Transoral Approaches to the Cervical Spine

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Chapter 175 Transoral Approaches to the Cervical Spine

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.1,2 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. En bloc resections, however, are reserved mainly for primary tumors in which long-term local control may improve survival or neurological 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.3 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.

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 (Figure 175-1).47 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,8 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.2,4,7,9

Technique

The patient is placed supine on the operating table. This should be done while the patient is awake to monitor for neurological changes. Intubation can proceed orotracheally or by tracheostomy. Orotracheal intubation should be performed fiberoptically in patients with cervical instability or cervicomedullary compression, and the tube must be retracted away, out of view. After inducing general anesthesia, a feeding tube is placed to allow for enteral nutrition postoperatively. The neck is placed in gentle extension, and the head is supported on a horseshoe support or rigidly held in place with a Mayfield skull clamp. It is our practice to continuously monitor the motor evoked potentials and somatosensory potentials if possible throughout the operation. Direct palpation of the posterior oropharynx can allow identification of the anterior arch of C1 and the body of C2, thus defining the operative anatomy. Gauze throat packs are placed to occlude the larynx and esophagus to prevent leakage of blood into the stomach. The oral cavity is then cleansed with chlorhexidine gluconate oral solution, and intravenous antibiotics are provided. The patient’s face is draped to allow access to the mouth and the nasal cavity. A self-retaining oral retractor is then placed over the teeth and expanded to keep the mouth open. Self-retaining retractors are then attached to the retractor to keep the tongue depressed (Figure 175-2). Tongue retraction should be released at 30-min intervals to prevent lingual congestion from venous and lymphatic compression.

A red rubber catheter can be placed through the nose and then sutured to the uvula. The catheter can then be withdrawn through the nose to retract the uvula out of the field. If further exposure is necessary, the soft palate can be split at the midline to extend the superior and lateral exposure and prevent obscuration by the uvula. This procedure allows for improved exposure of the lower clivus. The soft palate is divided at its midline extending from the junction with the hard palate to the base of the uvula with deviation off the midline to preserve the uvula itself. Silk sutures are placed and fastened to the mouth gag to provide retraction of the soft palate flaps. The posterior pharyngeal mucosa is infiltrated with 1% lidocaine with 1:100,000 epinephrine, and a midline posterior incision is made extending from the base of the clivus to the upper border of the third cervical vertebra. The anterior tubercle of C1 can help determine the midline, although this landmark is misleading in cases of rotational subluxation. The extent of this incision can be guided by using lateral fluoroscopy or frameless navigation systems.2 Some authors suggest creating an inferiorly based, U-shaped pharyngeal flap such that the apex of the flap lies just above the superior aspect of the anterior arch of C1. This incision can be extended superiorly if more rostral clivus must be exposed. The pharyngeal mucosa, pharyngeal constrictor musculature, and longus colli and longus capitis musculature are then elevated as a single myomucosal flap from the underlying anterior longitudinal ligament. The anterior ligaments of the spine are then coagulated with an insulated electrocautery device and dissected off the inferior clivus, the anterior arch of C1, and the anterior bodies of C2 and C3. The pharyngeal flaps are retracted using Crockard self-retaining retractors.

The transoral-transpharyngeal approach allows lateral exposure of roughly 15 to 20 mm bilaterally off the midline from the inferior clivus to the C3 body (Figure 175-3 and 175-4). Further lateral exposure risks trauma to the Eustachian tube orifice, hypoglossal nerve, Vidian nerve, and carotid artery as it passes in front of the C1 arch laterally.

Following exposure of the upper cervical spine, the anterior arch of the C1 ring is resected. The lateral margins of bone removal are toward the lateral aspects of the odontoid. The odontoid is then resected by drilling in a top-down fashion until the entire odontoid has been removed. Drilling in this fashion keeps the odontoid attached to the base of C2 until the rostral portions have been removed and prevents the odontoid from becoming free-floating, as would be the case if it were truncated at its base. After the odontoid has been completely removed, any pannus and/or the transverse, apical, and posterior longitudinal ligaments can then be resected if need be.

Several complications that may be encountered during the transoral-transpharyngeal approach deserve specific attention. Cerebrospinal fluid (CSF) leaks can lead to devastating complications, should be avoided when possible, and should be managed aggressively when encountered. The treatment of certain spinal pathologies is more likely to lead to inadvertent dural transgression. For example, during transoral odontoidectomies for basilar invagination, as the dens migrates further rostrally, the apical aspect of the dens may erode through the dura and may lead to CSF leaks. The posterior longitudinal ligament and tectorial membrane may be thinned by the dens or a pannus and be more susceptible to inadvertent violation. Because the pannus may resolve after posterior fusion, aggressive resection of the pannus may not be necessary. If the dura is transgressed, primary closure is performed with placement of fascia and bovine pericardium if possible, and the dural closure is reinforced with fat and/or fibrin glue as needed. In such cases, a lumbar subarachnoid drain is placed for CSF diversion. Watertight closure of the posterior pharynx should be performed in two layers (pharyngeal musculature and pharyngeal mucosa), as a single-layer closure may be susceptible to dehiscence.10 Continued leakage of CSF can be difficult to manage and may lead to meningitis.

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