Intra-cochlear Flushing Reduces Tissue Response to Cochlear Implantation.

INTRODUCTION: Intraoperative trauma leading to bleeding during cochlear implantation negatively impacts residual hearing of cochlear implant recipients. There are no clinical protocols for the removal of blood during implantation, to reduce the consequential effects such as inflammation and fibrosis which adversely affect cochlear health and residual hearing. This preclinical study investigated the implementation of an intra-cochlear flushing protocol for the removal of blood. METHODS: Three groups of guinea pigs were studied for 28 days after cochlear implantation; cochlear implant-only (control group); cochlear implant with blood injected into the cochlea (blood group); and cochlear implant, blood injection, and flushing of the blood from the cochlea intraoperatively (flush group). Auditory brainstem responses (ABRs) in addition to tissue response volumes were analyzed and compared between groups. RESULTS: After implantation, the blood group exhibited the highest ABR thresholds when compared to the control and flush group, particularly in the high frequencies. On the final day, the control and blood group had similar ABR thresholds across all frequencies tested, whereas the flush group had the lowest thresholds, significantly lower at 24 kHz than the blood and control group. Analysis of the tissue response showed the flush group had significantly lower tissue responses in the basal half of the array when compared with the blood and control group. CONCLUSIONS: Flushing intra-cochlear blood during surgery resulted in better auditory function and reduced subsequent fibrosis in the basal region of the cochlea. This finding prompts the implementation of a flushing protocol in clinical cochlear implantation. LEVEL OF EVIDENCE: N/A Laryngoscope, 134:1410-1416, 2024.

Cochlear implant surgery facilitates intracochlear distribution of perioperative systemic steroids.

BACKGROUND: Systemically administered steroids are widely utilised for hearing preservation therapies. More recently, steroids have been administered to achieve hearing protection after cochlear implant surgery. Currently there is a lack of understanding as to which administration route offers most therapeutic efficacy, local or systemic administration. Paramount to this are observations in animal studies that systemic administration following implantation offers hearing protection and reduced cochlear fibrosis, despite observations that perilymphatic levels are up to 10-fold higher after local administration in non-implanted cochleae. AIMS/OBJECTIVES: This paper explores the impact that cochlear implantation and associated acute inflammation has on steroid distribution and uptake following systemic administration of dexamethasone. MATERIAL AND METHODS: Eight guinea pigs received systemic dexamethasone 60 min prior to cochlear implantation. Implanted and contralateral non-implanted cochlea were harvested for tissue immunohistochemistry and detection of dexamethasone. RESULTS: Cochleostomy with scala tympani implantation resulted in a significant increase in cochlear dexamethasone signal. This was most notable at the organ of Corti, stria vascularis, and blood product in the scala tympani. CONCLUSIONS AND SIGNIFICANCE: This study demonstrates that the inner ear distribution of systemically administered steroids is enhanced following surgery for cochlear implantation and provides rationale for systemic perioperative steroids in hearing preservation surgery.

Spironolactone Ameliorates Cochlear Implant Induced Endolymphatic Hydrops.

BACKGROUND: Endolymphatic hydrops (EH) has been observed in both animal and human cochleae following cochlear implant (CI) surgery. We tested whether EH could be eliminated by administration of mineralocorticoid steroid antagonist spironolactone and explored the electrophysiological consequences of this. METHODS: Sixty-four adult guinea pigs underwent cochlear implantation with a dummy electrode. Animals then survived either 2, 7, or 28 days. Auditory function was monitored by recording electrocochleography from the round window membrane preimplantation, and on the last day of the experiment. Spironolactone or control solution was added to animals' feed for 7 days (if they survived that long) beginning immediately prior to surgery. The presence of EH was determined using thin-sheet laser imaging microscopy. RESULTS: Treatment with spironolactone resulted in significant reduction in EH in the second cochlear turn 7 days postimplantation. In all animals, the compound action potential (CAP) threshold was elevated 2 days postimplantation, but for most frequencies had recovered substantially by 28 days. There was no treatment effect on CAP thresholds. SP/AP ratios were elevated at day 2. The amplitude growth of the CAP did not differ between test and control groups at any time after implantation. CONCLUSIONS: EH can be suppressed by antagonism of mineralocorticoid receptors in the week after cochlear implantation. Reduction in EH did not lead to any change in hearing, and there was no indication of synaptopathy signalled by reduced CAP amplitude at high sound intensities. We found no electrophysiological evidence that EH early after implantation impacts negatively upon preservation of residual hearing.

A new method for three-dimensional immunofluorescence study of the cochlea.

Visualisation of cochlear histopathology in three-dimensions has been long desired in the field of hearing research. This paper outlines a technique that has made this possible and shows a research application in the field of hearing protection after cochlear implantation. The technique utilises robust immunofluorescent labelling followed by effective tissue clearing and fast image acquisition using Light Sheet Microscopy. We can access the health of individual components by immunofluorescent detection of proteins such as myosin VIIa to look at cochlear hair cells, NaKATPase alpha 3 to look at spiral ganglion neurons, and IBA1 to look at macrophages within a single cochlea, whilst maintaining the integrity of fine membranous structures and keeping the cochlear implant in place. This allows the tissue response to cochlear implantation to be studied in detail, including the immune reaction to the implant and the impact on the structure and health of neural components such as hair cells. This technique reduces time and labour required for sectioning of cochleae and can allow visualisation of cellular detail. Use of image analysis software allows conversion of high-resolution image stacks into three-dimensional interactive data sets so volumes and numbers of surfaces can be measured. Immunofluorescent whole cochlea labelling and Light Sheet Microscopy have the capacity to be applied to many questions in hearing research of both the cochlea and vestibular system.

Nanomechanical mapping reveals localized stiffening of the basilar membrane after cochlear implantation.

Cochlear implantation leads to many structural changes within the cochlea which can impair residual hearing. In patients with preserved low-frequency hearing, a delayed hearing loss can occur weeks-to-years post-implantation. We explore whether stiffening of the basilar membrane (BM) may be a contributory factor in an animal model. Our objective is to map changes in morphology and Young's modulus of basal and apical areas of the BM after cochlear implantation, using quantitative nanomechanical atomic force microscopy (QNM-AFM) after cochlear implant surgery. Cochlear implantation was undertaken in the guinea pig, and the BM was harvested at four time-points: 1 day, 14 days, 28 days and 84 days post-implantation for QNM-AFM analysis. Auditory brainstem response thresholds were determined prior to implantation and termination. BM tissue showed altered morphology and a progressive increase in Young's modulus, mainly in the apex, over time after implantation. BM tissue from the cochlear base demonstrated areas of extreme stiffness which are likely due to micro-calcification on the BM. In conclusion, stiffening of the BM after cochlear implantation occurs over time, even at sites far apical to a cochlear implant.

Adjuvant agents enhance round window membrane permeability to dexamethasone and modulate basal to apical cochlear gradients.

Glucocorticoids have direct anti-inflammatory, anti-oxidant and anti-apoptotic effects on cochlear hair cells. Cochlear glucocorticoid therapy has gained particular attention for its ability to enhance the protection of residual hearing following hearing preservation cochlear implantation. Local drug delivery methods achieve high drug concentrations within the inner ear fluids but are reliant upon diffusion across the round window membrane. Diffusion has been shown to demonstrate large individual variability. This study explores the role of "adjuvant agents", which when administered with glucocorticoids, enhance inner ear absorption and distribution. Guinea pig cochleae were administered either dexamethasone alone or in combination with hyaluronic acid, histamine, or combination histamine and hyaluronic acid, targeted at the round window membrane. Control subjects received saline. Perilymph was sampled from the cochlear apex, and basal to apical dexamethasone concentrations recorded with mass spectroscopy. Cochleae were harvested, and immunohistochemistry employed to explore dexamethasone tissue penetration and distribution. Basal to apical gradients were observed along the scala tympani, with higher dexamethasone concentrations observed at the cochlear base. Gradients were more pronounced and uniform when administered on a hyaluronic acid sponge, while histamine increased absolute concentrations reaching the inner ear. Tissue penetration correlated with perilymph concentration. Our results demonstrate that adjuvant agents can be employed to enhance dexamethasone absorption and distribution in the inner ear, thus proposing therapeutic strategies that may enhance steroid facilitated hearing protection.

A comparison of cochlear distribution and glucocorticoid receptor activation in local and systemic dexamethasone drug delivery regimes.

Local and systemically delivered glucocorticoids are commonly administered to protect the cochlea against damage associated with a variety of insults. There is reason to believe that dexamethasone administered by these routes may arrive at cochlear target sites via different pathways. Clinically, there is a lack of clarity as to which route is more effective in any specific circumstance. This study explores dexamethasone distribution within the guinea pig cochlea following local and systemic delivery methods. A combination of mass spectroscopy and immunohistochemistry were employed to compare both perilymph distribution, tissue uptake and receptor activation. Local administration of dexamethasone to the round window membrane resulted in greater perilymph concentrations, with a basal to apical gradient that favours the cochlear base. Tissue immunofluorescence was intimately related to perilymph concentration following local administration. Systemic administration resulted in much lower perilymph concentrations, with an inverse basal to apical gradient favouring the cochlear apex. Lower perilymph concentrations following systemic administration were associated with minimal tissue immunofluorescence. Despite this, GR activation of the SGNs was equivalent in both administration regimes. These results bring into question the efficacy of measuring perilymph concentrations alone as a surrogacy for dexamethasone distribution and activity in the cochlea, suggesting that the steroid ligand may arrive at its target receptor via alternative pathways. Our results suggest an equivalence in efficacy between local and systemic administration routes early after drug delivery, when the ultimate outcome of GR activation is the goal.

Cannula-based drug delivery to the guinea pig round window causes a lasting hearing loss that may be temporarily mitigated by BDNF.

Sustained local delivery of drugs to the inner ear may be required for future regenerative and protective strategies. The round window is surgically accessible and a promising delivery route. To be viable, a delivery system should not cause hearing loss. This study determined the effect on hearing of placing a drug-delivery microcatheter on to the round window, and delivering either artificial perilymph (AP) or brain-derived neurotrophic factor (BDNF) via this catheter with a mini-osmotic pump. Auditory brainstem responses (ABRs) were monitored for 4 months after surgery, while the AP or BDNF was administered for the first month. The presence of the microcatheter - whether dry or when delivering AP or BDNF for 4 weeks - was associated with an increase in ABR thresholds of up to 15 dB, 16 weeks after implantation. This threshold shift was, in part, delayed by the delivery of BDNF. We conclude that the chronic presence of a microcatheter in the round window niche causes hearing loss, and that this is exacerbated by delivery of AP, and ameliorated temporarily by delivery of BDNF.

Brain-derived neurotrophic factor modulates auditory function in the hearing cochlea.

Neurotrophins prevent spiral ganglion neuron (SGN) degeneration in animal models of ototoxin-induced deafness and may be used in the future to improve the hearing of cochlear implant patients. It is increasingly common for patients with residual hearing to undergo cochlear implantation. However, the effect of neurotrophin treatment on acoustic hearing is not known. In this study, brain-derived neurotrophic factor (BDNF) was applied to the round window membrane of adult guinea pigs for 4 weeks using a cannula attached to a mini-osmotic pump. SGN survival was first assessed in ototoxically deafened guinea pigs to establish that the delivery method was effective. Increased survival of SGNs was observed in the basal and middle cochlear turns of deafened guinea pigs treated with BDNF, confirming that delivery to the cochlea was successful. The effects of BDNF treatment in animals with normal hearing were then assessed using distortion product otoacoustic emissions (DPOAEs), pure tone, and click-evoked auditory brainstem responses (ABRs). DPOAE assessment indicated a mild deficit of 5 dB SPL in treated and control groups at 1 and 4 weeks after cannula placement. In contrast, ABR evaluation showed that BDNF lowered thresholds at specific frequencies (8 and 16 kHz) after 1 and 4 weeks posttreatment when compared to the control cohort receiving Ringer's solution. Longer treatment for 4 weeks not only widened the range of frequencies ameliorated from 2 to 32 kHz but also lowered the threshold by at least 28 dB SPL at frequencies ≥16 kHz. BDNF treatment for 4 weeks also increased the amplitude of the ABR response when compared to either the control cohort or prior to treatment. We show that BDNF applied to the round window reduces auditory thresholds and could potentially be used clinically to protect residual hearing following cochlear implantation.