Intra-cochlear electrode position impacts the preservation of residual hearing in an animal model of cochlear implant surgery.

INTRODUCTION: Preservation of residual hearing after cochlear implantation remains challenging. There are several approaches to preserve residual hearing, but the configuration of the implant electrode array seems to play a major role. Lateral wall electrode arrays are seemingly more favorable in this context. To date, there are no experimental data available which correlate the spatial electrode position in the scala tympani with the extent of hearing preservation. METHODS: Based on µCT imaging data, this study analyses the exact position of a pure- silicone electrode array inserted into the cochlea of four guinea pigs. Array position data were correlated with the extent of hearing loss after implantation, measured using auditory brainstem measurements in the frequency range of the area occupied by the electrode array area as well as apical to the array. RESULTS: The use of pure-silicone arrays without electrodes resulted in artifact-free, high-resolution µCT images that allowed precise determination of the arrays' positions within the scala tympani. The electrode arrays' locations ranged from peri-modiolar to an anti-modiolar. These revealed a correlation of a lower postoperative hearing loss with a higher spatial proximity to the lateral wall. This correlation was found in the low-frequency range only. A significant correlation between the interindividual differences in the diameter of the scala tympani and the postoperative hearing loss could not be observed. CONCLUSION: This study demonstrates the importance of the intra-cochlear electrode array's position for the preservation of residual hearing. The advantage of such an electrode array's position approximated to the lateral wall suggests, at least for this type of electrode array applied in the guinea pig, would be advantageous in the preservation of residual hearing for the apical part of the cochlea, beyond the area occupied by the electrode array.

Short-term overstimulation affects peripheral but not central excitability in an animal model of cochlear implantation.

Objective: Upper current limits (C-levels) are sometimes extremely increased over time since this procedure can enhance speech perception. It should be clarified if a larger amount of electrical stimulation is tolerated by the remaining peripheral and central auditory pathway.Materials and Methods: An animal electrode array was inserted in mechanically deafened guinea pigs. C-levels were adjusted to a mean of approximately 10 CL ('LOS' group), 40 CL ('MOS' group) or 60 CL ('HOS' group) above the electrode specific electrically evoked compound action potential (eCAP) threshold. The stimulation was performed via a sound processor in standardized auditory environment. Implanted and not stimulated animals served as controls.Results: A significant eCAP threshold shift was observed in the 'HOS'-group aftereight hours of stimulation at basal electrodes. Electrically evoked auditory brainstem thresholds were stable over time in all stimulation groups. The ratio between eCAP- and eABR threshold shifts was significantly enhanced in the 'HOS'- group.Conclusion: Even short-time overstimulation reduces the excitability of peripheral but not central auditory structures. The changed relationship between the excitability of spiral ganglion neurons and inferior colliculus neurons seems to indicate an overstimulation. The results are of utmost importance for a safe CI-processor fitting especially in children or non-compliant patients.

Cytomegalovirus Seropositivity as a Potential Risk Factor for Increased Noise Trauma Susceptibility.

Context: Cytomegalovirus (CMV) represents the leading congenital viral infection in humans. Although congenital CMV due to vertically transmitted infections is the main cause of CMV-related diseases, adult CMV infections might still be of clinical significance. It is still discussed how far CMV seropositivity, due to horizontal infection in immunocompetent adults, is able to induce significant dysfunction. The present study investigates in how far CMV seropositivity is an additional risk factor for an increasing susceptibility to sensorineural hearing loss induced by acoustic injury during adulthood in a guinea pig CMV (GPCMV) model of noise-induced hearing loss (NIHL). Methods: Two groups (GPCMV seropositive vs. seronegative) of normal hearing adult guinea pigs were exposed to a broadband noise (5-20 kHz) for 2 hours at 115 dB sound pressure level. Frequency-specific auditory brainstem response recordings for determination of auditory threshold shift were carried out and the number of missing outer hair cells was counted 2 weeks after the noise exposure. Results: The data show a slightly increased shift in auditory thresholds in seropositive animals compared to the seronegative control group in response to noise trauma. However, the observed difference was significant at least at high frequencies. The differences in threshold shift are not correlated with outer hair cell loss between the experimental groups. Conclusion: The results point to potential additional pathologies in a guinea pig NIHL model in correlation to GPCMV seropositivity, which should be taken into account when assessing risks of latent/reactivated CMV infection. Due to the relatively slight effect in the present data, the aim of future studies should be a more detailed consideration (e.g., larger sample size) and to localize possible target structures as well as the significance of the infection route.

Neuroprotective Effect of Near-Infrared Light in an Animal Model of CI Surgery.

INTRODUCTION: The preservation of residual hearing has become an important consideration in cochlear implant (CI) recipients in recent years. It was the aim of the present animal experimental study to investigate the influence of a pretreatment with near-infrared (NIR) light on preservation of sensory hair cells and residual hearing after cochlear implantation. METHODS: NIR was applied unilaterally (15 min, 808 nm, 120 mW) to 8 guinea pigs, immediately before a bilateral scala tympani CI electrode insertion was performed. The nonirradiated (contralateral) side served as control. Twenty-eight days postoperatively, auditory brainstem responses (ABRs) were registered from both ears to screen for hearing loss. Thereafter, the animals were sacrificed and inner hair cells (IHCs) and outer hair cells (OHCs) were counted and compared between NIR-pretreated and control (contralateral) cochleae. RESULTS: There was no IHC loss upon cochlear implantation. OHC loss was most prominent on both sides at the apical part of the cochlea. NIR pretreatment led to a statistically significant reduction in OHC loss (by 39.8%). ABR recordings (across the frequencies 4-32 kHz) showed a statistically significant difference between the 2 groups and corresponds well with the apical structural damage. Hearing loss was reduced by about 20 dB on average for the NIR-pretreated group (p ≤ 0.05). DISCUSSION/CONCLUSION: A single NIR pretreatment in this animal model of CI surgery appears to be neuroprotective for residual hearing. This is in line with other studies where several NIR posttreatments have protected cochlear and other neural tissues. NIR pretreatment is an inexpensive, effective, and noninvasive approach that can complement other ways of preserving residual hearing and, hence, should deserve further clinical evaluation in CI patients.