Acoustic Removal Posterior Fossa

Acoustic Neuroma (also known as: Vestibular Schwannoma) is a tumor that can affect hearing and movement of the facial muscles.

Many of the complications of acoustic removal by a posterior fossa approach and fusion are common to all surgeries of the posterior fossa:

Table: Neurosurgical Risks & Complications

 

Complications particular to acoustic removal by a posterior fossa approach include:

 

 

facial nerve injury

 

A collection of blood at the site of anterior neck dissection for anterior cervical discectomy has bled from any of a number of candidate arteries and veins that might have been encountered in dissecting through the neck to the vertebral column. 

 

Surgical anatomy of small vestibular schwannoma shows proximity of 7th and 8th nerves to tumor making them vulnerable to injury during tumor resection

meningitis

Post operative infections of the meninges following resection of an acoustic neuroma result from iatrogenic contamination of the cerebrospinal fluid either directly during surgery, or indirecly by creating a defect through an infection defense barrier, providing a route of entry for organisms that invade the  meninges inciting an immune inflammatory response associated with vasculitis and infarction. 

Delayed post operative:  

Meningitis complicating resection of a vestibular schwannoma (acoustic neuroma) is typically not manifest clinically (CONSEQUENCE below) for at day 

Delayed:  

Meningitis complicating resection of a vestibular schwannoma (acoustic neuroma)

Meningitis complicating resection of a vestibular schwannoma (acoustic neuroma) Meningitis can result when bacteria gain entry into the cerebrospinal fluid through an opening in the dura mater.   

Adequate closure is important to prevent persistent CSF leak and meningitis.

Patient — physiology

Patient — disease:  

 

If the operating room is not sterile infecting organisms may get into the CSF on instruments or from the surgical field. 

Post-operative meningitis complicating resection of a vestibular schwannoma (acoustic neuroma) may be of no clinical significance whatsoever.  Some post op patients whose CSF is sampled at the time of placement of a lumbar drain (to prevent or stop CSF leakage), have evidence of ongoing infection (bacteria, white blood cells) on microscopic examination of the fluid but are totally asymptomatic — without the headache, stiff neck, and fever that clinically characterize meningitis.

Intraoperative and acute postoperative:  Antibiotics are usually given intravenously just before, during, and immediately after (for up to 72 hours) surgery to prevent meningitis complicating resection of a vestibular schwannoma (acoustic neuroma) as part of routine prophylaxis.  They should definitely be given if a patient has symptoms (headache, stiff neck) and/or signs (fever, nuchal rigidity, Kernig or Brudzinski) of meningitis post-operatively.

The frequency of meningitis complicating resection of a vestibular schwannoma (acoustic neuroma) is low because of sterile technique in the operating room that prevents contamination and use of perioperative antibiotics that can prevent and eliminate most of the common bacteria (Staph aureas  is number 1)  that cause post-craniotomy meningitis.

Peri-operative (pre-, intra- and post-operative) antibiotics may help prevent meningitis complicating resection of a vestibular schwannoma (acoustic neuroma).

“Water tight” dural closure is also important to close a potential pathway for entry of skin and other external environmental bacteria into the area of tumor resection.   

 

Vestibular schwannoma (also known as “acoustic neuroma”) is a benign tumor that arises from the 8th cranial nerve and grows into sizeable masses (2, 3, or more centimeters in diameter) in the cerebellopontine (CP) angle.  Complications associated with resection of vestibular schwannomas include injury to the facial nerve, cerebrospinal fluid (CSF) leak, and meningitis. 

 

 

facial nerve injury

Acoustic neuromas (AKA: vestibular schwannomas) are tumors of the vestibular portion of the vestibulocochlear (8th cranial) nerve.  The vestibulocochlear and facial (7th cranial) nerve travel together from the brainstem, through the subarachnoid space in the cerebellopontine angle to the internal acoustic meatus (the site at which vestibular schwannomas (acoustic neuromas) arise.  During dissection of the tumor from the vestibular portion of the 8th nerve this nerve is frequently injured or destroyed.  In dissection of larger tumors it may be impossible to completely spare the 7th nerve from trauma and injury as well.

Intraoperative:  Facial nerve injury during acoustic removal may be recognized intraoperatively by inspection of the nerve — after a long dissection of a large tumor from an already severely thinned out and tenuous facial nerve, as the last of the tumor is removed two stumps on either side of a gap may be all that remains.   functional deficit can result from manipulation injury to a facial nerve that visually remains intact by facial nerve monitoring.  Decreased transmission of electrical signal through the nerve triggers an alarm that alerts the surgeon that something he or she is doing is compromising the nerve.

Immediate post operative:  Visual inspection and intra-operative monitoring monitoring do not detect all surgical facial nerve injury, however.  A mild or moderate facial weakness (House-Brackmann Grading system) can be detected immediately post-operatively in some patients whose nerves were intact to inspection and by monitoring throughout the course of their vestibular schwannoma resection.

Facial nerve injury during acoustic removal is less likely with smaller tumors and when the operating surgeon has greater experience and expertise in the dissection and removal of these lesions.

Selection of dissection instruments and strategies are important for optimal outcome (minimizing injury to the facial nerve). 

Experience with the nuances of dissection in the cerebellopontine angle and with the complex anatomy (brainstem, cranial nerves [5, 7, 8], arteries [superior inferior cerebellar, internal auditory, labyrinthine, etc.] also minimize the risk of injury. 

Facial nerve injury during acoustic removal may be related to the anatomic relationship between the tumor and the facial nerve.

Dissection and removal of larger tumors is more likely to result in facial nerve injury.  Facial nerve injury is also more likely with dissection of vestibular  tumors that are more densely adherent to adjacent tissues (such as the facial nerve).

Neurologic deficit:  The severity of the facial nerve injury (House-Brackmann Grading system) following removal of a vestibular schwannoma is variable and depends on such factors as the size and extent of the tumor, the condition of the nerve preoperatively, and the experience and skill of the surgeon.  Some patients have a deficit that is detectable only by careful observation of facial movements, some have no movement whatsoever.  

Not all deficits are permanent.  Some will last only a few days or weeks, some are permanent.  In general, deficits seen after a procedure during which there was no transection or major manipulation of the nerve, and only minimal activity by the facial nerve monitor, are most likely to resolve over a few days to weeks.  

The degree to which a facial nerve deficit resolves following injury is also variable and multfactorial making confident prediction of full recovery, particularly in cases with large tumors and difficult dissections,  problematic.

Prolongation of surgery:  facial nerve injury during acoustic removal  

Reoperation:  If facial nerve weakness is severe (House-Brackmann Grading system) a surgical reanimation of the face joining the proximal end of an intact cranial nerve (such as number 12 — hypoglossal) may be tried.  

Intraoperative:  

If it it recognized intra-operatively that the facial nerve has been divided, it is possible to perform a hypoglossal (12th cranial) nerve to distal facial nerve anastomosis connecting the distal segment of the facial nerve to the proximal portion of the hypoglossal nerve.

Some injuries occur without obvious disruption of the nerve.  Steroids are administered intra- and immediately post-operatively to try to protect the nerve from injury due to manipulation that bruises but does not divide it.

Immediate post operative:  facial nerve injury during acoustic removal  

Long term post operative:  

Lost facial movement on one side of the face can sometimes be restored by creating a connection (anastomosis) between the proximal end of an intact nearby nerve and the distal end (beyond the site of injury) of the facial nerve.  The most common anastomosis is between the hypoglossal (cranial nerve #12) — “hypoglossal-facial anastomosis”; but spinal accessory (cranial nerve #11) and phrenic to facial anastomosis are also used.

The frequency of facial nerve injury from vestibular schwannoma resection is 

Monitoring:  Facial nerve injury during acoustic removal  can sometimes be avoided or mitigated by intra-operative facial nerve monitoring.

Planning:  Facial nerve injury during acoustic removal is less likely when the surgeon has more room in which to maneuver around the tumor.  A translabyrinthine approach gives better exposure of the tumor and facial nerve but requires complete destruction of the hearing apparatus.  For patients already deaf or with only minimal hearing at the time of surgery, translabyrinthine offers a better chance of facial nerve preservation with no significant additional functional neurologic loss.   

Technique:  Facial nerve injury during acoustic removal  is more likely when tumors are large, dissections are difficult, and surgeons are inexperienced.

 

cerebrospinal fluid leak

Cerebrospinal fluid (CSF) is encountered once the dura is open to expose the vestibulocochlear nerve and the tumor growing in it.  The dura must be closed in order to prevent leakage of CSF locally and potentially through the incision or into the mastoid air cells.

Immediate post operative:  

A cerebrospinal fluid leak following acoustic neuroma resection is usually not noticed until at least several hours post-operatively — it takes this long for the CSF that was drained in the course of the operation to reaccumulate (at 1/3 “cc” per hour (hyperlink to physiology of CSF) and refill the CSF space of the cerebellopontine angle which is the usually site of the leak.  These leaks that occur immediately upon re-accumulation of physiologic volumes of CSF occur in areas where the dura remains open (was not closed) during the closure.  If there has been a closure of the dura the pressure behind it must be supraphysiologic (higher than normal) for CSF to lead past the dura into the scalp incision, out onto the skin as a clinically apparent leak.  

Delayed post operative:  

A leak may be delayed in onset beyond the time cerebrospinal fluid leak acoustic neuroma resection

Delayed:  Cerebrospinal fluid leak complicating acoustic neuroma resection may not be manifested for several days or even weeks post-operatively due, presumably,  to breakdown over this time period of the dura closure.

Cerebrospinal fluid leak following acoustic neuroma resection primarily occurs following retromastoid and translabyrinthine approaches.

Cerebrospinal fluid leak following acoustic neuroma resection becomes life threatening only if a secondary meningitis develops.

Neurologic deficit

Reoperation may be necessary to identify and close a hole in the dura through which CSF is leaking.

Management of cerebrospinal fluid leak following acoustic neuroma resection in the acute post operative period includes:

Head elevation

Spinal tap

Lumbar drain

Ventriculoperitoneal or lumboperitoneal shunt.

Antibiotics are usually part of the management of post op CSF leak following resection of acoustic neuroma, not for any effect they might have on the leak because they help to decrease the incidence of meningitis associated with CSF leakage.

The frequency of cerebrospinal fluid leak following acoustic neuroma resection

Pharmacology:  

Antibiotics are usually part of the management of post op CSF leak following resection of acoustic neuroma, not for any effect they might have on the leak because they help to decrease the incidence of meningitis associated with CSF leakage.

Technique:  Closure of the dura so that it is “water tight” (no cerebral spinal fluid leaking through the closure) is essential to prevention of cerebrospinal fluid leak following acoustic neuroma (vestibular schwannoma) resection.  CSF leak prevention will help prevent infection of the CSF post operatively by spread of a wound infection of the skin and scalp.

Equipment:

Sterile technique is important  Instruments and equipment used in the surgical field must be sterilized,  Surgeons must wear sterile gloves over adequately scrubbed hands.  Hats, masks