31 Foramen Magnum Meningiomas
Introduction
Although meningiomas account for three quarters of benign tumors of the foramen magnum (FM), as a group they account for only 1.8 to 3.2% of all meningiomas.1,2 Like other meningiomas, they occur much more frequently in females and they rarely occur in childhood. First reported by Hallopeau in 1874,3 their indolent development at the craniospinal junction makes clinical diagnosis complex and often leads to a long interval between onset of symptoms and diagnosis. The sensitivity of this region to surgical manipulation has sparked debate as to the most advantageous surgical approach. This chapter provides an overview of the relevant surgical anatomy, clinical features, and nuances of the management of FM meningiomas.
Foramen Magnum Anatomy
Several excellent reviews of FM anatomy have been published.4–13 By definition, FM meningiomas arise from the arachnoid at the craniospinal junction. The borders of this zone, as defined by George6 and George and colleagues9 range anteriorly from the lower third of the clivus, to the upper margin of the body of C2, laterally from the jugular tubercle to the upper margin of the C2 laminae, and posteriorly from the anterior edge of the squamous occipital bone to the C2 spinous process.
The FM contains several critical neuroanatomical and vascular structures of which the surgeon must be aware ( Fig. 31.1 ). The neural structures include the cerebellar tonsils, inferior vermis, fourth ventricle, caudal aspect of the medulla, lower cranial nerves (IX through XII), rostral aspect of the spinal cord, and upper cervical nerves (C1 and C2). The ninth through 11th cranial nerves arise as a series of rootlets along the anterior medulla, with the spinal component of the 11th cranial nerve arising midway between the anterior and posterior spinal rootlets of the spinal cord. The spinal accessory rootlets coalesce and ascend rostral to join the ninth, 10th, and cranial portion of the 11th nerve. Together, these nerves exit the skull through the jugular foramen. The 12th cranial nerve exits the medulla more anteriorly than the other lower cranial nerves and passes posterior to the ipsilateral vertebral artery (VA) on its course to the hypoglossal canal, located within the superior and anteriormost portion of the occipital condyle.
Major arterial structures located within the FM include the VAs, posterior inferior cerebellar arteries (PICAs), anterior and posterior spinal arteries, and meningeal branches of the vertebral, external, and internal carotid arteries. The suboccipital segment of the VA (also called V3) that courses from C2 to the dura consists of a vertical portion between C2 and C1 foramina transversaria (FT), a horizontal portion from the FT of C1 to the sulcus arteriosus (SA) of C1 and an oblique portion from the SA to the dura. The third portion of the V3 segment lies within the floor of the suboccipital triangle, curves above the lateral aspect of the posterior arch of C1, and proceeds rostral to pierce the dura mater just inferior to the lateral edge of the FM adjacent to the occipital condyle.
Understanding the anatomy of the suboccipital triangle is a key to a safe approach to V3 ( Fig. 31.2 ). The V3 segment is fixed at the FT and at the entry into the dura but can be mobilized by freeing the periosteal sheath of the FT or the dural entry point. There are several other important anatomical considerations of V3: (1) in the neutral position the vertical and horizontal portions are perpendicular, but with head rotation they can run parallel, only separated by C1 lamina; (2) V3 can end in the PICA or the occipital artery; and (3) the proatlantal congenital anastomosis between the internal carotid and the VA is associated with an atretic VA and an extradural origin of the PICA.14 Branches of the VA that arise in this V3 segment can supply neural parenchyma (PICA has an extradural origin in 5 to 20% of cases) or provide supply to the tumor so a close review of preoperative vascular-based imaging is required to avoid vascular complications.15 The V4 or intradural segment of the VA typically gives rise to the posterior spinal artery and PICA and then traverses anterior to the dentate ligament (DL) and anterior to the lower cranial nerves to join the contralateral VA beyond the hypoglossal canal at the vertebrobasilar (VB) junction.
The meningeal reflections around the FM are important to the surgical management of these lesions. The DL forms from a lateral projection of the pia into a flange that crosses the subarachnoid space. It connects the spinal cord to the dura at the FM via a series of “teeth,” which are attached to the spaces between issuing nerve roots, and are situated posterior to the VA.16 The uppermost dentate attachment is usually at the same level as the entrance of the VA intradurally.13 The cervical contribution to nerve XI lies on the posterior surface of the DL, and the rootlets of C1 and C2 lie dorsal to XI. The VA moves from its dural entrance just anterior to the DL to move anterior to nerve XII and the nerves that enter the jugular foramen. From the surgeon’s perspective via a posterior approach, XII is deep to the DL and sitting atop the VA, and the cervical part of XI and the high cervical rootlets sit on top of the DL. The arachnoid of the cisterna magna extends laterally to encompass the VA at its entrance from the dura. Maintaining this arachnoid plane allows the surgeon to dissect an apparently tumor-encased VA without hazard. If the tumor originates extradurally, tumor can extend between the adventitia of the VA and the arachnoid, thereby stenosing the VA. In these cases, dissection of the VA may place the adventitia at potential risk. It is sometimes difficult to identify the origin of a “transdural” tumor. At times, what looks like a “totally encased” VA may, in fact, be easily differentiable if this arachnoid plane is pressed around the VA. The surgeon should always determine whether the tumor is adherent to the VA; if it is not, it is likely that the arachnoid plane is present and the tumor will thus often be dissectible. If the VA is stentic, the arachnoid plane is often absent.
Classification of Foramen Magnum Meningiomas
FM meningiomas originate primarily from within the confines of the FM, or secondarily they invade the region after originating elsewhere. They most often arise intradurally but can also extend transdurally or rarely occur solely extradurally. We also classify the primary tumors according to their anteroposterior and lateromedial orientations. At surgery, the spinal DL delineates the anterior and posterior compartments. Of the intradural lesions, most (68 to 98%)2 arise anterolaterally; a posterolateral origin is the second most frequent, purely posterior lesions are the third, and the least common are entirely anterior.
In thinking about the surgical management of the lesions, it is also advantageous to classify them according to their effects on critical structures, such as the VA, cervicomedullary parenchyma, and cranial nerves.
These lesions can also be classified in relation to the VA. Those that begin in the spine, so called “spinocranial” meningiomas, displace the VA superiorly. Those that begin at the entrance of the VA will encase the VA and displace the VA away from the petrous bone and the medulla away from the VA. Those that begin cranially, anterolaterally, or anterior to the entrance of the VA displace the VA and the medulla away from the petrous bone. In these cases, the VA and the medulla are in close apposition if the origin was along the lower clivus at or above the VB junction ( Fig. 31.3 ). If the origin is lower on the clivus than the VB junction, the tumor may displace apart the relation between the medulla and the VA.
We also classify the site of origin of these lesions in relation to the neural foramina and cranial nerves. The spinocranial lesions will always originate below the FM and thereby displace the cranial nerves (and the VA) to the superior pole of the tumor. The surgeon can work from below to entirely decompress the neural structures in these cases to ultimately reach them at the end of the tumor re-section. The purely anterior cranial lesions originating at the anterior lip of the FM will originate medial to the hypoglossal and jugular foramen and so displace the cranial nerves posterolaterally, and the surgeon will encounter nerves IX, X, and XII anterior or ventral to the dentate before the tumor’s origin. If the origin is between the jugular and hypoglossal foramina, XII will be found medial and IX, X, and XI laterally. Most tumors originate inferolateral to the jugular foramen at the FM and so are similar to spinocranial meningiomas in their effect on the nerves.
Finally, we also recommend classifying these lesions based on their size relative to that of the FM (small: one third the transverse dimension of the foramen magnum; medium: one third to one half its dimension; large: more than one half the dimension of the FM). As a natural corollary of this size classification, we also classify the lesions according to the “surgical corridor” that they have created.13 The surgical corridor 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 medulla oblongata in a confined space such as the FM. As tumors enlarge ( Fig. 31.4 ), the corridor also enlarges, allowing access to the origin of the lesion more easily without retraction of the medulla or upper cervical spinal cord. The goal of the transcondylar approach is to enlarge the surgical corridor. By combining these classifications, we may describe a lesion such as that in Fig. 31.2C as intradural, spinocranial, with an adequate corridor, with encasement of the VA.
Clinical Presentation
The clinical presentation of FM meningiomas is protean, and the mean length of symptoms before diagnosis is 30.8 months, even in the era of magnetic resonance imaging (MRI).16 The clinical differential diagnosis includes multiple sclerosis, amyotrophic lateral sclerosis, syringomyelia, and cervical spondylosis.17–19 In a cursory examination the physician may miss subtle findings early in the stage of tumor progression, but later symptoms are often advanced, undeniable, and lead to significant and often permanent neurological deficit. Early features of FM meningiomas include occipital headache and upper cervical pain, which is often exacerbated by neck flexion or Valsalva maneuvers. Classic FM syndrome is defined by development of unilateral arm sensory and motor deficits, which progress to the ipsilateral leg, then the contralateral leg, and finally the contralateral upper extremity. Long tract findings characteristic of upper motor lesions are found paradoxically in the presence of atrophy in the intrinsic muscles of the hands. Motor loss is usually more pronounced ipsilateral to the lesion. Later findings include spastic quadriparesis and lower cranial nerve palsies. Patients may complain of cold or burning in one or both lower or upper limbs before other features arise. Slowly progressive lesions like these allow the development of accessory muscles to replace trapezius and sternocleidomastoid function. We therefore highly recommend that the patient be undressed and the sterno-cleidomastoid and trapezius muscles be closely inspected for atrophy. Likewise, the tongue should be inspected at rest for atrophy and fasciculation. Close attention to sensory testing of the C2 dermatome will help establish the diagnosis. Patients attest to initial sensory disturbances such as cold or burning dysesthesias, astereognosis, and anesthesia but often do not seek medical attention until intractable pain, motor deficits, or ataxia ensue. Terminal progression includes quadriplegia, an inability to maintain airway protection with secondary pneumonitis, and ultimately respiratory arrest.
Imaging Features
The role of neuroimaging is to confirm the clinical diagnosis and to allow the planning of a surgical approach. MRI is the modality of choice for defining tumors of the FM because it provides high-resolution images of soft tissue anatomy that is not susceptible to degradation by the surrounding skull base, a pitfall of computed tomographic (CT) scanning. Although plain T1-weighted MRI scans demonstrate excellent anatomical detail, they provide little discrimination between tumor and brain stem because the former may appear isointense, mildly hypointense, or hyperintense to surrounding brain. On T2-weighted images, meningiomas appear as isointense to slightly hyperintense compared with brain. The T2-weighted images should be carefully inspected for the presence of an arachnoid plane between the tumor, brain stem, and spinal cord. Edema depicted within the neuroparenchyma on T2-weighted sequences suggests that the pial membrane has been invaded; this should prompt an attempt at function preservation in which a near-total re-section leaves a small thin plating of tumor intact.20 The use of T1-weighted gadolinium-enhanced contrast imaging is particularly helpful in defining the dural attachment site of the tumor; additionally, it provides ready discrimination between tumor and brain stem, with often dramatic demonstration of brain stem distortion. Contrast-enhanced magnetic resonance or computed tomographic angiography should also be performed,21 if available, to help demonstrate vascular anatomy, collateral vessels, and the effect of the tumor on the VAs. A VA that is encased and narrowed suggests that the adventitia of the artery has been invaded, and the surgeon needs to assess whether residual tumor will be left in the adventitia or whether reconstruction is necessary. In our experience, subtotal resection is the preferred approach in this instance.
Although MRI provides clearly superior soft tissue assessment, CT scanning with osseous algorithms remains the tool of choice for identifying calcification, hyperostosis, and osseous anatomy. Axial CT scanning allows planning of the extent of bone resection required to resect tumor safely because of the sharp contrast between bone and soft tissues. It is sometimes difficult to outline bone margins on MRI scans, and this technique may overestimate the size of the surgical corridor available for extirpation. It is clearly evident that optimal surgical planning requires both CT and MRI to assess appropriately bone and soft tissues, respectively.
An additional imaging modality that may assist surgery is CT angiography or conventional angiography with optional embolization of vessels that supply tumor exclusively. The dural blood supply typically arises as posterior and anterior meningeal branches from the VAs with the support of meningeal branches via ascending pharyngeal and occipital arteries. The tumor may derive its vascular supply from a dominant vessel, which, when subjected to contrast injection, is visualized as a “blush.” If the vessel is accessible to endovascular catheterization, one might opt for preoperative embolization to diminish intraoperative bleeding during tumor debulking.22 As well, angiography may help to define in the rare circumstance whether sacrifice of the VA is possible by defining collateral flow.
Preoperative Assessment
As is true with all oncology, neurosurgeons must remember that they are treating people and not just resecting tumors. Sometimes this requires consideration of subtotal resection or monitoring. In initial discussions about surgery, one must not assume that patients wish to undergo surgery because they are present at a neurosurgeon’s office or that imaging has demonstrated a lesion. Often patients seek consultation to gather information about their situation, and depending on their age, ethnic background, and personal values, their decision-making process may not coincide with that of a neurosurgeon. It is therefore vital to reach an understanding of the patient’s expectations of surgery as well as the individual’s philosophy regarding quality of life issues. The possibility of residual tumor and subsequent treatment must also be discussed in relation to the risks of surgery.
A careful and detailed history will often demonstrate that symptoms appeared long before the chief complaint. As the lesion progresses in size, the clinical course may seem to accelerate because compensatory mechanisms are exhausted and the neural compromise reaches critical levels. A history in which symptoms are rapidly progressive without a longer prodrome should raise clinical suspicion that lesions such as carcinoma or infectious/inflammatory entities may be present.
Potential surgery-related risks are not insignificant, and a careful assessment will give the astute clinician a better understanding of them in a particular patient. Care should be directed to lower cranial nerve examination. Deficits of any magnitude suggest neural compression and potential vasa nervosum involvement, thereby making the nerves more vulnerable to surgical manipulation. The ninth and 10th cranial nerves represent the afferent and efferent arms of the gag reflex, respectively, and play a pivotal role in protecting from aspiration pneumonia. Patients with unilateral preoperative gag deficits are often able to adapt because of the deficit’s slow growth pattern and its chronic nature, which allow time for compensatory mechanisms to develop. An acute disruption of the gag reflex, however, can be lethal due to aspiration pneumonia. Thus preoperative and immediate postoperative endoscopic inspection of the pharynx and vocal cords should be performed to assess laryngeal function. New postoperative dysfunction should be treated using aggressive support measures, primarily by placing a gastric or jejunal feeding tube, and early tracheostomy if necessary. We routinely order an early otolaryngology and speech pathology consultation to test patients for vocal cord and swallowing functions preoperatively and postoperatively. Acute 11th cranial nerve deficits are sure to be problematic for the patient, especially in terms of shoulder abduction, but often the only symptom is shoulder pain. Patients with FM meningiomas rarely present preoperatively with an acute disturbance of this nerve, and its presence should lead one to consider alternate pathologies. Rapid onset with acute symptomatology related to XI often suggests an alternate pathology, such as carcinoma. Unilateral paralysis of the 12th cranial nerve may also be overlooked if the tongue is not inspected at rest within the mouth. Ipsilateral tongue deviation and furrowing are late and often irreversible signs of 12th cranial nerve palsy.
Planning for resection of FM meningiomas relies heavily on imaging findings and the clinical scenario.23–25 Because the two posterior approaches to anterior FM meningiomas require dissection of skin and muscles in anatomically distinct regions, one must decide preoperatively which approach is most suitable for the given tumor. Selection is based on the basic skull base surgery principle of removing bone to provide a corridor of access to the tumor to allow total tumor resection and to preclude retraction of neurological structures. Evaluation of MRI data allows one to determine the relationship of tumor to the brain stem and its possible site of dural attachment. CT scans provide data regarding the osseous anatomy in relation to tumor. Most importantly, an assessment of the surgical corridor is made at this stage. Because the majority of FM meningiomas are anterolaterally situated, their growth tends to displace the brain stem in a posterior and contralateral direction.
In our opinion, this in situ retraction made by the tumor creates an adequate surgical corridor for resection of most of these lesions. In our experience, no drilling of the occipital condyle has been necessary to achieve resection, and in the cases in which residual tumor remained, resection of the condyle would not have affected the degree of resection.
Imaging also clearly displays the relationship of tumor to vascular structures. Encasement of the VA is not uncommon and should not be surprising intraoperatively during tumor debulking. Provided that encasement of the VA exists, proximal control of the vessel is prudent and may require mobilization of the VA at the C1 trans-verse foramen or, rarely, below, particularly if a transcondylar approach is needed. Assessment of the PICA is also of importance in some cases, particularly those in which one encounters an encased VA. Contrast-enhanced magnetic resonance angiography or CT angiography provides the best degree of noninvasive resolution and should be used to assess for encasement, contralateral VA, and focal narrowing that could indicate adventitial invasion. If it is hypoplastic, the surgeon may decide to leave residual tumor on the vessel if necessary or perform a bypass, rather than simply resect the affected segment. Conventional angiography with greater resolution is rarely necessary but can be revealing. An additional advantage of preoperative imaging is to determine whether the PICA originates above, at, or below the level of the FM.13 Careful identification of PICA during routine posterior neck dissection is indicated if the PICA originates extradurally.
Surgical Management
Intraoperative management requires a careful assessment of how the lesion has affected the normal anatomy. Understanding the relations of nerves, vessels, the medulla, the spinal cord, the DL, arachnoid, and bone are critical to performing optimal surgery ( Fig. 31.5 ). Surgical dissection in which the cranial nerves and vascular structures are preserved is integral to FM tumor management. Every attempt should be made to keep the arachnoid with the patient and covering these structures. Perform dissection on the tumor’s side of the nerves and vessels during surgery. Even in cases involving encasement of the VA, if the dissection is deliberate and selective to identify and preserve the arachnoid, successful complete dissection is frequently possible in nonstenosed VAs. It may be tempting to cauterize small vessels overlying tumor capsule, in light of the concept that the tumor is parasitizing blood supply from the meninges. If possible, however, these vessels should be left intact in the arachnoid because they may actually mislead the surgeon into moving outside the ideal plane where coagulation could potentially produce brain stem perforator ischemia. The cranial nerves, vessels, and neural parenchyma are generally on the patient-side of the arachnoid and not on the tumor side of the arachnoid. Our general principle is to “leave the arachnoid with the patient” and “take the tumor from the patient.”
Intraoperative monitoring is intended to aid the neuro-surgeon in preserving neurological function. Somatosensory evoked potentials provide a measure of ascending pathways within the surgical field, whereas electromyo-graphic recordings in the sternocleidomastoid muscle and tongue reflect 11th and 12th cranial nerve activity, respectively. If either of these modalities demonstrates a change, then the surgeon is alerted to a potentially threatening maneuver and may pursue a different manner of dissection. Although we have found electromyographic monitoring of the 11th cranial nerve useful, stimulation of the 12 cranial nerve occasionally will cause protrusion of the tongue, which, if not returned to position by the anesthesia staff, can lead to postoperative tongue swelling. There is insufficient evidence to support the use of routine evoked potential monitoring in this location. Changes, if noted, are always noted after the event has occurred, and if retraction is minimized, they rarely change intraoperative management. Currently, these modalities have not gained absolute clinical acceptance; their use instead is based on surgeon preference.
Surgical Approaches to Foramen Magnum Meningiomas
The FM can be approached via anterior, lateral, and posterior approaches. Each approach serves an important function and each was developed to deal with specific problems. The anterior transoral approach to the FM is rarely conducted to reach intradural lesions, such as meningiomas, because of problems with dural repair, risk of CSF leakage, and meningitis. Debate about FM meningioma resection primarily involves the posterior suboccipital craniectomy and posterolateral approaches, which necessitate drilling of the occipital condyle ( Fig. 31.6 ). We limit our discussion to these approaches.
To simplify understanding of approaches to this region, we use the terms suboccipital craniotomy and transcondylar approach. Both require laminectomy, although the transcondylar is more commonly associated with mobilization of the VA from its lateral attachments to widen the surgical corridor. Terms such as far lateral and extreme lateral have only conjured up confusion and in our opinion should be avoided.
Suboccipital Craniotomy
Patient position: prone, head flexed on neck, neck kept neutral
Lateral decubitus, head turned 20 to 30 degrees toward floor
Craniotomy: suboccipital
With or without C1 laminectomy
Suboccipital craniotomy, or craniectomy, with or without cervical laminectomy, represents the classic approach to the FM meningiomas and is familiar to most neurosurgeons. For posteriorly situated lesions, we place the patient prone. The anethetist should always place a padding between the teeth to avoid clenching down on the endotracheal tube during flexion of the neck. For lateral or anterolateral lesions, the patient is placed in the lateral decubitus position with the vertex of the head displaced slightly downward to open the space between the occiput and the cervical spine. We also turn the head ~20 to 30 degrees toward the floor, depending on the extent to which the tumor is laterally situated. Because we use a diploscope mount to our surgical microscope, this allows both surgeon and assistant to access the surgical space and effectively use four hands to remove the tumor rather than only the surgeon’s two hands. For the lateral position, we place the patient’s contralateral shoulder overhanging the end of the operating table and allow the dependent arm to hang down from the bed, resting in a well-supported sling or armrest. Regardless of the approach, we use preoperative prophylactic antibiotics.
The surgical corridor defined on preoperative imaging must be easily within reach of the surgeon. The corridor should not be hidden under a large bulk of paracervical muscles deflected laterally. Sufficient soft tissue dissection to create access to the corridor is essential. Routine use of computerized neuronavigation helps to demonstrate subtle variations of anatomical distortions caused by these sessile meningiomas.
For midline posterior lesions, we make a midline incision. For posterolateral lesions that require exposure up to the condyle, we make a “hockey-stick” or inverted L–shaped extension laterally at the superior end of our incision just beneath the superior nuchal line. An S-shaped incision placed laterally can also be utilized. In addition, especially if a mastoidectomy is planned, a large C-shaped incision of the skin with the base toward the ear and a downward deflection of the suboccipital muscles can be performed. The bony exposure should include the superior and inferior extent of the lesion so that, at a minimum, the C1 lamina and superior part of the C2 are routinely exposed. Whichever the incision, cutting of the C2 nerve branches and the 11th cranial nerve distally in the neck should be avoided.
The VA is easily identifiable as it curves above the arch of the atlas, in the depth of the suboccipital triangle, providing proximal vascular control if required. We use neuro-navigation to help determine the extent of the craniotomy needed. Although some authors prefer to conduct a craniectomy, we prefer a craniotomy because the incidence of postoperative occipital pain, we believe, is limited by replacing a firm protective covering over the dura, even if it covers only a fraction of the exposed dura.26 If the surgical corridor to the tumor cannot be safely accessed as determined by neuronavigation before dura opening, more bone can be removed laterally toward the condyle. The craniotomy almost always has to be combined with a laminectomy to the inferior aspect of the tumor. At C1 the laminectomy should encompass at least the SA (i.e., the VA groove) in the lateral aspect of the C1 lamina. Care should be taken not to injure the thin-walled vertebral plexus of veins that surround the thick-walled VA. Of help in this procedure is bipolar coagulation with constant saline irrigation to avoid sticking of the instrument tips.
The advantage of suboccipital craniotomy includes visualization of the VA, brain stem, cranial nerves, and tumor in a safe, simple, and rapid manner. Criticisms of this approach primarily relate to the interposition of brain stem, cranial nerves, and vessels between an anterior tumor and the surgeon. The main problem we have seen with the simpler suboccipital craniotomy approach is the inability to mobilize the muscle mass of suboccipital muscles sufficiently laterally to get adequate lateral exposure. This can be overcome by extending the L-shaped incision into more of an inverted U and carrying the inferior limbs of the incision further inferiorly. Failure to do this results in the surgeon unduly retracting the neural structures to access anterolateral tumor or leaving residual. The purely anterior midline tumor without an adequate surgical corridor is completely obscured by these structures. The limitation of the unmodified suboccipital craniotomy approach is that it may necessitate undue retraction of critical neurological structures in cases in which the lesion is purely anterior with a narrow corridor. Fortunately, these purely ventrally located tumors are the rarest and can be dealt with using a suboccipital craniotomy combined with a partial condyle resection—what we call the transcondylar approach.
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