The larynx

Fig. 1. 38 Lateral radiograph of the neck: soft-tissue view showing pharynx and larynx.

1. Nasopharynx 2. Oropharynx 3. Laryngopharynx 4. Soft palate 5. Tongue

G. Epiglottis 7. Base of tongue 8. Vallecula 9. Aryepiglottic fold 10. Ventricle 11. Irregular calcification in superior cornu of thyroid cartilage 12. Calcification in lamina of thyroid cartilage 13. Inferior cornu of thyroid cartilage 14. Cricoid cartilage 15. Calcification in arytenoid cartilage 16. Body of hyoid bone 17. Greater cornu of hyoid bone 18. Anterior arch of C, 19. Odontoid process of C2 20. Body of C2 2 1. Prevertebral space 22. Prevertebral space 23. Trachea

Palatal studies and videofluoroscopic feeding studies The movement of the palate during phonation can be imaged by radiography in the lateral projection, where the position of the soft palate can be seen outlined by air. Movement of the palate during complex speech is studied by videofluoroscopy w i th or without coating the nasal and oral surfaces of the palate w i th barium.

Videofluoroscopy is also used to study the movement of the tongue, palate and pharynx during feeding by using foods and liquids of various consistencies mixed w i th barium.

Cross-sectional imaging CT and MRI provide excellent detail of the pharynx, its fascial planes and its related spaces (see Figs 1. 34, 1. 35). Pathology in this region tends to cause asymmetry, which is readily detected. Images are usually obtained in the axial plane and a good understanding of the cross-sectional anatomy is therefore required.

Coronal images may also be obtained by both modalities but are easier to obtain using MRI, as this may be performed without moving the patient. Pathology, especially carcinoma, of the nasopharynx spreads along the fascial planes and may extend intracranially through the many foramina described. For this reason, imaging in both axial and coronal planes is necessary to evaluate the base of the skull. CT yields excellent axial images of the base of the skull and provides good bone detail. Both CT and MRI demonstrate the muscles and soft-tissue planes and the examinations are complementary.

THE LARYNX (Figs 1. 38-1. 42) The larynx forms the entrance to the airway and is responsible for voice production. It extends from the base of the tongue to the trachea, lying anterior to the third to sixth cervical vertebrae. It lies between the great vessels of the neck and is covered anteriorly by the strap muscles of the neck, fascia and skin. It is lined by mucosa, which is continuous w i th that of the pharynx above and the trachea below. Its framework is composed of three single and three paired cartilages, which articulate w i th each other and are joined by muscles, folds and connective tissue.

The anchor cartilage of the larynx is the cricoid cartilage This is shaped like a signet ring, w i th a flat, wide lamina posteriorly and an arch anteriorly. It is joined to the thyroid cartilage above by the cricothyroid membrane, and to the trachea below by the cricotracheal membrane. The paired pyramidal arytenoid cartilages sit on the superolateral

margin of the signet posteriorly. These bear anteroinferior vocal processes, which give rise to the vocal ligaments of the true vocal cords.

The thyroid cartilage forms the anterior and lateral boundary of the larynx. It is formed by a pair of laminae, which are joined anteriorly forming an angle and are separated above to form the superior thyroid notch. This notch is at approximately C4 level. The posterior parts of the laminae have upper and lower projections known as the superior and inferior horns or cornua. The inferior horns project down posterolaterally to articulate w i th the signet of the cricoid cartilage w i th a synovial joint. The vocal ligaments are attached to the inner surface of the thyroid cartilage near its lower margin.

The epiglottis is a leaf-shaped cartilage whose narrow base or petiole is attached to the inner surface of the thyroid cartilage at the same point as the anterior extremity of the vocal cords. It projects up behind the base of the tongue and directs boluses laterally into the piriform fossae during deglutition, thus protecting the larynx. A pair of mucosal folds, called the pharyngeal folds, pass laterally from the epiglottis to the pharyngeal wall. Three mucosal folds, the glossoepiglottic folds - namely, a central and two lateral folds - pass from the anterior surface of the epiglottis to the base of the tongue. These form paired recesses between the base of the tongue and the epiglottis known as the valleculae. A further pair of mucosal folds pass from the lateral margin of the epiglottis posteriorly to the arytenoid cartilages separating the larynx from the piriform fossae. These are the aryepiglottic folds which, together w i th the epiglottis, define the entrance to the larynx.

Two further pairs of cartilages lie in the aryepiglottic folds. The corniculate cartilages sit on top of the arytenoid cartilages, and the cuneiform cartilages lie immediately laterally in the free margin of the fold.

The cavity of the larynx is divided into three parts by upper and lower pairs of mucosal folds. The upper pair of folds are the vestibular or false cords. The space between the laryngeal entrance and the false cords is known as the vestibule or the sinus of the larynx. The lower pair of folds are the true cords and contain the vocal ligaments, which are responsible for voice production. The space between the false and true vocal cords is the laryngeal ventricle. This

space may extend anterosuperiorly, forming a small pouch known as the saccule of the larynx. The term glottis refers to the true vocal cords and the triangular space between them when open (rima glottidis). The lowest part of the larynx is the infraglottic larynx, which lies between the true cords and the trachea. It is elliptical in cross-section superiorly, and circular inferiorly as it merges w i th the trachea.

The hyoid bone is a U-shaped bone between the mandible and the thyroid cartilage. It has a small central body and a long extension on each side called the greater cornua or horns. Smaller lesser cornua arise from its upper surface. The tip of each greater cornu is attached to the styloid process by the stylohyoid ligament.

The entire larynx is slung from the hyoid bone by the thyrohyoid membrane and ligaments. Anteriorly the thyrohyoid membrane arises from an oblique line on the thyroid cartilage and inserts into the inferior part of the hyoid bone. A midline thickening forms the thyrohyoid ligament. The thickened posterior part of the same membrane passes from the superior horns of the thyroid cartilage to the greater horns of the hyoid bone, forming the lateral thyrohyoid ligaments. The hyoid bone, in turn, is attached to the mandible, tongue and styloid process, and to the pharynx by the middle pharyngeal constrictor.

Cross-sectional anatomy of the larynx

(Figs 1. 41 and 1. 42)

Supraglottic level (Figs 1. 41a and 1. 42a) The larynx is anterior to the piriform sinuses, separated from them by the aryepiglottic folds. At a higher level, the thyroid cartilage and the hyoid bone, w i th its greater horns laterally, may be seen. The epiglottis, pharyngoepiglottic folds, valleculae and base of tongue may also be identified posterior to the hyoid bone. The sternocleidomastoid muscles are seen posterolaterally w i th the carotid and internal jugular vessels medial to them.

Glottic level (Figs 1. 41b and 1. 42b) A complete ring of cartilage is seen at this level - the thyroid cartilage anteriorly and the lamina of the cricoid and arytenoid cartilages posteriorly. The vocal processes of the arytenoids may be identified giving attachment to the vocal ligaments and defining the level of the glottis.

The anterior fusion of the vocal cords is known as the anterior commissure and is very thin when the cords are abducted. Similarly, the posterior commissure, which is seen between the arytenoids, is thin in abduction of the cords. Both commissures may appear thickened during adduction of the cords (phonation). The larynx is elliptical in shape at the level of the true cords and triangular at the level of the false cords, which are at a slightly higher level. Here the false cords form the lateral wall of the larynx, and the aryepiglottic folds form the posterior wall. The thyroid cartilage is anterior. The paralaryngeal space is between the larynx and the thyroid cartilage.

Infraglottic level (Figs 1. 41c and 1. 42c) Just below the cords the larynx is elliptical. The lamina of the cricoid is posterior, w i th the cricothyroid membrane anterior. The inferior thyroid horns and part of the thyroid lamina are posterior and lateral to the cricoid. At a lower level the larynx is more circular and the cricoid forms a complete ring. Part of the lobes of the thyroid gland may be seen laterally, w i th the neck vessels situated postero-. laterally. The external jugular vein may be seen anterior to the sternocleidomastoid muscle.

Radiology of the larynx (Figs 1. 38, 1. 39 and 1. 42)

Plain radiography This is a relatively simple method of demonstrating the anatomy of the larynx.

Lateral views are the most useful as the larynx is not obscured by overlying bone. The air in the pharynx and larynx provides intrinsic contrast w i th the soft-tissue walls and mucosal folds (see Fig. 1. 38).

On the lateral view, the hyoid bone and cartilages of the larynx are seen. The thyroid, cricoid and arytenoid cartilages are composed of hyaline cartilage and may calcify or undergo true ossification. Calcification is often irregular, but may be homogeneous w i th a dense cortex and lessdense medulla. The other cartilages are composed of yellow elastic fibrocartilage and do not calcify. The lateral thyrohyoid ligaments may contain triticeal cartilages, which can calcify and should not be mistaken for foreign bodies. The base of the tongue, vallecula, epiglottis, aryepiglottic folds, and true and false cords may be identified. The vestibule, ventricle and infraglottic larynx are those spaces above, between and below the cords.

Tomography This is useful in the AP plane when overlying bony densities are blurred to allow better detail (see Fig. 1. 39). The true and false cords and laryngeal ventricle are best seen in this view. The piriform fossae are seen on either side of the proximal larynx, between it and the thyroid cartilage. Symmetry of the soft-tissue planes is important.

CT and MRI

Cross-sectional imaging using CT (see Figs 1. 34 and 1. 42) provides excellent anatomical detail of the larynx and surrounding structures. Scans are usually obtained in the