23 Medial Parapharyngeal Approach

The parapharyngeal space is a suprahyoid infracranial fascial space of the neck, whose shape is typically described as an inverted pyramid. ¹ The superior boundary (i.e., the base of the pyramid) corresponds to the inferior surface of the petrous portion of the temporal bone, whereas the apex lies on the posterior end of the greater horn of the hyoid. The parapharyngeal space is enclosed between the prevertebral space, posteriorly, infratemporal fossa (or deep masticatory space) and submandibular fossa, anterolaterally, the parotid space and nodal levels IB and IIA, posterolaterally, and the nasopharynx and oropharynx medially. ² The parapharyngeal space is further divided into prestyloid and retrostyloid compartments (also called real parapharyngeal space and carotid space, respectively) with respect to the styloid process and related muscles and ligaments. ¹ Moreover, the parapharyngeal space can be divided into upper, middle, and lower portions based on the two horizontal planes passing through the inferior border of the lateral pterygoid muscles and mandibular angles. ^{2 , 3}

Only the upper parapharyngeal space can be properly exposed via the transnasal pathway, namely, through the lower transpterygoid approach. ² From a surgical viewpoint, the transnasal parapharyngeal space approach can be divided into a medial and lateral corridor with respect to the lateral pterygoid plate. The medial corridor targets the eustachian tube and related muscles, while the lateral corridor leads to the area of the external carotid foramen, jugular foramen, and related neurovascular structures. This chapter focuses on the endoscopic transnasal approach to the medial portion of the upper parapharyngeal space.

Given the strict relationship with the nasopharynx, the medial portion of the upper parapharyngeal space is particularly prone to be invaded by nasopharyngeal tumors. Moreover, lateral extension of tumors arising into the nasopharynx is favored by the presence of a defect within the pharyngobasilar fascia (called sinus of Morgagni), corresponding to the area where the eustachian tube and levator veli palatini muscle pass from the parapharyngeal space to the nasopharynx forming the torus tubarius. On the other hand, tumors primarily arising from the parapharyngeal space and invading the upper compartment usually extend in the lateral portion (or in both medial and lateral portions), which is composed of smooth structures that are easier to be displaced and compressed by the tumor compared to the eustachian tube and the pterygoid process.

In recent years, the transnasal endoscopic approach to the upper parapharyngeal space has progressively acquired a role in the resection of selected lesions of this area, especially when nonamenable for radical nonsurgical treatment. ^{4 , 5} The endoscopic surgical excision of the nasopharynx has been defined as the “nasopharyngeal endoscopic resection” (NER) and classified into three types depending on the extent of resection: ^{6 , 7} Type 1 resection is limited to the posterior nasopharyngeal wall; type 2 resection also includes the removal of the nasopharyngeal vault and sphenoidal floor; type 3 resection further requires the resection of the medial portion of the upper parapharyngeal space. The surgical technique is currently employed for recurrent/persistent nasopharyngeal carcinomas originally treated with primary (chemo)radiation, as well as for minor salivary gland tumors, papillary adenocarcinomas, plasmacytomas, sarcomas, mucosal melanomas, and other tumors or tumorlike lesions. ^{7 – 11}

The main concern of the surgeon when approaching the medial upper parapharyngeal space is avoiding injury to the parapharyngeal tract of the internal carotid artery. In fact, the combination of a narrow surgical corridor, two-dimensional view, and need to manipulate irradiated tissues makes bleeding from the internal carotid artery a dramatic event. To minimize the chance of damaging this vessel, some concepts should be kept in mind: (1) Knowledge of bony anatomical landmarks (including the lateral pterygoid plate, foramen ovale, foramen spinosum, musculotubal canal) ^{6 , 7 , 12 , 13} and fascial anatomical planes (such as the plane guiding to the internal carotid artery between the medial pterygoid muscle and the tensor veli palatini muscle) ¹⁴ is of utmost importance to identify the position of the internal carotid artery in the surgical field. (2) The endoscopic perspective adopted during the critical phases of dissection should be carefully chosen to avoid disorientation and favor safe directions for surgical instruments; in particular, the ipsilateral perspective provides the best combination in terms of reliability of anatomical landmarks and safety of dissection trajectories. ¹⁵ (3) An in-depth analysis of the course of the internal carotid artery at preoperative imaging and use of a Doppler probe and navigation system are extremely helpful to identify unfavorable anatomic situations (e.g., medial kinking of the internal carotid artery) and localize the vessel at surgery, respectively.

The endoscopic transnasal approach to the upper parapharyngeal space requires the harvesting of a transmaxillary corridor to laterally displace the content of the pterygopalatine fossa and expose the pterygoid process. Up to this step, the dissection can be performed taking advantage of the avascular subperiosteal planes to maintain a clean surgical field. The lower transpterygoid route (i.e., the route through pterygoid plates, sparing the base of the pterygoid process) is then employed to gain access to the pterygoid fossa and subsequently reach the parapharyngeal space. During this part of the dissection, several venous plexuses, vessels, and muscles are encountered, which are a considerable source of bleeding.

Given the impossibility to reach the entire parapharyngeal space via a transnasal route, the approach presented in this chapter fits particularly well with the concept of multiportal surgery. In lesions of the parapharyngeal space also invading the related skull base, surgery should be accurately planned, considering the possibility to combine nondisruptive surgical corridors (i.e., transnasal, transoral, transorbital, transcervical, transpetrosal) to manage advanced lesions. Likewise, the endoscopic parapharyngeal space approach can be combined with transnasal corridors targeting adjacent areas (i.e., infrapetrous, suprapetrous, transcondylar–transjugular tuberculum, medial petrous apex, infratemporal) when addressing extended skull base lesions.

As a final remark, it is worth mentioning that, especially in irradiated patients, wide resections of the upper parapharyngeal space with exposure of the overlying skull base and internal carotid artery frequently require reconstruction to avoid severe complications such as skull base osteitis/osteomyelitis or vessel blowout. Therefore, the surgical team performing the resection should also plan to resurface the defect with one of the many available local/regional reconstructive options (i.e., nasoseptal flap, lateral nasal wall flap, temporoparietal fascia flap).

The reader is asked to perform dissection of the upper parapharyngeal space both focusing on the numerous anatomical details of this anatomical area and keeping in mind the surgical aspects that have been emphasized in the present introduction.

Related posts:

1 Classification of Endoscopic Transnasal Approaches to the Skull Base and Adjacent Areas 9 Transsellar Approach 10 Transsellar Transdorsal Approach 17 Medial Transcavernous Approach 22 Transcondylar/Transjugular Tuberculum (“Far-Medial”) Approach 18 Lateral Transcavernous Approach

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