Surgical Management of Tumors of the Foramen Magnum

Clinical Presentation

The clinical presentation associated with FM tumors is insidious. Because of their slow-growing nature, their indolent behavior, and the wide subarachnoid space at this level, the mean duration of symptoms before diagnosis is 30.8 months. ^, In the early stages, patients complain of occipital headache and cervical pain. This pain is described as deep and is aggravated by neck motion, coughing, and straining. As the tumor grows, sensory and motor deficits develop. The classical syndrome of FM tumors, mainly of meningiomas located anteriorly, is an asymmetric deficit defined by weakness, paresthesias, and spasticity, first in the ipsilateral arm and then progressing to the ipsilateral leg, then to the contralateral leg, and finally to the contralateral arm. Long tract signs characteristic of upper motor lesions include the presence of atrophy in the intrinsic muscles of the hands. Later findings include spastic quadriparesis, respiratory dysfunction, and lower cranial nerve deficits. ^, In extradural tumors, especially in cranial-base chordomas, diplopia is the symptom most commonly reported, and headache is the second most common.

Classification of Tumors

Tumors of the FM are classified according to their origin. They can arise in the FM itself or secondarily from surrounding areas. Most of the classifications focus on meningiomas and usually do not address bony tumors. Among the many classifications of meningiomas of the FM, the one most frequently used by neurosurgeons is that of Bruneau and George. The main objective of this system is to define the surgical strategy preoperatively. Based on this classification, meningiomas of the FM are classified as intradural, extradural, or intra- and extradural. According to their insertion on the dura, meningiomas are anterior if insertion happens on both sides of the anterior midline; anterolateral if insertion occurs between the midline and the dentate ligament; or posterior if insertion is posterior to the dentate ligament. The other landmark used for classification is the tumor’s relation to the VA, as meningiomas of the FM can develop above, below, or on both sides of the VA. Intradural meningiomas are the most common type and most of them arise anterolaterally ; these are followed by posterolateral tumors. Tumors that arise purely posteriorly and anteriorly are very rare.

A new classification system was recently developed by Yamahata et al., aiming for a better preoperative assessment. They divided tumors into three types: clival (lower third of clivus), foraminal (attached at the level of FM), and atlantal (below the hypoglossal canal). Regarding foraminal and atlantal tumors, tumor growth results in displacement of the neuraxis, creating a large corridor, so that the mass effect of the tumor is a natural retractor. Clival tumors, on the other hand, usually need retraction.

The surgical approach to extradural tumors is based on their relation to the C1 lateral mass, OC, clivus, intradural extension, cavernous sinus, jugular foramen, retropharyngeus, VA, and carotid artery. The size, position, and nature of the tumor defines the surgical approach and steps, such as drilling the lateral wall of the FM and transposing the VA. The definition of the space between the cervicomedullary junction and the lateral wall of the FM, the so-called surgical corridor, is also an important consideration. Large tumors, either anterior or anterolateral, push the cervicomedullary junction posteriorly, hence creating a surgical avenue for tumor removal. In contrast, small tumors and an elongated FM may require additional space, which can be obtained via the condyle or the lateral mass of the C1, with transposition of the VA.

Preoperative Imaging

Preoperative workup includes MRI and CT. With the availability of CT angiography and MR angiography, conventional angiography is rarely indicated unless embolization is planned in a highly vascularized tumor. Preoperative imaging studies allow for planning of the surgery; for this, the following information must be retrieved from the images: the nature of the tumor (intra- and/or extradural), location and attachment, relation to the cervicomedullary junction, caudal and rostral extension, position and possible involvement of the VA and its branches, shape of the FM, dominance of the VA, venous drainage patterns and dominance, and bony involvement. A T ₁ -weighted MRI with contrast enhancement clearly defines the tumor and the dural attachment site and discriminates between the tumor and brain stem. T ₂ -weighted MRI provides information on the arachnoid plane between the tumor and the cervicomedullary junction. CT using sagittal, coronal, and axial viewing and bone window remains the tool of choice for the study of bone involvement, the shape of the FM, and the surgical “corridor.” ^{, ,}

Choosing the Best Approach

The principal factors that determine access to lesions at the craniovertebral junction are the nature, position, and size of the tumors and the shape of the FM. Tumors located posteriorly or posterolaterally to the cervicomedullary junction can be approached from the posterior midline, which allows an extensive sagittal view from the skull base to the entire cervical spine; however, this approach does not work well for tumors located anterolaterally . This midline route does not enable control of the VA when the bone must be removed ventrolaterally. Anterior approaches via transcervical or transoral routes have been used but are not widely accepted. The transoral approach is essentially a midline and extradural approach to the inferior clivus and upper cervical spine , which—combined with maxillotomy or labiomandibulotomy and glossotomy—can provide access from the superior clivus to the midcervical spine. Nevertheless, this approach is limited laterally from both carotid arteries and the VA at the clival and spinal levels. Removal of an intradural pathology carries a high risk of cerebrospinal fluid (CSF) leak. The dura is difficult to repair because at this point it comprises a limited amount of soft tissue and the subarachnoid space is exposed to the contaminated field. Anterior approaches are suitable for small extradural and bony lesions without VA and carotid artery involvement. ^{, , ,}

However, minimizing cervicomedullary retraction and the risk of CSF leakage, ensuring a firm watertight closure, and managing the OC and VA are the main factors to be considered when this approach is chosen. Among the available approaches to the FM, the so-called far lateral approach, the extreme transcondylar approach, and variants of the lateral suboccipital approach meet these criteria. These approaches can be combined with petrosal, retrosigmoid, transtubercular, transfacetal, and infratemporal approaches, according to the tumor’s rostrocaudal extension and nature. ^,

Extradural tumors located frontal to the cervicomedullary junction that present with or without involvement of the VA and the lower cranial nerves, that invade the dura and/or destroy the OC and the articulation between OC, C1, and C2, can be approached via the same route; however, these tumors often require combined approaches. ^{, , ,}

The endoscopic endonasal transclival approach is a good alternative for ventral tumors. This route to the craniovertebral junction was first described by Kassam ; subsequently many other authors improved the technique. ^, It provides direct access to ventral (and ventrolateral) lesions without the need to mobilize the spinal cord, brain stem, and lower cranial nerves. Preoperative assessment is a very important step and should include a careful study on imaging, particularly the relation between the tumor, lower cranial nerves, basilar artery, and VAs. Midline tumors at the ventral portion of the FM with no lateral extension (beyond the OC) and no extension to the level of C2 are the best indication for this approach.

Far Lateral Approach