Mee Hyun Song, MD, PhD; Sang Cheol Kim, MD; Jinna Kim, MD, PhD; Jin Woo Chang, MD, PhD; Won-Sang Lee, MD, PhD; Jae Young Choi, MD, PhD

Transcription

1 The Laryngoscope VC 2011 The American Laryngological, Rhinological and Otological Society, Inc. The Cochleovestibular Nerve Identified During Auditory Brainstem Implantation in Patients with Narrow Internal Auditory Canals: Can Preoperative Evaluation Predict Cochleovestibular Nerve Deficiency? Mee Hyun Song, MD, PhD; Sang Cheol Kim, MD; Jinna Kim, MD, PhD; Jin Woo Chang, MD, PhD; Won-Sang Lee, MD, PhD; Jae Young Choi, MD, PhD Objectives/Hypothesis: To analyze the value of preoperative diagnostic tools in predicting the status of the cochleovestibular nerve (CVN) in patients with narrow internal auditory canals (IAC). Study Design: Retrospective case series at a tertiary hospital. Methods: Eight profoundly deaf patients with narrow IACs who received auditory brainstem implantation were included in this study. The results of preoperative imaging, electrophysiologic, and auditory tests were correlated with the CVN status identified during auditory brainstem implantation. Results: Temporal bone computed tomography (CT) findings, including the patency of the bony cochlear nerve canal and the diameter of the IAC, were limited in accurately reflecting the status of the CVN. Magnetic resonance imaging (MRI) and preoperative auditory responses to either pure tone or environmental sounds were more accurate markers for detecting the presence of a CVN than CT; however, there were limitations in cases with a very thin CVN or combined severe mental retardation. Absence of promontory or intracochlear electrically evoked auditory brainstem responses were not always indicative of an absent CVN. Conclusions: Visualization on MRI and detection of auditory responses suggested the presence of a CVN in patients with narrow IACs; however, the possibility of the presence of a CVN should be considered even when there is no clear evidence of a CVN on preoperative evaluations. Therefore, physicians should be prudent when determining candidacy for cochlear implantation or auditory brainstem implantation in patients with narrow IACs. Key Words: Cochleovestibular nerve, narrow internal auditory canal, auditory brainstem implant, magnetic resonance imaging, computed tomography, electrically evoked auditory brainstem response. Level of Evidence: 3b. Laryngoscope, 121: , 2011 INTRODUCTION With technical advances, cochlear implantation has provided patients with congenital sensorineural hearing loss the benefit of auditory stimulation and speech development, even in cases of severe inner ear malformations. 1 However, the outcome of cochlear implantation has been unsatisfactory in certain populations, including patients with narrow internal auditory canals (IAC) who often have a poor prognosis after cochlear implantation due to frequently combined From the Department of Otorhinolaryngology (M.H.S.), Kwandong University College of Medicine, Goyang, South Korea; and Department of Otorhinolaryngology (S.C.K., J.Y.C., W-S.L.), Yonsei University College of Medicine, Seoul, South Korea, Department of Radiology (J.K.), Yonsei University College of Medicine, Seoul, South Korea, Department of Neurosurgery (J.W.J.), Yonsei University College of Medicine, Seoul, South Korea. Editor s Note: This Manuscript was accepted for publication February 15, This study was supported by a grant of the Korea Healthcare Technology R&D Project, Ministry for Health, Welfare & Family Affairs, Republic of Korea (A090200). The authors have no conflicts of interest to disclose. Send correspondence to Dr. Jae Young Choi, Department of Otorhinolaryngology, Yonsei University College of Medicine, 134 Shinchon-dong, Seodaemun-gu, Seoul, , South Korea. jychoi@yuhs.ac DOI: /lary cochlear nerve deficiency. 1,2 Until the 1990s, narrow IAC was considered to be a contraindication to cochlear implantation because of the possibility of an absent cochlear nerve, 3 and controversy still exists regarding the candidacy of cochlear implantation in these patients. Recently, more promising results have been reported in cases of narrow IACs after cochlear implantation, even when the cochlear nerve is not definitely visualized on preoperative magnetic resonance imaging (MRI). 4,5 Auditory brainstem implants (ABI) were initially implanted in neurofibromatosis type-2 patients with hearing loss, which resulted in very limited speech perception. 6 Review of data for more than a decade has revealed that better outcomes after ABI can be anticipated in nontumor patients compared with those having tumorous conditions. 6 Especially, ABI in patients with narrow IACs and deficient cochlear nerves has shown promising auditory outcomes in recent reports. 7 Considering that satisfactory results may be achieved by cochlear implantation in some of these patients, it is often difficult to decide whether to perform cochlear implantation or auditory brainstem implantation as the initial treatment in patients with narrow IACs because preoperative evaluations often fail to provide accurate information regarding the presence of a functioning cochlear nerve. 1773

2 TABLE I. Demographic Data of Patients with Narrow Internal Auditory Canal. Patient No. Age* (Y;M) IAC Diameter (mm) Combined Disease Inner Ear Anomaly CI History (Side) ABI Site 1 1;6 1.5 Incomplete FN palsy (L), mild MR R 2 5; CHARGE, blindness, MR IP-II, SCC aplasia þ (R) R 3 18; þ (L) R 4 2; Incomplete FN palsy (R) þ (R) L 5 9;4 1.8 þ (R) R 6 6;1 2.3 Sacral deformity þ (R) R 7 2;5 2.4 SCC dysplasia R 8 7;5 2.3 CHARGE, MR IP-II, SCC aplasia þ (L) L *Age at auditory brainstem implantation. IAC ¼ internal auditory canal; CI ¼ cochlear implantation; ABI ¼ auditory brainstem implantation; Y ¼ year; M ¼ month; FN ¼ facial nerve; MR ¼ mental retardation; IP-II ¼ incomplete partition type II; SCC ¼ semicircular canal. Various preoperative evaluations are currently used to predict the status of the cochleovestibular nerve (CVN), including imaging and electrophysiologic testing. The patency of the bony cochlear nerve canal (BCNC) is considered to be an important parameter on temporal bone computed tomography (CT), whereas the number of nerves seen within the IAC on the parasagittal view of the high-resolution temporal MRI provides information about the status of the CVN. 4,8 10 Electrophysiologic testing, including promontory or intracochlear electrically evoked auditory brainstem response (EABR), is thought to have prognostic value in predicting the results of cochlear implantation in patients with narrow IACs. 11,12 However, some patients with suspected cochlear nerve deficiency who did not demonstrate any clear response on promontory EABR have been reported to achieve limited auditory responses after cochlear implantation. 13 Therefore, there is a need to more accurately analyze the value of preoperative evaluations in patients with narrow IACs by correlating these findings with the actual anatomical status of the CVN as identified during surgery. Based on surgical findings during ABI in patients with narrow IACs, the authors have correlated the anatomic status of the CVN to the results of preoperative imaging, electrophysiologic and auditory testing to determine the predictive value of diagnostic tools for the evaluation of CVN deficiency. MATERIALS AND METHODS Subjects Eight patients with profound sensorineural hearing loss and narrow IACs received ABI at Severance Hospital from July 2008 to July A narrow IAC was defined when the diameter of the IAC on an axial view of temporal bone CT was less than 3 mm (range: mm). Six of eight patients had received cochlear implants prior to ABI either on the ipsilateral or contralateral side. The reasons for initially performing cochlear implantation instead of ABI in these patients were as follows (Table I): (1) the patient had been implanted at another instituion (Patient 3), (2) a CVN could be visualized on MRI on the ipsilateral (Patients 6 and 8) or contralateral (Patient 4) side as ABI, (3) partial responses were detected on preoperative auditory steady state response test (Patient 5), or (4) the parents strongly wanted to try cochlear implantation despite extensive counseling about the expected poor outcome in patients with narrow internal auditory canals (Patient 2). The ages of the patients at the time of ABI surgery ranged from 18 months to 19 years. Various degrees of developmental delay were found in all of the patients, whereas five patients had additional combined conditions such as mental retardation, incomplete facial nerve palsy, blindness, sacral deformity, or CHARGE syndrome (Table I). Imaging Study The temporal bone CT scan was performed with a 16 multidetector row CT scanner (Somatom Sensation 16; Siemens, Erlangen, Germany) using a standard temporal bone protocol. Contiguous 0.7-mm scans of the temporal bone were acquired in the axial plane and reformatted coronally with 1.0-mm increments. CT images were performed, digitally stored, and displayed by using the Picture Archiving Communication System (PACS) (Centricity; GE Healthcare, Milwaukee, WI). The narrowest portion of the IAC diameter on axial images was measured, and the width of the BCNC located at the fundus of the IAC was obtained at the midportion between the base of the modiolus and the inner margin of the fundus of the IAC on axial images, by using the electronic calipers provided by the PACS. 14 The BCNC was considered stenotic if the width was <1.4 mm. 15,16 MRI was acquired by using a 3.0-T (Achieva; Philips Medical Systems, Best, The Netherlands) or 1.5-T system (Intera; Philips Medical Systems, Best) with a six-channel sensitivity encoding (SENSE) head coil. The targeted parasagittal scan perpendicular to the long axis of the IAC was obtained with T2- weighted three-dimensional (3D) turbo spin-echo (TSE) sequence with driven equilibrium RF reset pulse (DRIVE), following routine MR sequences with spin-echo T1- and T2- weighted images. The sequence parameters for the T2-weighted 3D FSE sequence with DRIVE were as follows: repetition time (TR)/echo time (TE) ¼ 1,500/200 milliseconds, 256 acquisition/ 256 reconstruction, 15-cm field of view, 1.5-mm section thickness with a 0.75-mm overlap, number of acquisitions ¼ 2, and the scan time was less than 5 minutes. EABR Promontory EABR was performed in all patients before they underwent either cochlear implantation or ABI. In the four patients who either had not previously received cochlear implantation (Patients 1 and 7) or had undergone cochlear implantation on the contralateral side prior to ABI (Patients 3 and 4), only the 1774

3 Fig. 1. Patency of the bony cochlear nerve canal (BCNC) and the status of the cochleovestibular nerve (CVN) identified during auditory brainstem implant surgery. (A) The presence or absence of the CVN during surgery is presented in patients with obliterated or patent BCNC. (B) The BCNC was found obliterated (black arrow) on temporal bone computed tomography of the right side in Patient 3. (C) A thin CVN (white arrowhead) was identified during auditory brainstem implant surgery of the right ear in Patient 3. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary. com.] promontory EABR could be performed. Intracochlear EABR was performed in three of four patients who had received cochlear implantation prior to ABI on the same side, in the same manner as previously described using SCLIN 2000 software (Advanced Bionics Corp., Sylmar, CA) and Custom Sound EP software (Cochlear Corp, NSW, Australia) for Clarion and Nucleus devices, respectively. 12 The EABR waves were recorded using a Navigator Pro AEP (Bio-logic Systems Corp., Mundelein, IL). Audiologic Evaluations Prior to cochlear implantation or ABI, younger children underwent auditory brainstem response testing, whereas pure tone audiometry was administered to older children who were more cooperative. In patients who had received cochlear implants prior to ABI, behavioral response to sound and/or pure tone audiometry under sound-field conditions was assessed at least 12 months following cochlear implantation. Data Analysis The results of preoperative evaluations, including temporal bone CT, temporal MRI and EABR, were correlated with the surgical findings of CVN status at the cerebellopontine angle during ABI surgery. of age. An auditory response was achieved for approximately 1 to 2 years after cochlear implantation, which implied the presence of a CVN on the left side; however, the patient s hearing deteriorated progressively afterward, leading to ABI on the right side. Despite an obliterated BCNC on the right side, a thin CVN was identified during ABI surgery (Fig. 1C). The diameters of IAC measured on axial images of temporal bone CT were correlated with the presence or absence of the CVN. In two patients who showed an IAC diameter of mm (Patients 1 and 2), a CVN was not found at the cerebellopontine angle during ABI (Fig. 2). Two of three patients with IAC diameters ranging from mm (Patients 3 and 5) revealed a visible CVN during surgery. Similarly, of the three patients exhibiting IAC diameters of more than 2 mm, two patients (Patients 6 and 8) presented with an identifiable CVN during the ABI procedure. These findings suggest that the presence or absence of the CVN could not be accurately predicted by the diameter of the IAC measured on RESULTS Temporal Bone CT Findings and the Status of the CVN The BCNC was obliterated in five of eight patients with narrow IACs (Patients 1 3, 6, and 8) and was patent in three patients (Patients 4, 5, and 7) (Fig. 1A). In the three patients with patent BCNC, the widths were 0.7, 1, and 1 mm, which are considered stenotic according to the criteria proposed by Stjernholm et al. 15 The CVN was surgically identified at the cerebellopontine angle in three of five patients despite obliteration of the BCNC on temporal bone CT, whereas only one of three patients with patent BCNC demonstrated an identifiable CVN during surgery (Fig. 1A). An 18-year-old female patient (Patient 3) who had an obliterated BCNC on both sides on temporal bone CT (Fig. 1B) had received cochlear implantation on the left side at 7 years Fig. 2. The correlation between internal auditory canal (IAC) diameter and the status of the cochleovestibular nerve (CVN). 1775

4 TABLE II. MRI and Surgical Findings of Neural Components in Narrow IAC Patients. MRI Finding Patient No. ABI Site Axial Parasagittal Surgical Finding 1 R Single nerve Single nerve FN only 2 R Single nerve Single nerve FN only 3 R Single nerve NA Thin CVN, FN 4 L No visible nerve in IAC No visible nerve in IAC Empty IAC 5 R Single nerve Single nerve Thin CVN, FN 6 R 1 normal-sized nerve 1 normal-sized nerve Thin CVN, FN 1 small-sized nerve 1 small-sized nerve 7 R Single nerve Single nerve FN only 8 L 1 normal-sized nerve 1 normal-sized nerve Thin CVN, FN 1 small-sized nerve 1 small-sized nerve ABI ¼ auditory brainstem implantation; IAC ¼ internal auditory canal; FN ¼ facial nerve; NA ¼ not available; CVN ¼ cochleovestibular nerve. temporal bone CT, although cases with narrow IACs measuring less than 1.5 mm may be more frequently associated with the absence of the CVN. Temporal MRI Findings and the Status of the CVN Table II and Figure 3 show MRI and surgical findings of neural components identified at the cerebellopontine angle. In one patient (Patient 4), no neural component entering into the IAC was found on MRI, and the facial nerve was suspected to pass through a separate canal (Figs. 4A C). Surgical findings revealed an empty IAC with the facial nerve entering into a separate canal, consistent with the MRI findings (Fig. 4D). Four of the patients presented with a single neural component within the IAC on MRI, corresponding with the surgical findings in all but one patient (Patient 5) in whom a very thin CVN and a facial nerve were identified entering the IAC (Figs. 5B and C). In one patient (Patient 3), the parasagittal view was not available, and the axial view of MRI showed only a single nerve within the IAC; however, a very thin CVN was identified at the cerebellopontine angle in the surgical field (Fig. 1C). In Patients 6 and 8, a CVN much smaller than the facial nerve was visualized on axial and parasagittal views of MRI, and consistently, a thin CVN was also observed during surgery (Figs. 6A C). Together, although MRI findings were often correlated with the surgical findings regarding the presence or absence of the CVN, a very thin CVN identified during ABI was not able to be seen on MRI in one patient. the cochlear implants (Patients 2, 5, 6, and 8). Of the two patients (Patients 2 and 8) who did not demonstrate any response on intracochlear EABR, one patient (Patient 8) revealed a very thin CVN during ABI surgery (Fig. 6C D). In another Patient (Patient 5), only muscle potentials were demonstrated on intracochlear EABR without any auditory response, and a CVN was displayed during ABI surgery (Figs. 5C D). Auditory Response and the Status of the CVN The audiologic test results were analyzed to determine how accurately they reflected the status of the CVN (Table III). In Patients 5 and 6, although a response was detected on preoperative auditory brainstem response or auditory steady-state response testing and environmental sound detection could be achieved after cochlear implantation, facial twitching made it impossible to continue auditory rehabilitation using a cochlear implant. As expected, a thin CVN was observed during ABI in these cases. In Patient 3, the auditory brainstem response test that had been performed at 2 EABR Findings and the Status of the CVN Figure 7 demonstrates the correlation between the results of EABR and the presence or absence of a CVN during ABI. Promontory EABR failed to show any consistent response in any of the patients. Despite the lack of response on promontory EABR in any of these patients, a CVN was identified during ABI in four patients. Intracochlear EABR was able to be carried out in three patients who received ABI on the same side as Fig. 3. The correlation between magnetic resonance imaging (MRI) findings and the status of the cochleovestibular nerve (CVN). NA ¼ not available. 1776

5 Fig. 4. Magnetic resonance imaging (MRI) and surgical findings during auditory brainstem implantation in Patient 4. (A B) Axial view of MRI showed no neural component entering the internal auditory canal (IAC) at the cerebellopontine angle (thick arrow; A) or within the IAC (thin arrow; B). (C) On parasagittal view of MRI, no neural component was found entering the IAC at the cerebellopontine angle (black arrow). (D) During auditory brainstem implantation, no neural component was observed entering the IAC (black arrowhead) and the facial nerve (asterisk) was found entering into a separate canal. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] years of age showed a response at a threshold of 100 db nhl on the right side, and the pure tone audiometry performed at 17 years of age demonstrated a threshold of 100 db HL at 250 Hz. During ABI surgery, a thin CVN was also detected in this patient (Fig. 1C). The three patients (Patients 1, 4, and 7) in whom no response was detected on either preoperative auditory brainstem response or behavioral testing to environmental sound demonstrated an absence of CVN during ABI surgery. Nevertheless, a 7-year-old patient (Patient 8) who exhibited mental retardation in addition to hearing loss revealed a thin CVN during ABI surgery despite a lack of any response on preoperative auditory brainstem response or on behavioral testing to environmental sound after cochlear implantation. DISCUSSION Preoperative radiographic evaluation of cochlear implant candidates is clinically important for determining whether the patient is suitable for implantation and for predicting auditory performance after cochlear implantation together with other prognostic factors. Recent improvement of image resolution and sequence development have made MRI the most important diagnostic tool for detection of the cochlear nerve, and many studies have emphasized the necessity of MRI as a part of the preoperative evaluation before cochlear implantation The cross-sectional images of the IAC make more detailed identification of the neural components possible. Nevertheless, there have been some reports of cases in which an auditory response was achieved after cochlear Fig. 5. Findings of preoperative evaluations and surgical findings in Patient 5. (A) Stenotic bony cochlear nerve canal (thick arrow) and narrow internal auditory canal were seen on temporal bone computed tomography. (B) A single nerve was found at cerebellopontine angle (black arrow) and within the internal auditory canal (white arrow) on the axial view of magnetic resonance imaging. (C) Surgical finding revealed a very thin cochleovestibular nerve (white arrowheads) and a facial nerve (black arrowhead) at the cerebellopontine angle. (D) Intracochlear electrically evoked brainstem response showed only muscle potentials without any identifiable auditory response. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.] 1777

6 Fig. 6. Findings of preoperative evaluations and surgical findings in Patient 8. (A B) A cochleovestibular nerve (arrowhead) and a facial nerve (arrow) were identified on axial and parasagittal views of magnetic resonance imaging. (C) During auditory brainstem implantation, the presence of a thin cochleovestibular nerve (black arrowheads) and a facial nerve (black arrow) was surgically confirmed. (D) No response was identified on intracochlear electrically evoked brainstem response. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary. com.] implantation even in the absence of a visible cochlear nerve on MRI. 16 In this study, a thin CVN not visualized on MRI was identified during ABI surgery in one patient. Therefore, MRI has limited ability to accurately determine the cochlear nerve status. The reasons may be due to limitation of spatial resolution to detect a very thin CVN and subtle motion artifacts. High-resolution temporal bone CT is the primary tool for the diagnosis of a narrow IAC. Many studies have clearly demonstrated that the auditory performance after cochlear implantation is worse in patients with narrow IACs than it is in those with normal inner ears or with other inner ear malformations. 1,2,20 According to the results of the present study, the presence or absence of the CVN could not be accurately predicted based on the diameter of the IAC, although diameter of less than 1.5 mm seemed more associated with the absence of a CVN. The BCNC has been regarded as another marker identifiable on temporal bone CT that may provide information about the status of the cochlear nerve and postoperative outcome after cochlear implantation. 1,8,16,21 In this study, three of the five patients who displayed an obliterated BCNC on temporal bone TABLE III. Auditory Response and Surgical Findings of Internal Auditory Canal. Fig. 7. The correlation between the results of electrcially evoked auditory brainstem response (EABR) and the surgical findings of auditory brainstem implantation. Promontory EABR (P-EABR) failed to show any consistent response in any of the patients regardless of the presence or absence of the cochleovestibular nerve (CVN). One of the two patients with negative response and one patient showing muscle potentials on intracochlear EABR (C- EABR) demonstrated thin CVNs during surgery. NA ¼ not applicable, M ¼ muscle potential. Patient No. PTA or ABR Response to Environmental Sound Surgical Finding of CVN 1 ABR ( ) Absent 2 ABR ( ) Absent 3* PTA (þ; 100 db at 250 Hz) þ Thin CVN 4 ABR ( ) Absent 5 PTA (þ; 95 db at 500 Hz) þ Thin CVN 6 PTA (þ; db at 250 1k Hz) þ Thin CVN 7 ABR ( ) Absent 8 ABR ( ) Thin CVN *Response to pure tone and environmental sound convert to negative after age of 9 years. These two patients were not able to use cochlear implants due to facial nerve twitching. PTA ¼ pure tone audiometry; ABR ¼ auditory brainstem response; CVN ¼ cochleovestibular nerve. 1778

7 CT demonstrated thin CVNs during ABI surgery, supporting the limitations of using the BCNC as an indicator of cochlear nerve deficiency. Therefore, IAC morphology analyzed according to temporal bone CT findings may be an unreliable surrogate marker of CVN integrity. The response to electrical stimulation is another important diagnostic method for predicting the functional status of the cochlear nerve. Promontory EABR has been employed in many cochlear implantation centers to help determine candidacy or for the preoperative choice of implant site. Some reports have shown that the pattern of response on promontory EABR has prognostic significance in predicting auditory outcome after cochlear implantation in patients with congenital inner ear anomalies. 11 However, the results of this study refute the clinical usefulness of promontory EABR because four of eight patients exhibiting negative response on promontory EABR demonstrated thin CVNs during ABI surgery. The high impedance that is inevitable in promontory EABR is suspected to be the reason for the negative responses in cases with thin CVNs. The authors have previously reported that intracochlear EABR can more accurately predict the outcome of cochlear implantation in narrow IAC patients because of lower impedance and frequency-specific stimulation. 12 Nevertheless, even intracochlear EABR was shown to have limitations in precisely predicting the presence or absence of the CVN in this study. In particular, it was not possible to acquire any auditory response in a patient due to artifacts induced by muscle potentials resulting from stimulation of the facial nerve. Residual response on pure tone audiometry and behavioral response to environmental sounds appeared to be more accurate markers for predicting the presence or absence of the CVN compared to imaging or electrophysiologic testing because all three patients who showed a response to sound stimuli demonstrated thin CVNs during surgery. In one patient who was shown to have a thin CVN during surgery despite a negative response on preoperative auditory brainstem response and an absence of any behavioral response to pure tones or environmental sounds after cochlear implantation, it was suspected that an accurate audiologic evaluation may not have been performed because of concomitant mental retardation. Although the presence of an auditory response is highly suggestive of the anatomical presence of a CVN, there is the possibility of false negative or false positive results when using audiologic findings as a marker of CVN integrity, especially if the audiologist is inexperienced. The CVNs identified by the authors during ABI surgery were very thin, measuring approximately 10% to 20% of the diameter of the facial nerves. Therefore, it is challenging to preoperatively predict the presence of these CVNs. Accordingly, the possibility that a thin CVN may be present should always be kept in mind, even if a CVN is not clearly identified on preoperative evaluations, including imaging, electrophysiologic testing, or audiologic testing. Nevertheless, the question of whether patients with a thin CVN should be considered candidates for cochlear implantation needs more careful consideration because the auditory stimulation acquired by cochlear implantation may often not be sufficient for speech development in most of these cases with narrow IACs. CONCLUSIONS Visualization on MRI and detection of auditory responses suggested the presence of a CVN in patients with narrow IACs. However, the possibility of presence of a CVN should always be considered even in cases without clear evidence of a CVN on imaging, audiologic, or electrophysiologic tests. Therefore, physicians should be very prudent when interpreting the results of preoperative diagnostic tools and determining candidacy for cochlear implantation or ABI in patients with narrow IACs. BIBLIOGRAPHY 1. Papsin BC. Cochlear implantation in children with anomalous cochleovestibular anatomy. Laryngoscope 2005;115: Kim LS, Jeong SW, Huh MJ, Park YD. Cochlear implantation in children with inner ear malformations. Ann Otol Rhinol Laryngol 2006;115: Shelton C, Luxford WM, Tonokawa LL, Lo WW, House WF. The narrow internal auditory canal in children: a contraindication to cochlear implants. Otolaryngol Head Neck Surg 1989;100: Govaerts PJ, Casselman J, Daemers K, De Beukelaer C, Yperman M, De Ceulaer G. Cochlear implants in aplasia and hypoplasia of the cochleovestibular nerve. Otol Neurotol 2003;24: Warren FM 3rd, Wiggins RH 3rd, Pitt C, Harnsberger HR, Shelton C. Apparent cochlear nerve aplasia: to implant or not to implant? Otol Neurotol 2010;31: Colletti V. Auditory outcomes in tumor vs. nontumor patients fitted with auditory brainstem implants. Adv Otorhinolaryngol 2006;64: Colletti L, Zoccante L. Nonverbal cognitive abilities and auditory performance in children fitted with auditory brainstem implants: preliminary report. Laryngoscope 2008;118: Miyasaka M, Nosaka S, Morimoto N, Taiji H, Masaki H. CT and MR imaging for pediatric cochlear implantation: emphasis on the relationship between the cochlear nerve canal and the cochlear nerve. Pediatr Radiol 2010;40: Glastonbury CM, Davidson HC, Harnsberger HR, Butler J, Kertesz TR, Shelton C. Imaging findings of cochlear nerve deficiency. AJNR Am J Neuroradiol 2002;23: Trimble K, Blaser S, James AL, Papsin BC. Computed tomography and/or magnetic resonance imaging before pediatric cochlear implantation? Developing an investigative strategy. Otol Neurotol 2007;28: Kileny PR, Zwolan TA. Pre-perioperative, transtympanic electrically evoked auditory brainstem response in children. Int J Audiol 2004; 43(Suppl 1):S16 S Song MH, Bae MR, Kim HN, Lee WS, Yang WS, Choi JY. Value of intracochlear electrically evoked auditory brainstem response after cochlear implantation in patients with narrow internal auditory canal. Laryngoscope 2010;120: Nikolopoulos TP, Mason SM, Gibbin KP, O Donoghue GM. The prognostic value of promontory electric auditory brain stem response in pediatric cochlear implantation. Ear Hear 2000;21: Fatterpekar GM, Mukherji SK, Alley J, Lin Y, Castillo M. Hypoplasia of the bony canal for the cochlear nerve in patients with congenital sensorineural hearing loss: initial observations. Radiology 2000;215: Stjernholm C, Muren C. Dimensions of the cochlear nerve canal: a radioanatomic investigation. Acta Otolaryngol 2002;122: Adunka OF, Jewells V, Buchman CA. Value of computed tomography in the evaluation of children with cochlear nerve deficiency. Otol Neurotol 2007;28: Chiang MJ, Wu CM. Cochlear nerve aplasia and hypoplasia: diagnosis with three-dimensional magnetic resonance imaging. Cochlear Implants Int 2004;5(Suppl 1): Parry DA, Booth T, Roland PS. Advantages of magnetic resonance imaging over computed tomography in preoperative evaluation of pediatric cochlear implant candidates. Otol Neurotol 2005;26: Cerini R, Faccioli N, Cicconi Det al. Role of CT and MRI in the preoperative evaluation of auditory brainstem implantation in patients with congenital inner ear pathology. Radiol Med 2006;111: Kang WS, Lee JH, Lee HN, Lee KS. Cochlear implantations in young children with cochlear nerve deficiency diagnosed by MRI. Otolaryngol Head Neck Surg 2010;143: Buchman CA, Copeland BJ, Yu KK, Brown CJ, Carrasco VN, Pillsbury HC 3rd. Cochlear implantation in children with congenital inner ear malformations. Laryngoscope 2004;114: