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

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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