Transoral Approaches to the Cervical Spine

Introduction

Multiple anterior approaches to the craniocervical junction have been described to allow exposure to the midline and lateral aspects of both the cranial base and upper cervical spine. The transoral-transpharyngeal approach, a technique well known to many spine surgeons, provides surgical access to the anterior clivus, C1, and C2. Transoral approaches provide the fundamental techniques upon which the more extensive approaches are based. Although it provides more limited exposure, the transoral-transpharyngeal approach may be appropriate for biopsy, drainage of infections, bony decompression at the craniocervical junction, surgery for small tumors, and surgery for larger tumors that can be removed piecemeal. When a more aggressive surgical approach is required, potentially for local control of low-grade malignant tumors of the skull base, more complex maxillotomy and mandibulotomy approaches can be used. In general, such approaches to this area can be classified as lone transoral approaches with or without palatotomy, approaches that proceed through the maxilla (maxillary osteotomies), and combined transoral-transmandibular approaches associated with displacement of the mandible (mandibular swing transcervical and bilateral mandibular osteotomies). This chapter discusses the transoral approach, the extensions of the transoral approach, and alternative anterior approaches to the ventral craniocervical junction.

Pathology

Numerous pathologies of the anterior craniocervical junction have been approached surgically. Degenerative conditions leading to basilar invagination, odontoid fractures or nonunion, C1–C2 instability with pannus formation of basilar invagination with brain stem compression, odontoid hypoplasia, and tumors involve the majority of such surgically treatable lesions. ^, Understanding the pathology to be treated at the ventral craniocervical junction is critical in determining the optimal surgical approach. In general, the ideal surgical approach to this area is dictated by whether a particular pathological entity is extradural and whether it will be optimally treated in a piecemeal fashion or as an en bloc resection. Most degenerative conditions, for instance, are extradural and can be treated anteriorly with piecemeal decompression of the brain stem and spinal cord. This may also be the case for small, benign, extradural lesions. However, en bloc resections are reserved mainly for primary tumors in which long-term local control may improve survival or neurologic function, such as with primary bone tumors. En bloc resections require more extensive exposure to allow the surgeon access to the entire pathology and thus may be associated with significant morbidity.

Anatomy

It is important to understand the anatomy of the pharyngeal wall and the vertebral artery in this region. The pharyngeal wall is composed of two layers: the mucosa and the prevertebral fascia. Between these two layers is the retropharyngeal space, which contains pharyngeal branches of the carotid, palatine, and pharyngeal arteries as well as the pharyngeal veins. Posterior to the prevertebral fascia lies the prevertebral space, at the center of which lies the anterior tubercle of C1. Prevertebral musculature, including the longus capitis and longus cervicis muscles, course inferolaterally to superomedially. Retraction of the prevertebral muscles reveals the anterior longitudinal ligament overlying the osseous spine. The atlanto-occipital membrane connects the foramen magnum to the anterior arch of the atlas and is a continuation of the anterior longitudinal ligament. After lateral retraction of the prevertebral fascial muscles and mucosa, the atlantoaxial joints can be visualized.

Carotid artery injury can be avoided with thorough understanding of its anatomy. The common carotid artery is contained within the carotid sheath, which is formed from the deep cervical fascia. The common carotid artery bifurcates into the internal and external carotid arteries at the level of the fourth cervical vertebra. The internal carotid artery runs anterior to the transverse processes of the upper vertebrae. During transoral approaches to the cervical spine, the internal carotid artery is at most risk in the vicinity of C1, where it resides anterolaterally to the C1 arch before it enters the skull base.

Knowledge of the course of the vertebral artery in the upper cervical spine as it enters the foramen magnum is critical during anterior approaches to the craniocervical junction. The vertebral artery enters the osseous cervical spine, typically at C5, and is encased in transverse foramina as it ascends to C2. It then runs posterolaterally and enters the C1 transverse foramen. The artery continues medially along the vertebral artery groove that lies on the superior aspect of the posterior atlas posteriorly. It then advances superiorly and enters the foramen magnum.

It is important to note that the C2 segment of the vertebral artery lies more anteromedially than the C1 segment. This should be kept in mind when drilling C1 and C2, to avoid vertebral artery injury. However, it is also critical to acknowledge that the course of the vertebral artery in the craniocervical junction can be anomalous, and all imaging studies should be reviewed carefully to avoid encountering the vertebral artery unexpectedly.

Preoperative Considerations

Several issues should be considered before patients undergoing transoral-transpharyngeal procedures enter the operating room. Optimal oral and dental hygiene may help to limit bacterial contamination of the operative field. Dental caries may be a nidus for infection and should be treated appropriately. Loose dentition should be protected with dental guards or removed prophylactically. Clinicians should examine patients for intraoral or perioral sores that may be a contraindication to proceeding with surgery.

Brain stem and cranial nerve function are commonly affected by diseases of the craniocervical junction and should be explicitly tested preoperatively. Tracheotomy should be considered in patients with vagal, hypoglossal, and/or glossopharyngeal nerve dysfunction. Patients with impaired swallowing dysfunction may be malnourished, and preoperative nutritional support should be considered to improve postoperative wound healing.

Transoral-transpharyngeal techniques can be particularly challenging to use in some children and even in adults who are unable to open their mouths sufficiently wide to provide an adequate surgical corridor. Jaw excursion should be at least 2.5 cm for adequate exposure. Transmaxillary and/or transmandibular approaches may be needed to provide adequate surgical exposure if the patient’s jaw is unable to be opened far enough or if more extensive exposure of the clivus or upper cervical spine is needed.

Transoral-Transpharyngeal Approach With or Without Palatotomy

Traditionally, the anterior craniocervical junction has been approached by using a transoral-transpharyngeal route, known also as the “buccopharyngeal approach.” This approach provides access to the lower clivus, foramen magnum, anterior arch of C1, and the underlying odontoid process of C2, and down to the C3 vertebral body in some patients. The general goal of transoral operations is to correct irreducible anterior compression at the cervicomedullary junction ( Fig. 160.1 ). In this way, it may provide the most direct approach to an abnormality ventral to the brain stem and upper cervical cord. Since 1917, when Kanavel first described the removal of a bullet lodged at the craniocervical junction by using the transoral-transpharyngeal approach, transoral approaches have been used extensively for a wide range of pathological entities. Largely due to advancements in imaging techniques and improved availability of the operating microscope, there have been significant improvements in the efficacy and safety of the transoral approach over the past several decades. ^{, , ,}