Oral cavity and the oropharynx

Technique

The oropharynx including the base of the tongue and the floor of the mouth, is despite its relationship to the mouth difficult to evaluate clinically. Computed tomography has therefore become an important complement to the clinical examination when assessing solid lesions of this region. Ultrasound can be used to confirm cystic lesions of the floor of the mouth and the neighbouring neck.

Anatomy

The hard and soft palate forms the upper boundary of the oropharynx while the hypopharynx and the supraglottic airway forms the lower boundary. The three pharyngeal constrictors forming the palatoglossal arches makes up the lateral and posterior margins of the oropharynx. The palatine tonsils are found between the palatoglossal and palatopharyngeal arches (anterior and posterior tonsillar pillars).

The bulk of the tongue is formed by paired intrinsic and extrinsic tongue muscles. The interdigitating intrinsic muscles consist of longitudinal as well as oblique and transverse muscles. The three extrinsic tongue muscles anchor the tongue to the surrounding structures and help move the tongue. These three muscles are the genioglossus, the hyoglossus and the styloglossus muscles. The genioglossus muscles take their origin from the midline genial tubercles on the inside of the mandible. The hyoglossus muscles originates from the lateral margins of the hyoid bone while the styloglossus muscles descend from the styloid processes and joins the hyoglossus muscles forming the lateral borders of the base of the tongue (Fig. 19a).The circumvallate papillae is the dividing landmark between the oral free portion and the base of the tongue. The tongue base is rich in lymphoid tissue forming the lingual tonsil.

The floor of the mouth is also made up of three paired muscles. The converging mylohyoid muscles forms the supporting floor meeting in a midline raphe. These muscles take their origin along the broad mylohyoid line on the inside of the mandible. The geniohyoid muscles run in the midline on the oral side of the mylohyoid muscles. The anterior bellies of the digastric muscles run parallel just off the midline superficial to the mylohyoid muscles inserting on the inside of the mandible (Fig. 19b).

a

Figure 19. Normal oropharyngeal anatomy - computed tomography a) Transverse section through the base of the tongue. The paired genioglossus muscles can be seen on both sides of the fibrous lingual septum (crossed arrow). b) Coronal section through the floow of the mouth. The mylohyoid muscles (mh) coming from the inside of the mandible (crossed arrow) con verge in the midline. (m=mandible; mh=mylohyoiod muscle; hg=hyoglossus muscle; dg=anterior belly of the digastric muscle; sm=submandibular gland; a=internal carotid artery; v=internal jugular vein)

b

Pathology

Benign lesions

Congenital cysts can be found in the midline of the tongue base and the floor of the mouth as well as further down in the neck. The thyroid gland migrates down from the foramen caecum of the tongue. Along this route ductal remnants can form a thyroglossal cyst. These are often discovered after a upper respiratory infection in childhood. The cyst is always midline in location spreading the midline muscles apart. The cyst often has contact with the body of the hyoid bone and computed tomography can show this relationship. This is of importance since surgical removal should include the cyst together with the body of the hyoid bone and the ductal remnant up to the foramen caecum in order to avoid any recurrence. Both epidermoid and dermoid cyst can occur in the midline.

	Figure 20. Epidermoid cyst - computed tomography Transverse section through the floor of the mouth. A large cyst with mixed content that is layering out can be seen in the midline.
	Figure 21. Base of tongue thyroid - computed tomography Transverse section at the level of the hyoid bone (h). A round, well demarcated hyperdense tumor compatible with thyroid tissue is seen in the floor of the mouth in front of the hyoid bone (arrows).

Epidermoids lined by squamous epithelium are filled by fluid, while dermoid cyst can have a more fatty content (Fig. 20).

Residual thyroid tissue in the tongue base will give rise to a lingual thyroid. In 70% of these patients this represent a failure of thyroid migration and the lingual thyroid being the only thyroid tissue. Before attempting any removal of a lingual thyroid a thyroid scan should be done to map all thyroid tissue. Like the thyroglossul duct cyst the lingual thyroid can be found in the midline of the tongue base. Because of he natural iodine content the glandular tissue can be visualized on computed tomography without contrast enhancement (Fig 21).

Figure 22. Carcinoma of the base of the tongue - computed tomography Transverse section through the tongue base. A large inhomogenously enhancing tumor is seen on the left side deep in the tongue crossing the midline (black arrow). Bilateral lymph node metastases can also be seen in front of the carotid sheaths (white arrows).

Malignant tumors

Squamous cell carcinoma accounts for more than 90% of the malignancies of the oropharynx. Malignant lesions originating from the lips, buccal mucosa, alveolar ridge, floor of the mouth and the oral free portion of the tongue can usually be detected and assessed by direct inspection and palpation. Plain radiographs or computed tomography can give additional information regarding any bone invasion. Carcinoma originating in the base of the tongue or the tonsillar area are much more difficult to detect and evaluate clinically. Therefore tumors originating out of these two areas have neck metastases in a high percentage when they are discovered. Computed tomography or magnetic resonance imaging can detect tumors in the base of the tongue and the tonsil at an earlier stage and give a better assessment of the size and extent of the primary tumor as well as detect any coexisting metastatic lymphadenopathy (Fig. 22).

Sven G. Larsson and Anthony A. Mancuso