The history of development of surgery for acoustic neuroma dates back to Cushing and Dandy. 

Various surgical approaches are employed.

They are: suboccipital, trans labyrinthine, and middle fossa approach.

The patient's age, hearing status, tumor size, and above all, the surgeon's preference decide the approach. On occasions, a combination above approaches can, effectively, be employed.

 1) Suboccipital (retrosigmoid transmeatal) approach:  

It is the most widely used approach.

Anatomy of the CP angle:  

The CP angle or fissure is V shaped and is formed by the folding of petrosal surface of the cerebellum lateral side of the pons and medial cerebellar peduncle. The floor of the space is formed by medial peduncle.  

The cerebello medullary cistern is situated between the cerebellar tonsils and medulla and communicates with the CP angle cistern near the foramen of luschka. The trigeminal, the abducent, the facial, the vestibulocochlear and glasso-pharyngeal arise between the superior and inferior limbs of the CP angle.

The internal acoustic meatus has a width of 9 to 10mm and height of 3 to 6mm. The internal auditory canal has a length of 6 to 7mm and the height of 3 to 7mm. The falciform or transverse   crest   divides   the meatus into the superior and inferior portion.

There are 4 nerves at the IAM-the facial, the cochlear, the superior and inferior vestibular nerves. The facial nerve and the superior vestibular nerve are superior to the crest with the facial nerve placed anteriorly. The cochlear nerve and the inferior vestibular nerve are in the inferior portion of the   crest with cochlear nerve anteriorly. 

The facial nerve arises from brainstem near the lateral end of the ponto-medullary sulcus, 1 to 2 cm anterior to the point where vestibulo-cochlear nerve enters the brainstem at the lateral end of the same sulcus. The facial nerve arises 2 to 3 cm above the emergence of the superior most rootlet of lower   cranial nerves from the brainstem.  The intra cistern length of the facial is 9 to 26mm.

The vestibular-cochlear  nerve enters the brainstem 13 - 17  mm  from  the   midline   and its  intra  cistern  length  is  about  14.9  mm. While entering the brainstem, the cochlear part is lateral most and the superior vestibular the most medial with inferior vestibular in between.

The length of the trigeminal nerve in the CP angle and the posterior fossa is 12.3mm for the sensory root and 14.1mm for the motor root. It exits from the posterior fossa through the dural opening situated at the anterior end of the medial surface of the tentorium cerebelli. The superior petrosal sinus is closely associated with the nerve and superior cerebellar artery also forms a close   relationship to the nerves.  

The abducent nerve emerges from the brainstem approximately 3.9mm lateral to midline. The 9th and 10th cranial nerves merge caudal to pondomedullary sulcus. The anterior inferior cerebellar artery is closely related to the facial and vestibular cochlear nerve.

Internal auditory artery, the recurrent perforating   arteries and the subarcuate artery are the branches of AICA. The superior petrosal vein (Dandy's) is the principle draining vein of antero-lateral posterior fossa structure. The vein is 1 to 2mm in diameter. The inferior petrosal vein courses along the vagus nerve.

Pathological anatomy:

Schwannomas 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%.The origin of the tumor is from junctional  (Obersteiner  Redlich) zone where the central and peripheral myelin meet. This zone is situated at the region of IAM or within the internal auditory canal. The tumor grows initially within the canal and thereafter, extrudes into the CP angle. Inside the petrous  bone,  the  tumor  may   compress the   cochlear  component  of  the  nerve  or the labyrinthine artery, causing  sudden  severe  hearing  loss. Growth of the tumor in to the CP angle leads to the anterior displacement of the facial and cochlear nerve. The relationship of   the tumor to the   vestibular   cochlear   nerve varies. In about 50%, the nerve fibers are intimately involved with the tumor, making separation impossible. In 40%, though the nerve is in the form of bundle initially, it becomes adherent and a part of the   tumor capsule making functional preservation impossible, and in 10%, uninvolved portion of the nerve maintain anatomical integrity. Anatomically  the  last  group  present with preservation of hearing  and  in  this group, the vestibular cochlear nerve is displaced  inferiorly in 80%,anteriorly in18% and posteriorly.

Depending on the direction of growth of the tumor, the facial nerve may be  displaced  anterosuperiorly or anteroinferiorly. The facial nerve may run one of 4 courses around the acoustic neuroma. The nerve runs anterior to the tumor in 70%, superior in about 10%, posterior in 7% and inferior in 13%.

The position of the facial nerve is most constant at the lateral end of the IAM. The nerve may be anatomically distorted by the tumor in about 2/3rd of cases, the nerve maintains the shape of a thin bundle, while in about a 3rd of cases, and the nerve fibers are splayed over the tumor capsule. Since the tumor arise from outside the CSF space, it pushes the lateral layer of the arachnoid inwards till it comes into contact with the middle layer. The double layer thus formed contains the important vessels and nerves of the CP angle.

Management of pre-operative hydrocephalus:

Patient with acoustic neuroma with obstructive hydrocephalus are shunted using Chabbra medium pressure tube as a first stage. This helps in reducing the intracranial tension, and prevents post operative CSF leak due to increased intracranial pressure. This can be done few days earlier to surgery.

Pre-operative preparation:

Pre-operative steroids are   advocated by many. I have   practiced   giving steroids only per-operatively and post-operatively for a  short  duration. The previous night of surgery, broad spectrum antibiotic is given along with sedation.

On the day of surgery, 45 minutes  before surgery, premedication is given using  Pethidine 100gms IM and Atropine 0.6 mgs I.M. 

At the time of inducing anesthesia, another dose antibiotic is   given I.V.  Minimum 3  IV  lines  are  maintained, of which,  one is  a central   line (CVP). Indwelling Foley's catheter is inserted and crepe bandage applied for both lower limbs.

Monitoring of the patient:

Continuous ECG, PaO2 NIBP monitoring is done. Electrophysiological monitoring of facial nerve function if available is useful.

The stethoscope with a long tube is attached and fixed with dynaplast to the chest over pre-cardial  region for auscultation of the heart by the anesthetist  for  any  air  embolism is practiced in our unit.  

Positioning:

Though various positions are employed, such as semi sitting, lateral or sitting position, surgeon's preference should be taken into consideration for appropriate positioning of the patient. I usually do all patients in sitting posture with head fixed with 3 pin fixations.

Both legs should be kept slightly flexed at the knee level as keeping them straight may cause stretching of sciatic nerve and post   operative sciatic pain.

Adequate side support on the sides with pillows and one pillow just below the knees are kept to make the position comfortable to the patient and support the patient adequately in sitting position.

It should be made sure that no part of the body comes in contact with any metallic part of the operation table as this may cause electric shock/electric burn while using monopolar diathermy.

Also due attention paid while flexing the neck under anesthesia - 2 fingers should  be  kept  between the chin  and the chest and the head is turned to the ipsilateral side of the tumor for about 20 to 30 degrees, so that, when the surgeon  approaches the tumor, his vision is in line with the tumor/brainstem.

Those elderly patient who do not tolerate sitting posture due to fall in BP, surgery can be done in lateral position. Before making the patient sit up slowly, about 1500cc of fluids should be given IV so that there is no fall in BP while making the patient sits up. Adequate time should be taken to make the patient sit up with periodical BP monitoring.

Iodine solution is used liberally. Initially iodine scrub solution is used and the operating site is scrubbed for a minimum period of 5 minutes. This is followed with another preparation with 10% iodine solution.

Procedure: click for intraoperative video clippings

It is always advantageous to have an accessibility to the ventricle post operatively and also for patients who undergo surgery in sitting posture to prevent pneumocephalus. It is better to have a burr hole before proceeding for tumor excision. A 2 cm horizontal skin incision is made 7 cm above and 3cm lateral to the EOP on the ipsilateral side of the tumor. The burr hole is done and the dura is opened after cautery in a cruciate manner. Pia is also cauterized and opened. Hemostasis achieved and the wound is closed in single layer.

After adequate relaxation of the cerebellum, it will be possible to retract the cerebellar surface easily. A broad lint or soft roll or thin rubber sheet from the gloves may be used to cover the cerebellar surface and the retraction to the cerebellum is gently applied to expose the surface of the tumor.

Most often, there is thin layer of arachnoid seen covering the tumor. The good result of the surgery is achieved by maintaining the integrity of the arachnoid layer. As long as it is maintained, it is easy to protect the nerves and vessels encountered.

Debulking may be done using bipolar coagulation, scoop or CUSA if available. While using CUSA, one should be careful, not to try to debulk the tumor too fast or take a CUSA probe too much anteriorly as it may suck in the facial nerve. If one is careful, then the CUSA can be used advantageously and reduces a lot of operating time.

After adequate debulking using fine dissectors, it will be possible to separate the capsule from the arachnoid.

Most of the time, the tumor is not adherent to the brainstem. But the arachnoid layer can be missed. This is the situation where difficulty will be encountered in separating the tumor from the brainstem. Also small nubbin of the tumor may be indenting the brainstem and there will be difficulty in separating such nubbins from the brainstem, if the arachnoid layer is not intact. Also if arachnoid is breached, it will be difficult to separate and preserve the veins going into the brainstem. Hence one has to be careful in separating the arachnoid layer and preserving it intact.

After debulking the tumor, the capsule of the tumor is lifted from below upwards exposing the lower cranial nerves. The nerves can be easily separable from the tumor capsule by fine dissectors and protected by covering with lint pieces. At this stage, one can see whether there is any blood supply from the vertebral to the tumor or from AICA. Large branches from inferior cerebellar artery are sometime embedded in the tumor capsule which can usually be dissected free by dividing the small branch directly supplying the   tumor. 

Subsequently the dissection carried out superiorly separating the greater petrosal vein from the tumor surface. Sometimes it may be necessary to cauterize and cut this vein and it does not cause any undue sequelae. While separating the arachnoid superiorly, the trigeminal nerve will come into view. It can easily be identified by its thickness. This could easily be separated from the tumor surface, using fine dissectors.

Periodical debulking and excision of the capsule is done so that adequate space is created and the dissection is made easy.

While separating the tumor medially from the brainstem, the DREZ zone of the V, VII, and VIII nerve complex will be encountered. 

I do not use gelfoam over the  brainstem or the nerve roots to get hemostasis as they may cause post-op complications. 

After hemostasis, a bit of fat and fascia are kept within IAM to prevent CSF leak. Then the dura is closed using 5.0 prolene. If dura is not closed, it is better to harvest pericranium or fascia lata and cover the dural defect to prevent CSF leak post operatively. The bone bits collected during craniectomy and bone dust while doing burr hole are sandwiched between split gelfoam and replaced extradurally and wound is closed in 4 layers. A Ryle's tube is passed before extubation and the tube is kept for few daystill the swallowing is tested.

Post operative management:

Constant vigil is kept over patient’s conscious level. If the level of conscious deteriorates, immediate CT scan is done to rule out post operative hematoma which may need immediate evacuation.

Occasionally CSF leak may occur, if the dura is not completely closed leaving behind a small hole which may act as ball valve. If dura   could not be closed it is better to leave the dura totally open and not to close partially. This will prevent CSF leak. Sometime, there may be CSF leak due to local collection which may stop in a day or two.

With those patients who have not undergone shunt surgery, one has to keep an eye for the development of hydrocephalus which may  need shunting.  

Meningitis though not common, may develop and needs immediate lumbar punctures to drain CSF and broad spectrum antibiotics   coverage. Post operatively, all patients are kept in tapering dose of steroids (4mgs of Dexamethozone twice daily for 3 days and  gradually  reduced over a period of three days and then stopped).

Even, if facial nerve is preserved anatomically, there is likely development of facial nerve paralysis. This may need torsorraphy to protect cornea and prevent exposure karatitis. 

In elderly patients, active chest physiotherapy and early mobilization are necessary to prevent DVT and pneumonia.

In our experience, 95% of the tumor can be excised totally and another 5% radically. Facial nerve preservation is possible only in 60 to 70% of the cases as the tumors encountered are very large and the facial nerve complex is splayed out. In these circumstances, it is difficult to preserve the facial nerve anatomically.

 Translabyrinthine approach:

This approach, discussed elsewhere, may be used in larger tumor with no hearing. The goal is to preserve the facial nerve.

 Middle fossa approach:

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. The goal is to preserve hearing.