eISSN 2444-7986
DOI: https://doi.org/10.14201/orl.33246

REVIEW ARTICLE

COCHLEAR IMPLANTATION IN PATIENTS WITH INTERNAL AUDITORY CANAL DUPLICATION: A LITERATURE REVIEW

Implante coclear en pacientes con duplicación del CAI: Revisión de la literatura

María SAN MILLÁN-GONZÁLEZ; Paula PECIÑA-MELGOSA; Victoria DUQUE-HOLGUERA; Jaime SANTOS-PÉREZ

Otorhinolaryngology-Head and Neck Surgery Department, Clinic University Hospital, Valladolid, Spain

Correspondence: msanmillang@saludcastillayleon.es

Reception date: May 7, 2025
Date of Acceptance: July 8, 2025
Publication date: July 22, 2025
Date of publication of the issue: September 30, 2025

Conflict of interest: The authors declare no conflicts of interest
Images: The authors declare that the images have been obtained with the permission of the patients
Rights policy and self-archive: the self-archive of the post-print version (SHERPA / RoMEO) is allowed
License CC BY-NC-ND. Creative Commons Attribution-Noncommercial-NoDerivate Works 4.0 International
University of Salamanca. Its commercialization is subject to the permission of the publisher

SUMMARY: Introduction and Objectives: Duplication of the internal auditory canal (IAC) is a rare congenital anomaly associated with profound sensorineural hearing loss. Once considered a relative contraindication to cochlear implantation (CI), modern diagnostic tools have expanded therapeutic options. This review examines current evidence on CI in patients with IAC duplication. Methods: A narrative review of four clinical studies published within the last 10 years, in order to ensure the inclusion of up-to-date diagnostic and surgical practices. Anatomical features, functional findings, and postoperative outcomes were analyzed. Risk of bias was assessed using the ROBINS-I tool. Results: Four pediatric patients with IAC duplication and confirmed cochlear nerve presence were identified. All underwent CI (unilateral or bilateral) with favorable hearing outcomes, language development, and CAP scores ranging from 5 to 6. No major complications were reported. Discussion: IAC duplication does not preclude CI if cochlear nerve function is verified by imaging and electrophysiology. Early intervention and structured rehabilitation appear beneficial. Conclusions: CI is a viable option in selected cases of IAC duplication. Surgical indication should prioritize functional over purely anatomical criteria.

KEYWORDS: internal auditory canal duplication; cochlear implantation; sensorineural hearing loss; internal auditory canal malformations

RESUMEN: Introducción y objetivo: La duplicación del conducto auditivo interno (CAI) es una malformación congénita rara asociada a hipoacusia neurosensorial profunda. Tradicionalmente considerada una contraindicación relativa para el implante coclear (IC), nuevos enfoques diagnósticos han ampliado sus posibilidades terapéuticas. Esta revisión analiza la evidencia disponible sobre IC en pacientes con duplicación del CAI. Métodos: Revisión narrativa de cuatro estudios clínicos, la búsqueda se limitó a estudios publicados en los últimos 10 años, con el objetivo de incluir prácticas diagnósticas y quirúrgicas actualizadas. El riesgo de sesgo fue valorado mediante ROBINS-I. Resultados: Se incluyeron cuatro pacientes pediátricos con duplicación del CAI y nervio coclear presente en al menos un canal. Todos recibieron IC (unilateral o bilateral) con resultados auditivos positivos, desarrollo del lenguaje y puntuaciones CAP entre 5 y 6. No hubo complicaciones relevantes. Discusión: La duplicación del CAI no excluye el IC si se confirma la presencia funcional del nervio coclear mediante imagen y pruebas electrofisiológicas. La intervención temprana mejora el pronóstico. Conclusiones: El IC es una opción válida en casos seleccionados de duplicación del CAI. La indicación debe basarse en criterios funcionales más que anatómicos.

PALABRAS CLAVE: duplicación del conducto auditivo interno; implante coclear; hipoacusia neurosensorial; malformaciones del conducto auditivo interno

INTRODUCTION

Duplication of the internal auditory canal (IAC) is a rare congenital anomaly often associated with cochlear nerve dysplasia and profound sensorineural hearing loss [1, 2]. This anomaly is frequently associated with cochlear nerve aplasia or hypoplasia, and with other inner ear abnormalities such as cochlear hypoplasia and vestibular dysplasia [1–3]. Traditionally considered a relative contraindication for cochlear implantation (CI), recent advances in imaging and intraoperative testing have enabled more accurate identification of cochlear nerve presence, expanding CI candidacy [2, 3]. Although few cases have been reported, several demonstrate favorable auditory outcomes following implantation [1, 3]. This review summarizes current evidence on CI in patients with IAC duplication, highlighting diagnostic challenges and postoperative results to guide clinical decision-making. Understanding the functional prognosis of CI in patients with this anomaly is essential for refining selection criteria and counseling families [1–3].

MATERIAL AND METHODS

A systematic review of the literature using a narrative synthesis approach with PubMed, Embase, and Scopus databases. The search strategy included the following terms: ("internal auditory canal duplication" OR "internal auditory meatus duplication" OR "duplicated internal auditory canal" OR "IAC duplication" OR "duplicated IAC") AND ("cochlear implantation" OR "cochlear implant" OR "cochlear implant outcomes" OR "cochlear nerve" OR "sensorineural hearing loss").

The inclusion criteria comprised articles reporting cases in which cochlear implantation was performed in patients with duplicated internal auditory canal (IAC). The search was limited to studies published within the last 10 years, in order to ensure the inclusion of up-to-date diagnostic and surgical practices. Studies were excluded if they did not involve IAC duplication, or if, despite the presence of IAC duplication, cochlear implantation was not undertaken. Data were extracted on patient demographics, defect characteristics, surgical technique, postoperative outcomes, and complications. Two independent reviewers screened the articles, and discrepancies were resolved by consensus. A PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) 2020 flow diagram detailing the study selection process is provided [4]. Figure 1. The methodological quality of the included non-randomized studies was assessed using the ROBINS-I (Risk Of Bias In Non-randomized Studies of Interventions) tool. Each study was evaluated across seven domains. Overall risk of bias was classified as low, moderate, or serious, based on the cumulative assessment.

RESULTS

As shown in the PRISMA flow diagram, the described methodology resulted in 14 articles (see Figure 1) [4]. Following the detailed screening process, four studies were selected: three case reports and a case series of two patients. We created a table specifying the authors, country, year of publication, study type, and the main results analyzed. Table 1. While there are a few articles addressing duplicated internal auditory canal, we have chosen to focus exclusively on those in which cochlear implantation was performed, which explains the limited number of articles selected. Eleven articles were excluded because they addressed internal auditory canal malformations without performing cochlear implantation, or because the presence of IAC duplication was not confirmed. All included studies presented either moderate or serious risk of bias, mostly due to confounding and selection bias as assessed by the ROBINS-I tool. In all four studies, selection bias was rated as moderate because participants were not randomly selected, but rather identified retrospectively based on the availability of complete imaging and surgical outcome data. This could overestimate the benefit of cochlear implantation by including only patients with favorable anatomical or functional criteria. Similarly, confounding was a concern in all cases due to the lack of control for variables such as age at implantation, rehabilitation intensity, and coexisting inner ear anomalies. These limitations should be considered when interpreting the results. Table 2.

DISCUSSION

Overview and Pathophysiological Background

Duplication of the internal auditory canal (IAC) is an exceptionally rare congenital anomaly, identified in a small number of reported cases worldwide [1–4]. This malformation, characterized by the presence of two bony canals within the petrous portion of the temporal bone, is frequently associated with cochlear nerve aplasia or hypoplasia and profound sensorineural hearing loss (SNHL) [1, 2, 4]. Historically, such anatomical abnormalities were considered a poor prognostic factor for cochlear implantation (CI), based on the assumption that the absence of a functional cochlear nerve would render the implant ineffective [2–4].

Diagnostic Strategies and Anatomical Evaluation

High-resolution imaging plays a central role in the diagnosis and management of duplicated IAC. Computed tomography (CT) enables identification of the bony canal duplication, while magnetic resonance imaging (MRI)—particularly parasagittal T2-weighted sequences—is essential for assessing the presence and morphology of the cochlear nerve. In the case described by Kishimoto et al. [1], bilateral cochlear nerves were clearly visualized in the anterior duplicated canals, providing the rationale for proceeding with implantation. Similarly, Binnetoglu et al. [2] reported a hypoplastic nerve on the right and absence on the left, highlighting the need for side-specific planning. Thompson and Birman’s report [3] employed a comprehensive diagnostic protocol, including MRI, auditory brainstem response (ABR), electrocochleography (ECochG), and trans-tympanic electrically evoked ABR, which enabled robust confirmation of neural viability prior to bilateral implantation. Manchanda et al. [4] also emphasized the value of correlating HRCT and 3D MRI sequences to differentiate between hypoplasia and aplasia of the cochlear nerve, which was decisive in selecting the side for implantation.

Surgical and Functional Outcomes

Despite anatomical concerns, outcomes following CI in patients with duplicated IAC can be favorable when cochlear nerve fibers are present. Kishimoto et al. [1] reported hearing thresholds of 40–45 dB five months after implantation, indicating effective auditory stimulation. Binnetoglu et al. [2] described slower yet meaningful progress in a child implanted on the side with a hypoplastic cochlear nerve. Notably, Thompson and Birman’s report [3] demonstrated that bilateral implantation was not only feasible but also associated with progressive speech development, with CAP scores reaching 5 and 6 over two to three years of follow-up. Importantly, both children in their case series underwent simultaneous bilateral cochlear implantation during a single surgical procedure, which may have contributed to the favorable auditory and language outcomes observed. This approach supports the hypothesis that early, bilateral auditory stimulation during critical periods of neuroplasticity enhances rehabilitation potential. Manchanda et al. [4] reported a successful cochlear implantation in a six-year-old patient with bilateral complete duplication of the IAC and hypoplastic cochlear nerve on the implanted side, reinforcing the idea that hypoplasia does not exclude favorable outcomes when a residual functional nerve is present.

Electrophysiological Testing and Intraoperative Considerations

Functional testing is critical in both preoperative and intraoperative decision-making. Intraoperative neural response telemetry and electrically evoked ABR, as used by Kishimoto et al. [1] and Thompson and Birman’s report [3], provide real-time confirmation of auditory pathway responsiveness. Binnetoglu et al. [2] further emphasized the importance of promontory stimulation testing, especially in anatomically ambiguous cases. These techniques help identify candidates with residual cochlear nerve function who may benefit from CI, even when imaging is inconclusive. Nevertheless, anatomical variations may complicate surgery, as illustrated in the Japanese case, where a bifurcated facial nerve and cerebrospinal fluid (CSF) gusher were encountered [1], and in Thompson and Birman’s report [3], where postoperative facial nerve stimulation necessitated device reprogramming. Manchanda et al. [4] also highlighted the role of MRI in pre-surgical mapping to avoid complications in cases with associated malformations such as lateral semicircular canal aplasia or optic nerve abnormalities. Similar intraoperative confirmation strategies have been proposed in other series involving IAC stenosis or hypoplasia, suggesting broader applicability in congenital inner ear malformations.

Prognostic Factors and Clinical Implications

These cases collectively challenge the traditional exclusion of patients with duplicated IAC from CI candidacy. The presence of a hypoplastic nerve, previously considered a negative prognostic indicator, may still support auditory stimulation and language development if addressed through a multidisciplinary strategy and early intervention [2–4]. Furthermore, bilateral implantation—especially when performed simultaneously, as in Thompson and Birman’s report [3]—appears to be both safe and potentially advantageous for enhancing binaural input and optimizing central auditory development.

Limitations in the Evidence

Despite encouraging findings, the current literature is limited by methodological constraints. All four studies are observational in nature, with small sample sizes, no control groups, and moderate to serious risk of bias according to the ROBINS-I assessment. Bias due to confounding, selective reporting, and lack of blinding is inherent in retrospective case reports and small case series. Moreover, outcome measures vary considerably, ranging from ABR thresholds to CAP scores and subjective speech assessments, complicating cross-study comparisons [1–4]. Given the small number of available studies, the use of the ROBINS-I tool should be interpreted with caution. Although it allows for structured assessment, its applicability is limited in contexts where case reports and small series predominate and outcome data are heterogeneously reported.

CONCLUSIONS

The clinical scenarios described in this review illustrate the need for renewed flexibility in the selection criteria for cochlear implantation in patients with rare inner ear malformations. Duplication of the internal auditory canal, once regarded as a definitive contraindication, has demonstrated variable yet promising outcomes when supported by appropriate diagnostic pathways and early intervention. Rather than adhering to rigid anatomical norms, future efforts should prioritize functional viability and patient-specific potential for auditory rehabilitation. Advancing the evidence base will require standardized reporting of outcomes, inclusion of long-term follow-up data, and international cooperation to build a robust foundation for clinical decision-making in these exceptional cases.

REFERENCES

1. Kishimoto I, Moroto S, Fujiwara K, Harada H, Kikuchi M, Suehiro A, Shinohara S, Naito Y. Bilateral duplication of the internal auditory canal: a case with successful cochlear implantation. Int J Pediatr Otorhinolaryngol. 2015 Sep; 79(9):1595-8. Epub 2015 Jul 13. PMID: 26209350. https://doi.org/10.1016/j.ijporl.2015.07.004

2. Binnetoğlu A, Bağlam T, Sarı M, Gündoğdu Y, Batman Ç. A Challenge for Cochlear Implantation: Duplicated Internal Auditory Canal. J Int Adv Otol. 2016 Aug; 12(2):199-201. PMID: 27716607. https://doi.org/10.5152/iao.2016.1440

3. Thompson MR, Birman CS. Bilateral duplication of the internal auditory canals and bilateral cochlear implant outcomes and review. Int J Pediatr Otorhinolaryngol. 2019 Apr; 119:41-46. Epub 2019 Jan 12. PMID: 30665175. https://doi.org/10.1016/j.ijporl.2019.01.015

4. Manchanda S, Bhalla AS, Kumar R, Kairo AK. Duplication Anomalies of the Internal Auditory Canal: Varied Spectrum. Indian J Otolaryngol Head Neck Surg. 2019 Sep; 71(3):294 298. Epub 2017 Feb 2. PMID: 31559193; PMCID: PMC6737140. https://doi.org/10.1007/s12070-017-1087-4

5. Page MJ, McKenzie JE, Bossuyt PM, Boutron I, Hoffmann TC, Mulrow CD, et al. The PRISMA 2020 statement: an updated guideline for reporting systematic reviews. BMJ. 2021; 372:n71. https://doi.org/10.1136/bmj.n71