Acoustic Neuroma

 

Acoustic  Neuroma / Vestibular Schwannoma

 

Introduction

 

  • Benign tumour arising from abnormally proliferative schwann cells, which envelope the lateral portion of the vestibular nerve in the internal acoustic meatus

 

History

 

  • First described by Sandifort in 1777
  • William House introduced the translabrynthine and middle fossa approach
  • At 1992 consensus conference à name  ’vestibular schwannoma’ adopted

 

Epidemiology

 

  • 8% of all intracranial tumors
  • 90% of CP angle tumors
  • > 95% sporadic (unilateral)
  • 5%  Neurofibromatosis type 2 (bilateral)
  • Gender F:M= 3:2
  • Age- fourth and sixth
  • Incidence in general population  10 – 15 / million/ year
  • Incidence in temporal bone collections 1-2%

 

Pathogenesis

 

  • Usually arise from vestibular portion of eight nerve
  • Commonly from IAC portion of the nerve, from its OBERSTEINER-REDLICH zone which is junction of central myelin produced by glial cells & and peripheral myelin produced by schwann cells. Also is the glial & neurilemmal junction.
  • Superior division more common (some books mention equal frequency)

 

Histopathology

 

  • Macroscopic
    • Benign
    • Yellowish to pinkish grey
    • Firm to rubbery consistency
    • Encapsulated
    • Nodular surface
    • Well defined plane of separation
    • Cysts formation within substance of tumor common à may form bulk of tumor
    • Cyst may have –CSF/hemorrhagic fluid
    • Microscopic
      • Two patterns
        • Antoni A à closely packed cells with small spindle-shaped and densely stained nuclei. A whirled appearance of Antoni type A cells is called a Verocay body
        • Antoni B à looser cellular aggregation of vacuolated pleomorphic cells
  • Positive immunostaining with S100 à differentiates from meningioma

 

Tumor spread

 

  • —  From schwann cells of vestibular nerve in IAC
  • —  Expands è widens & erodes IAC è cerebellopontine angle
  • —  Involves cranial nerve V or IX, X, XI depending upon the direction of spread
  • —  Later, brainstem, cerebellum è increased intra cranial pressure

 

 

 

 

GROWTH RATE AND GROWTH PATTERN

 

  • Mean growth rate à 1.1 mm / yr (Scott Brown)
  • Growth patterns

 

    1. Continuous growth
    2. No measurable growth
    3. No measurable growth followed by continuous growth
    4. Negative growth
    5. Various positive growth patterns

 

Classification

 

  • Jackler system ( adopted at Tokyo consensus conference 2001)

 

Extrameatal Size (mm)
Grade 1 Small 1-10
Grade 2 Medium 11-20
Grade 3 Moderately Large 21-30
Grade 4 Large 31-40
Grade 5 Giant >40

 

  • Note should also be made if fundus empty or filled by tumor
  • Also if VS sac was cystic

 

Clinical Features

 

  • Stage 1 – Otological Stage
    • Progressive unilateral SNHL with tinnitus, more in higher frequency
    • In 5 – 10 % can be SSNHL
    • Very poor speech discrimination
    • Imbalance, unsteadiness
    • Nystagmus +/-
    • Facial nerve involvement à hypoaesthesia post EAC, taste loss, decreased lacrimation
  • STAGE 2 – Trigeminal nerve involvement
    • Corneal reflex loss
    • Facial paraesthesia
  • STAGE 3 – Brain stem & cerebellar compression
    • Ataxia, motor/sensory loss over extremities
    • Cerebellar ataxia, +ve romberg, dysdiadochokinesis, incoordination
  • STAGE 4 – Increasing intracranial pressure
    • Headache, nystagmus, vertigo, diplopia, papilledema
  • STAGE 5 – Terminal stage

 

INVESTIGATIONS

  • PTA

-          Asymmetric unilateral SNHL

-          Slow progressive

-          More in higher frequencies

  • Audiological tests

-          Speech discrimination poor; Roll over phenomenon present

-          SISI low à recruitment absent

-          ABLB à no recruitment

-          Tone decay à high à retrocochlear hearing loss

-          Stapedial reflex absent

  • Diagnostic bekesy audiometry

-          Types III & IV àretrocochlear loss

  • Electrocochleography

-          Broad waveform

  • ABR

-          Very sensitive à 95% -100% detection rates

-          Interaural latency of >0.2 msec in wave V between 2 ears is significant

-          I-V interpeak latency > 4ms

  • Caloric tests & electronystagmography

-          Canal paresis in 96 % of cases

-          Inferior vestibular nerve schwannoma may not show changes in caloric response

  • Conventional x-ray

-          Views

    • Stenver projection
    • Chamberlain-towne projection
    • Caldwell view

-         Look for

    • Dilated IAC
    • Funelling of IAC
    • Erosion of posterior lip of IAC
    • CT Scan

-          Can detect posterior fossa tumors upto 0.5 cm

-          Combined with intra thecal air (oxygen cisternography), even intra meatal tumors can be detected

  • MRI with gadolinium contrast

-          Superior to CT Scan

-          Gold standard in imaging for vestibular schwannoma

-          Intracanalicular tumor of even a few mms can be detected.

-          Advantages

    • High intrinsic contrast between tissues
    • Absence of bone artefacts
    • Multiplanar imaging
    • No radiation

Differential diagnosis of CP angle tumor

  • Acoustic Neuroma 90%
  • Meningioma
    • Broad base on posterior surface of temporal bone or petrous bone
    • Imaging features
  • Primary Cholesteatoma
    • From congenital epithelial rest cell in temporal bone of PCF
    • Presents with progressive facial palsy or hemifacial spasm
    • Imaging
  • Arachnoid cyst
    • Congenital malformation
    • Infection-adhesive aracnoiditis
    • Trauma
    • Have CSF-so hypointense on T1
  •  Schwannoma of other cranial nerves
    •  Low in Jugular foramen(IX, X)
    • Facial paresis
  • Lipoma
  • Choroid plexus papilloma
    • Von Hippel Lindou disease
  • Hemangioma
  • Hemangiopericytoma – Bad prognosis
  • Glomus Jugulare(type IV)

Treatment decision factors

  • Patient  à age, general health, status of hearing in the contralateral ear, preference
  • Tumor   à size, location, extent, growth rate, bilateral, recurrence
  • Surgeon skill & preference
  • Treatment options
    • Observation
    • Stereotactic radiation therapy
    • Complete surgical excision
    • Currently, no randomized, prospective clinical trial has compared the three treatment options and there are no clearly accepted, evidence-based, best practices for managing acoustic neuroma

Conservative management  (Wait & Scan)

  • Indications
    • Patients with small tumors
    • Advanced age
    • Poor general condition not fit for surgery
    • Unwilling for surgery
    • Yearly scanning advised (MRI)
    • Disadvantage
      • Risk of losing useful hearing (10-43%) in spite of no growth on MRI

Surgery

  • 3 main approaches
    • Middle fossa approach
    • Translabyrinthine approach
    • Retrosigmoid approach
  • Middle Fossa approach
    • Young pt
    • Hearing to be preserved
    • Small tumour < 2.5 cms
    • Good exposure of lateral IAC, CPA, and clivus
    • Drilling is extradural decreasing morbidity
    • Disadv
      • Temporal lobe retraction
      • Must dissect around facial nerve due to its superior position
      • Limited posterior fossa exposure
  • Steps
    • Position: Supine with head turned to affected side
    • Incision: Front of ear at level of zygomatic arch & curves upwards and backwards to sup temporal line expose the squamous temporal bone
    • 4X4 cm craniotomy, 2/3 ant to EAC, 1/3 post to it.
    • Middle fossa dura elevated off surface of petrous apex
    • Middle meningeal artery identified, followed till GSPN
    • Arcuate eminence identified
    • House approach – Follow GSPN to geniculate ganglion à trace back facial nerve to reach IAM
    • Fisch approach – angle between line of GSPN and plane of SCC (arcuate eminence) bissected à gives line of IAM
    • IAC identified, dura of IAC opened to long axis
    • Tumor dissected free, sup and inf vestibular nerve totally ablated
    • Internal auditory artery preserved
  • Translabyrinthine approach
    • In cases with unserviceable hearing
    • Wide exposure of posterior fossa
    • No size limit for resection
    • Facial nerve easily identified throughout
    • Ease of facial nerve repair if damaged/resected during removal
    • Low recurrence
    • Low headaches
    • Disadv
      • Residual hearing is sacrificed
      • Requires abdominal fat graft
  • Steps
    • Extension of Std postauricular incison upper end upto anterior wall of external meatus

 

      • Lower limit : 2cm behind mastoid tip
      • Superiorly based periosteal flap
      • Complete Mastoidectomy done, labyrynthectomy – IAC dissection
      • Dura of IAC opened, transverse crest & Bill’s bar identified
      • Debulking of tumor done & gradual complete dissection
      • Intracapsular removal of tumor using ( House Urban rotatory dissector)
      • Meticulous closure middle ear with muscle, fascia graft for dura and antrum, mastoid defect with abdominal fat
  • Retrosigmoid/Suboccipital Approach
    • Large size tumor
    • Hearing preservation possible
    • Wide exposure of brainstem and lower cranial nerves
    • Neurosurgeon familiarity
    • Consistent facial nerve identification
    • Disadv
      • Must be medially located
      • Lateral tumors risk injury to endolymphatic sac and  vestibular labyrinth
      • Cerebellar retraction
  • Steps
    • Position: Modified Park Bench ( supine with ipsilateral shoulder and hip bumped with rolls and padding, head flexed and rotated to opp shoulder)
    •  ‘C’ shaped curvilinear incision from upper edge of pinna to spine of C2
    • 4×4 cm Craniotomy performed
      • Anterior limit: Sigmoid sinus
      • Superior limit: transverse sinus
      • Dura opened by triradiate incision
      • CSF run off or anaesthetist decreases CSF pressure
      • Cerebellum falls under own pressure plus retractor can be applied
      • Ant based U shaped opening made in dura
      • Intracranial segment of tumor debulked
      • Prevent injuy to V nerve, AICA, Lower Cr nerves
      • Another Laterally or medially  based dural flap opened over IAC
      • Meatus drilled-exposing lesion till Bill’s bar & transverse crest
      • If hearing preservation desired drill medial to lateral
      • Seal with Bone wax, fibrin glue after closing dura

Conventional Radiotherapy – No role

Gamma knife radiosurgery

  • Principle à To deliver a single precise, conformed dose of radiation tailored to margins of tumor
  • Indication
    • No fixed guidelines
    • Smaller lesion to medium
    • Older individual
    • Method
      • Rigid stereotactic frame attached to patient’s head
      • X, Y & Z co ordinates
      • Combined with radiological images, usually MRI
      • Targets pathological structures
      • Uses ionizing radiation (array of cobalt 201 sources or linear accelerator)
      • Median peripheral dose of 15 Gy
      • Directed through a variable array of collimators onto a single point
      • Delivers single high dose of radiation
      • Fate of tumor
        • Tumor remains in situ but growth is stabilized
        • Modest degree of shrinkage
        • May swell in intial 6-18 months due to radiation oedema
        • Gadolium enhancement in centre is reduced
        • 5% regrowth
        • Upto 92% control rate has been achieved
        • Complications
          • Cerebral oedema
          • Hydrocephalus
          • Vertigo
          • Seizures
          • Headache
          • Secondary oncogenesis- 5 cases reported

Fourth pathway

  • Stereotactic RT after microsurgical debulking

 

Bilateral Vestibular Schwannoma (Neurofibromatosis-2)

  • Treatment options vary
  • Bilateral-small tumor (less than 2 cm) & good hearing is a candidate for hearing preservation
    • Surgery on one side (larger tumor) or with worse hearing & if hearing preserved à surgery other side in 6 months
    • Observation without surgery
      • Small tumor
      • Unilateral hearing ear

Recent advances

  • Ultrasonic aspiration
  • Minimally invasive endoscopic neurosurgery
  • KTP-532 CO2,Argon laser


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