Cochlear nerve dysplasia in unilateral severe to profound congenital sensorineural hearing loss - Prevalence in Australian children and the impact of socioeconomic disadvantage on its management.

INTRODUCTION: Congenital unilateral sensorineural hearing loss (cuSNHL) carries potentially significant social, educational, and developmental consequences. Early diagnosis enables investigation, and consideration of options for management and early intervention, helping to mitigate the effects of hearing loss. Cochlear nerve dysplasia (CND) is a prominent cause of cuSNHL and may affect candidacy for cochlear implantation. Socioeconomic disadvantage may impact on a patient's family's capacity to participate in necessary intervention and follow-up. METHODS: Infants with severe-profound cuSNHL referred to a large Australian quaternary pediatric center between October 2004 and December 2020 were retrospectively included. Audiometric and clinical data, and the presence of hearing loss risk factors were obtained from a prospectively collated database. In Australia MRI scans are provided free-of-charge to citizens and residents. MRI scans were reviewed to determine the status of the nerves within the internal acoustic meatus (IAM grade) along with attendance rates. Travel distance to the hospital was also calculated. Reasons for non-attendance at MRI were obtained from patient medical records and correspondence. Socioeconomic, educational, and occupational indices, and travel distances were obtained using patient residential postcodes with reference to Australian Bureau of Statistics data. RESULTS: A total of 98 patients were reviewed, 64.3% (n = 63) of whom underwent MRI. The median age at diagnosis was 40 days (IQR 27). The prevalence of CND was 75% (n = 47). Importantly, there was no significant difference in the degree of hearing loss between IAM grades (F(4,57) = 1.029, p = 0.405). Socioeconomic indices were significantly lower in patients not attending MRI investigations compared with patients who did attend. Travel distance was not significantly different between the two groups. CONCLUSION: CND is a prominent cause of cuSNHL in Australian infants. MRI at a young age allows parent education regarding management options and timely intervention where indicated. Socioeconomic disadvantage significantly impacts on participation in further routine assessment of cuSNHL, potentially limiting management options for these children long term.

Monitoring Cochlear Health With Intracochlear Electrocochleography During Cochlear Implantation: Findings From an International Clinical Investigation.

OBJECTIVES: Electrocochleography (ECochG) is emerging as a tool for monitoring cochlear function during cochlear implant (CI) surgery. ECochG may be recorded directly from electrodes on the implant array intraoperatively. For low-frequency stimulation, its amplitude tends to rise or may plateau as the electrode is inserted. The aim of this study was to explore whether compromise of the ECochG signal, defined as a fall in its amplitude of 30% or more during insertion, whether transient or permanent, is associated with poorer postoperative acoustic hearing, and to examine how preoperative hearing levels may influence the ability to record ECochG. The specific hypotheses tested were threefold: (a) deterioration in the pure-tone average of low-frequency hearing at the first postoperative follow-up interval (follow-up visit 1 [FUV1], 4 to 6 weeks) will be associated with compromise of the cochlear microphonic (CM) amplitude during electrode insertion (primary hypothesis); (b) an association is observed at the second postoperative follow-up interval (FUV2, 3 months) (secondary hypothesis 1); and (c) the CM response will be recorded earlier during electrode array insertion when the preoperative high-frequency hearing is better (secondary hypothesis 2). DESIGN: International, multi-site prospective, observational, between groups design, targeting 41 adult participants in each of two groups, (compromised CM versus preserved CM). Adult CI candidates who were scheduled to receive a Cochlear Nucleus CI with a Slim Straight or a Slim Modiolar electrode array and had a preoperative audiometric low-frequency average thresholds of ≤80 dB HL at 500, 750, and 1000 Hz in the ear to be implanted, were recruited from eight international implant sites. Pure tone audiometry was measured preoperatively and at postoperative visits (FUV1 and follow-up visit 2 [FUV2]). ECochG was measured during and immediately after the implantation of the array. RESULTS: From a total of 78 enrolled individuals (80 ears), 77 participants (79 ears) underwent surgery. Due to protocol deviations, 18 ears (23%) were excluded. Of the 61 ears with ECochG responses, amplitudes were < 1 µV throughout implantation for 18 ears (23%) and deemed "unclear" for classification. EcochG responses >1 µV in 43 ears (55%) were stable throughout implantation for 8 ears and compromised in 35 ears. For the primary endpoint at FUV1, 7/41 ears (17%) with preserved CM had a median hearing loss of 12.6 dB versus 34/41 ears (83%) with compromised CM and a median hearing loss of 26.9 dB ( p < 0.014). In assessing the practicalities of measuring intraoperative ECochG, the presence of a measurable CM (>1 µV) during implantation was dependent on preoperative, low-frequency thresholds, particularly at the stimulus frequency (0.5 kHz). High-frequency, preoperative thresholds were also associated with a measurable CM > 1 µV during surgery. CONCLUSIONS: Our data shows that CM drops occurring during electrode insertion were correlated with significantly poorer hearing preservation postoperatively compared to CMs that remained stable throughout the electrode insertion. The practicality of measuring ECochG in a large cohort is discussed, regarding the suggested optimal preoperative low-frequency hearing levels ( < 80 dB HL) considered necessary to obtain a CM signal >1 µV.