The Extreme Lateral Approach for Chordomas and Chondrosarcomas of the Craniovertebral Junction

Abstract

The craniovertebral junction is a complex region that includes the lower clivus, foramen magnum, C1, and C2. Chordomas and chondrosarcomas are slowly growing tumors that arise from notochord rests in the clivus and cartilaginous synchondrosis, respectively. Given their slow growth, they are often large and involve multiple anatomic compartments at clinical presentation. The extreme lateral approach is ideal for accessing tumors that involve the lower clivus, C1/C2, and the retropharyngeal space. Limitations of this approach are access to the contralateral condyle and the clivus above the jugular foramen. Tumors that extend beyond the limits of the approach should be treated with a combined approach. If the occipital condyle or C1/C2 joint are resected, occipital cervical fusion is necessary. Careful planning of the order and timing of multiple approaches and stabilization is necessary for these complex tumors.

Keywords

Chondrosarcoma, Chordoma, Extreme lateral, Occipital condyle, Occipitocervical fusion, Skull base

Outline
Introduction 221
Pathophysiology 221
Surgical Anatomy 222
Indications 223
Preoperative Evaluation 223
Operative Procedure 223
- Intraoperative Monitoring 223
- Positioning 223
- Skin Incision and Soft Tissue Dissection 224
- Exposure and Isolation of the Vertebral Artery 224
- Bone Removal 225
- Tumor Resection 226
- Closure 226
- Stability of the Craniovertebral Junction 226
Discussion 226
- Case Example 1 227
- Case Example 2 228
Conclusion 228
References 229

Introduction

The craniovertebral junction, a complex anatomical region, is the site of a variety of tumors that require surgical resection. These tumors range from benign to malignant and may be intradural, extradural, or both. George et al. found 28 chordomas and 19 other bony tumors in a series of 230 tumors of the foramen magnum. In a recent series of 212 tumors of the lower clivus, there were 73 chordomas and 23 chondrosarcomas. These tumors arise from the bone anterior or anterolateral to the neuraxis.

Pathophysiology

Chordomas arise from notochordal rests within the clivus, and as such arise in the midline. Chondrosarcomas arise from the cartilage of the skull base, and as a result originate lateral to the midline. As tumors that originate in bone, they begin as entirely extradural and often do not present until they have become quite large. As they grow, they destroy bone and extend to neural foramina, occipital condyle, and the occipitoatlantal joint capsule. Over time they may grow into and through the dura. Tumors may even be found between the meningeal and periosteal layer of the dura. These tumors may adhere to the adventitia of the vertebral artery but rarely invade the artery itself.

Although generally slowly growing, both types of tumors have a propensity for local invasiveness and recurrence. As a result, radical surgical resection is often necessary to achieve long-term control. The goal of the surgical approach and exposure is to provide the ability to obtain a safe, complete surgical resection with wide tumor-free margins. Occasionally, maximal direct access may require a combination of approaches.

Surgical Anatomy

The craniovertebral junction is a complex region consisting of the lower clivus, foramen magnum, and C1 and C2 vertebrae. The lower clivus is the portion of the basioccipital bone that extends from the jugular foramen to the foramen magnum ( Fig. 19.1A ). Contained within this region are multiple neurovascular foramina. The jugular foramen is bordered by the occipital bone medially and the petrous temporal bone laterally. The jugular foramen transmits cranial nerves (CNs) IX, X, and XI and the jugular bulb. The hypoglossal canal is located inferomedial to the jugular foramen within the middle third of the occipital condyle. Between the jugular foramen and the hypoglossal canal there is a protuberance known as the jugular tubercle ( Fig. 19.1A ). The occipital condyles are located within the anterior half of the circumference of the foramen magnum. Inferiorly the occipital condyle articulates with the C1 lateral mass. The anterolateral surface of the C1 lateral mass continues to become the transverse process of C1. This is an important landmark that can be palpated inferior to the mastoid tip. The C1 transverse process is the attachment site for the superior oblique, inferior oblique, levator scapulae, and rectus capitis minor muscles. Between the C1 transverse process and the lateral mass is the foramen transversarium through which the vertebral artery passes before wrapping around the medial surface of the C1 lateral mass ( Fig. 19.1B ). Anteromedial from the C1 lateral mass is the anterior arch of C1. This articulates with the odontoid process of C2 and is stabilized by the transverse, cruciate, and alar ligaments. The inferior aspect of the C1 lateral mass articulates with the superior articulating facet of C2.

The craniovertebral junction contains the third segment of the vertebral artery, which starts at the transverse foramen of C2 and ends as the artery pierces the dura at the foramen magnum ( Fig. 19.1B and C ). Just proximal to the C2 transverse foramen, the vertebral artery courses under the pars of C2 before turning laterally to exit the transverse foramen. There is often redundancy within the segment of artery between the C2 and C1 transverse foramina to allow for proper rotation of the spine ( Fig. 19.1D ). Once the artery passes through the C1 transverse foramen, it passes within the suboccipital triangle and then runs within the groove of the sulcus arteriosis over the posterior arch of C1. The artery is surrounded by a periosteal sheath as well as significant venous plexus. Within this segment the artery may give off muscular branches. In approximately 5% of patients there will be an extradural origin of the posterior inferior cerebellar artery (PICA) within this segment. This must be recognized preoperatively to avoid injury and neurologic deficit.

Indications

The extreme lateral approach is utilized for resection of chordomas and chondrosarcomas located anteriorly or anterolaterally within the craniovertebral junction. It is ideal for tumors that involve or extend lateral to the occipital condyle. When the vertebral artery on one side is involved by the tumor, this approach allows the surgeon to control and mobilize the artery and remove the tumor around it. Additionally it can be utilized to approach the anterolateral aspect of C1 and C2.

Preoperative Evaluation

A thorough preoperative evaluation of the extent of tumor involvement and bone destruction is necessary for successful surgical resection of tumors in this region. Magnetic resonance (MR) imaging with constructive interference in steady state sequencing will display the extent of tumor involvement and relation of the tumor and the lower CNs. Chordomas are iso/hypointense on T1-weighted imaging and hyperintense on T2/fluid-attenuated inversion recovery–weighted imaging. There is heterogeneous enhancement with the administration of gadolinium. A thin-cut computed tomographic (CT) scan of the skull base and upper cervical spine will depict the degree of bony destruction and inform regarding the possible need for instrumentation and stabilization. Vascular imaging such as CT angiogram or MR angiogram may be helpful in identifying the path of the vertebral artery, artery dominance, and an extradural PICA. Additionally, an MR venogram is helpful in identifying venous sinus dominance. In some cases, a cerebral angiogram is necessary to fully resolve any vascular questions. Patients should also have a full evaluation of the lower CNs with a laryngoscopic examination and swallow evaluation.

Operative Procedure

Intraoperative Monitoring

Patients are monitored with standard motor evoked potentials and somatosensory evoked potentials. If substantial compression exists at the craniocervical junction, baseline evoked potentials are obtained prior to positioning. The process of positioning may lead to increased compression, which can be revealed by a change or decrease in neurologic potentials. CN VII and the lower CNs (IX, X, XI, XII) are monitored with direct electromyographic electrodes. Monitoring of the larynx is performed through electrodes placed within a specialized endotracheal tube. A stimulating probe is utilized for direct nerve stimulation in identifying nerves to the face, larynx, trapezius, and tongue.

Positioning

The patient is placed in a lateral decubitus position with the approach side up. The dependent portions of the body are well padded to avoid skin breakdown as they are usually in one position for a long time. An axillary roll is placed against the dependent chest wall to avoid injury to the brachial plexus. The head is held by three-point fixation. To avoid cervicomedullary compression, ventral flexion is avoided. The head is slightly inclined toward the dependent shoulder, and the ipsilateral shoulder is taped down. This opens the working corridor and improves the anterior and superior view of the operating microscope. When taping the ipsilateral shoulder down, excessive traction should be avoided to prevent brachial plexus injury. The patient must be properly secured with side-to-side taping and straps to allow for bed rotation during the procedure.

A variation of the lateral decubitus, the lateral park bench ( Fig. 19.2 ), is sometimes utilized in large patients. In this configuration, the dependent arm hangs over the edge of the bed and is supported by a padded sling. This increases the ability to flex the head toward the dependent shoulder, thus opening the interval between the skull base and the ipsilateral shoulder.