Preservation of Useful Hearing in Resection of
Acoustic Neuroma (Vestibular Schwannoma)

by Robert G Ojemann, M.D. and Joseph B. Nadol, Jr., M.D.
Stephen B. Tatter,M.D., Ph.D. , HTML editor

The introduction of computerized tomography and magnetic resonance imaging has made possible the diagnosis of small acoustic neuromas. This as well as refinement in microsurgical technique have made consideration of hearing preservation during acoustic neuroma resection feasible. Success in preservation of usable hearing has been reported by the suboccipital and middle fossa approaches in a number of centers.

Several important questions remain unresolved. Can truly useful hearing be preserved often enough to make this worthwhile, particularly in unilateral cases? Are there preoperative clinical indicators that can identify the likelihood of success in a given patient? How helpful is intraoperative neurophysiologic monitoring? Does an attempt to preserve hearing compromise tumor excision, making recurrence more likely? Is initial success in preserving measurable hearing rewarded by long term preservation?

In a recent study at the Cranial Base Center of 144 cases of patients who underwent total removal of unilateral acoustic neuroma via the suboccipital approach with an attempt to preserve auditory function, some insight on these questions was achieved. All cases were done by the suboccipital approach by a team including neurosurgeon, neurotologist, and neurophysiologic monitoring using intraoperative evoked potentials and facial nerve electromyography. Graphic display of pre- and post-operative hearing as measured by speech discrimination scores is displayed in Figure 3 for the most recent series of 78 cases. As can be seen in 12 patients (15%) postoperative hearing was within 15 percentage points of preoperative levels. In 4 patients (5%) hearing was actually improved. In the entire series, usable hearing is defined by a postoperative speech discrimination score of 15% or better and was achieved in 25 patients or 32% of the total.

Figure. Preoperative and postoperative speech discrimination scores in the current series of 78 cases. The central line corrresponds to parity of preoperative and postoperative scores; the other parallel lines indicate +/-15 percentage point change. Thus, all cases to the left of the central line represent postoperative decrenient in speech discnmination score and all cases to the right of the central line represent improvement in speech discrimination score.

Logistic regression analysis demonstrated that there were clear cut predictors of success. The most significant predictor of preservation of useful hearing was a relatively small tumor and relatively high preoperative speech discrimination score. Thus for tumors that extended less than 5 millimeters into the posterior fossa, hearing was unchanged or better in 41% of cases and deemed useful in 50% of cases.

lntraoperative monitoring of evoked potential provided reliable prognostic data concerning hearing outcome and in the judgement of the operating surgeons was of use in the attempt to save hearing. Thus, when wave V was unchanged at the end of the resection, even if it had been transiently lost during surgery, useful hearing was invariably preserved. In 4 patients who enjoyed significant improvement in hearing (greater than 15 percentage point increase in speech discrimination) no changes were detected in intraoperative evoked potentials that would have predicted this outcome. The strong positive correlation between small tumor size and preservation of hearing was likewise observed in preservation of facial nerve function. Thus, in cases where tumor size was 5 millimeters or less 100% had normal facial function at one year, whereas in those with tumors exceeding 25 millimeters of extension to the posterior fossa, 75% had normal facial function at one year. In the overall group of 78 patients, 90% of patients had normal facial function at one year.

In a recent follow up of a subset of these patients in whom hearing was initially preserved, stable hearing was achieved in 78% whereas further deterioration of hearing was found in 22%, with a mean follow-up period of 5 years.

To date, there is no evidence that an attempt to save hearing predisposes the patient to an increased risk of clinical recurrence of tumor. Our current results support the notion that an attempt to save hearing, particularly in small acoustic neuromas, is worthwhile and safe for patients.

References:

1. Nadol JB,Jr, Chiong CM, Ojemann RG, McKenna MJ, Martuza RL, Montgomery WW, Levine RA, Ronner SF, Glyrin RG: Preservation of hearing and facial nerve function in resection of acoustic neuroma. Laryngoscope 102:1153-1158;1992
2. McKenna MJ, Halpin C, Ojemann RG, Nadol JB Jr., Montgomery WW, Levine RA, Carlisle E, Martuza R: Long lerm hearing results in patients after surgical removal of aco~stic tumors with hearing preservation. Am J Otol 13:134-136;1992.
3. Ojemann RG, Levine RA, Montgomery WW, McGaffigan P: Use of intraoperative auditory evoked potentials to preserve hearing in unilateral acoustic neuroma removal. J Neurosurg 61: 938-948, 1984
4. Ojemann RG: Management of acoustic neuroma (Vestibular schwannoma) Clin Neurosurg 40:498-533, 1993.

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