Sensitivity of the cochlear nerve to acoustic and electrical stimulation months after a vestibular labyrinthectomy in guinea pigs

Efficient delivery of adeno-associated virus (AAV) into inner ear in vivo via trans-stapes route in adult guinea pig

Adeno-associated virus (AAV) are potent vectors to achieve treatment against hearing loss resulting from genetic defects. However, the effects of delivery routes and the corresponding transduction efficiencies for clinical applications remain elusive. Here, we screened AAV vectors of three serotypes (AAV 8, 9 and Anc80L65) into the inner ears of adult normal guinea pigs through trans-stapes (oval window) and trans-round window delivery routes in vivo. Trans-stapes route is akin to stape surgeries in humans. Then, auditory brainstem response (ABR) measurements were conducted to evaluate postoperative hearing, and inner ear tissues were harvested for transduction efficiency analysis. Results showed that AAV8 targeted partial inner hair cells (IHCs) in cochlear basal turn; AAV9 targeted IHCs in cochlear basal and second turn, also a part of vestibular hair cells (VHCs). In contrast, Anc80L65 contributed to GFP signals of 80%-95% IHCs and 67%-91% outer hair cells (OHCs), as well as 69% VHCs via the trans-round window route, with 15-20 dB ABR thresholds shifts. And, via trans-stapes (oval window) route, there were 71%-90% IHCs and 42%-81% OHCs, along with 64% VHCs demonstrating GFP positive, and the ABR thresholds shifts were within 10 dB. This study revealed AAV could be efficiently delivered into mammalian inner ear cells in vivo via trans-stapes (oval window) route with postoperative hearing preservation, and both delivery routes showed promise of virus-based clinical translation of hearing impairment treatment.

Hearing loss versus vestibular loss as contributors to cognitive dysfunction

In the last 5 years, there has been a surge in evidence that hearing loss (HL) may be a risk factor for cognitive dysfunction, including dementia. At the same time, there has been an increase in the number of studies implicating vestibular loss in cognitive dysfunction. Due to the fact that vestibular disorders often present with HL and other auditory disorders such as tinnitus, it has been suggested that, in many cases, what appears to be vestibular-related cognitive dysfunction may be due to HL (e.g., Dobbels et al. Front Neurol 11:710, 2020). This review analyses the studies of vestibular-related cognitive dysfunction which have controlled HL. It is suggested that despite the fact that many studies in the area have not controlled HL, many other studies have (~ 19/44 studies or 43%). Therefore, although there is certainly a need for further studies controlling HL, there is evidence to suggest that vestibular loss is associated with cognitive dysfunction, especially related to spatial memory. This is consistent with the overwhelming evidence from animal studies that the vestibular system transmits specific types of information about self-motion to structures such as the hippocampus.

Cochlear implant in immune mediated inner ear diseases: Impedance variations and clinical outcomes

OBJECTIVE

Immune-mediated inner ear disease (IMIED) might cause severe/profound hearing loss and these patients are considered ideal candidates to cochlear implant (CI) surgery. The aim of the study was to evaluate impedance changes over time.

METHOD

The Study Group (SG) was composed of CI IMIED patients (31 ears) and a Control Group (CG) of CI patients with hearing loss not related to their immune system (31 ears). Audiological performance and impedance values were measured and compared amongst groups at 3, 6, 12 and 18 months following the fitting sessions.

RESULTS

Speech perception was significantly better for SG in word and sentence recognition in quiet. Impedance values were, on average, significantly higher for apical and middle electrode segments in SG compared to CG at the 3- month follow-up and were maintained over time. Additionally, a subset of SG patients (active patients) experienced significantly greater impedance fluctuation corresponding to clinical symptom reactivation.

CONCLUSION

IMIED patients achieve good audiological performance. However, the relapsing inflammation could change the inner ear environment, causing impedance fluctuations and, consequently, more frequent CI fittings. Additionally, impedance evaluation could be utilized as an early warning sign of IMIED recurrence and as an aid to therapeutic decision-making.

Surgical Labyrinthectomy and Cochlear Implantation in Menière's Disease

OBJECTIVE

The aim of this study was to analyze the results of labyrinthectomy and cochlear implantation (CI) on hearing, vertigo, and tinnitus and evaluate the adequacy of labyrinthectomy and CI for the treatment of end stage Menière's Disease (MD).

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center.

PATIENTS

Charts of 22 patients undergoing labyrinthectomy and CI in the same ear for intractable vertigo and hearing loss with both preoperatory and postoperatory documentation available, were reviewed.

INTERVENTION(S)

Therapeutic.

MAIN OUTCOME MEASURE(S)

Auditory outcomes were assessed with pure tone and speech audiometry, and compared with the preoperatory audiometric evaluation. Dizziness was graded according to the Dizziness Handicap Inventory Questionnaire (DHI). Tinnitus outcomes were assessed by the tinnitus handicap inventory (THI).

RESULTS

Post-CI pure tone average had a statistically significant improvement (p = 0.035, paired t test). Speech audiometry resulted in a non-statistically significant speech discrimination score variation (p = 0.056, paired t test). Postoperatory THI had a statistically significant variation (p = 0.0001, paired t test). Sixty seven per cent of the patients had complete resolution of the vestibular symptoms in their operated ear, however, patients over 70 years old had significantly more failures as evinced by the postoperative DHI (p = 0.0109, Fisher's exact test).

CONCLUSIONS

Patients affected by end stage MD or secondary MD, with vertigo and severe hearing loss can successfully undergo labyrinthectomy and CI. Caution should be reserved in elderly patients for a risk of persistent instability. The CI confers significant benefit in hearing rehabilitation and tinnitus suppression.

Late electrically-evoked compound action potentials as markers for acute micro-lesions of spiral ganglion neurons

Cochlear implants (CIs) are the treatment of choice for profoundly hearing impaired people. It has been proposed that speech perception in CI users is influenced by the neural health (deafferentation, demyelination and degeneration) of the cochlea, which may be heterogeneous along an individual cochlea. Several options have been put forward to account for these local differences in neural health when fitting the speech processor settings, however with mixed results. The interpretation of the results is hampered by the fact that reliable markers of locally restricted changes in spiral ganglion neuron (SGN) health are lacking. The aim of the study was (i) to establish mechanical micro-lesions in the guinea pig as a model of heterogeneous SGN deafferentation and degeneration and (ii) to assess potential electrophysiological markers that can also be used in human subjects. First, we defined the extent of micro-lesions in normal hearing animals using acoustically-evoked compound action potentials (aCAPs); second, we measured electrically-evoked CAPs (eCAPs) before and after focal lesioning in neomycin-deafened and implanted animals. Therefore, we inserted guinea pig adjusted 6-contact CIs through a cochleostomy in the scala tympani. The eCAP was recorded from a ball electrode at the round window niche in response to monopolar or bipolar, 50 µs/phase biphasic pulses of alternating anodic- and cathodic-leading polarity. To exclude the large electrical artifact from the analysis, we focused on the late eCAP component. We systematically isolated the eCAP parameter that showed local pre- versus post-lesion changes and lesion-target specificity. Histological evaluation of the cleared cochleae revealed focal damage of an average size of 0.0036 mm³ with an apical-basal span of maximal 440 µm. We found that the threshold of the late N2P2 eCAP component was significantly elevated after lesioning when stimulating at basal (near the lesion), but not apical (distant to the lesion) CI contacts. To circumvent the potentially conflicting influence of the apical-basal gradient in eCAP thresholds, we used the polarity effect (PE=cathodic-anodic) as a relative measure. During monopolar stimulation, but not bipolar stimulation, the PE was sensitive to the lesion target and showed significantly better cathodic than anodic thresholds after soma lesions. We conclude that the difference in N2P2 thresholds in response to cathodic versus anodic-leading monopolar stimulation corresponds to the presence of SGN soma damage, and may therefore be a marker for SGN loss. We consider this electrophysiological estimate of local neural health a potentially relevant tool for human applications because of the temporal separation from the stimulation artifact and possible implementation into common eCAP measurements.

Cochlear Implantation in Ménière's Disease With and Without Labyrinthectomy

OBJECTIVE

To investigate outcomes of cochlear implantation (CI) in patients with Ménière's disease (MD) with and without surgical labyrinthectomy.

STUDY DESIGN

Retrospective study.

SETTING

Multiple tertiary referral centers.

SUBJECTS

Thirty one ears from 27 patients (17 men, 10 women, aged 42-84) with CI in ipsilateral MD ear.

INTERVENTION

CI in ears with intact labyrinths (Group 1), CI with simultaneous surgical labyrinthectomy (Group 2), and CI sequential to surgical labyrinthectomy (Group 3).

MAIN OUTCOME MEASURE

Within-subject improvement on Bamford Kowal Bench test or City University of New York open set sentence tests.

RESULTS

Majority of ears achieved excellent open-set speech recognition by 12 months post-CI, irrespective of intervention group. Preoperative details including patient age and sex, implant, MD and previous intervention, and audiological test results did not significantly affect outcomes. Patients with MD undergoing CI only may experience vestibular dysfunction which may cause long-term concerns. Incidental finding was noted of eight ears with fluctuating symptoms in ipsilateral ear during 12-month period post-CI, with five of eight ears showing objective fluctuating impedances and mapping.

CONCLUSION

CI in MD can yield good hearing outcomes in all three groups and this is possible even after a long delay after labyrinthectomy. Bilateral MD patients are complex and prospective quality of life (QoL) measures would be beneficial in being better able to manage the vestibular outcomes as well as the audiological ones.

Electrophysiological Measurements of Peripheral Vestibular Function-A Review of Electrovestibulography

Electrocochleography (EcochG), incorporating the Cochlear Microphonic (CM), the Summating Potential (SP), and the cochlear Compound Action Potential (CAP), has been used to study cochlear function in humans and experimental animals since the 1930s, providing a simple objective tool to assess both hair cell (HC) and nerve sensitivity. The vestibular equivalent of ECochG, termed here Electrovestibulography (EVestG), incorporates responses of the vestibular HCs and nerve. Few research groups have utilized EVestG to study vestibular function. Arguably, this is because stimulating the cochlea in isolation with sound is a trivial matter, whereas stimulating the vestibular system in isolation requires significantly more technical effort. That is, the vestibular system is sensitive to both high-level sound and bone-conducted vibrations, but so is the cochlea, and gross electrical responses of the inner ear to such stimuli can be difficult to interpret. Fortunately, several simple techniques can be employed to isolate vestibular electrical responses. Here, we review the literature underpinning gross vestibular nerve and HC responses, and we discuss the nomenclature used in this field. We also discuss techniques for recording EVestG in experimental animals and humans and highlight how EVestG is furthering our understanding of the vestibular system.