20.1 Surgical Anatomy

The anatomy of the parapharyngeal space is either wonderfully intricate or frighteningly complicated, depending on the operator’s level of experience. Failure to appreciate the anatomic relationships can lead to selection of an incorrect surgical approach. The result may be inadequate access, difficult tumor removal, damage to vital structures, or tumor spillage and thus recurrence of neoplasms.

The parapharyngeal space is often described as an inverted pyramid with its base at the skull and apex at the greater cornu of the hyoid bone (Fig. 20.1). The parapharyngeal space is further compartmentalized by thick fascial layers that direct tumor growth. Prior descriptions of these fascial layers have varied.1

The superior border of the parapharyngeal space is a small portion of the temporal bone (Fig. 20.2). The superomedial wall is enclosed by a fascial connection from the medial pterygoid plate to the spine of the sphenoid. This fascia passes lateral to the foramen ovale and the foramen spinosum. These foramina are not included in the superior limits of the parapharyngeal space but rather are in the infratemporal fossa. The inferior boundary of the parapharyngeal space ends at the junction of the posterior belly of the digastric muscle and the greater cornu of the hyoid bone. The firm fascial attachments in this area limit parapharyngeal space extension inferior to the hyoid bone. This fascia, however, can be weak and may be an ineffective barrier to the spread of infection.

The posterior border of the parapharyngeal space is formed by the fascia over the vertebral column and paravertebral muscles. The anterior limit is composed of the pterygomandibular raphe and medial pterygoid fascia.

The medial wall of the parapharyngeal space is made up of the fascia overlying the medial pterygoid muscle and the ramus of the mandible. The fascia of the medial pterygoid muscle superiorly incorporates the sphenomandibular ligament that extends from the spine of the sphenoid to the lingula of the mandible. This dense fascia then continues to the skull base and separates the parapharyngeal space from the inferior alveolar nerve, the lateral pterygoid muscle, and the condyle of the mandible. The retromandibular portion of the deep lobe of the parotid gland also forms a small portion of the lateral border, as does a portion of the posterior belly of the digastric muscle.

Superiorly, the medial border is formed by the approximation of the fascia from the tensor veli palatini muscle to the medial pterygoid muscle. The pharyngobasilar fascia forms the posteromedial border of the poststyloid space near the levator palatini muscle. Inferiorly, the medial border is contiguous with the fascia over the superior constrictor muscle and contains attachments of the stylopharyngeal aponeurosis. The inferomedial wall continues with fascia that joins the styloglossus and stylopharyngeus muscles.

The parapharyngeal space is divided into a prestyloid and a poststyloid compartment (Fig. 20.2) by fascia that extends from the styloid process to the tensor veli palatini muscle. Posteriorly, this fascial plane blends with the styloid muscles. The prestyloid space extends superiorly into a blind pouch formed by the joining of the medial pterygoid fascia to the tensor veli palatini fascia. This space contains a variable portion of the retromandibular deep lobe of the parotid gland. In addition, a small branch of cranial nerve V crosses this area to reach the tensor veli palatini muscle. Most of the prestyloid parapharyngeal space is composed of fat. As a result, tumors in this area are generally limited to salivary lesions, lipomas, and rare neurogenic tumors (Table 20.1).

The poststyloid compartment contains the carotid artery and jugular vein located posterolateral to the artery at the skull base. Cranial nerves IX through XII accompany these vessels, with X occupying a position between the artery and the vein. Cranial nerve XI crosses the vein anteriorly or posteriorly, and IX crosses the carotid artery laterally. This compartment also contains the sympathetic chain, lymph nodes, and glomus tissue. The retropharyngeal space is separated from the poststyloid space by a thin fascial layer that is a minimal barrier to the spread of tumor or infection.

The stylomandibular ligament is fascia that unites the styloid process to the mandibular ramus and forms one of the boundaries of the stylomandibular tunnel.2 The remaining borders include the skull base and ascending ramus of the mandible. Extension of tumors through the rigid opening of the stylomandibular tunnel will often be noted as a result of constricted tumor growth in this narrow area.

The parapharyngeal space has numerous lymphatics that drain the paranasal sinuses, the oropharynx, the oral cavity, and a portion of the thyroid gland. These nodes are connected superiorly to the node of Rouvière, situated in the retropharyngeal space, which drains the nasopharynx, upper oropharynx, and sinuses.

20.2 Regional Pathology and Differential Diagnosis

Tumors of the parapharyngeal space include primary neoplasms, direct extension from adjacent regions, and metastatic disease (Table 20.2). The majority of neoplasms in this area are of salivary gland origin, followed in order of frequency neurogenic, lymphatic, and metastatic lesions.3 ^,​ 4 ^,​ 5 Malignant tumors can invade the parapharyngeal space from the nasopharynx, oropharynx, mandible, maxilla, oral cavity, or parotid gland. Primary parapharyngeal tumors can extend intracranially through the jugular foramen or into the retropharyngeal space.

20.3 Clinical Assessment

Tumors of the parapharyngeal space most often present as an asymptomatic mass in the neck or oropharynx. They are often discovered on routine physical examination. Their presence should be suspected when a subtle fullness is noted in the soft palate or tonsillar region or when there is mild fullness near the angle of the jaw. The tumors often must grow to 3 cm before they can be palpated.

Small tumors in the parapharynx cause few symptoms. As the tumors enlarge and extend superiorly, they may cause symptoms related to the Eustachian tube. As tumors expand medially, voice change, nasal obstruction, aspiration, and dyspnea may occur. Rarely, tumors have been found that require immediate tracheotomy for relief of upper airway obstruction. As tumors enlarge, they may compress cranial nerves IX, X, XI, or XII, causing hoarseness, dysphagia, and dysarthria. Horner’s syndrome may also be produced by tumor pressure on the superior cervical ganglion. For a benign tumor to cause significant nerve deficits, it must enlarge to a considerable degree. Pain, trismus, or cranial nerve palsy often suggests malignancy.

Malignant tumors of the parapharyngeal space can cause carotid sinus hypersensitivity and glossopharyngeal neuralgia. Asystole, bradycardia, and hypertension have been reported.13

Tumors of the parapharyngeal space are often misdiagnosed as infections or tonsil tumors. Patients often complain of a mild sore throat or globus sensation, and sometimes they complain of dysphagia. The swelling in the tonsillar and soft palatal region maybe misdiagnosed as a peritonsillar abscess.

Delays in diagnosis have also occurred because patients were being treated for presumed nasal obstruction, Eustachian tube dysfunction, or serous otitis media. Patients have been known to be misdiagnosed with temporomandibular joint pathology when they actually had a parapharyngeal space lesion.

A complete head and neck examination is an essential part of the evaluation of parapharyngeal space tumors. However, the anatomic location of the parapharyngeal space makes it difficult to accurately assess tumor presence and size. Bimanual palpation with one finger in the patient’s mouth and the other hand on the patient’s neck assesses mobility, pulsation, and tumor extent. The finding of a pulsatile mass is generally not a helpful differentiating sign, as many of the tumors will transmit carotid pulsations. If a preauricular mass is noted at physical examination in addition to a lateral pharyngeal mass, this indicates a parotid tumor extending through the stylomandibular tunnel. The presence of cervical lymphadenopathy may indicate malignancy. Cranial nerve function should be noted.

Flexible nasal endoscopy can help determine the inferior extent of tumor, and it can also offer information about the function of cranial nerves IX and X. The status of the patient’s airway should be assessed and the expected difficulty of orotracheal intubation determined.

20.4 Diagnostic Imaging

Radiographic study of all parapharyngeal space tumors is essential. CT scan with and without contrast medium or a MRI study with gadolinium should be performed in all cases. Angiographic procedures may also be necessary in select cases. The results of these studies aid significantly in diagnosis and treatment planning.

CT imaging is capable of displaying the soft tissues of the parapharyngeal space extremely well. One of the most important features in radiographic evaluation of parapharyngeal space lesions is to assess whether they lie anterior or posterior to a plane from the styloid process to the medial pterygoid plate. This is the plane of the fascia of the tensor veli palatini muscle, which divides the parapharyngeal space into a prestyloid and poststyloid compartment. A lesion in the prestyloid compartment will be anterior to the carotid artery and posterior to the medial pterygoid muscle (Fig. 20.3).14 Prestyloid tumors are usually salivary gland neoplasms that displace the carotid sheath contents posteriorly. Poststyloid tumors are normally of neurogenic or vascular origin. Poststyloid tumors displace the internal carotid artery in an anteromedial direction.

The exception to this rule is schwannomas, which can displace the carotid artery in different directions because of the unpredictable tumor position between the great vessels and the site of origin of the tumor. Vessel displacement depends on the nerve of origin and on whether the tumor arises near the base of the skull or in the inferior portion of the parapharyngeal space. A recent review found that neurogenic tumors that splayed the internal carotid artery and internal jugular vein with medial displacement of the carotid carried an 86% probability of vagal nerve origin, whereas the absence of vessel splaying and lateral carotid displacement had a 91% chance of sympathetic chain origin.15

CT scans may be helpful in separating prestyloid deep-lobe parotid tumors from extraparotid salivary neoplasms. The best way to distinguish between these two lesions is the finding of a fat plane between the deep lobe of the parotid gland and the posterolateral aspect of a mass. When seen, the tumor is extraparotid. Unfortunately, for lesions larger than 4 cm, the fat plane is obliterated.

It is important to look for evidence of skull base or cervical vertebral erosion and extension through the jugular foramen into the cranial cavity as well. Benign prestyloid tumors can cause erosion of the pterygoid plate, a finding that is not pathognomonic for malignant lesions. Radiographically, low-grade malignancies are difficult to distinguish from benign parapharyngeal space tumors.

Large pleomorphic adenomas have a less homogeneous appearance on CT and contain irregular areas of minimal enhancement, which can give them an appearance similar to that of many neuromas. Neurilemmomas also often have areas of hemorrhage, cystic necrosis, and fatty deposition.16 Lesions that show enhancement on CT with contrast include paragangliomas, hemangiomas, hemangiopericytomas, aneurysms, and neurilemmomas.

Irregular tumor margins, spread into surrounding tissues and fat planes on CT, and evidence of enlarged necrotic cervical or retropharyngeal nodes indicate malignancy.

The capability of MRI to image the coronal and sagittal planes directly through MR scan is a significant advantage over CT. MRI also has the ability to distinguish lesions based on their signaling and enhancement. Pleomorphic adenomas, for instance, have low signal on T1-weighted images and bright signal on T2-weighted images, and paragangliomas often demonstrate flow voids within areas of high vascularity (Fig. 20.4; Fig. 20.5; Fig. 20.6). The presence of flow voids on imaging of a poststyloid lesion can suggest paraganglioma over less vascular schwannomas.

Distinguishing between malignant and benign parapharyngeal lesions based on images is difficult, but some findings may suggest malignancy. Irregular borders and obliteration of fat planes, particularly around nerves, can suggest malignancy. Invasion of the skull base and dura is often an indicator of malignancy, as is the presence of lymphadenopathy.

If a paraganglioma is suspected, then patients should undergo angiography. Angiography can help confirm the diagnosis and allow identification of feeding vessels for which simultaneous embolization can be performed. Embolization is used by the authors for extensive paragangliomas extending to the skull base and for some carotid body tumors. If embolization is planned, it should be performed within 24 hours of the definitive operative therapy to prevent revascularization and inflammatory reaction from complicating tumor removal. It may also be necessary to perform a carotid occlusion study during the angiogram. This must be done when a malignant tumor involves the poststyloid portion of the parapharyngeal space or for extensive vascular tumors that surround the carotid artery at the base of skull.

Ultrasound is rarely indicated for assessment of tumors of the parapharyngeal space, with the exception of possible use to aid in fine-needle aspiration (FNA) biopsy. PET may be useful in assessment of malignant tumors in some circumstances.