Background

Reports on the use of transoral robotic surgery (TORS) for approaches to the nasopharynx and anterior skull base are present in the literature. As compared to the well-documented approaches using TORS to the oropharynx, hypopharynx, and supraglottis, the approaches to the nasopharynx remain limited to scattered case reports and cadaveric studies. Patient selection is paramount, and preoperative imaging and planning remain the mainstay of safe surgery. Specifically, given the anatomical limits of the nasopharynx, combined approaches are necessary to obtain adequate exposure and access for instrumentation. Some considerations include the lack of a drilling apparatus as part of the TORS system, thereby limiting access through the bony skull base. Some of these restrictions can be overcome by combining a transnasal endoscopic approach with TORS.

Indications

No well-established indications exist for use of TORS in the nasopharynx, although appropriate patient selection dictates success of surgery. The curved instrumentation of the robot allows access to lateral tumors that supersedes endonasal instrumentation, although some limits to lateral resection are present. Although small tumors located in the middle of the posterior nasopharyngeal wall are ideal, case reports describing resection of nasopharyngeal tumors extending to the pharyngeal fossa (of Rosenmüller), infratemporal fossa, anterior skull base, sphenoid sinus, clivus, middle cranial fossa, and craniocervical junction are available in the literature.

Contraindications include radiologic evidence of tumor adjacent to the common or internal carotid artery, any T-stage tumor with fixed invasion of tissues lateral to the pharyngeal constrictor muscles or posterior to the prevertebral fascia, unresectable node involvement, and dermal metastasis. Non– tumor-related contraindications include trismus preventing adequate exposure, a retropharyngeal internal carotid artery, and any medical comorbidities precluding the patient for general anesthesia or surgery. Dentition of the patient should also be taken into consideration when evaluating extent of oral opening for access.

Anatomical Considerations

The nasopharynx is located 10 cm from the surface of the skin. It is bounded laterally by the medial pterygoid plates and medial pterygoid muscle, posteriorly by the superior pharyngeal constrictor and prevertebral muscles, and superiorly by the floor of the sphenoid sinus. The pharyngeal fossa is medial to the opening of the auditory (eustachian) tube and torus tubarius. The auditory tube exits the skull just medial to the spine of the sphenoid bone at the junction of the infratemporal fossa and poststyloid space. Fig. 5.1a and Fig. 5.1b demonstrate sagittal and axial illustrations of these structures.

The serpentine course of the internal carotid artery (ICA) is key in maintaining safe dissection. The ICA is encased in the petrous portion of the temporal bone after it enters the carotid canal at the base of the skull. It then travels in the vertical direction until it passes medial to the osseous part of the auditory tube, where it takes a turn in the anteromedial direction to form the horizontal segment. The horizontal segment ends when the ICA takes a vertical turn to pass along the lateral wall of the sphenoid sinus. It then traverses the cavernous venous sinus before entering the intracranial space.

Nasopharyngectomy can involve dissection into the skull base depending on the extent of the tumor. The anterior cranial base extends from the frontal bone, over the orbital roof, to the anterior edge of the greater wing of the sphenoid bone. The roof of the ethmoid and sphenoid sinuses forms the floor of the anterior cranial base between the orbits. The planum sphenoidale is the roof of the sphenoid sinus and is the most posterior limit of the anterior cranial base. Lateral to this are the optic canals. The middle cranial base is composed of the greater wing and body of the sphenoid, the petrous bone anterior to the petrous ridge, and the squamous portion of temporal bone. The sella turcica, which houses the pituitary gland, sits behind the posterior wall of the sphenoid sinus and below the optic chiasm.

Tumors of the nasopharynx can also extend laterally into the infratemporal fossa. The infratemporal fossa is bounded anteriorly by the posterior buttress of the malar eminence and maxillary sinus, posteriorly by the glenoid fossa and mandibular condyle, and medially by a plane extending from the lateral pterygoid plate to the spine of the sphenoid bone. ¹ The fossa contains the insertion of the temporalis muscle, the pterygoid muscles, the foramen ovale and mandibular nerve (CN V₃), and the foramen spinosum and middle meningeal artery. The auditory tube exits the skull just medial to the spine of the sphenoid bone at the junction of the infratemporal fossa and poststyloid space. The poststyloid space contains the entrance to the carotid canal and jugular foramen (with the glossopharyngeal and hypoglossal nerves, CN IX/XII). It is located behind a plane that connects the medial pterygoid plate to the styloid process. The pterygopalatine fossa is a space between the posterior wall of the maxillary sinus and pterygoid plates. The inferior orbital fissure opens into the pterygopalatine fossa, which contains the foramen rotundum (with the maxillary nerve, CN V₂) and the nerve of pterygoid canal (vidian nerve).

Described Approaches

Well-documented transnasal endoscopic approaches have been described for removal of small tumors of the midline nasopharynx and skull base; however, the extent of resection is limited for lateral access because of the limitations of currently available endoscopic instruments. TORS can overcome some of these constraints with the advantage of wristlike mobility of the robot instruments utilizing two-handed surgery, which can provide lateral access as well as the availability of wide-angled magnified 3-D view and the ability to see “around corners.” The use of TORS for the nasopharynx has been documented in several cadaveric studies and case reports, cataloging several approaches to the nasopharynx utilizing TORS. These include the isolated transoral approach with and without transpalatal incision, the combined transnasal approach, and the transcervical approach.

Transoral Approach

Approaching the parapharyngeal space and infratemporal fossa using TORS alone is described in one human patient as well as cadaveric and live canine studies. ² The approach begins with appropriate intraoral exposure using a Crowe-Davis retractor; a 30° angled high-magnification lens is required. All robotic arms are docked in the mouth. Electrocautery is used to make an incision lateral to the anterior tonsillar pillar. Dissection is carried through the palatopharyngeus and palatoglossus muscles to the parapharyngeal fat. The tonsillar branches from the external carotid artery are then dissected and ligated with endoscopic surgical clips. The glossopharyngeal nerve is dissected and preserved. The internal maxillary artery is then encountered and dissected anterolaterally. Access to the poststyloid and infratemporal region is then gained (as described in Chapter 3). Further dissection identifies the internal carotid artery and the glossopharyngeal, vagus, and accessory nerves (CN IX, X, XI) in the infratemporal fossa. Entry into the jugular bulb within the bony skull base is also identified.

Although this approach allows exposure of the skull base, with identification of foramina and respective cranial nerves, there are limitations to the bony work that can be performed. Current lack of robotic drills and burrs prohibits resection of the cranial base and intracranial work with a TORS-only approach. Of note, dissection could not be carried inferiorly below the level of the carotid bifurcation with this approach. ² Although this approach is suited for benign lesions such as cysts, schwannomas, or adenomas, the wide resection needed for malignant or invasive neoplasms is not possible. The lack of drilling limits ability to establish bony margins or to access the cranial cavity.

More recently, with the advent of flexible robotic endoscopes such as the Medrobotics Flex robot (Raynham, MA, USA), these areas can also be accessed transorally with flexible instruments accessing the region posterior to the palate. Fig. 5.3 demonstrates the view achievable with this instrumentation. Additionally, with this tool, the surgical field can extend from torus to torus.

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