Simultaneous rather than retrograde spiral ganglion cell degeneration following ototoxically induced hair cell loss in the guinea pig cochlea

Severe damage to the organ of Corti leads to degeneration of the spiral ganglion cells (SGCs) which form the auditory nerve. This degeneration starts at the level of synaptic connection of the peripheral processes (PPs) of SGCs with the cochlear hair cells. It is generally thought that from this point SGC degeneration progresses in a retrograde fashion: PPs degenerate first, followed by the SGC soma with a delay of several weeks to many months. Evidence for this course of events, both in animals and in humans, is not unambiguous, while this knowledge is important since the presence or absence of the different neural elements may greatly influence the response to electrical stimulation with a cochlear implant (CI). We therefore aimed to provide a comprehensive account of the course of SGC degeneration in the guinea pig cochlea after ototoxic treatment. Histological analysis of eighteen healthy and thirty-three deafened cochleas showed that the degeneration of SGCs and their peripheral processes was simultaneous rather than sequential. As the site of excitation for electrical stimulation with a CI may depend on the course of degeneration of the various neural elements, this finding is relevant both for understanding the electrophysiological mechanisms behind cochlear implantation and for recent efforts to induce PP resprouting for improved electrode-neural interface. Since excitation of the PPs is often thought to result in (secondary) longer-latency activity, we tested the hypothesis that having relatively many PPs produces a larger N2 peak in the electrically evoked compound action potential (eCAP); the present findings however do not support this theory. The course of the degeneration process may vary among species, and may depend on the cause of deafness, but the present findings at least indicate that gradual retrograde degeneration of the auditory nerve is not an elemental process following severe damage to the organ of Corti

Changes in the Electrically Evoked Compound Action Potential over time After Implantation and Subsequent Deafening in Guinea Pigs

The electrically evoked compound action potential (eCAP) is a direct measure of the responsiveness of the auditory nerve to electrical stimulation from a cochlear implant (CI). CIs offer a unique opportunity to study the auditory nerve's electrophysiological behavior in individual human subjects over time. In order to understand exactly how the eCAP relates to the condition of the auditory nerve, it is crucial to compare changes in the eCAP over time in a controlled model of deafness-induced auditory nerve degeneration. In the present study, 10 normal-hearing young adult guinea pigs were implanted and deafened 4 weeks later, so that the effect of deafening could be monitored within-subject over time. Following implantation, but before deafening, most examined eCAP characteristics significantly changed, suggesting increasing excitation efficacy (e.g., higher maximum amplitude, lower threshold, shorter latency). Conversely, inter-phase gap (IPG) effects on these measures - within-subject difference measures that have been shown to correlate well with auditory nerve survival - did not vary for most eCAP characteristics. After deafening, we observed an initial increase in excitability (steeper slope of the eCAP amplitude growth function (AGF), lower threshold, shorter latency and peak width) which typically returned to normal-hearing levels within a week, after which a slower process, probably reflecting spiral ganglion cell loss, took place over the remaining 6 weeks (e.g., decrease in maximum amplitude, AGF slope, peak area, and IPG effect for AGF slope; increase in IPG effect for latency). Our results suggest that gradual changes in peak width and latency reflect the rate of neural degeneration, while peak area, maximum amplitude, and AGF slope reflect neural population size, which may be valuable for clinical diagnostics

BDNF-mediated preservation of spiral ganglion cell peripheral processes and axons in comparison to that of their cell bodies

Treatment with neurotrophins prevents degeneration of spiral ganglion cells (SGCs) after severe hair cell loss. In a previous study we demonstrated a long-lasting effect with brain-derived neurotrophic factor (BDNF) after cessation of treatment. In that study the survival of the SGC cell bodies was examined. Here we address the question whether their peripheral processes and central processes (axons) were protected by this treatment as well in the cochleas of the aforementioned study. Guinea pigs were deafened by co-administration of kanamycin and furosemide. Two weeks after deafening the right cochleas were implanted with an intracochlear electrode array combined with a cannula connected to an osmotic pump filled with BDNF solution. Four weeks later the treatment was stopped by surgically removing the osmotic pump. At that point, or another four or eight weeks later, the animals were sacrificed for histological analysis. Control groups consisted of normal-hearing animals, and three groups of deafened animals: two-weeks-deaf untreated animals, and six- and fourteen-weeks-deaf sham-treated animals. Cochleas were processed for analysis of: (1) the myelinated portion of peripheral processes in the osseous spiral lamina, (2) the cell bodies in Rosenthal's canal, and (3) axons in the internal acoustic meatus. Packing densities and cross-sectional areas were determined using light microscopy. Up to eight weeks after treatment cessation the numbers of peripheral processes and axons were significantly higher than in untreated cochleas of control animals. Whereas the numbers of cell bodies and axons were similar to those at the start of treatment, the peripheral processes were significantly less well preserved. This smaller protective effect was found mainly in the apical turns. Strategies to prevent SGC degeneration after hair cell loss should consider the differential effects on the various neural elements

Intratympanic gentamicin treatment for Ménière's disease: A randomized, double-blind, placebo-controlled trial on dose efficacy - results of a prematurely ended study

Background: Gentamicin is used as a therapeutic agent for Ménière's disease because of its vestibulotoxicity causing chemo-ablation of the vestibular sensory epithelia. Its use has increased in recent years. However, there is still no consensus about the dose regimen of gentamicin in the treatment of Ménière's disease. In this study two different dose regimen treatment protocols are compared in a placebo controlled study design. The primary objective is to quantify the treatment effect on dizziness, the secondary objective is hearing evaluation.Methods: We performed a randomized, double-blind, placebo-controlled study in adults with unilateral Ménière's disease according to the AAO-HNS guidelines resistant to conservative medication. Three groups received four injections, administered weekly (four intratympanic injections with 40 mg/mL gentamicin solution, two injections gentamicin solution and two injections of placebo in random order, or four injections with placebo). Outcome measures were the score on the Dizziness Handicap Inventory and pure tone audiometry (PTA). Intended follow-up was 2 years.Results: During follow-up one patient exceeded the accepted amount of hearing loss. Further, enrollment was very slow (until 12 months between two patients) and new insights showed an apparent benefit of intratympanic gentamicin treatment (ITG). Therefore we performed an unscheduled interim analysis which showed that PTA threshold shifts reached the stopping criteria in two more patients. Because of this, this study was ended. Of the three patients with the significant PTA threshold shift two were enrolled in the gentamicin group.Conclusion: No conclusions can be drawn concerning doses regimens. Now that new publications have shown that ITG treatment can be an effective and safe treatment, a placebo-controlled randomized controlled trial may not pass the ethical committee because of these recent reports in literature. Still, a dose regimen study (without placebo) on ITG treatment needs to be performed.Trial registration: This trial was registered in The University Medical Center Utrecht/ Gelre hospital Apeldoorn. Protocol ID: 07/343, EudraCT number 2006-005913-37

The Wnt and Notch signalling pathways in the developing cochlea : Formation of hair cells and induction of regenerative potential

The Wnt and Notch signalling pathways control proliferation, specification, and cell fate choices during embryonic development and in adult life. Hence, there is much interest in both signalling pathways in the context of stem cell biology and tissue regeneration. In the developing ear, the Wnt and Notch signalling pathways specify otic cells and refine the ventral boundary of the otic placode. Since both signalling pathways control events essential for the formation of sensory cells, such as proliferation and hair cell differentiation, these pathways could hold promise for the regeneration of hair cells in adult mammalian cochlea. Indeed, modulating either the Wnt or Notch pathways can trigger the regenerative potential of supporting cells. In the neonatal mouse cochlea, Notch-mediated regeneration of hair cells partially depends on Wnt signalling, which implies an interaction between the pathways. This review presents how the Wnt and Notch signalling pathways regulate the formation of sensory hair cells and how modulating their activity induces regenerative potential in the mammalian cochlea

Local delivery of brain-derived neurotrophic factor on the perforated round window membrane in Guinea pigs : A possible clinical application

Hypothesis/Background: Local delivery of neurotrophic factors on the intact round window membrane (RWM) of hair cell-deprived cochleas reduces degeneration of the cochlear nerve. In an animal model of profound hearing loss, we investigated whether this otoprotective effect could be enhanced by perforation of the RWM. Such method could be highly relevant for future clinical applications. Methods: Guinea pigs were deafened by coadministration of kanamycin and furosemide. Two weeks after deafening, Gelfoam cubes infiltrated with brain-derived neurotrophic factor (BDNF) were deposited onto the RWM of the right cochlea. In the experimental condition, the RWM was perforated. Electrically evoked auditory brainstem responses (eABRs) were recorded weekly. Two or four weeks after Gelfoam placement, both left (untreated) and right (BDNF-treated) cochleas were processed for histology. Results: In BDNF-treated cochleas, both with and without perforation, neural survival in the basal turn of the cochlea was significantly larger than in untreated cochleas. Amplitudes of electrically evoked auditory brainstem responses were larger in BDNF-treated cochleas with an RWM perforation than in those without a perforation and comparable to those of normal-hearing controls. Perforation did not lead to collateral cochlear damage. Conclusion: When considering clinical applications of neuroprotective agents such as BDNF, delivery on a perforated RWM seems to be a safe and effective option

Recovery characteristics of the electrically stimulated auditory nerve in deafened guinea pigs : Relation to neuronal status

Successful cochlear implant performance requires adequate responsiveness of the auditory nerve to prolonged pulsatile electrical stimulation. Degeneration of the auditory nerve as a result of severe hair cell loss could considerably compromise this ability. The main objective of this study was to characterize the recovery of the electrically stimulated auditory nerve, as well as to evaluate possible changes caused by deafness-induced degeneration. To this end we studied temporal responsiveness of the auditory nerve in a guinea pig model of sensorineural hearing loss. Using masker-probe and pulse train paradigms we compared electrically evoked compound action potentials (eCAPs) in normal-hearing animals with those in animals with moderate (two weeks after ototoxic treatment) and severe (six weeks after ototoxic treatment) loss of spiral ganglion cells (SGCs). Masker-probe interval and pulse train inter-pulse interval was varied from 0.3 to 16ms. Whereas recovery assessed with masker-probe was roughly similar for normal-hearing and both groups of deafened animals, it was considerably faster for six weeks deaf animals (τ≈1.2ms) than for two weeks deaf or normal-hearing animals (τ≈3-4ms) when 100-ms pulse trains were applied. Latency increased with decreasing inter-pulse intervals, and this was more pronounced with pulse trains than with masker-probe stimulation. With high frequency pulse train stimulation eCAP amplitudes were modulated for deafened animals, meaning that amplitudes for odd pulse numbers were larger than for even pulses. The relative refractory period (τ) and the modulation depth of the eCAP amplitude for pulse trains, as well as the latency increase for both paradigms significantly correlated with quantified measures of auditory nerve degeneration (size and packing density of SGCs). In addition to these findings, separate masker-probe recovery functions for the eCAP N1 and N2 peaks displayed a robust non-monotonic or shoulder-shaped course in all animals. The time interval between the N1 and N2 correlated with neuronal refractoriness, suggesting that the N2 peak reflects a second firing of part of the SGC population. We conclude that - compared to the commonly used masker-probe recovery functions - recovery functions obtained with pulse train stimulation may provide a means to augment differences and, by doing so, to more potently discriminate between auditory nerve conditions

Temporary Neurotrophin Treatment Prevents Deafness-Induced Auditory Nerve Degeneration and Preserves Function

After substantial loss of cochlear hair cells, exogenous neurotrophins prevent degeneration of the auditory nerve. Because cochlear implantation, the current therapy for profound sensorineural hearing loss, depends on a functional nerve, application of neurotrophins is being investigated. We addressed two questions important for fundamental insight into the effects of exogenous neurotrophins on a degenerating neural system, and for translation to the clinic. First, does temporary treatment with brain-derived neurotrophic factor (BDNF) prevent nerve degeneration on the long term? Second, how does a BDNF-treated nerve respond to electrical stimulation? Deafened guinea pigs received a cochlear implant, and their cochleas were infused with BDNF for 4 weeks. Up to 8 weeks after treatment, their cochleas were analyzed histologically. Electrically evoked compound action potentials (eCAPs) were recorded using stimulation paradigms that are informative of neural survival. Spiral ganglion cell (SGC) degeneration was prevented during BDNF treatment, resulting in 1.9 times more SGCs than in deafened untreated cochleas. Importantly, SGC survival was almost complete 8 weeks after treatment cessation, when 2.6 times more SGCs were observed. In four eCAP characteristics (three involving alteration of the interphase gap of the biphasic current pulse and one involving pulse trains), we found large and statistically significant differences between normal-hearing and deaf controls. Importantly, for BDNF-treated animals, these eCAP characteristics were near normal, suggesting healthy responsiveness of BDNF-treated SGCs. In conclusion, clinically practicable short-term neurotrophin treatment is sufficient for long-term survival of SGCs, and it can restore or preserve SGC function well beyond the treatment period

Systematic review of compound action potentials as predictors for cochlear implant performance

Objectives/Hypothesis: The variability in speech perception between cochlear implant users is thought to result from the degeneration of the auditory nerve. Degeneration of the auditory nerve, histologically assessed, correlates with electrophysiologically acquired measures, such as electrically evoked compound action potentials (eCAPs) in experimental animals. To predict degeneration of the auditory nerve in humans, where histology is impossible, this paper reviews the correlation between speech perception and eCAP recordings in cochlear implant patients. Data Sources: PubMed and Embase. Review Methods: We performed a systematic search for articles containing the following major themes: cochlear implants, evoked potentials, and speech perception. Two investigators independently conducted title-abstract screening, full-text screening, and critical appraisal. Data were extracted from the remaining articles. Results: Twenty-five of 1,429 identified articles described a correlation between speech perception and eCAP attributes. Due to study heterogeneity, a meta-analysis was not feasible, and studies were descriptively analyzed. Several studies investigating presence of the eCAP, recovery time constant, slope of the amplitude growth function, and spatial selectivity showed significant correlations with speech perception. In contrast, neural adaptation, eCAP threshold, and change with varying interphase gap did not significantly correlate with speech perception in any of the identified studies. Conclusions: Significant correlations between speech perception and parameters obtained through eCAP recordings have been documented in literature; however, reporting was ambiguous. There is insufficient evidence for eCAPs as a predictive factor for speech perception. More research is needed to further investigate this relation

Systematic review of compound action potentials as predictors for cochlear implant performance

Objectives/Hypothesis: The variability in speech perception between cochlear implant users is thought to result from the degeneration of the auditory nerve. Degeneration of the auditory nerve, histologically assessed, correlates with electrophysiologically acquired measures, such as electrically evoked compound action potentials (eCAPs) in experimental animals. To predict degeneration of the auditory nerve in humans, where histology is impossible, this paper reviews the correlation between speech perception and eCAP recordings in cochlear implant patients. Data Sources: PubMed and Embase. Review Methods: We performed a systematic search for articles containing the following major themes: cochlear implants, evoked potentials, and speech perception. Two investigators independently conducted title-abstract screening, full-text screening, and critical appraisal. Data were extracted from the remaining articles. Results: Twenty-five of 1,429 identified articles described a correlation between speech perception and eCAP attributes. Due to study heterogeneity, a meta-analysis was not feasible, and studies were descriptively analyzed. Several studies investigating presence of the eCAP, recovery time constant, slope of the amplitude growth function, and spatial selectivity showed significant correlations with speech perception. In contrast, neural adaptation, eCAP threshold, and change with varying interphase gap did not significantly correlate with speech perception in any of the identified studies. Conclusions: Significant correlations between speech perception and parameters obtained through eCAP recordings have been documented in literature; however, reporting was ambiguous. There is insufficient evidence for eCAPs as a predictive factor for speech perception. More research is needed to further investigate this relation