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What is Acoustic Neuroma Surgery?

There are three basic surgical approaches currently used for removal of the acoustic neuroma. The middle fossa exposure allows a view of the lateral end of the internal auditory canal and, thus, total removal of a smaller acoustic neuroma with a chance for preservation of hearing. The suboccipital exposure, which is used by many surgeons to treat all sizes of acoustic neuromas, does not enter the inner ear unless drilling violates the posterior semicircular canal. This semicircular canal, however, prevents viewing of the lateral end of the internal auditory canal. The translabyrinthine exposure always destroys remaining hearing, if any. However, this technique allows dissection of the acoustic neuroma from the facial nerve in the internal auditory canal and allows a direct seventh-seventh nerve attachment when necessary if the facial nerve is cut during removal of the acoustic neuroma. In addition, the anterior brainstem region can be viewed and retraction of the cerebellum is minimized when the translabyrinthine approach is used for removal of the acoustic neuroma.

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Acoustic Neuroma Surgery

History

In 1977 Maddox described the lateral approach to the acoustic neuroma REFERENCE. The highlights were:

  • The facial nerve was preserved in 88% of the patients using this approach for removal of the acoustic neuroma and the mortality rate was 2%.

  • The most common complication of surgical removal of the acoustic neuroma was cerebrospinal fluid leakage.

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Surgical Approach vs. Size of Acoustic Neuroma

Similarly, Thomsen and colleagues REFERENCE examined the choice of approach in surgery for the acoustic neuroma.

  • All acoustic neuromas measuring 25 mm or more on MRI were operated via the translabyrinthine approach.

  • All patients with PTA (pure tone average) poorer than 30 db, and SDS (speech discrimination score) poorer than 70% were operated via the translabyrinthine approach.

  • Acoustic neuromas less than 10mm extrameatally, and PTA (pure tone average) better than 30 db and SDS (speech discrimination score) better than 70% were removed via the middle fossa route.

  • Acoustic neuromas measuring 10-25 mm and PTA (pure tone average) better than 30 db and SDS (speech discrimination score) better than 70% were operated via the suboccipital route.

  • Consequences of postponing surgical treatment of the acoustic neuroma were described, demonstrating that 74% of 127 tumors continued to grow with a mean of 3.4 mm increased tumor diameter per year.

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Suboccipital Approach to Acoustic Neuroma:

A straight incision is placed approximately 2 cm medial to the mastoid process (back of ear).
line drawing
The suboccipital (back of the head) muscles and fascia are incised in line with the incision and separated from their attachments to the bone using dissection and electrocautery (electrical tool for dissection) . A burr hole (single hole drilled into the skull) is placed, the dura (fibrous, tough covering of the brain) carefully separated from the overlying bone, and a bone flap cut. This opening exposes the dura over the lateral two thirds of the cerebellar hemisphere and exposes the transverse sinus (large vein at back of brain). Further bone is removed as needed to expose the turn from the transverse sinus to the sigmoid sinus (another large vein) and the edge of the petrous bone (bone containing acoustic and vestibular nerves) laterally. This allows the edge of the sinus (vein) to be retracted, with the sutures placed to hold the dural flaps.

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The distinction between the acoustic neuroma and the facial nerve is often unclear. As is shown, the facial nerve can be inside the acoustic neuroma, rendering visualization difficult. This can result in cutting the facial nerve. For removal, the cerebellum is then gently elevated, the arachnoid is opened, and cerebrospinal fluid is allowed to drain. This allows exposure and visualization of the cerebellopontine angle with minimal retraction. Following placement of the self-retaining retractors, the operating microscope is positioned.

line drawing
The position of the seventh (facial) nerve is confirmed with stimulation. An internal decompression (gutting the inside of the acoustic neuroma, then working around the edges) of the acoustic neuroma may be done using sharp dissection to make the exposure better. Dissection along the facial and cochlear nerves is done with fine straight or curved microdissectors and sharp dissection is done with microscissors. Dissection is alternated from different directions. When the cochlear and facial nerves have been clearly seen, the vestibular nerves coming into the tumor are divided on both the medial and lateral aspects of the tumor. In some patients the lateral end of the tumor may not be exposed because of the limitation in bone removal. In these patients the tumor is transected near the end of the canal and the lateral extent of the tumor is removed.

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Translabyrinthine Approach

The facial nerve is identified in the lateral internal auditory canal at the transverse crest, and the superior and inferior vestibular and cochlear nerves are disarticulated from the lateral canal. The nerves and tumor are dissected in a lateral-to-medial direction with lateral dissection of the facial nerve and intermittent tumor debulking. Frequent facial nerve identification and stimulation for monitoring response is performed throughout the procedure. Once the entire tumor is removed, the dura is approximated with sutures and abdominal fat is packed into the dural opening for a watertight closure. The eustachian tube and antrum are also packed with muscle, fat, or Surgicel.

Advantages of the Translabyrinthine Approach

  • Identification of the facial nerve early in the procedure via a constant bony landmark.

  • Increased preservation of the anatomical and functional integrity of the facial nerve.

  • Minimal retraction of the cerebellum and brainstem.

  • Lower incidence of postoperative headaches than the suboccipital approach.

  • Easy access to the tumor bed after the operation for management of any postoperative complications.

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Facial Nerve

Facial Nerve Preservation

Facial nerve preservation has always been considered to be a primary concern in acoustic neuroma surgery, because the nerve is frequently adherent to the surface of the tumor.

  • Facial nerve anatomical preservation rates reported for the suboccipital approach range from 65 to 89%. However, anatomical preservation of the facial nerve is not the same as functional preservation. For acoustic neuromas, the larger the tumor size, the more difficult it is to maintain the integrity of the facial nerve.

  • Reported rates of functional preservation when using the translabyrinthine approach vary greatly, ranging from 20 to 82%, with the majority of authors reporting rates above 60% and as high as 82%. Similarly, published results for the suboccipital approach range from 56 to 88%.

  • In one study of large AN, facial nerve function was assessed immediately after surgery, at the time of discharge, and at 3 to 4 weeks and 1 year after discharge. Excellent function (House-Brackmann facial nerve Grade I or II) was present in only 55%, 33.9%, 38.8%, and 52.6% of the patients for each time interval, respectively, REFERENCE.

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Hearing Preservation

Hearing Preservation vs. Size of Acoustic Neuroma

Nadol et. al. described the surgical results in 69 patients with unilateral tumors of the cerebellopontine angle or internal auditory canal in whom total tumor removal was accomplished, and in whom an attempt was made to preserve hearing REFERENCE. The success rate of preservation of hearing and facial nerve function was correlated with the size of the tumor.

  • Useful hearing, as defined by speech reception threshold no poorer than 70 db and a discrimination score of at least 15%, was preserved in 73% of cases in which the tumor extension to the posterior fossa was no greater than 0.5 cm.

  • In contrast, useful hearing was preserved in 22% of cases in which posterior fossa extension was greater than 2.5 cm.

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A decade later, Glasscock et. al. presented "A systematic approach to the surgical management of acoustic neuroma." REFERENCE

  • Total removal of tumor was accomplished in more than 99% of patients with a mortality rate of less than 1%.
  • Preservation of hearing was unlikely when the tumor was larger than 2 cm.

When hearing preservation is to be attempted, some surgeons prefer the middle fossa approach to a posterior fossa approach because the retrosigmoid operation leaves tumor behind in the lateral aspect of the internal auditory canal and places the facial nerve at greater risk.

Monitoring

Monitoring of facial nerve function is routine in order to try to preserve the function of the nerve during the operation. The facial nerve function is monitored by continuous recording of electromyographic (muscle twitches after electrical stimulation) activity with two electrodes, one in the orbicularis oculi (orbit muscle of the eye) and the other in the orbicularis oculus muscles (another eye muscle). Monopolar (single lead) stimulation is used to locate the seventh (facial motor) nerve. Fifth (trigeminal: facial sensory) nerve function is monitored with electrodes placed in the masseter and temporalis (muscles for closure of the jaw) muscles.

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Postoperative Complications

Hematoma (Bleeding) and Cerebellar Infarction (Stroke)

During the initial part of the operation attention is focused on occluding the arterial vessels in the muscles as they are encountered. Then during the closure the muscles are again carefully checked for bleeding. A significant blood clot can result with hemorrhage from these blood vessels.

Prior to closure of the dura, the systemic blood pressure is elevated to approximately 140-150 mm Hg to see if it will cause bleeding. Postoperatively the blood pressure is controlled, for as long as necessary.

If the cerebellum is unusually full at the end of the operation and there has been good cerebrospinal fluid drainage, cerebellar stroke or blood clot could be the reason. In this situation a resection of the lateral 1-2 cm of the cerebellum may need to be done.

If the patient does not recover promptly from anesthesia or there is an unexpected significant neurological deficit or delayed neurological worsening, a scan is done immediately to look for cerebellar bleeding or stroke. Prompt removal of a significant blood clot or area of infarction may be necessary.

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Cerebrospinal Fluid Leak

If a cerebrospinal fluid leak develops, a lumbar drain is placed for 72 hours; this often resolves the problem. When the leak persists, an operation through the mastoid using an adipose tissue graft is needed.

In one series for translabyrinthine removal of large acoustic neuromas, the rate of spinal fluid leak was 14%, and the rate of meningitis was 4% (Lanman, Brackman et. al: Report of 190 consecutive cases of large acoustic tumors (vestibular schwannoma) removed via the translabyrinthine approach; J. Neurosurgery April 1999 Volume 90 Number 4).

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Hydrocephalus

Fortunately, persistent hydrocephalus is rare. Most patients recover spontaneously, a few require a temporary lumbar drain, and only occasionally is a ventriculoperitoneal shunt needed.

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Meningitis

When there is postoperative fever with headache or stiffness in the neck, the possibility of either aseptic or bacterial meningitis must be considered. A scan is done to look for an area that might suggest a local infection. A lumbar puncture is then done and administration of broad-spectrum antibiotics is started. Subsequent treatment is guided by the results of the cerebrospinal fluid examination and cultures. If the findings suggest aseptic meningitis, steroids are used.

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Wound Infection

When the infection is superficial and the organism is sensitive to antibiotics, it may not be necessary to remove the bone flap. If the infection is extensive, debridement of the wound and removal of the bone flap is necessary.

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Neurological Complications

If there is any significant postoperative disability the patient is seen by physical and occupational therapists. Difficulty swallowing due to impaired function in the ninth and tenth nerves should be carefully evaluated with a modified barium swallow and followed by a specialist in swallowing disorders. Often the patient can be given instructions that facilitate their swallowing and prevent aspiration. On rare occasion a gastrostomy is needed.

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Headache

After suboccipital operation a percentage of patients complain of persistent headache. The magnetic resonance imaging (MRI) scan rarely shows a structural abnormality such as hydrocephalus. In most patients it seems to be a myofascial problem, in a few a neuroma of the occipital nerve, and in some cervical spine degenerative disease. Many patients are benefited by a program of physical therapy and local blocks as indicated.

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