Larynx

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CHAPTER 34 Larynx

The larynx is an air passage, a sphincter and an organ of phonation, and extends from the tongue to the trachea. It projects ventrally between the great vessels of the neck and is covered anteriorly by skin, fasciae and the hyoid depressor muscles. Above, it opens into the laryngopharynx and forms its anterior wall; below, it continues into the trachea (see Fig. 33.2). It is mobile on deglutition. At rest, the larynx lies opposite the third to sixth cervical vertebrae in adult males; it is somewhat higher in children and adult females. In infants between 6 and 12 months, the tip of the epiglottis (the highest part of the larynx) lies a little above the junction of the dens and body of the axis vertebra. Until puberty, male and female larynges are similar in size. After puberty, the male larynx enlarges considerably in comparison with that of the female: all the cartilages increase in both size and weight, the thyroid cartilage projects in the anterior midline of the neck, and its sagittal diameter nearly doubles. The male thyroid cartilage continues to increase in size until 40 years of age, after which no further growth occurs.

SKELETON OF THE LARYNX

The skeletal framework of the larynx is formed by a series of cartilages interconnected by ligaments and fibrous membranes, and moved by a number of muscles (Figs 34.134.3). The hyoid bone is attached to the larynx: it is usually regarded as a separate structure with distinctive functional roles, and is described on page 436. The laryngeal cartilages are the single thyroid, cricoid and epiglottic cartilages, and the paired arytenoid, cuneiform, corniculate and tritiate cartilages.

In relation to the surface anatomy of the larynx, the levels of the laryngeal cartilages worth noting are: C3 (level of body of hyoid and its greater cornu); C3–4 junction (level of upper border of thyroid cartilage and bifurcation of common carotid artery); C4–5 junction (level of thyroid cartilage); C6 (level of cricoid cartilage).

The corniculate, cuneiform, tritiate and epiglottic cartilages and the apices of the arytenoid are composed of elastic fibrocartilage, with little tendency to calcify. The thyroid, cricoid and the greater part of the arytenoid cartilages consist of hyaline cartilage and may undergo mottled calcification as age advances, starting about the 25th year in the thyroid cartilage and somewhat later in the cricoid and arytenoids. By the 65th year, these cartilages commonly appear patchily dense in radiographs.

EPIGLOTTIS

The epiglottis is a thin leaf-like plate of elastic fibrocartilage which projects obliquely upwards behind the tongue and hyoid body, and in front of the laryngeal inlet (Figs 34.2, 34.3; see Fig. 34.5). Its free end, which is broad and round, and occasionally notched in the midline, is directed upwards. Its attached part, or stalk (petiolus), is long and narrow and is connected by the elastic thyroepiglottic ligament to the back of the laryngeal prominence of the thyroid cartilage just below the thyroid notch. Its sides are attached to the arytenoid cartilages by aryepiglottic folds (which contain the aryepiglottic muscle). Its free upper anterior, or lingual, surface is covered by mucosa (the epithelium is non-keratinized stratified squamous), which is reflected onto the pharyngeal aspect of the tongue and the lateral pharyngeal walls as a median glossoepiglottic, and two lateral glossoepiglottic, folds. There is a depression, the vallecula, on each side of the median fold. The lower part of its anterior surface, behind the hyoid bone and thyrohyoid membrane, is connected to the upper border of the hyoid by an elastic hyoepiglottic ligament, and separated from the thyrohyoid membrane by adipose tissue, which constitutes the clinically important preepiglottic space. The smooth posterior, or laryngeal, surface is transversely concave and vertically concavo-convex, and is covered by ciliated respiratory mucosa: its lower projecting part is called the tubercle. This surface forms the oblique anterior wall of the laryngeal vestibule. The cartilage is posteriorly pitted by small mucous glands (Fig. 34.3D) and is perforated by branches of the internal laryngeal nerve and fibrous tissue, which means that the posterior surface of the epiglottis is in continuity through these perforations with the pre-epiglottic space.

THYROID CARTILAGE

The thyroid cartilage is the largest of the laryngeal cartilages (Figs 34.134.3). It consists of two quadrilateral laminae with anterior borders that fuse along their inferior two-thirds at a median angle to form the subcutaneous laryngeal prominence (‘Adam’s apple’). This projection is most distinct at its upper end, and is well marked in men but scarcely visible in women. Above, the laminae are separated by a V-shaped superior thyroid notch or incisure. Posteriorly, the laminae diverge, and their posterior borders are prolonged as slender horns, the superior and inferior cornua. A shallow ridge, the oblique line, curves downwards and forwards on the external surface of each lamina: it runs from the superior thyroid tubercle lying a little anterior to the root of the superior cornu, to the inferior thyroid tubercle on the inferior border of the lamina. Sternothyroid, thyrohyoid and thyropharyngeus (part of the inferior pharyngeal constrictor) are attached to the oblique line, usually as little more than a tendon (Fig. 34.3A).

The internal surface of the lamina is smooth. Above and behind, it is slightly concave and covered by mucosa. The thyroepiglottic ligament, the paired vestibular and vocal ligaments, the thyroarytenoid, thyroepiglottic and vocalis muscles, and the stalk of the epiglottis are all attached to the inner surface of the cartilage, in the angle between the laminae. The true vocal folds lie 6–9 mm below the median thyroid notch. The superior border of each lamina is concave behind and convex in front, and the thyrohyoid membrane is attached along this edge (Figs 34.1, 34.2). The inferior border of each lamina is concave behind and nearly straight in front, and the two parts are separated by the inferior thyroid tubercle. Anteriorly, the thyroid cartilage is connected to the cricoid cartilage by the anterior (median) cricothyroid ligament, which is a thickened portion of the cricothyroid membrane.

The anterior border of each thyroid lamina fuses with its partner at an angle of approximately 90° in men and approximately 120° in women. The shallower angle in men is associated with the larger laryngeal prominence, the greater length of the vocal cords, and the resultant deeper pitch of the voice. The posterior border is thick and rounded and receives fibres of stylopharyngeus and palatopharyngeus. The superior cornu, which is long and narrow, curves upwards, backwards and medially, and ends in a conical apex to which the lateral thyrohyoid ligament is attached. The inferior cornu is short and thick, and curves down and slightly anteromedially. On the medial surface of its lower end there is a small oval facet for articulation with the side of the cricoid cartilage: this facet is variable and is well defined only sometimes.

During infancy, a narrow, rhomboidal, flexible strip, the intra-thyroid cartilage, lies between the two laminae, and is joined to them by fibrous tissue.

CRICOID CARTILAGE

The cricoid cartilage is attached below to the trachea, and articulates with the thyroid cartilage and the two arytenoid cartilages by synovial joints. It forms a complete ring around the airway, the only laryngeal cartilage to do so (Fig. 34.3B). It is smaller, but thicker and stronger, than the thyroid cartilage, and has a narrow curved anterior arch, and a broad, flatter posterior lamina.

ARYTENOID CARTILAGE

The paired arytenoid cartilages articulate with the lateral parts of the superior border of the cricoid lamina (Figs 34.2, 34.3). Each is pyramidal, and has three surfaces, two processes, a base and an apex. The posterior surface, which is triangular, smooth and concave, is covered by transverse arytenoid. The anterolateral surface is convex and rough, and bears, near the apex of the cartilage, an elevation from which a crest curves back, down and then forwards to the vocal process. The lower part of this arcuate crest separates two depressions (foveae). The upper is triangular (fovea triangularis), and the vestibular ligament is attached to it. The lower is oblong (fovea oblonga), and vocalis and lateral cricoarytenoid are attached to it. The medial surface is narrow, smooth and flat, and is covered by mucosa: its lower edge forms the lateral boundary of the intercartilaginous part of the rima glottidis. The base is concave, with a smooth surface for articulation with the lateral part of the upper border of the cricoid lamina. Its round, prominent lateral angle, or muscular process, projects backwards and laterally: it gives attachment to posterior cricoarytenoid behind, and lateral cricoarytenoid in front. The vocal ligament is attached to its pointed anterior angle (vocal process), which projects horizontally forward. The apex curves backwards and medially and articulates with the corniculate cartilage.

JOINTS

CRICOTHYROID JOINT

The joints between the inferior cornua of the thyroid cartilage and the sides of the cricoid cartilage are synovial. Each is enveloped by a capsular ligament strengthened posteriorly by fibrous bands (Figs 34.1, 34.2, 34.4). Both capsule and ligaments are rich in elastin fibres. The primary movement at the joint is rotation around a transverse axis which passes transversely through both cricothyroid joints. The effect of this rotation is to move the cricoid and thyroid cartilages relative to one another in such a way as to bring together the lamina of the thyroid cartilage and the arch of the cricoid cartilage (‘closing the visor’). There is some controversy as to which cartilage moves, but it seems most likely that the cricoid cartilage rotates to a greater extent. When the joint is in a neutral position, the ligaments are slack and the cricoid can glide, to a limited extent, in different directions on the thyroid cornua. The effect of these movements is to lengthen the vocal folds, provided the arytenoid cartilages are stabilized at the cricoarytenoid joint. This may also increase vocal fold tension.

CRICOARYTENOID JOINT

The cricoarytenoid joints are a pair of synovial joints between the facets on the lateral parts of the upper border of the lamina of the cricoid cartilage and the bases of the arytenoids. Each joint is enclosed by a capsular ligament and strengthened by a ligament that, although traditionally called the posterior cricoarytenoid ligament, is largely medial in position (Figs 34.134.4).

The cricoid facets are elliptical, convex and obliquely directed laterally, anteriorly and downwards. The long axes of the two facets intersect posteriorly at an angle of about 50°. Two movements occur at this joint. The first is rotation of the arytenoid cartilages at right angle to the long axis of the cricoid facet (dorso-medio-cranial to ventro-latero-caudal), which, because of its obliquity, causes each vocal process to swing laterally or medially, thereby increasing or decreasing the width of the rima glottidis. This movement is sometimes referred to as a rocking movement of the arytenoid cartilages. There is also a gliding movement, by which the arytenoids approach or recede from one another, the direction and slope of their articular surfaces imposing a forward and downward movement on lateral gliding. The movements of gliding and rotation are associated, i.e. medial gliding occurs with medial rotation and lateral gliding with lateral rotation, resulting in adduction or abduction of the vocal folds respectively. When viewed from above, foreshortening can give the illusion that the arytenoid cartilages are rotating about their vertical axes, but the shape of the facets prevents such movement occurring. The posterior cricoarytenoid ligaments limit forward movements of the arytenoid cartilages on the cricoid cartilage. It has been suggested that the ‘rest’ position of the cricoarytenoid ligament is a major determinant of the position of a denervated vocal cord.

SOFT TISSUES

The skeletal framework of the larynx is joined to surrounding structures by extrinsic membranes. It is also interconnected by intrinsic ligaments and fibroelastic membranes, of which the thyrohyoid, quadrangular and cricothyroid membranes and the conus elasticus are the most significant. The thyrohyoid membrane is external to the larynx, whereas the paired quadrangular membranes, the cricothyroid membrane and the conus elasticus are internal. The named ligaments are the median (anterior) cricothyroid ligament, the hyoepiglottic and thyroepiglottic ligaments and the cricotracheal ligament.

EXTRINSIC LIGAMENTS AND MEMBRANES

Thyrohyoid membrane

The thyrohyoid membrane is a broad, fibroelastic layer attached below to the superior border of the thyroid cartilage lamina and the front of its superior cornua, and above to the superior margin of the body and greater cornua of the hyoid (Figs 34.1, 34.2, 34.5). It thus ascends behind the concave posterior surface of the hyoid, separated from its body by a bursa which facilitates the ascent of the larynx during swallowing. Its thicker part is the median thyrohyoid ligament. The more lateral, thinner, parts are pierced by the superior laryngeal vessels and internal laryngeal nerves (Fig. 34.1). Externally, it is in contact with thyrohyoid and omohyoid and the body of the hyoid bone. Its inner surface is related to the lingual surface of the epiglottis and the piriform fossae of the pharynx. The round, cord-like, elastic lateral thyrohyoid ligaments form the posterior borders of the thyrohyoid membrane, and connect the tips of the superior thyroid cornua to the posterior ends of the greater hyoid cornua (Fig. 34.1).

INTRINSIC LIGAMENTS AND MEMBRANES

The fibroelastic membrane of the larynx lies within the cartilaginous skeleton of the larynx, beneath the laryngeal mucosa (Fig. 34.5). It forms a discontinuous sheet separated on both sides of the larynx by a horizontal cleft between the vestibular and vocal ligaments. Its upper part, the quadrangular membrane, extends between the arytenoid cartilages and the sides of the epiglottis. Its lower part, the cricothyroid membrane and conus elasticus, connects the thyroid, cricoid and arytenoid cartilages.

Cricothyroid membrane and the conus elasticus

The cricothyroid ligament is composed mainly of elastic tissue. It consists of two parts: the cricothyroid membrane below and the conus elasticus above.

LARYNGEAL CAVITY

The laryngeal cavity extends from the laryngeal inlet (from the pharynx) down to the lower border of the cricoid cartilage, where it continues into the trachea (Figs 34.5, 34.6). The walls of the cavity are formed of the fibroelastic membranes described above and are lined with mucous membrane which folds over the free edges of these fibroelastic membranes within the larynx. On either side, the continuity of the fibroelastic membrane is interrupted between the upper and lower folds.

The folds project into the lumen of the cavity and divide it into upper and lower parts, separated by a middle portion between the two sets of folds leading into the laryngeal ventricle. The upper folds are the vestibular (ventricular or false vocal) folds; the median aperture which they guard is the rima vestibuli. The lower pair are the (true) vocal folds (or vocal cords), and the fissure between them is the rima glottidis or glottis. The true vocal folds are the primary source of phonation, whereas the vestibular folds normally do not contribute directly to sound production. The clinical term supraglottis refers to the part of the larynx that lies above the glottis and comprises the laryngeal inlet formed of the laryngeal surface of the epiglottis and arytenoid cartilages and the laryngeal aspects of the aryepiglottic folds, the laryngeal vestibule (introitus) and the vestibular folds.

MICROSTRUCTURE OF THE LARYNX

The laryngeal mucosa is continuous with that of the pharynx above and the trachea below. It lines the entire inner surface of the larynx, including the ventricle and saccule, and is thickened over the vestibular folds, where it is the chief component. Over the vocal folds it is thinner, and is firmly attached to the underlying vocal ligaments. It is loosely adherent to the anterior surface of the epiglottis, but firmly attached to its anterior surface and the floor of the valleculae. On the aryepiglottic folds it is reinforced by a considerable amount of fibrous connective tissue, and it adheres closely to the laryngeal surfaces of the cuneiform and arytenoid cartilages.

The laryngeal epithelium is mainly a ciliated, pseudostratified respiratory epithelium where it covers the inner aspects of the larynx, including the posterior, laryngeal surface of the epiglottis, and it provides a mucociliary clearance mechanism shared with most of the respiratory tract (see Ch. 57). However, the vocal folds are covered by non-keratinized, stratified squamous epithelium where they contact each other: this important variation protects the tissue from the effects of the considerable mechanical stresses that act on the surfaces of the vocal folds. The exterior surfaces of the larynx, which merge with the laryngopharynx and oropharynx (including the anterior, lingual surface of the epiglottis and the aryepiglottic folds), are subject to the abrasive effects of swallowed food, and are therefore covered by non-keratinized, stratified squamous epithelium.

The laryngeal mucosa has numerous mucous glands, especially over the epiglottis, where they pit the cartilage, and along the margins of the aryepiglottic folds anterior to the arytenoid cartilages, where they are known as the arytenoid glands. Many large glands in the saccules of the larynx secrete periodically over the vocal folds during phonation. The free edges of these folds are devoid of glands, and their stratified epithelium is vulnerable to drying and requires the secretions of neighbouring glands: hoarseness as a result of excessive speaking is due to partial temporary failure of this secretion. The epithelial surfaces are ridged and this may help retain the lubricating secretions over the surfaces of the edges of the folds. Poorly lubricated folds offer increased resistance to airflow, which means that higher subglottal pressures are needed to initiate phonation. Taste buds, like those in the tongue, occur on the posterior epiglottic surface, aryepiglottic folds and less often in other laryngeal regions.

UPPER PART

The upper part of the laryngeal cavity contains the laryngeal inlet (aditus), the aryepiglottic fold and the laryngeal vestibule (introitus).

Aryepiglottic fold

The aryepiglottic fold contains ligamentous and muscular fibres. The ligamentous fibres represent the free upper border of the quadrangular membrane (Fig. 34.5). The muscle fibres are continuations of the oblique arytenoids. The posterior part of the aryepiglottic fold contains two oval swellings, one above and in front, the other behind and below, that mark the positions of the underlying cuneiform and corniculate cartilages respectively. They are separated by a shallow vertical furrow which is continuous below with the opening of the laryngeal ventricle.

MIDDLE PART

The middle part of the laryngeal cavity is the smallest, and extends from the rima vestibuli above to the rima glottidis below. On each side it contains the vestibular folds, the ventricle and the saccule of the larynx.

Vestibular folds and ligaments

The narrow vestibular ligament represents the thickened lower border of the quadrangular membrane (Fig. 34.5). It is fixed in front to the thyroid angle below the epiglottic cartilage and behind to the anterolateral surface of the arytenoid cartilage above its vocal process. With its covering of mucosa, it is termed the vestibular (ventricular or false vocal) fold (Figs 34.5, 34.6). The presence of a loose vascular mucosa lends the vestibular folds a pink appearance in vivo, as they lie above and lateral to the vocal cords.

Ventricle (sinus) of the larynx

The laryngeal ventricle is a slit between the vestibular and vocal cords (Figs 34.5, 34.6). It opens into a fusiform recess on each side of the larynx and extends upwards into the laryngeal wall lateral to the vestibular fold, opening into the saccule.

Saccule of the larynx

The saccule is a pouch which ascends forwards from the ventricle, between the vestibular fold and thyroid cartilage, and occasionally reaches the upper border of the cartilage (Fig. 34.6). It is conical, and curves slightly backwards; 60–70 mucous glands, sited in the submucosa, open onto its luminal surface. The orifice of the saccule is guarded by a delicate fold of mucosa, the ventriculosaccular fold.

The saccule has a fibrous capsule that is continuous below with the vestibular ligament. It is covered medially by a few muscular fasciculi from the apex of the arytenoid cartilage which pass forwards between the saccule and vestibular mucosa into the aryepiglottic fold, and laterally it is separated from the thyroid cartilage by the thyroepiglottic muscle. The latter compresses the saccule, expressing its secretion onto the vocal cords, which lack glands, to lubricate and protect them against desiccation and infection. Saccules occasionally protrude through the thyrohyoid membrane.

Vocal folds (cords) and ligaments

The free thickened upper edge of the conus elasticus forms the vocal ligament. It stretches back on either side from the mid level of the thyroid angle to the vocal processes of the arytenoids. When covered by mucosa, it is termed the vocal fold or vocal cord (cord is the preferred clinical term) (Figs 34.5, 34.6). The vocal folds form the anterolateral edges of the rima glottidis and are concerned with sound production. Each fold consists of five layers, namely mucosa, lamina propria (three layers) and the vocalis muscle (Fig. 34.7).

The mucosa overlying the vocal ligament is thin and attached to the underlying lamina propria by a basement membrane. It lies directly on the ligament, and so the vocal fold appears pearly white in vivo. The lamina propria consists of three layers. The most superficial consists of loose collagen and elastic fibres, and is only loosely attached to the underlying vocal ligament, an arrangement that produces a potential space (Reinke’s space) that extends along the length of the free margin of the vocal ligament and a little way onto the superior surface of the cord: oedema fluid readily collects here in disease. The intermediate layer consists of elastic fibres, and the deep layer is formed of collagen fibres; these two layers collectively form the vocal ligament. Fibres of the vocalis muscle form the fifth layer of the vocal folds. The site where the vocal folds meet anteriorly is known as the anterior commissure, and is the region where fibres of the vocal ligament pass through the thyroid cartilage to blend with the overlying perichondrium, forming Broyle’s ligament. Since Broyle’s ligament contains blood vessels and lymphatics, it represents a potential route for the escape of malignant tumours from the larynx.

Reinke’s oedema

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