Surgery for Acoustic Neuromas

Elizabeth H. Toh

There are three basic surgical approaches for resection of acoustic neuromas. The approach selected is dependent on tumor location, tumor size, and hearing status of the affected ear. The middle fossa and retrosigmoid approaches offer the potential for hearing preservation, whereas the translabyrinthine approach necessarily sacrifices any residual hearing in the affected ear. Surgery is usually performed with neurosurgical support for intracranial tumor dissection.

• Surgical removal is the preferred treatment modality for large acoustic neuromas. For small growing tumors, surgery offers the option for potential long-term hearing preservation with complete cure.
  
• Hearing preservation approaches are offered if the patient has any aidable residual hearing in the affected ear (pure tone average threshold better than 50 dB and word recognition score over 50%). Strict audiologic criteria for useful hearing may vary with different centers.
  
• Treatment alternatives, including observation with serial imaging, stereotactic radiation, and, occasionally, tumor decompression only for (neurofibromatosis 2[NF2] patients) should be extensively discussed with the patient prior to surgery.
  
• The overall health status, age, and preferences of the patient should be taken into consideration, along with tumor characteristics, in selecting the best treatment modality for each individual patient.

• A complete pure tone and speech audiogram should be obtained prior to surgery. Although auditory brainstem response testing may be helpful in prognosticating postoperative hearing, it is not routinely performed.

• A gadolinium-enhanced magnetic resonance imaging (MRI) scan of the internal auditory canals is the gold standard for diagnosing acoustic neuromas. Care should be taken to review the unenhanced T1 images to rule out the rare lipoma.
  
• Additional MRI of the entire spine should be routinely performed in all NF2 patients.

• Caloric testing may be helpful in predicting postoperative dizziness. The greater the extent of reduced vestibular response in the affected ear, the less likely the patient is to be dizzy after surgery. This, however, assumes a superior vestibular nerve origin for the tumor, which may not always be the case.

• Surgery is performed under general anesthesia, with the use of muscle relaxants limited to the initial induction phase only.
  
• Intraoperative facial nerve monitoring is routinely used. Brainstem evoked audiometry responses are monitored in hearing preservation approaches only. The additional monitoring of somatic sensory evoked potentials is optional for small tumors but helpful with surgery for larger tumors.
  
• The middle fossa and translabyrinthine approaches may be performed without the use of a Mayfield holder and pins. The retrosigmoid approach is usually performed using head pins for positioning.
  
• The left lower quadrant of the abdomen is also prepared and draped at the beginning of the procedure to harvest an abdominal fat graft.
  
• Perioperative medications administered at the time of anesthesia induction include broad-spectrum antibiotics, steroids, and diuretics (Lasix/mannitol).
  
• A Foley catheter, nasogastric tube, and Venodyne boots (Microtek Medical Inc., Columbus, mississippi) are used for every case.
  
• Bacitracin irrigation is used only after bony dissection has been completed because bothersome foaming will occur if used in the irrigation fluid for drilling.

• Any size tumor with no useful hearing in the affected ear.
  
• Larger tumors filling the internal auditory canal (IAC) and extending more than 1 cm into the cerebellopontine angle (CPA), regardless of hearing status, because hearing preservation is unlikely using any approach in such cases.