Endoscopic Approaches to the Craniovertebral Junction

Overview

Surgical access to the craniovertebral junction (CVJ) is challenging because of its deep, anatomically protected location and the diverse pathology involved in this region. Examples of lesions that involve the CVJ include rheumatoid pannus, lower clival chordomas and chondrosarcomas, congenital skull base malformations and basilar invagination, metastatic disease, and even intradural pathology such as meningiomas and vascular malformations. The most direct approach to the anterior CVJ is by the ventral transoral route. Initially described by Fang and Ong in 1962 and refined by Menezes, Crockard and Hadley, this approach is the preferred workhorse for pathology in this location; however, it has several limitations. Specifically, the microscopic transoral route creates a deep and narrow working channel that can produce inadequate visualization of the pathologic process and surgical bed. To improve visualization and access, additional approaches to the craniocervical junction include transfacial and high-cervical retropharyngeal approaches. In addition, lateral and posterior approaches to the CVJ have been described, but we focus our discussion on ventral approaches in this chapter.

Based on the significant morbidity associated with the more traditional microsurgical approaches, surgeons have pioneered less-invasive techniques to approach the CVJ. Although the operative microscope provides direct illumination of the operative field, illumination and visualization are restricted to a narrow cone of direct light. In contrast, endoscopes can provide direct illumination with a wider, panoramic field of view. Fortunately, endoscopes have become commonplace in hospitals and operating rooms across the world, and the technology is easily available in almost any operating room. Endoscopic illumination is at the end of a long glass rod, allowing light to penetrate deeper and closer to the target. Modern endoscopes can also capture a large field of view up to about 80 degrees, giving the surgeon a panoramic perspective.

The familiar drawback of the endoscope is its ability to give only a two-dimensional (2D) image, which can be somewhat overcome with movement of the scope and manual palpation to provide secondary depth perception clues. In addition, resolution of an endoscope coupled to a camera is only as good as the camera and screen (e.g., 1080 light pairs), whereas a microscope allows for direct visualization using the human retina, which has dramatically more resolving power than even the best high-definition video. Nevertheless, rapid improvements in video technology and the introduction of three-dimensional (3D) endoscopes have continued to provide advancements in the field. Endoscopes can be used free-hand, or they can be placed in a holding system to allow the surgeon and assistant full use of both hands during the procedure. Endoscopes come in a wide variety of angles but typically only a zero-degree, 30-degree upviewing, and 30-degree downviewing endoscope are needed for neurosurgery, because greater-angle scopes may provide adequate visualization without the corresponding ability to perform the manual dissection.

Given the improvements in endoscopic access to the ventral skull base and ventral CVJ, this chapter will review three endoscopic approaches: 1) endoscopic endonasal; 2) endoscopic transoral, including transoral robotic surgery (TORS); and 3) endoscopic transcervical. All use endoscopy to aid in visualization of the CVJ, and all attempt to avoid the morbidity usually associated with the more “open” approaches discussed above.

Surgical Anatomy

Because the ventral transoral route is the standard approach to the CVJ, we only briefly review the anatomy here with a focus on endoscopic nuances.

Endoscopic Endonasal Approach

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This approach allows access to the skull base superiorly but is limited to the upper C2 inferiorly ( Fig. 7-1 ).

Figure 7-1

The limits of surgical access for transnasal and transoral endoscopic surgery.
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When entering the nasal cavity, the turbinates must be lateralized, and the nasal septum must be resected, followed by a sphenoidectomy.
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By removing the posterior nasal septum, space is made for four instruments to pass through both nares.
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The floor of the sella, the carotid protuberances, and the upper clivus can be visualized after removal of the posterior sphenoid sinus wall using a zero-degree endoscope; a 30-degree endoscope can be used to visualize more superiorly or inferiorly as necessary.
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A midline incision in the posterior pharyngeal mucosa should be kept above the Passavant ridge to avoid subsequent swallowing difficulty. The mucosa can be retracted out of the way of surgical dissection with use of long, thin, vocal cord retractors that can be placed either endonasally or transorally to allow the surgeon to continue to use the endonasal access for surgical instruments.

Endoscopic Transoral Approach

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The transoral approach is typically limited by mouth opening and by the concern for disarticulation of the temporomandibular joint (TMJ) with excessive mouth opening. In addition, the hard palate remains a physical barrier for superior access along the lower clivus. Both mouth opening and restricted view secondary to the hard palate are less of an issue with use of an endoscope, which can provide illumination and visualization that is more panoramic and not necessarily limited to line of sight.
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The soft palate can obscure the view, but it can be retracted and elevated using red rubber catheters taken out through the nose and the mouth ( Fig. 7-2 ).

Figure 7-2

Zero-degree endoscopic view of the uvula and soft palate being retracted superiorly.
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Splitting of the soft palate for better superior or inferior visualization is rarely needed with angled endoscopes; if necessary, it can be performed by carrying an incision from the posterior soft palate anteriorly through the uvula to the hard palate, securing both flaps laterally ( Fig. 7-3 ).

Figure 7-3

Splitting of the soft palate by carrying an incision from the posterior soft palate anteriorly through the uvula and reflecting the divided tissue laterally.
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The anterior arch of the atlas can be visualized through the soft tissues of the pharyngeal mucosa and can be palpated with soft pressure (see Fig. 7-3 ).
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A midline incision in the posterior pharyngeal mucosa can be carried down to the bone at C1 and extended inferiorly as an incision in the midline, a relatively avascular plane ( Fig. 7-4 ).

Figure 7-4

A midline incision is made in the posterior pharyngeal mucosa.
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A U-shaped incision, preferred by some surgeons, may be made, but it must be limited to the mucosal layers only, because the deeper muscular layers contain the pharyngeal plexus of the vagus nerve ( Fig. 7-5 ).

Figure 7-5

Endoscopic view of the landmarks used in the endoscopic transoral approach, including the eustachian tubes, anterior atlas, and soft palate. A U-shaped incision can be made in the posterior pharyngeal wall but must be limited to the superficial muscular layers to avoid injury to the pharyngeal plexus of the vagus nerve.
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The surgeon must be aware of the possibility of aberrant vertebral arteries, although these usually lie at least 1 cm from the midline ( Fig. 7-6 ).

Figure 7-6

Bony anatomy and vertebral artery course encountered during a ventral approach to the craniovertebral junction. The vertebral arteries lie laterally but may be aberrantly medial in some cases.
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The eustachian tubes are an excellent landmark, although their position may be altered as a result of the pathologic process; they represent the lateral extent of the subperiosteal dissection at the level of C2, because they are relatively fixed in the skull (see Fig. 7-5 ).
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The endoscopic transoral approach allows access to the lower one third of the clivus and inferiorly to the C2–C3 disk space (see Fig. 7-1 ).

Endoscopic Transcervical Approach

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The endoscopic transcervical approach permits access to the anterior tubercle of C1 superiorly and the lower cervical spine inferiorly. This approach theoretically has no lower limit, because a wide transverse cervical incision can expose the cervical spine to the cervicothoracic junction.
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Anatomy via this approach is well known to spine surgeons; it mirrors the anatomy encountered when performing an anterior approach to the lower cervical spine. Careful attention to the location of the digastric muscle and hypoglossal nerve will help to minimize traction injury to that nerve.
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A tubular retractor system helps to minimize overly aggressive retraction, and the use of a 30-degree endoscope positioned at the superior portion of the tubular retractor can look “down” on the anatomy, providing the familiar perspective of a head-on view of the ventral cervical spine.
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Visualized anatomy in this approach is limited to the surface anatomy that can be visualized through the tubular retractor. If there is a need to see an area greater than that provided by the tubular retractor, the retractor must be repositioned along with the endoscope. This may make it difficult to see anatomic relationships to neighboring structures.

Operative Techniques

Endoscopic Endonasal Approach

Rationale

Transnasal endoscopic surgery has become the workhorse of minimally invasive approaches to the ventral anterior skull base and is primarily used to tackle tumors in the anterior skull base and the sellar/suprasellar region. Endoscopic transclival surgery is technically feasible, and approaches through the endonasal route to the CVJ are possible, including both extradural and intradural pathology. It is best used for tumors of the lower clivus that secondarily involve the CVJ.

Preparation and Positioning

When approaching the CVJ via the endonasal route, it is important to take into account that this approach is not capable of providing access to the lower portion of the body of C2 (see Fig. 7-1 ). If a large dural opening is planned for intradural pathology, such as a foramen magnum meningioma, closure techniques must be addressed prior to surgery. A lumbar drainage device may be placed for cerebrospinal fluid (CSF) diversion and may be used as an aid to dural closure and healing, although our group has gradually moved away from this as a routine adjunct for dural closure. In addition, proper consideration of vascularized closure with mucosal flaps must be made in conjunction with a rhinology or head and neck ENT colleague.

The patient is positioned supine on the operating table. If imaging guidance is to be used, the preferred system may be set up at this time, which may require rigid skull fixation. A surgeon and assistant stand on either side of the patient at the level of the patient’s shoulders; viewing monitors are situated at the head of the bed ( Fig. 7-7 ). The patient is given a broad-spectrum antibiotic; the nares are not usually prepped with any specific topical antibiotic. The nasal mucosa is subjected to cocaine or cottonoids soaked in Afrin, and major mucosal feeding vessels and mucosa are injected with a vasoconstrictor, such as lidocaine, to reduce bleeding and mucosal bulk.