Foramen magnum meningiomas (FMM)

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Meningioma

Definition

• Origin from meninges between the upper border of the arch of C2 and the junction of the lower and the middle third of clivus, laterally extending to the jugular tubercle.

• Arise from arachnoid at the craniospinal junction

Numbers

• Rare and constitute only 2% of all intracranial meningiomas.

Classification

• Based on location
• Anteroposterior location
• Anterior to the dentate ligament
• Most common 68 to 98%
• Lateromedial location
• Posterior to the dentate ligament
• 2nd most common
• Pure posterior
• 3rd most common
• Pure anterior
• Least common

• Based on effects on critical structures,
• VA
• Cervicomedullary parenchyma
• Cranial nerves

 

• Based on their size relative to that of the FM
• Small: 1/3 the transverse dimension of the FM
• Medium: 1/3 - 1/2 its dimension;
• Large: >1/2 the dimension of FM

 

Clinical features

Early signs

• Occipital headache and upper cervical pain
• Exacerbated by neck flexion or Valsalva maneuvers.

Late signs

Delayed diagnosis

• Mean length of symptoms before diagnosis is 30.8 months

• Because of similarity with symptoms of commoner conditions affecting musculoskeletal and nervous system in this region.

Neural compression

• Brainstem compression
• Long tract signs
• Earliest sign cold or burning in one or both lower or upper limbs before other features arise.
• 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.
• Bulbar signs
• CN11
• Pain mainly
• Sometimes as Shoulder abduction weakness
• If there is acute disturbance of CN11 look for other causes like Carcinoma
• Slowly progressive lesions like these allow the development of accessory muscles to replace trapezius and sternocleidomastoid function.
• Undressed and inspect the sternocleidomastoid and trapezius muscles for atrophy.
• CN9/10
• Sleep apnea
• Swallowing difficulties
• Gag reflex
• 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
• Inability to maintain airway protection with secondary pneumonitis, and ultimately respiratory arrest.
• CN12
• The tongue should be inspected at rest (in the mouth) for atrophy and fasciculation

• Spinal cord compression
• Cruciate paresis, also known as hemiplegia cruciata, is often mentioned in the context of more anteriorly placed FMMs, but in practice it is rarely encountered.

• Nerve root compression
• Close attention to sensory testing of the C2 dermatome will help establish the diagnosis
• Initial: cold or burning dysesthesias, astereognosis, and anesthesia
• Later: Intractable pain, motor deficits, or ataxia ensue

Radiology

MRI

• T1
• 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.

• T2
• 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;

CT

• Identifying calcification, hyperostosis, and osseous anatomy.

• Extent of bone resection required to resect tumor safely because of the sharp contrast between bone and soft tissues.

Angiography

• 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.

Differential diagnosis

• Multiple sclerosis

• Amyotrophic lateral sclerosis

• Syringomyelia

• Cervical spondylosis

Surgical management

Adjuncts

• IOM
• SSEP
• MEP
• CN9,10,11,12 monitoring
• CN12: stimulation of the 12 cranial nerve occasionally will cause protrusion of the tongue, which, if not returned to position by the anaesthesia staff, can lead to postoperative tongue swelling. There is insufficient evidence to support the use of routine evoked potential monitoring in this location

Approaches

General

• Surgical approach depends on the location of the meningioma
• Circumferentially AND
• Posterior vs.
• Posterolateral vs.
• Anterolateral vs.
• Anterior
• Cranio-caudally
• Intracranial extension vs.
• Spinal extension

• Tumour growth causing in situ cord retraction provides an adequate route to the tumour
• The top left diagram shows the narrow space between the medulla and the bony wall of the FM (double-headed arrow) in a purely anterior FM meningioma.
• As the tumor enlarges (bottom left), the brain stem is displaced and the corridor widens.
• Often the patient begins to become symptomatic at this point but is rarely diagnosed until the corridor is widely established (bottom right).

Anterior

• Transoral approach
• Very rarely used in intradural surgery due to
• Risk of CSF leak
• Meningitis

Posterior

Suboccipital craniotomy

• Use a mouth block to avoid clenching down on the endotracheal tube during flexion of the neck

• Patient position:
• Prone, head flexed on neck, neck kept neutral
• Lateral decubitus
• Head turned 20 to 30 degrees toward floor
• Vertex of the head displaced slightly downward to open the space between the occiput and the cervical spine
• For lateral or anterolateral lesions.

• Bone work:
• Craniotomy vs craniectomy
• Prefer craniotomy to reduce post op occipital pain
• Even if the bone is very small and covers a small portion of the dura
• With or without C1 laminectomy

• Skin incision
• For midline posterior lesions
• A midline incision.
• For posterolateral lesions that
• Require exposure up to the condyle:
• 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.
• Require a mastoidectomy
• 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.
• 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

• 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
• 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.
• VA is freed from collagenous tissue at the C1 foramen transversarium and adjacent to the condyle by using fine micro-dissection under surgical magnification.
• A fine Prolene stitch can be used to secure the VA in a medial position.

• Advantage
• Advantage of suboccipital craniotomy includes visualization of the VA, brain stem, cranial nerves, and tumor in a safe, simple, and rapid manner.

• Disadvantage
• There will be brainstem, cranial nerve and vessels between anterior tumour and the surgeon
• 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

 

Far-lateral approach

• Indication
• For the more anterior locations a far lateral approach is ideal.
• However, the need and degree of the condylar resection (transcondylar approach) should be tailored to the location and size of the lesion.
• Even modestly sized meningiomas often create a sufficient surgical corridor by displacing the medulla and spinal cord thus obviating the need for condylar resection.

• Technique
• Removal of the FM rim toward the condyle + excision of the ipsilateral atlantal arch
• For those extending mainly to the upper cervical canal and foramen magnum at the level of the C1 tubercle, condylar resection is not needed.
• Tumours with superior extension up to the jugular foramen often have the jugular tubercle obscuring the view of the tumour.
• The attachment just medial to the dural entry of the vertebral artery (VA) poses another difficulty with the VA obscuring this attachment.
• Releasing the first denticulate ligament which separates the spinal accessory nerve from VA as it enters the intradural space, opens the surgical window wider onto the anteriorly placed meningiomas.
• In many cases where the dural origin is not resected a persistent enhancing dural base on postoperative MRIs tend to be stable in the long term.

Transcondylar approach

• Indications
• Small anterior lesion: where the lesion is not large enough to displace the cord laterally to create a safe surgical corridor
• For those extending mainly to the upper cervical canal and foramen magnum at the level of the C1 tubercle, condylar resection is not needed.

• Resection of some or all of the occipital condyle

• Patient Position:
• Lateral decubitus
• Three-quarter prone,
• Head turned 20 to 30 degrees toward floor

• Technique
• Skin incision
• Inverted U– shaped incision with one limb of the U in the midline and the other along the anterior border of the sternocleidomastoid muscle.
• Sternocleidomastoid muscle is detached from the mastoid process and reflected as laterally as possible to avoid hindering access to the skull base.
• The superficial splenius capitis, semispinalis capitis, and longissimus capitis muscles are reflected downward to expose the underlying suboccipital triangle
• Bone work
• Suboccipital
• Craniotomy should extend from the medial to the sigmoid sinus, to the most medial aspect of the lesion, and to just above the rim of the FM.
• The residual bone over the sigmoid and FM is removed using rongeurs or a high-speed drill.
• Anterior condylar resection can include liberating the hypoglossal nerve from its canal if necessary to create an adequate surgical corridor
• Occipitocervical fusion is recommended in condylar resections of > 50%
• C1 laminectomy
• Extends out to the foramen transversarium, which is unroofed separately
• If necessitated by the anatomy of the lesion, the foramen transversarium and C2 lamina are also decompressed
• Durotomy
• Because bone removal is extended much more laterally than in the suboccipital craniotomy, the ipsilateral VA is situated in the center of, or medially in, the “surgical corridor” before dural opening.
• Dura is opened by making an incision that parallels the lateral margins of the craniotomy, with the base of the flap located medially.
• A ring of dura can be left attached to the VA where it is pierced. This maneuver allows the artery to be retracted away from the surgical corridor, thereby providing a clear view of the anterior portion of the brain stem and rostral cord.