Consequences of unilateral hearing loss: time dependent regulation of protein synthesis in auditory brainstem nuclei

The role of diffusion tensor imaging in idiopathic sensorineural hearing loss: is it significant?

Purpose

To assess the role of diffusion tensor imaging metrics in the evaluation of the microstructural integrity of the central auditory tract in patients with idiopathic sensorineural hearing loss (SNHL), and to compare these patients with healthy controls.

Material and methods

This prospective study, which was conducted upon 30 subjects (21 males, 9 females; age range from 16 to 65 years, mean age 45years) with SNHL proven by audiometric tests. Ten age- and sex-matched healthy volunteers were included as a control group. Patients (n = 30) and volunteers (n = 10) underwent conventional magnetic resonance imaging and diffusion tensor imaging of the brain. Both fractional anisotropy and mean diffusivity (MD) of 3 points along the acoustic tract (inferior colliculus, lateral lemniscus and superior olivary nucleus) were measured bilaterally in all patients and correlated with controls.

Results

Mean fractional anisotropy (FA) values were reduced bilaterally at the superior olivary nucleus and/or lateral lemniscus and more significantly at the inferior colliculus of subjects with SNHL in comparison to the volunteers. In patients of unilateral SNHL, similar results were obtained in the contralateral side when compared to controls with statistically significant difference at the 3 regions (p = 0.001). No significant changes were noticed in the MD parameters either in patient or control groups.

Conclusions

The FA value was a valuable non-invasive biomarker in evaluating the subtle microstructural abnormalities of the central auditory tract in idiopathic SNHL and correlated well with hearing impairment.

Transient Conductive Hearing Loss Regulates Cross-Modal VGLUT Expression in the Cochlear Nucleus of C57BL/6 Mice

Auditory nerve fibers synapse onto the cochlear nucleus (CN) and are labeled using the vesicular glutamate transporter-1 (VGLUT-1), whereas non-auditory inputs are labeled using the VGLUT-2. However, the underlying regulatory mechanism of VGLUT expression in the CN remains unknown. We examined whether a sound level decrease, without primary neural damage, induces cellular and VGLUT expression change in the CN, and examined the potential for neural plasticity of the CN using unilateral conductive hearing loss models. We inserted earplugs in 8-week-old mice unilaterally for 4 weeks and subsequently removed them for another 4 weeks. Although the threshold of an auditory brainstem response significantly increased across all tested frequencies following earplug insertion, it completely recovered after earplug removal. Auditory deprivation had no significant impact on spiral ganglion and ventral CN (VCN) neurons’ survival. Conversely, although the cell size and VGLUT-1 expression in the VCN significantly decreased after earplug insertion, VGLUT-2 expression in the granule cell lamina significantly increased. These cell sizes decreased and the alterations in VGLUT-1 and -2 expression almost completely recovered at 1 month after earplug removal. Our results suggested that the cell size and VGLUT expression in the CN have a neuroplasticity capacity, which is regulated by increases and decreases in sound levels. Restoration of the sound levels might partly prevent cell size decrease and maintain VGLUT expression in the CN.

Transcutaneous Osseointegrated Implants for Pediatric Patients With Aural Atresia

Importance

Patients with aural atresia typically have maximal conductive hearing loss, which can have negative academic and social consequences. Transcutaneous osseointegrated implants (TOIs) can potentially restore hearing on the affected side.

Objectives

To review the demographic, audiological, and surgical outcomes of TOI placement in pediatric patients with aural atresia and to describe a modification in incision technique in anticipation of later auricular reconstruction.

Design, Setting, and Participants

This retrospective case series reviewed 41 cases of TOI placement in pediatric patients between January 1, 2014, and September 30, 2016, at Lurie Children's Microtia and Aural Atresia Clinic. Patients, all younger than 18 years and with atresia or microtia, received at least 6 months of follow-up and underwent testing before and after surgery.

Main Outcomes and Measures

Patient age, indication for procedure, ear sidedness, case length, incision type, complications, and other postoperative events. Audiological outcomes before and after implantation were measured using pure-tone averages and the Hearing In Noise Test for Children, presented in variable signal to noise ratios.

Results

In total, 46 TOIs were performed in 38 pediatric patients, but only 41 implantations in 34 patients were included in this study. Of the 34 patients, 13 (38%) were males and 21 (62%) were females, with a mean age of 8.9 (range, 5-17) years at the time of TOI placement. Microtia on the implanted side was present in 39 cases (95%). A modified posterior-superior scalp incision technique was used in 30 (73%) of 41 ears, all in cases of microtia. One perioperative surgical complication occurred: a seroma requiring drainage. Two patients developed minor skin irritation and erythema at the magnet site related to the overnight use of the processor, which resolved when removed while sleeping. The mean (SD; range) score for the Speech In Noise test at 5 dB signal to noise ratio improved from 75.3% (14.4%; range, 50%-92%) correct in unaided/preoperative condition to 93.6% (6.95%; range, 80%-100%) correct in the aided/postoperative condition. The mean improvement in score was 18.3% (95% CI, 10.8%-25.9%), with an effect size of 1.62 (95% CI, 0.95-2.29). The mean pure-tone averages (SD; range) similarly improved from 63.7 (13.2; range, 25-11) dB to 9.6 (4.9; range, 5-15) dB.

Conclusions and Relevance

Transcutaneous osseointegrated implantation has a low complication rate among pediatric patients with atresia or microtia and can provide excellent audiological results. It should be included as a treatment option for this population of patients who meet audiological criteria.

Reversible external auditory canal ligation (REACL): A novel surgical technique to induce transient and reversible hearing loss in developing rats

BACKGROUND

Cases of sensory loss provide an excellent model to study the plastic nature of cortical sensory systems. Models of reversible sensory loss are particularly useful for establishing the timeline of various critical periods for cortical plasticity. However, there currently is an absence of adequate methods to produce reversible hearing loss in neonatal and developing rodents.

NEW METHOD

We propose a novel and reversible adaptation of an existing surgical technique-external auditory canal ligation (EACL)-that produces a reliable and moderate hearing loss.

RESULTS

Auditory brainstem responses (ABRs) were used to measure both the magnitude of the hearing loss induced by EACL and the auditory thresholds following hearing restoration. The EACL and reopening procedures, as assessed by visual inspection, had success rates of 81% and 78%, respectively. The average hearing thresholds, as assessed with ABRs, increased by nearly 40 decibels across all tested frequencies. Hearing thresholds returned to normal levels following the reopening procedure.

COMPARISON WITH EXISTING METHODS

Our procedure yields similar benefits to other methods, such as producing a reliable and moderate hearing loss that is entirely reversible. Furthermore, to our knowledge, it is the first that can be performed in neonatal rodents, thus allowing researchers the opportunity to assess the effects of sensory loss on behavior and cortical neurophysiology during developmental critical periods.

CONCLUSION

Our modified technique of reversible external auditory canal ligation offers an easy, and reliable method to induce a transient state of hearing loss that mimics naturally occurring congenital conductive hearing loss.

Neural Plastic Changes in the Subcortical Auditory Neural Pathway after Single-Sided Deafness in Adult Mice: A MEMRI Study

Single-sided deafness (SSD) induces cortical neural plastic changes according to duration of deafness. However, it is still unclear how the auditory cortical changes accompany the subcortical neural changes. The present study aimed to find the neural plastic changes in the cortical and subcortical auditory system following adult-onset single-sided deafness (SSD) using Mn-enhanced magnetic resonance imaging (MEMRI). B57BL/6 mice (postnatal 8-week-old) were divided into three groups: the SSD-4-week group (postnatal 12-week-old, n = 11), the SSD-8-week group (postnatal 16-week-old, n = 11), and a normal-hearing control group (postnatal 8-week-old, n = 9). The left cochlea was ablated in the SSD groups. White Gaussian noise was delivered for 24 h before MEMRI acquisition. T1-weighted MRI data were analyzed from the cochlear nucleus (CN), superior olivary complex (SOC), lateral lemniscus (LL), inferior colliculus (IC), medial geniculate body (MG), and auditory cortex (AC). The differences in relative Mn2+-enhanced signal intensities (Mn2+SI) and laterality were analyzed between the groups. Four weeks after the SSD procedure, the ipsilateral side of the SSD showed significantly lower Mn2+SI in the CN than the control group. On the other hand, the contralateral side of the SSD demonstrated significantly lower Mn2+SI in the SOC, LL, and IC. These decreased Mn2+SI values were partially recovered at 8 weeks after the SSD procedure. The interaural Mn2+SI differences representing the interaural dominance were highest in CN and then became less prominently higher in the auditory neural system. The SSD-8-week group still showed interaural differences in the CN, LL, and IC. In contrast, the MG and AC did not show any significant intergroup or interaural differences in Mn2+SI. In conclusion, subcortical auditory neural activities were decreased after SSD, and the interaural differences were diluted in the higher auditory nervous system. These findings were attenuated with time. Subcortical auditory neural changes after SSD may contribute to the change in tinnitus severity and the outcomes of cochlear implantation in SSD patients.

Conductive Hearing Loss Has Long-Lasting Structural and Molecular Effects on Presynaptic and Postsynaptic Structures of Auditory Nerve Synapses in the Cochlear Nucleus

Sound deprivation by conductive hearing loss increases hearing thresholds, but little is known about the response of the auditory brainstem during and after conductive hearing loss. Here, we show in young adult rats that 10 d of monaural conductive hearing loss (i.e., earplugging) leads to hearing deficits that persist after sound levels are restored. Hearing thresholds in response to clicks and frequencies higher than 8 kHz remain increased after a 10 d recovery period. Neural output from the cochlear nucleus measured at 10 dB above threshold is reduced and followed by an overcompensation at the level of the lateral lemniscus. We assessed whether structural and molecular substrates at auditory nerve (endbulb of Held) synapses in the cochlear nucleus could explain these long-lasting changes in hearing processing. During earplugging, vGluT1 expression in the presynaptic terminal decreased and synaptic vesicles were smaller. Together, there was an increase in postsynaptic density (PSD) thickness and an upregulation of GluA3 AMPA receptor subunits on bushy cells. After earplug removal and a 10 d recovery period, the density of synaptic vesicles increased, vesicles were also larger, and the PSD of endbulb synapses was larger and thicker. The upregulation of the GluA3 AMPAR subunit observed during earplugging was maintained after the recovery period. This suggests that GluA3 plays a role in plasticity in the cochlear nucleus. Our study demonstrates that sound deprivation has long-lasting alterations on structural and molecular presynaptic and postsynaptic components at the level of the first auditory nerve synapse in the auditory brainstem.

SIGNIFICANCE STATEMENT

Despite being the second most prevalent form of hearing loss, conductive hearing loss and its effects on central synapses have received relatively little attention. Here, we show that 10 d of monaural conductive hearing loss leads to an increase in hearing thresholds, to an increased central gain upstream of the cochlear nucleus at the level of the lateral lemniscus, and to long-lasting presynaptic and postsynaptic structural and molecular effects at the endbulb of the Held synapse. Knowledge of the structural and molecular changes associated with decreased sensory experience, along with their potential reversibility, is important for the treatment of hearing deficits, such as hyperacusis and chronic otitis media with effusion, which is prevalent in young children with language acquisition or educational disabilities.

Amblyaudia: Review of Pathophysiology, Clinical Presentation, and Treatment of a New Diagnosis

OBJECTIVE

Similar to amblyopia in the visual system, "amblyaudia" is a term used to describe persistent hearing difficulty experienced by individuals with a history of asymmetric hearing loss (AHL) during a critical window of brain development. Few clinical reports have described this phenomenon and its consequent effects on central auditory processing. We aim to (1) define the concept of amblyaudia and (2) review contemporary research on its pathophysiology and emerging clinical relevance.

DATA SOURCES

PubMed, Embase, and Cochrane databases.

REVIEW METHODS

A systematic literature search was performed with combinations of search terms: "amblyaudia," "conductive hearing loss," "sensorineural hearing loss," "asymmetric," "pediatric," "auditory deprivation," and "auditory development." Relevant articles were considered for inclusion, including basic and clinical studies, case series, and major reviews.

CONCLUSIONS

During critical periods of infant brain development, imbalanced auditory input associated with AHL may lead to abnormalities in binaural processing. Patients with amblyaudia can demonstrate long-term deficits in auditory perception even with correction or resolution of AHL. The greatest impact is in sound localization and hearing in noisy environments, both of which rely on bilateral auditory cues. Diagnosis and quantification of amblyaudia remain controversial and poorly defined. Prevention of amblyaudia may be possible through early identification and timely management of reversible causes of AHL.

IMPLICATIONS FOR PRACTICE

Otolaryngologists, audiologists, and pediatricians should be aware of emerging data supporting amblyaudia as a diagnostic entity and be cognizant of the potential for lasting consequences of AHL. Prevention of long-term auditory deficits may be possible through rapid identification and correction.

Central plasticity and dysfunction elicited by aural deprivation in the critical period

The acoustic signal is crucial for animals to obtain information from the surrounding environment. Like other sensory modalities, the central auditory system undergoes adaptive changes (i.e., plasticity) during the developmental stage as well as other stages of life. Owing to its plasticity, auditory centers may be susceptible to various factors, such as medical intervention, variation in ambient acoustic signals and lesion of the peripheral hearing organ. There are critical periods during which auditory centers are vulnerable to abnormal experiences. Particularly in the early postnatal development period, aural inputs are essential for functional maturity of auditory centers. An aural deprivation model, which can be achieved by attenuating or blocking the peripheral acoustic afferent input to the auditory center, is ideal for investigating plastic changes of auditory centers. Generally, auditory plasticity includes structural and functional changes, some of which can be irreversible. Aural deprivation can distort tonotopic maps, disrupt the binaural integration, reorganize the neural network and change the synaptic transmission in the primary auditory cortex or at lower levels of the auditory system. The regulation of specific gene expression and the modified signal pathway may be the deep molecular mechanism of these plastic changes. By studying this model, researchers may explore the pathogenesis of hearing loss and reveal plastic changes of the auditory cortex, facilitating the therapeutic advancement in patients with severe hearing loss. After summarizing developmental features of auditory centers in auditory deprived animals and discussing changes of central auditory remodeling in hearing loss patients, we aim at stressing the significant of an early and well-designed auditory training program for the hearing rehabilitation.

Glia-related mechanisms in the anteroventral cochlear nucleus of the adult rat in response to unilateral conductive hearing loss

Conductive hearing loss causes a progressive decline in cochlear activity that may result in functional and structural modifications in auditory neurons. However, whether these activity-dependent changes are accompanied by a glial response involving microglia, astrocytes, or both has not yet been fully elucidated. Accordingly, the present study was designed to determine the involvement of glial related mechanisms in the anteroventral cochlear nucleus (AVCN) of adult rats at 1, 4, 7, and 15 d after removing middle ear ossicles. Quantitative immunohistochemistry analyses at light microscopy with specific markers of microglia or astroglia along with immunocytochemistry at the electron microscopy level were used. Also, in order to test whether trophic support by neurotrophins is modulated in glial cells by auditory activity, the expression and distribution of neurotrophin-3 (NT-3) and its colocalization with microglial or astroglial markers was investigated. Diminished cochlear activity after middle ear ossicle removal leads to a significant ipsilateral increase in the mean gray levels and stained area of microglial cells but not astrocytes in the AVCN at 1 and 4 d post-lesion as compared to the contralateral side and control animals. These results suggest that microglial cells but not astrocytes may act as dynamic modulators of synaptic transmission in the cochlear nucleus immediately following unilateral hearing loss. On the other hand, NT-3 immunostaining was localized mainly in neuronal cell bodies and axons and was upregulated at 1, 4 and 7 d post-lesion. Very few glial cells expressed this neurotrophin in both control and experimental rats, suggesting that NT-3 is primarily activated in neurons and not as much in glia after limiting auditory activity in the AVCN by conductive hearing loss.

Remodelling at the calyx of Held-MNTB synapse in mice developing with unilateral conductive hearing loss

Structure and function of central synapses are profoundly influenced by experience during developmental sensitive periods. Sensory synapses, which are the indispensable interface for the developing brain to interact with its environment, are particularly plastic. In the auditory system, moderate forms of unilateral hearing loss during development are prevalent but the pre- and postsynaptic modifications that occur when hearing symmetry is perturbed are not well understood. We investigated this issue by performing experiments at the large calyx of Held synapse. Principal neurons of the medial nucleus of the trapezoid body (MNTB) are innervated by calyx of Held terminals that originate from the axons of globular bushy cells located in the contralateral ventral cochlear nucleus. We compared populations of synapses in the same animal that were either sound deprived (SD) or sound experienced (SE) after unilateral conductive hearing loss (CHL). Middle ear ossicles were removed 1 week prior to hearing onset (approx. postnatal day (P) 12) and morphological and electrophysiological approaches were applied to auditory brainstem slices taken from these mice at P17-19. Calyces in the SD and SE MNTB acquired their mature digitated morphology but these were structurally more complex than those in normal hearing mice. This was accompanied by bilateral decreases in initial EPSC amplitude and synaptic conductance despite the CHL being unilateral. During high-frequency stimulation, some SD synapses displayed short-term depression whereas others displayed short-term facilitation followed by slow depression similar to the heterogeneities observed in normal hearing mice. However SE synapses predominantly displayed short-term facilitation followed by slow depression which could be explained in part by the decrease in release probability. Furthermore, the excitability of principal cells in the SD MNTB had increased significantly. Despite these unilateral changes in short-term plasticity and excitability, heterogeneities in the spiking fidelity among the population of both SD and SE synapses showed similar continuums to those in normal hearing mice. Our study suggests that preservations in the heterogeneity in spiking fidelity via synaptic remodelling ensures symmetric functional stability which is probably important for retaining the capability to maximally code sound localization cues despite moderate asymmetries in hearing experience.

Monaural conductive hearing loss alters the expression of the GluA3 AMPA and glycine receptor α1 subunits in bushy and fusiform cells of the cochlear nucleus

The impact of conductive hearing loss (CHL), the second most common form of hearing loss, on neuronal plasticity in the central auditory pathway is unknown. After short-term (1 day) monaural earplugging, the GluA3 subunits of the AMPA receptor (AMPAR) are upregulated at auditory nerve synapses on the projection neurons of the cochlear nucleus; glycine receptor α1 (GlyRα1) subunits are downregulated at inhibitory synapses in the same neuronal population. These data suggest that CHL affects receptor trafficking at synapses. We examined the impact of 7 days of CHL on the general expression of excitatory and inhibitory receptors by quantitative biochemistry and immunohistochemistry, using specific antibodies to detect AMPAR subunits (GluA1, GluA2, GluA2/3, and GluA4), GlyRα1, and the GABA(A) receptor subunits β2/3. Following monaural earplugging and an elevation of the hearing threshold by approximately 35 dB, the immunolabeling of the antibody for the GluA2/3 subunits but not the GluA2 subunit increased on bushy cells (BCs) and fusiform cells (FCs) of the ipsilateral ventral and dorsal cochlear nuclei. These same cell types showed a downregulation of the GlyRα1 subunit. Similar results were observed in the contralateral nuclei. The expression levels of GABA(A) β2/3 were unchanged. These findings suggest that, following longer periods of monaural conductive hearing loss, the synthesis and subsequent composition of specific glutamate and glycine receptors in projection neurons and their synapses are altered; these changes may contribute to abnormal auditory processing.

Evaluating the perceptual and pathophysiological consequences of auditory deprivation in early postnatal life: a comparison of basic and clinical studies

Decades of clinical and basic research in visual system development have shown that degraded or imbalanced visual inputs can induce a long-lasting visual impairment called amblyopia. In the auditory domain, it is well established that inducing a conductive hearing loss (CHL) in young laboratory animals is associated with a panoply of central auditory system irregularities, ranging from cellular morphology to behavior. Human auditory deprivation, in the form of otitis media (OM), is tremendously common in young children, yet the evidence linking a history of OM to long-lasting auditory processing impairments has been equivocal for decades. Here, we review the apparent discrepancies in the clinical and basic auditory literature and provide a meta-analysis to show that the evidence for human amblyaudia, the auditory analog of amblyopia, is considerably more compelling than is generally believed. We argue that a major cause for this discrepancy is the fact that most clinical studies attempt to link central auditory deficits to a history of middle ear pathology, when the primary risk factor for brain-based developmental impairments such as amblyopia and amblyaudia is whether the afferent sensory signal is degraded during critical periods of brain development. Accordingly, clinical studies that target the subset of children with a history of OM that is also accompanied by elevated hearing thresholds consistently identify perceptual and physiological deficits that can endure for years after peripheral hearing is audiometrically normal, in keeping with the animal studies on CHL. These studies suggest that infants with OM severe enough to cause degraded afferent signal transmission (e.g., CHL) are particularly at risk to develop lasting central auditory impairments. We propose some practical guidelines to identify at-risk infants and test for the positive expression of amblyaudia in older children.

Monaural deprivation disrupts development of binaural selectivity in auditory midbrain and cortex

Degraded sensory experience during critical periods of development can have adverse effects on brain function. In the auditory system, conductive hearing loss associated with childhood ear infections can produce long-lasting deficits in auditory perceptual acuity, much like amblyopia in the visual system. Here we explore the neural mechanisms that may underlie "amblyaudio" by inducing reversible monaural deprivation (MD) in infant, juvenile, and adult rats. MD distorted tonotopic maps, weakened the deprived ear's representation, strengthened the open ear's representation, and disrupted binaural integration of interaural level differences (ILD). Bidirectional plasticity effects were strictly governed by critical periods, were more strongly expressed in primary auditory cortex than inferior colliculus, and directly impacted neural coding accuracy. These findings highlight a remarkable degree of competitive plasticity between aural representations and suggest that the enduring perceptual sequelae of childhood hearing loss might be traced to maladaptive plasticity during critical periods of auditory cortex development.