Preoperative, Intraoperative,and Postoperative AuditoryEvaluation of Patients withAcoustic Neuroma

Roberto A. Cueva, MDa,b

KEYWORDS

� Acoustic neuroma � Preoperative auditory evaluation� Postoperative auditory evaluation � Intraoperative monitoring for acoustic neuroma� Audiometry � Neurotology

PREOPERATIVE EVALUATION OF ACOUSTIC NEUROMABehavioral Audiometry

Because patients with acoustic neuroma (AN) typically present with unilateral senso-rineural (SNHL) hearing loss as their most common presenting symptom, most of themwould already have had behavioral audiometry. This test evaluates the entirety of theauditory system, including the tympanic membrane/middle ear, cochlea, cochlearnerve, dorsal and ventral cochlear nuclei, trapezoid body and its nucleus, superior oli-vary nuclei, lateral lemniscus and its nuclei, inferior colliculus, medial geniculate body,auditory radiations via the posterior limb of the internal capsule, and finally the auditorycortex in the transverse gyri of Heschl. For the purpose of this section, the focus is ontests that may be done as part of the preoperative assessment of a patient with AN.Pure tone auditory thresholds and speech discrimination score (SDS) are the mostimportant factors in preoperative decision making. Even patients with relatively poorpure tone thresholds would remain candidates for attempted hearing-preservationsurgery if their SDS were good enough to allow successful amplification. Traditionallythe 50/50 rule is used as a guideline in this decision making; that is, a patient withhearing equal to or better than 50 dB pure tone average (PTA) and better than 50%SDS may be considered for hearing-preservation surgery. This also, of course,depends on tumor size because the likelihood of hearing preservation is inverselyproportional to increasing tumor size.

a Kaiser Permanente Medical Center, Department of Head and Neck Surgery, 5893 CopleyDrive, San Diego, CA, USAb University of California, San Diego, CA, USAE-mail address: roberto.a.cueva@kp.org

Otolaryngol Clin N Am 45 (2012) 285–290doi:10.1016/j.otc.2011.12.002 oto.theclinics.com0030-6665/12/$ – see front matter � 2012 Elsevier Inc. All rights reserved.

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Auditory Brainstem Response

Some investigators advocate the use of auditory brainstem response (ABR) testing asa way to prognosticate chances for hearing preservation.1 In the absence of a good-quality ABR, some surgeons would advise translabyrinthine surgery because therewould be no ABR signal to monitor during attempted hearing-preservation surgery.On the other hand, those surgeons who routinely use direct eighth-nerve monitoring(DENM) during surgery have found that a cochlear nerve action potential (CNAP) isroutinely recorded even when preoperative ABR is poor or absent.2 Therefore, ABRis thought to be of little utility for preoperative assessment by surgeons usingDENM because it does not influence their decision to attempt hearing preservationor their ability to monitor hearing intraoperatively.ABR is a far-field technique for monitoring sound-evoked electrical activity in the

auditory system from the cochlea/cochlear nerve through the brainstem. Becausesignal amplitude is negatively affected by increasing distance from the source of theneuronal activity, ABR has relatively small amplitudes on the order of tenths of amicro-volt. In addition, averaging of hundreds to thousands of stimulus/response events isnecessary to separate the desired signal from the random background electricalactivity of the brain. Given the necessity of averaging many stimulus/response events,obtaining an ABR can take from 2 to 5minutes. Pure tone thresholds less than 70 dB at2 kHz and more are usually required to achieve an interpretable ABR. In patients withhearing worse than this, waveforms III through V may be of poor quality or absent.However, poor-quality or absent ABR waveforms do not preclude attempted hearingpreservation during surgery because DNEM is often useful in this setting. Since theidentification of ABR as a reliable tool in the assessment of the auditory system,a significant amount of research has endeavored to identify the location of the neuralgenerators for the waveforms.Animal studies investigating ABR, primarily in cats, were conducted in the mid- to

late 1970s.3–5 Correlation of ABR waveforms with their neural generators in humanswas investigated in the mid-1970s and continued into the 1990s by studies lookingat ABR abnormalities related to known lesions of the central nervous system6–8 andstudies using intracranially recorded auditory responses during surgery.9–11 Basedon these investigations, generally accepted neural generators for ABR waves I andII have been established. However, the precise identity of the neural generators forwaves III, IV, and V remains unclear because of conflicting reports from various labo-ratories. Fig. 1 demonstrates a normal ABR with the waveforms labeled with Romannumerals I through V. Table 1 indicates the first 5 ABR waveforms, their typical laten-cies, and their proposed neural generators.12–14 The most important waveformsregarding preoperative and intraoperative assessment of the auditory system inpatients with ANs are I, III, and V. These waveforms correlate with the functional statusof anatomic structures from the cochlea to the inferior colliculus. If these waveformsare present preoperatively then reasonable cochlear nerve integrity can be assumed.Likewise, if these waveforms are present at the conclusion of acoustic tumor surgery,the likelihood of hearing preservation is good.

Otoacoustic Emissions

Otoacoustic emissions (OAEs) in their different variations (distortion product OAE andtransient-evoked OAE) can differentiate cochlear from noncochlear hearing lossesand, thus, may be useful in a retrocochlear screening test battery.15 Although theexpected pattern would be one of poor hearing and intact OAEs, it was somewhatsurprising to discover that more than half of ANs had reduced cochlear function as

Fig. 1. ABR tracing with numbered waves I through V.

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measured by OAEs. It has been shown that OAEs can be preserved in the setting ofsevere cochlear nerve dysfunction.16 Some tumors result in abnormally large OAE,presumably because of loss of efferent suppressions from tumor compression ofthe eighth cranial nerve. Intraoperative assessment of the auditory system is relatedto acoustic tumors because they are a measure of cochlear (outer hair cell) function

Table 1Proposed neural generators of ABR waveforms

Waveforms Proposed Neural Generators Latency (ms)

I Cochlear nerve: modiolus/IAC 1.7 � 0.15

II Cochlear nerve: proximal CPA portion 2.8 � 0.17

III Cochlear nucleus 3.9 � 0.19

IV Superior olive/lateral lemniscus 5.1 � 0.24

V Terminal fibers of lateral lemniscus intoinferior colliculus

5.7 � 0.25

Abbreviations: CPA, cerebellopontine angle; IAC, internal auditory canal.Data from Latency values adapted from Martin WH, Pratt H, Schwegler JW. The origin of the

human auditory brain-stem response wave II. Electroencephalogr Clin Neurophysiol1995;96:357–70; and Grundy BL, Jannetta PJ, Procopio PT, et al. Intraoperative monitoring ofbrain-stem auditory evoked potentials. J Neurosurg 1982;57:674–81.

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and do not provide information regarding the auditory system more proximal to thecochlea. Nevertheless, intraoperative monitoring of OAE responses has beensuccessfully accomplished, and responses were faster than ABR, suggesting theirpotential use within a battery of measures during surgery.17

INTRAOPERATIVE AUDITORY ASSESSMENTAuditory Brainstem and Direct Eighth-Nerve Intraoperative Monitoring

The 2 most commonly used techniques for intraoperative auditory monitoring duringAN resection are ABR and DENM. Because of early challenges maintaining electrodeplacement on the cochlear nerve during DENM, the technique fell into disfavor. Thisresulted in ABR becoming the most widely used auditory monitoring technique duringAN surgery in the 1980s.18,19 However, the operating room poses several challengesto ABR monitoring. First, there can be a tremendous amount of electrical interferencein the operating room, and ensuring that all equipment is properly grounded is neces-sary to minimize 60-Hz electrical interference that may overwhelm the ABR tracing.This susceptibility to electrical interference is related to the small response amplitudesfound in far-field techniques, such as ABR. Another disadvantage of ABR for intrao-perative monitoring during surgery is the inherent delay in obtaining a tracing. Tocompensate for the small response amplitudes, literally thousands of stimulus repeti-tions are required. At a stimulus rate of 11 per second, 2 to 3 minutes are typicallyneeded to obtain an ABR tracing during surgery. This degree of time delay in the oper-ative setting can allow irreversible damage to occur to the cochlear nerve or thecochlear blood supply. Injury to the latter is thought to be the predominant cause ofhearing loss during AN removal.20

The development of an electrode achieving more reliable and atraumatic positioningon the cochlear nerve in the late 1990s stimulated resurgence in the use of DENM.21,22

To perform DENM, the scalp electrode setup is the same as for performing ABR. Thedifference is that once the cochlear nerve is exposed, an electrode is placed on thecochlear nerve, preferably proximal to the area of tumor involvement. Because theelectrode is placed directly on the nerve of interest, this is a near-field monitoring tech-nique and therefore response amplitudes are much stronger than ABR reducing (butnot eliminating) susceptibility to electrical interference. Typical CNAP responses arefrom 1 to 50 mV. Furthermore, because response amplitudes are larger, less averagingis needed to generate a reliable CNAP. DENM requires on average less than 20 stim-ulus/response events to record a CNAP. This translates to about 1 to 2 seconds togenerate a CNAP, resulting in near instantaneous intraoperative feedback during ANsurgery. Such rapid feedback regarding the integrity of the cochlea and cochlear nerveduring tumor dissection allows for interruption of surgical maneuvers that may beplacing undue stress on the nerve or cochlear blood supply. As mentioned earlier,studies have indicated that the CNAP, recorded from the proximal portion of thecochlear nerve, corresponds to wave II of the ABR.11

In both ABR and DENM, the first sign of stress on the cochlear nerve or cochlearblood supply is a prolongation of the waveform latencies. Should further deteriorationin function occur, response amplitudes begin to diminish. An exception to this canoccur during DENM, when a drop in amplitude without a significant change in latencycan occur if the area where the electrode is contacting the cochlear nerve is bathed inexcess blood or cerebrospinal fluid (CSF). Blood in the area of the electrode/nerveinterface appears to affect the CNAP recording more than CSF does. Preservationof CNAP at the conclusion of tumor removal is highly correlated with preservation ofhearing. It is not uncommon to see CNAP amplitudes improve after tumor removal.

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The absence of a CNAP at the conclusion of tumor removal is likewise correlated withloss of hearing. Sometimes, however, hearing may be preserved despite an absentCNAP. This is likely caused by transient spasm of the internal auditory artery. Thepresence of good ABR waveforms is also highly correlated with hearing preservation,but the absence of a good ABR does not particularly predict hearing loss.

POSTOPERATIVE AUDITORY ASSESSMENT

Initial assessment of patients’ hearing after resection of their AN is often performedwith a tuning fork or whisper test in the operated ear with the contralateral ear canaloccluded by a finger. This action can be easily performed at the bedside once thepatient has recovered from general anesthetic. More formal evaluation of auditoryfunction is performed using behavioral audiometry. The current reporting standardsfor postoperative auditory function (American Academy of Otolaryngology—Headand Neck Surgery, Gardner-Robinson) tend to overweigh PTA in determining “classof hearing.” It is becoming increasingly recognized that the SDS is, by far, more impor-tant in a patient’s ability to successfully use his or her postoperative hearing. It isobvious that if the SDS is preserved at a good level but PTA declines, amplificationcan successfully rehabilitate the loss. If, however, PTA is preserved but SDS ispoor, amplification is less useful.

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