Changes in cochlear function during acute endolymphatic hydrops development in guinea pigs

Meniere's disease: Pathogenesis, treatments, and emerging approaches for an idiopathic bioenvironmental disorder

Meniere's disease (MD) is a severe inner ear condition known by debilitating symptoms, including spontaneous vertigo, fluctuating and progressive hearing loss, tinnitus, and aural fullness or pressure within the affected ear. Prosper Meniere first described the origins of MD in the 1860s, but its underlying mechanisms remain largely elusive today. Nevertheless, researchers have identified a key histopathological feature called Endolymphatic Hydrops (ELH), which refers to the excessive buildup of endolymph fluid in the membranous labyrinth of the inner ear. The exact root of ELH is not fully understood. Still, it is believed to involve several biological and bioenvironmental etiological factors such as genetics, autoimmunity, infection, trauma, allergy, and new theories, such as saccular otoconia blocking the endolymphatic duct and sac. Regarding treatment, there are no reliable and definitive cures for MD. Most therapies focus on managing symptoms and improving the overall quality of patients' life. To make significant advancements in addressing MD, it is crucial to gain a fundamental understanding of the disease process, laying the groundwork for more effective therapeutic approaches. This paper provides a comprehensive review of the pathophysiology of MD with a focus on old and recent theories. Current treatment strategies and future translational approaches (with low-level evidence but promising results) related to MD are also discussed, including patents, drug delivery, and nanotechnology, that may provide future benefits to patients suffering from MD.

Using x-ray micro computed tomography to quantify intracochlear fibrosis after cochlear implantation in a Guinea pig model

Cochlear implants (CIs) allow individuals with profound hearing loss to understand speech and perceive sounds. However, not all patients obtain the full benefits that CIs can provide and the cause of this disparity is not fully understood. One possible factor for the variability in outcomes after cochlear implantation, is the development of fibrotic scar tissue around the implanted electrode. It has been hypothesised that limiting the extent of fibrosis after implantation may improve overall CI function, and longevity of the device. Currently, histology is often used to quantify the extent of intracochlear tissue growth after implantation however this method is labour intensive, time-consuming, often involves significant user bias, and causes physical distortion of the fibrosis. Therefore, this study aimed to evaluate x-ray micro computed tomography (μCT) as a method to measure the amount and distribution of fibrosis in a guinea pig model of cochlear implantation. Adult guinea pigs were implanted with an inactive electrode, and cochleae harvested eight weeks later (n = 7) and analysed using μCT, to quantify the extent of tissue reaction, followed by histological analysis to confirm that the tissue was indeed fibrotic. Cochleae harvested from an additional six animals following implantation were analysed by μCT, before and after contrast staining with osmium tetroxide (OsO4), to enhance the visualisation of soft tissues within the cochlea, including the tissue reaction. Independent analysis by two observers showed that the quantification method was robust and provided additional information on the distribution of the response within the cochlea. Histological analysis revealed that μCT visualised dense collagenous material and new bone formation but did not capture loose, areolar fibrotic tissue. Treatment with OsO4 significantly enhanced the visible tissue reaction detected using μCT. Overall, μCT is an alternative and reliable method that can be used to quantify the extent of the CI-induced intracochlear tissue response and will be a useful tool for the in vivo assessment of novel anti-fibrotic treatments.

Impact of endolymphatic hydrops on DPOAE in subjects with normal to mild hearing loss

Objective

The increased endolymph volume affects a shift in the organ of Corti and basilar membrane in ears with endolymphatic hydrops (EH), which might affect distortion-product otoacoustic emissions (DPOAE) by altering the operating point of the outer hair cells. We investigated how changes in DPOAE are related to the distribution site of EH.

Study Design

Prospective study.

Methods

Among 403 patients with hearing or vestibular symptoms who underwent contrast-enhanced magnetic resonance imaging (MRI) for the diagnosis of EH and subsequent DPOAE testing, subjects whose hearing levels on pure tone audiometry were ≤35 dB at all frequencies were included in this study. In patients with EH on MRI, the presence and amplitude of DPOAE were evaluated between groups with hearing levels of ≤25 dB at all frequencies versus hearing levels of >25 dB at one or more frequencies.

Results

There were no differences in the distribution of EH between groups. The amplitude of DPOAE had no clear correlation with the presence of EH. However, in both groups, there was a significantly higher probability of the presence of a DPOAE response from 1001 to 6006 Hz in cases with EH in the cochlea.

Conclusion

Among patients whose hearing levels were ≤35 dB at all frequencies, better responses on DPOAE testing were found in subjects with EH in the cochlea. Alteration of DPOAEs in the early stages of hearing impairment could indicate morphological changes in the inner ear with altered basilar membrane compliance due to EH.

Level of Evidence

4.

Low Dose Betahistine in Combination With Selegiline Increases Cochlear Blood Flow in Guinea Pigs

OBJECTIVE

Betahistine is frequently used in the pharmacotherapy for Menière's Disease (MD). Little is known about its mode of action and prescribed dosages vary. While betahistine had an increasing effect on cochlear microcirculation in earlier studies, low dose betahistine of 0.01 mg/kg bw or less was not able to effect this. Selegiline inhibits monoaminooxidase B and therefore potentially the breakdown of betahistine. The goal of this study was to examine whether the addition of selegiline to low dose betahistine leads to increased cochlear blood flow.

METHODS

Twelve Dunkin-Hartley guinea pigs were anesthetized, the cochlea was exposed and a window opened to the stria vascularis. Blood plasma was visualized by injecting fluoresceinisothiocyanate-dextrane and vessel diameter and erythrocyte velocity were evaluated over 20 minutes. One group received low dose betahistine (0.01 mg/kg bw) and selegiline (1 mg/kg bw) i.v. while the other group received only selegiline (1 mg/kg bw) and saline (0.9% NaCl) as placebo i.v.

RESULTS

Cochlear microcirculation increased significantly (P < .001) in guinea pigs treated with low dose betahistine combined with selegiline by up to 58.3 ± 38.7% above baseline over a period of up to 11 minutes. In one guinea pig, the increase was 104.6%. Treatment with Selegiline alone did not affect microcirculation significantly.

CONCLUSIONS

Low dose betahistine increased cochlear microcirculation significantly when combined with selegiline. This should be investigated in further studies regarding dose-effect relation in comparison to betahistine alone. Side effects, in particular regarding circulation, should be considered carefully in view of the clinical applicability of a combination therapy in patients with MD.

Green porous benzamide-like nanomembranes for hazardous cations detection, separation, and concentration adjustment

Green biomaterials play a crucial role in the diagnosis and treatment of diseases as well as health-related problem-solving. Typically, biocompatibility, biodegradability, and mechanical strength are requirements centered on biomaterial engineering. However, in-hospital therapeutics require an elaborated synthesis of hybrid and complex nanomaterials capable of mimicking cellular behavior. Accumulation of hazardous cations like K⁺ in the inner and middle ear may permanently damage the ear system. We synthesized nanoplatforms based on Allium noeanum to take the first steps in developing biological porous nanomembranes for hazardous cation detection in biological media. The 1,1,1-tris[[(2'-benzyl-amino-formyl)phenoxy]methyl]ethane (A), 4-amino-benzo-hydrazide (B), and 4-(2-(4-(3-carboxy-propan-amido)benzoyl)hydrazineyl)-4-oxobutanoic acid (B1) were synthesized to obtain green ligands based on 4-X-N-(…(Y(hydrazine-1-carbonyl)phenyl)benzamide, with X denoting fluoro (B2), methoxy (B3), nitro (B4), and phenyl-sulfonyl (B5) substitutes. The chemical structure of ligand-decorated adenosine triphosphate (ATP) molecules (S-ATP) was characterized by FTIR, XRD, AFM, FESEM, and TEM techniques. The cytotoxicity of the porous membrane was patterned by applying different cell lines, including HEK-293, PC12, MCF-7, HeLa, HepG2, and HT-29, to disclose their biological behavior. The morphology of cultured cells was monitored by confocal laser scanning microscopy. The sensitivity of S-ATP to different cations of Na⁺, Mg²⁺, K⁺, Ba²⁺, Zn²⁺, and Cd²⁺ was evaluated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) in terms of extraction efficiency (η). For pH of 5.5, the η of A-based S-ATP followed the order Na⁺ (63.3%) > Mg²⁺ (62.1%) > Ba²⁺ (7.6%) > Ca²⁺ (5.5%); while for pH of 7.4, Na⁺ (37.0%) > Ca²⁺ (33.1%) > K⁺ (25^.7%). The heat map of MTT and dose-dependent evaluations unveiled acceptable cell viability of more than 90%. The proposed green porous nanomembranes would pave the way to use multifunctional green porous nanomembranes in biological membranes.

Similarities and Differences Between Vestibular and Cochlear Systems - A Review of Clinical and Physiological Evidence

The evoked response to repeated brief stimuli, such as clicks or short tone bursts, is used for clinical evaluation of the function of both the auditory and vestibular systems. One auditory response is a neural potential - the Auditory Brainstem Response (ABR) - recorded by surface electrodes on the head. The clinical analogue for testing the otolithic response to abrupt sounds and vibration is the myogenic potential recorded from tensed muscles - the vestibular evoked myogenic potential (VEMP). VEMPs have provided clinicians with a long sought-after tool - a simple, clinically realistic indicator of the function of each of the 4 otolithic sensory regions. We review the basic neural evidence for VEMPs and discuss the similarities and differences between otolithic and cochlear receptors and afferents. VEMPs are probably initiated by sound or vibration selectively activating afferent neurons with irregular resting discharge originating from the unique type I receptors at a specialized region of the otolithic maculae (the striola). We review how changes in VEMP responses indicate the functional state of peripheral vestibular function and the likely transduction mechanisms allowing otolithic receptors and afferents to trigger such very short latency responses. In section "ELECTROPHYSIOLOGY" we show how cochlear and vestibular receptors and afferents have many similar electrophysiological characteristics [e.g., both generate microphonics, summating potentials, and compound action potentials (the vestibular evoked potential, VsEP)]. Recent electrophysiological evidence shows that the hydrodynamic changes in the labyrinth caused by increased fluid volume (endolymphatic hydrops), change the responses of utricular receptors and afferents in a way which mimics the changes in vestibular function attributed to endolymphatic hydrops in human patients. In section "MECHANICS OF OTOLITHS IN VEMPS TESTING" we show how the major VEMP results (latency and frequency response) follow from modeling the physical characteristics of the macula (dimensions, stiffness etc.). In particular, the structure and mechanical operation of the utricular macula explains the very fast response of the type I receptors and irregular afferents which is the very basis of VEMPs and these structural changes of the macula in Menière's Disease (MD) predict the upward shift of VEMP tuning in these patients.

A Synchrotron and Micro-CT Study of the Human Endolymphatic Duct System: Is Meniere's Disease Caused by an Acute Endolymph Backflow?

Background: The etiology of Meniere's disease (MD) and endolymphatic hydrops believed to underlie its symptoms remain unknown. One reason may be the exceptional complexity of the human inner ear, its vulnerability, and surrounding hard bone. The vestibular organ contains an endolymphatic duct system (EDS) bridging the different fluid reservoirs. It may be essential for monitoring hydraulic equilibrium, and a dysregulation may result in distension of the fluid spaces or endolymphatic hydrops. Material and Methods: We studied the EDS using high-resolution synchrotron phase contrast non-invasive imaging (SR-PCI), and micro-computed tomography (micro-CT). Ten fresh human temporal bones underwent SR-PCI. One bone underwent micro-CT after fixation and staining with Lugol's iodine solution (I2KI) to increase tissue resolution. Data were processed using volume-rendering software to create 3D reconstructions allowing orthogonal sectioning, cropping, and tissue segmentation. Results: Combined imaging techniques with segmentation and tissue modeling demonstrated the 3D anatomy of the human saccule, utricle, endolymphatic duct, and sac together with connecting pathways. The utricular duct (UD) and utriculo-endolymphatic valve (UEV or Bast's valve) were demonstrated three-dimensionally for the first time. The reunion duct was displayed with micro-CT. It may serve as a safety valve to maintain cochlear endolymph homeostasis under certain conditions. Discussion: The thin reunion duct seems to play a minor role in the exchange of endolymph between the cochlea and vestibule under normal conditions. The saccule wall appears highly flexible, which may explain occult hydrops occasionally preceding symptoms in MD on magnetic resonance imaging (MRI). The design of the UEV and connecting ducts suggests that there is a reciprocal exchange of fluid among the utricle, semicircular canals, and the EDS. Based on the anatomic framework and previous experimental data, we speculate that precipitous vestibular symptoms in MD arise from a sudden increase in endolymph pressure caused by an uncontrolled endolymphatic sac secretion. A rapid rise in UD pressure, mediated along the fairly wide UEV, may underlie the acute vertigo attack, refuting the rupture/K+-intoxication theory.

Hidden hearing loss is associated with loss of ribbon synapses of cochlea inner hair cells

The present study aimed to observe the changes in the cochlea ribbon synapses after repeated exposure to moderate-to-high intensity noise. Guinea pigs received 95 dB SPL white noise exposure 4 h a day for consecutive 7 days (we regarded it a medium-term and moderate-intensity noise, or MTMI noise). Animals were divided into four groups: Control, 1DPN (1-day post noise), 1WPN (1-week post noise), and 1MPN (1-month post noise). Auditory function analysis by auditory brainstem response (ABR) and compound action potential (CAP) recordings, as well as ribbon synapse morphological analyses by immunohistochemistry (Ctbp2 and PSD95 staining) were performed 1 day, 1 week, and 1 month after noise exposure. After MTMI noise exposure, the amplitudes of ABR I and III waves were suppressed. The CAP threshold was elevated, and CAP amplitude was reduced in the 1DPN group. No apparent changes in hair cell shape, arrangement, or number were observed, but the number of ribbon synapse was reduced. The 1WPN and 1MPN groups showed that part of ABR and CAP changes recovered, as well as the synapse number. The defects in cochlea auditory function and synapse changes were observed mainly in the high-frequency region. Together, repeated exposure in MTMI noise can cause hidden hearing loss (HHL), which is partially reversible after leaving the noise environment; and MTMI noise-induced HHL is associated with inner hair cell ribbon synapses.

[Various types of vertigo and balance disorders in patients with Meniere's disease]

Vertigo and balance disorders in Meniere's disease (MD) may have various etiology. The aim of the review is discussing pathogenetic mechanisms of the typical vertiginous paroxysms in MD, resulting from endolymphatic hydrops as well as analysis of etiology, pathogenesis, clinical course and basic treatment of paroxysmal and permanent forms of vertigo and balance disorders, caused by other conditions, associated with MD. We discussed the course of MD complicated by vestibular migraine, benign positional paroxysmal vertigo, functional dizziness, bilateral vestibulopathy and vestibular drop-attacks.

Utricular Sensitivity during Hydrodynamic Displacements of the Macula

To explore the effects of cochlear hair cell displacement, researchers have previously monitored functional and mechanical responses during low-frequency (LF) acoustic stimulation of the cochlea. The induced changes are believed to result from modulation of the conductance of mechano-electrical transduction (MET) channels on cochlear hair cells, along with receptor potential modulation. It is less clear how, or if, vestibular hair cell displacement affects vestibular function. Here, we have used LF (<20 Hz) hydrodynamic modulation of the utricular macula position, whilst recording functional and mechanical responses, to investigate the effects of utricular macula displacement. Measured responses included the Utricular Microphonic (UM), the vestibular short-latency evoked potential (VsEP), and laser Doppler vibrometry recordings of macular position. Over 1 cycle of the LF bias, the UM amplitude and waveform were cyclically modulated, with Boltzmann analysis suggesting a cyclic modulation of the vestibular MET gating. The VsEP amplitude was cyclically modulated throughout the LF bias, demonstrating a relative increase (~20-50 %; re baseline) and decrease (~10-20 %; re baseline), which is believed to be related to the MET conductance and vestibular hair cell sensitivity. The relationship between macular displacement and changes in UM and VsEP responses was consistent within and across animals. These results suggest that the sensory structures underlying the VsEP, often thought to be a cranial jerk-sensitive response, are at least partially sensitive to LF (and possibly static) pressures or motion. Furthermore, these results highlight the possibility that some of the vestibular dysfunction related to endolymphatic hydrops may be due to altered vestibular transduction following mechanical (or morphological) changes in the labyrinth.

Experimental Animal Models for Meniere's Disease: A Mini-Review

Several novel animal models that represent the pathophysiological process of endolymphatic hydrops (ELH) of Meniere's disease (MD) have been developed. Animal models are important to identify and characterize the pathophysiology of ELH and to corroborate molecular and genetic findings in humans. This review of the current animal models will be useful in understanding the pathophysiology of and developing proper treatments for MD. Surgical animal models will be replaced by medication-induced animal models. Study models previously developed in guinea pigs will be developed in several smaller animals for ease of conducting molecular analysis. In this review, we provided updated resources including our previous studies regarding the current and desirable animal models for MD.

Cochlear compound action potentials from high-level tone bursts originate from wide cochlear regions that are offset toward the most sensitive cochlear region

Little is known about the spatial origins of auditory nerve (AN) compound action potentials (CAPs) evoked by moderate to intense sounds. We studied the spatial origins of AN CAPs evoked by 2- to 16-kHz tone bursts at several sound levels by slowly injecting kainic acid solution into the cochlear apex of anesthetized guinea pigs. As the solution flowed from apex to base, it sequentially reduced CAP responses from low- to high-frequency cochlear regions. The times at which CAPs were reduced, combined with the cochlear location traversed by the solution at that time, showed the cochlear origin of the removed CAP component. For low-level tone bursts, the CAP origin along the cochlea was centered at the characteristic frequency (CF). As sound level increased, the CAP center shifted basally for low-frequency tone bursts but apically for high-frequency tone bursts. The apical shift was surprising because it is opposite the shift expected from AN tuning curve and basilar membrane motion asymmetries. For almost all high-level tone bursts, CAP spatial origins extended over 2 octaves along the cochlea. Surprisingly, CAPs evoked by high-level low-frequency (including 2 kHz) tone bursts showed little CAP contribution from CF regions ≤ 2 kHz. Our results can be mostly explained by spectral splatter from the tone-burst rise times, excitation in AN tuning-curve "tails," and asynchronous AN responses to high-level energy ≤ 2 kHz. This is the first time CAP origins have been identified by a spatially specific technique. Our results show the need for revising the interpretation of the cochlear origins of high-level CAPs-ABR wave 1. NEW & NOTEWORTHY Cochlear compound action potentials (CAPs) and auditory brain stem responses (ABRs) are routinely used in laboratories and clinics. They are typically interpreted as arising from the cochlear region tuned to the stimulus frequency. However, as sound level is increased, the cochlear origins of CAPs from tone bursts of all frequencies become very wide and their centers shift toward the most sensitive cochlear region. The standard interpretation of CAPs and ABRs from moderate to intense stimuli needs revision.

Saccular otoconia as a cause of Ménière's disease: hypothesis based on two theories

BACKGROUND

The cause of Ménière's disease remains enigmatic after 156 years. Schuknecht's rupture and potassium intoxication theory of attacks was based on histological studies.

OBJECTIVES

This paper aimed to: present the most contemporary evidence indicating that ruptures do not usually occur, and discuss the possibility that detached saccular otoconia are the main cause of Ménière's disease; and to establish an unequivocal definition of the age of Ménière's disease onset.

METHOD

The paper reviews the electrophysiological basis of the Gibson-Arenberg drainage theory used to explain vertigo attacks. The current, limited knowledge of the likely fate of detached saccular otoconia is discussed.

RESULTS

Electrophysiological studies during attacks do not support endolymph ruptures, but rather endolymph flowing in one direction and then in the opposite direction. Age of onset for Ménière's disease parallels that for benign paroxysmal positional vertigo.

CONCLUSION

The similarity of age of onset spectrum for Ménière's disease and benign paroxysmal positional vertigo raises the possibility that the two conditions have the same fundamental cause.

Putative Auditory-Evoked Neurophonic Measurements Using a Novel Signal Processing Technique: A Pilot Case Study

With changes to cochlear implant candidacy and improvements in surgical technique, there is a need for accurate intraoperative assessment of low-frequency hearing thresholds during cochlear implantation. In electrocochleography, onset compound action potentials (CAPs) typically allow estimation of auditory threshold for frequencies above 1 kHz, but they are less accurate at lower frequencies. Auditory nerve neurophonic (ANN) waveforms, on the other hand, may overcome this limitation by allowing phase-locked neural activity to be tracked during a prolonged low-frequency stimulus rather than just at its onset (Henry, 1995). Lichtenhan et al. (2013) have used their auditory nerve overlapped waveform (ANOW) technique to measure these potentials from the round windows of cats and guinea pigs, and reported that in guinea pigs these potentials originate in the cochlear apex for stimuli below 70 dB SPL (Lichtenhan et al., 2014). Human intraoperative round window neurophonic measurements have been reported by Choudhury et al. (2012). We have done the same in hearing impaired awake participants, and present here the results of a pilot study in which we recorded responses evoked by 360, 525, and 725 Hz tone bursts from the cochlear promontory of one participant. We also present a modification to the existing measurement technique which halves recording time, extracting the auditory neurophonic by recording a single averaged waveform, and then subtracting from it a 180° group-delayed version of itself, rather than using alternating condensation and rarefaction sound stimuli. We cannot conclude that the waveforms we measured were purely neural responses originating from the apex of the cochlea: as with all neurophonic measurement procedures, the neural responses of interest cannot be separated from higher harmonics of the cochlear microphonic without forward masking, regardless of electrode location, stimuli or post-processing algorithm. In conclusion, the extraction of putative neurophonic waveforms can easily be incorporated into existing electrocochleographic measurement paradigms, but at this stage such measurements should be interpreted with caution.

The Auditory Nerve Overlapped Waveform (ANOW) Detects Small Endolymphatic Manipulations That May Go Undetected by Conventional Measurements

Electrocochleography (ECochG) has been used to assess Ménière's disease, a pathology associated with endolymphatic hydrops and low-frequency sensorineural hearing loss. However, the current ECochG techniques are limited for use at high-frequencies only (≥1 kHz) and cannot be used to assess and understand the low-frequency sensorineural hearing loss in ears with Ménière's disease. In the current study, we use a relatively new ECochG technique to make measurements that originate from afferent auditory nerve fibers in the apical half of the cochlear spiral to assess effects of endolymphatic hydrops in guinea pig ears. These measurements are made from the Auditory Nerve Overlapped Waveform (ANOW). Hydrops was induced with artificial endolymph injections, iontophoretically applied Ca²⁺ to endolymph, and exposure to 200 Hz tones. The manipulations used in this study were far smaller than those used in previous investigations on hydrops. In response to all hydropic manipulations, ANOW amplitude to moderate level stimuli was markedly reduced but conventional ECochG measurements of compound action potential thresholds were unaffected (i.e., a less than 2 dB threshold shift). Given the origin of the ANOW, changes in ANOW amplitude likely reflect acute volume disturbances accumulate in the distensible cochlear apex. These results suggest that the ANOW could be used to advance our ability to identify initial stages of dysfunction in ears with Ménière's disease before the pathology progresses to an extent that can be detected with conventional measures.

Effect of Endolymphatic Hydrops on Sound Transmission in Live Guinea Pigs Measured with a Laser Doppler Vibrometer

Objective. This study aimed at describing the mechanism of hearing loss in low frequency and the different dynamic behavior of the umbo, the stapes head, and the round window membrane (RWM) between normal guinea pigs and those with endolymphatic hydrops (EH), using a laser Doppler vibrometer (LDV). Methods. Cochlear sections were stained with hematoxylin and eosin (HE) to evaluate the hydropic ratio (HR). Auditory brainstem responses (ABR) and whole-mount immunostaining were measured. Displacement of the umbo, stapes head, and RWM in response to ear-canal sound was evaluated using a LDV. Results. Mean HR values in EH model of all the turns are larger than the control group. The ABR threshold of the EH group was significantly higher than that of the control. Strong positive correlation was found between HR at apical turn and ABR threshold elevation at 1000 Hz and at subapical turn and ABR threshold elevation at 2000 Hz. FITC-phalloidin immunostaining of the cochlear basilar membrane in the apical, subapical, and suprabasal turns showed missing and derangement stereocilia of third-row outer hair cells. The umbo, stapes head, and RWM displacement in ears with EH was generally lower than that of normal ears. The EH-induced differences in stapes head and RWM motion were significant at 0.5 kHz. Conclusion. The LDV results suggested that the higher inner ear impedance in EH affected the dynamic behavior of the two opening windows of the cochlea and then reduced the vibration of the ossicular chain by increasing the afterload, resulting in acoustic dysfunction. The vibration reduction mainly occurred at low frequencies, which has related with the morphology changes of the apical and subapical turns in EH model.

Endolymph movement visualized with light sheet fluorescence microscopy in an acute hydrops model

There are a variety of techniques available to investigate endolymph dynamics, primarily seeking to understand the cause of endolymphatic hydrops. Here we have taken the novel approach of injecting, via a glass micropipette, fluorescein isothiocyanate-dextran (FITC-dex) and artificial endolymph into scala media of anaesthetized guinea pigs, with subsequent imaging of the inner ear using Light Sheet Fluorescence Microscopy (LSFM) as a means to obtain highly resolved 3D visualization of fluid movements. Our results demonstrate endolymph movement into the utricle, semicircular canals and endolymphatic duct and sac when more than 2.5 μl of fluid had been injected into scala media, with no apparent movement of fluid into the perilymphatic compartments. There was no movement of endolymph into these compartments when less than 2.5 μl was injected. The remarkable uptake of the FITC-dex into the endolymphatic duct, including an absorption into the periductal channels surrounding the endolymphatic duct, highlights the functional role this structure plays in endolymph volume regulation.

Abnormal fast fluctuations of electrocochleography and otoacoustic emissions in Menière's disease

The responses of cochlear hair cells to sound stimuli depend on the resting position of their stereocilia bundles, which is sensitive to the chemical and mechanical environment. Cochlear hydrops, a hallmark of Menière's disease (MD), which is likely to come with disruption of this environment, results in hearing symptoms and electrophysiological signs, such as excessive changes in the cochlear summating potential (SP) and in the postural shifts of distortion-product otoacoustic emissions (DPOAEs). Here, SP from the basal part of the cochlea and DPOAEs from the apical part of the cochlea were recorded concomitantly in 73 patients with a definite MD, near an attack (n = 40) or between attacks with no clinical symptoms (n = 33), to compare their sensitivities to posture and evaluate their stability. The phase of the 2f1-f2 DPOAEs was monitored during body tilt, with stimuli f1 = 1 kHz and f2 = 1.2 kHz at 72 dB SPL. Extratympanic electrocochleography was performed in response to 95-dBnHL clicks. The normal limits of the DPOAE phase shift with body tilt, [-18°, +38°], and of the SP to action-potential (AP) ratio, <0.40, were exceeded in 75% and 60% of patients, respectively, near an attack. In these patients, but not in the asymptomatic ones, both tests reveal fluctuating cochlear responses from one data sample to the next. They emphasize how hydrops hinders normal hair-cell operation and may generate fast fluctuations in inner-ear functioning. If these fluctuations also occur on shorter time scales, it might explain the imperfect diagnostic sensitivity of SP and DPOAE tests, as averaging procedures would tend to level out transient fluctuations characteristic of hydrops.

Changes in utricular function during artificial endolymph injections in guinea pigs

Various theories suggest endolymphatic hydrops may cause a rupture of the membranous labyrinth or may force open the utriculo-saccular duct, resulting in a sudden change in inner ear function. Here, we have used slow injections of artificial endolymph into either scala media or the utricle of anaesthetised guinea pigs to investigate the effects of hydrops. Vestibular function was continuously monitored in addition to the measurements of cochlear function developed in our laboratory (Brown et al. Hear Res, 2013). Scala media injection induced consistent functional changes, which occurred in two stages. Initial changes involved were associated with an increased hydrostatic pressure in scala media that only affected cochlear function. After 3-4 μl of endolymph had been injected, cochlear function spontaneously recovered, and was often shortly followed by a transient increase or decrease in utricular sensitivity, with the effects varying between animals. Endolymph injection directly into the utricle produced variable effects across animals, although in 2 experiments it produced similar changes as those observed for scala media injections, suggesting that the fluid pathway between scala media and the utricle was continuous in these animals. The mechanism underlying the sudden, spontaneous functional changes is not yet clear, but we tentatively suggest that in some cases it may be caused by the utriculo-saccular duct suddenly opening to alleviate an elevated hydrostatic pressure in the pars inferior, resulting in a change in utricular function due to an increase in its volume. These changes are comparable to the sudden or fluctuating functional changes in Ménière's sufferers, and support the hypothesis that endolymphatic hydrops can directly cause some symptoms of this syndrome.