The Microsurgical Transoral Approach

7 The Microsurgical Transoral Approach

José Alberto Landeiro and Gustavo Augusto Porto Sereno Cabral

Abstract

The transoral approach is a safe and effective surgical procedure for pathologies of the craniovertebral junction (CVJ). It provides direct access from the inferior third of the clivus to the C3 body and allows excellent decompression of the ventral brainstem and upper cervical spinal cord. Understanding the complex anatomy and biomechanics of this region is critical for surgical planning. The choice for this approach depends on patient and pathology details, as with all surgical techniques. The main indications are irreducible ventral compression, with the most common being basilar invagination. There are several peculiarities in the preoperative evaluation, like assessment of oral and dental hygiene, and nutritional. Postoperative care requires evaluation of instability of the CVJ. In recent years, endoscopic endonasal approach has emerged as a valuable option for pathologies of the CVJ.

Keywords: Keywords: craniovertebral junction, transoral approach, microsurgery, atlantoaxial instability, skull base

7.1 Introduction

Pathologies located on the ventral side of the craniovertebral junction (CVJ) are still challenging, despite the progress made over the last decades. Anatomy, joint configuration, shape, and orientation make the CVJ unique in comparison with the rest of the cervical spine.1 This complex musculoskeletal organization provides protection of the brainstem and upper cervical spinal cord, and it allows complex movements of the head and neck.

Remarkable progress has been made in treatment of CVJ pathology over the years, since Kanavel, in 1917, who first described the transoral-transpharyngeal approach to CVJ to remove a bullet lodged between the atlas and the clivus.² Reduction and decompression strategies, and fixation techniques received advancements and refinements throughout the last decades.^{3,4,5,6,7,8,9,10,11,12,13,14} Endoscopic approach has emerged in recent years, providing another option for CVJ pathology.

7.2 Anatomy of the Craniovertebral Junction

The CVJ represents the transition between the brain and the cervical spine and it is composed of the occiput, atlas, and axis. This area contains vital neurovascular structures and it is responsible for the majority of the spine’s rotation, flexion, and extension.^15,16,17 The complexities of anatomy and biomechanics demand a good understanding for evaluation and treatment of the pathologies of the CVJ.

The clivus, the foramen magnum, and the occipital condyles belong to the occipital bone (Fig. 7.1). Besides the horizontal support to the pons, the clivus form the basion. The pontomedullary junction can be revealed after resection of the inferior clivus.¹⁸

Fig. 7.1 Superior (a) and inferior (b) view of the occipital bone and foramen magnum. The basilar part of the occipital bone is referred to as the clivus (1). Foramen magnum is formed by the occipital bone and can have different shapes (oval, round). The occipital condyles (2), located at the condylar part of the occipital bone, are situated lateral to the foramen magnum.

The foramen magnum can have distinct shapes^19,20 and corresponds to the transition between the brainstem as the medulla, entering the vertebral canal as the spine. The occipital condyles articulate with the atlas, permitting extension and flexion of the cranium (Fig. 7.2). The hypoglossal canal is located medial and superior to the occipital condyle.²¹

Fig. 7.2 Superior view of the C1–C2 segment. The superior articular facet of the atlas articulates with the occipital condyle and the inferior articular facet articulates with the superior facet of the axis. There is an articular facet in front of the dens that articulates with the facet on the back of the anterior arch of the atlas.

The atlantoaxial segment is responsible for the rotation of the skull through articulation between atlas and axis. The transverse ligament gives great contribution to the CVJ stability, preventing the dens from folding into the midbrain during flexion. The cruciform ligament is formed by the transverse ligament and the superior and inferior crura (both arise from the transverse ligament, attaching to the anterior foramen magnum and to the body of C2).

The alar ligaments also contribute to the CVJ stability, with primary function of restricting rotation of the skull.

The tectorial membrane represents the continuation of the posterior longitudinal ligament as it extends superiorly, attaching to the clivus before incorporating with the dura mater. Restriction of extension and flexion, and protection of the dura from the dens are some of the functions proposed for this membrane.^22,23,24,25

The apical ligament is proposed to be vestigial and does not have integrity to impact forces of flexion and extension. The anterior atlantooccipital membrane is continuous with the articular joint capsules, from the anterior tubercle of the atlas to the occiput.

The C1 nerve exits through a groove in the posterior ring of atlas, shared with the vertebral artery. The C2 nerve exits more laterally than C1. The C3 nerve and distal branches of C2 nerve have a medial and superior course, passing to the posterior CVJ as the greater, lesser, and C3 nerve.

7.3 Indications

Regarding the vital neural structures contained in the CVJ, instability or mass effect caused by any pathology can result in neurological deficit. Therefore, surgical access to the CVJ is crucial for decompression and stability.10 The transoral approach provides the most direct access to pathologies located ventrally at the CVJ.^{12,26,27,28,29,30,31}

A wide range of congenital, developmental, and acquired pathologies located at the anterior portion of the CVJ can be approached transorally:

●Basilar invagination (Fig. 7.3)

●Odontoid fracture or nonunion

●C1–C2 instability (Fig. 7.4)

●Odontoid hypoplasia

●Tumors

Piecemeal decompression of brainstem and spinal cord can be achieved in small, benign extradural lesions and degenerative conditions. More extensive exposure is required for en bloc resection, which is associated with morbidity.

Fig. 7.3 Sagittal plane, CT scan showing a higher position of the tip of the odontoid process.

Fig. 7.4 T2-weighted midsagittal MRIs showing important compression of the brainstem and medulla oblongata.

7.4 Preoperative Evaluation

Imaging studies should include CT scans and MRI to provide a better visualization of the lesion and to allow a good planning. Dynamic radiographs are useful for assessment of atlantoaxial instability.

It is very important to address oral and dental hygiene in order to limit bacterial contamination of the operative filed. Dental caries should be treated correctly.

Brainstem and cranial nerve functions should be examined. Patients with vagal, hypoglossal, and/or glossopharyngeal disfunction should be considered to receive tracheostomy. Preoperative nutritional support is necessary for malnourished patients with impaired swallowing.

Limited mouth opening can make transoral-transpharyngeal techniques very challenging. In this situation, transmaxillary and/or transmandibullary techniques may be needed to provide adequate exposure.

7.5 Operative Technique

Transoral surgery is performed under general anesthesia and all patients receive prophylactic antibiotics. Intraoperative somatosensory evoked-potential monitoring is useful for assessment of the spinal cord and brainstem status during the surgery.

Patient is placed in the supine position, with the head slightly extended and secured with three-point skull fixation. The operation table is placed in a slight Trendelenburg position. The surgeon usually stands behind or on the right side of the patient.

A nasotracheal armored tube is inserted to provide adequate ventilation. Wide exposure of the posterior oropharynx is achieved with a self-retaining retractor system (Fig. 7.5). The tongue and endotracheal tube are depressed, and the soft palate is elevated and compressed backwards, assisted by two nasal tubes fixed to the palate. Teeth protectors are useful and help to fix the self-retaining retractors during the surgery.

Posterior pharyngeal mucosa is infiltrated (xylocaine with adrenaline solution) to reduce mucosal bleeding. A midline incision in the posterior mucosa and the pharyngeal muscles is done in one single plane up to the anterior longitudinal ligament (Fig. 7.6). The anterior tubercle of the atlas is an important landmark that can be palpated before the incision. Therefore, an incision of 2.5 cm below and above the anterior tubercle is usually sufficient to reach the body of the axis and the lower clivus.

After the incision, the pharyngeal muscles and mucosa are retracted. Longitudinal ligaments are dissected from the bone (Fig. 7.7), with periosteal dissectors, releasing the anterior facets of the axis body, the arch of C1, and the inferior border of the clivus (Fig. 7.8).