Contemporary Approaches to Diagnostics and Treatment of Foramen Magnum Meningiomas

There has been a significant change in the structure of intracranial tumors this past decade. According to CBTRUS, in 1994, gliomas totaled 46.5% (including glioblastomas 23.3%) of all brain neoplasms, meningiomas 24.8%, other tumors 28.7%, in 2007-2011 the share of gliomas decreased to 27.8% (including glioblastomas 15.6%), but the meningiomas percentage increased up to 35.8% and other tumors to 36.4% [1, 2]. Intracranial meningiomas, in the overwhelming majority, belong to the benign neoplasms (WHO Grade I-II) [3-5] with slow growth and progredient progression. A separate group consists of skull base meningiomas characterized by complex anatomical interrelations and high sensitivity of surrounding structures to surgical manipulations. The neoplasm removal is associated with solving the function preservation problem of intracranial neural and vascular structures, which is ensured using wide possibilities of microneurosurgery: surgical optics, neurophysiological monitoring, etc. [6]. Due to specific symptoms, complicated surgical anatomy and unique operating conditions, a special group of meningiomas is distinguished meningiomas of the craniocervical junction or the Foramen Magnum (FM).


Introduction
There has been a significant change in the structure of intracranial tumors this past decade. According to CBTRUS, in 1994, gliomas totaled 46.5% (including glioblastomas -23.3%) of all brain neoplasms, meningiomas -24.8%, other tumors -28.7%, in 2007-2011 the share of gliomas decreased to 27.8% (including glioblastomas -15.6%), but the meningiomas percentage increased up to 35.8% and other tumors to 36.4% [1,2]. Intracranial meningiomas, in the overwhelming majority, belong to the benign neoplasms (WHO Grade I-II) [3][4][5]   people per year) [6,7]. It is believed that among all meningiomas, anterior -from the lower third of the clivus to the upper edge of C2 vertebra body; lateral -from the jugular tubercle of the occipital bone to the upper edge of C2 vertebra arc; posterior -from the anterior margin of the occipital bone scales to C2 vertebra spinous process [8][9][10][11][12][13].

Materials and Methods
This study includes 34 observations; patients aged 18 to 75 years (median age 52.8 years) in the period from 1991 to 2016. There were 7 men, 27 women (ratio M: F -1:3.9). This amounted to about 1.6% of the total patient number with symptomatic meningiomas. It is believed that only 25% of meningiomas are symptomatic [9], and the rest are identified as incidental findings in routine examinations or autopsy. In our study, all tumors were symptomatic; there was not a single incidental detection of FM Meningioma on Magnetic resonance imaging (MRI) or CT. The duration of manifestation prior to diagnosis was 2.5 to 48 months (median duration 14.8 months). Clinical symptoms are presented in Table 1.    meningiomas. An invasive growth into the bone tissue was observed in 4 (11.8%) cases. Bone changes (hyperostosis) caused by FM meningiomas did not have a significant size in our study. These neoplasms were not characterized by perifocal changes, which are not very typical for meningiomas. The frequency of such changes caused by meningiomas with different localizations is 50-83% [6].
The possibilities of dual-energy SCT method allowed to not only provide 3D bone reconstruction with the tumor node projection on them, but also SCT -angiography with the projection of the main vessels onto the tumoral nodes ( Figure 2). with blood vessels in it [6]. In FM meningiomas, unlike other meningiomas localizations, this specific sign was not detected in all cases. Full or partial obturation is caused by the arachnoid sheath adhesions with the underlying superficial parts of the brain, which are caused by proliferative changes developed at the tumor edge.
The use of contrast enhancement increased the abilities to diagnose dura mater neoplasm invasion. This symptom was noted in 7 (20.6%) patients. According to MRI data, tumor growth all around vertebral artery was detected in 12 (35.3%) cases ( Figure 3). But this is not completely correct data, intraoperatively the dura mater invasion around the vertebral artery was observed in 18 (52.9%) observations.

Figure 3:
Tumor grows around vertebral artery (MRI data) (basilar artery is marked with arrows).
We used the midline posterior (suboccipital) in 27 (79.4%) and posterolateral transcondylar 7 (20.6%) surgical approaches with lateralization. The midline posterior approach is considered optimal for posterior meningiomas -in these cases the brain structures are shifted anteriorly from the tumor. Posterolateral (far lateral or lateral) approach was first proposed by Koos WTH et al. [13], and Babu RP et al. [14], and modified by Heros RC [15] and Perneczk A [16]. It provides a lower-lateral approach, if necessary -double-sided, to the ventral surface of the brainstem without brain retraction. The described variants differ only in the condyles resection degree and mobilization of the vertebral artery. After the VA mobilization, it was displaced downwards, and the resection of the occipital condyle posterior part was performed with the drill. We avoided the junction resection more than 50% to avoid craniocervical instability. It is believed that partial condylectomy can significantly expand the surgical corridor.
The performed incisions of dura mater were Y-shaped or horseshoe-shaped, retracted with stitches. According to matrix accessibility, tumor resection was started with the devascularization of its posteroinferior portion. Then meningioma was enucleated in parts and separated from the rootlets of the spinal cord and lower cranial nerves. The medial and anterior tumor parts were removed last, after complete devascularization took place, in a dry operative field, with a minimal brainstem displacement. If the matrix was inaccessible, the removal was started with tumor enucleation using ultrasonic aspirator within visible limits with thorough gradual hemostasis. The enucleation area was expanded toward the matrix, which was coagulated, starting with its middle part. Gradually, the posterior tumor portion was devascularized and removed, releasing lower cranial nerves, cervical rootlets.
Thus, surgical corridor was expanded significantly, which provided well-controlled manipulations on the remaining medial tumor portion and its matrix. After resection of the latter and cutting it off from dura mater the medial portion of the tumor was gradually dislocated and removed in parts. In 3 (11.3%) cases, the control of manipulations was provided by using an assisting endoscopy.
According to the dural insertion with the tumorous tissue, a partial or complete resection of its affected areas (anterior or posterior to the vertebral artery) was performed -7 (20.6%) cases. Vertebral arteries mobilization from the tumor mass is associated with a risk of ischemia; therefore, it required thorough intraoperative neurophysiological monitoring. We succeeded in VAs mobilizing and dissecting them from the tumor all the way from the site of its penetration through the dura mater in 14 (41.2%) patients. In 4 (11.8%) cases, the VA mobilization and tumor resection were limited by neuromonitoring indications (a sharp decrease of somatosensory evoked potentials amplitude) or anesthesiological limitations (persistent bradycardia and blood pressure decrease). Muscle patch was used to close dura mater.
Muscular and aponeurosis layers were tightly closed.

Results
To assess the extent of surgical treatment, we used the gradation system Simpson [17]. Total removal of FM meningiomas There were no hemorrhagic complications in any case.
Postoperative radiosurgery (gamma-knife) was carried out in 1 (2.9%) case (after subtotal tumor removal), radiotherapy -in 4 (11.8%) cases after partial and subtotal FM meningioma resection. There were no gross neurological disorders, which significantly restricted the ability to work and live. All patients were or continued to be monitored, according to clinical protocols. They were recommended to undergo a control MRI at least once a year.
Catamnesis was up to 19 years. There were no repeated complaints about recurrence or continued FM meningioma growth in either case, even after subtotal or partial tumor removal.
Bulbar disorders and deep right-sided hemiparesis were observed in postoperative period in a single patient after separation of tumor from brain stem and vertebral artery (total tumor resection). Autologous MSCs from adipose tissue (6.0-8.010^6 cells) were used in complex postoperative treatment (submucous perineural implantation [18][19][20][21]). Migration of MSCs led to distinct progressive recovery of functions: bulbar disorders completely regressed in four months, hemiparesis disappeared, and motor coordination recovered. Patient wears heels and likes dancing 4 months later. Information on the ability of stem cells to differentiate in a neuron-like direction after microglia activation is the fundamental basis for this tactic [22]. There are experimental data [23] confirming that implanted stem cells form synaptic contacts with endogenous brain neurons close to neurodestruction area after differentiation into neuron-like elements. By the way, it is paradoxical, but stem cells have antitumor effect also [24].
FM meningiomas are limited tumors that spread within the anatomical area without significant bone invasion, the removal of which usually does not require modified surgical approaches.
Mainly ventral neoplasms location suggests the use of anterior transoral approaches. However, they proved to be ineffective due to significant drawbacks: increased risk of CSF fistula and infectious complications, poor access to laterally extending tumors resulting in incomplete resection, the risk of postoperative craniocervical instability and soft palate paralysis [12,26]. Currently surgical approaches are used for FM meningiomas resection, in accordance to the need of lateral surgical corridor extension: midline posterior, posterolateral transcondylar and ultra-lateral transcondylar. The most optimal for posterior FM meningiomas resection (posterior to the dentate ligament and medial to the vertebral artery) is the midline posterior approach [2,9].
The surgery is performed in patient's "sitting" position (preferable position, to reduce venous bleeding) or "on the side" with rigid head fixation. The risk of venous air embolism is easily solved by anesthetic measures. Skull trepanation (usually resection) is performed in the lower part of the occipital bone with partial resection of C1 posterior arch. Dura mater is incised T-or Y-shaped, retracted with stitches. Goel A et al. [34] believe that even ventral FM meningiomas can be removed using this approach. With posterolateral access, there is a risk of vertebral artery damage at the condylotomy stage, or stricture formation due to excessive coagulation. Expanding the approach to the jugular tubercle is dangerous due to a high risk of lower cranial nerves damage [2,7,9,13,16,27,32,33,35,49].
It is believed that anterolateral (extreme-lateral) access (anterolateral approach or extreme-lateral approach) is indicated by the extradural extension of ventral and ventrolateral FM meningiomas. It was first described by Sen CN, Sekhar LN [39], modified by Salas E et al. [49] and Rhoton AL Jr [32]. It This ensures a safe resection of atlanto-occipital joint and the jugular tubercle. Often, the periosteum around the vertebral artery is calcified and requires careful manipulation. Retrosigmoid craniotomy is performed down to FM and the sigmoid sinus is exposed to the jugular vein bulb. If necessary, the VA transposition [47] is performed, especially with extradural meningioma expansion. C1 and C2 laminectomy is performed to the lower pole of the tumor. After this, the extradural component and the altered bone structures of FM anterior part can be removed. Tumor invasion in this case indicates the volume of bone resection to ensure maximum tumor resection. Then the dura mater is incised, and tumor resection begins [2,3,5,7,13,17,27,3s8,49].
The use of intraoperative neurophysiological monitoring of somatosensory evoked potentials, short latency auditory evoked potentials and electroneuromyography of lower cranial nerves by recording through the intubation tube (CN X) and with the needle in the sternocleidomastoid muscle (CN XI) and the tongue (CN XII), is considered appropriate [9,13]. Bassiouni H et al. [41], all surgical approaches are divided into two types-retrocondylar, when the articular processes are not resected, and transcondylar, when condyle resection of any volume is performed. In recent years, advantages and disadvantages of these methods have been critically evaluated, and many specialists have begun to use less laborintensive posterior suboccipital access to remove ventrolateral and even ventral FM meningiomas [34].
The level of transitory postoperative complications ranges from 39.3%, permanent -from 7.1%. Usually these are lower cranial nerves dysfunctions and ischemic problems are associated with the vertebral artery. The surgical medulla oblongata trauma of neuraxis is rare. The sublingual nerve and jugular vein are most often traumatized during occipital condyle resection [3,36,39,40].
Conclusion. Suboccipital lateralized approach with laminectomy to the lower tumor pole is enough to provide adequate microsurgical FM meningiomas removal without atlanto-occipital junction resection. Matrix approach should be performed after partial tumor resection without brain stem traction. The use of intraoperative neuromonitoring provides stem functions control at all stages of tumor removal and spinal artery mobilization.