Cerebello-Pontine Angle Tumors

Shikher Shrestha

History..

Challenge despite benign – complexity of the anatomy

Sir Charles Balance (1894) – first successful complete removal(Right posterior fossa craniectomy and removal through finger insertion) – patient lived for 18 years post surgery

Krause – faradic stimulation to differentiate facial from audiovestibular nerve

Cushing – reduced morbidity and mortality by intracapsular removal; however with high recurrence rate

History

Dandy – introduced currently widely accepted concept

“benign tumors should be removed completely to prevent future recurrence even at the expense of a somewhat higher perioperative mortality rate”

Now – much lower morbidity and mortality

advent of newer imaging modality, intraoperative electrophysiological monitoring, more reliable and safe operative and anesthetic technique

CP angle Anatomy

CP angle: Triangular space bounded by –

pyramid anteriorly

tent superiorly

pons medially and

cerebellum dorsally

CP angle Anatomy

CP angle cistern is located between

anterolateral surface of pons &cerebellum and posterior surface of the petrous bone

Contains: CN V, VI, VII and VIIISCA and AICAvariable number of draining veinsflocculus of the cerebellumchoroid plexus that protrudes through the foramen of

Luschka

VII Nerve exits from the lateral part of the pontomedullary sulcus; 1-2 mm anterior to the entry zone of VIII nerve.

IX, X and XI CNs are located inferiorly

5 nerves pass through the internal auditory canal

FacialVestibular (superior and inferior)Cochlear andNervus intermedius (accompanied by labyrinthine artery and occasionally by branches of AICA or a loop of the AICA itself)

vii sv

coch iv

Ant

superior

Histopathology of Vestibular Schwannoma

Antoni A fibers – narrow elongated bipolar cells

Antoni B fibers – loose reticulated fibers

Verocay bodies – acellular areas surrounded by parallel arrangement of spindle shaped schwann cells

Tumors of the Cerebello Pontine Angle

5-10% of all intracranial tumors

VS – the commonest (80-95%)Followed by

Meningioma (3-10%)Epidermoids (2-4%)

Others: schwannomas of other CNs; paragangliomas (glomus jugulare/glomus tympanicum) , chordomas, chondrosarcomas, arachnoid or neuroenteric cysts, dermoid and metastasis.

Mnemonic:S – schwannomasA - aneurysm, arachnoid cystsM – meningiomas, metsE - epidermoids

Tumors of the Cerebello Pontine Angle

CP angle secondarily involved by:

tumors from brainstem or fourth ventricle: gliomas, ependymomas, choroid plexus papillomas, medulloblastomas or lymphomas

Bilateral CP angle tumors: characteristics of NF2: typical histology: VS

Imaging

Things to consider:

site of origin

location

size, shape and margins

density and signal intensity

contrast enhancement characteristics

Imaging

Non enhancing extra axial CP angle lesions:

CysticEpidermoid cysts, arachnoid cysts, Neurenteric cyst

Containing FatDermoid cyst, Lipoma

Intrinsic brain tumor with a significant exophytic extension into the CP angle may be difficult to differentiate from an extra axial lesion based on imaging

Bone window thin slice CT: bony changes of the pyramid and IAC – essential in surgical planning

Erosion or dilatation of IAC seen in 70-90% of VS

Nevertheless, the Diagnostic tool of choice: MRI

MRI Characteristics of VS:

T1- weighted sequences:isointense to slightly hypointense

T2- weighted sequences:hyperintense

Gadolinium contrast:intense and homogenous enhancement with exception of

cystic portion

Intrameatal VSs are best visualized with gadolinium enhancement.

MRI characteristics of Meningiomas:

T1-weighted images:isointense to slightly hypointense to brain parenchyma

T2-weighted images:higher intensity than VS and with homogenous contrast

enhancement

Criteria for radiological differential between VS and CPA meningioma

Meningiomas are

centered usually away from the IAC

broad contact with petrous bone or the tent

obtuse angle between tumor and the pyramid

IAC not widened and tumor very rarely extends into the IAC (secondary invasion of the IAC might be observed in 10-20%)

Criteria for radiological differential between VS and CPA meningioma

Calcification and cystic changes are frequent findings

Tail of enhancement along the dura (the dural ‘tail’ sign) – 60-70% meningiomas

Calcifications are extremely rare in VS

Imaging of Epidermoids

Hypodense on CT

Rim calcification observed in 25% cases

T1 + T2 weighted MRI: isointense to cerebrospinal fluid

Signal intensity related to cyst content. Eg. If cholesterol predominates, the cysts are hyperintense on T1 weighted images and hypointense on T2

Because of similar characteristics, it might be difficult to differentiate it with arachnoid cyst.

Epidermoid vs Arachnoid cyst

Two major differences

smaller mass effect by arachnoid cyst

arachnoid cysts have more homogenous signal intensity on T2 weighted images

DWI restriction in epidermoids

Evaluation of patient with suspected CPA mass

MRI with and without contrast; FIESTA MRI if available (Fast Imaging Employing Steady State Acquisition): uses CSF as the contrast agent and NOT gadolinium

CT scan if MRI contraindicated

Audiometric evaluation

Pure tone AudiogramSpeech Discrimination EvaluationPatients with small VSs (<15 mm diameter) gets ENG, VEMP

and ABR

ENG (Electronystagmography)

tests horizontal semicircular canal

hence, assesses the superior vestibular nerve which innervates it

normally, each ear contributes an equal portion of the response

abnormal if >20% difference between the two sides

Vestibular Evoked Myogenic Potential (VEMP):

assesses inferior vestibular nerve by testing the saccule.

independent of hearing (can be done even with severe SNHL)

Auditory brainstem responses (ABR):

aka BAER

prognosticates chance of hearing preservation

most common findings – prolonged I-III and I-V interpeak latencies

poor wave morphology correlates with lower chance of preserving hearing (even with good hearing).

Pure Tone Audiogram (PTA)

First step screening test

Air conduction assesses the entire system

Bone conduction assesses from the cochlea and proximally

PTA assesses the functionality of hearing (to help in treatment decision making) + baseline for future comparison

The single numerical score is an average of the thresholds for frequencies across the audio spectrum

Pure Tone Audiogram (PTA)

Xs denote the left ear (AS) and Os denote the right ear (AD)

Progressive unilateral or asymmetric SNHL of high tones occurs in >95% of VS

DDx: age related and noise related hearing loss (usually symmetrical)

Other differentials of asymmetric hearing loss:meningiomainner ear lesionsintraaxial lesions leading to infarctionsmultiple sclerosis

PTA difference of >10-15 dB between 2 ears should be investigated

Speech Discrimination Evaluation

SD is maintained in conductive hearing loss

Moderately impaired in cochlear lesion

Worst with retrocochlear lesion

A score of 4% suggests retrocochlear lesion

Open set word recognition score (WRS) is a more sensitive measure of communication ability than PTA.

Class WRS %I 70-100%II 50-69%III 1-49%IV 0

Gardener and Robertson modified hearing classification(modification of Silverstein and Norrell system)

Class Description Pure tone audiogram (dB)

Speech discrimination

I Good-excellent 0-30 70-100%II Serviceable 31-50 50-69%III Non

serviceable51-90 5-49%

IV Poor 91-max 1-4%V None Not testable 0

NB: class > or = III is generally considered non serviceable hearing

American Academy of Otolaryngology- Head and Neck Surgery Foundation hearing classification system

Class Pure tone threshold (dB)

Speech Discrimination (%)

A < or = 30 > Or = 70B >30 and < or = 50 > Or = 50C >50 > Or = 50D Any level <50

Class A & B are considered “useful”, Class C is “aidable”, & Class D is “non functional”

Definition of serviceable hearing

Even non serviceable hearing can offer some benefit

If WRS is good (> or = 70) but PTA is poor, a hearing aid may provide significant benefit.

Definition of serviceable hearing

1. AAO-HNS class A or B

2. “50/50 rule”: Gardner-Robertson class I or II (pure tone audiogram threshold < or = 50 dB and speech discrimination score > or = 50%)

3. Some prefer a 70/30 rule (70% WRS, 30 dB PTA)

4. In a patient with good hearing in the contralateral ear, a speech discrimination score (SDS) of <70% in the affected ear is not considered good hearing; whereas if the contralateral ear is totally deaf, a SDS of > or = 50% can be useful.

Addendum to CT scan imaging…

Small lesions may be visualized by introducing 3-4 ml of subarachnoid air via lumbar puncture

Scanning is done with the affected side up (to trap air in region of IAC)

Non filling of IAC is indicative of an intracanalicular mass.

Although many VSs enlarge the ostium of the IAC (called trumpeting) (normal diameter of the IAC is 5-8 mm), 3-5% of VSs do not enlarge the IAC on CT

Advantage over MRI: bony anatomy delineation and thus, useful for planning translabyrinthine approach.

Vestibular Schwannomas

Histopathologically benign typically slow growing neoplasms

Originates from the transition zone between central and peripheral myelin that usually is located in the medial part of the IAC – Obersteiner Reidlich zone

2 different forms:

Sporadic unilateral (95%)Bilateral associated with NF2 (5%)

Vestibular Schwannomas

SPORADIC NF 2Presents at 5th or 6th decade Usually younger at presentation

Predisposition to multiple tumors like meningiomas, ependymomas, neurofibromasAutosomal dominant inheritanceMutation in chromosome 22q12: affects gene encoding schwannomin/merlin: implicated in cellular remodeling and growth regulation

Tumors just displaces VIIIth nerve without infiltrating

NF2 tumors form grape like clusters that may infiltrate the nerve fibers

VS progression – Four Stage Concept of Growth

Gradually fills the IAC first

Occupy the CP angle cistern, displacing VII and VIII cranial nerves and AICA

Compresses the brainstem and the cerebellum

Hydrocephalic stage with obstruction of IV th ventricle and its outlets

Symptomatology of patient differs according to the stage of progression

Symptoms

Intrameatal: vestibulocochlear dysfunction: hearing loss, tinnitus or

vestibular dysfunctionHearing loss is insidious and high frequency sensorineural

type

Cisternal stage:progressive hearing losssense of dysequilibrium gradually replaces vertigo

Later stage:trigeminal symptoms, headache, ataxia and obstructive

hydrocephalus

Symptomatolgy and signs

With further brainstem compression:

contralateral long tract signs

severe gait disturbance

lower cranial nerve palsies

signs of intracranial hypertension

Sign %

Abnormal corneal reflex

33

Nystagmus 26

Facial hypoesthesia 26

Facial weakness 12

Abnormal eye movement

11

Papilledema 10

Babinski sign 5

Classification of VS based on size or degree of extension into CP angle

According to size:

Intrameatal

up to 1 cm

1 to 2.5 cm

from 2.5 to 4 cm

larger than 4 cm

INI (International Neuroscience Institute) or Samii Classification of Vestibular Schwannoma extension:

Tumor Grade

Tumor Extension

T1 Purely intrameatalT2 Intra-, extrameatalT3a Filling the CPA cisternT3b Reaching the brainstemT4a Compressing the brainstemT4b Severe compression and distortion of brainstem and

fourth ventricleT5 Giant tumors (maximal diameter >4 cm), extension

over the midline

House and Brackmann Facial Nerve Grading System

Management

Expectant management

Radiation therapy

Surgery

Chemotherapy

Expectant management

Follow symptoms, hearing (audiometrics) and tumor growth on serial imaging

Intervention is performed for progression

Growth patterns observed:

a. little or no growth: applies to most (83%) VSs confined within the IAC and 30% extending into CPA

b. Slow growth ~2 mm/yrc. Rapid growth > or = 10 mm/yrd. A few actually shrink

Intracanalicular or CPA tumors < or = 20 mm diameter that are non cystic and non NF2: observation with serial imaging and hearing tests

Presence of cysts: cystic tumors may display sudden and dramatic growth; high recurrence

Imaging repeated if shows >2mm growth then intervention

Q6mo for 2 yrs; if stable then annually until year 5 after diagnosis

If stable then at years 7, 9 and 14 after diagnosis

Radiation therapy (alone or in conjunction with surgery):

a. External Beam Radiation Therapy (EBRT)

b. Stereotactic radiation

a. Stereotactic radiosurgery (SRS): single doseb. Stereotactic radiotherapy (SRT): fractionated

Recommended current dose of radiation 12-14 Gy to reduce cranial n. morbidityTumor control achieved in 93-98%Facial neuropathy in 1 to 20% and trigeminal dysfunction in 2-30% casesHearing preservation in 40-75% cases2-7% - tumor enlargement occurs even after radiotherapy

Post radiation changes obliteration of dissection plane difficulty in functional preservation of cranial nerves if microsurgical removal required in later dates

Larger VS: staged procedure: microsurgical debulking and brainstem decompression followed by radiosurgery of the remnant as second stage

Surgery: Approaches…

Retrosigmoid (aka suboccipital): may be able to spare hearing

Translabyrinthine (and its several variations): sacrifices hearing, may be slightly better for sparing VII

Middle fossa approach (extradural subtemporal): only for small lateral VSs

Complete tumor removal achieved in 80-99% casesRecurrence rate: 0.5 to 5%Facial nerve preservation in >90% cases

Chemotherapy:

Early promise for NF2 related VS

Bevacizumab (Avastin; it is an anti VEGF vascular endothelial growth factor monoclonal antibody)

In 6 patients, 4 had radiographically significant tumor shrinkage and 4 had improvement in auditory word recognition score

Surgical approaches…

MIDDLE FOSSA APPROACH/ Kawase approach

Hearing preserving techniqueDirect access to lateral end of IACMost lateral part of the tumor can be safely resectedLow risk of CSF leaksApplicable to small tumors

Necessitates temporal lobe retractionEndangers vein of LabbeRestricted access to the CP angle

TRANSLABYRINTHINE APPROACH: Rudolf Panse, 1904

Avoidance of cerebellar retractionShorter distance to the tumorEarly identification of the facial nerve at the lateral end of IAC

Sacrifice hearingRestricted access to the CP angleDifficult dissection and hemostasis close to the brainstemPoor visualization and access to the caudal cranial nerves

RETROSIGMOID APPROACH: Fedor Krause, 1903

Allows hearing preservation even in large VSsExcellent visualization of whole CP angleIncreased safety during dissection from brainstem and lower cranial nervesVII/VIII n complex can be identified both proximally towards brainstem and distally towards IAC

Needs cerebellar retractionDifficulty visualizing the most lateral part of the IAC without endangering inner ear integrityHigher rate of postoperative headache

A d d i t i o n a l r e m o v a l o f t h e s u p r a m e a t a l t u b e r c l e ( S A M I I Te c h n i q u e ) 1 9 8 2 ; p r o v i d e s a c c e s s t o t u m o r e x t e n s i o n s i n t o M e c k e l ’ s c a v e , t h e p e t r o c l i v a l a r e a

a n d e v e n t h e p o s t e r i o r c a v e r n o u s s i n u s .

Booking the case – retrosigmoid craniectomy

Position: supine with shoulder roll, Mayfield 3 pin head rest

Equipment:microscopeultrasonic aspiratorimage guided navigation system

ENT to assist IAC drilling

Neuromonitoring: facial EMG, direct cochlear nerve monitoring and SSEPs

Post op: ICU

Consent:procedure in lay term

alternatives: non surgical management with follow up MRIs, other surgical approaches, radiation (stereotactic radiosurgery)

complications: CSF leak with possible meningitis, loss of hearing in ipsilateral ear (if not already lost), paralysis of facial muscles on the side of surgery with possible need for surgical procedures to help correct (correction is often far from perfect), post-op balance difficulties/vertigo, brainstem injury with stroke.

Technique Summary..

Position: 30 degree elevation of head

Percutaneous lumbar drain (optional)

Incision is shaped like the pinna of the ear, 3 finger breadths behind the external auditory canal

The craniotomy has to be lateral enough to expose part of the sigmoid and part of the transverse sinuses.

To prevent CSF leak, seal all bone edges with bone wax

Dural opening Y shaped making base to sinuses

Exposure is enhanced by opening the CP angle cistern and the cisterna magna under the microscope and draining CSF (20-40 ml of CSF can also be drained via a lumbar subarachnoid catheter)

The petrosal vein is often sacrificed at the beginning of the procedure to allow the cerebellum to relax and fall back and to avoid tearing off the transverse sinus. Be careful not to coagulate the SCA that often runs with the petrosal vein.

Using the facial nerve stimulator, the posterior aspect of the tumor is inspected to make sure the facial nerve has not been pushed posteriorly

The thin layer of arachnoid that covers most tumors is identified. Vessels within the arachnoid may contribute to cochlear function and may be preserved by keeping them with the arachnoid.

The plane between tumor and cerebellum may be followed to the brainstem, and occasionally to the VII nerve (this plane is harder to follow once bleeding from tumor debulking occurs)

Posterolateral tumor capsule is opened, and internal decompression is performed. The tumor is collapsed inward and the capsule is kept intact and is rolled laterally off of VII and is eventually removed.

The most difficult area to separate VII from tumor is just proximal to the entrance to the porus acousticus.

Large tumors: capsule adherent to brainstem portion must be left; recurrence rate: 10-20% may involve V superiorly and IX, X, XI inferiorly; these cranial nerves are spared by protecting with cottonoids

After extracanalicular portion of tumor is removed, the dura over the IAC incised

IAC drilled open and tumor removed from this portion

Bony labyrinth must not be violated to preserve hearing

The most vulnerable structure is posterior semicircular canal

Preop CT help determine maximum amount of temporal bone drilling

Any opening of labyrinth should be plugged with bone-wax or muscle

CP angle Meningiomas

Meningiomas located lateral to trigeminal nerve regardless of the site of dural attachment are termed CP angle meningiomas

If located medial to trigeminal n. petroclival meningioma

Slow growing, benign

Women in 5th to 6th decade

Arise from groups of arachnoid cells located in high concentration around the IAC, lower border of superior petrosal sinus, lateral border of inferior petrosal sinus, around cranial n. foramina and in the region of the geniculate ganglion.

Classified as premeatal and retromeatal in relation to the IAC

The more medially located the tumor, the greater is the surgical challenge and the poorer is the outcome

Further subclassified as premeatal, postmeatal, suprameatal, inframeatal and those centered at the IAC.

Signs:

Hearing loss 30-73%

Trigeminal nerve signs 13-49%

Cerebellar signs 25-52%

Tinnitus 10-12%

Signs of increased ICP 16-29%

Hydrocephalus 20-31%

Three percent of the cases of trigeminal neuralgia are due to tumor compression at the root entry zone

Premeatal tumors are diagnosed earlier and consequently are smaller. Their clinical presentation is with trigeminal signs and facial and cochlear nerve signs

Retromeatal meningiomas present with cerebellar signs

Total surgical removal is the optimal treatment option but should not be achieved at the expense of new neurological dysfunctions or worsening quality of life.

Different surgical approaches: retrosigmoid, translabyrinthine, transpetrosal

Principles:adequate exposureinterruption of the blood supply along the dural attachmentinternal decompressioncautious dissection of the tumor capsule from the brainstem

and cranial nerves at the arachnoid plane

With the retrosigmoid approach hearing is preserved in 82-90.8% of the patients

Compared to VS, CP angle meningiomas have higher tendency toward recurrence, with a recurrence rate between 0-9.5%

Radiotherapy or radiosurgery proposed for small meningiomas

Surgery may still be required in case of treatment failure or secondary trigeminal neuralgia after irradiation

Epidermoids

Might expand through the tentorial incisura into the middle cranial fossa, grow toward the contralateral CP angle, or extend toward the foramen magnum

Are sometimes densely adherent to the neurovascular structures

Usually present between the third and fifth decades of life with long standing history of tinnitus and hearing loss.

Cause relatively more frequently trigeminal neuralgia or hemifacial spasm when compared to VS and meningiomas; lead to facial nerve signs much earlier than VS.

Treatment of choice is surgical and preferred approach is retrosigmoid

Endoscopic assisted microsurgical technique

Small remnants of capsule firmly attached to important structures should not be removed for risk of additional neurological deficit

Subtotal resected epidermoid have late recurrence up to 20-30 years and thus mandates close follow up on a long term basis.

Thank you!!!