Anatomy

The oropharynx extends from the level of the hard palate above to the hyoid bone below (Figure 6-1). On a practical basis, this region may be divided into the palatine arch consisting of the soft palate, uvula and the anterior faucial pillar, and the oropharynx proper2 as tumors of the arch tend to be less virulent than those arising at other subsites. For the purpose of tumor classification, however, 4 main anatomical subdivisions are described (Table 6-1)

The anterior wall of the oropharynx is formed by the base or posterior third of the tongue bounded anteriorly by the v-shaped line of circumvallate papillae. Numerous lymphatic aggregates give the base of the tongue its characteristic nodularity, a normal feature that may cause great difficulty in the diagnosis of early lesions of this region. Lymphatics from the tongue base course downward toward the hyoid bone where they pierce the pharyngeal wall to drain into the upper deep cervical chain or level II nodes. The jugulodigastric lymph node, the largest of these nodes, is frequently the first to be involved by metastatic tumor followed by those at levels III and IV. Disruption of normal lymphatic channels by the presence of a tumor or surgery to the neck may result in aberrant patterns of spread to levels I and V,

Figure 6-1. The anatomical boundaries of the oropharynx.

Table 6-1. ANATOMICAL SUBDIVISIONS OF THE OROPHARYNX

Anterior wall (glossoepiglottic area) Tongue posterior to the circumvallate papillae (base of tongue) Vallecula excluding the lingual surface of the epiglottis Lateral wall Tonsil

Tonsillar fossa and faucial pillars Glossotonsillar sulcus Posterior wall Superior wall Inferior surface of the soft palate Uvula or to the contralateral side of the neck. Crossover patterns of lymphatic drainage have been demonstrated and tumors involving or growing close to the midline exhibit bilateral nodal involvement in approximately one-third of patients.

The lateral wall of the oropharynx includes the tonsil, the tonsillar fossae, the faucial pillars and more posteriorly, the lateral pharyngeal wall that blends into the posterior wall. Immediately lateral to the lateral pharyngeal wall lies the inverted cone-shaped parapharyngeal space with its base at the temporal bone and its apex at the greater cornu of the hyoid bone. This potential space contains several important neurovascular structures such as the carotid artery, the internal jugular vein, the sympathetic chain, and cranial nerves IX through XII (Figure 6-2). Involvement of this space not only results in cranial nerve deficits and trismus, but also provides tumors access to the base of skull superiorly or the neck inferiorly.

The tonsil is the largest aggregation of lymphoid tissue in Waldeyer's ring and is characterized by deep crypts in which squamous carcinomas may arise without causing obvious surface ulceration. The tonsils have a rich lymphatic network that drains directly through the pharyngeal wall into the upper deep cervical (jugulodigastric) nodes. Lymph node metastasis is less frequent from primary tumors of the tonsillar pillars compared to tumors of the tonsillar fossa. Lesions of the posterior tonsillar pillar are more likely to metastasize to the spinal accessory and upper posterior triangle nodes. Metastatic squamous carcinoma deposits involving nodes from an asymp

Ste mo mastoid muscle

Digastric muscle Parotid gland Styloid process Retromandibular vein

Stylohyoid muscle

Styloglossus muscle Medial pterygoid muscle Lingual nerve

Masseter muscle

Buccinator muscle

Ste mo mastoid muscle

Masseter muscle

Buccinator muscle

Accessory nerve Internal jugular vein

Vagus nerve Sympathetic chain Internal carotid artery

Stylopharyngeus muscle Tonsil

Sublingual gland

— Stylohyoid muscle

Figure 6-2. Cross-section of the oropharynx demonstrating its relationship to the parapharyngeal space.

Accessory nerve Internal jugular vein

Vagus nerve Sympathetic chain Internal carotid artery

Stylopharyngeus muscle Tonsil

Sublingual gland

— Stylohyoid muscle

Figure 6-2. Cross-section of the oropharynx demonstrating its relationship to the parapharyngeal space.

tomatic, small tonsillar primary have a tendency to undergo cystic degeneration. Fine needle aspiration of the cystic mass may draw fluid that is often acel-lular or non-diagnostic, prompting local excision of the mass with a mistaken diagnosis of branchial cyst. Lymphatics from the upper part of the lateral wall drain to the retropharyngeal nodes, of which the only constant one is the node of Rouviere situated close to the skull base between the internal carotid artery and the lateral wall of the pharynx.

The posterior pharyngeal wall extends from the level of the hard palate and Passavant's ridge superiorly to the level of the hyoid bone inferiorly where it becomes continuous with the hypopharynx. In contrast to other areas of the oropharynx, the mucosa is smooth and contains only occasional small aggregates of lymphoid tissue. The primary echelons of drainage from posterior pharyngeal wall tumors are the retropharyngeal nodes and the nodes at levels II and III. The risk of lymph node metastasis ranges from 25 percent for T1 lesions to over 75 percent for T4 tumors.3

The roof of the oropharynx is formed by the curved arch of the inferior surface of the soft palate and the uvula in the midline. Tumors of the soft palate drain lymph to the upper jugulodigastric and the retropharyngeal nodes. About a third of patients present with clinically positive neck nodes and involvement of the tonsillar fossa increases this risk. Occult nodal metastases occur in 16 percent of patients, and about 15 percent of patients who have a midline primary lesion will have bilateral or contralateral neck metastases.4

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