Summary
The complex anatomy of the craniovertebral junction is critical both to allow necessary movement of the head and to prevent excess motion that would injure the neurologic and vascular structures at the cervicomedullary junction. Pathology in this area may disrupt the stability of the craniovertebral junction or require surgical intervention that introduces instability. Surgical approaches have been developed to access the region from any direction. Thorough knowledge of the regional anatomy, surgical approaches and their variants, and stabilization techniques is critical to safely manage tumors in this location. This chapter reviews the relevant osseoligamentous, vascular, and neurologic structures; describes investigations and selection of surgical approach; and discusses potential pitfalls of carrying out surgery in this location.
27 Tumors of the Craniovertebral Junction
27.1 Introduction
Management of tumors that arise at the craniovertebral junction (CVJ) is made challenging by the complex osseoligamentous, vascular, and neural anatomy of the region as well as the need to balance dynamic function with mechanical stability. Both erosive neoplasms and extensive operative approaches can destroy bone, joints, and ligaments, resulting in excess movement that encroaches on the traversing neurovascular elements. Tumor compression or invasion, surgical manipulation, or ischemia can compromise the cervicomedullary junction, which contains critical autonomous neurologic functions, the motor decussation, and multiple cranial nerve nuclei, as well as the cranial nerves themselves. A number of surgical approaches have been developed to access the CVJ from all directions, using corridors from different trajectories to decrease morbidity on neighboring structures. This chapter describes the basic anatomy of the CVJ, outlines the primary surgical approaches for gaining access to the region, and discusses considerations for stabilization.
27.2 Surgical Anatomy
The CVJ, which comprises the occiput, atlas, and axis and their associated joints and ligaments, surrounds vital neural and vascular structures that represent the transition between the brain and spinal cord. The majority of the rotation, flexion, and extension of the head relative to the spine occurs at the CVJ, yet a high degree of stability is critical to the integrity of the neurovascular components.
The occipital bone forms the clivus, which leads caudally to the basion at the anterior aspect of the foramen magnum. The dorsal surface of the clivus is concave, the extent of which can affect the exposure achieved from posterolateral surgical approaches. Flanking the anterior half of the foramen magnum are the occipital condyles, which extend in an inferomedial-to-superolateral orientation from anterior to posterior. Immediately rostral to the condyle lies the hypoglossal canal, opening medially one-third of the distance from the posterior edge of the condyle and exiting extracranially one-third of the distance behind its anterior edge. The squamous occipital bone completes the dorsal ring of the foramen magnum, a thickened keel arising in the midline to the inion. Projecting laterally from the posterior aspect of the condyle is the jugular process, which bears on its anterior surface a notch that forms the posterior portion of the jugular foramen.
The articular surface of the condyle is convex and is received by the concave superior articular surface of the lateral mass of the atlas. The lateral masses are joined by an anterior and posterior arch, the latter of which in particular may contain a developmental defect.1 The posterior arch also bears on its rostral surface the groove of the sulcus arteriosus, over which courses the vertebral artery (VA). The transverse processes and transverse foramina of the atlas extend farther laterally than at the levels below. The inferior articular surface of the lateral mass meets the superior articular process of the axis in a biconvex configuration, allowing for extensive movement at the atlantoaxial segment. At a third interface, the ventral aspect of the dens apposes the dorsal surface of the anterior atlas arch. The pars interarticularis, lamina, and prominent bifid spinous process complete the dorsal ring of the axis.
Much of the stability of the craniocervical junction is provided by ligamentous structures, which can be disrupted by both pathology and surgical dissection. The transverse ligament of the cruciate complex is the strongest ligament of the spine and thus is a critical source of stability for the CVJ.2 It inserts on the medial aspect of each lateral mass of the atlas, arching dorsal to the dens and holding it against the anterior arch while permitting rotation at the atlantoaxial joint. The other major stabilizers of the CVJ are the paired alar ligaments, which extend from the upper dens to the region of the medial occipital condyle and anterolateral foramen magnum.3 Each alar ligament limits contralateral axial rotation and lateral flexion. They are second in tensile strength only to the transverse ligament and in the setting of disruption of the latter are responsible for preventing atlantal subluxation.4 Additional structures, including the vertical part of the cruciate ligament, which inserts on the clivus rostrally and posterior axis body caudally, and the apical ligament, which extends from the tip of the dens to the basion, are consistently identified but contribute little to CVJ stability.
The tectorial membrane sits posterior to the cruciate ligament, blending with dura rostrally at the spheno-occipital synchondrosis and attaching caudally to the posterior axis body. It helps prevent posterior impingement of the odontoid process during flexion and demarcates the posterior border of the supraodontoid space.5 The anterior border of this space is formed by the anterior atlanto-occipital membrane, a thin structure attached to the anterior atlas arch and foramen magnum rim that lies immediately posterior to the prevertebral muscles. The posterior atlanto-occipital membrane attaches the posterior foramen magnum to the atlas arch and is related to the posterior atlantoaxial membrane caudally, the dura ventrally, and the rectus capitis posterior minor muscle dorsally.
The dorsal myoligamentous complex, which includes the interspinous ligament, nuchal ligament, rectus capitis posterior minor and major muscles, and obliquus capitis superior and inferior muscles, supports the stability of the CVJ.6 The latter three muscles form the suboccipital triangle, the deepest layer of musculature in the posterior neck. The rectus capitis posterior major takes its origin from the spinous process of the axis and inserts on the occipital bone to form the medial border of the triangle. The obliquus capitis superior also inserts on the occipital bone after arising from the transverse process of the atlas, and the obliquus capitis inferior arises from the spinous process of the axis and inserts on the transverse process of the atlas, respectively forming the superior and inferior borders of the triangle. They are covered by the semispinalis capitis and splenius capitis muscles, which are in turned overlaid by the trapezius and sternocleidomastoid (SCM) muscle. Lateral to the occipital condyle, the rectus capitis lateralis arises from the transverse process of the atlas and inserts onto the jugular process. From the anterior aspect of the atlas lateral mass arises the rectus capitis anterior, which inserts onto the occipital bone anterior to the condyle and is covered anteriorly by the longus capitis. In the midline roughly 1 cm anterior to the foramen magnum, the pharyngeal tubercle provides the point of attachment for the fibrous raphe of the superior pharyngeal constrictor.
The predominant movements at the atlanto-occipital joint are flexion and extension, contributing 23 to 24.5° of the range of the skull on the spine.7 Other movements are limited, as the cupped configuration of the condyles in the lateral masses provide stability to this segment. In contrast, the atlantoaxial joint allows for a wide range of motion, adding 10.1 to 22.4° of flexion/extension and providing 25 to 30° of axial rotation.8 The basion and dens act as a central pillar around which the occiput and atlas rotate, and overrotation is prevented by the ligaments and joint capsules. Less than 10° of lateral bending occurs, and translation, distraction, and compression are minimal at the CVJ.
Although the VA takes a roughly vertical course as it ascends the subaxial spine, movement at the CVJ requires greater mobility and redundancy of the vessel. Above the transverse foramen of the axis, the VA travels laterally to pass through the transverse foramen of the atlas, then turns posteromedially behind the lateral mass. It crosses over the sulcus arteriosus in the depths of the suboccipital triangle, passing under the inferior border of the posterior atlanto-occipital membrane, and finally turns anteriorly to penetrate the dura. The artery is surrounded by a rich venous plexus, which can cause brisk bleeding during surgical dissection. Although the intradural course of each VA can be highly variable, they typically give rise to the posteroinferior cerebellar arteries (PICAs), ascend ventral to the hypoglossal nerves, and join to form the basilar artery at the pontomedullary junction.
The jugular foramen is situated between the occipital and petrous temporal bones, its intracranial opening superolateral to that of the hypoglossal canal and its extracranial opening lateral to the anterior occipital condyle. The glossopharyngeal, vagus, and accessory nerves exit in the anteromedial portion of the foramen, whereas the jugular bulb transitions to the internal jugular vein (IJV) and exits the posterolateral portion anterior to the rectus capitis lateralis, posteromedial to the styloid process and posterolateral to the opening of the carotid canal. Ascending within the carotid sheath, the internal carotid artery lies anterior to the transverse processes of the axis and atlas and the longus capitis.
27.3 Regional Pathology and Differential Diagnosis
Neoplasms of the CVJ may arise from neural elements or from the surrounding osseous or soft tissue structures. As with other parts of the skull base, intradural lesions such as meningiomas, schwannomas, neurofibromas, epidermoids, and paragangliomas occur in this region, as do extradural lesions such as metastases, plasmacytomas, and giant cell tumors. More specific to the CVJ are chordomas and chondrosarcomas and extensions of pituitary tumors, craniopharyngiomas, and nasopharyngeal malignancies. Lesions that should be distinguished from skull base neoplasms include exophytic intra-axial tumors, rheumatoid pannus, fibrous dysplasia, and congenital segmentation abnormalities.
27.4 Clinical Assessment
Because the cross-sectional area of the foramen magnum and spinal canal at the CVJ is much larger than that of the traversing neurovascular structures, by the time symptoms are apparent a slow-growing lesion may have attained a large size. Clinical presentation may follow a pattern in keeping with posterior fossa pathology, abnormality of the cervicomedullary junction, or high myelopathy and may additionally manifest hydrocephalus, syringohydromyelia, or vascular compromise. The most common initial symptom is pain in the suboccipital region, referred to the C2 dermatome, that is aggravated by head and neck motion.9 The head is held flexed, and torticollis may be apparent. In contrast, bone erosion and mechanical instability can result in nondermatomal pain. Such nonspecific complaints may lead to delay in diagnosis, sometimes for years, until other findings develop.
Cranial neuropathies may arise due to involvement of brainstem nuclei, subarachnoid space segments, or intraforaminal nerves. The glossopharyngeal, vagus, and hypoglossal nerves are most commonly affected, manifesting as dysarthria, dysphagia, or repeated aspiration, with secondary pneumonia and weight loss. Involvement of the accessory nerve may result in SCM and trapezius weakness and atrophy. Abnormal or diminished pain, temperature, and deep pressure sensation in the face may result from compression of the caudal extent of the spinal trigeminal nucleus. Compression or traction of the vestibulocochlear nerve may result in vertigo, tinnitus, or hearing loss.
Effects on the ascending long tracts may produce a variety of sensory deficits. Paresthesias or dysesthesias are common, often in a suspended pattern affecting the hands first. Although typically suggestive of intramedullary disease, a dissociated pattern of decreased pain and temperature sensation with preserved joint position and vibration sensation can be seen with extradural lesions.
Weakness and clumsiness associated with spasticity is a common motor finding, although atrophy of intrinsic hand muscles may also be seen. A pattern of weakness beginning in the ipsilateral upper extremity, progressing to the ipsilateral lower extremity and then the contralateral lower extremity, and finally affecting the contralateral upper extremity, is a classic syndrome associated with pathology of the lateral craniocervical junction.10 The syndrome of cruciate paralysis, with upper extremity weakness and preserved lower extremity power, has been described in tumors of the CVJ.
Vascular compromise can result in a variety of vertebrobasilar syndromes. Ataxia, nystagmus, vertigo, dysarthria, dysphagia, diplopia, hemiparesis, or paraparesis may occur, transiently or progressively. Venous obstruction may cause spinal cord edema, with findings of myelopathy. Hydrocephalus may result from obstruction of cerebrospinal fluid (CSF) flow or absorption.
27.5 Diagnostic Imaging
CT is often the first imaging obtained when pathology of the CVJ is suspected, and it is invaluable for delineating osseous anatomy and involvement. Planning of surgical approach and targeting extent of bone removal is facilitated by multiplanar thin-slice CT studies. Patterns of hyperostosis, erosion, lysis, and sclerosis help in narrowing the differential diagnosis. CT venography and angiography can be used to evaluate the caliber, course, dominance, and compromise of adjacent vessels and potential segments at risk during surgical resection. Particularly in older patients, the course of the vertebral and carotid arteries may become tortuous as they approach the skull base, and imaging studies should be reviewed to anticipate any anomalies.
MRI can distinguish tumors of the CVJ from intramedullary and nonneoplastic lesions that present in similar fashion. Patterns of enhancement with gadolinium contrast, as well as patterns on susceptibility-weighted imaging, diffusion-weighted imaging, and steady-state sequences, can further narrow the differential diagnosis, identify the likely anatomical site of origin, and provide detailed information about the location of neighboring vascular and neural structures. Fluid-attenuated inversion recovery sequences can reveal the presence of edema within the brainstem or spinal cord. MR venography and angiography can also be obtained in the same session.
Digital subtraction angiography may be needed for dynamic evaluation of collateral circulation and to assess whether a patient will tolerate vascular occlusion. Highly vascular tumors such as paragangliomas may benefit from preoperative embolization of feeding arteries.
Additional imaging can identify instances in which the tumor has caused destruction of the bony and ligamentous structures that provide stability at the CVJ. Surgical stabilization should be considered when patients are found to have instability before resection—that is, instability caused by the tumor itself. Such instability can be identified using both static and dynamic X-rays. Static X-rays of the CVJ allow many widely used parameters to be drawn and measured. These measurements, such as the Chamberlain and McRae lines and the basion–dens and basion–atlanto intervals, can help distinguish whether instability is present.11 Dynamic X-rays, such as flexion–extension X-rays, allow for identification of translation, which is also indicative of instability.
27.6 Preoperative Preparation
In patients who have decreased hearing on presentation, formal evaluation using an audiogram will determine baseline function and may identify those at greatest risk of complete hearing loss. Patients who have glossopharyngeal, vagal, and/or hypoglossal nerve palsies may require tracheostomy and gastrostomy. Long-standing dysphagia may have resulted in malnourishment, and nutritional support may be required preoperatively to lessen problems of wound healing. The patient should be examined for development of neurological symptoms with the head in rotation or flexion to ensure that they will tolerate surgical positioning. Prior to transnasal or transoral approaches, examination for and treatment of infections that may lead to contamination of the surgical field should be undertaken.
Even in the absence of preoperative mechanical instability, disruption of the CVJ caused by the surgical approach warrants stabilization. The posterior midline approach can cause instability by disrupting the posterior tension band, as well as if the bony resection is taken laterally to involve the condyle. Lateral approaches involve drilling of the condyle, with stabilization often performed if more than 70% of the condyle is drilled.12 The anterolateral approach can require drilling of the lateral mass of the atlas, as well as the atlanto-occipital and atlantoaxial joints. This bony removal can also lead to instability. Finally, the transoral route may lead to resection of a significant portion of the bony and ligamentous complex that unites the clivus, atlas, and dens. This results in instability in the majority of patients who undergo this approach.13 The extent of bone removal may be determined intraoperatively, but if the approach may be destabilizing, a plan should be in place to perform surgical stabilization at the same session or to maintain alignment until stabilization is carried out at later session.
The anesthesiologist should be made aware of patients who have CVJ instability or cervicomedullary compression and should modify the intubation technique accordingly. Standard protocols to facilitate brain relaxation are employed, including mild hyperventilation, mannitol administration, and head elevation. A ventriculostomy may be considered in patients who have obstructive hydrocephalus. Electrophysiologic monitoring of somatosensory, motor and brainstem–auditory evoked potentials can provide early warning of neurologic compression, retraction, or ischemic injury. Electromyographic monitoring of cranial nerve function may also be useful, particularly when the tumor has caused displacement of normal structures.
27.7 Surgical Technique
Numerous surgical approaches have been developed to access the CVJ, allowing for circumferential access to the region. In selecting a technique, factors for consideration include (1) likely histology and possibility of complete resection, (2) site of origin and direction of growth and neurovascular compression/displacement, (3) pathological or surgical breach of dura and need for CSF containment, and (4) mechanical craniospinal stability. Slow-growing tumors have often pushed away the neurovascular structures, creating working space and allowing for reduced surgical manipulation.
The available corridors can be considered in four categories: posterior midline, far lateral, anterolateral, and anterior transoral–transpharyngeal. For dorsal and paramedian tumors, the posterior midline suboccipital craniotomy is a routine and familiar approach and is often combined with C1 and C2 laminectomy. It allows for bony decompression of the foramen magnum, provides wide access to the CVJ on both sides of midline, and is essentially unlimited in potential rostrocaudal extension. It can be used for intradural pathology, as the dura is easily repaired, and dorsal instrumentation and fusion can be performed in the same exposure. For lateral or anterolateral tumors, a posterolateral trajectory is needed, which can be achieved using a far lateral approach (Fig. 27.1).
Further ventral exposure is gained with drilling of the occipital condyle, C1 lateral mass, or jugular tubercle, although extensive removal of the O–C1 joint may cause instability requiring fusion,14 and the dorsal jugular bulb can be accessed with removal of the jugular process. Because the dura is also readily closed, intradural pathology to and past the ventral midline can be treated using this approach. Anterior and anterolateral tumors can be directly accessed by an anterolateral approach, also called extreme lateral—a useful technique, particularly for extradural tumors or dumbbell lesions having a large extradural component and when a posterolateral approach has previously been performed.15 If the prevertebral region anterior to the CVJ is exposed, this approach carries risk of pharyngeal dysfunction as well as injury to the extracranial segments of the hypoglossal nerves. As in any setting of extensive O–C1 or C1–C2 joint removal, stabilization may be required. For primarily extradural ventral pathology, the transoral–transpharyngeal approach is direct and relatively simple, avoiding manipulation of the cervicomedullary junction, cranial nerves, and VA. However, morbidity of the approach includes postoperative tube feeding while the pharynx heals as well as potential long-term velopharyngeal insufficiency. Endoscopic endonasal approaches have become increasingly applied to directly ventral pathology, as they may result in less pharyngeal morbidity. Detailed discussions of endoscopic techniques are provided elsewhere in this volume.
27.7.1 Posterior Midline Approach
Positioning
The patient is placed in the Concorde position, with the head fixed in flexion at the atlanto-occipital segment to increase working space at the posterior CVJ, taking care to avoid obstruction of cerebral venous outflow. The upper chest and pelvis are supported, allowing free movement of the chest and abdominal wall, and the operating table is placed in the reverse Trendelenburg position to reduce cerebral venous congestion.
Incision
The incision is placed in the midline, from 2 cm above the inion to the spinous process of C2. The pericranial layer above the superior nuchal line is preserved.
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