SURGICAL MANAGEMENT

OF VESTIBULAR SCHWANNOMA

- DR. DHIRENDRA V. PATILM.S. (ENT)

J.N.M.C., Aligarh Muslim University.

INTRODUCTION

For many years the management of vestibular schwannoma (VS) did, in fact, nearly always mean surgical management.

The main reason for this was late diagnosis.

Most large centres now adopt a ‘wait and rescan’ policy for tumours confined to the internal auditory meatus (IAM), or with limited extension into the cerebellopontine angle (CPA).

HISTORY

It remains unclear whether it was Ballance in London, or Annandale in Edinburgh who performed the first successful VS removal.

They performed these operations through the suboccipital approach.

William House, in the 1960s, proposed surgery as soon as the diagnosis could be made and suggested the translabyrinthine approach to the CPA.

The logic of letting a small- or medium-sized tumour to become a large or giant tumour before performing an operation was unacceptable to him.

House endured many hostile confrontations with the ‘neurosurgical community’ whose objections were as much due to the fact that ‘otologists’ were becoming involved with this type of surgery as with the approach itself.

The ‘best’ approach is the approach that gives the best results in the hands of the individual surgeon.

RELEVANT ANATOMY

An axial view of the skull through the level of the IAC and CPA.

Various Approaches

Most vestibular schwannomas originate in the region of the IAC, enlarging the porus and extending into the cerebellopontine angle.

IAC

Tumour Behaviour

Schematic illustration of initial growth of VS

Schematic illustration of intrameatal expansion of VS

Schematic illustration of expansion of VS in the cerebellopontine angle

SURGICAL APPROACHES TO THECEREBELLOPONTINE ANGLE

SURGICAL APPROACHES TO THECEREBELLOPONTINE ANGLE

Translabyrinthine Approach

Translabyrinthine approach

This is now the favoured approach for the removal of VS for the majority of neurotologists.

The key stages in the operation are

1. skin and periostial flaps; 2. extended cortical mastoidectomy; 3. bony labyrinthectomy; 4. skeletonization of the jugular bulb and

vertical portion of the facial nerve; 5. skeletonization of the internal auditory

meatus;

6. identification of the facial nerve at the lateral end of the internal meatus;

7. opening of the posterior fossa through the dura of the posterior surface of the petrous bone;

8. removal of tumour using standard neurosurgical techniques;

9. closure with obliteration of the middle ear and petrosectomy defect, usually with abdominal fat.

Skin incision

A curved incision above and behind the pinna is planned, it can be about 3-4 cm behind the postauricular sulcus.

Vid-1

Cortical mastoidectomy

Using cutting and coarse diamond paste burs, bone is removed up to the middle fossa dura, exposing it widely.

This allows easy retraction of the dura with the instruments during tumour removal.

In a similar manner, bone is removed from the sigmoid sinus and from the bone overlying the posterior fossa dura for 2 or 3 cm behind the sinus.

Cortical mastoidectomy

The secret of the operation is the extent of the bone removal (Figure)

Dural elevation permits insertion of retractors, which augments the exposure of deeper structures.

Some surgeons like to leave an island of thin bone over the sinus that can be retracted with the sinus and provides some protection for the sinus.

Bony labyrinthectomy

A standard total bony labyrinthectomy is performed (Figure)

Removal of the semicircular canals is commenced in the sinodural angle.

Care must be taken in drilling out the ampulla of the posterior canal, which lies medial to the second genu of the facial nerve.

The ampulla of the superior semicircular canal should be retained, as it is a landmark for the superior vestibular nerve (SVN).

Labyrinthectomy is then deepened to open and then remove the SCCC.

The endolymphatic duct can be traced from the vestibule along the line of the common crus where it turns though 90 degree towards the posterior fossa dura and widens out to become the sac.

At this stage the endolymphatic sac and duct are exposed.

The duct wraps around the common crus on its J shaped route to the vestibule.

Labyrinthectomy is completed with a diamond burr to identify the horizontal and second genu portions of FN.

Vid-2 (Labyrinthectomy)

Skeletonization of the jugular bulb and the

verticalportion of the facial

nerve

The jugular bulb is the lower limit of bone removal and in nearly all cases bone should be removed down to its level.

Once the facial nerve is located, the remaining portion of sigmoid sinus is uncovered, in a direction of jugular bulb.

The IAC lies in the deep bone to the labyrinth(fig).

The retrofacial air cells are exenterated and bone may be removed over the vertical portion of the facial nerve until the sheath is visible through the bone.

Vid-3 (left ear)

Skeletonization of the internal meatus

Exposure of the internal auditory canal commences with a cutting burr.

Canal courses posteriorly after taking its origin at the vestibule.

Once the plane of the IAC has been identified, troughs are drilled above and below the canal, parallel to its long axis.

Bone is removed along the posterior petrous face medial to the porus acosticus.

A diamond burr is used to remove the last eggshell thin piece of bone over the canal dura.

Before removing the remaining bone from the floor and roof of canal, the dura is elevated from the remaining bony shell.

The remaining superior and inferior bony plates are removed with a diamond burr while gently displacing the contents of canal.

The transverse crest is a prominent bony landmark separating SVN and IVN.

A U-shaped gutter is drilled above, behind and below the internal meatus (Figure).

The extent of bone removal should be approximately 270 degrees round the meatus, and is much faster if the temporal bone is well pneumatized.

One should constantly keep in mind the position of the facial nerve in the anterosuperior quadrant of the meatus, and remember that in an expanded meatus it may, in fact, be very close to the middle fossa.

At the lateral end of the meatus the transverse crest and the canal for the SVN should be sought.

The latter runs from the lateral end of the meatus towards the retained ampulla of the superior semicircular canal, and is a constant and reliable landmark.

VID 4 (IAC skeletanisation)

Opening the posterior cranial fossa

Opening the posterior cranial fossa

This is done through a U-shaped dural flap, based laterally close to the lateral sinus.

The upper limb is close to the superior petrosal sinus and the lower limb close to the jugular bulb.

The medial limb is at the level of the porus.

The dura of the internal meatus should be cut from lateral to medial at the level of the transverse crest.

Dural incision obtains wide exposure of both the IAC and CPA.

The IAC dura is then opened with upbiting and angled scissors.

The IAC and CPA incisions are then connected at the level of porus acusticus.

VID 5 (Dura incised)

Identification of the facial nerve

Identification of the facial nerve

The facial nerve is displaced from its normal position by the tumour, but in the majority of cases it is displaced in a fairly predictable way.

It runs along the anterosuperior quadrant of the meatus as far as the porus.

In the translabyrinthine approach, the tumour is usually between the surgeon and the facial nerve; however, this is not always the case.

The routine identification of Bill’s bar, the vertical crest separating the SVN from the facial nerve, has been abandoned by many surgeons now because of availability of reliable facial nerve monitors.

But it may be useful to do so in cases of doubt, and at the institutes where monitors are not available.

Access to the anterosuperior part of the meatus may be helped by careful debulking of the tumour in the lower half of the meatus.

It is also useful to try to identify the facial nerve on the brainstem at the earliest opportunity (Figure).

Tumour removal

Tumour removal

With tumours confined to the internal meatus or with little intracranial extension, dissection can start at the fundus and proceed medially, keeping to the arachnoid plane.

Little difficulty should be encountered although even small tumours may be surprisingly adherent to the facial nerve just at and medial to the porus, and sharp dissection may be needed (Figures).

With larger tumours debulking of the inside of the tumour is carried out so that the tumour is converted from a solid ball to a hollow ball.

This technique is based on the fact that as the tumour expands all important structures such as the facial nerve and AICA are pushed before the tumour and are to be found in the arachnoid sheath on the outside of the tumour capsule.

A number of techniques and instruments can be used for debulking.

If the inside tumour is very soft it is possible to reduce the volume quite rapidly with suction alone.

More solid tumours may require the use of the ultrasonic surgical aspirator (CUSA) or the cutting bipolar loops.

Vid 5 (US debulking)

As the tumour bulk reduces it becomes progressively easier to manipulate the tumour capsule.

After tumour removal, haemostasis must be secured.

Care must be taken with the use of the bipolar diathermy in the vicinity of the facial nerve.

CLOSURE

Closure

This is one of the most important steps in the translabyrinthine operation.

CSF fistula remains one of the most common postoperative problems.

To minimize the risk, careful obliteration of the middle ear and the temporal bone defect is essential.

The middle ear, Eustachian tube and vestibule are obliterated with muscle and bone wax.

The supra- and inframeatal gutters are obliterated with fat and obvious air cell tracts sealed with bone wax.

The temporal bone defect is obliterated with abdominal fat either in strips or in one large piece

Fat grafts are harvested from the anterior abdominal graft.

In woman, fat can be harvested from hip region.

A schematic axial view demonstrating placement of the FIRST fat strip into the craniotomy defect.

A schematic axial view demonstrating additional fat strips into the craniotomy defect.

The periosteal flap is then sutured back over the fat and the skin closed in two layers.

A firm-pressure dressing is applied and kept in place for a week.

VID-6 (CLOSURE)

Middle Fossa

Approach

Schematic coronal view through Temporal lobe and roof of petrous pyramid.

Temporal lobe retraction provides excellent view of IAC.

The middle fossa approach is one of the possible routes of access for hearing preservation surgery.

Its advantages and disadvantages are summarized in Table.

It has the advantage of allowing good visualization of the lateral extent of the internal meatus.

The approach is however somewhat cramped and access to the posterior fossa limited.

Maximum size of tumour that can be removed is approx. 1–1.5 cm in intracranial diameter.

There is a small but real risk of epilepsy following extradural retraction of the temporal lobe (Aggarwal et al).

The key stages in the middle fossa approach

are: 1. skin and soft tissue incisions; 2. middle fossa craniectomy; 3. extradural approach to upper surface

of temporal bone and to posterior fossa; 4. skeletonization of internal meatus; 5. identification of facial and vestibular

nerves; 6. removal of tumour; 7. closure.

The surgeon is seated at the head end of the bed during middle fossa surgery.

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.

Craniectomy

A 5 x 5 cm square bone flap is cut with about two-thirds in front of the intermeatal line and one-third behind it.

The dura over the temporal lobe is exposed.

Eevation of dura from anterior face of petrous pyramid proceeds from posterior to anterior.

Eventual target is crest at the ridge of the petrous pyramid (dashed line) and Porus acousticus (solid circle).

Anteriorly, GSPN is encountered, if it is adhered to the dura it must be liberated sharply.

Middle meningeal artery bleeding can be controlled with bipolar cautery and packing bone wax into its foramen.

Transparent view of petrous contents as shown from

above.

Picture after the completion of dural elevation.

Note the GSPN and Arcuate eminence overlying the SSCC.

Temporal lobe retractors

How to localise the IAC?

Several methods are available for localising the IAC in the middle fossa floor.

One commonly used method is to first identify the porus acousticus.

Then drill the rest of the IAC wall from medial to lateral to expose the full length of the canal.

A deep trough is drilled into the apical petrous bone, well anterior to anticipated location of canal.

Another method to locate IAC is by tracing the GSPN back to the geniculate ganglion.

Then follow the labyrinthine segment of nerve to the fundus.

This method is ‘House method’.

IAC may be located by beginning with ‘blue lining’ of SSCC (arcuate eminence).

SSCC is fairly constant angular relation to the IAC(Between 45 and 60 degrees).

This is ‘FISCH method’.

Once the dura of IAC has been opened, an intracanalicular tumour can be visualised.

Often the FN lies on the superior surface of the tumour.

CLOSURE

The cavity created during IAC exposure is filled with a free tissue graft (eg. temporalis muscle or fat graft).

The bone flap is then replaced and wired in position.

Vid 7 (Middle fossa approach)

Retrosigmoid Approach

Retrosigmoid approach

This approach has evolved from the classic suboccipital operation that was favoured by neurosurgeons for the removal of all tumours but particularly for large ones.

The advantages and disadvantages of the approach are summarized in Table.

Postoperative headache is more common with this approach than the translabyrinthine operation.

Incision

A vertical or slightly curving incision is made about 3 cm behind the mastoid process, from above the level of the transverse sinus to the level of the tip of the mastoid.

Craniotomy and exposure of the tumour

A 5 x 5 cm craniotomy is made using the drill, taking the mastoid emissary vein as the starting point and retaining the bone dust for closure.

The anterior and superior limits of bone removal are the sigmoid and the transverse sinuses, respectively.

Dural incision is made in such a way to facilitate suture closure of the dura.

Before posterior retraction of the cerebellum, it is necessary to release the CSF pressure by opening the cisterna magna.

Premature retraction of the cerebellum, before decompressing the cistern, risks inducing massive cerebellar swelling.

Opening of the cisterna magna with a suction lancet.

Axial schematic view of Retrosigmoid approach to the CPA and IAC.

Note the cerebellar retraction.

Closure

Dural closure should be very thorough.

Bone wax is applied to transected mastoid air cells.

Craniotomy defect is repaired.

COMPLICATIONS

COMPLICATIONS (Shambaugh)

lntraoperative Complications :

1. Cranial Nerve Injury2. Bleeding3. Brain Edema4. Venous Air Embolism5. Cardiac Arrhythmias6. Brain Herniation

Postoperative Complications:1. Hemorrhage2. Infarction3. Cerebrospinal Fluid Leak4. Meningitis5. Tension Pneumocephalus

Stereotactic Radiation Therapy

ln 1951, the Swedish neurosurgeon Leksell developed the first open stereotactic instrument by focusing multiple radiation beams on a single target.

Currently, stereotactic radiation is the principal alternative active treatment for vestibular schwannomas.

The goals of stereotactic radiation therapy are the :

1) Long-term prevention of tumor growth,

2) Maintenance of neurologic function, and

3) Prevention of new neurologic deficits.

The word stereotaxis is derived from two Greek words:

Stereos - “three-dimensional,” Taxis - “orderly arrangement.” A high dose of radiation can be

delivered to a defined region, usually within a well-immobilized system that conforms closely to the 3D shape of the target volume.

Stereotactic irradiation can be performed by using any one of the high-energy forms of radiation, whether

1) Radiographs (x-rays),2) Gamma rays, or3) Charged-particle irradiation.

It involves fixing a Rigid Stereotactic Frame to the patient’s head, which then acts as a reference, defining the volume which it encloses in a set of X, Y and Z coordinates.

By combining this setup with radiological images (generally MRI), it can be used to target pathological structures including skull base tumours.

How is Radiosurgery different from Radiotherapy?

Radiosurgery differs fundamentally from radiotherapy in that it is delivered as a single high dose of radiation, rather than as a fractionated course of treatment.

Advantages of Stereotactic Radiation

Potential advantages of stereotactic radiation over Microsurgical resection include :

1) Decreased hospitalization time,2) A quicker return to work, and,3) A reduced cost of treatment.4) Can be considered for elderly or

medically unfit patients.

Stereotactic Radiation Disadvantages

Three concerns are frequently raised about radiosurgery, particularly in comparison with surgery.

1. The need for long-term follow-up.2. The risk of radiation causing

malignant transformation.3. The possibility that radiosurgery

causes peritumoural scarring that makes subsequent surgery more difficult.

In summary, although acceptable outcomes have been reported with stereotactic radiation therapy for the treatment of vestibular schwannomas, long-term outcomes at current levels of radiation have not been well documented.

THANK YOU

-drdhiru456@gmail.com

References :

1) Scott-Brown’s Otorhinolaryngology: Head and Neck Surgery : 7th edition.

2) Shambaugh : 6th edition.3) Neurotology and skull base

surgery: R. K. Jackler.4) Cumming’s

otorhinolaryngology : 5th edition.