Introduction

The larynx is a complex constricting and dilating gateway to the trachea. The three primary functions of the larynx are (1) protection of the airway, (2) respiration, and (3) phonation. Laryngeal closure also allows the patient to build up intra-thoracic pressure (the Valsalva maneuver) prior to coughing. It is essential for physicians performing diagnostic or treatment procedures in or through the upper aerodigestive tract to be familiar with the larynx. The purpose of this chapter is twofold: (1) to discuss laryngeal anatomy and function, and (2) to suggest an approach to laryngeal examination.

Anatomy

Although it sits on top of the trachea, the larynx is suspended from the sternum, clavicles, skull base, mandible, and anterior vertebral column by a group of extrinsic muscles. Its skeleton is a series of cartilages held together by ligaments, elastic membranes, and the intrinsic laryngeal muscles.

Cartilages

The cricoid cartilage forms the base of the larynx. Although the larynx is a tubular structure, the cricoid is the only complete cartilaginous ring in the laryngeal skeleton. Thus, injury to the larynx in the region of the cricoid ring from trauma, tumor, or iatrogenic causes may quickly lead to laryngeal collapse and airway obstruction. The cricoid has a signet ring shape and is much taller posteriorly (20–30 mm) than anteriorly (5–7 mm). While the anterior cricoid is palpable below the thyroid cartilage during neck examination, the posterior cricoid rises past the inferior level of the thyroid cartilage (Figure 3.1).

Figure 3.1 The larynx in cross section.

Posteriorly, the cricoid supports the arytenoids: paired, lightweight pyramid-shaped cartilages that are the mobile posterior attachment point for the vocal cords (Figure 3.2). The cricoarytenoid articulation is a synovial joint that allows the arytenoids to move sideways as well as to slide downward toward the midline. In combination, these movements create the vocal cord abduction (outward movement away from midline) and adduction (inward movement towards midline) essential for proper laryngeal function. Like any other synovial joint, the cricoaryntenoid joint is subject to a wide spectrum of pathology including arthritis (infectious or inflammatory), ankylosis (fixation), and dislocation, which can impede vocal cord mobility. The corniculate cartilages sit atop the arytenoids and project posteromedially. During vocal cord adduction, they approach each other in the midline and exert laterally directed elastic recoil on the posterior vocal cords. The cuneiform cartilages are suspended in the aryepiglottic fold and are often visible as raised white masses during endoscopy, sometimes mistaken for malignancy or a submucosal cyst.

Figure 3.2 The larynx viewed from above.

The thyroid cartilage is the largest and most prominent laryngeal cartilage. It is shaped like a shield and appropriately named (thyrus = shield, Greek) as it protects the internal anatomy of the larynx. The thyroid cartilage has paired superior and inferior horns, or cornu. The superior cornu have ligamentous attachments to the hyoid above, an important part of the laryngeal suspension. The inferior cornu articulate with the lateral cricoid cartilage. This cricothyroid joint is also a synovial joint that allows anteriorposterior rocking of the thyroid cartilage on the cricoid; therefore, it is also susceptible to the pathologies mentioned earlier.

The epiglottis contains the leaf-shaped epiglottic cartilage that is suspended anteriorly from the hyoid and inferiorly from the thyroid by a series of ligaments. As the larynx tips anterosuperiorly during swallowing, the epiglottic base is pushed against the tongue base causing the epiglottis to fold over the laryngeal inlet. Although the hyoid is not typically considered part of the larynx, it is intimately associated with the superior cartilages and important in the laryngeal suspension. The hyoid, thyroid, cricoid, and arytenoids are composed of hyaline cartilage and progressively ossify. The hyoid is completely ossified by age two; the thyroid begins ossifying in the early twenties, and the cricoid and arytenoids follow shortly thereafter.

Membranes

There are two important submucosal elastic membranes in the larynx: the quadrangular membrane superiorly and the conus elasticus inferiorly. The quadrangular membrane joins the lateral epiglottis, the arytenoids, and corniculate cartilages. The inferior border joins the vestibular ligament above the vocal fold to form the “false vocal cord” while the superior border and its membranous covering form the aryepiglottic fold. The conus elasticus joins the superior border of the cricoid to the thyroid and arytenoid cartilages. The superior edge of the conus elasticus runs between the midline posterior thyroid cartilage (anterior commissure) and the vocal process of the arytenoids. This free edge is the vocal ligament, the elastic component of the true vocal cord.

Muscles

The intrinsic muscles of the larynx cause the configuration changes in the laryngeal cartilages that open or close and loosen or tighten the vocal cords. The paired thyroarytenoid, lateral cricoarytenoid, and unpaired interartenoid muscles adduct the vocal cords while the paired posterior cricoarytenoid muscles are the only abductors. The paired cricothyroid muscles tip the thyroid cartilage anteriorly on the cricoid, tightening the vocal cords further.

The extrinsic muscles of the larynx suspend it in the soft tissues of the neck. The laryngeal elevators raise and anteriorly displace the larynx during swallowing to prevent aspiration. These include the stylohyoid, digastric, and stylopharyngeus from the skull base and the geniohyoid and mylohyoid from the mandible. The elevating forces exerted on the hyoid are transmitted to the larynx by the thyrohyoid muscle and ligaments. The laryngeal depressors displace the larynx downward during inspiration and include the remaining strap muscles – omohyoid, sternohyoid, and sternothyroid. The esophageal constrictors and cricopharyngeus insert on the posterior larynx and are critical to the swallowing reflex.

Mucosa

The larynx is lined by two different types of mucosa. The free edges of the true vocal cords, upper edges of the aryepiglottic folds and superior epiglottis are covered in stratified squamous epithelium while the remainder of the larynx is covered in pseudo-stratified ciliated columnar respiratory epithelium. The mucosa is a continuous sheet as it wraps over top of the epiglottis and reflects onto the tongue base, forming the vallecula. A layer of loose connective tissue is immediately deep to the mucosa in the bulk of the larynx. However, this layer of connective tissue is absent on the laryngeal (posterior) surface of the epiglottis which accounts for the marked anterior epiglottic swelling that can be seen in epiglottitis or supraglottitis.

The submucosal layers of the vocal cord are completely distinct from the remainder of the larynx, and deserve special attention. The vocal cord consists of three main layers: the stratified squamous cover, the trilamellar lamina propria, and the vocalis muscle. The superficial layer of the lamina propria is a very loose connective tissue that has the consistency of a soft gelatin. This is the primary layer in which vibration during phonation occurs and is also the layer most likely to become filled with edematous fluid due to its loose configuration. The intermediate and deep layers of the lamina propria consist of elastic and collagen fibers, respectively. These layers form the vocal ligament, and are the termination of the conus elasticus.