Acoustic Neuroma

History

• 1.

  History of present illness

  • a.

    Gradual hearing loss

  • b.

    Five percent of AN patients present with sudden hearing loss.

  • c.

    Asymmetric hearing loss should be assumed to be an AN until proven otherwise.

  • d.

    Vestibular symptoms are usually motion intolerance rather than acute vertigo.

  • e.

    Facial nerve dysfunction is rarely a presenting symptom in AN.

  • f.

    Patients with a large AN may have cranial nerve dysfunction leading to hypoesthesia (CN V), diplopia (VI), dysphagia (CN IX, X), hoarseness, and aspiration. Such tumors can cause increased intracranial pressure leading to headaches and papilledema.

• 2.

  Past medical history

  • a.

    Previous schwannoma or meningioma: Neurofibromatosis-2 (NF-2) is an autosomal dominant but a previous AN on the contralateral side or meningioma or a family history of NF-2 requires thorough evaluation. NF-2 management is different from sporadic AN management, which is the subject of this chapter.

  • b.

    Medical illness

  • c.

    Surgery

  • d.

    Family history

  • e.

    Medications, vitamins, and supplements

Physical Examination

In addition to a complete head and neck examination,

• 1.

  Microscopic otoscopy: A tympanic membrane perforation raises the risk of postoperative cerebrospinal fluid (CSF) otorrhea and the possibility of an inflamed mastoid, which would affect the selection of a transtemporal approach.

• 2.

  Cranial nerve: A systematic examination of all nerves (especially VII)

• 3.

  Nystagmus: Bruns’ nystagmus in large tumors with brainstem compression reveals coarse, slow-beating nystagmus on ipsilateral gaze (brainstem induced) and fine beating nystagmus on contralateral gaze (vestibular induced).

• 4.

  Vestibular examination: Gait testing, Fukuda stepping test and head thrust tests identify vestibulopathy.

Relevant Testing

• 1.

  Audiogram: Air, bone, and speech audiometry objectively document hearing levels. Fifty decibels Speech Response Threshold (SRT) (or Pure Tone Average (PTA)) and 50% speech discrimination are the lower limit in considering hearing preservation surgery.

• 2.

  Vestibular: AN patients undergo a complete vestibular testing battery that includes ROTO (rotational chair), VEMP (vestibular evoked myogenic potentials), and VNG (videonystagmography). Documentation of pretreatment vestibular function is a baseline for future comparison and patient education about postoperative symptoms and a guide for perioperative vestibular physical therapy.

• 3.

  Facial: A preoperative electroneuronography (ENoG) abnormality identifies preoperative facial nerve compromise.

• 4.

  ABR (audiometric brainstem response) is used in large lesions as a baseline for intraoperative contralateral brainstem monitoring during surgery and to assess the preoperative status of the cochlear nerve if hearing preservation surgery is being considered.

Imaging

• 1.

  MRI with gadolinium of the IAC is the gold standard for diagnosing AN. The sensitivity approaches 100%; however, false positive results are possible.

• 2.

  T2 sequences can identify the nerve of origin of the tumor and whether there is a cap of CSF at the fundus that may facilitate preservation of hearing in an MF or RS approach.

• 3.

  CT with contrast is not as sensitive as MRI. Currently MRI with contrast can identify lesions as small as 1 to 2 mm.

• 4.

  CT combined with ABR: A recent meta-analysis found that pooled sensitivity for ABR was 82.0% for all tumors, and for tumors greater than 1 cm this rose to 95.6%. For patients that cannot complete MRI, ABR with CT may offer an alternative means of diagnosis.

• 5.

  CT without contrast is used for surgical planning in all three surgical approaches. For TL surgery, the mastoid pneumatization, distance between the sigmoid and the IAC, and the height of the jugular bulb relative to the IAC are the important features. For the MF approach, the relationship between the arcuate eminence and superior canal (SSC) and the pneumatization over the IAC are important. In RS surgery, the pneumatization of the petrous bone, position of the sigmoid sinus, jugular bulb, and vertical height of the IAC are important features.

Indications

• 1.

  Treatment strategy: Patients with AN are offered three management options: observation, SR, and microsurgical resection. The surgeon individualizes recommendations in every case.

• 2.

  Observation (“watch and scan”) is increasingly used as initial tumor management for AN. A central tenant of skull base surgery is to avoid any additional morbidity with treatment, which is greater than what would be expected with the natural course of the disease. The Denmark experience reveals that within the first 5 years, 17% of intrameatal tumors and 28.9% of extrameatal tumors grew, while 45% lost class A hearing after 5 years regardless of tumor growth. The critical issue with observation is that it this strategy requires close follow-up, as we are not currently able to predict which tumors will grow.

• 3.

  SR for AN includes stereotactic radiosurgery, which is a one-time treatment, and hypofractionated SR (radiotherapy) in which the treatment is divided into three to five doses. Based on Lars Leksell’s innovations, the goal of SR is prevention of tumor growth to preserve neurologic function. The initial high-dose treatment plans (16 Gy marginal) have been replaced with lower dose 12 Gy single dose or 18 Gy fractionated with similar tumor control rates as previous regimens but with less risk of cranial neuropathy or brainstem complications. Since no pathologic specimen is obtained, SR risks radiating a malignant lesion with inadequate dosage; close follow-up is essential. Special considerations in radiation include the following:

  • a.

    Control rates: Control in growing tumors is approximately 80% to 85% versus 90% to 95% when all lesions are included.

  • b.

    Radiation-induced malignancy: There have been eight cases of radiation-induced malignancy confirmed with microsurgical resection following stereotactic radiotherapy documented in the literature.

  • c.

    Hearing preservation: Initial hearing preservation deteriorates over time—only 20% retain useful hearing after five years in the Mayo series.

  • d.

    Type of radiation: Gamma Knife versus CyberKnife versus other LINAC systems:

  • In our practice, only tumors with documented growth are radiated, and younger patients are typically not radiated. We use CyberKnife or Gamma Knife Icon because it is a frameless stereotactic system that permits fractionation with high precision.

• 4.

  Microsurgical resection: The only curative treatment for AN is microsurgical resection. Patients who undergo gross total resection (GRT) benefit from excellent tumor control, management of mass effect, pathologic diagnosis of benign schwannoma, and less intense imaging regiment postoperatively. The best results are obtained at skull base centers that treat a high volume of AN. Tumors may be managed by one of three lateral skull base approaches: TL, MF, and RS craniotomy. Age, hearing status, tumor size, and location all play a role in determining the approach for each patient.

  • a.

    TL: Popularized by the pioneering work of Drs. William House and William Hitselberger, TL is the prototypical neurotologic, collaborative skull base surgical approach.

    • 1)

      TL sacrifices hearing as a part of the approach.

    • 2)

      TL is recommended for patients without serviceable hearing and in cases where hearing preservation is unlikely.

    • 3)

      The primary advantages of TL are little to no brain retraction, reliable identification of the facial nerve at the meatal foramen, and no intradural drilling, thus reducing the risk of postoperative headache.

  • b.

    RS

    • 1)

      Hearing preservation: The modern RS is a versatile approach allowing for a wide view of the posterior fossa with hearing preservation for tumors with limited lateral extension into the IAC. MRI T2 sequence imaging can determine the extent of tumor penetration into the IAC.

    • 2)

      Hearing preservation with posterior fossa extension: One advantage of RS over MF is that RS allows for hearing preservation in instances where tumors extend more than 0.5 cm into the CPA.

    • 3)

      When hearing preservation is not an option, the decision between RS and TL for medium and larger tumors is the surgeon’s preference. The authors usually select TL due to less brain manipulation than with the RS approach.

  • c.

    MF approach:

    • 1)

      Hearing preservation: MF is an extradural subtemporal technique to access the IAC and is used for hearing preservation with small laterally based lesions.

    • 2)

      Limitation: The major limitations to MF are that it provides poor access to the posterior fossa and requires temporal lobe retraction. The addition of temporal lobe retraction adds a minimal risk of seizure, aphasia, and stroke specifically to this approach, and caution should be used in elderly patients due to age-related changes in the dura. Depending on the nerve of origin of the tumor, the risk of facial paresis may be slightly greater in the short term with the MF when compared to other approaches; however, by 1 year the difference is not significant.

• 5.

  Treatment selection

  • a.

    Patient education: Detailed information is presented regarding all treatment strategies.

  • b.

    Risks:

    • 1)

      Observation: Patients are educated that their tumor may grow, and there is no good means of determining which tumors grow prior to observation, that hearing may deteriorate, and that they may still require either microsurgical resection or stereotactic radiotherapy.

    • 2)

      Surgery: Specific risks include bleeding, infection, facial paralysis, hearing loss, balance disturbance, CSF leak, stroke, prolonged headache, need for tracheostomy, gastrostomy, and locked-in syndrome or death.

    • 3)

      Informational materials: Patients are provided with a booklet, recommendations for web-based link(s), and a book chapter on CPA tumor management as reliable resources for them to refer to after their appointment.

    • 4)

      Radiation: Patients are advised that the risks are similar to surgery but may take years to manifest. Risks specific to radiation include radiation-induced hydrocephalus, hearing loss, facial nerve dysfunction, and the risk of radiation-induced malignancy.

Contraindications

• 1.

  Contraindications to microsurgery are mainly related to medical issues. If a patient cannot tolerate general anesthesia, then the tumor is observed; and if it demonstrates growth, stereotactic radiotherapy is offered with possible CSF diversion for hydrocephalus.

• 2.

  For patients with tumors that reach or compress the brainstem, microsurgical resection is offered in place of stereotactic radiotherapy or observation. Tumor swelling is common for 18 months following SR. Accordingly any brainstem or cerebellar compression identified prior to treatment contraindicates radiation.

Preoperative Preparation

• 1.

  See “ Relevant Test ” section for audiologic and imaging requirements.

• 2.

  Cessation of anticoagulants

• 3.

  Preoperative consultation for medical clearance

Operative Period