Auditory Nerve Research Articles

MRI surveillance after translabyrinthine vestibular schwannoma resection and cochlear implantation: is it feasible?

PurposeCochlear implantation in patients with vestibular schwannomas is of increasing importance and interest. Two remaining challenges are the assessment of conduction of the cochlear nerve and the possibility of postoperative surveillance with magnetic resonance imaging. The aim of the current study was to assess follow-up imaging and determine the visibility of the internal auditory canal after vestibular schwannoma resection and cochlear implantation as well as in patients with persistent vestibular schwannomas and cochlear implants in place. Visibility of the internal auditory canal, cerebellopontine angle, and labyrinth were evaluated and graded.MethodsFor this retrospective study, 15 MR examinations of 13 patients after translabyrinthine vestibular schwannoma resection and ipsilateral cochlear implantation were included. All patients had been implanted with an MED-EL cochlear implant. Magnetic resonance imaging was carried out on a 1.5T device. All patients were prepped according to the manufacturer’s recommendations.ResultsAll 15 examinations were carried out without any adverse event during imaging, such as pain, magnet dislocation, or malfunction. The internal auditory canal and the cerebellopontine angle were sufficiently visible in all cases to allow for vestibular schwannoma follow-up.ConclusionMagnetic resonance imaging surveillance of the internal auditory canal following vestibular schwannoma resection and cochlear implantation is feasible and safe with modern implants with a 1.5T magnetic resonance imaging device using metal artifact reduction sequences. Necessary follow-up imaging should not be a contraindication for cochlear implantation in patients with vestibular schwannomas.

Does vestibular function correlate with objective MRI findings after vestibular schwannoma surgery?

Vestibular schwannoma surgery leads to acute unilateral vestibular loss. In some patients, however, the process of post-operatively initiated central compensation proceeds more rapidly than in others. This study aimed to evaluate post-operative vestibular function and correlate it with morphological findings of MRI scans. The study included 29 patients who underwent surgery for vestibular schwannoma. Vestibular function was analysed post-operatively by video head impulse test (vHIT). Subjective symptoms were evaluated using validated questionnaires. All patients underwent MRI 3 months post-operatively, and the presence of the facial and vestibulocochlear nerves in the internal auditory canal was assessed. The vestibulo-ocular reflex gain measured by the vHIT correlated positively with audiological findings. Subjective perception of vestibular disorder did not correlate with objectively measured vestibular impairment or with MRI findings. After the resection of vestibular schwannoma, some patients may still have preserved vestibular function as measured by vHIT. The preserved function does not correlate with subjective symptoms. Patients with partially deteriorated vestibular function showed lower sensitivity to combined stimuli.

Diffusion tensor imaging and auditory tractography to evaluate cochlear implant candidacy: a pilot study

Background Conventional radiological evaluation does not evaluate the functional status of the auditory pathway in patients scheduled for cochlear implantation (CI). Objectives Determine the value of diffusion tensor imaging [DTI] in the preoperative evaluation of some patients scheduled for CI. Material and Methods Patients with profound SNHL and inner ear and/or cochlear nerve anomalies or long standing SNHL were selected. They underwent conventional MRI images of the brain in three orthogonal planes, MR arterial spin labelling (ASL), perfusion, and DTI auditory tractography to determine functional status of the auditory pathways. Results Ten patients were included. Seven with bilateral SNHL, one with fluctuating hearing loss and one with long standing single sided deafness. In 8 patients the auditory pathway could be traced and functional maps could determine the side of possible better function. In the patient with progressive hearing loss DTI revealed major central pathway problems and CI was discouraged. In the patient with SSD, DTI revealed a robust intact pathway and CI was advised. Conclusions DTI and auditory tractography can help in outlining the functional integrity of the 33auditory pathway and assist in decision making before CI.

The Relationship Between Cochlear Nerve and Cochlear Nerve Canal Dimensions in Incomplete Partition Types

The Relationship Between Cochlear Nerve and Cochlear Nerve Canal Dimensions in Incomplete Partition Types

Cochlear synaptopathy impairs suprathreshold tone-in-noise coding in the cochlear nucleus.

Hearing impairment without threshold elevations can occur when there is damage to high-threshold auditory nerve fibre synapses with cochlear inner hair cells. Instead, cochlear synaptopathy produces suprathreshold deficits, especially in older patients, which affect conversational speech. Given that listening in noise at suprathreshold levels presents significant challenges to the ageing population, we examined the effects of synaptopathy on tone-in-noise coding on the central recipients of auditory nerve fibres, i.e. the cochlear nucleus neurons. To induce synaptopathy, guinea pigs received a unilateral sound overexposure to the left ears. A separate group received sham exposures. At 4weeks post-exposure, thresholds had recovered but reduced auditory brainstem response wave1 amplitudes and auditory nerve synapse loss remained on the left side. Single-unit responses were recorded from several cell types in the ventral cochlear nucleus to pure-tone and noise stimuli. Receptive fields and rate-level functions in the presence of continuous broadband noise were examined. The synaptopathy-inducing noise exposure did not affect mean unit tone-in-noise thresholds, nor the tone-in-noise thresholds in each animal, demonstrating equivalent tone-in-noise detection thresholds to sham animals. However, synaptopathy reduced single-unit responses to suprathreshold tones in the presence of background noise, particularly in the cochlear nucleus small cells. These data demonstrate that suprathreshold tone-in-noise deficits following cochlear synaptopathy are evident in the first neural station of the auditory brain, the cochlear nucleus neurons, and provide a potential target for assessment and treatment of listening-in-noise deficits in humans. KEY POINTS: Recording from multiple central auditory neurons can determine tone-in-noise deficits in animals with quantified cochlear synapse damage. Using this technique, we found that tone-in-noise thresholds are not altered by cochlear synaptopathy, whereas coding of suprathreshold tones-in-noise is disrupted. Suprathreshold deficits occur in small cells and primary-like neurons of the cochlear nucleus. These data provide important insights into the mechanisms underlying difficulties associated with hearing in noisy environments.

Intraoperative Cochlear Nerve Monitoring in Cochlear Implantation after Vestibular Schwannoma Resection.

The use of a cochlear implant (CI) for hearing rehabilitation after vestibular schwannoma (VS) resection is widely spreading. The procedure is usually performed simultaneously to tumor resection with a translabyrinthine approach. To ensure the best device function, assessing the integrity of the cochlear nerve is of primary importance. A narrative review of the literature on the present topic was carried out up to June 2022. Finally, nine studies were considered. Electrically evoked auditory brainstem responses (eABR) is the most widely used method of intraoperative monitoring of cochlear nerve (CN) during VS resection, although its limits are known. It can be assessed through the CI electrode array or through an intracochlear test electrode (ITE). Variations of the graph are evaluated during the surgical procedure, in particular the wave V amplitude and latency. As tumor dissection progresses, the parameters may change, informing of the CN status, and the surgical procedure may be modulated. An eABR positive result seems to be reliably correlated with a good CI outcome in those cases in which a clear wave V is recorded before and after tumor removal. On the contrary, in those cases in which the eABR is lost or altered during the surgical procedure, the positioning of a CI is still debatable.

Central projections of auditory nerve fibers in the western rat snake (Pantherophis obsoletus).

Despite the absence of tympanic middle ears, snakes can hear. They are thought to primarily detect substrate vibration via connections between the lower jaw and the inner ear. We used the western rat snake (Pantherophis obsoletus) to determine how vibration is processed in the brain. We measured vibration-evoked potential recordings to reveal sensitivity to low-frequency vibrations. We then used tract tracing combined with immunohistochemistry and Nissl staining to describe the central projections of the papillar branch of the VIIIth nerve. Applications of biotinylated dextran amine to the basilar papilla (homologous to the organ of Corti of mammals) labeled bouton-like terminals in two first-order cochlear nuclei, a rostrolateral nucleus angularis (NA) and a caudomedial nucleus magnocellularis (NM). NA formed a distinct dorsal eminence, consisted of heterogenous cell types, and was parvalbumin positive. NM was smaller and poorly separated from the surrounding vestibular nuclei. NM was distinguished by positive calbindin label and included fusiform and round cells. Thus, the atympanate western rat snake shares similar first-order projections to tympanate reptiles. Auditory pathways may be used for detecting vibration, not only in snakes but also potentially in atympanate early tetrapods.

Immunohistochemistry Reveals TRPC Channels in the Human Hearing Organ-A Novel CT-Guided Approach to the Cochlea.

TRPC channels are critical players in cochlear hair cells and sensory neurons, as demonstrated in animal experiments. However, evidence for TRPC expression in the human cochlea is still lacking. This reflects the logistic and practical difficulties in obtaining human cochleae. The purpose of this study was to detect TRPC6, TRPC5 and TRPC3 in the human cochlea. Temporal bone pairs were excised from ten body donors, and the inner ear was first assessed based on computed tomography scans. Decalcification was then performed using 20% EDTA solutions. Immunohistochemistry with knockout-tested antibodies followed. The organ of Corti, the stria vascularis, the spiral lamina, spiral ganglion neurons and cochlear nerves were specifically stained. This unique report of TRPC channels in the human cochlea supports the hypothesis of the potentially critical role of TRPC channels in human cochlear health and disease which has been suggested in previous rodent experiments.

Cochlear Implantation in Very Young Children With Single-Sided Deafness.

Cochlear implants (CIs) for single-sided deafness (SSD) have only been approved for patients 5 years and older despite data supporting that younger children can also benefit from implantation. This study describes our institution's experience with CI for SSD in children 5 years and younger. Case series with chart review. Tertiary referral center. A case series with chart review identified 19 patients up to age 5 years who underwent CI for SSD between 2014 and 2022. Baseline characteristics, perioperative complications, device usage, and speech outcomes were collected. The median age at CI was 2.8 (range, 1.0-5.4) years, with 15 (79%) patients being below age 5 at implantation. Etiologies of hearing loss were idiopathic (n = 8), cytomegalovirus (n = 4), enlarged vestibular aqueduct (n = 3), hypoplastic cochlear nerve (n = 3), and meningitis (n = 1). The median preoperative pure-tone average was 90 (range, 75-120) and 20 (range, 5-35) dB eHL in the poorand better hearing ears, respectively. No patients had postoperative complications. Twelve patients achieved consistent device use (average, 9 h/d). Three of the seven who were not consistent users had hypoplastic cochlear nerves and/or developmental delays. The three patients with available preoperative and postoperative speech testing showed significant benefits, and five patients with available postoperative testing demonstrated speech recognition in the implanted ear when isolated from the better ear. CI can safely be performed in younger children with SSD. Patients and families accept early implantation, as evidenced by consistent device use, and derive notable benefits in speech recognition. Candidacy can be broadened to include SSD patients under age five years, particularly individuals without hypoplastic cochlear nerves or developmental delay.

Anatomical Features of Children With Mondini Dysplasia: Influence on Cochlear Implantation Performance.

To analyze the long-term auditory performance after cochlear implantation (CI) and identify anatomical features of Mondini dysplasia associated with post-CI outcomes. Retrospective study. Tertiary care academic center. We enrolled 49 ears with Mondini dysplasia who underwent CI with more than 7 years of follow-up and age at CI- and sex-matched control group with radiologically normal inner ears. The development of auditory skills after CI was evaluated using word recognition scores (WRSs). The anatomical features were measured based on temporal bone computed tomography and magnetic resonance imaging, involving the width of the bony cochlear nerve canal (BCNC), cochlear basal turn, enlarged vestibular aqueduct, cochlear height, and diameter of the cochlear nerve (CN). CI in ears with Mondini dysplasia showed comparable benefits and improvement of auditory performance to controls during the 7 years of follow-up. In Mondini dysplasia, four (8.2%) ears showed narrow BCNC (<1.4 mm) with poorer WRS (58 ± 17%) than those with normal-sized BCNC, which had WRS (79 ± 10%) comparable to that of the control group (77 ± 14%). In Mondini dysplasia, the maximum ( r = 0.513, p < 0.001) and minimum ( r = 0.328, p = 0.021) CN diameters had positive correlations with post-CI WRS. The maximum CN diameter ( β = 48.347, p < 0.001) and BCNC width ( β = 12.411, p = 0.041) were significant factors that influence the post-CI WRS in multiple regression analysis. Preoperative anatomical evaluation, especially BCNC status and CN integrity, may serve as predictive markers for post-CI performance.

Symptoms: Sudden Sensorineural Hearing Loss and Aural Fullness

Symptoms: Sudden Sensorineural Hearing Loss and Aural Fullness

Interaural stimulation timing mismatch in listeners provided with acochlear implant and ahearing aid : Areview focusing on quantification and compensation

Bimodal provision of patients with asymmetric hearing loss with ahearing aid ipsilaterally and acochlear implant (CI) contralaterally is probably the most complicated type of CI provision due to avariety of inherent variables. This review article presents all the systematic interaural mismatches between electric and acoustic stimulation that can occur in bimodal listeners. One of these mismatches is the interaural latency offset, i.e., the time difference of activation of the auditory nerve by acoustic and electric stimulation. Methods for quantifying this offset are presented by registering electrically and acoustically evoked potentials and measuring processing delays in the devices. Technical compensation of the interaural latency offset and its positive effect on sound localization ability in bimodal listeners is also described. Finally, most recent findings are discussed which may explain why compensation of the interaural latency offset does not improve speech understanding in noise in bimodal listeners.

Light at the end of the tunnel of Corti

Light at the end of the tunnel of Corti

Round-window delivery of lithium chloride regenerates cochlear synapses damaged by noise-induced excitotoxic trauma via inhibition of the NMDA receptor in the rat.

Noise exposure can destroy the synaptic connections between hair cells and auditory nerve fibers without damaging the hair cells, and this synaptic loss could contribute to difficult hearing in noisy environments. In this study, we investigated whether delivering lithium chloride to the round-window can regenerate synaptic loss of cochlea after acoustic overexposure. Our rat animal model of noise-induced cochlear synaptopathy caused about 50% loss of synapses in the cochlear basal region without damaging hair cells. We locally delivered a single treatment of poloxamer 407 (vehicle) containing lithium chloride (either 1 mM or 2 mM) to the round-window niche 24 hours after noise exposure. Controls included animals exposed to noise who received only the vehicle. Auditory brainstem responses were measured 3 days, 1 week, and 2 weeks post-exposure treatment, and cochleas were harvested 1 week and 2 weeks post-exposure treatment for histological analysis. As documented by confocal microscopy of immunostained ribbon synapses, local delivery of 2 mM lithium chloride produced synaptic regeneration coupled with corresponding functional recovery, as seen in the suprathreshold amplitude of auditory brainstem response wave 1. Western blot analyses revealed that 2 mM lithium chloride suppressed N-methyl-D-aspartate (NMDA) receptor expression 7 days after noise-exposure. Thus, round-window delivery of lithium chloride using poloxamer 407 reduces cochlear synaptic loss after acoustic overexposure by inhibiting NMDA receptor activity in rat model.

ANTIOXIDANT THERAPY IN THE ELDERLY WITH TINNITUS: SYSTEMATICS REVIEW

Tinnitus is one of the most prevalent and annoying disorders that affect the ear and the auditory system. It is one of the most important symptoms in neurootology, along with hearing loss, vertigo, dizziness, and nausea. It can cause a variety of somatic and psychological disorders that interfere with a person's quality of life. There is a correlation between oxidative stress and tinnitus. Hair cell apoptosis, cochlear degeneration, alterations to supporting structures and stria vascularis, changes to the nerve fibers of the acoustic nerve, irregular neuronal activity in the auditory pathway, and malfunction in the central cortex are all possible outcomes of oxidative stress. Animal models of aging, noise damage, and ototoxicity have been used to investigate the relationship between oxidative stress and the death of hair cells. Antioxidants are substances that can take on a wide variety of chemical structures, but they are categorized as a single class because they share the ability to neutralize the effects of highly reactive and potentially damaging free radicals. These radicals are produced when foods undergo the necessary oxidation reactions that are a natural part of the digestion process. Antioxidant supplementation with vitamins, minerals, phytochemicals, and alpha-lipoic acid (ALA) might have therapeutic effects on tinnitus by reducing the subjective discomfort and the level of severity caused by the ringing in the ears.

Metabolic decline in an insect ear: correlative or causative for age-related auditory decline?

One leading hypothesis for why we lose our hearing as we age is a decrease in ear metabolism. However, direct measurements of metabolism across a lifespan in any auditory system are lacking. Even if metabolism does decrease with age, a question remains: is a metabolic decrease a cause of age-related auditory decline or simply correlative? We use an insect, the desert locust Schistocerca gregaria, as a physiologically versatile model to understand how cellular metabolism correlates with age and impacts on age-related auditory decline. We found that auditory organ metabolism decreases with age as measured fluorometrically. Next, we measured the individual auditory organ's metabolic rate and its sound-evoked nerve activity and found no correlation. We found no age-related change in auditory nerve activity, using hook electrode recordings, and in the electrophysiological properties of auditory neurons, using patch-clamp electrophysiology, but transduction channel activity decreased. To further test for a causative role of the metabolic rate in auditory decline, we manipulated metabolism of the auditory organ through diet and cold-rearing but found no difference in sound-evoked nerve activity. We found that although metabolism correlates with age-related auditory decline, it is not causative. Finally, we performed RNA-Seq on the auditory organs of young and old locusts, and whilst we found enrichment for Gene Ontology terms associated with metabolism, we also found enrichment for a number of additional aging GO terms. We hypothesize that age-related hearing loss is dominated by accumulative damage in multiple cell types and multiple processes which outweighs its metabolic decline.

The Relationship Between Cochlear Implant Speech Perception Outcomes and Electrophysiological Measures of the Electrically Evoked Compound Action Potential.

This study assessed the relationship between electrophysiological measures of the electrically evoked compound action potential (eCAP) and speech perception scores measured in quiet and in noise in postlingually deafened adult cochlear implant (CI) users. It tested the hypothesis that how well the auditory nerve (AN) responds to electrical stimulation is important for speech perception with a CI in challenging listening conditions. Study participants included 24 postlingually deafened adult CI users. All participants used Cochlear Nucleus CIs in their test ears. In each participant, eCAPs were measured at multiple electrode locations in response to single-pulse, paired-pulse, and pulse-train stimuli. Independent variables included six metrics calculated from the eCAP recordings: the electrode-neuron interface (ENI) index, the neural adaptation (NA) ratio, NA speed, the adaptation recovery (AR) ratio, AR speed, and the amplitude modulation (AM) ratio. The ENI index quantified the effectiveness of the CI electrodes in stimulating the targeted AN fibers. The NA ratio indicated the amount of NA at the AN caused by a train of constant-amplitude pulses. NA speed was defined as the speed/rate of NA. The AR ratio estimated the amount of recovery from NA at a fixed time point after the cessation of pulse-train stimulation. AR speed referred to the speed of recovery from NA caused by previous pulse-train stimulation. The AM ratio provided a measure of AN sensitivity to AM cues. Participants' speech perception scores were measured using Consonant-Nucleus-Consonant (CNC) word lists and AzBio sentences presented in quiet, as well as in noise at signal-to-noise ratios (SNRs) of +10 and +5 dB. Predictive models were created for each speech measure to identify eCAP metrics with meaningful predictive power. The ENI index and AR speed individually explained at least 10% of the variance in most of the speech perception scores measured in this study, while the NA ratio, NA speed, the AR ratio, and the AM ratio did not. The ENI index was identified as the only eCAP metric that had unique predictive power for each of the speech test results. The amount of variance in speech perception scores (both CNC words and AzBio sentences) explained by the eCAP metrics increased with increased difficulty under the listening condition. Over half of the variance in speech perception scores measured in +5 dB SNR noise (both CNC words and AzBio sentences) was explained by a model with only three eCAP metrics: the ENI index, NA speed, and AR speed. Of the six electrophysiological measures assessed in this study, the ENI index is the most informative predictor for speech perception performance in CI users. In agreement with the tested hypothesis, the response characteristics of the AN to electrical stimulation are more important for speech perception with a CI in noise than they are in quiet.

Electrical promontory stimulation test using a portable peripheral nerve stimulator with an ear canal electrode

ObjectiveAn intact cochlear nerve is necessary for successful cochlear implantation (CI). Although the promontory stimulation test (PST) using a promontory stimulator (PS) and a transtympanic needle electrode is invasive, it is still commonly used to verify cochlear nerve function. PSs are currently unavailable because they are no longer manufactured; however, considering that PST continues to be beneficial in certain situations, alternative equipment is needed. The PNS-7000® (PNS) was developed as a neurologic instrument for stimulating the peripheral nerves. This study investigated the usefulness of the ear canal stimulation test (ECST) using PNS with a silver ball ear canal electrode, which is a new noninvasive alternative technique to the PST. MethodsECST was performed from November 2013 to December 2018 using PS and PNS for patients with severe to profound sensorineural hearing loss. The electrical threshold, most comfortable loudness level, uncomfortable loudness level, dynamic range, and gap detection were measured in the ECST. The results of these measured PNS items were compared with PS. ResultsECST was performed in 61 ears of 35 patients (age, 59.9 ± 20.1 years) using PS and PNS. The sound sensation was elicited in 51 (83.6%) and 52 (85.2%) ears with PS and PNS, respectively. All items excluding GAP were measured in 46 (75%) and 43 (70%) ears at 50 and 100 Hz, respectively. GAP was measured in 33 ears by the ascending and descending methods using PS and PNS. Spearman's rank-order correlation coefficient revealed a significant positive linear correlation between the PS and PNS results in all measurements. No significant difference was found between the PS and PNS thresholds in all measured items. ConclusionsPNS is a useful instrument for performing ECST as a new alternative to PS. ECST using a silver ball electrode is a less invasive and easier test than PST.

Beat encoding at mistuned octaves within single electrosensory neurons

Beats are slow periodic amplitude modulations resulting from the superposition of two spectrally close periodic signals. The difference frequency between the signals sets the frequency of the beat. A field study in the electric fish Apteronotus rostratus showed the behavioral relevance of very high difference frequencies. Contrary to expectations from previous studies, our electrophysiological data show strong responses of p-type electroreceptor afferents whenever the difference frequency approaches integer multiples (mistuned octaves) of the fish's own electric field frequency (carrier). Mathematical reasoning and simulations show that common approaches to extract amplitude modulations, such as Hilbert transform or half-wave rectification, are not sufficient to explain the responses at carrier octaves. Instead, half-wave rectification needs to be smoothed out, for example by a cubic function. Because electroreceptive afferents share many properties with auditory nerve fibers, these mechanisms may underly thehuman perception of beats at mistuned octaves as described by Ohm and Helmholtz.

Hearing Rehabilitation in Vestibular Schwannoma.

The most common complaint among patients with vestibular schwannoma (VS) is hearing loss. This significantly affects the quality of life before, during, and after treatment for patients with VS. Untreated hearing loss in VS patients may even lead to depression and feelings of social isolation. A variety of devices are available for hearing rehabilitation for patients with vestibular schwannoma. These include contralateral routing of hearing signals (CROSs), bone-anchored hearing devices, auditory brainstem implants (ABI), and cochlear implants. In the United States, ABI is approved for patients 12 years of age and older with neurofibromatosis type 2. In the past few years, cochlear implantation has been offered simultaneously or sequentially with tumor resection or irradiation, or even to patients whose VS have been monitored with serial imaging. However, determining the functional integrity of the auditory nerve in patients with vestibular schwannoma is a challenge. This review article consists of (1) the pathophysiology of vestibular schwannoma (VS), (2) hearing loss in VS, (3) treatment of VS and associated hearing loss, (4) options for auditory rehabilitation in patients with VS with their individual benefits and limitations, and (5) challenges in hearing rehabilitation in this cohort of patients to determine auditory nerve functionality. (6) Future directions.