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Foramen Magnum Meningiomas: Concepts, Classifications, and Nuances

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Foramen Magnum Meningiomas: Concepts, Classifications, and Nuances Neurosurg Focus 14 (6):Article 10, 2003, Click here to return to Table of Contents Foramen magnum meningiomas: concepts, classifications, and nuances MELFORT R. BOULTON, M.D., PH.D., AND MICHAEL D. CUSIMANO, M.D., PH.D., F.R.C.S.(C) Division of Neurosurgery, St. Michael’s Hospital, and the University of Toronto, Ontario, Canada Foramen magnum meningiomas represent a common histological tumor in a rare and eloquent location. The authors review the clinical presentation, relevant anatomical details of the foramen magnum region, neuroimaging features, the posterior and posterolateral surgical approaches for resection, and outcomes. Based the experiences of the senior author (M.D.C.) and a review of the literature, they introduce the concept of a “surgical corridor,” discuss the classification of these tumors, and the nuances of care for patients with these challenging lesions. KEY WORDS • meningioma • foramen magnum • craniovertebral junction • skull base • suboccipital craniotomy Meningiomas are slow-growing benign tumors that from the lower third of the clivus, to upper margin of the arise at any location where arachnoid cells reside. Al- body of C-2, laterally from the jugular tubercle to the though meningiomas account for a sizable proportion of upper margin of the C-2 laminae, and posteriorly from the all primary intracranial neoplasms (14.3–19%),46 only 1.8 anterior edge of the squamous occipital bone to the C-2 to 3.2% arise at the foramen magnum.2 Their indolent spinous process. development at the craniospinal junction makes clinical The foramen magnum contains several critical neu- diagnosis complex and often leads to a long interval be- roanatomical and vascular structures of which the surgeon tween onset of symptoms and diagnosis. The sensitivity of must be aware. The neural structures include the cerebel- this region to surgical manipulation has sparked recent lar tonsils, inferior vermis, fourth ventricle, caudal aspect debate as to the most advantageous surgical approach. The of the medulla, lower cranial nerves (ninth–12th), rostral intent of this paper is to provide an overview of the rele- aspect of the spinal cord, and upper cervical nerves (C-1 vant surgical anatomy, clinical features, and nuances of and C-2). The ninth through 11th cranial nerves arise as a the management of foramen magnum meningiomas. We series of rootlets along the anterior medulla, with the introduce the concept of the “surgical corridor” and dis- spinal component of the 11th cranial nerve arising mid- cuss its importance in the management of these challeng- way between the anterior and posterior spinal rootlets of ing lesions. the spinal cord. The spinal accessory rootlets coalesce and ascend rostral to join the ninth, 10th, and the cranial por- tion of the 11th nerve. Together, these nerves exit the skull FORAMEN MAGNUM ANATOMY through the jugular foramen. The 12th cranial nerve exits the medulla more anteriorly than the other lower cranial Several excellent reviews of foramen magnum anatomy 3,10,14–17,29–31 nerves and passes anterior to the ipsilateral VA on its have been published. By definition, foramen course to the hypoglossal canal, located within the superi- magnum meningiomas arise from arachnoid at the cra- or and anterior–most portion of the occipital condyle. niospinal junction. The borders of this zone, as defined 14,17 Major arterial structures located within the foramen by George and George and colleagues range anteriorly magnum include the VAs, PICAs, anterior and posterior spinal arteries, and the meningeal branches of the verte- Abbreviations used in this paper: CSF = cerebrospinal fluid; bral, external, and internal carotid arteries. The VA cours- CT = computerized tomography; GKS = gamma knife surgery; es through the foramen tranversalis until reaching C-1 MR = magnetic resonance; PICA = posterior inferior cerebellar where it curves above the lateral aspect of the posterior artery; QOL = quality of life; VA = vertebral artery. arch and proceeds rostral to pierce the dura mater just Neurosurg. Focus / Volume 14 / June, 2003 1 Unauthenticated | Downloaded 10/01/21 10:34 AM UTC M. R. Boulton and M. D. Cusimano inferior to the lateral edge of the foramen magnum adja- cent to the occipital condyle. Its intradural portion typi- cally gives rise to the posterior spinal artery and PICA, although the origin of the PICA has been reported to vary, arising at, above, or below the foramen magnum.13 CLASSIFICATION OF FORAMEN MAGNUM MENINGIOMAS We describe foramen magnum meningiomas as having originated primarily from within the confines of the fora- men magnum or having secondarily invaded the region but originating elsewhere. We also classify the primary tumors according to their anteroposterior and lateromedi- al orientations. The spinal dentate ligament delineates the anterior and posterior compartments. Most lesions (68– 98%)2 arise anterolaterally; a posterolateral origin is the second most frequent, purely posterior lesions the third, Fig. 1. Illustration of the surgical corridor. A: Normal rela- and least common are entirely anterior. We also classify tionship of brainstem to foramen magnum. oc = occipital condyle; these lesions based on their size relative to that of the fora- cmj = cervicomedullary junction. B–D: As the tumor (T) en- men magnum (small, Ͻ one third the transverse dimen- larges, it displaces the brainstem posteriorly and typically to one sion of the foramen magnum; medium, one third to one side, naturally creating a widened surgical corridor. B: A narrow half its dimension; large, Ͼ one half). corridor of less than 1 cm between the condyle and cervicomed- Most importantly, when managing these lesions, we de- ullary junction. C: Adequate corridor with a 1- to 2-cm distance. D: Large corridor (Ͼ 2 cm) that allows relatively easy access to the fine an avenue that we term the “surgical corridor.” This anterior foramen magnum. is defined as “the space for surgical access to a lesion.” It describes the space that the surgeon will work through to access the lesion. The surgical corridor can be enlarged naturally by a tumor displacing normal structures like the undeniable, and lead to significant and often permanent medulla oblongata in a confined space such as the fora- neurological deficit. Early features of foramen magnum men magnum. Alternatively, one goal of a surgical route meningiomas include occipital headache and upper cervi- such as the transcondylar approach would be to enlarge cal pain, which is often exacerbated by neck flexion or the surgical corridor. Valsalva maneuvers. Classic foramen magnum syndrome In the management of foramen magnum meningiomas, is defined by development of unilateral arm sensory and the surgical corridor involves the space between the later- motor deficits, which progress to the ipsilateral leg, then al margin of the cervicomedullary junction and the medi- the contralateral leg, and finally contralateral upper ex- al aspect of the occipital condyle. The corridor can and tremity. Long tract findings characteristic of upper motor should be assessed on preoperative imaging. In these le- lesions are found paradoxically in the presence of atrophy sions, the corridor represents the view of a lesion that a in the intrinsic muscles of the hands. Later findings surgeon obtains via a posterolateral approach after remov- include spastic quadriparesis and lower cranial nerve pal- ing only the occipital bone and leaving the condyle intact. sies. Slowly progressive lesions such as these allow the We define a corridor as “narrow” if it provides a diameter development of accessory muscles to replace trapezius of access to the tumor of less than 1 cm; “adequate” if it is and sternocleidomastoid function. We therefore highly re- greater than 1 cm but less than 2 cm; and “large” if greater commend that the patient be undressed and the sternoclei- than 2 cm (Fig. 1). Thus, a tumor arising primarily from domastoid and trapezius muscles be closely inspected for the posterior lip of the foramen magnum and extending atrophy. Likewise, the tongue should be inspected at rest laterally has a large corridor of access, whereas one pure- for atrophy and fasciculation. Close attention to sensory ly anterior with no displacement of the cervicomedullary testing of the C-2 dermatome will help establish the diag- junction has a narrow corridor. Most tumors, once ad- nosis. Patients attest to initial sensory disturbances such as vanced enough to produce symptoms, have at least an ade- cold or burning dysesthesias, astereognosis, and anesthe- quate corridor. sia but often do not seek medical attention until intractable pain, motor deficits, or ataxia ensue. Terminal progression includes quadriplegia, an inability to maintain airway pro- CLINICAL PRESENTATION tection with secondary pneumonitis, and ultimately respi- ratory arrest. The clinical presentation of foramen magnum meningi- omas is protean, and the mean length of symptoms prior IMAGING FEATURES to diagnosis is 30.8 months, even in the era of MR im- aging.16 The clinical differential diagnosis includes multi- The role of neuroimaging is to confirm the clinical ple sclerosis, amyotrophic lateral sclerosis, syringomyelia, diagnosis and to allow the planning of a surgical approach. and cervical spondylosis.11,20,36 In a cursory examination Magnetic resonance imaging is the modality of choice for the physician may miss subtle findings early in the stage defining tumors of the foramen magnum because it pro- of the progression, but later symptoms are often advanced, vides high-resolution images of soft-tissue anatomy that is 2 Neurosurg. Focus / Volume 14 / June, 2003 Unauthenticated | Downloaded 10/01/21 10:34 AM UTC Foramen magnum meningiomas not susceptible to degradation by the surrounding skull resect tumor safely because of the sharp contrast between base, a pitfall of CT scanning. Although plain T1-weight- bone and soft tissues. It is sometimes difficult to outline ed MR images demonstrate excellent anatomical detail, bone margins on MR images, and this technique may they provide little discrimination between tumor and brain- overestimate the size of the surgical corridor available for stem because the former may appear isointense, mild- extirpation.
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