Krista Fitzgerald, Au.D., CCC-A, CNIM
Assistant Clinical Professor and Director of Clinical Education, University of Florida Doctor of Audiology Program
IONM liaison for SpecialtyCare, LLC
Intraoperative neurophysiologic monitoring (IONM) is the use of electrophysiological measures during surgery in an attempt to reduce the risk of neurological insult involving the peripheral and central nervous system (Devlin & Schwartz 2007; Sing et al. 2016). IONM can be helpful in many ways, such as justifying that a neurological deficit is present pre-operatively, identifying deficits that may become apparent during surgical positioning, injury occurring during surgery, or use in an attempt to preserve hearing during neuro-otologic cases.
Surgeries involving IONM can utilize one or multiple modalities at time. The modalities chosen are specific to the structures at risk and surgeon preference. The structural function, anesthetic depth, perfusion to the brain, poor positioning of limb can all be detected by various tests. In my clinical experience, the modalities that are utilized the most include somatosensory evoked potentials (SSEPs), transcranial motor evoked potentials (tceMEPs), triggered and spontaneous electromyography (sEMG and tEMG), and the brainstem auditory evoked response (BAER), also termed auditory brainstem response (ABR).
Among the many professional invested in the field of intraoperative neuromonitoring (IONM), it is well understood that audiologists built the foundation upon which current professionals continue to build off. Paul R. Kileny, PhD began work in the mid-1980s on the use of the BAER in hearing preservation during various otologic procedures, along with observing physiologic and anesthetic effects on the evoked response. Eventually the use of BAER and electrocochleography were utilized together in assessing change in cochlear function in otologic surgical procedures (Kileny & McIntyre, 1985; Kileny et al. 1988). In fact, the Ad Hoc Committee in “The Scope of Practice in Audiology” has listed IONM as part of audiologist’s diagnostic procedures. It states, “This assessment includes measurement and professional interpretation of sensory and motor evoked potentials, electromyography, and other electrodiagnostic tests for purposes of neurophysiologic intraoperative monitoring and cranial nerve assessment.”
Typically, when most individuals consider IONM during otologic procedures, the first procedure they may think of is the removal of an acoustic neuroma and the modality most utilized are BAERs. The intervention that takes place largely depends on the size of the tumor. If a surgeon elects to use IONM during these procedures, the operative goals would include gross tumor resection while protecting the adjacent cranial nerves and the preservation of hearing (Oh et al. 2012). Therefore, BAERs may not be the only modality utilized; cochlear nerve action potentials (CNAPs), electrocochleography (EcochG), and facial nerve electromyography may also be useful. For the purpose of this review, I will be discussing BAERs. Depending on the size of the tumor, surgeons may elect to monitor the growth via magnetic resonance imaging (MRI) and follow-up audiologic test measures or they may use noninvasive gamma knife stereotactic radiosurgery to restrain tumor growth while preserving neurological function (Popp & Kraus 2007).
BAERs are used during neuro-otologic surgery when the eighth cranial nerve and/or the brainstem are at risk. In some cases, perfusion to the brainstem may be necessary to monitor when the surgeon needs to apply retraction. The auditory nuclei are susceptible to change during certain degrees of retraction and the continuous running of the BAERs can give valuable information to the surgeon during this stage. Typical neuro-otologic surgeries where BAERS are utilized include acoustic neuroma resection and removal of cerebellopontine angle tumors. The technique implemented in surgery is similar the way we would run a diagnostic BAER with a few differences.
During these cases, a two-channel electrode montage is implemented. Electrode placement can be difficult during these cases. For example, the mastoid typically will be unavailable for electrode placement, so the non-inverting electrode can be placed anterior to the tragus. The vertex (Cz) to contralateral (nonsurgical) ear channel should be used to assist in peak identification. This channel can be used if the non-surgical side is to be monitored as well (see Figure 1 for recording montage and waveforms). The time window that should be implemented is 15 to 20 milliseconds. This will compensate for any underlying pathology that may affect the latency of the waveforms. The filter settings suggested are a high pass filter of 100 Hz to 150 Hz and low pass cut off values of 2500-3000 Hz. The high frequency filter can be reduced to 1000 Hz. This will reduce the 60Hz inference without degrading the BAER (Doyle & Hyde 1989; John et al. 1982). A click stimulus would be utilized at presentation levels ranging from 60-70 HL. If hearing thresholds are unknown, one when can use 70 dB nHL to 95dB nHL depending on the presence of preexisting sensorineural hearing loss, which can be concluded from preoperative audiogram and case history. Stimulus presentation rates should be relatively fast (30.1 clicks/sec) to permit continuous rapid recordings. However, if the BAER demonstrates poor waveform morphology and/or low amplitudes, a slower stimulus rate can be used. The number of trials to be averaged depends on the amount of noise present in the recordings and/or small amplitudes, typically 500-1000 are collected. Insert earphones should be securely placed so they do not fall out during surgery. Bone wax, gauze pads and Tegaderm are all great tools to use as long as you make sure not to kink the tubing. A baseline recording should always be obtained prior to surgical incision so that the audiologist has a true baseline to compare to throughout the surgical process. I recommend 3-4 complete runs per ear (please see Table 1 for summary and details of parameters).
BAERs are exceptionally resistant to the effects of anesthesia. Inhalational agents, such as isoflurane have been shown to have mild effects on BAER latency and amplitude (Doyle & Hyde 1987; Drummond & Todd, 1985). Core body temperature can greatly affect the latency and amplitude of all BAER waveforms. Studies have shown that these negative effects take place when the core body temperature is below 35 degrees Celsius (Stockard et al. 1978; Markand et al. 1990). Regardless of whether or not the anesthestic regiment affects the BAER, the audiologist should take notes on any changes in anesthestic agents, mean arterial blood pressure, and body temperature during the case. This information can better direct your understanding when interpreting an intraoperative change in signals.
In my current practice as a neurophysiologist and audiologist, the criteria set for intraoperative changes in the BAER response are a 1msec increase in the wave I-V interval and/or 50% decrease in Wave V amplitude as compared to baselines (ACNS Guidelines, 2009) These changes are then further broken down into the structures that are being compromised. Wave I changes would indicated a sound conduction issue. In other words, a decrease in intensity, kinked tubing, fluid in the outer or middle ear, or the transducer fell out. Of course, this could also mean that there is an issue with the neural transduction of sound, such as a cochlear lesion/ischemia to the cochlea. Wave I-III changes would suggest problems with the auditory nerve and wave III-V changes would suggest problems with conduction through the pons/midbrain. Earlier I mentioned cerebellar retraction. This is required during CPA surgeries. Retraction can significantly increase the wave I-V inter-peak interval on the surgical side. These changes are typically reversible but should not be ignored. If ignored, it can result in postoperative hearing loss.
BAERs are most frequently used during cases that involve hearing preservation and preserving brainstem function. For relatively small tumors on the eighth cranial nerve, it is possible to preserve some hearing. This also applies to microvascular decompression procedures of the eighth cranial nerve. You may come across cases with relatively large brainstem tumors where the preservation of hearing is unlikely, but the function of the brainstem is of main concern. In such cases, a change in wave V amplitude and/or latency contralateral to the surgical side could represent change in function of the pons to midbrain region on the surgical side. If identified, the surgical team will then be informed so that proper intervention can take place.
The utilization of BAERs during the above-mentioned surgical procedures can be useful to the surgeon when the auditory nerve or brainstem are at risk. Monitoring can assist in reducing postoperative deficits, providing information to the surgeon to change surgical technique and provide information on the functional status of the structures. For an audiologist who would like to become more involved in the operating room, it is important not only train on the technical side, but to also understand the anatomy and physiology at risk so you can better interpret and deduce why the changes are occurring.
Figure 1: Right sided ABR responses: Ipsilateral and contralateral responses.
Table 1: Parameters for utilizing BAERs in the operating room
|Electrode montage (2 channel)||Noninverting electrodes: A1 and A2, Inverting electrode Cz, ground (shoulder or lower forehead)|
|Stimulus Intensity||60-70 dB HL or 70-95 dB nHL|
|Stimulus Type||Click, 100 microsecond monophasic square wave pulse|
|Time window||15 to 20 msec|
|Filter settings||100-150 to 2500-3000 Hz (1000 Hz)|
|60 Hz noise notch||OFF|
|Repetition rate||30.1 clicks/sec (faster repetition, wave I and V distinguishable); 5-12 clicks/sec (optimal resolution of I, III, and V)|
|Number of trials||500-1000 (depends noise present and BAER signal)|
- American Clinical Neurophysiology Society.(2019). Guideline 11 C: Recommended Standards for Intraoperative Monitoring of Auditory Evoked Potentials. Available from https://www.acns.org/pdf/guidelines/Guideline-11C.pdf.
- American Speech-Language-Hearing Association. (2018). Scope of Practice in Audiology [Scope of Practice]. Available from www.asha.org/policy.
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