4C). This arrangement is comparable to the levator ani that forms the floor of the pelvis.³

Muscle Tone and the “Peardrop” Phenomenon

Tongue flaccidity and adhesion of the blade to the tongue complicate finding and controlling the epiglottis. Lifting the blade before each advance can break adhesion. Steps are kept small to avoid bypassing the epiglottis. Loss of lingual muscle tone requires additional attention. The tongue has a globular shape when the mouth is closed and muscle tone is normal (Fig. 1-4A). With loss of muscle tone, the tongue is easily distorted dorsally by contact with the blade as it is being inserted, and the tongue and epiglottis are readily pressed against the posterior pharyngeal wall (Fig. 1-5).

In code situations, the tongue may be found “slurred” up against the palate as well as the posterior pharyngeal wall, impairing bag-mask-ventilation (BMV) as well as laryngoscopy. Passing the blade between the blade and palate can cause trauma to soft tissues. A gauze pad enables grasp of the flaccid tongue by its broad tip, to be pulled forward and then maintained forward by continuous positive airway pressure with BMV or by the laryngoscopy blade.

Larynx and Epiglottis

The larynx is a 4 cm long structure below an almost 2 cm inlet (Fig. 1-6). It overlies the 4th, 5th, and 6th cervical vertebra in adult males, higher in females and children. It is suspended from the hyoid bone via a flexible sheet of ligament, the thyrohyoid membrane (Fig. 1-6). The hyoid bone is secured dorsally to the skull via the SHL. The hyoid and thyroid cartilage are open-ring structures that form the anterior wall of the lower pharynx. The laryngeal skeleton can swing well forward of the posterior pharyngeal wall, because it is secured to the somatic skeleton primarily by the SHL above and the cricopharyngeus muscle at C6. The upper esophageal sphincter is formed from fibers of the cricopharyngeus muscle, which extends from one side of the cricoid arch to the other; the cricoid cartilage is secured to the posterior pharyngeal wall by the tone of the cricopharyngeus muscle.³,⁴

FIGURE 1-1 X-ray laryngoscopy supine and during DL in sniffing position (aligned at dorsal C6), and corresponding illustrations. A: In the neutral position, the hyoid bone (HB) is dorsal to the thyroid cartilage (TC) and therefore also to the glottis. The SHL functions as a cable by which the larynx is suspended from the styloid process (SP), which can be seen in A, just behind the anterior arch of the atlas. The posterior extensions of the hyoid bone and lateral walls of the thyroid cartilage abut the posterior pharyngeal wall. B: During DL in the sniffing position (head elevation and a-o extension), the entire larynx is rotated forward. Both the hyoid and thyroid cartilage are lifted forward from the pharyngeal wall. In particular, notice the hyoid is lifted anterior to the glottis (compare C and D). Because LOS requires the hyoid forward of the thyroid cartilage, it appears that release of tension in the SHL may be a mechanism by which head elevation facilitates DL. (A modified from Fuller MJ http://www.wikiradiography. com/page/Lateral+Soft+Tissue+Neck+for+Foreign+Body, Case #1with permission;B modified from Nishikawa K, Yamada K, Sakamoto A. A new curved laryngoscope blade for routine and difficult tracheal intubation. Anesth Analg. 2008;107:1248-52 with permission.)

FIGURE 1-2 View into mouth.A: The anterior and posterior tonsillar pillars and palatine tonsil form an isthmus between mouth and pharynx. Note the open space cephalad, under the palate. B: Laryngoscopy with a curved blade usually is over the right dorsum of the tongue, directed to contact its broad leading edge against the midline fold of the HEL to “flip” the epiglottis up against the blade (M).²⁴ Laryngoscopy from the right corner of the mouth and along the base of the tongue (paraglossal) with the straight blade often enables glottal view not possible with a curved blade, as it bypasses the bulk of the tongue, shortens the distance to the larynx, and improves the angle of approach to the glottis (P). See chapter on Direct Laryngoscopy. (B revised from Netter FH. Atlas of Human Anatomy. 4th ed. Philadelphia, PA: Saunders/Elsevier; 2006 with permission.).

FIGURE 1-3 Hypertrophic lingual tonsils (in the top 1/3 to ½ of each example above) can extend to or cover the tip and lateral edges of the epiglottis (in the center). Lower part of each image is the soft palate. Manipulation of the friable tissue can cause copious bleeding (Modified from Ovassapian A, Glassenberg R, Randel Gl, The unexpected difficult airway and lingual tonsil hyperplasia: a case series and a review of the literature. Anesthesiology. 2002;97(1):124-132 with permission.)

FIGURE 1-4 Muscles of the tongue. The hyoid normally is palpable at the junction of the neck and chin (A). DL in the midline requires the tongue, epiglottis and hyoid displaced (arrows) across a line from the teeth to the glottis (dashed line). Although limited by dentition and elasticity of the mouth, blade insertion from the corner of the mouth may provide a better angle (dotted line). A-o extension (compare A and B) lengthens the space into which the tongue is displaced and stretches the tissues of the submandibular space. Displacement of the tongue requires stretch of the floor of the mouth (anterolateral mylohyoid (C, D, E) and midline geniohyoid (A, B), and of the suspending elements: the palatoglossus, styloglossus, stylohyoid, digastric muscles, and tenses the SHL. Ease of passing the endotracheal tube from the right side of the mouth or adjacent to the base of the tongue (paraglossal) is strongly influenced by whether molars are present (E).

FIGURE 1-5 X-ray laryngoscopy: normal vs. the “peardrop phenomenon”. A: The diagram shows the expected position of the laryngoscope blade (shaded) relative to the tongue (cross-hatched) in the sniffing position. Tongue is shown dorsal to the blade because the blade displaces the tongue laterally as well as forward. The blade tip reaches to just behind the hyoid. The dashed line represents the anterior delineation of the tongue. B: The “peardrop” phenomenon. The tongue has “slurred” dorsally, to a shape called peardrop by Horton et al.²³ The blade tip is beyond the hyoid and held well back from it by the tongue, which is trapped by the blade. C: X-ray laryngoscopy of the peardrop phenomenon. (A modified from Horton A, Fahy L, Charters P. Factor analysis in difficult tracheal intubation: laryngoscopy-induced airway obstruction. Br J Anaesth.

Only gold members can continue reading. Log In or Register to continue