Keywords
Foramen magnum - meningioma - brainstem
Introduction
Neurosurgeons have to deal with the challenging anatomy of the craniocervical junction
and its surrounding neurological and vascular structures.[1] Meningioma is the most common intradural tumor of the foramen magnum (FM).[2]
[3] Its growth induces mass effect on vascular and nervous structures and modifies the
surgical anatomy of this region. Once a patient suffers from neurological symptoms
related to the mass effect, the neurosurgeon has to perform a challenging surgical
resection. The main goal of this functional-sparing surgery is the stabilization or
the improvement in existing neurological symptoms without compromising neurovascular
structures. What is more, there is no place for technical challenge anymore as the
exhilaration of gross total resection must be balanced with the possibility of postoperative
radiotherapy or simply long-term follow-up.
Case Presentation
A 67-year-old female patient, treated with hormone replacement therapy by nomegestrol,[4] was referred for gait disturbance and lower limbs pain. Clinical examination showed
a tetrapyramidal syndrome with Babinski sign, Hoffmann sign, and hyperactive reflexes,
and a posterior cord syndrome with proprioceptive ataxia and a Romberg sign, a thermoalgesic
anesthesia, distal tingling, and numbness. There was a left lower limb weakness ranked
4/5 on the modified Medical Research Council scale. Magnetic resonance imaging of
the brain revealed a meningioma measuring 31 mm (craniocaudal) × 23 mm (anteroposterior) × 16 mm
(lateral) inserted on the anterior and lateral left part of the FM, between the lower
clivus and the upper half part of the odontoid process ([Fig. 1]). The medulla oblongata was displaced backward on the right and the left vertebral
artery (VA) was completely displaced superiorly.
Fig. 1 Preoperative postcontrast T1-weighted magnetic resonance imaging shows the meningioma
inserted on the anterior and lateral left part of the foramen magnum. The medulla
oblongata is displaced backward on the right, and the left vertebral artery is displaced
externally and superiorly.
The patient was operated on in a right park-bench position, the head being slightly
rotated on the right to better expose the left side of the posterior fossa and the
upper spine. A left far-lateral approach (FLA) was used to expose a bone window with
a suboccipital craniectomy passing through the FM, the removal of the posterior arch
of the atlas from the posterior tubercule to the vertebral groove to expose the V3
segment (extradural) of the VA, and the superior part of the lamina of the axis. The
dura matter was opened in a hockey stick fashion. A dural slot was cut until the opening
of the transdural passage of the VA for optimal vascular control. The cisterna magna
was opened, then the arachnoid was loosened around the medulla oblongata. This allowed
a complete visualization of the meningioma. The left VA in its V4 segment (intracranial)
was displaced superiorly and externally. The lower cranial nerves (LCN) (CN IX, X,
cranial XI, and XII) were not seen at first. The spinal accessory nerve (CN XI) was
stretched laterally by the tumor and running directly against it. Finally, C2 nerve
root was stretched by the inferior pole of the tumor ([Fig. 2]).
Fig. 2 Intraoperative view using a left far-lateral approach before the resection of the
meningioma. (1) Vertebral artery; (2) medulla oblongata; (3) cerebellum; (4) C1 root of spinal accessory nerve; (5) C2 root of spinal accessory nerve; (6) ventral root of C2; (7) spinal accessory nerve; (8) arachnoid membrane; (9) dura mater; (10) posterior arch of the atlas; (11) lamina of the axis; (12) meningioma. (Pencil drawing created by Nathan Beucler.)
Because of the firmness of the tumor, the ultrasonic surgical aspirator was of limited
support. We performed a progressive tumoral fragmentation between the different cervical
roots of CN XI; we were lucky enough to find an arachnoid cleavage plane that helped
to preserve this cranial nerve. This allowed the resection of a large portion of the
center of the tumor and the loosening of tension against the medulla oblongata. Then,
we could gently isolate the posterior wall of the meningioma from the medulla oblongata
by following the arachnoid cleavage plane; we did not find any particular tumoral
adherence either. The surgical excision was not impeded by excessive bleeding. Another
main surgical concern was the wide insertion of the tumor from the lower clivus to
the upper part of the odontoid process. We had a proper vision on the lower part of
the insertion. For the upper part, we had to tilt the operative microscope to remove
small tumoral portions through the suboccipital craniectomy within a thin corridor
limited by the LCN anteriorly, the cerebellum superiorly, and the VA loop from V3
to V4 segment posteriorly and inferiorly. The opening of the transdural passage of
the VA allowed us to displace the artery safely to better expose this remote part
of the insertion of the tumor. The whole tumor insertion was eventually coagulated
leading to a Simpson 2 resection.[5] Pathology report was consistent with a grade 1 transitional meningioma with a proliferation
index (Ki67) of 3%.
The complete removal of the meningioma led to a proper visualization of the major
structures of the bulbopontine junction. The left LCN appeared during their course
to join the jugular foramen.[6] The left VA was seen in its intracranial V4 portion. Its first branch, the posterior
inferior cerebellar artery (PICA), was also visualized. Because of the remaining footprint
of the tumor on the medulla oblongata, the contralateral VA and the contralateral
dura mater were visible. A large portion of spinal CN XI was seen with its roots arising
from C1, C2, and C3. In the same way, the ventral root of C2 was loosened ([Figs. 3] and [4]).
Fig. 3 Intraoperative view using a left far lateral approach after the resection of the
meningioma. (1) Left vertebral artery; (2) right vertebral artery; (3) left posterior inferior cerebellar artery; (4) lower cranial nerves; (5) cerebellum; (6) medulla oblongata; (7) C1 root of spinal accessory nerve; (8) C2 root of spinal accessory nerve; (9) ventral root of C2; (10) C3 root of spinal accessory nerve; (11) spinal accessory nerve; (12) posterior arch of the atlas; (13) lamina of the axis; (14) dura mater; (15) contralateral dura mater; (16) insertion of the meningioma coagulated. (Pencil drawing created by Nathan Beucler.)
Fig. 4 Postoperative postcontrast T1-weighted magnetic resonance imaging shows gross total
resection of the meningioma. Note the remaining footprint of the tumor on the medulla
oblongata. The axial slice exposed here is situated in the lower aspect of the clivus
at the superior part of the tumor. Thus, it is different for that of [Fig.’1] that was situated at the level of the odontoid process, which explains that the
medulla oblongata is not displaced backward on the right as it could be expected.
The dura mater was closed in a watertight fashion using individual stitches completed
with biological glue, then the wound was closed layer by layer. The postoperative
course was uneventful, and the patient was sent to neurological rehabilitation.
Discussion
The Far-Lateral Approach: Historical Considerations
Historically, the surgical removal of anteriorly-inserted FM meningiomas was considered
as a life-saving procedure.[7] The surgical technique has evolved over time from a posterior medial approach that
forced the surgeon to retract noble structures and leave some tumor anteriorly, to
a lateral approach[8] and finally a FLA that led to a better control on the VA and an easier access to
the anterior insertion of the tumor.[9] This rethinking of the surgical corridor allowed a precise analysis of the relation
between the tumor, the medulla oblongata, the VA, and the LCN.[10] The surgeon was given the possibility to perform gross total resection (GTR) whenever
possible, or to leave a few tumoral remnants in case of tumor adherence to nervous
structures, with a thought of functional-sparing procedure.[11]
The Far-Lateral Approach: Surgical Advantages
The FLA provides a particular three-quarters backside view over C1 lateral mass, C2
articular process, and the trajectory of the VA in its V3 and V4 segments ([Fig. 5]). Although the FLA has scarce indications, it is the ideal opportunity to understand
the relations between the atlas, the axis, and the VA. In the case of FM meningiomas,
the challenge for the operator is to perform a precise subarachnoid dissection between
the tumor, the main vascular structures, and the LCN. It is the same issue in the
case of anteriorly located epidermoid cysts of the cerebellopontine angle, or clival
chordomas. This approach is also particularly adapted for microsurgical clipping of
PICA aneurysms, because PICA lateral medullary segment is to be found right after
the opening of the dura mater.[12] Finally, the FLA can be used for the microsurgical resection of cavernous malformations
located laterally to the lower brainstem.[13]
Fig. 5 Postoperative three-dimensional computed tomography scanner in lateral view and three-quarters
backside view illustrating the bone window in the far lateral approach. (C1) atlas;
(C2) axis; (C3) third cervical vertebra; (CRA) suboccipital craniectomy; (Co) occipital
condyle; (FM) foramen magnum; (Ma) mastoid process.
In a way, the FLA for the resection of tumors of the anterior part of the upper cervical
spine canal is comparable to Georges' oblique transcorporeal approach to the lower
cervical spine.[14]
Perspectives Regarding the Surgical Approach
Nowadays, neurosurgeons work in facilities that provide sufficient medical and material
support to perform extremely demanding skull base procedures. With this in mind, the
surgical approach should be planned to obtain the sufficient exposure for the safe
resection of the tumor with minimal retraction over noble neurovascular structures,
and without compromising the stability of craniocervical junction. For lesions located
anteriorly to the FM, the choice between the FLA and the extreme far-lateral transcondylar
approach depends on the tumor's insertion on the clivus, the jugular foramen, and
the contralateral portion of the FM. For such lesions, an extreme FLA may sometimes
be required, but the operator must keep in mind that the surgical exposure takes longer
and can theoretically induce instability of the craniocervical junction depending
on the portion of occipital condyle that is drilled. Also, the extreme FLA suffers
from higher postoperative complication rates (LCN deficits and especially CN XII,
infection, cerebrospinal fluid leak) compared with the FLA.[15]
[16]
Conclusion
In the modern surgery era, even with the advent of minimal invasive procedures, some
tumors located anteriorly to the FM still require large opening for proper exposure
and safe resection with minimal retraction over neurovascular structures. Indeed,
the surgical paradigm for such tumors has evolved over 50 years from a life-saving
procedure to a functional-sparing surgery. The FLA provides an exceptional view over
the VA in its V3 and V4 segment, the bulbopontine junction, and the LCNs. Beyond its
interest for the safe resection of lesions located anteriorly to the FM, this approach
is an anatomical lesson.
