Brain Tumor Education Resource

Brain tumor series #4: SCHWANNOMA

This series covers:

Trigeminal Schwannoma
Foramen magnum Schwannoma
Jugular foramen Schwannoma
Vestibular Schwannoma (acoustic neuroma)

For each of these cases, the following format is used:

Brain tumor background: An introduction to the type of brain tumor being case-presented.
Brain tumor clinical presentation: What were the brain tumor 's symptoms and signs?
Brain tumor diagnostic workup: What investigations were used to diagnose the brain tumor?
Brain tumor treatment paradigm: What were the options and proposed treatment for the brain tumor?
Brain tumor operative procedure and approach: What specific approach did the surgeon use for the brain tumor?
Brain tumor technical nuances and potential surgical pitfalls: What were important considerations for the brain tumor surgery?
Where available, brain tumor pre- and post- and/or intra-operative radiological images will be shown for the following brain tumor case examples.

Please note:

For neurosurgical patients and their families, reader-friendly and practical details of basic brain anatomy, symptoms and signs of neurosurgical lesions, step-by-step investigation and operative procedure details, operative risk counselling, informed consent, recovery and rehabilitation issues, follow-up and salvage recommendations, FAQs and other case histories are all presented elsewhere.
The information given below is for patients and physicians alike. However, certain areas such as those pertaining to the surgical aspects of the case are more medical jargon-intensive, being particularly suited for neurosurgical trainee teaching purposes. See elsewhere for comprehensive patient-oriented surgical information.

Introduction to Schwannomas

A Schwannoma is a tumor that arises from Schwann cells, named after their discoverer, Theodor Schwann (a 19th Century German histologist and physiologist). Schwann cells are cells that are derived from a primitive (embryological) structure known as the neural crest. Schwann cells, like the central nervous system's (CNS) oligodendrocytes ( see glioma section), lay down the myelin sheath. Myelin is a fat- or lipid-rich substance that is "spun" and eventually "wrapped" in concentric layers around each conducting portion of a nerve cell or neuron referred to as its axon. Such myelin sheaths allow axons to conduct faster. Schwann cells are found mainly in the peripheral nervous system (PNS), but also at junctions where the CNS interfaces with the PNS. At such junctions, each of which is referred to as the Obersteiner-Redlich Zone (ORZ), the oligodendrocytes of the CNS are interspersed with Schwann cells of the PNS. The ORZ includes the regions of the root entry zones (REZ) of nerves such as the vestibular (VIIIth) and trigeminal (Vth) (cranial) nerves and nerve roots of the spinal nerves. Theoretically, Schwannomas can arise anywhere that a Schwann cell is found. However, in the brain and spinal cord, Schwannomas tend to arise mainly at the REZ of the following:

The VIIIth (vestibulocochlear) cranial nerve - where they are referred to as vestibular Schwannomas (or acoustic "neuromas");
The Vth (trigeminal) cranial nerve - where they are referred to as trigeminal Schwannomas;
The lower cranial nerves (IX, X or XI - i.e., glossopharyngeal, vagus, or accessory) at the jugular foramen - where they are referred to as jugular foramen Schwannomas;
The spinal cord's spinal nerve roots (especially sensory or "dorsal" nerve roots) - where they are referred to as spinal Schwannomas.

Many of these tumors are referred to as skull base tumors because of their tendency to frequent the base of the skull (like certain meningiomas, chordomas and paragangliomas). At a gross or macroscopic level, Schwannomas tend to be firm and encapsulated with a whitish-yellow hue. At a microscopic level, Schwannomas tend to have a dual or "bimorphic" pattern referred to as Antoni A (dense collection of fusiform cells in a reticulin and collagen "connective tissue stroma") and Antoni B (loose collection of stellate/round cells in connective tissue stroma). Schwannomas themselves tend not to contain axons, but may be cystic, hemorrhagic, and with or without fat deposits.

What causes them to form is largely unknown and presumed to be an inherent "genetic defect". Such tumours frequently occur in the genetically inherited condition known as neurofibromatosis (NF), especially NF Type II. However, it has recently been reported ( see elsewhere for details) that in certain individuals, electromagnetic or radiofrequency radiation from cell phone devices may be a significant risk factor for developing an "acoustic neuroma" (a vestibular schwannoma -- see below -- involving the apparatus for hearing and balance).

Schwannomas are typically regarded as benign tumors, but can regrow following treatment. They cause symptoms by compressing adjoining nerve bundles or adjoining nerve tissue. Their growth rates vary, but are typically not considered to be "rapid". Treatment is open surgery or stereotactic radiosurgery (SRS). The latter is gaining popularity, but its use can depend on many factors including the size and location of the Schwannoma, and the age of the patient (rule of thumb: whenever possible, avoid SRS/radiation in the young).

Brain Tumor section; Case 16 - Trigeminal Schwannoma

Background: Trigeminal schwannomas arise from the Vth cranial (trigeminal) nerve. They grow anywhere from the origin of the trigeminal nerve from the brainstem (REZ of V; see above) to the trigeminal ganglion sitting in the skull base's middle cranial fossa (in a boney enclave referred to as Meckel's cave) through to one or more divisions of the trigeminal nerve. Although these tumors are rare, when they do occur, they frequently occupy two areas of the skull known as the middle cranial fossa (MCF) and the posterior cranial fossa (PCF). The bulk of the tumor tends to lie in the MCF, with an adjoining tongue of tumor present in the PCF. Such tumors present with some degree of sensory dysfunction on one side of the face. The dysfunction may be one or more of numbness, tingling, stabbing or burning discomfort; rarely, more severe "trigeminal neuralgia"-type pain (tic douloureux) can occur, which may be episodic severe electrical or stabbing pain in one or more distributions of the trigeminal nerve in the face.

Clinical presentation: A 37 year old female presented with a progressive left facial burning and tingling discomfort that was chronic (nonepisodic). Symptoms were mainly confined to the maxillary (Vth cranial nerve - V2 division) and mandibular (Vth cranial nerve - V3 division) regions (i.e., lower 2/3 of the face) on the left. Physical examination showed mildly reduced sensation along the left V2 and V3 regions, but no motor disturbance involving the muscles of mastication. The corneal reflex was preserved. There were no other sensory or motor findings.

Diagnostic workup: A brain MRI with and without contrast was carried out. This showed a contrast-enhancing mass whose epicenter was in Meckel's cave (see imaging below). The mass was mainly confined to the middle cranial fossa, but a portion of it was also located in the posterior fossa, with very mild mass effect on the ventral surface of the left pons.

Treatment paradigm: The clinical presentation and imaging features were consistent with a trigeminal Schwannoma. Surgery was offered as an appropriate first-line treatment for this lesion, although some discussion was made regarding stereotactic radiosurgery (SRS). Given the age of the patient and the size of the lesion with mild mass effect on the left pons, surgery was favored over SRS.

Operative procedure/operative approach: A left temporal craniotomy with zygomatic osteotomy and apical petrosectomy was carried out under stereotactic MRI guidance. The goal was gross-total resection. Intraoperatively, Vth and VIIth cranial nerve monitoring was used, as was CSF drainage via a lumbar drain placed immediately prior to incision. The head position was almost true contralateral, with the malar eminence relatively high and the vertex tilted to the floor in order to allow the temporal lobe to elevate gravitationally off the middle cranial fossa floor. Discussion of the technical aspects of the craniotomy itself are beyond the scope of this Site. An extradural approach to the lesion was used. The dura of the middle fossa was elevated off the floor, and the base of the petrous bone was identified, as was the tentorium and the superior petrosal sinus along the petrous ridge. The GSPN and Kawase's triangle were indentified and the petrous apex drilled. A contiguous durotomy was made from the temporal to the exposed posterior fossa dura. The stereotactic guidance was used to identify the tumor, which was entered with a number 11 scalpel parallel to the fibers of V. Microdissecting instruments and small punches were used to manipulate and resect the lesion. The posterior fossa durotomy, aided by the mesial/apical petrosectomy, allowed the posterior-most tongue of tumor to be resected off the trigeminal nerve itself.

Technical nuances and potential surgical pitfalls: Intraoperative CSF drainage, a low (i.e., to middle fossa floor) craniotomy with zygomatic osteotomy, and optimal head positioning will all be synergistic in minimizing the need for significant temporal lobe retraction. Careful identification and preservation of the vein of Labbe and the trochlear nerve during the combined middle and posterior fossa durotomy are essential. Excessive GSPN traction can result in facial paresis. The GSPN should be freed off the dura to minimize traction. The internal carotid artery (petrous segment) lies deep to and lateral to this structure in the petrous bone (Glasscock's triangle), therefore drilling the petrous (apex) should be undertaken medially (Kawase's triangle). Entry into the tumor parallel to the fibers of V is also important, and stereotaxis is very helpful in directing the surgeon to the tumor.

Imaging:

Image 16.1 (above). Trigeminal Schwannoma. Preoperative MRI axial and coronal T1 post-contrast sequence. Tumor encircled. Most of the tumor is in the middle fossa in the vicinity of Meckel's cave. Arrow heads point to a nodule of tumor in the posterior fossa and causing mild effacement of the left pons (seen best in the top left image).

Image 16.2 (above). Trigeminal Schwannoma. Preoperative MRI axial and coronal T1 post-contrast sequence (top and bottom left). Tumor encircled. Postoperative MRI axial and coronal T1 post-contrast sequences (top and bottom right). Tumor has been gross-totally resected. Resection cavity marked by tips of arrow heads.

Brain Tumor section; Case 17 - Foramen magnum Schwannoma

Background: The foramen magnum is an aperture at the very bottom of the skull through which the brainstem (medullary region) and the spinal cord (upper cervical cord) meet (i.e., become contiguous at the cervicomedullary junction). Schwannomas in this region are called foramen magnum Schwannomas. These tumors tend to arise here from Schwann cells in sheaths surround one of the lower cranial nerves or upper cervical nerves. Tumors here can cause a variety of disturbances depending on their exact origin and size. Symptoms may be voice and swallowing impairment, weakness or paralysis of one or more limbs (including "cruciate paralysis" which can be due to a meningioma or Schwannoma or other mass in this region), headache from obstructive hydrocephalus, and so forth.

Clinical presentation: A 32 year old female presented with headache. There were no other symptoms. Physical examination was entirely normal.

Diagnostic workup: A brain MRI was carried out, with and without contrast. This revealed a contrast-enhancing mass in the region of the lower brainstem (cervicomedullary junction) on the right side. There was no obvious dural tail. The mass was located in the vicinity of the right 12th cranial and C1 nerves, there was no obstructive hydrocephalus, and therefore the imaging did not explain the patient's headache. A CT angiogram (CTA) was subsequently ordered just to confirm that this mass was not an unusual presentation of a brain aneurysm or other vascular abnormality. The CTA suggested this was a solid tumor.

Treatment paradigm: Owing to the age of the patient, the highly probable presence of a tumor per the imaging, and despite the asymptomatic nature of the mass, the patient was recommended for treatment as opposed to "observation". Surgery was favored over stereotactic radiosurgery owing to the surgical accessibility of the mass, the potential for avoidance of brainstem radiation in a relatively young person, and to confirm the diagnosis of Schwannoma versus some other possibility such as a dural-based metastasis (i.e., a tumor that has spread from another region; click here to go to the metastasis section; or a meningioma; click here to go to the meningioma section).

Operative procedure/operative approach: A right far lateral craniotomy using a modified "park bench position" was carried out. There was no need for mobilization of the vertebral artery or for occipital condyle drilling in this case. Stereotactic MRI image-guidance was used, as was lower cranial nerve monitoring (electrodes were placed for cranial nerves 11 and 12; an endotracheal tube with electrophysiological recording capacity was used to monotor cranial nerves 9 and 10). In addition, somatosensory- and motor-evoked potential monitoring (SSEP, MEP) was carried out. Immediate preoperative lumbar drain placement was not required owing to early and direct access to the cisterna magna. The far lateral approach in the park bench position allowed excellent visualization and access to the structures of the craniocervical junction, with a 45 degree trajectory across the right anterolateral medulla. The lesion was identified as a Schwannoma arising from a fascicle of the right XIIth cranial nerve, and a complete resection was carried out. A watertight dural closure was obtained, bolstered by fibrin glue (Tisseel) and local muscle; lumbar drain placement was not necessary here. The original bone-flap was restored.

Technical nuances and potential surgical pitfalls: Meticulous patient positioning and padding for this approach cannot be overstated. The potential for disorientation from the midline can be minimized using intraoperative MRI image-guidance, and following the usual anatomical midline landmarks which should be visualized intraoperatively - i.e., inion, midline external occipital keel and the C1-posterior arch's midline tubercle (all of which may still appear "shifted" or rotated in the modified park bench position). Particular caution must also be paid to the vertebral artery during the exposure - the surrounding venous plexus and yellow-brown fat are good clues to its proximity. CSF leak is a potential complication of posterior fossa surgery, and its incidence can be reduced by measures mentioned above. Lower brainstem cranial nerve monitoring as indicated above is advised for tumors in this location. Motor-evoked potential monitoring is recommended for brainstem surgery, but requires excellent neuroanesthesia experience and coordination.

Imaging:

Image 17.1 (above). Foramen magnum Schwannoma. Preoperative MRI sagittal and axial T1 post-contrast sequence. Lines point to circular tumor at the craniocervical junction, ventrolateral to the medulla on the right side.

Image 17.2 (above). Foramen magnum Schwannoma. Postoperative MRI sagittal and axial T1 post-contrast sequence. Arrow heads point to resection bed of previously present tumor.

Brain Tumor section; Case 18 - Jugular foramen Schwannoma

Background: The jugular foramen is a part of the occipital bone at the base of the skull. It lies superior and lateral to the hypoglossal canal (this canal is lateral to the occipital condyle of the foramen magnum region). The jugular foramen lies inferior and adjacent to the petrous temporal bone's internal auditory meatus and cochlear canaliculus. The jugular foramen has three compartments: The anterior compartment contains the IXth cranial nerve and the inferior petrosal (venous) sinus; the middle compartment contains the Xth and XIth cranial nerves; the posterior compartment is the junction of the sigmoid sinus and jugular bulb. Schwannomas in this region are known as jugular foramen Schwannomas, and presumably arise from Schwann cells associated with the lower cranial nerves. Tumors arising from the jugular foramen (such as Schwannomas, paragangliomas and meningiomas) can cause a variety of "jugular foramen syndromes". In essence, these syndromes involve dysfunction of the lower cranial nerves (IX-XII). For example, impaired taste, throat numbness, vocal cord and palate paralysis, tongue weakness, and neck muscle weakness are some of the symptoms and signs that patients with jugular foramen tumors present with. The imaging modality of choice is an MRI brain with and without contrast. On an MRI, the jugular foramen can be found by following the transverse and sigmoid sinuses inferiorly as they sweep towards the jugular bulb (which itself lies immediately inferior to the jugular foramen). Depending on the type of tumor suspected and its extent, a CT scan (including bone windows) and an angiogram (either a "conventional catheter angiogram" or a "CT angiogram") may be included in the diagnostic workup. The treatment of Schwannomas in this region is generally surgical, with stereotactic radiosurgery (SRS) reserved for postoperative remnants or recurrences.

Clinical presentation: A 40 year old male presented with voice (phonation) difficulty, impaired swallowing a tongue weakness. Physical examination revealed unilateral tongue atrophy and deviation, right palatal numbness and evidence for vocal cord paresis. Sternocleidomastoid and trapezius functions were preserved. There was mild left sided hyperreflexia, but no overt spasticity or weakness.

Diagnostic workup: A brain MRI with and without contrast was ordered. This revealed a mass in the right jugular foramen, that enhanced somewhat non-uniformly. There was mild mass effect on the lower right pons but no obstructive hydrocephalus. The mass appeared to occupy the right lower cerebellopotine angle (CPA), with passage through the jugular foramen into the region of the right jugular bulb. A diagnostic limited cerebral angiogram was carried out to rule out hypervascularity consistent with a jugular paraganglioma (glomus jugulare tumor). The lesion was minimally vascular. Together, the imaging findings were consistent with a jugular Schwannoma.

Treatment paradigm: Surgery was offered over SRS as a first-line treatment owing the size of the lesion, its accessibility and the age of the patient. It was discussed that the likely residual tumor confined to the jugular bulb region would be irradiated with SRS postoperatively.

Operative procedure/operative approach: A right retrosigmoid craniotomy was carried out with intraoperative stereotactic MRI guidance. The craniotomy was extended towards the foramen magnum. A lumbar drain was placed immediately preoperatively. Somatosensory evoked potential (SSEP) monitoring was used, as was monitoring of cranial nerves VII, VIII (BAER), and IX-XII. An ultrasonic aspirator was present, as was an Ojemann-type nerve stimulator. The tumor was near-totally resected, the exception being the tongue of tumor that lay in the jugular bulb region. Following this, the dura was closed in a watertight manner; Duraseal was used to augment the dural closure. The original bone flap was restored, approximated with additional titanium plates.

Imaging:

Image 18.1 (above). Jugular foramen Schwannoma. Preoperative MRI coronal and axial T1 post-contrast sequence. Red arrow heads point to contrast enhancing tumor mass. Yellow arrow heads point to mass effect on right side of pons.

Image 18.2 (above). Jugular foramen Schwannoma. Postoperative MRI coronal and axial T1 post-contrast sequence. Red arrow heads point to resection bed of previously present tumor.

Brain Tumor section; Case 19 - Vestibular Schwannoma (acoustic neuroma)

Background: Vestibular Schwannomas are Schwannomas that arise along the VIIIth cranial nerve, known as the vestibulocochlear nerve. Please note that these tumors are not neuromas, and the vast majority do not arise from the acoustic (or cochlear) division of the VIIIth cranial nerve, therefore calling them acoustic neuromas is, in fact, technically incorrect. Despite this, the term acoustic neuroma persists. Here they will be referred to as vestibular schwannomas (VS). Most VS arise from the sheath surrounding the superior or inferior vestibular nerve. A handful of case reports exist about (very rare) VS that arise from the sheath surrounding the cochlear nerve. Regardless of their nerve division of origin, VS arise from the ORZ region of the nerve division (see above). These tumors are regarded as benign, and most occur on one side of the patient. In patients in whom VS are found on both sides, a diagnosis of neurofibromatosis (NF) type II (NF2) is typically made. Genetic aberrations predisposing to Schwannomas and NF are described elsewhere. Evidence for an association between heavy cell phone use and vestibular schwannoma formation is given elsewhere. These tumors usually present with impairment of hearing and balance with or without ringing in the ear (tinnitus). Less frequently, facial weakness and numbness can occur, as can headache with nausea and vomiting from hydrocephalus (raised intracranial pressure associated with large VS that obstruct the flow of cerebrospinal fluid). They grow at a rate of 1-10mm per year. Clinical investigations in patients with VS include formal hearing assessment (audiometry) and brain MRI with and without contrast. Treatment options are surgery and stereotactic radiosurgery (SRS). The treatment of choice is surgery but, in essence, the choice of treatment/approach varies according to many factors including tumor size and "extent", patient age and preference, risk to hearing and facial function, and so forth. There are three surgical approaches, the most common of which is "retrosigmoid" (the other approaches are "translabyrinthine" and "extradural subtemporal/middle fossa"). Each of these approaches has specific advantanges and risks/disadvantages which should be discussed with your treating physician.

Clinical presentation: A 42 year old female presented with progressive right-sided hearing impairment. Physical examination was unremarkable except for significant right-sided hearing loss.

Diagnostic workup: An MRI of the brain with and without contrast revealed a contrast-enhancing mass in the right internal auditory canal (IAC) with extension laterally into the inner ear structures (labyrinth) and medially into the cerebellopontine angle (CPA). The mouth of the IAC at the porous acousticus was expanded. There was no obvious dural tail to the lesion. Findings were consistent with a vestibular Schwannoma. A preoperative audiogram showed marked hearing loss (nonserviceable hearing) on the right side.

Treatment paradigm: Owing to the size and extent of the tumor, the age of the patient, and the lack of meaningful hearing on the right side, surgery was offered over stereotactic radiosurgery. Of the multiple surgical approaches to the region, a combined presigmoid (translabyrinthine approach) and retrosigmoid craniotomy were offered owing to:

the need for facial nerve preservation;
the clinical presentation with near-complete loss of hearing on the right side; and
to the wide (mediolateral) extent of the tumor from the CPA to the labyrinth itself.

Operative procedure/operative approach: A combined right presigmoid (translabyrinthine approach) and retrosigmoid craniotomy were carried out by a neurosurgeon operating with an neurotologist. The head was turned 90 degrees contralaterally with appropriate body padding. Intraoperative stereotactic MRI guidance was used. A lumbar drain was placed immediately preoperatively. Somatosensory evoked potential (SSEP) monitoring was used, as was monitoring of cranial nerves VII, IX and X. An Ojemann-type nerve stimulator was used. The sigmoid sinus was skeletonized and bony labyrinth drilled; the dura was opened anterior and posterior to this sinus. The tumor was gross-totally resected. Following this, the dura was closed in a watertight manner; Duraseal was used to augment the dural closure. The original bone flap was restored, approximated with additional titanium plates.

Technical nuances and potential surgical pitfalls: Identifying the peritumoral arachnoid (maintaining it on the brainstem and neurovascular structures rather than on the tumor itself) and meticulous microdissection and "freeing-up" of surrounding neurovascular structures are key. Intraoperative CSF egress via a lumbar drain is helpful in gaining posterior fossa relaxation early in the procedure. Nerve stimulation is useful in planning the entry site into the tumor. Debulking the central part ("coring out") of the tumor first allows for more room to be made in the posterior fossa, and facilitates better identification and handling of its capsule against the surrounding neurovascular structures. Traction, ultrasonic aspiration and bipolar cautery all carry potential for additional morbidity in these cases. At the porous acousticus/IAC, stuctures to be carefully identified and preserved are the facial nerve and the internal arcuate/arcuate/subarcuate vessels. Watertight dural closure, and postoperative transient lumbar drainage are important.

Imaging:

Image 19.1 (above). Vestibular Schwannoma (acoustic neuroma). Preoperative MRI axial T1 postcontrast sequence. Tumor encircled. Yellow arrow heads point to the distal end of the tumor in the inner ear structures (vestibule and cochlea). Red arrow head points to proximal end of tumor at the root entry zone of the VIII nerve complex on the right side of the pons. Here, tumor mushrooms into the cerebellopontine angle (CPA) giving a classic "ice-cream cone" appearance. White lines mark the portion of tumor located in the internal auditory canal (IAC), whose medial entrance is the internal auditory meatus/porous acousticus. Note the normal structures (VII/VIII complex and inner ear components) on the patient's left side (right side of the above image).