Electrically Evoked Auditory Brainstem Response–Based Evaluation of the Spatial Distribution of Auditory Neuronal Tissue in Common Cavity Deformities

Objective In a common cavity (CC) deformity, the cochlea and vestibule are confluent to form a single cavity without internal architecture, and distribution of auditory neuronal tissue is unclear. The purposes of this study are to reveal the spatial distribution of auditory neuronal tissue in CC deformity using electrically evoked auditory brainstem response (EABR) during cochlear implantation. Study Design Retrospective case review. Setting Cochlear implant (CI) center at a tertiary referral hospital. Patients Five patients with CC deformity who underwent cochlear implantation and intraoperative EABR testing. Main Outcome Measures Spatial distribution of electrodes that elicited an evoked wave V (eV) in EABR testing was evaluated in each CC deformity. Results Electrically evoked auditory brainstem response testing demonstrated that electrodes attached on the inner wall of the anteroinferior cavity of the CC deformity successfully elicited a reproducible eV in all cases, and the latency of each eV was an approximately 4 ms, which is similar to those reported in patients without an inner ear malformation. Interestingly, in Case 1 with the lowest percentage of eV-positive electrodes (31.8%), CI-aided audiometric thresholds were changed, depending on the frequency allocation to eV-positive electrodes in the programming. Cochlear implant–mediated facial nerve stimulation was observed in 3 of 5 cases, and results of EABR testing were useful for optimizing the device program to decrease facial nerve stimulation without sacrificing CI-mediated auditory performance. Conclusion The results of EABR testing suggested that auditory neuronal elements are distributed to the anteroinferior part of CC deformity, mainly around or near the inner wall of the cavity. In cases with CC deformity, EABR testing is useful to achieve the optimal electrode array placement and to adjust programming parameters of the implanted device, which might be essential to maximize CI outcomes and to decrease facial nerve stimulation.

Objective: In a common cavity (CC) deformity, the cochlea and vestibule are confluent to form a single cavity without internal architecture, and distribution of auditory neuronal tissue is unclear. The purposes of this study are to reveal the spatial distribution of auditory neuronal tissue in CC deformity using electrically evoked auditory brainstem response (EABR) during cochlear implantation. Study Design: Retrospective case review. Setting: Cochlear implant (CI) center at a tertiary referral hospital. Patients: Five patients with CC deformity who underwent cochlear implantation and intraoperative EABR testing. Main Outcome Measures: Spatial distribution of electrodes that elicited an evoked wave V (eV) in EABR testing was evaluated in each CC deformity. Results: Electrically evoked auditory brainstem response testing demonstrated that electrodes attached on the inner wall of the anteroinferior cavity of the CC deformity successfully elicited a reproducible eV in all cases, and the latency of each eV was an approximately 4 ms, which is similar to those reported in patients without an inner ear malformation. Interestingly, in Case 1 with the lowest percentage of eV-positive electrodes (31.8%), CI-aided audiometric thresholds were changed, depending on the frequency allocation to eV-positive electrodes in the programming. Cochlear implantYmediated facial nerve stimulation was observed in 3 of 5 cases, and results of EABR testing were useful for optimizing the device program to decrease facial nerve stimulation without sacrificing CI-mediated auditory performance. Conclusion: The results of EABR testing suggested that auditory neuronal elements are distributed to the anteroinferior part of CC deformity, mainly around or near the inner wall of the cavity. In cases with CC deformity, EABR testing is useful to achieve the optimal electrode array placement and to adjust programming parameters of the implanted device, which might be essential to maximize CI outcomes and to decrease facial nerve stimulation. Key Words: Auditory nerveVCochlear implantVCommon cavityVElectrically evoked auditory brainstem responseVInner ear malformationVIntraoperative.
Inner ear malformations account for about 20% to 30% of congenital, severe, and profound hearing loss, and many children with an inner ear malformation are undergoing cochlear implantation (1,2). In 1987, Jackler et al. (3) originally proposed a classification of inner ear malformations based on the hypothesis in which termination of ordinary inner ear development leads to inner ear malformations, and the type of malformations, including Michel deformity (labyrinth aplasia), cochlear aplasia, common cavity (CC) deformity, cochlear hypoplasia, and incomplete partition, corresponds to each step of inner ear development. Later, Sennaroglu developed Jackler's classification and further divided cochlear hypoplasia and incomplete partition into cochlear hypoplasia type I-III and incomplete partition type I-III (2,4). In addition to accumulating evidences for the association between inner ear malformations and genetic mutations (5Y8), a limited variety in the shape of a malformed bony labyrinth between patients with the same class of malformations suggests that the majority of inner ear malformations are a result of genetic etiology (2). Common cavity deformity is the second most frequent inner ear malformation in which the cochlea and vestibule are confluent to form a single cystic cavity without internal architecture. Cochlear implantation in children with CC deformity is a challenge to clinicians because of its difficulty in array placement, high risk of cerebrospinal fluid (CSF) gusher and misinsertion into the internal auditory canal (IAC), and high incidence of facial nerve abnormalities (1,2,9). In 1997, McElveen et al. (10) described a transmastoid labyrinthotomy approach for cochlear implantation in CC deformity, which has been widely accepted as a surgical method for the placement of the electrode array in CC deformity while minimizing risk of injury to the facial nerve.
Cochlear implant (CI) electrically stimulates cell bodies of spiral ganglion neurons and their fibers to restore afferent input to the auditory central nervous system. Effective CI-mediated stimulation of auditory neuronal elements should be necessary to maximize CI outcome, but the spatial distribution of spiral ganglion neurons and auditory nerve fibers is unclear in CC deformity because of no differentiation between the cochlea and vestibule in addition to the lack of a modiolus that would contain spiral ganglion neurons. Hearing performance with CI was reported to vary widely among CC deformity cases (1), which might be caused by the difference in the distribution of the auditory neuronal elements. Recently, electrically evoked auditory brainstem responses (EABRs) using CI-mediated stimulus are used for the objective evaluation of auditory neuronal responses in the brainstem in patients with or without an inner ear malformation (1,11,12). In this study, we investigated the spatial distribution of auditory neuronal tissue in CC deformity using EABR and examined the utility of EABR data in optimizing programming parameters of the implanted device.

MATERIALS AND METHODS
We retrospectively examined 5 patients with CC deformity (Cases 1Y5) with congenital profound sensorineural hearing loss who underwent cochlear implantation at Kobe City Medical Center General Hospital from 2005 to 2013 (Table 1). Mean age at implantation was 27.4 months, and the mean follow-up period was 26.0 months. The type of inner ear malformations was determined using the computed tomographyYbased classification published by Sennaroglu (2). Discrimination between CC deformity and cochlear aplasia is sometimes difficult (2), and in this classification, cochlear aplasia is distinguished from CC deformity by a normal or dilated vestibule and semicircular canals. Moreover, cochlear aplasia is located at the posterolateral part of the IAC fundus, whereas the IAC enters at the center of a cavity in CC deformity. Our cases were classified as CC deformity based on the lack of at least 1 semicircular canal and the presence of a cavity in the anteroinferior direction to the fundus. Because the long axis of the CC deformity was often not parallel to the axial plane, coronal computed tomographic images were more effective for identifying the anterioinferior cavity, which was visualized inferior to the fundus of the IAC. Because the width of the midpoint of the IAC was greater than 2.5 mm in each case, no CC deformity was associated with a narrow IAC (2). Magnetic resonance imaging confirmed the presence of a vestibulocochlear nerve, but an isolated cochlear nerve bundle was not detected in all patients. The use of human subjects in this study was approved by the Research Ethics Committee of the Kobe City Medical Center General Hospital.
Case 1 initially underwent cochlear implantation with the standard transmastoid labyrinthotomy (10), followed by reimplantation using a modified transmastoid labyrinthotomy with a 3.0-mm-diameter hole at the posterolateral wall of the CC deformity for the electrode insertion. In the standard labyrinthotomy approach, a labyrinthotomy is created at the area where the lateral semicircular canal would normally be situated. The size of the labyrinthotomy is approximately 1.0 mm, which is large enough for electrode insertion. The modified labyrinthotomy approach was developed from the standard labyrinthotomy approach by changing the size of a labyrinthotomy. In the modified labyrinthotomy approach, most of the posterosuperior wall of the CC deformity was removed to make a large hole of 3.0 mm in diameter, which allowed better access into the anteroinferior part of the CC deformity. The IAC fundus could be identified through the large labyrinthotomy, and a prebent electrode array was inserted into the anteroinferior cavity beyond the fundus of the IAC with the curved end of the electrode array foremost to prevent intrameatal placement or undesirable folding of the tip. Immediately after electrode insertion, we gently filled the cavity of the CC deformity with soft tissue to push the electrode array in an anteroinferior direction for attaching electrodes on the wall of the anteroinferior cavity. The hole of labyrinthotomy was then covered by small pieces of bone and sealed by a thin layer of bone pate with fibrin glue. Continuous facial nerve monitoring was used to prevent facial nerve injury during mastoidectomy and labyrinthotomy. In the other patients, this modified transmastoid labyrinthotomy approach was conducted in the initial implantation. Nucleus device with 22 active electrodes (Ch1YCh22), including CI24RST, CI24REST, or CI422, was implanted in all cases. The first 2 of these devices have full-banded electrodes, while CI422 has halfbanded electrodes that are originally designed to point toward the modiolus of the cochlea. CI422 was used in Cases 4 and 5, in which the electrode array was turned around to position halfbanded electrodes close to the inner wall of the cavity. Intraoperative EABR testing was performed with Nucleus Custom Sound EP software (Cochlear, Corp., NSW, Australia). The biphasic electrical stimuli with a stimulus pulse width of 50 to 100 Ks and 200 to 230 current levels were delivered at 20 Hz of pulse rate using MP 1 + 2 mode. Other conditions were defaults in the autoNRT program. The EABR was recorded by Neuropack (Nihon Koden, Tokyo, Japan) with a filter setting of 20 Hz to 3 kHz on the opposite side to minimize artifacts of the implanted device. At least 500 sweeps were averaged. All EABR tests were performed without neuromuscular blockade to detect CI-mediated facial nerve stimulation (FNS). The presence of an evoked wave V (eV) was determined by (i) reproducible responses with amplitude greater than 0.1 KV, (ii) a currentdependent increase in amplitude, and (iii) latency of the wave greater than 3 ms, which are developed from the criteria in the previous report (13). Once a putative eV was identified, an intensity of CI-mediated stimulus was increased stepwise by 3 or 5 current levels to confirm the current-dependent increase in the amplitude of the putative eV. During these sessions, only a few step increases of current intensity sometimes resulted in emergence of an unusually large biphasic response. This response differed from the putative eV, which showed a currentdependent increase in the amplitude in the previous sessions FIG. 1. Results of EABR testing in Case 1 before and after the reimplantation. A, EABR testing after the initial implantation with the standard labyrinthotomy approach showing eVs in Ch17 and Ch19 among the 11 tested electrodes with an odd number. The latency of these eVs is approximately 5 ms (arrowheads). B, A maximum intensity projection of the T2-weighted magnetic resonance image of the CC deformity on the implanted side. The anteroinferior part of the CC deformity (AI) is smaller than the posterosuperior part (PS). C, Radiograph of the initial implantation demonstrating that Ch17 and Ch19 with a positive eV in EABR testing (circles) seem to be outside but near the anteroinferior part of the CC deformity (dotted line). D, EABR testing after the reimplantation with the modified labyrinthotomy approach showing a distinct eV in 7 of 22 electrodes. The latency of these eVs ranges from 3.8 to 4.1 ms (arrowheads). E, Radiograph after the reimplantation demonstrating that electrodes with a positive eV (circles) are located at the curved end of the prebent electrode array inserted in the anteroinferior part of the CC deformity (dotted line). CC indicates common cavity; EABR, electrically evoked auditory brainstem response.
with a lower intensity of stimulus, with respect for its biphasic waveform, relatively large amplitude, and late latency (9 4 ms). These characteristic features suggest that this response might result from myogenic activity, probably facial myogenic compound action potential, although an obvious facial twitching was not observed during the EABR testing. As reported previously, the threshold for a facial myogenic compound action potential is significantly lower than the threshold for observable facial movement, supporting this conclusion (14). Comparison of results in EABR tensing between with and without the use of a neuromuscular blocking agent would be effective in evaluating contamination of myogenic compound action potentials in EABR, but we did not conduct EABR testing under neuromuscular blockade because of the limitation in time. In Case 1, EABR tests were also performed under sedation at 1 year after the initial implantation. The position of the inserted electrode array was evaluated by intraoperative X-ray and postoperative computed tomography. Hearing outcomes with the CI were evaluated by hearing thresholds, speech discrimination scores of closed-set Japanese infant words, and category of auditory performance (CAP) scores (15) at 40 months after the reimplantation in Case 1 and at 29, 22, 18, and 9 months after the implantation in Cases 2, 3, 4, and 5, respectively. The speech discrimination test was performed only if the CAP score was 4 or higher (Table 1).

EABR Testing After the Initial CI in Case 1
In Case 1, the radiograph obtained during the initial cochlear implantation demonstrated that most of the electrodes were located within the CC deformity, but the CI-aided performance was still poor (CAP score 1) even after 1 year of use of CI. Electrically evoked auditory brainstem response testing under sedation at that time elicited a reproducible eV only at 2 of 11 tested electrodes with an odd number (18.2%, Ch17 and Ch19). The latency of the detected eVs was approximately 5 ms (Fig. 1A) and considerably longer than 4.05 ms, which was previously reported in patients without an inner ear malformation at 1 year after implantation (16). Postoperative computed tomographic images indicated that the electrode array was fully inserted in the CC deformity but did not reach the anteroinferior cavity owing to stacking at the fundus of the IAC (Fig. 2, A and B). The eV-positive Ch17 and Ch19 appeared to be located outside but near the anteroinferior cavity of the CC deformity (Fig. 1, B and C). These results suggested that suboptimal electrical stimulation of auditory neuronal elements was responsible for the inadequate auditory performance.

EABR Testing During the Reimplantation in Case 1
To overcome this problem, we performed reimplantation using the modified transmastoid labyrinthotomy with a larger hole for electrode insertion to achieve better access into the anteroinferior cavity. Electrically evoked auditory brainstem response testing during the reimplantation revealed a clear eV at 7 of 22 tested electrodes (31.8%, Ch10YCh16), and the latency of eV ranged from 3.8 to  (Fig. 1D), similar to 4.05 ms that was reported in patients without an inner ear malformation (16). Radiographs and computed tomographic images demonstrated that these 7 electrodes with a positive eV were located at the curved end of the bent array that was inserted into the anteroinferior part of the CC deformity (Fig. 1E), attaching on the inner wall of this area (Fig. 2, C and D).

EABR-Based Programming in Case 1
After the reimplantation, frequency-specific hearing thresholds in Case 1 changed depending on the frequencyto-electrode allocation in the programming. When frequencies of 188 to 7,938 Hz were allocated to all 22 electrodes, the hearing thresholds were 40 dB at 1,000 and 2,000 Hz, for which Ch10 to Ch16 with a positive eV were responsible, but greater than 80 dB for other tested frequencies. On the other hand, when frequencies from 188 to 5,063 Hz were allocated to 8 sequential electrodes, including eV-positive Ch10 to Ch16, the hearing thresholds ranged from 30 to 50 dB at all tested frequencies (Fig. 3). Even after the reimplantation, CI-mediated FNS was induced by electrical stimulation of Ch10 to Ch12 and Ch16 to Ch20. Because Ch17 to Ch20 failed to elicit eV and seemed not to contribute to auditory perception, these electrodes were deactivated. Regarding Ch10 to Ch12 and Ch16 with a positive eV, the maximum stimulation level was set to a value below the threshold of CI-mediated FNS.

EABR Testing and EABR-Based Programming in the Other CC Deformity Cases
In the other 4 patients who underwent the modified transmastoid labyrinthotomy at the initial implantation, postoperative computed tomographic images showed the optimal position of the electrode array (data not shown). Although the size and shape of each CC deformity differed among the cases (Fig. 4, AYD), electrodes inserted in the anteroinferior cavity successfully elicited eVs in all 4 cases, similarly to Case 1 (Fig. 4, EYH). As shown in Figure 4, I to L, reproducible eVs were detected in 9 of 11 tested electrodes with an even number (81.8%) in Case 2, 8 of 11 with an odd number (72.7%) in Case 3, 12 of 22 electrodes (54.5%) in Case 4, and 5 of 11 with an even number (45.5%) in Case 5. The latency of the detected eVs was approximately 4 ms in all cases (Fig. 4, IYL). Using a program in which several electrodes without eV at the most distal and/or proximal part of the electrode array were deactivated, their pure-tone hearing thresholds were 40 or 45 dB at 500, 1,000, and 2,000 Hz except for 50 dB at 2,000 Hz in Case 5 who had used his CI for only 9 months. Cases 2 and 4 experienced CI-mediated FNS, and almost all electrodes were responsible for this aversive symptom. In both patients, deactivation of some electrodes without eV in addition to a decrease in current levels of the other electrodes including eV-positive ones was effective in reducing FNS.

CI Outcomes and Other CI-Related Problems
in CC Deformity Cases Before implantation, no patient could detect sounds, indicating that their preoperative CAP score was zero, but auditory perception improved after activation of the CI in all patients ( Table 1). The postoperative CAP score reached to 6 in Cases 1 and 2 who had used their CI for more than 2 years, indicating that they understood common phrases without sign language or lip reading. Speech discrimination scores of closed-set infant words were 76% and 80% in Cases 1 and 2, respectively. The other 3 patients, Cases 3, 4, and 5, who had used their CI for less than 2 years, showed CAP scores of 4, 3, and 3, respectively, and Case 3 showed 40% of the infant word discrimination score (Table 1).
Minor CSF leakage occurred during implantation in Case 4, which was easily stopped by gently packing several pieces of soft tissue inside the cavity followed by sealing the labyrinthotomy site with periosteum and bone pate. After implantation, Cases 1 and 2 experienced dizziness, which spontaneously disappeared within a week (Table 1).

DISCUSSION
The present study demonstrated that reproducible eVs were elicited by electrical stimulation of electrodes that were located at the anteroinferior part of the CC deformity in all patients. Although the electrode array was almost FIG. 3. Change in CI-aided hearing thresholds in Case 1, depending on the frequency-to-electrode allocation in the programming of the device. When frequencies from 188 to 7,938 Hz are allocated to all 22 electrodes, the hearing thresholds are 40 dB for 1,000 and 2,000 Hz, for which eV-positive Ch10 to Ch16 are responsible, but greater than 80 dB for other tested frequencies (white triangles). However, when frequencies from 188 to 5,063 Hz are allocated to only 8 sequential electrodes including Ch10 to Ch16 with a positive eV in the EABR testing, the hearing thresholds range from 30 to 50 dB for all tested frequencies (black triangles). CI indicates cochlear implant; EABR, electrically evoked auditory brainstem response. fully inserted in each CC deformity using the same surgical procedure, the percentage of electrodes with a positive eV varied widely between patients, ranging from 31.8% to 81.8%. Although the percentage of eV-positive electrodes seems to be influenced by the size of the anteroinferior part of CC deformity, it is difficult to predict the exact number and position of eV-positive electrodes on the basis of radiographic findings, suggesting the importance of EABR testing in cases with CC deformity. Interestingly, in Case 1 with the lowest percentage of eV-positive electrodes, CIaided audiometric thresholds were changed, depending on which frequency was allocated to the eV-positive electrodes in the program of the implanted device. Both these electrophysiological and audiometric data indicate that auditory neuronal elements are mainly distributed in the anteroinferior part of the CC deformity. Common cavity deformity is thought to be caused by an arrest in differentiation of the otic vesicle during the fourth gestational week (3,4). In the normal development of an inner ear, the ventral portion of the otic vesicle elongates in the ventral direction, initiating cochlear development (17); therefore, the anteroinferior part of CC deformity might FIG. 4. Results of EABR testing in Cases 2 to 5. The upper, second, third, and bottom columns show data for Cases 2 to 5, respectively. A to D, Maximum intensity projection of T2-weighted magnetic resonance images of the CC deformity on the implanted side. AI and PS indicate the anteroinferior and posterosuperior parts of the CC deformity, respectively. E to H, Electrodes with a positive eV (circles) are located at the curved end of the prebent electrode array inserted in the anteroinferior part of the CC deformity. I to L, EABRs for 3 representative electrodes at the curved end of the prebent electrode array. The latency of these eVs is approximately 4 ms in all cases (arrowheads). CC indicates common cavity; EABR, electrically evoked auditory brainstem response. be programmed to differentiate to a cochlea. These findings support our conclusion regarding the anteroinferior distribution of auditory neuronal tissue in CC deformity.
In Case 1, moving the electrode array closer to the inner wall of the anteroinferior part of the CC deformity shortened the eV latency from approximately 5.0 ms to 3.8 to 4.1 ms, which is close to 4.05 ms measured in patients without inner ear malformations (12). Given that changes in stimulus intensity do not significantly affect latencies of evoked waves in EABR testing (12), the eV latencies of this study are comparable to those of the previous study, although the amplitude and pulse width of the EABR stimulus are slightly different between studies. Thus, the eV latency of Case 1 at the reimplantation suggests that the auditory neuronal elements were mainly distributed around or near the inner wall of the anteroinferior part of the CC deformity. The longer distance between a stimulating electrode and neuronal tissue at the initial implantation might have caused ineffective auditory stimulation in Case 1. This conclusion is supported by the previous histologic study, which reported that neural elements are likely to lie in the wall of CC deformity (18). Another possible explanation for the change in the latency of the waves after the reimplantation is that the detected waves in the initial EABR were not auditory neural responses, but the myogenic compound action potential caused by FNS. However, the Ch17-or Ch19-mediated facial twitching was not observed in the EABR testing, and electrical stimulation of other electrodes located nearer the facial nerve, such as Ch11 and Ch13, elicited no obvious response, suggesting that the waves elicited by the stimulation of Ch17 and Ch19 in the initial EABR testing may be auditory neural responses rather than myogenic responses.
We would also emphasize that magnetic resonance imaging failed to identify an isolated cochlear nerve bundle, but the vestibulocochlear nerve contained sufficient cochlear nerve fibers to transmit the CI-mediated auditory signals to the brainstem in all CC deformity cases. Because CC deformity is thought to be caused by developmental arrest before differentiation between the cochlea and vestibule (3,4), it is possible that a cochlear nerve is not separated from the vestibular nerves to constitute a single vestibulocochlear nerve because the cochlea and vestibule are represented by a single chamber.
In the present study, 3 (60%) of the 5 subjects experienced CI-mediated FNS, which is consistent with the previous study reporting the high frequency of FNS among patients with inner ear malformations who had implants (14,19,20). Because electrical stimulation close to the facial nerve as well as exceeding or leaking electrical current causes FNS, deactivating the responsible electrodes or reducing current levels in these electrodes is usually effective in reducing FNS. On the other hand, in cases with a severe inner ear malformation, high current level and/or increased pulse width are often required to achieve good auditory performance (11,20), suggesting a necessity to adjust the current level to an appropriate value that is high enough to provide sufficient auditory input but lower than the threshold for FNS. Usually, the programming parameters of the device are adjusted based on the patients auditory behavioral responses. Cases 1, 2, and 4, however, experienced FNS before improvement of their auditory performance, and furthermore, many electrodes were responsible for the FNS, showing difficulty in making the appropriate program for their CI. Previous studies demonstrated that nonbehavioral measures including EABR are useful in determining useful cochlear implant stimulation levels, particularly in young children and infants with limited auditory experience (21). In this study, although precise EABR thresholds were not examined, we gradually and carefully increased the current level and pulse width of the electrical stimulus in each electrode by referring to the amplitude and pulse width of the electrical stimulus that had evoked eV in the intraoperative EABR testing, and if FNS was observed, we decreased the current levels (amplitudes) of the responsible electrodes below the threshold of FNS or sometimes deactivated these electrodes. This method might be useful in decreasing FNS without sacrificing CI-mediated auditory responses especially before the patients show clear auditory responses.
Based on the experience of Case 1, we used the modified transmastoid labyrinthotomy with a large hole in the other 4 CC deformity cases at the initial implantation. Although the shape of the CC deformity varied among these cases, the optimal electrode array placement was achieved, and a reproducible eV with approximately 4 ms of latency was elicited by electrical stimulation of the electrodes in the anteroinferior cavity in all cases. These results suggest the effectiveness of this approach for cochlear implantation in CC deformities, especially when the anteroinferior cavity is small like the CC deformity in Case 1. Regarding this modified transmastoid labyrinthotomy approach, however, 3 points require attention: (i) damage to the auditory neuronal tissue in the CC deformity, (ii) probable difficulty in the control of CSF leaks, and (iii) damage to the vestibular system. Atraumatic round window insertion techniques have attracted recent attention in preserving residual hearing (22,23). In cases with CC deformity, patients are usually deaf before implantation, but it is possible that less trauma in the auditory neuronal elements in CC deformity leads to better CI-aided outcomes. In the present study, the results of EABR testing in Case 1 clearly demonstrated improved auditory brainstem responses after reimplantation with the modified transmastoid labyrinthotomy, suggesting that optimal array placement attached on the inner wall of the anteroinferior cavity might be more important than atraumaticity at least in this patient. In Case 4, the IAC and CC deformity were connected through a small defect of the IAC fundus and CSF leakage after the labyrinthotomy was stopped by plugging the IAC fundus with pieces of soft tissue. However, if CC deformity widely communicates with the IAC, a risk of a postoperative CSF leakage would increase because the larger labyrinthotomy leads to more difficulty in achieving a complete seal to manage a CSF leak. The third issue is that the posterior part of the CC deformity, which may correspond to a primitive vestibule, is destroyed in the modified transmastoid labyrinthotomy. A previous study showed vestibular evoked myogenic potentials in some cases with CC deformity (24), suggesting that destruction of the posterior part of the cavity might impair vestibular function on the side that is operated on. In fact, Cases 1 and 2 showed mild disequilibrium after surgery, but their symptom disappeared within a few weeks, probably due to vestibular adaptation (25) or recovery of some vestibular function. However, because the long-term effect and safety of the modified transmastoid labyrinthotomy approach have not been established, we have to closely monitor auditory and vestibular performance of our 5 cases of CC deformity for a long time.
Postoperative CAP scores varied widely between patients in this study. Since the follow-up period was different between patients, the short duration of CI use might be responsible for the immature auditory development especially in Cases 4 and 5. Long-term observation for up to 4 years will be required to lead to a definite conclusion as described by a previous study (26). What is noteworthy here is that Case 1 who showed eV only at 7 (31.8%) of 22 electrodes exhibited 6 in CAP score and 76% in the infant word discrimination test at 4 years after the initial implantation, which are similar to those observed in the 2-year postoperative Case 2 who showed eV at almost all electrodes (81.8%). These data suggest that even if the only limited number of electrodes shows eV in EABR testing, the patient might achieve sufficient CI-aided auditory performance after long-term use of the CI with an appropriate program. Previous studies using patients with CI without an inner ear malformation demonstrated that the speech discrimination saturated around 8 electrodes and did not improve when more electrodes were activated (27,28). In Case 1, at least 7 electrodes successfully activated auditory neurons in the brainstem, and this number might be enough to achieve sufficient CAP and infant word discrimination scores, although spatial distribution of auditory neuronal elements might be different between the CC deformity and a cochlea without a malformation. Because we examined only 5 patients with CC deformity and the follow-up duration was short in this study, further investigation should be necessary to reveal a relationship between the number of eV-positive electrodes and CImediated auditory performance.

CONCLUSION
The present study using EABR testing demonstrated that auditory neuronal tissue is distributed in the anteroinferior part of CC deformity, mainly around or near the inner wall of the cavity in all cases, regardless of the fact that the shape of the CC deformity was widely different between patients. In cases with CC deformity, EABR testing is useful to achieve the optimal electrode array placement and to adjust programming parameters of the implanted device, which might be essential to maximize CI outcomes and to decrease FNS.