Fig. 33.1
Classification of foramen magnum tumors according to the position in the anteroposterior plane as anterior (a), lateral (b), and posterior (c)
The last aspect includes the relative position to the VA, in which tumors develop above, below, or on both sides of the artery (Fig. 33.2) [7, 13, 14]. FMMs are more often located below the VA (45.7 %); nevertheless, in some series tumors located above and below the VA had the same occurrence (45.7 %) [7, 14]. Thus, the surgical strategy in caudal tumors is relatively straightforward due to expected cranial and posterior displacement of the lower cranial nerves [7]. On the other hand, in the case of tumors growing above or above and below the VA, the position of the lower cranial nerves cannot be anticipated in the way that such nerves can be displaced in any direction [7]. In addition, VA dural entry point may also be invaded by tumor, which is extremely adherent to the adventitia of the VA ultimately precluding complete resection [7, 34].
Fig. 33.2
Schematic coronal demonstration of the classification system of foramen magnum tumors according to the relation to the vertebral artery. Tumors may develop above (a), below (b), and above and below (c) the artery
Moreover, Margalit et al. [25] consider FM morphological characteristics for intradural tumors, as determined by the imaging studies, by defining FM into three types, namely, deep, when the anteroposterior (AP) diameter exceeds the lateral one in 3 mm or more; round, when the difference between AP and lateral diameters are less than 3 mm; and wide, when the lateral diameter exceeds the AP one in 3 mm or more. Round FM shape predominates (44.5 %), followed by the wide shape in 33.3 % and the deep shape in 22.2 % of the patients evaluated [25]. The surgical implications of such classification lie in the distance of the anterior FM to the lateral exposure so that patients harboring deep-shaped FM and small anterior tumors require partial condyle resection in order to facilitate access to the area in front of the VA [25]. In patients having wide and round-shaped FM, there is generally no need for condyle resection [25].
33.2.2 Foramen Magnum Neurinomas
George’s system can also be applied for neurinomas having the same surgical implications as FMMs [14]. Neurinomas of the FM are mostly extradural (47.3 %), lateral (100 %), and arise below the VA (84.2 %) [15]. When analyzed separately, Cavalcanti et al. [8] found that hypoglossal schwannomas are frequently intra-extradural (67 %) and schwannomas of the upper cervical roots are mostly intradural (46 %). Nonaka et al. [29] reported 13 patients suffering from hypoglossal schwannomas and provided a detailed literature review on that field. Overall, hypoglossal schwannomas are typically intra- (31.7 %) and intra-extradural (38.6 %) lesions [29]. Finally, George and Lot [15] studied 42 patients affected by C1 and C2 neurinomas in a multicentric cooperation and described intra-extradural lesions as the most common ones (45 %), whereas 38 % were exclusive extradural.
Based on an in-depth literature review, Nonaka et al. [29] proposed a new classification system for hypoglossal schwannomas. They stratified tumors in type A, when the tumor is strictly intradural; type B, a dumbbell-shaped tumor; type C, extracranial tumor with osseous involvement; and type D, peripheral tumor without osseous involvement (Fig. 33.3) [29]. According to intra- or extracranial extension, different surgical approaches can be anticipated [29].
Fig. 33.3
Classification of hypoglossal schwannomas: type A (a), intradural tumor; type B (b), dumbbell-shaped lesion; type C (c), extradural lesion with bone remodeling; and type D (d), peripheral tumor. Note that the vertebral artery is displaced in type A and type B tumors, while the carotid artery may be displaced in types B, C, and D tumors. HC hypoglossal canal, HN hypoglossal nerve
33.3 Clinical Remarks
FMMs are typically slow-growing benign tumors that are often large at diagnosis because of their indolent development, misleading clinical manifestations, and the wide subarachnoid space at the CCJ [6–8, 24]. There is a characteristic sex distribution, in which women are much more affected than men [13]. The woman-to-man ratio varies in different series but at least 60 % of patients suffering from FMMs are women [2–5, 13, 16, 17, 22, 24, 25, 30, 34, 44]. Such sex distribution is not so clear concerning hypoglossal schwannomas, as well as C1 and C2 neurinomas [8, 15, 29, 49]. The patient’s average age at diagnosis and average time interval to diagnosis are 34–59 years and 9–45 months, respectively [2–5, 13, 16, 17, 22, 24, 25, 28–30, 34, 43, 44].
Early findings are generally upper neck pain and occipital headache exacerbated by neck flexion or Valsalva maneuver, as well as neck stiffness [6, 13, 24]. As the tumor progresses, motor and sensory symptoms develop [13, 24]. Lower cranial nerve dysfunction and sphincter disturbances are late complains [13]. Classic FM syndrome is an asymmetrical clockwise motor and sensory deficit defined by initial ipsilateral arm dysfunction, followed by ipsilateral leg, then the contralateral leg, and finally the contralateral arm [6]. Of note, however, is the relative high occurrence of severe symptoms, namely, tetraplegia (8 %), hemiplegia (3.5 %), and paraplegia (0.7 %) at admission [13]. It is worth mentioning that there is a recent trend toward minor symptoms at the time of diagnosis [13]. Table 33.1 summarizes presenting symptoms in published series.
Signs and symptoms | Numberof cases (%) |
|---|---|
Motor weaknessa | 249 (59.6) |
Neck pain/headache | 207 (49.5) |
Gait imbalance and myelopathy | 158 (37.8) |
Numbness (hypesthesia, hemihypesthesia) | 108 (25.8) |
Dysphagia | 82 (19.6) |
XII weakness/tongue atrophy | 58 (13.9) |
Hearing deficit | 45 (10.8) |
Paresthesia | 44 (10.5) |
Sphincter disturbances | 42 (10) |
Hand deficit/clumsinessof hands/hand atrophy | 36 (8.6) |
Trapezius weakness | 31 (7.4) |
Speech dysfunction | 23 (5.5) |
Vomiting | 17 (4) |
Hydrocephalus/raised ICP/cognitive | 15 (3.6) |
No neurological deficit | 11 (2.6) |
Facial palsy | 8 (1.9) |
Diplopia | 2 (0.5) |
A useful clinical classification is that proposed by Yasargil et al. [47], in which patients are grouped into four grades, namely, grade I, single symptom; grade II, mild to moderate symptoms; grade III, pronounced cranial nerves, brain stem, and cerebellar symptoms; and grade IV, patients are confined to bed.
Regarding hypoglossal schwannomas, some observations are worth discussing. Ipsilateral hemiatrophy is observed in 90–100 % of all patients revised by Nonaka et al. [29]. Interestingly, as unilateral hypoglossal palsy causes minimal symptoms, patients rarely seek medical attention due to speech, chewing, or swallowing problems [29]. Tongue atrophy is rather an incidental finding during regular dental or otolaryngological consultations [29]. Because of such unspecific clinical presentation, patients harboring hypoglossal schwannomas present quite late, by which time cerebellar, brain stem, or cranial nerves symptoms, or even intracranial hypertension, are already noted [29].
33.4 Diagnostic Imaging
Preoperative imaging includes computed tomography (CT) and magnetic resonance imaging (MRI). CT scanning is especially useful for identifying calcification, bone remodeling at the site of origin of the FMM and the hypoglossal canal, and FM shape [3, 6, 13, 25, 29]. MRI has a complementary role on CT by demonstrating the compartment of development, tumor location and attachment, tumor relationship with the CCJ, the position and potential involvement of the VAs and their branches, as well as arterial and venous dominance and the surgical corridor [6, 13, 24]. T1-weighted imaging provides adequate anatomical details but little discrimination between neurovascular structures [6]. Meningiomas appear mildly hypointense, isointense, or even hyperintense to surrounding brain [6]. T1-weighted imaging with contrast enhancement is particularly helpful in defining the tumor, the neural structures, and the dural attachment [6, 24]. The arachnoid plane and edema are better defined in T2-weighted imaging [6, 24].
Conventional angiography should be used at the discretion of the medical team in an individual basis [34]. With the advent of CT and MR angiography, tumoral vascularity and the size and displacement of the VAs and their branches are generally well demonstrated reserving conventional angiography for planned embolization of highly vascularized tumors [13, 24, 44]. Additional indications for conventional angiography comprise limited visualization with noninvasive imaging studies [2] and the need for performing a balloon occlusion test in cases of VA encasement [7]. When performed, the main blood supply for meningiomas usually comes from the meningeal branches arising from the extradural VA at C2 level with some participation of meningeal branches via ascending pharyngeal and occipital arteries [3, 6, 13, 44].
In cases with complete engulfment of the VA, preoperative embolization of the tumor and the segment of VA may be required so as to permit complete tumor removal [26]. VA occlusion has never been necessary for Bruneau and George [7], however. Although preoperative embolization has been described by many authors in the scope of FMMs [13, 44], it is noteworthy that endovascular techniques and agents as well as their efficacy and safety are largely unreported.
33.5 Surgical Management
Surgery is the definitive treatment and the standard of care for most tumors affecting the CCJ [13, 24]. The main goal is to relieve medulla and spinal cord compression with no or minimal injury to the neuraxis [13]. Similar to tumor distribution in the anteroposterior plane, surgical approaches to the FM are circumferentially assigned as anterior, anterolateral, posterolateral, and posterior. Lateral approaches were developed due to limitations of the anterior and posterior approaches in order to obtain better visualization of the neuraxis and the tumor with minimal or no brain retraction [13].
Lateral approaches nomenclature comprises a particular source of controversy and confusion in the literature since terms such as posterolateral, far-lateral, and extreme lateral have been used indiscriminately and what is lateral depends on the perspective of the surgeon [7, 13]. Rhoton [31] solved the issue by defining the far-lateral approach, also called posterolateral approach or lateral suboccipital approach, as those directed posteriorly to the sternocleidomastoid muscle and the VA and medially to the occipital condyles and the atlantooccipital joint. The extreme lateral approach is an otherwise anteriorly directed approach with posterior retraction of the sternocleidomastoid muscle and just behind the internal jugular vein [31]. These approaches can be extended cranially (retrosigmoid craniotomy or infratemporal approach), anteriorly (transfacetal, transtubercle, transcondylar, paracondylar, or transjugular), and laterally (petrosectomy, mastoidectomy) or eventually combined according to tumoral extension [13, 22, 24, 32, 39]. There is no evidence in literature to support any superiority of extreme lateral approach over far-lateral approach or even retrocondylar over transcondylar in terms of clinical outcomes, complications, and extent of surgical resection [3, 28, 32].
33.5.1 Anterior and Anterolateral Approaches
Several different anterior cervical and transoral approaches have been described for the resection of FM tumors [13]. Although they appear very straightforward approaches providing a direct route to the tumor, previous series have demonstrated significant drawbacks, namely, narrow surgical exposure due to lateral limitations from the carotid arteries and VAs, difficulty in dural repair and high risks of cerebrospinal fluid (CSF) leakage and meningitis, and the need for spine fusion after vertebral corpectomies [5, 6, 13, 17, 24, 39]. The advent of endoscopic skull base surgeries has revived the anterior route but limitations remained the same [5]. Recently, Zhang et al. [49] described three radical resections of dumbbell-shaped hypoglossal schwannomas via a purely endoscopic transoral approach with stimulating results. Reconstruction is technically demanding but neither CSF leakage nor meningitis occurred [49]. It is worth noting that all tumors in their series had small intracranial extension.
The extreme lateral approach was described by Sen and Sekhar [38] back in 1990 for FMMs and requires the VA transposition as a sine qua non condition among other technical nuances. Since VA transposition carries the risk of neurological disability due to vascular mobilization [3], we share George’s opinion that anterolateral approaches are most suitable for FM extradural tumors or meningiomas with bony invasion [7, 13, 14]. Anterior and anterolateral approaches are rarely used in our departments.
33.5.2 Far-Lateral Approach
The far-lateral approach promotes the lateral extension necessary for reaching the most anterior portion of the FM. Positioning is of utmost importance for satisfactory surgical results [24]. Orotracheal intubation under fibroscopy is extremely advisable to avoid neck manipulation [5]. The patient may be placed in a supine, three-quarter prone (JAL’s preference), lateral, or semi-sitting position (MT’s preference). The surgical approach is planned in accordance with tumoral laterality and arterial and venous dominance. Pressure points are adequately padded in order to avoid peripheral nerve injury, and the patient is fixed to the operating table. The head is placed in a three-point Mayfield head holder and slightly flexed anteriorly and laterally up to 30° so that the patient is facing the floor [24]. Intraoperative monitoring of somatosensory evoked potentials (SEP) is particularly useful for surgical positioning, either lateral or semi-sitting [34]. SEP deterioration should be promptly identified for proper correction in the surgical positioning [34]. Besides SEP, brain stem auditory evoked potentials and electromyography of the facial, glossopharyngeal, vagus, accessories, and hypoglossal nerves as well are monitored continuously. Muscle motor evoked potential of the hand, foot, facial nerve, and lower cranial nerves is routinely used to monitor major motor pathways. The semi-sitting position is addressed separately in this book in the Retrosigmoid Approach to the Posterior and Middle Fossa section.
An inverted hockey stick skin incision is marked starting medially from the C4 spinous process to the inion, then turned laterally through the superior nuchal line. At the level of the pinna, the incision is moved downward to the mastoid process and over the posterior border of the sternocleidomastoid muscle. Paramedian straight and slightly curved incisions can also be used. The skin flap is created taking care to preserve the pericranium as it can be used for dural closure.
Muscle dissection is the next surgical step. We summarize this stage in three main steps: (1) elevation of the superficial muscle layers to expose the suboccipital triangle, (2) dissection of the suboccipital triangle in order to identify the VA, and (3) transposition of the VA as required. Some authors advocate separate muscle dissection in order to avoid CSF leakage and to minimize trauma to lower cranial nerves [2, 25, 28]. We do not recommend dissection of each muscle due to the likelihood increase in the dead space, risk of infection, and muscle ischemia, which leads to wound dehiscence [24].
The first or superficial muscle layer (sternocleidomastoid and trapezius muscles) is opened in line with the skin incision leaving a small cuff for reinsertion. The second or middle layer (splenius capitis, longissimus capitis, and semispinalis capitis) is retracted laterally to expose the suboccipital triangle, which is formed by the superior and inferior oblique muscles and the rectus capitis posterior major muscle. The superior and inferior oblique muscles are detached from the transverse process of C1, while the rectus capitis posterior major and the superior oblique muscles are detached from the inferior nuchal line and retracted posteriorly. Cautery is used at the discretion of the surgical team but should be utilized with care at the posterior arch of C1 and the suboccipital triangle in order to avoid VA injury. Muscle detachment brings the VA, its venous plexus, and C1 root to the surgical field. The main source of bleeding and air embolism in this step is the venous network [24].
The next surgical step is exposure and transposition of the V3 or horizontal segment of VA, which are rarely required in the basic far-lateral approach [24]. Manipulation of the venous plexus during VA transposition can be particularly troublesome postponing the intradural stage for a second sitting [2]. For VA transposition, unroofing of the C1 transverse process is mandatory [24]. It was necessary in approximately 20 % of intradural tumors and 36 % of extradural tumors in our series [24]. VA mobilization should be done with utmost care in order to avoid thrombosis [22, 28] or rupture [2, 25, 30].
The osseous stage involves exposure of the transverse and sigmoid sinus borders, resection of the lateral rim of the FM, resection of the squamous part of the occipital bone to the midline or slightly thereafter, and resection of the ipsilateral posterior arch of C1 after detachment of the VA. The size of craniotomy or craniectomy and hemilaminectomy should be tailored for cranial or caudal tumoral extension. Additional lateral room can be gained with occipital condyle (OC) drilling, which is also rarely necessary during basic far-lateral approaches [24]. Navigation is a useful tool to help define subtle variations of the surgical anatomy caused by tumor growth [6] and to modify the size of bone resection.
OC drilling is one of the most controversial aspects of the far-lateral approach. OC is an oval-shaped osseous structure that lies at the base of the occipital bone. The posterior part terminates at the mid-level of the FM and blocks the angle of view toward the anterior FM and CCJ [24]. Thus, several studies support the need for OC drilling on the assumption of increased surgical visibility at the most anterior part of the FM based on the findings of some anatomical observations showing improved lateral exposure, angle of exposure, and shortening of the distances with condylar drilling [1, 46]. The amount of bone drilling is also controversial in the way that no, up to one-third, up to one-half, and complete, condyle drilling have been reported [24]. It is noteworthy that anatomical studies did not consider the space created by the tumor or surgical corridor [3, 22, 33]. Besides, endoscope-assisted microsurgical resection is becoming routine today leading to surgical view “around the corner” of osseous structures, as reported for the jugular tubercle in jugular foramen meningiomas [36].
In most instances, drilling of the condyle and mobilization of the VA are not required to obtain sufficient access, even in patients with anterior meningiomas. Only in small tumors in this location is drilling of the posterior third of the condyle indicated, from our point of view. Otherwise, the tumor itself provides all the space required to visualize all of the structures adequately. Incomplete resections are not the result of inadequate exposure with this technique, but instead are related to the involvement of vertebral and basilar arteries and perforating vessels, as well as to the involvement of cranial nerves as mentioned in this article. Samii [33]Related posts:
Preoperative Visualization of the Facial Nerve Using Diffusion Tensor Imaging Fibre Tracking in Patients with Large Vestibular Schwannomas
Endoscope-Assisted Microsurgery
Restoration of Locomotion in Post-traumatic Paraplegics: The Neurosurgeon’s Personal View
Endoscopic Transnasal Surgery for Clival Chordoma
Diagnosis and Treatment of Adult Hydrocephalus
The Surgical Management of Trigeminal Schwannomas
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