Intraoperative assessment of cochlear implant and auditory brainstem implant device function

Operative Techniques in Otolaryngology (2005) 16, 131-139

Intraoperative assessment of cochlear implant and auditory brainstem implant device function Jill B. Firszt, PhD, Christina L. Runge-Samuelson, PhD, P. Ashley Wackym, MD, FACS, FAAP From the Department of Otolaryngology and Communication Sciences, Medical College of Wisconsin, Milwaukee, Wisconsin. KEYWORDS Cochlear implant; Auditory brainstem implant; Electrophysiology; Electrical compound action potential; Electrical auditory brainstem response

Intraoperative objective measures play an important role in the treatment of patients who receive cochlear implants. This article will address the recording and interpretation of electrically evoked auditory potentials via traditional and brainstem cochlear implant devices, as well as electrical measures of device function and stapedial reflexes obtained at implant surgery. Several examples of intraoperative electrically evoked compound action potential and electrically evoked auditory brainstem response measures are shown for both the pediatric and adult populations using the three implantable prostheses available in the United States. The effects of stimulating and recording parameters, as well as patient variation that influence the electrophysiologic responses are presented. © 2005 Elsevier Inc. All rights reserved.

A number of objective measures can be used to assess both cochlear implant device function and patient physiology at surgical implantation. Electrical measures such as electrode impedance, compliance voltage, and averaged electrode voltages provide information about device function, and integrity of the electrode contacts and current delivery. Stapedial reflex measures evoked with electrical stimulation can be acquired intraoperatively and indicate stimulation of the auditory system by the implant. The reflex thresholds may be used to estimate current levels for speech processor settings. In addition, auditory evoked potentials represent the summed response of many neurons and can be obtained to assess human physiology evoked with electrical stimulation. The most commonly used electrically evoked measures at surgery include the electrically evoked compound action potential (ECAP) and the electrically evoked auditory brainstem response (EABR). These electrophysiologic responses may be obtained for various purposes, such as evaluation of auditory system integrity as well as estimation of programming levels needed for the externally worn speech processor. Because of the increase in the numAddress reprint requests and correspondence: Christina L. RungeSamuelson, PhD, Department of Otolaryngology and Communication Sciences, Medical College of Wisconsin, 9200 West Wisconsin Avenue, Milwaukee, WI 53226. E-mail address: [email protected]. 1043-1810/$ -see front matter © 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.otot.2005.04.002

ber of young children recommended for cochlear implantation who have little auditory experience, limited communication abilities, and/or secondary handicaps, there is continued interest in the application of objective electrophysiologic methods using electrical stimulation. This article will address the recording and interpretation of electrically evoked auditory potentials via traditional and brainstem cochlear implant devices, as well as electrical measures of device function and stapedial reflexes obtained at implant surgery.

General setup in the operating room Figure 1 shows the equipment and connections for evoked potential testing in the operating room. For EABR testing, needle recording electrodes are placed in the scalp, nape of the neck, and the skin anterior to the lobules of the pinnae before draping the patient. The locations of the electrode leads are labeled near the connectors and taped under the operating table for later retrieval. Before insertion of the electrode array into the scala tympani, the recording electrode leads are identified and plugged into the corresponding locations on the preamplifier. Often, a radio frequency filter is placed in line with the electrode leads to reduce the artifact that occurs from the radio frequency transmission signal once the transmission coil is placed. The preamplifier is connected to the recording/averaging system. The aver-

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Figure 1

Stimulating and recording equipment, and connections for evoked potential testing in the operating room.

aging system must be synchronized to the stimulus, and this is accomplished by connecting the averager to the trigger output from the stimulus generator. The transmission coil and speech processor are connected to the manufacturer stimulus software via an interface unit. Electrical measures of device function and physiologic responses are obtained after the electrode array is inserted into the scala tympani and the transmission coil, placed inside a sterile ultrasound drape, has been placed in contact with the implant receiver, either on top of or under the skin flap. If the transmitter coil is placed directly over the receiver coil, a moist Ray-Tec sponge (Johnson & Johnson, New Brunswick, NJ) is interposed between the implant and the transmission coil. The speech processor, which is connected between the transmission coil via a long cable and the interface unit, is placed outside the sterile field. If intraoperative testing consists of ECAP measures only, recordings are obtained from the intracochlear electrodes, therefore, there is no need for placement of needle electrodes or use of the evoked potential averager. However, the stimulus software, interface unit, and transmission coil are necessary.

Electrical measures of device function Because of the incorporation of telemetry in cochlear implant systems after 1990, current devices allow information to be obtained concerning integrity of the internal device, electrode array, and radio frequency connection. The current conduction by the electrode contacts can be assessed immediately after insertion of the array by measuring the impedances of the electrodes. Typically, each electrode is evaluated with either broad bipolar or monopolar stimulation compared with the extracochlear ground electrode. High values may suggest an open electrode circuit. Although irrigation of scala tympani with lactated Ringer solution is performed before electrode insertion to remove air bubbles and bone debris, air bubbles around an electrode can form and may create a temporary high impedance value that mimics an open circuit. Because the electrode array is fragile, difficult electrode insertions can result in damaged electrodes and, consequently, high impedances. Open-cir-

cuit electrodes cannot be stimulated and, therefore, are not included in subsequent electrophysiologic measures in the operating room. Often the small air bubbles surrounding a basal electrode contact will clear after a few minutes have passed. It is also possible to stimulate individual electrode contacts in a monopolar mode and measure the voltage from each nonstimulated electrode along the array. This results in a distribution of voltages for activated and nonactivated electrodes (eg, with the MED-EL COMBI 40⫹ cochlear implant system [Innsbruck, Austria]) and allows for improved isolation of short-circuited electrodes. These compliance voltage measures can be easily obtained in the operating room within a few minutes. Compliance telemetry values indicate whether the implant current source is able to output the required stimulation level. In most situations, the implant can deliver the desired amount of electrical current. However, under some circumstances (eg, high tissue impedance), the voltage that is available is not sufficient, and an objective indication via compliance telemetry is beneficial. Electrical current passed between electrodes produces an electrode voltage that can be averaged over multiple samples, resulting in averaged electrode voltages.1 Averaged electrode voltages represent the peak-to-peak voltages of each electrode along the array and can be measured from surface electrodes placed on the scalp.2 Averaged electrode voltages provide a measure of the function of the internal stimulator and electrode array, and can be recorded with any of the three implant devices available in the United States. Averaged electrode voltages were recorded extensively at surgery with the Nucleus® 22 implant (Cochlear Corp, Lane Cove, Australia); however, this procedure is less common today because the current devices can provide information about device function through telemetry.

Electrically evoked stapedial reflex measures Stapedial reflex measures can be made intraoperatively (although they are rarely performed) and after implantation with electrical stimulation,3 and may serve as an objective guideline for programming cochlear implant devices, which may be especially useful in the pediatric population. After

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insertion of the electrode array and before closing, electrical current with adequately intense stimulation is sent to specific intracochlear electrodes. By using either immittance measures or visual detection, the contraction of the stapedius muscle can be recorded. A correction factor has been suggested that is applied to the intraoperative reflex threshold to account for the effects of anesthetic agents during surgery.4 Compared with behavioral judgments of loudness, the stapedial reflex threshold obtained after surgery and initial stimulation tends to fall between most comfortable loudness and loudness discomfort levels.5 In a certain percentage of cases (ie, 25% to 35%), the reflex can not be obtained, particularly when there is ossification of the cochlea or middle ear disease (in the case of recordings after implantation).

Electrically evoked auditory potentials Electrophysiologic responses can be recorded in the operating room via electrical stimulation to confirm that the electrode array is stimulating the auditory nerve and that the auditory system of the individual has sufficient synchrony to activate the auditory pathway. The presence of these responses is also comforting to parents and family members because confirmation of auditory activity in response to electrical stimulation can be given immediately and corresponds with a high likelihood that the patient will be able to hear sound through the implant. For some parents, this may be the first time they have evidence of an auditory stimulus that resulted in an auditory physiologic response for their child. If an internal device has failed or when reimplantation is necessary, electrophysiologic responses at surgery can be especially informative. In the case of auditory brainstem implantation, the presence of the EABR is critical for determining the most beneficial placement of the electrode array on the auditory cochlear nucleus. Both the ECAP and EABR can be recorded intraoperatively and used to assist in the initial programming of the speech processor for traditional cochlear implants. When clinicians and surgeons have experience working as a team in the operating room, there is an increased likelihood of successfully recording responses, particularly in more difficult cases such as auditory brainstem implant (ABI) and reimplantation. Recording electrophysiologic responses in the operating room can be challenging because of variables such as electrical interference in the room and time constraints, but these challenges can be overcome, and the benefits are substantial.

ECAP The ECAP is a short-latency (ie, 0.2-0.5 ms) neural response that consists of a negative trough (N1) and represents activation of the peripheral auditory nerve. The response is obtained by sending electrical current to one intracochlear electrode and recording the response for another nearby electrode within the cochlea. The amplitude of the N1 response is large at suprathreshold levels and diminishes with subsequent decreases in stimulus level. The de-

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Figure 2 Screen from the NRT 3.0 software for measuring the ECAP with the Nucleus® Contour device. The left panel displays the waveforms recorded from several different electrodes (3, 7, 11, 17, and 21) from stimulation presented at various levels (210, 220, and 230 clinical units [CU]). Waveform amplitude measurements are indicated by the lines marking N1 and baseline for each waveform, and the amplitude scale is indicated by the scale bar (in this case, 185 ␮V). The upper right panel shows the stimulating and recording parameters. The middle right panel plots the amplitude/level, or input/output, function for electrode 21, and the lower right panel shows an overlay of a waveform selected from the left panel. (Color version of figure is available online.)

crease in amplitude can be followed to a threshold level, and this value may be useful in supplementing device fitting. Several sources provide a complete review of the ECAP response.6,7 Because the latency of the ECAP is very early, the electrical stimulus artifact often overlaps in time with and obscures the ECAP response waveform. Therefore, methods of artifact rejection have been developed so that the ECAP may be a useable measure with electrical stimulation. Recording of the ECAP with a subtraction technique that reduces electrical stimulation artifact using a percutaneous implant and research software was first described by Brown et al.8 Today, the ECAP response can be obtained with commercial software known either as Neural Response Telemetry (NRT) for the Nucleus® 24 device (Cochlear Corp) or Neural Response Imaging (NRI) for the Advanced Bionics CII (Advanced Bionics Corp, Sylmar, CA) and HiRes 90K device. At this writing, the Auditory nerve Response Telemetry (ART) software for the MED-EL Pulsar implant was not yet available commercially in the United States but has been implemented in a research platform in Europe. The ECAP is not affected by sleep or anesthetics, and therefore is conducive to obtaining a response intraoperatively. Because the response is recorded from intracochlear electrodes, it is not necessary to place surface or needle electrodes on the scalp or ears before draping the patient. Once the electrode array has been inserted into scala tympani, the transmission coil is placed within a sterile ultrasound drape and placed on the implant receiver. The coil is connected to an external processor, interface unit, and computer with software for generating the electrical signals. Figure 2 (left panel) shows ECAP responses recorded in the operating room with the Nucleus® NRT 3.0 software from a patient with varied stimulus levels (210, 220, 230 clinical

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Operative Techniques in Otolaryngology, Vol 16, No 2, June 2005 with Advanced Bionics that can be used to record ECAP responses on multiple electrodes in the operating room, which may assist in the determination of programming levels for fast rate stimulation.

EABR

Figure 3 Screen from the NRI software for measuring the ECAP with the Advanced Bionics device. The left panel shows the ECAP waveforms recorded for a single electrode at various stimulation levels. ECAP amplitudes are marked with circles placed at N1 and the following positive peak. The upper right panel indicates the stimulating and recording parameters, and the lower right panel displays the input/output function for the tested electrode. (Color version of figure is available online.)

units [CU]) and on several electrodes (3, 7, 11, 17, 21). The x-axis represents time in milliseconds (ms), and the amplitude of the response is measured in microvolts (␮V) (in this case, 185 ␮V for the bar length indicated). The upper right panel of Figure 2 displays the stimulating and recording parameters; the middle right panel shows the input/output function for electrode 21, which is a plot of the ECAP amplitude recorded for each stimulus level; and the lower right panel displays an overlay of selected waveforms. A regression line is fit to the input/output function in the middle panel and the x-intercept of the line is calculated. This x-intercept value is referred to as the NRT threshold and may be used in device fitting. ECAP responses using the Advanced Bionics NRI system are shown in the left panel of Figure 3 for stimulating electrode 8 and recording electrode 6. As the amount of current decreases, starting at 282 CU, the N1 response amplitude (marked with an open circle) decreases until the response is no longer present at 153 CU. In the right panel, the ECAP response amplitudes recorded in the operating room are plotted as a function of stimulus level to compose an ECAP input/output function. As with NRT, a regression line is fit to the function to calculate the x-intercept, or NRI threshold, which in this case is 124 CU for electrode 8. Recent psychophysical procedures for programming the HiResolution strategy have been introduced by Advanced Bionics, whereby electrodes are stimulated in a group, or banded, rather than stimulating single electrodes. In programming, comfortable loudness measures are obtained on banded electrodes that mimic the stimulation of electrodes when speech occurs (ie, activation of multiple electrodes at one time or in rapid sequence). The ECAP also can be recorded to stimuli delivered simultaneously to multiple electrode contacts. ECAP input/output functions for single and multiple electrodes for a patient are shown in Figure 4. As the number of stimulated electrodes increases from 1-4, the slopes of the input/output functions also increase, which is consistent with higher neural activity for multiple-electrode stimulation. Commercial software is in development

The EABR reflects neural activity from the eighth nerve through the brainstem via electrical stimulation. The EABR primarily consists of 3-4 vertex-positive peaks labeled as waves II, III, IV, and V. Wave I of the EABR is usually obliterated by electrical stimulus artifact at the beginning of the recording. In general, the latency of wave V is 1.0-1.5 ms earlier compared to the wave V latency of the ABR, which is elicited with acoustic stimuli. The EABR is affected by changes in stimulus intensity, showing decreases in amplitude and increases in latency with corresponding decreases in stimulus level, although the effects on latency are not as pronounced as those seen with acoustic ABR. Figure 5 displays a loudness growth function at implantation for a young child with a history of progressive hearing loss who received the Advanced Bionics CII cochlear implant device. The EABR waveforms were recorded from electrode 1 (apical location) at stimulus levels that ranged from 416 to 80 CU, with a pulse width of 75 ␮s per phase. Waves II, III, IV, and V are identifiable at higher stimulus levels, and wave V is followed to threshold, a level of 100 CU in this example. EABR thresholds can be obtained on a set of electrodes that represent apical, medial, and basal locations along the array at surgery. The thresholds can be used to assist the programming of the speech processor at initial stimulation in a manner similar to the use of the ECAP threshold, such as when fitting a child or adult who is unable to respond behaviorally. The EABR threshold level typically reflects a level that should be audible to the patient. If the EABR threshold level has been reached at initial activation and the patient has not responded, the clinician checks that all external equipment is functional, reinstructs the patient, changes activities, or proceeds to increase stimulation above

Figure 4 Input/output functions for single electrodes and those banded in groups of 2, 3, and 4 (as indicated) using the Advanced Bionics device.

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Figure 5 EABR waveform growth function from stimulation of an apical electrode for a child with progressive hearing loss who uses the Advanced Bionics device. Waves II, III, IV, and V are indicated. Stimulation levels were tested from 416 to 80 CU, with threshold determined as 100 CU.

the EABR threshold level carefully while observing for behavioral responses. Although wave V is typically the largest and most robust peak in the EABR waveform, there are examples in which this is not the case, particularly in children. Figure 6 displays EABR waveforms recorded intraoperatively for a 14month-old child implanted with the MED-EL COMBI 40⫹ device (left panel) and a 12-month-old child who received the Advanced Bionics CII implant (right panel). For both children, wave III is the most robust peak and can be easily followed to threshold. Although the clinical implications of a relatively large wave III are unclear, it may be consistent with a differential development of the components of the central auditory pathway. In recent years, acceptance has been growing regarding cochlear implantation as a treatment option for children with auditory neuropathy who have reduced speech recognition abilities and subsequent delays in communication

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development. Therefore, a number of centers are gaining experience with children diagnosed with auditory neuropathy who have received cochlear implants.9,10 Auditory neuropathy/dyssynchrony is a hearing disorder that constitutes normal outer hair cell function, as evidenced by normal otoacoustic emissions and/or cochlear microphonic, combined with possible inner hair cell or eighth nerve dysfunction documented by abnormal ABR results with acoustic stimulation.11 In Figure 7, ABR results evoked with acoustic and electrical stimulation for a young child diagnosed with auditory neuropathy are displayed. The acoustic ABR reveals the presence of the cochlear microphonic response, which inverts with stimulus polarity reversal, but absent neural responses. However, with electrical stimulation, neural synchrony is enhanced, and an intraoperative EABR is often elicited in these patients. The morphology of the EABR shown in Figure 7 suggests response patterns that are similar to other children whose profound hearing loss is sensory in nature (eg, Figure 5). In addition to neural properties, EABR morphology is likely to be affected by the anatomic structure of the cochlea. A child with congenital profound hearing loss may have a cochlear malformation that ranges from an apical incomplete partition and only 2 turns of the cochlea to a common cavity of the cochlea. In these cases, electrophysiologic testing at surgery is beneficial to identify electrodes that can be stimulated and will produce auditory sensations. Figure 8 provides EABR tracings recorded on apical (electrode 18), medial (electrode 10), and basal (electrode 5) electrodes at similar suprathreshold levels (205 or 200 CU) from a 5-year-old child with cochlear malformations who received the Nucleus® cochlear implant. Waves III and V can be identified for the medial and basal electrodes. The response on the apical electrode is not nearly as robust as those for the other electrodes. It is possible that the cochlear malformations contributed to the unexpected EABR measures. Electrophysiologic responses such as EABR can also be used to measure the effects of stimulating from different

Figure 6 Examples of EABR wave III amplitudes that are larger than wave V amplitudes in 2 children implanted with 2 different devices. The left panel shows the EABR growth function waveforms for a 14-month-old child who is implanted with the MED-EL device. The right panel shows the waveforms for a 12-month-old child who is implanted with the Advanced Bionics device.

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Figure 8 EABR waveforms from a 5-year-old child with cochlear malformations implanted with the Nucleus® device. Although stimulation levels were very similar, the response to apical electrode stimulation (electrode 18, top tracing) is not as robust as that for the medial and basal electrodes (middle and bottom tracings, respectively).

Figure 7 Acoustically evoked (top panel) and electrically evoked (bottom panel) ABR waveforms for the right ear of a child with auditory neuropathy. The ABR tracings in the top panel for the condensation and rarefaction click stimuli (presented at 90 dB nHL) show cochlear microphonic responses that reflect the opposing polarities of the stimuli. The response to 90 dB nHL alternating click stimuli clearly shows the absence of neural response to acoustic stimulation and, as would be expected, absence of the cochlear microphonic. The EABR waveform growth function in the lower panel displays neural responses to electrical auditory stimulation.

electrode distances from the modiolus. One goal of more recent intracochlear electrode designs (eg, Clarion® HiFocus electrode and positioner [Advanced Bionics], Nucleus® Contour and Contour Advance electrode [Cochlear Corp], helix electrode [Advanced Bionics]) is to position the electrode closer to the modiolus of the cochlea, as opposed to positioning along the lateral cochlear wall. EABR measured in cats has been used to determine whether experimental changes in intracochlear electrode position affect neural excitation within the scala tympani.12 In human beings, the EABR has been shown to reflect electrophysiologic changes relative to lateral-to-medial changes in intracochlear electrode position as a result of the positioner for the HiFocus electrode13 or the stylet of the Nucleus® Contour electrode.14 In Figure 9, EABR waveforms are shown before and after stylet removal for apical electrode 18 in the Nucleus® device; removal of the stylet allows the electrode array to

curl around the modiolus, placing the electrode array in a perimodiolar position. Increases in wave V suprathreshold amplitude are seen by comparing the top (before) and middle (after) waveforms, both of which were in response to the same stimulus level (210 CU). The wave V amplitude more than doubled from the before to after stylet condition, from 0.41 to 0.86 ␮V, respectively. In addition, well-defined early latency peaks are seen after stylet removal. The bottom waveform shows the EABR after stylet removal to a lower level stimulus (175 CU), which has a similar wave V amplitude as that for the pre-stylet response recorded for 210 CU. Although the wave V amplitudes are similar, the

Figure 9 Effects of perimodiolar placement on EABR waveforms for apical electrode 18 in an adult with the Nucleus® device. A suprathreshold (210 CU) waveform obtained before stylet removal, when the electrode was in the lateral position, is shown in the top tracing. The bottom tracings are recordings obtained after stylet removal for levels of 210 (middle tracing) and 175 (bottom tracing) CU. Wave V amplitudes for each waveform (in ␮V) are indicated. Note the increase in suprathreshold amplitude as well as the presence of early latency waves for the recordings for the post-stylet condition.

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graphy is highly recommended for all cochlear implantation procedures.

EABR and the ABI

Figure 10 EABR tracings show an interfering late latency response (arrows) in an adult implanted with the MED-EL device. The waveform growth functions for the apical (top tracings) and medial (bottom tracings) electrodes illustrate how a late latency potential can obscure wave V at higher stimulation levels.

early latency waves are apparent in the post-stylet waveform. A 60-CU decrease in EABR threshold was also observed for this electrode as a result of perimodiolar placement. When recording the EABR, there can be obstacles that interfere with the acquisition of the response. One obstacle previously mentioned is stimulus artifact, which is large and can obscure the response, particularly the short latency potentials. This problem is somewhat alleviated with the use of short biphasic pulses that are alternating in polarity. Because implant systems use radio frequency signals to transmit information across the skin, the radio frequency signal can be picked up by the recording electrodes and may contribute to the artifact problem. The use of a radio frequency filter can assist in successful recording of targeted responses. Nonauditory potentials, such as facial nerve stimulation and muscle artifact, can also interfere with recording the early latency potentials such as EABR. Figure 10 displays EABR waveforms recorded intraoperatively for a 62-year-old adult at implantation with the MED-EL COMBI 40⫹ device. The top 3 tracings show responses for electrode 1 (apical location) at current levels of 419, 296, and 196. Likewise, the bottom 3 tracings display the same current levels used to evoke EABR from medial electrode 5. In both cases, at suprathreshold levels (ie, 419 and 296 CU) a large peak is recorded at approximately 5.5 ms (arrows). This peak obscures wave V, which is observed at approximately 3.5 and 4.5 ms for electrodes 1 and 5, respectively. For both electrodes at the lower current level of 196 CU, the facial stimulation is no longer apparent, and, therefore, the morphology of wave V is not affected. As seen in this example, stimulation of nonauditory sites can produce waveforms that resemble those from auditory stimulation and can obscure the desired response. The ability to recognize these responses that are contaminated by muscle and facial nerve stimulation is needed for accurate waveform interpretation. Facial nerve electromyo-

The intraoperative EABR is used to confirm neural activation after placement of the ABI on the surface of the posteroventral and dorsal cochlear nucleus, and to identify the location of optimized auditory responses.15 As with traditional cochlear implants, the ABI is activated by placing the transmission coil on the scalp over the internal receiver and sending electrical current through the implanted electrodes directly to the cochlear nucleus. In Figure 11, an EABR waveform growth function obtained in the operating room after removal of an acoustic neuroma and placement of the Nucleus® 24 ABI is shown for a 52-year-old male diagnosed with neurofibromatosis type 2. Looking at the top waveform, a typical EABR waveform elicited with an ABI may be described. Because the electrodes are placed more centrally relative to a traditional cochlear implant, the waves present are waves III and V, which reflect neural responses from central auditory structures of the cochlear nucleus (wave III), and superior olive and lateral lemniscus (wave V).16 The waveforms in the growth function are easily identified at high stimulus levels, and the waveforms show amplitude decreases and latency increases with decreases in stimulus level. The patient for whom these recordings were obtained had a history of normal hearing thresholds in the contralateral ear and mild sensorineural hearing loss in the ipsilateral ear just before surgery. While in the operating room, the presence of an EABR for an ABI is examined by recording on one pair of electrodes using broad bipolar stimulation between electrodes spaced farthest apart. Once a response is obtained, EABR measures are attempted across the electrode pad to compare responses along the distal, proximal, and central locations of the electrode array. Figure 12 shows the EABR recorded for different electrode pairs in a female patient with neurofibromatosis type 2 who received a Nucleus® ABI immediately after a secondside acoustic tumor removal. Electrode pair 17 to 18 (active

Figure 11 EABR waveform growth function obtained intraoperatively with a Nucleus® ABI. The central location of the ABI electrode placement is reflected by EABR waves III and V.

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Figure 12 Effects of ABI electrode placement on EABR recordings in a patient with neurofibromatosis type 2. The waveforms recorded for initial and final electrode placements are shown for bipolar electrode pairs 17 to 18, stimulated with 150 CU (left panel), and 21 to 2 stimulated with 180 CU (right panel).

and ground electrodes, respectively) shows no response when stimulated after the initial ABI placement (left, upper panel), whereas a response is present for the same pair after repositioning the ABI (left, lower panel). Likewise, in the same subject, stimulation of electrode pair 21 to 2 results in a small and somewhat questionable response (right, upper panel) compared with that obtained after the repositioning of the ABI (right, lower panel) using the same amount (180 CU) of electrical current. Note the difference in waveform morphology for this patient, where one positive peak, wave III, is seen, compared with the presence of both waves III and V displayed for a different ABI recipient in Figure 10. The varied morphology may reflect activation of different neural structures in the brainstem across individuals, as well as different ABI placements and corresponding stimulation of the cochlear nucleus. To interpret waveform morphology during ABI cases, separate monitoring of cranial nerves V, VII, IX, X, and XI is performed.

To summarize the differences between ABR waveforms for different stimuli and devices, Figure 13 illustrates the changes in morphology, latency, and amplitude that can be observed in the waveforms of the ABR recorded with acoustic stimulation (top tracing), electrical stimulation through a traditional cochlear implant (middle tracing), and electrical stimulation through an ABI (bottom tracing). As seen, the waveforms are similar, although there are shorter latencies and fewer peaks present with electrical stimulation compared with acoustic stimulation, with the largest difference apparent for the ABI condition.

Implications for surgical decision making There are several implications of intraoperative assessment of cochlear implant and ABI device function for the surgical procedure. With complete electrode insertion and successful EABR and/or ECAP measures, it is not necessary to obtain an intraoperative radiograph to confirm electrode position. The time required to perform EABR and/or ECAP measures during cochlear implant surgery is less than that required to obtain a plain radiograph. The advantages of knowing that the implant recipient is able to have auditory information delivered to the central nervous system and that the device is functional cannot be overstated. Parents of children or families of adult cochlear implant recipients find the objective evidence of cochlear implant and auditory system function reassuring. Similarly, the use of EABR in ABI electrode placement is critical for optimizing outcome.17

Conclusion Figure 13 Examples of auditory brainstem responses evoked acoustically (top tracing), electrically through a cochlear implant (middle tracing), and electrically through an ABI (bottom tracing). These examples highlight the differences in morphology and latency across the different stimulation parameters.

Intraoperative objective measures play an important role in the treatment of patients who receive cochlear implants. Electrical and electrophysiologic measures of impedance, compliance voltage, stapedius reflex, ECAP, and EABR, to name a few, provide valuable information about the func-

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tion of the internal device and electrode array, as well as corresponding activation of the auditory nerve and brainstem pathways. Research continues at many institutions to investigate the relationships between intraoperative measures and postoperative outcomes. As new techniques and clinical applications are made, our understanding of human physiology evoked with electrical stimulation will advance the field of cochlear implantation.

Acknowledgments The authors thank Wolfgang Gaggl, MSE, for assistance with data collection in the operating room during cochlear implantations.

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5. Hodges AV, Balkany TJ, Ruth RA, et al: Electrical middle ear muscle reflex: Use in cochlear implant programming. Otolaryngol Head Neck Surg 117:255-261, 1997 6. Abbas PJ, Brown CJ, Shallop JK, et al: Summary of results using the Nucleus CI24M implant to record the electrically evoked compound action potential. Ear Hear 20:45-59, 1999 7. Brown CJ: The electrically evoked whole nerve action potential, in Cullington HE (ed): Cochlear Implants: Objective Measures. Philadelphia, PA, Whurr Publishers, 2003, pp 96-129 8. Brown CJ, Abbas PJ, Gantz B: Electrically evoked whole nerve action potentials: Data from human cochlear implant users. J Acoust Soc Am 88:1385-1391, 1990 9. Peterson A, Shallop J, Driscoll C, et al: Outcomes of cochlear implantation in children with auditory neuropathy. J Am Acad Audiol 14: 188-201, 2003 10. Rance G, Cone-Wesson B, Wunderlich J, et al: Speech perception and cortical event related potentials in children with auditory neuropathy. Ear Hear 23:239-253, 2002 11. Starr A, Picton TW, Sininger Y, et al: Auditory neuropathy. Brain 119:741-753, 1996 12. Shepherd RK, Hatsushika S, Clark GM: Electrical stimulation of the auditory nerve: The effect of electrode position on neural excitation. Hear Res 66:108-120, 1993 13. Firszt JB, Wackym PA, Gaggl W, et al: Electrically evoked auditory brain stem responses for lateral and medial placement of the Clarion HiFocus electrode. Ear Hear 24:184-190, 2003 14. Wackym PA, Firszt JB, Gaggl W, et al: Electrophysiologic effects of placing cochlear implant electrodes in a perimodiolar position in young children. Laryngoscope 114:71-76, 2004 15. Waring MD: Intraoperative electrophysiologic monitoring to assist placement of ABI. Ann Otol Rhinol Laryngol 104:33-36, 1995 (suppl 166) 16. Moeller AR, Janetta PJ: Interpretation of brainstem auditory evoked potentials: Results from intracranial recordings in humans. Scand Audiol 12:125-133, 1983 17. Wackym PA, Runge-Samuelson CL, Firszt JB: Multidisciplinary approach for recipients: Surgical, electrophysiologic, and behavioral outcomes, Miyamoto RT (ed): Cochlear implants. Proceedings of the 8th International Cochlear Implant Conference. New York, NY, Elsevier, 2004, pp 433-436