The carotid-vertebral space: an 'extended' lateral window to the ventromedial cranial base and lower craniocervical junction.

Abstract

We describe a unique method of accessing the ventromedial skull base and lower craniocervical junction. Our method employs a trajectory between that of the more anterior transoral or retropharyngeal approaches and the various posterior or posterolateral skull base approaches. This "extended" lateral approach allows surgeons to resect very large tumors of the skull base through a single incision. The operative field is more extensive than that achieved with other approaches; it extends from the cerebellar hemisphere to the extradural ventral upper cervical spine, and it provides access to tissue outside the spinal canal, such as the ventral strap muscles. We describe our use of this approach during a single-stage resection of a large hemangiopericytoma in a 37-year-old man.

Introduction

The osseous framework of the ventromedial cranial base is made up of the caudal segment of the clivus, the upper portion of the dens, the medial component of the occipital condyle, and both the superior articular facet and the anterior tubercle of C1. Critical neurovascular structures--including the carotid and vertebral arteries and the lower cranial nerves--are intimately involved in this complex bony anatomy. Gaining access to this region thus presents a formidable challenge to skull base surgeons.

The traditional approaches to gaining access to this area are the ventral (transoral or transmaxillary) approach (1-3) and the posterolateral approach. (4-8) Both have significant limitations. The ventral approach does not provide sufficient posterolateral exposure, thereby preventing adequate vascular control and limiting resection of any lateral skull base component. Conversely, the posterolateral approach restricts access to the most medial and caudal portions of the ventral skull base. In 1998, a unique posterior approach to this region was described by Grundy and Gill, but they were handicapped by an inability to reach anterior to C1. (9)

We believe that mobilizing the lower cranial nerves and then creating a window between the internal carotid artery and the vertebral artery provides direct controlled access to this location. In this article, we report our experience with this approach in facilitating a complete resection of a hemangiopericytoma that affected the occipital condyle, clivus, upper dens, and paravertebral strap muscles ventral to the ring of C1.

Case report

A 37-year-old right-handed man presented with a 2-month history of progressive left-sided lower cranial neuropathies. He initially developed ipsilateral hypoglossal paralysis, which was followed sequentially by spinal accessory weakness, a change in voice quality, and swallowing dysfunction that limited his oral intake. Examination demonstrated dysfunction of cranial nerves X through XII.

Computed tomography (CT) revealed that an extensive destructive lesion was centered in the hypoglossal canal and had invaded the lateral and ventral skull base (figure 1, A). The lesion had destroyed the occipital condyle and traveled through the clivus. It also extended caudally into the paravertebral soft tissue surrounding C1 and C2. Magnetic resonance imaging (MRI) demonstrated that intradural extension resulted in brainstem compression and created an intimate relationship between the lesion and both the vertebral artery and the cervicomedullary junction (figure 1, B); the tumor also extended rostrally to the middle cerebellar peduncle (figure 1, C). Angiography revealed that the lesion was extremely vascular (figure 1, D). The ascending pharyngeal supply was embolized, but the muscular branches from the vertebral artery and the small branches from the petrous carotid artery were not accessible. The patient underwent an open biopsy via a retromastoid approach, and analysis of the specimen identified it as a hemangiopericytoma. The patient was scheduled for surgery.

[FIGURE 1 OMITTED]

Preoperatively, an elective tracheotomy and a feeding jejunotomy were performed to optimize perioperative nutrition. The following week, the patient returned to the operating room for resection. He was placed in a semilateral position with a roll under the left shoulder and the head rotated to the right. Positioning was performed under somatosensory evoked potential monitoring to mitigate against the consequences of potential instability and brainstem compression by the tumor mass.

A standard extended retroauricular incision was made from the midtemporal area through the retromastoid area (incorporating the previous biopsy incision) and then carried medially across the anterior border of the sternocleidomastoid muscle (figure 2). We first used the ventral cervical portion of the incision to expose the internal carotid artery and trace it up to the skull base, thus providing proximal vascular control. In the process, the cervical portions of cranial nerves IX through XII were isolated from the strap muscles, and the jugular vein was transected.

[FIGURE 2 OMITTED]

To further assist in achieving proximal vascular control and to devascularize the tumor early on, we next isolated the vertebral artery. In brief, the lamina of C1 was identified and traced laterally to the transverse process. The venous plexus and the fat pad over C1 were used to identify the vertebral artery in this area. The transverse process of C1 was resected, and the foramen transversarium was unroofed with a high-speed drill. This provided complete access to the vertebral artery and its muscular branches, which were systematically transected to devascularize the tumor and untether the vertebral artery for subsequent caudal mobilization.

To gain further rostral access to the tumor (given its involvement with the posterior temporal lobe), the retrocochlear mastoid area was drilled away. This was undertaken posterior and inferior to the cochlea and the facial nerve; the nerve was isolated and protected without any need for transposition. Next, the sigmoid sinus was isolated at the level of the petrosal system, suture-ligated, and resected. The venous phases on the preoperative angiogram had confirmed the patency of the torcula and dominance of the contralateral sigmoid sinus, assuring us that the ipsilateral sigmoid sinus could be resected safely. Labbe's vein was still able to drain in a retrograde direction through the transverse sinus and out the contralateral jugular system. Such a maneuver is not possible if a unilateral sigmoid sinus is demonstrated on preoperative imaging or if the torcula is not patent. The transsigmoid exposure we were able to create not only provided complete access to the superior extent of the tumor, it also facilitated exposure of the internal carotid artery as it entered the petrous canal.

At this point, the fine tumor vessels along the course of the internal carotid artery were easily taken to further devascularize the tumor. This approach to gaining access to the intracranial extent of the tumor is not unlike a post-auricular transtemporaljugular foramen type of approach, but it would not by itself permit sufficient access to the extradural, more caudal component of the tumor.

Indeed, in starting resection at the tumor's superior extradural pole, we found that part of this area could have been accessed through a posterolateral approach. However, as we moved caudally, we observed that the tumor had extended toward the dens, which necessitated a ventral access that would not be possible with a posterolateral approach alone. The previously isolated lower cranial nerves and the upper portion of the internal carotid artery were then gently mobilized anteriorly, and the untethered vertebral artery was retracted posteriorly (figure 3). This maneuver is the key to achieving the desired exposure, and it greatly enhances access to the clivus and to the most medial portion of the occipital condyle, which we resected up to the hypoglossal canal. Perhaps most important, this maneuver provides ventral and caudal access to the top of the dens and to the related anterior tubercle of C1 and the associated paraspinal strap muscles. Resection was continued in this difficult area until negative tumor margins were obtained. This required resection of the tip of the dens and the synovial joint capsule that articulated the dens with the anterior tubercle of C1.

[FIGURE 3 OMITTED]

The exposure we have described allowed for access to the tumor from multiple angles. Once the dura was opened, this exposure obviated the need for retraction because the intradural tumor could then be approached from a low and lateral angle. Furthermore, the circumferential devascularization we had undertaken while achieving this exposure made resection of the intradural portion of the tumor a relatively straightforward procedure despite its vascularity. Following gross total resection, the dural defect was reconstructed with a cadaveric pericardial graft and augmented with fibrin sealant. The bony defect was recontoured with hydroxyapatite cement.

Postoperatively, the patient recuperated uneventfully and did not experience any new neurologic deficits. In fact, we noted marginal improvements in his spinal accessory and hypoglossal nerve function, and we were able to remove his tracheotomy and jejunotomy tubes. No residual enhancement was seen on imaging, confirming that the resection was complete (figure 4).

[FIGURE 4 OMITTED]

The patient was discharged 1 week following surgery after receiving a Miami J cervical collar. Adjuvant radiotherapy was held in abeyance pending an assessment of his progress. He returned 1 week following discharge to undergo an occiput-to-C3 fusion through a separate posterior midline incision.

Discussion

Fortunately, lesions that affect the ventromedial skull base are rare. When they do occur, an individually tailored approach is required to achieve controlled access while preserving neurovascular function. When these lesions extend caudally and ventrally to involve the further reaches of the craniocervical junction, critical neurovascular structures can be mobilized to provide circumferential access to the lesion. This provides the surgeon with the best chance of preserving neurologic function.

The key to achieving this exposure is complete dissection of the neurovascular structures, including release from their respective bony canals. This requires mobilization of the carotid artery anteriorly and the vertebral artery posteriorly to create an effective window into the lower craniocervical junction. If necessary, rostral access into the ventromedial cranial base can be achieved by ligation of the sigmoid sinus. Mobilization of these structures dramatically enhances the previously described posterolateral approaches by providing access to the occipital condyle and the upper cervical spine as far medially as the top of the dens and the paravertebral strap muscles along the ventral body of C2.

We believe that our modification of the posterolateral approach may obviate the need for second-stage ventral procedures, thus reducing morbidity.

References

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(8.) Ruckenstein M J, Denys D. Lateral skull-base surgery--A review of recent advances in surgical approaches. J Otolaryngol 1998;27: 46-54.

(9.) Grundy PL, Gill SS. Odontoid process and C1-C2 corrective ostcotomy through a posterior approach: Technical case report. Neurosurgery 1998;43:1483-6, discussion 1486-7.

From the Department of Neurological Surgery (Dr. Kassam, Dr. Patel, Dr. Welch, and Dr. Balzer) and the Department of Otolaryngology (Dr. Snyderman, Dr. Hirsch, and Dr. Carrau), University of Pittsburgh Medical Center.

Reprint requests: Amin Kassam, MD, Department of Neurological Surgery, UPMC Presbyterian, Suite B-400,200 Lothrop St., Pittsburgh, PA 15213. Phone: [phone]; tax: [phone]; e-mail: [e-mail]