Intracranial Schwannomas:   

 

Dr. A. Vincent Thamburaj,   

Neurosurgeon, Apollo Hospitals,  Chennai , India.

The solitary benign schwannoma is the most common tumor of the cranial nerves. They are typically found in adults; when present in childhood they are usually associated with NF2. Schwannomas are typically well circumscribed with a thick, collagenous capsule. Cystic degeneration and lipid accumulation are common.

 

Microscopically,   these tumors exhibit areas of compact elongated bipolar cells arranged in fascicles (Antoni A pattern) that occasionally form palisades (Verocay bodies) and areas of loose, spongy tissue with a degenerative appearance (Antoni B pattern). These hypocellular areas usually have an abundant number of xanthoma cells. Other degenera­tive features, such as thick and hyalinized blood vessels and hemosiderin-laden macrophages, are common. Schwannomas are uniformly and strongly reactive for S-100 protein,

Leu-7, and vimentin.

Antoni A pattern

Antoni B pattern

 

Intracranial malignant peripheral nerve sheath tumors and granular cell tumors have been reported but are very rare.

 

 Acoustic Schwannomas  (neurilemmoma or neurinoma):

 

Surgical anatomy of vestibulocochlear nerve: The 7th and the 8th nerves, with the intervening nervus intermedius, leave the junction of pons and medulla and pass laterally and somewhat upwards to enter the internal acoustic meatus. The labyrinthine artery from the basilar or the anterior inferior cerebellar artery, lies with them. The internal acoustic meatus is a foramen directed laterally in the posterior surface of the obliquely set petrous bone. Its fundus consists of a plate of bone, the lamina cribrosa, divided by a horizontal crest into an upper and lower semicircle. The fibrous layer of the dura matter is everywhere fused with the endosteum within the meatus. The facial nerve and nervus intermedius pierce the front of the upper part, the cochlear nerve the front of the lower part. The vestibular nerve pierces the plate posteriorly, by the upper and lower divisions that lie behind the facial nerve foramen, and the spiral cochlear foramina respectively.

 

Harvey Cushing in his monograph ‘Tumors of the Nervus Acusticus and Syndrome of the Cerebellopontine Angle’, credited Satifort with the earliest mention to this tumor in 1777. Curveilheir in several reports from 1835 to 1842 described  the clinical and pathological findings in detail.

 

Acoustic schwannomas account for 80% to 90% of tumors found in the CPA and are common tumors of the posterior fossa,second only to the gliomas. Overall, they account for 6% to 8% of all intracranial tumors. Typically benign, slowly growing tumors, they arise  most   commonly   from  the vestibular  nerve  (80%),  occasionally  from  the cochlear   (5to 7%).The inferior vestibular nerve is involved in 70%, superior  vestibular in 20% and cochlear nerve in 10%. Cystic changes are rare. The site of origin of the tumor is believed to be the point of transition between the central and peripheral myelin, where Schwann cells meet oligodendrocytes (Obesteiher-Redlich zone) inside the internal auditory canal (IAC) near Scarpa’s ganglion.

 

Acoustic schwannomas have epidemiological and biochemical similarities with meningiomas. They are more frequently found in females than males, presenting around the 4th-5th decades. They often exhibit defects in, or complete loss of one copy of chromosome 22 and express the C-six and fibroblast growth factor genes. 

 

Patients with bilateral acoustic schwannomas usually become symptomatic earlier than patients with single tumors.  In 5% of the cases the tumors are bilateral, and are associated with the so-called central neurofibromatosis or NF type2, an autosomal dominant disorder due to a genetic abnormality located in chromosome 22. The possibility of NF2 should be entertained in any patient with  a parent, sibling, or child with neurofibromatosis-2 and either unilateral eigth nerve mass or any two of the following neurofibroma, meningioma, glioma, schwannoma, juvenile posterior subcapsular lenticular opacity.

 

However, acoustic schwannomas may also develop in patients with neurofibromatosis type I, where the abnormality is located in chromosome 17.

 

Clinical presentation:

 

Most patients present with progressive unilateral hearing loss, usually over a period of months or years.  In fact, hearing deficit is characterized by mild to moderate pure-tone sensorineural loss, greater in the higher frequencies and a loss of speech discrimination out of proportion to the pure-tone impairment.

 

Further growth of the tumor may cause occlusion of the internal auditory artery, with permanent loss of functional hearing.  This can be associated with tinnitus and, as the tumor enlarges, with unsteadiness and loss of balance, without true rotatory vertigo.

 

As the tumor grows into the C-P angle, trigeminal nerve compression may occur with re­sulting facial hypesthesia or, more commonly, diminution of the corneal reflex. True trigeminal neuralgia is rare, but facial hyposthenia is usually present when the tumor is larger than 3 cm in diameter, and particularly when it grows upwards. 

 

Facial nerve function usually remains preserved until the tumor reaches a large size. Involvement of lower cranial nerves is rare.

 

Cerebellar signs and symptoms occur late. 

 

As a rule papilledema and hydrocephalus also occur only in late stages, but are not necessarily related of the size of the lesion.

 

Investigations:

 

Formal hearing evaluation will usually disclose a pattern consistent with sensorineural hearing loss. Speech discrimination is decreased, while the speech reception threshold is increased. Brain-stem auditory evoked response latencies are increased (especially wave V). 

MRI with gadolinium is the imaging of choice. The tumor is usually iso or hypodense relative to the brain. it enhances brightly with gadolinium and the intracanalicular portion is well visualized.On T2-weighted images, schwannomas tend to exhibit increased signal intensity.

 

High resolution, CT scanning is helpful prior to surgical management of these tumors. It allows assessment of mastoid aeration, posi­tion of the jugular bulb, and measurement of the distaqnce from the posterior lip of the internal auditory meatus to the posterior semicircular canal.

Left Acoustic neuroma-MRI

Koos et al have graded the tumor according to size:

 

Grade  I:  

 Intracanalicular and intracisternal tumor with a longitudinal diameter of 1 - 10  mm

Grade  II: 

Intracanalicular and intracisternal tumor with a longitudinal diameter of up to 20  mm

Grade  III:

Intrameatal and intracisternal  tumor with a longitudinal diameter of up to 30 mmn  (reaching the  brainstem).

Grade   IV: 

Intrameatal and  intracisternal tumor with a  longitudinal diameter of more than 30mm, indenting and displacing the brainstem.

Emann et al classified the tumors as:

a)  small tumors than extend less than two cms into the cerebellopontine cistern and generally cause only eighth  nerve  symptoms,  

b)  medium sized tumors which extend 2-3  cms into the cerebellopontine cistern, cause trigeminal symptoms and   reach  up  to  the  brainstem, and   

c)  large tumors which  extend for more than three cms into the cistern and produce signs of brainstem  compression and of raised intracranial pressure.

 

Management:

 

Options for management of a patient with a vestibular schwannoma include observation, microsurgical removal, and radiosurgery. The approach chosen must take into account the patient's age and medical condi­tion, quality of the patient's hearing, tumor size, experience of the surgeon, and, of course, the patient's wishes.

 

Not all patients with a vestibular schwannoma require treatment. Elderly patients with ipsilateral hearing loss  without evidence of

brainstem compression or hydroocephalus may simply be followed clinically with either CT or MRI performed at regular intervals.

 

Progression of symptoms and signs of tumor growth may require more aggressive treatment.

 

Microsurgical excision:

 

Cushing advocated an intracapsular subtotal removal, together with a bilateral suboccipital craniectomy for decompression.  The first successful total removal was reported by Dandy in 1917.  Later, he described a technique for unilateral suboccipital exposure, internal decompression, unroofing of the internal auditory meatus and total removal. Currently, the ideal treatment for symptomatic patients with vestibular schwannoma is complete microsurgical excision of the tumor. Concomitant goals include preservation of facial nerve function, low morbidity and mortality, and, when possible, preservation of hearing.

 

The choice surgical approach should take into account the size and the amount of extension of the tumor into the IAC, as well as the patient' hearing ability and the experience of the surgical team. Every attempt should be made to preserve useful hearing, although it must be realized that when the tumor is greater than 2 cm in size or when it fills the fundus of the IAC, this goal is far jess frequently realized. Each of these surgical approaches has inherent advan­tages and disadvantages.

 

The retrosigmoid, transmeatal approach is the most familiar route. Following a retro sigmoid craniotomy and exposure of the CPA, the posterior lip of the IAC is removed and the cranial nerves are identified.  Following removal of the intracanalicular component of the tumor, the tumor in the CPA is centrally debulked. The capsule is dissected from the brainstem, cranial nerves, and cerebellum. The facial and vestibulocochlear nerves can be identified at the lateral end of the pontomesencephalic sulcus, just rostral to the glossopharyngeal nerve and just anterosuperior to the foramen of Luschka, the flocculus, and the choroid plexus protruding from the lateral recess. Dissection is performed in a medial to lateral direction, to avoid traction on the cochlear nerve fibers.(click for intraoperative video)

 

A small group of surgeons recommend a middle fossa approach  to tumors of the IAC (intracanalicular) or those with no more than 1cm of CPA extension. Although this approach allows excellent facial nerve control; complete visualization of the IAC, allowing tumor removal under direct vision and, above all, resection of intracanalicular tumors with the possibility of hearing preservation. The technique involves a subtemporal extradural approach with identification and microsurgical unroofing of the IAC. The tumor is gently dissected from the facial and cochlear nerves and the edges of the IAC. The vestibular nerves are divided and the tumor removed.

 

When the tumor is larger and hearing is absent, the translabyrinthine approach (click for intraoperative video) can be used. It improves the anterolateral exposure of the tumor by removal of the posterolateral temporal bone, including the bony labyrinth. Also, by completely exposing the distal end of the IAC, the facial nerve is positively identified in its location anterior to the vertical crest. The tumor can then be dissected from the nerve under direct vision. The dural opening extends from the IAC to the presigmoid dura thus exposing the CPA. With central debulking of the tumor, the capsule folds in and it can be dissected from the surrounding cranial nerves, blood vessels, and brainstem. Wound closure is usually achieved by packing the dural defect and mastoidectomy defect with fat..  It is important that the eustachian tube be obliterated to avoid CSF leakage.

 

With any of the above described surgical approaches, the rate of anatomical preservation of the facial nerve is in excess of 90% to 95%. Functional facial preservation  for tumors less than 3 cm in size is also in excess of 75% to 80%, no matter which approach is utilized. Facial nerve monitoring has improved facial nerve outcomes. Hearing preservation, when attempted, has been possible in 35% to 80% of selected patients. Recurrence rates of 3% or less have been reported following complete tumor excision.

 

Radiosurgery:

 

Radiosurgery must be considered an option in the treat­ment of patients with vestibular schwannomas of 3 cm or less in size. In most instances, however, this form of treatment is reserved for patients who refuse microsurgery, the medically infirm, symptomatic patients of advanced age, patients in whom the tumor has recurred following microsurgical excision, and residual disease following subtotal surgical resection.

 

Reports suggest that hearing is preserved in 100% of patients when assessed immediately post-treatment and drops to about 45% at both 1 and 2 years following treatment. Tumor control rate is about 95% at 2 years follow up. Useful facial nerve function is 100% post treatment, dropping to about 95% at 2 year follow up. Persisting mild trigeminal nerve sensory symptoms are noted in about 25& at 2 years follow up.

 

A new onset of trigeminal and/or facial nerve palsy may occur, and worsening of a preexisting trigeminal and/or facial nerve paresis is noted in about 10%. and hydrocephalus requiring a shunt in about 5%.

The risk of complication was related to both dose and tumor diameter.

 

Treatment of bilateral acoustic schwannomas raises different problems because of the risk of bilateral impairment of both the seventh and eighth nerves in such cases. Preservation of these structures is often more difficult to achieve.  In general, the first side to be operated should be the one showing progressive hearing loss in the audiogram of an enlarging tumor on serial studies. Radiosurgery is now a reasonable alternative in these situations.

 

 

 Trigeminal Schwannomas:

Bil.Acoustic neuromas-MRI

 

Surgical anatomy of the trigeminal nerve:  The trigeminal root (consisting of a larger sensory and a smaller motor root) arises from the lateral side of the pons and courses obliquely through the anterior part of the cerebellopontine angle to its dural exit portal (the porus trigeminus) situated just inferior to the lateral tentorial attachment and the superior petrosal sinus. The root then becomes wholly extradural in position and ends in the trigeminal (Gasserian) ganglion. The ganglion is crescent-shaped and is located in a bony recess (Meckel’s cave) on the anterior surface of the petrous apex. The dural reflections at this region result in the ganglion being enveloped by a double layer of dura. The foramen lacerum separates the ganglion from the internal carotid artery and the eustachian tube.  The sensory root rotates anteriorly as it passes into the ganglion and the motor root is, therefore, related to the inferior surface of the ganglion as it passes forwards to be incorporated in the mandibular nerve. The greater superficial petrosal nerve passes anteriorly beneath the ganglion and is liable to be injured during surgery in this area. The dural reflection around the ganglion is continuous anteriorly with the lateral wall of the cavernous sinus. The ophthalmic and maxillary divisions pass from the ganglion, via the lateral wall of the cavernous sinus to their exit foramina, while the mandibular division containing the motor fibres exits near the posterior end of the cavernous sinus.

 

Schwannomas of the trigeminal nerve account for 0.07% to 0.36% of intracranial tumors and 0.8% to 8% of intra-cranial Schwann cell tumors. The first description of an intracranial trigeminal neuroma was by Dickson in 1846. 

 

Clinical presentation:

 

Jefferson classified these tumors in relation to their anatomical location:1) middle fossa tumors- type A, 2) predominantly posterior fossa tumors- type B, and 3) dumbbell tumors with major components in both the middle and the posterior fossae-type C.

 

Half are located primarily in the middle fossa arising from the ganglionic segment of the trigeminal nerve (Jefferson's type A tumors). Trigeminal sensory dysfunction is the presenting complaint in about 60 per cent of patients and is present in 80-90 per cent by the time they come to the hospital.  The initial symptoms are typically pain and/or paraesthesia in the distribution of one or more divisions of the trigeminal nerve.  Facial pain, as a symptom, is more common in patients with tumors involving the ganglion than those involving the root.  Pain is followed by sensory loss in the particular division.  Complete anesthesia is rare.  Weakness of the muscles of mastication may be seen in about 30-45 per cent of patients.  When there is total trigeminal anesthesia and severe trigeminal motor involvement including atrophy of the masticatory muscles, one must suspect a malignant tumor involving the ganglion rather than a benign neuroma.  Ten to seventy percent of patients may have no symptoms or signs of trigeminal nerve dysfunction.  This is explained by displacement rather than destruction of the trigeminal nerve fibres by the tumor. The third, fourth and sixth cranial nerves may be involved by the tumor.  Oculomotor nerve palsy may be seen in 15-30 per cent of patients with middle fossa tumors.  Rarely, if the tumor extends up to the superior orbital fissure, exophthalmos may occur.  Compression of the optic nerve at the optic foramen may cause visual loss.  Visual loss, together with erosion of the bone near the pituiary fossa, may stimulate a pituitary tumor. Seventh nerve dysfunction in middle cranial fossa tumors has been thought to be due to stretching of the greater superficial petrosal nerve, by traction on the geniculate ganglion or by erosion of the bone surrounding the facial nerve in the fallopian canal with subsequent nerve compression.  Hearing loss from middle fossa tumors is generally due to erosion of the anterior surface of the petrous bone with subsequent conductive deafness due to eustachian tube obstruction or a sensory neural deafness due to direct involvement of the cochlea.

 

Tumors of the trigeminal root account for 20-30% of trigeminal schwannomas and are usually confined to the posterior fossa (Jefferson's type B tumors). The clinical presentation is usually a combination of hearing loss, tinnitus, and facial nerve and cerebellar dysfunction. Up to 10% of patients with a vestibular schwannoma initially present with trigeminal nerve dysfunction. As a corollary, 6% of the patients with trigeminal schwannomas initially complain of hearing loss. Impairment of lower cranial nerve function and long tract signs are noted at diagnosis in 30% to 50% of patients.

 

Dumbbell shaped tumors that occupy both the middle and posterior fossae make up 15% to 25% of all trigeminal schwannomas (Jefferson's type C tumors). Their clinical presentation is a composite of the symptoms and signs of tumors occurring in the ganglionic segment and in the posteriorfossa, long tract signs may predominate due to cerebral peduncle compression at the tentorial hiatus.

 

Schwannomas may also arise from the branches of the trigeminal nerve.  It is occasionally difficult to differentiate these tumors from those originating from the ganglion and extending out along the branches.  These ‘branch’ neuromas may be located entirely in the wall of the cavernous sinus, or may involve the cavernous sinus.  They may otherwise arise more distally and extend extracranially, exhibiting signs of an extracranial mass.  They commonly arise from the first division of the trigeminal nerve and present with proptosis, ocular nerve involvement and occasionally, visual loss.  The exact site or division of origin of the tumor may not be clear  with the tumor extending into the pterygopalatine fossa, infratemporal fossa, sphenoid sinus or nasopharynx.  Rarely these tumors may arise in the extracranial portion of the branches of the trigeminal nerve.  As they grow, they may erode the skull base and then extend intracranially.

 

Investigations:

 

On T1 weighted MR images, the lesions appear hypointense with signal intensities equal to or slightly lower than that of brain cortex.  With T2 weighting, the large lesions have conspicuously high signal intensity.  MRI affords a better assessment of the displacement of the internal carotid and/or the basilar arteries.  The signs of a small tumor on T1 weighted images include fusiform or global enlargement of the trigeminal root, obliteration of CSF spaces within Meckel’s cave and widening of the foramen ovale and rotundum.

 

CT scan may delineate the bony changes better.

Rt.Trigeminal Schwannoma-

MRI coronal

Trigeminal Schwannoma thru' Left Foramen ovale-MRI axial

Rt.Trigeminal (Dumbbell) Schwannoma- MRI axial

 

Management:

 

The first attempted excision of a ganglionic trigeminal neuroma was, by Krogius, in 1895.  It was Frazier, in 1918, who successfully removed totally a ganglionic neuroma. These tumors, however, still pose major impediments to complete removal e.g., involvement of the cavernous sinus, adherence to major vascular structures and difficulty in achieving an adequate exposure.  Notwithstanding the above, favorable results have been reported. 

 

The potential for these tumors to extend into two cranial compartments presents difficulties regarding the choice of approach.  The decision regarding the approach is dependent purely on the anatomical location of the lesion. 

 

Tumors of the middle fossa can be excised via a subtemporal intradural approach.  The advantage of this approach is the excellent exposure of the floor of the middle cranial fossa and also easy access into the posterior fossa if required.  It must be kept in mind that in using this approach the surgeon has to incise the basal dura over the tumor to reach the capsule of the lesion and the correct plane for dissection.  Though it is possible to approach the lesion extradurally, the exposure provided is inadequate and only small lesions of the ganglion can be dealt with.  Access to the posterior fossa through the intradual subtemporal approach is achieved by incising the tentorium and ligating the superior petrosal sinus.  By doing this, a tumor which is anterior and superior in the cerebellopontine angle may be removed.  However, lesions which extend caudally and are inferior to the VII and VIII cranial nerves cannot be safely dealt with via this route. Though complete removal of the lesion is the goal, it may occasionally not be possible to remove part of the lesion adherent to the posterolateral wall of the cavernous sinus or where the carotid artery is adherent to  the lesion.  Especially at risk are the petrous, pre-cavernous and cavernous segments of the internal carotid artery, which may have no intervening bone between them and the tumor.  A subtotal intracapsular excision is an acceptable option in the elderly.

 

Tumors of the trigeminal root were generally approached through a retrosigmoid craniectomy, while a variety of approaches were necessary for the dumbbell-shaped, tumors. These approaches include combined approaches such as the petrosal, supra and infratentorial approach, or sub and infratemporal approach.

 

Morbidity consisted mainly of new or worsened trigeminal deficits in about 46% of patients. Other morbidity includes abducens nerve palsies, facial palsy, and hearing loss.

 

 Facial Nerve Schwannomas:

 

Facial nerve schwannoma was first reported in 1930.  These are most frequently found within the temporal bone or extracranially.  Isolated intracranial facial nerve schwannomas are very rare. When found in the internal auditory canal or intracranially, their origin is explained on the basis of embryonic rests of ganglionic cells from the geniculate ganglion. 

 

The tumors may present with 1) facial nerve paresis or palsy, 2) otologic symptoms and/or, 3) parotid mass. 

 

Facial paresis is most often seen with intratemporal lesions and is due to nerve compression within the bony canal.  Slowly progressive facial weakness is the typical clinical presentation of a facial nerve schwannoma. Sudden facial weakness, however, occurs in about 11% of cases, and a 27% of patients with facial nerve schwannomas never manifest facial weakness. Facial spasm has been reported in up to 17% of patients.

 

Otologic symptoms occur when the tumor presents intracranially.  Hearing loss of a conductive, sensorineural, or mixed- nature occurs in approximately 50% of patients. Tinnitus and vertigo, or dizziness, occur in 13% and 10%, respectively.

 

Extracranial tumors present as parotid masses which may be multiple or cystic. Only about 20 per cent of the tumor present with facial paralysis. 

 

Radiological diagnosis of these tumors is not simple and the main challenge is the differentiation of intracranial facial schwannomas from acoustic neuromas. When the CT or MRI show involvement of the geniculate ganglion, the diagnosis is apparent.  

 

Surgical excision is the treatment of choice. Most patient are left with a facial nerve palsy, as total tumor removal almost always requires resection of the facial nerve. Reconstruction of the facial nerve can be done variously with specific procedures depending upon individual preference.

 

 Other Schwannomas:

 

The clinical presentation and the surgical management of jugular foramen schwannoma are more a function of anatomical location than specific nerve origin (discussed elsewhere).

Schwannomas of the 3rd, 4th, 6th cranial nerves are extremely rare. The clinical presentation usually involves diplopia due to dysfunction of the nerve of origin. Other symptoms may include decreased visual acuity, hemiparesis, ataxia of gait, paresthesias, and symptoms of intracranial hypertension. Treatment is surgical resection, which may occasionally be associated with functionally normal eye movements.

Schwannomas, when they occur, are usually located entirely within the intracranial compartment and less commonly, have a dumbbell-shaped appearance with both intra and extracranial components. Purely extracranial tumors can also occur. Unilateral lingual atrophy, deviation, and fibrillation are nearly universal findings. Intracranial hypertension, long tract signs, ataxia, and dysfunction of the other lower cranial nerves may occur. Surgical removal is best accomplished via a transcondylar approach, either alone or in combination with an anterior transcervical ap­proach. Purely extracranial tumors can be removed via an anterior transcervical approach alone.

 

Orbital schwannoma with intracavernous extension-MRI

Hypoglosal schwannoma- MRI

Vagal schwannoma-MRI

 

Intraparenchyma schwannoma is a rare clinical entity and an histological curiosity. Radiologically, they can not be differentiated from other intraparenchymal tumors. They appear to be more common in the spinal cord. They have been attributed to disordered embryogenesis. It has also been suggested that the cell of origin is schwann cells from the telia choroidea. They may also probably arise from proliferating multipotential mesenchymal cells, from nerve crest cell rests or from hamartomatous lesions from pial cells. Treatment is like that of any other intraparenchymal tumors.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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