VESTIBULAR SCHWANNOMA

- DR. PRANEETH

CONTENTS

ANATOMY OF CPA & IAC

EPIDEMIOLOGY

CLASSIFICATION

PATHOLOGY

NATURAL HISTORY

CLINICAL FEATURES

SPECIAL MANIFESTATIONS

DIAGNOSIS

MANAGEMENT

ANATOMY OF CEREBELLOPONTINE ANGLE• Triangular shaped

• Filled with CSF

• Sup – tentorium

• Inf – cerebellar tonsil & medullary olives

• Ant – posterior dural surface of petrous bone & clivus

• Post – ventral surface of pons & cerebellum

• Med – cisterns of pons & medulla

• Apex – lateral recess of 4th ventricle

• Foramen of luschka – lateral opening of 4th ventricle opens into CPA

ANATOMY OF CEREBELLOPONTINE ANGLE

• CN V-XI traverse the cephalic and caudal extent of CPA.

• Central structures passing to and from the IAC are VII & VIII CN respectively.

• CN VII & VIII are covered with central myelin provided by neuroglial cells as they cross the CPA and carry a sleeve of posterior fossa dura into the IAC.

• Transition to peripheral myelin made by schwann cells occurs at the medial opening of the IAC.

• AICA is the main artery in the CPA and is the source of labyrinthine artery.

• Labyrinthine courses via IAC & is an end artery for hearing and balance organs.

INTERNAL ACOUSTIC MEATUS

• VIII CN divides into 3 nerves – cochlear, sup & inf vestibular nerves in the lateral extent of CPA or medial part of IAC.

• IAC is divided into 4 quadrants by a vertical crest called Bill’s bar , and a transverse crest.

• VII CN – anterosuperior

• SVN – posterosuperior

• IVN – posteroinferior

• Cochlear – antero inferior

EPIDEMIOLOGY

• Most common tumour involving the CPA.

• 80% of CPA tumours.

• 8% of all intracranial tumours.

• Incidence - 10 in 1 million individuals per year

• Age of presentation – 40-60 years

• 95% occur in sporadic fashion.

• Remaining 5% have NF2 or Familial VSs.

• Age of presentation in nonsporadic forms is 2nd-3rd decade.

CLASSIFICATION

• At the Consensus Meeting on Reporting Systems on Vestibular Schwannoma held at Keio University, Tokyo, in November 2001, the following classification scheme was proposed and recom mended for adoption.

• It was based on the size of the largest extrameatal diameter and it was further recommended that a note should be made if the fundus was empty or filled by tumour and whether the VS was cystic.

PATHOGENESIS

• VS originate from the schwann cells of the SVN or IVN at the transition zone of the peripheral and central myelin (obsteiner-Redlich zone)

• The transition zone is present in the lateral part of CPA or medial part of IAC.

• VSs most commonly arise in the IAC than CPA.

• The reason that VSs mostly arise from vestibular nerves is due to highest concentration of schwann cells in the vestibular ganglia in the IAC.

• VSs occur due to mutations in gene for the tumor suppressor protein merlin, located on chromosome 22q12.

• In NF2, patients inherit 1 mutated allele and 1 normal allele (in contrast to sporadic cases where both are inherited normal and requires mutation in both copies to get VS).

PATHOLOGY• Smooth surface, yellow to gray colour.

• Usually solid.

• Occasional cystic components.

• Firm to soft in consistency depending on components.

• Surface connective tissue capsule is 3-5 um in thickness.

• 2 regions are noted intermixed – Antoni A & Antoni B.

• Antoni A – densely packed cells with spindle shaped nuclei and fibrillary cytoplasm. The palisades of the nuclei are termed Verocay bodies.

• Antoni B - hypocellular areas containing vacuolated, pleomorphic cells.

• Prominent thick walled vessels are also seen. Angioneogenesis is visible at surgery, with new small vessels running from the porus to the extrameatal portion of the tumour.

• VS sections stain with S-100 immunoperoxidase.

NATURAL HISTORY

• Slow rate of growth in IAC and then into the cistern of the CPA.

• Periods of growth intermixed with periods of quiescence.

• Average growth rate is 1.8 mm/yr.

• This slow growth is the reason for insidious and progressive symptoms and signs as there is displacement, distortion and compression of structures first in the IAC and then in the CPA.

• Occasionally rapid expansion may occur due to cystic degeneration or haemorrhage into the tumor, causing rapid neurologic deterioration.

• Initial growth affects vestibulocochlear nerve in IAC and causes U/L hearing loss, tinnitus & vertigo. (typical features of any lesion in CPA).

• Motor fibres of facial nerve are resistant to injury during

intracanalicular phase and so patients have normal facial function.

• Then the tumor grows into CPA freely without any new symptoms because structures in CPA are initially displaced without injury.

• As the tumor approaches 3cm, it abuts on CPA boundaries and causes symptoms & signs.

• Compression of CN V cause corneal and midface numbness and pain.

• Further distortion of CN VIII and now CN VII causes further hearing loss and disequilibrium, as well as facial weakness or spasms.

• Brainstem distortion leads to narrowing of the 4th ventricle.

Further growth leads to CPA syndrome –

• U/L hearing loss

• Tinnitus

• Vertigo

• Hypoesthesia & Neuralgia

• Nystagmus

• Facial palsy

• Vocal cord palsy

• Dysphagia

• Diplopia

• Respiratory compromise

• Death

• Patient develops cerebellar signs due to compression of flocculus and cerebellar peduncle.

• Also obstructive hydrocephalus may develop due to closure of 4th ventricle.

• Increasing ICP manifests in ocular changes, headache, mental status changes, nausea and vomiting.

• If VS continues to grow, then death occurs due to respiratory compromise.

SYMPTOMS AND SIGNS

• Auditory

Hearing loss in 95% - slowly progressive with noise distortion. (20% will have sudden hearing loss). The improvement of hearing loss with or without treatment does not rule out retrocochlear disease. Level of hearing loss is not clearly related to its size.

Tinnitus in 65% - constant, high-buzzing pitch. Not reported by many due to focus on hearing loss.

• Vestibular

Self limiting episodes of vertigo in 60% - patients tolerate and adapt well to the disequilibrium because of the central compensation for slowly evolving vestibular injury.

• Trigeminal

Midfacial numbness (V2)

Absent corneal reflex

Motor supply is rarely affected

• Facial

17% of patients

Sensory 1st affected – numbness in posterior wall of external canal wall called hitzelberger sign.

• CN II, IV & VI

Decreased visual acuity, Diplopia

• Hydrocephalus

Headache, nausea, vomiting, altered mental status

• CN IX & X

Dysphagia, aspiration, hoarseness,

Poor gag reflex, VC paralysis

SPECIAL MANIFESTATIONS OF VS

• An extraordinary large internal acoustic meatus with extensive bone resorption is found in up to 11 percent of small- and medium-sized tumours with an extrameatal diameter of 1-25 mm.

• The extra meatal part of the large tumours can vary significantly in shape. Most often they are round with equal perpendi cular diameters, but sometimes the diameter along the axis of the meatus is longer, extending along the basis of the pyramid.

• Spontaneous haemorrhage and calcification within the tumour is seen occasionally.

MEDIAL VESTIBULAR SCHWANNOMA• The medial VS is defined as a tumour

without lateral extension into the internal acoustic meatus.

• The internal auditory canal is filled with yellow cerebrospinal fluid which has a high protein content. Typically, all four nerves are clearly visible in the empty fundus when the internal acoustic meatus is opened during translabyrinthine or middle fossa surgery.

• Tumour is located mainly extrameatally and the internal acoustic meatus is not widened in the majority cases.

• The smallest medial tumour had an extrameatal diameter of 15 mm and the size of medial tumours was significantly larger, and the involvement of the cerebellum and the brain stem significantly higher.

• Most of the medial tumours with normal internal acoustic meatuses were giant and large tumours with cerebellar and trigeminal symptoms and relatively better hearing.

CYSTIC VESTIBULAR SCHWANNOMA• Cyst formation within VS are seen regularly

and are easily detected by MR. This has been thought to represent degenerative change or coalescence of micro cysts in Antoni A tissue.

• More recently, it has been shown that cystic tumours contain an increased amount of Antoni B tissue which is surrounded by a membrane-like structure composed of Antoni A type cells.

• Therefore, three criteria are required to be present before a tumour can be termed 'cystic'. First, there must be a hypo dense/hypo intense area on CT /MR. Second, peroperative identification of the cystic elements must be achieved and, third, there must be histological verification of S-100 positive membrane.

• Surgical outcome of cystic VS is less favourable than that of solid tumours of comparable size.

DIAGNOSTIC EVALUATION

AUDIOLOGY –

• Pure tone audiometry

• Speech audiometry

• Threshold tone decay test

• SISI test

Stapedial reflex decay test

VESTIBULAR TESTING –

• ENG with CALORIC TESTING

Neurological testing –

• Complete examination of cranial nerves,

• cerebellar functions,

• brainstem signs of pyramidal and sensory tracts

AUDITORY BRAINSTEM RESPONSE

IMAGING –

• CT

• MRI

AUDIOLOGY

• PTA – asymmetric, down-sloping, high-frequency, SNHL in 70%.

• SDSs are lower than predicted by pure-tone thresholds, which are further accentuated when retested at higher speech intensity. This phenomenon is called roll over. Seen in nearly 50%.

• Loss of acoustic reflex decay is

found in many cases.• Short Increment Sensitivity Index

(SISI) test will show a score of 0–20% in 70–90% of cases. Patients with cochlear lesions distinguish smaller changes in intensity of pure tone better than normal persons and those with conductive or retrocochlear pathology. SISI test is thus used to differentiate a cochlear from a retrocochlear lesion.

• Recruitment phenomenon is absent. Recruitment is typically seen in lesions of the cochlea (e.g. Ménière’s disease and presbycusis) and thus helps to differentiate a cochlear from a retrocochlear sensorineural hearing loss.

VESTIBULAR TESTING

• Most commonly done is ENG with Caloric testing.

• ENG shows reduced caloric response in the affected ear.

• Extent of vestibular function present predicts amount of post-op vertigo.

• Location of VS on sup or inf vestibular nerve can also be predicted by ENG as LSCC innervated by sup vestibular nerve is primarily evaluated by ENG.

AUDITORY BRAINSTEM RESPONSE

• Measured electrical response of cochlea and its brainstem pathway to short-duration broad-band clicks. The evoked response is a characteristic waveform with 5 identifiable peaks(I-V)

• In these patients, ABR is partially or completely absent, or there is a delay in latency in wave V on affected side.

• An interaural delay of wave V > 0.2ms is abnormal.

• Sensitivity > 90%

• Specificity – 90%

IMAGING

• MRI with Gd contrast – gold std

• A series of T1W images, T1W with Gd, T2W images are obtained. (T1W – CSF dark & fat bright, T2W – CSF bright).

• A hypointense globular mass in T1W with Gd images is characteristic.

• High resolution fast spin echo T2W scans are being developed in which CSF is used as a contrast.

MANAGEMENT

• Surgical removal

• Observation & radiotherapy – who cant tolerate surgery or have a life span < 5 yrs.

• Surgical approaches – • Translabyrinthine,• Retrosigmoid,• Middle fossa craniotomyThe appropriate approach is based on the hearing status, size of tumor, extent of IAC involvement and experience of the surgeon.Retrosigmoid and Middle fossa approaches are hearing preserving. Middle fossa approach is best for patients with good hearing and tumor < 2cm.Retrosigmoid is best for good hearers & tumor < 4cm & not involving lateral part of IAC (as it is accessible by removing PSCC which leads to hearing loss).

• Translabyrinthine approach causes total hearing loss and is so

appropriate for patients with poor hearing (pure tone average > 30dB) or good hearers but tumor not accessible by other approaches.

• 3 critical issues inherent to all 3 techniques – • Extent of exposure of IAC & CPA• Identification & preservation of facial nerve• Extent of brain retraction

Translabyrinthine approach• Boundaries of the approach –

1. Ant – facial nerve & cochlear duct2. Sup – middle fossa dura3. Post – posterior fossa dura4. Inf – jugular foramen

• Position - supine position with the head turned 30° away from the surgeon and supported on a soft head ring.

• Incision – postauricular

• A complete canal up

mastoidectomy is done, with identification of incus, tegmen, sigmoid sinus and facial nerve.

• A complete labyrinthectomy is then performed with medial skeletonization of middle & posterior fossa dura and decompression of the sigmoid sinus to the jugular foramen.

• After bony skeletonisation of IAC, dura

of IAC is opened and facial nerve is identified medial to the vertical crest (bills bar) in the fundus or lateral aspect of IAC.

• Tumor removal is done from lateral to medial along the IAC.

• In large tumors, it is debulked internally and then capsule is removed from surrounding structures including facial nerve.

• Abdominal fat is placed at the defect site.

Tumor removal is done from lateral to medial along the IAC.

• 3 advantages of this approach –

• Tumors of all sizes can be removed• Minimal retraction of brain• Ability to visualise directly and preserve the facial nerve.

• Rate of facial nerve preservation is 97%

• Rate of CSF leak presenting under the incision or draining through the nose via the Eustachian tube is 5%.

• Majority of these leaks resolve with conservative management – mastoid dressing & fluid restriction.

• Minimal risk of meningitis associated with CSF leak.

Retrosigmoid approach• Modification of traditional suboccipital approach used by

neurosurgeons to address most posterior fossa lesions.

• It’s a versatile approach with a panoramic view of CPA from foramen magnum inferiorly to tentorium superiorly.

• Medial 2/3rd of IAC is also accessible without violating inner ear; therefore, hearing is preserved.

• Position - The patient is supine with the head on a head ring or in a neurosurgical clamp. The essential point is that the intermeatal line should be perpendicular to the floor. This ensures that the internal meatus is in the same plane as the external meatus and assists the surgeon's orientation in identifying the internal meatus.

• Incision – A 6-7 cm vertical or gently backward curving incision starts at the level of the zygomatic arch just in front of the pinna. The temporalis muscle is exposed and an inverted T -shaped incision is made though the muscle down to the skull.

• 5 x 5 cm craniotomy is performed with sigmoid sinus as anterior

boundary & transverse sinus the superior boundary.

• Bone is removed by drilling.

• Bone fragments are collected & will be replaced during closure.

• Bone fragments will reform a bone plate and prevent adherence of musculature to the dura.

• If decompression of sigmoid sinus is needed for exposure, a mastoidectomy may also be performed.

• Dura is then opened along the sigmoid sinus, and cerebellum is seen.

• CSF from cisterna magnum should be released prior to retracting cerebellum.

• Medial retraction of cerebellum allows visualisation of CPA.

• To see the IAC component of tumor, posterior bone of IAC should be removed.

• Bone dust created is carefully confined and removed to prevent meningeal irritation.

• Extent of IAC skeletonisation is limited by proximity to inner ear.

• Endolymphatic duct & sac serve as landmarks to the proximity of the PSCC and allow preservation of the inner ear and hearing.

• Facial nerve is normally anterior to the tumor, or its position is ascertained with facial nerve monitoring.

• Tumor removal done from lateral to medial along IAC.

• If large tumor, then debulking done internally and capsule removed

from surrounding structures including facial nerve.

• Air cells along IAC & mastoid closed with bone wax or bone cement to eliminate paths for CSF leak.

• A fat or muscle graft may also be placed into the petrosal defect to prevent CSF leak.

• Dura mater is closed and bone plate is replaced.

• Musculature & soft tissue are carefully closed.

• Advantages relative to translabyrinthine – 1. Hearing preservation2. Versatile approach to CPA & IAC

• Disadvantages compared with translabyrinthine –1. Persistent post-op headache2. Increased difficulty in resolving CSF leaks3. Need for cerebellar retraction4. Inability to have direct access to facial nerve

• Severe post-op headache is due to intradural drilling leading to meningeal irritation by bone dust & dissection of suboccipital musculature.

• In case of extensive pneumatisation of IAC & mastoid, air cells may be difficult to seal completely, and the inability to address the aditus ad antrum or ET causes CSF leaks to be persistent despite conservative treatment.

• Amount of cerebellar retraction increases with size of tumor.

• Surgical control of facial nerve is adequate in retrosigmoid approach but exposure is less compared to translabyrinthine.

Middle fossa approach• A hearing preserving approach to intracanalicular

tumors < 1 cm cisternal component.

• Incision – inverted U shaped centered over the ear.

• Temporal muscle reflected inferiorly to expose squamous portion of temporal bone.

• 5 x 5 cm temporal craniotomy is performed and is centred over zygomatic root.

• Extradural elevation of temporal lobe is done to reveal temporal bone.

• GSPN leading to geniculate ganglion reveals anterior, lateral boundary of IAC, and the arcuate eminence reveals posterior boundary of IAC

• IAC dura have to be identified medially by drilling towards porus acousticus.

• Bone removal is continued laterally till basal turn of cochlea anteriorly and SSCC posteriorly.

• IAC dura opened posteriorly to avoid injury to facial nerve.

• Tumor is dissected free of facial nerve in medial to lateral direction.

• Any air cells are sealed, and the dural defect is covered with a fat or muscle plug.

• Craniotomy bone flap is replaced, and incision closed.

• This approach is unique compared to others because 1. Entire IAC is accessible without violating inner ear, allowing

intracanalicular tumors removal while preserving hearing.

• Limitations include – 1. Tumors > 1cm cisternal component

• Disadvantages – 1. Temporal lobe retraction causing transient speech & memory

disturbances and auditory hallucinations.2. Poor surgical position of facial nerve relative to the tumor. If tumor

originates from IVN, then facial nerve will be between surgeon & tumor.

Intra operative Complications

• Vascular injury

• Air embolism (if occurs – patient positioned in left lateral & Trendelenburg to trap the air in right ventricle and then aspirated via a central venous catheter)

• Parenchymal brain injury

• Cranial nerve injury

• Only venous drainage of temporal lobe – vein of labbe & lower portion of cerebellum are vulnerable.

• Retraction injury to cerebellum during retrosigmoid & to temporal lobe during middle fossa approach can occur.

Post operative complications

• Hemorrhage, Stroke, VTE, SIADH, CSF leak, Meningitis.

• Post op haemorrhage manifest as neurologic & cardiovascular deterioration and will require evacuation.

• Post-op LMW heparin with compression stockings & intermittent pneumatic compression devices will reduce risk of TE without increasing risk of intracranial bleed.

• Most common complication is CSF leak (10-15%) , may occur via the wound or via a pneumatic pathway to ET. These leaks resolve with conservative care – placing sutures at leak site, replacing the mastoid dressing, decreasing ICP with acetazolamide, restricting fluid intake and resting in bed.

• Meningitis (2-10%) – aseptic / bacterial / lipoid due to irritation from fat graft.

Prognosis & rehabilitation• Most concerns are about deafness, imbalance, facial nerve weakness.

• Hearing preservation ranges from 20-70%.

• Patients with good C/L hearing tolerate U/L loss or may be helped with a CROS hearing aid, and patients with poor C/L hearing may be rehabilitated with a cochlear implant if cochlear nerve fibres preserved.

• Almost 50% will have vertigo. Those who have vertigo beyond post-op period should have vestibular rehabilitation therapy.

• Rehabilitation of facial nerve injury is based on general principles of nerve injury, recovery and rehabilitation. If nerve transected intra-op, it should be repaired primarily if possible or with a greater auricular interposition graft.

• Lack of facial function (grade 6) at 1 year and no reinnervation potentials on EMG should lead to hypoglossal-facial transposition, interposition nerve graft or a cross-facial graft.

• If there is electrical silence of facial muscle on EMG, muscle transpositions with temporalis or masseter to the lip give improved tone & symmetry to the lower face.

• Upper face can be rehabilitated with a brow lift & gold weight for the eyelid.

SUMMARY –

• All 3 approaches have mortalirty < 1% , >90% rate of tumor removal & facial nerve preservation.

• Translabyrinthine – 98% facial nerve preservation

• Hearing preservation – 50% in retrosigmoid & 70% in middle fossa approach.

• Recurrence rate < 1.5%

Stereotactic radiation

• Goal – prevent further growth of VS while preserving hearing & facial nerve function.

• Mechanism relies on delivering radiation to a specific intracranial target by using several precisely collimated beams of ionising radiation.

• Beams take various pathways to the target tissue, creating a sharp dose gradient between the target tissue & surrounding tissue.

• Ionising radiation causes necrosis & vascular fibrosis.

• Time course of effect is over 1-2 years.

• Ionising radiation is most commonly delivered by 201-source cobalt 60 gamma-knife system.

• Standard linear accelerator can also be used.

• Success depends on dose of radiation deliuvered.

• Rate of cranial neuropathies & hearoing loss is decreased by lowering the radiation dose.

• Hearing preservation rate decreases each year after radiation & stabilises after 3 years at 50%.

• Facial nerve dysfunction varies from 3-50% based on radiation dose at margin of tumor & length of facial nerve in radiation field.

• 20% will have trigeminal neuropathy.

• Hydrocephalus is also a complication of radiation.

• Radiation therapy is useful in patients in whom arrest of tumor growth is acceptable.

• These patients have either short life expectancies or high surgical risk.

• Stereotactic radiotherapy may improve hearing preservation in patients with 2-3 cm VSs compared to microsurgery.

• Radiation therapy in large (>3cm) tumors or those causing brain compression will exacerbate symptoms owing to initial tumor swelling.

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