Characteristics of the Summating Potential Measured Across a Cochlear Implant Array as an Indicator of Cochlear Function.

OBJECTIVES: The underlying state of cochlear and neural tissue function is known to affect postoperative speech perception following cochlear implantation. The ability to assess these tissues in patients can be performed using intracochlear electrocochleography (IC ECochG). One component of ECochG is the summating potential (SP) that appears to be generated by multiple cochlear tissues. Its qualities may be able to detect the presence of functional inner hair cells, but evidence for this is limited in human cochleae. This study aimed to examine the IC SP characteristics in cochlear implantation recipients, its relationship to preoperative speech perception and audiometric thresholds, and to other IC ECochG components. DESIGN: This is a retrospective analysis of 113 patients' IC ECochG recordings across the array in response to a 500 Hz tone burst stimulus. Responses to condensation and rarefaction stimuli were then subtracted from one another to emphasize the cochlear microphonic and added to one another to emphasize the SP, auditory nerve neurophonic, and compound action potential. Patients were grouped based on their maximum SP deflection being large and positive (+SP), large and negative (-SP), or minimal (0 SP) to further investigate these relationships. RESULTS: Patients in the +SP group had better preoperative speech perception (mean consonant-vowel-consonant phoneme score 46%) compared to the -SP and 0 SP groups (consonant-vowel-consonant phoneme scores 34% and 36%, respectively, difference to +SP: p < 0.05). Audiometric thresholds were lowest for +SP (mean pure-tone average 50 dB HL), then -SP (65 dB HL), and highest for 0 SP patients (70 dB HL), but there was not a statistical significance between +SP and -SP groups ( p > 0.1). There were also distinct differences between SP groups in the qualities of their other ECochG components. These included the +SP patients having larger cochlear microphonic maximum amplitude, more apical SP peak electrode locations, and a more spatially specific SP magnitude growth pattern across the array. CONCLUSIONS: Patients with large positive SP deflection in IC ECochG have preoperatively better speech perception and lower audiometric thresholds than those without. Patterns in other ECochG components suggest its positive deflection may be an indicator of cochlear function.

Intracochlear tPA infusion may reduce fibrosis caused by cochlear implantation surgery.

BACKGROUND: Experiments show that the extent of ongoing fibrotic change within the cochlea can be determined by the volume and pattern of bleeding within the first 24 h following cochlear implantation. Tissue-type plasminogen activator (tPA) is effective at reducing thrombus volume when administered both within and external to the systemic circulation. AIMS/OBJECTIVES: To determine if tPA delivered into the scala tympani immediately following implantation will reduce thrombus volume within the lower basal turn of the cochlea. MATERIALS AND METHODS: Guinea pigs were implanted with either 'soft' or 'hard' arrays and administered tPA or saline via an intra-cochlear infusion immediately after implantation. Hearing was checked prior to, and 2 weeks after implantation. Cochleae were then harvested and imaged. RESULTS: Animals implanted with 'soft' arrays had 4.2% less tissue response compared with animals implanted with 'hard' arrays. In animals receiving 'soft' arrays, tPA reduced the volume of tissue response (measured by the percentage of the lower basal turn of the scala tympani occupied by tissue response) compared with saline. CONCLUSIONS AND SIGNIFICANCE: tPA may be effective in reducing the overall volume of tissue response in routine 'soft' cochlear implantation and may have a greater effect in the event of significant surgical trauma.

The Early Postoperative Effects of Cochlear Implantation on Horizontal Semicircular Canal Function.

OBJECTIVES: To use video head impulse testing to examine the effect of cochlear implantation (CI) on horizontal SCC vestibulo-ocular reflex (VOR) gain early after surgery, and to relate outcomes to subjective imbalance. STUDY DESIGN: Prospective cohort study. SETTING: Academic tertiary referral center. PATIENTS: Thirty-seven (23F:14M) adult cochlear implant recipients (mean age, 55; age range, 20-80). INTERVENTION: Cochlear implantation. MAIN OUTCOME MEASURE: The VOR of the horizontal semicircular canal of both the operated and non-operated ears was examined using video head impulse testing before surgery and at days 1, 7, and 28 following surgery. VOR gain, VOR gain asymmetry, and the change in VOR gain from preoperative baseline where the primary outcome measures. Subjective imbalance was assessed using a structured questionnaire. RESULTS: VOR gain value for the operated ear was 0.88 ±â€Š0.21. Mean VOR gain on day 1 postoperatively was 0.86 ±â€Š0.19; on day 7, 0.87 ±â€Š0.17, and on day 30, 0.91 ±â€Š0.21. Before surgery median asymmery was -5.50%, on day 1 it was -5.30%, at day 7, -6.44%, and at day 30 it was -2.61%. There was no significant difference between these measures for the cohort across the four time points. Thirteen of 37 (35%) of patients experienced imbalance in the follow-up period, but this was not correlated to changes in VOR gain. CONCLUSION: Horizontal semicircular canal function is preserved in the immediate and early postoperative period. This suggests that horizontal semicircular canal impairment is not likely to be responsible for postoperative imbalance.

Defining the Hook Region Anatomy of the Guinea Pig Cochlea for Modeling of Inner Ear Surgery.

HYPOTHESIS: The aim of this study was to describe the hook region anatomy of the guinea pig cochlea to identify the optimal surgical approach for cochlear implantation and to determine what anatomical structures are at risk. BACKGROUND: Animal studies investigating hearing loss after cochlear implantation surgery are currently constrained by the lack of a reproducible implantation model. METHODS: Guinea pig cochleae were imaged using thin-sheet laser imaging microscopy. Images were stitched, reconstructed, and segmented for analysis. Insertion vectors were determined by tracing their paths to the outer wall and converting to Cartesian coordinates. Spherical surface and multiplane views were generated to analyze outer wall and radial forces of the insertion vector. RESULTS: Thin-sheet laser imaging microscopy enabled quantitative, whole specimen analysis of the soft and bony tissue relationships of the complex cochlear hook region in any desired plane without loss of image quality. Round window or cochleostomy approaches in the anteroinferior plane avoided direct damage to cochlear structures. Cochleostomy approach had large interindividual variability of angular depth and outer wall forces but predictable radial force. CONCLUSION: The guinea pig hook region and lower basal turn have similar structural relationships to humans. Careful cochleostomy placement is essentially for minimizing cochlear trauma and for ensuring a straight insertion vector that successfully advances around the outer wall. Experiments with guinea pigs that control for the surgical approach are likely to provide useful insights into the aetiology and the development of therapies directed at postimplantation hearing loss.

Outcomes After Cochlear Implantation in the Very Elderly.

OBJECTIVE: To evaluate the outcomes after cochlear implantation (CI) in the elderly population, with a particular emphasis on perioperative complications, dizziness, and speech perception outcomes. STUDY DESIGN: A retrospective cohort study of elderly cochlear implant patients. SETTING: Tertiary referral center (Cochlear Implant Clinic, Royal Victorian Eye and Ear Hospital, Melbourne). PATIENTS: All patients aged 75 and above at the time of first cochlear implant (N = 150). Comparison was made between groups aged 85+ to 80-84, and 75-79. INTERVENTIONS: All patients received Nucleus devices (either CI512 or CI24RE(CA)). MAIN OUTCOME MEASURES: Speech recognition scores both pre- and postimplantation, symptomatic dizziness and effects upon independent living after surgery, and the incidence of perioperative medical and surgical complications. Complications were classified as major (intrinsic device failure, device migration, extracochlear insertion, meningitis, surgical site infection requiring reoperation, wound breakdown, permanent facial nerve paralysis) and minor (tinnitus, transient facial nerve palsy, facial nerve stimulation, taste disturbance, delayed wound healing). RESULTS: All three cohorts had poor preoperative speech perception. There was significant improvement in postoperative word scores at 3 and 12 months across all groups. There was no statistically significant difference between the three cohorts in terms of speech recognition outcomes at 3 and 12 months. After surgery, more than 20% of patients at all ages experienced transient imbalance, although the incidence did not differ significantly between age groups (p = 0.71). In total, there were 13 major complications in 7 patients (4.7%), and 28 minor complications in 25 patients (16.7%). CONCLUSION: Postoperative disequilibrium was commonly observed in this elderly population, yet patients still benefited with improved speech perception after cochlear implantation. Elderly patients can benefit from cochlear implantation, and age should not be a limitation for CI surgery. Cochlear implantation can be done safely and provides significant patient benefits.

Early cochlear response and ICAM-1 expression to cochlear implantation.

AIM: To examine the early cochlear response and intercellular cell adhesion molecule-1 (ICAM-1) expression to implantation of a cochlear electrode into the scala tympani. BACKGROUND: Understanding the early response of the cochlea to implantation may inform the duration which drug therapies should be delivered to protect hearing. METHODS: Guinea pigs were implanted with a cochlear electrode and survived 1, 2, or 7 days before they were euthanized, cochleae harvested, processed, and cryosectioned for light microscopy or ICAM-1 immunohistochemistry. RESULTS: On hematoxylin and eosin staining, scala tympani was characterized by the presence of fibrin and blood clot at 1 to 2 days after surgery, with a leukocytic infiltrate, primarily of neutrophils and macrophage-like cells. By 7 days after surgery, fibroblasts had infiltrated the clot, and the numbers of red blood cells (RBCs) and neutrophils had diminished. ICAM-1 expression was greatest in the lateral cochlear wall with highest expression found in the basal turn in the region of the electrode at 24 hours postimplantation. CONCLUSION: The cochlear vasculature is maximally primed to recruit cells from the circulation, as evidenced by ICAM-1 expression levels, at 24 hours after cochlear implantation. This response is similar to that seen after other types of injury. Where cochlear implantation differs is the predominance of fibrin and clot early after electrode insertion before infiltration by fibroblasts by the end of the first postoperative week. These results suggest that anti-inflammatory drugs aimed at reducing the extravasation of immunecompetent cells into the cochlea must be effective over the first few days after surgery. Whether this can be achieved through preoperative treatment alone, or whether therapy will need to continue postoperatively, awaits further experimentation.

Systemic immunity influences hearing preservation in cochlear implantation.

HYPOTHESIS: To determine whether a systemic immune response influences hearing thresholds and tissue response after cochlear implantation of hearing guinea pigs. METHODS: Guinea pigs were inoculated with sterile antigen (Keyhole limpet hemocyanin) 3 weeks before cochlear implantation. Pure-tone auditory brainstem response thresholds were performed before implantation and 1 and 4 weeks later. Dexamethasone phosphate 20% was adsorbed onto a hyaluronic acid carboxymethylcellulose sponge and was applied to the round window for 30 minutes before electrode insertion. Normal saline was used for controls. Cochlear histology was performed at 4 weeks after implantation to assess the tissue response to implantation. To control for the effect of keyhole limpet hemocyanin priming, a group of unprimed animals underwent cochlear implantation with a saline-soaked pledget applied to the round window. RESULTS: Keyhole limpet hemocyanin priming had no significant detrimental effect on thresholds without implantation. Thresholds were elevated after implantation across all frequencies tested (2-32 kHz) in primed animals but only at higher frequencies (4-32 kHz) in unprimed controls. In primed animals, dexamethasone treatment significantly reduced threshold shifts at 2 and 8 kHz. Keyhole limpet hemocyanin led to the more frequent observation of lymphocytes in the tissue response to the implant. CONCLUSION: Systemic immune activation at the time of cochlear implantation broadened the range of frequencies experiencing elevated thresholds after implantation. Local dexamethasone provides partial protection against this hearing loss, but the degree and extent of protection are less compared to previous studies with unprimed animals.

Prediction and control of neural responses to pulsatile electrical stimulation.

This paper aims to predict and control the probability of firing of a neuron in response to pulsatile electrical stimulation of the type delivered by neural prostheses such as the cochlear implant, bionic eye or in deep brain stimulation. Using the cochlear implant as a model, we developed an efficient computational model that predicts the responses of auditory nerve fibers to electrical stimulation and evaluated the model's accuracy by comparing the model output with pooled responses from a group of guinea pig auditory nerve fibers. It was found that the model accurately predicted the changes in neural firing probability over time to constant and variable amplitude electrical pulse trains, including speech-derived signals, delivered at rates up to 889 pulses s(-1). A simplified version of the model that did not incorporate adaptation was used to adaptively predict, within its limitations, the pulsatile electrical stimulus required to cause a desired response from neurons up to 250 pulses s(-1). Future stimulation strategies for cochlear implants and other neural prostheses may be enhanced using similar models that account for the way that neural responses are altered by previous stimulation.