Atrophy of middle and short stereocilia on outer hair cells of guinea pig cochleas with experimentally induced hydrops

Whole-exome sequencing suggests multiallelic inheritance for childhood-onset Ménière's disease

The genetic background of Ménière's disease (MD) was studied in one patient with childhood-onset MD and his grandfather affected with middle age-onset MD. Whole-exome sequencing was performed and the data were compared to 76 exomes from unrelated subjects without MD. Thirteen rare inner ear expressed variants with pathogenic estimations were observed in the case of childhood-onset MD. These variants were in genes involved in the formation of cell membranes or the cytoskeleton and in genes participating in cell death or gene-regulation pathways. His grandfather shared two of the variants: p.Y273N in HMX2 and p.L229F in TMEM55B. HMX2 p.Y273N was considered the more likely candidate for MD, as the gene is known to affect both hearing and vestibular function. The variant in the HMX2 gene may affect inner ear development and structural integrity and thus might predispose to the onset of MD. As there was a significant difference in onset between the patients, an accumulation of defects in several pathways is probably responsible for the exceptionally early onset of the disease, and the genetic etiology of childhood-onset MD is most likely multifactorial. This is the first molecular genetic study of childhood-onset MD.

Change in endolymphatic hydrops 2 years after endolymphatic sac surgery evaluated by MRI

OBJECTIVE

This study was performed to determine whether endolymphatic sac surgery improves vestibular and cochlear endolymphatic hydrops 2 years after sac surgery and to elucidate the relationship between the degree of improvement of endolymphatic hydrops and the changes in vertigo symptoms, the hearing level, and the summating potential/action potential ratio (-SP/AP ratio) by electrocochleography (ECochG) in patients with Ménière's disease (MD).

METHODS

Twenty-one patients with unilateral MD who underwent sac surgery were included in this study. All patients underwent gadolinium-enhanced magnetic resonance imaging (Gd-MRI) before and 2 years after sac surgery. We evaluated the difference in vestibular and cochlear endolymphatic hydrops between before and after surgery in both ears and compared these findings with the frequency of vertigo attacks, hearing level, and ECochG findings.

RESULTS

In affected ears, the presence of vestibular endolymphatic hydrops and the frequency of vertigo attacks significantly decreased after surgery. However, affected ears showed no significant improvement in the presence of cochlear endolymphatic hydrops or the -SP/AP ratio by ECochG; there was also no significant improvement or deterioration in the hearing level.

CONCLUSION

The present findings suggest that sac surgery reduces vestibular endolymphatic hydrops and prevents aggravation of cochlear endolymphatic hydrops, and these changes lead to a reduction of vertigo attacks and suppress the progression of hearing impairment associated with vertigo attacks.

Cannula-based drug delivery to the guinea pig round window causes a lasting hearing loss that may be temporarily mitigated by BDNF

Sustained local delivery of drugs to the inner ear may be required for future regenerative and protective strategies. The round window is surgically accessible and a promising delivery route. To be viable, a delivery system should not cause hearing loss. This study determined the effect on hearing of placing a drug-delivery microcatheter on to the round window, and delivering either artificial perilymph (AP) or brain-derived neurotrophic factor (BDNF) via this catheter with a mini-osmotic pump. Auditory brainstem responses (ABRs) were monitored for 4 months after surgery, while the AP or BDNF was administered for the first month. The presence of the microcatheter - whether dry or when delivering AP or BDNF for 4 weeks - was associated with an increase in ABR thresholds of up to 15 dB, 16 weeks after implantation. This threshold shift was, in part, delayed by the delivery of BDNF. We conclude that the chronic presence of a microcatheter in the round window niche causes hearing loss, and that this is exacerbated by delivery of AP, and ameliorated temporarily by delivery of BDNF.

Endolymphatic hydrops: pathophysiology and experimental models

It is well established that endolymphatic hydrops plays a role in Ménière disease, even though the precise role is not fully understood and the presence of hydrops in the ear does not always result in symptoms of the disease. It nevertheless follows that a scientific understanding of how hydrops arises, how it affects the function of the ear, and how it can be manipulated or reversed could contribute to the development of effective treatments for the disease. Measurements in animal models in which endolymphatic hydrops has been induced have given numerous insights into the relationships between hydrops and other pathologic and electrophysiological changes, and how these changes influence the function of the ear. The prominent role of the endolymphatic sac in endolymph volume regulation, and the cascade of histopathological and electrophysiological changes that are associated with chronic endolymphatic hydrops, have now been established. An increasing number of models are now available that allow specific aspects of the interrelationships to be studied. The yclical nature of Ménière symptoms gives hope that treatments can be developed to maintain the ear in permanent state of remission, possibly by controlling endolymphatic hydrops, thereby avoiding the rogressive damage and secondary pathologic changes that may also contribute to the patient's symptoms.

Spiral ganglion loss outpaces inner hair cell loss in endolymphatic hydrops

OBJECTIVES/HYPOTHESIS

Neuronal toxicity is thought to be important in Meniere's disease and experimental endolymphatic hydrops (ELH). This study quantifies the relationship between neuronal degeneration and hair cell degeneration in ELH to evaluate the hypothesis that a primary neural insult would yield greater loss in the spiral ganglion than at the inner hair cell level.

STUDY DESIGN

Following induction and histopathologic confirmation of endolymphatic hydrops in guinea pigs, the degree of hydrops, spiral ganglion loss, and hair cell degeneration were quantified and compared.

METHODS

Guinea pigs with surgically induced unilateral hydrops were sacrificed and their cochleas preserved. Hydrops severity and spiral ganglion density were quantified using automated methods. Hair cells were counted manually. Values were normalized against the contralateral ear to create loss indexes.

RESULTS

Inner hair cell (IHC) loss at the apex is significantly lower than corresponding neuronal loss. IHC loss at the base is also lower than neuron loss, although not significantly. Regression analysis shows a significant, positive correlation between neuron loss severity and IHC loss severity at the apex, but not at the base. There is no correlation between hydrops severity and inner hair cell loss.

CONCLUSIONS

By confirming that spiral ganglion loss is more severe than hair cell loss, and that hair cell loss appears to worsen with neuronal degeneration, this study supports the theory that neuronal toxicity is the primary insult in ELH-related disorders, such as Meniere's disease, and may provide the basis for designing treatment strategies.

Spiral ganglion degeneration patterns in endolymphatic hydrops

OBJECTIVES

The mechanistic association between endolymphatic hydrops (ELH) and hearing loss (HL) is unclear. Although ELH severity has been shown to correlate in some studies with HL, injury of vital structures, including hair cells and the cochlear nerve, have failed to demonstrate correlation with ELH severity. The goal of this study is to evaluate the hypothesis that spiral ganglion cell degeneration is the principle pathologic site of ELH-related cochlear injury, correlates with ELH severity, and is most profound in the apical region.

STUDY DESIGN

Surgical induction of ELH in the guinea pig model was followed by histologic confirmation of ELH and subsequent correlation with segmental spiral ganglion cell densities.

METHODS

Guinea pigs (N = 14) were subjected to unilateral ELH induction and killed after 4 to 6 months. ELH severity and spiral ganglion densities were obtained using computer-aided morphometric analysis. Densities were normalized by calculating a spiral ganglion degeneration index (DI) for each animal.

RESULTS

The apical spiral ganglion demonstrated significantly greater degeneration than that noted in the basal spiral ganglion (DI: 1.93 vs. 1.13; P = .004). The degree of spiral ganglion degeneration in the apex correlates well with a total hydrops index (P = .006) and an apical hydrops index (P = .003). Basal spiral ganglion degeneration however, does not correlate well with hydrops severity (total hydrops index: P > .05; basilar hydrops index: P > .05).

CONCLUSIONS

ELH-related pathology appears to focus initially on the apical spiral ganglion and the degree of deterioration correlates well with the severity of ELH. These findings mirror some reports in the human condition, and imply that the mechanism of cochlear injury in ELH and secondary dysfunction appears to be a neural toxicity that begins in the apex of the cochlea.

Experimental estrogen-induced hyperprolactinemia results in bone-related hearing loss in the guinea pig

Our group (Horner KC, Guieu R, Magnan J, Chays A, Cazals Y. Neuropsychopharmacology 26: 135-138, 2002) has earlier described hyperprolactinemia in some patients presenting inner ear dysfunction. However, in that study, it was not possible to determine whether hyperprolactinemia was a cause or an effect of the symptoms. To investigate the effect of hyperprolactinemia on inner ear function, we first developed a model of hyperprolactinemia in estrogen-primed Fischer 344 rats and then performed functional studies on pigmented guinea pigs. Hyperprolactinemia induced, after 2 mo, a hearing loss of approximately 30-40 dB across all frequencies, as indicated by the compound action potential audiogram. During the 3rd mo, the hearing loss continued to deteriorate. The threshold shifts were more substantial in males than in females. Observations under a dissection microscope revealed bone dysmorphology of the bulla and the cochlea. Light microscopy observations of cryostat sections confirmed bone-related pathology of the bony cochlear bulla and the cochlear wall and revealed morphopathology of the stria vascularis and spiral ligament. Scanning electron microscopy revealed loss of hair cells and stereocilia damage, in particular in the upper three cochlear turns and the two outermost hair cell rows. The data provide the first evidence of otic capsule and hair cell pathology associated with estrogen-induced prolonged hyperprolactinemia and suggest that conditions such as pregnancy, anti-psychotic drug treatment, aging, and/or stress might lead to similar ear dysfunctions.

Diameter of the Cochlear Nerve in Endolymphatic Hydrops: Implications for the Etiology of Hearing Loss in Ménière's Disease

OBJECTIVE/HYPOTHESIS

Endolymphatic hydrops (ELH) is an important histopathological hallmark of Ménière's disease. Experimental data from human temporal bones as well as animal models of the disorder have generally failed to determine the mechanism by which ELH or related pathology causes hearing loss. Hair cell and spiral ganglion cell counts in both human and animal case studies have not, for the most part, shown severe enough deterioration to explain associated severe sensorineural hearing loss. However a limited number of detailed ultrastructural studies have demonstrated significant reductions in dendritic innervation densities, raising the possibility that neurotoxicity plays an important role in the pathology of Ménière's disease (MD) as well as experimental endolymphatic hydrops (ELH). This study tests the hypothesis that neurotoxicity is an important primary mediator of injury to the hydropic ear and is reflected in measurable deterioration of the cochlear nerve in the animal model of ELH. This study also explores the previously presented hypothesis that cochlear injury in ELH is mediated through the actions of nitric oxide (NO) by evaluating whether hearing loss or various measures of cochlear damage can be ameliorated by administration of an agent that limits excess production of NO.

STUDY DESIGN

Part one of the project involves the surgical induction of endolymphatic hydrops and correlation of long term hearing loss with histological parameters of ELH severity as well as cochlear nerve and eighth cranial nerve diameter measurements. In part two, aminoguanidine is administered orally to a separate set of hydropic animals in an attempt to limit cochlear injury presumably mediated by NO.

METHODS

Guinea pigs are subjected to surgical induction of unilateral endolymphatic hydrops after establishing baseline ABR thresholds at 2, 4, 8, 16, and 32 kHz. Threshold shifts are established prior to sacrifice at 4 to 6 months and temporal bones processed for light microscopy. Measurements of cochlear nerve and eighth cranial nerve maximal diameters as well as average maximal diameters are carried out and correlated to hearing loss and a semi-quantitative measure of hydrops severity. The identical experiments are carried out in animals treated with aminoguanidine, an inhibitor of inducible nitric oxide synthase.

RESULTS

: The mean maximal diameter (n = 14) of the hydropic cochlear nerve was significantly reduced (432.14 +/- 43.18 vs. 479.28 +/- 49.22 microns, P = .0025) as compared to the control nerve. This was also seen in measures of the eighth cranial nerve (855.71 +/- 108.82 vs. 929 +/- 81.53 microns, P = 0.0003). Correlation studies failed to show correlation between hydrops severity and a cochlear nerve deterioration index (r = -0.0614, P = .8348). Similarly, hearing loss severity failed to correlate with cochlear nerve deterioration (r = 0.1300, P = .6577). There was a significant correlation between hearing loss and hydrops severity (r = 0.6148, P = .0193). Aminoguanidine treated animals (n = 5) also sustained nerve deterioration to the same degree as non-treated animals and there appeared to be no protective effect (at the dosage administered) against ELH related hearing loss, hydrops formation, or nerve deterioration.

CONCLUSION

ELH results in significant deterioration of cochlear nerve and eighth cranial nerve maximal diameters in the guinea pig model. These findings are in accord with previous studies which detected ultrastructural evidence of dendritic damage and indicate that neural injury is of sufficient severity to result in light microscopic evidence of cochlear nerve and eighth cranial nerve deterioration. These data support the concept that the principle pathological insult in ELH is a form of neurotoxicity, especially in light of previous studies which indicate relative preservation of hair cells at similar points in time. The lack of correlation between the severity of hydrops and nerve deterioration suggests that nerve deterioration is independent of hydrops severity.

Acute endolymphatic hydrops generated by exposure of the ear to nontraumatic low-frequency tones

Low-frequency sounds presented at high nontraumatizing levels induce temporary hyperacusis in humans and animals. One explanation of this finding is that the basilar membrane operating point may be disturbed by an endolymph volume change. This possibility was investigated using volume and flow markers iontophoresed into the endolymphatic space of guinea pigs. Marker concentrations were measured with ion-selective microelectrodes placed apically and basally to the iontophoresis site during exposure of the ear to low-frequency tones. Concentration changes were interpreted quantitatively using a finite-element model of the endolymphatic space that allowed changes of endolymph cross-sectional area and flow to be derived. Stimulation with a 200 Hz tone at 115 dB SPL for 3 min produced marker concentration changes consistent with the induction of transient endolymphatic hydrops and a basally directed displacement of endolymph. Endocochlear potentials were greater than normal after the exposure when hydrops was present. During identical tone exposures of animals without marker, we found that action potential (AP) threshold changes and endolymph potassium changes associated with the hydropic state were small. Marker concentration changes were compared with changes in endocochlear potential and AP thresholds for a range of exposure frequencies and levels. AP hypersensitivity occurred with 200 Hz exposure levels below those inducing endolymph volume disturbances. Endolymph volume changes are thought to be the result of, rather than the cause of, changes in operating point of the cochlear transducer. The observations that auditory threshold and endolymph potassium changes are minimal under conditions where substantial endolymphatic hydrops is present is relevant to our understanding of the hearing loss in patients with Meniere's disease.

Pure-tone and speech audiometry in patients with Menière's disease

The aim of this study was to reinvestigate many of the claims in the literature about hearing loss in patients with Menière's disease. We carried this out on a well-defined group of patients under well-controlled circumstances. Thus, we were able to find support for some claims and none for many others. As part of a diagnostic protocol, pure-tone and speech audiometry was performed on 111 patients with Menière's disease according to the 'Definition Menière Groningen'. This was a prospective clinical cohort study. Affected ears of patients suffering from Menière's disease show reduced hearing, both in pure-tone and in speech audiometry. A classification method was devised to determine audiogram shape in an objective manner. The results of this method indicate that affected ears more frequently show 'low' or 'low + high' hearing losses (P = 0.006). The shape of the hearing loss does not depend on the duration of the affection of the disease. In combination with the fact that the average hearing loss does not correlate with the duration of the disease, this leads to the conclusion that, if a classification of the hearing loss in Menière's disease is possible, such a classification cannot be connected to the duration. This conclusion is further supported by the fact that no relationship is found between the duration of the disease and the classification of the hearing loss over the 3 months before hospital admission, as given by the patients in a questionnaire. A relationship between the (objective) audiometric data and the (subjective) classification of the hearing loss by the patient seems to be present, but is not very strong. Correlations between pure-tone and speech audiometry are present as in non-Menière ears (r = 0.899, P < 0.001), and no indications are found of reduced speech discrimination relative to the expectation based on pure-tone loss. The audiogram shape does not appear to play any additional role in speech discrimination (in addition to the influence of the average pure-tone loss).

Classification of patients with Ménière's disease using otoacoustic emissions

Otoacoustic emissions (OAEs) were studied in patients with Ménière's disease in order to assess their usefulness for distinguishing between different stages of the disease. An accurate classification of Ménière's patients is expected to allow optimalization of the treatment for each particular stage. Click-evoked and distortion product OAEs were evaluated in both ears for 70 Ménière's patients. Based on these measurements, Ménière's patients can be divided into four different categories. In patients with small hearing losses OAEs are found, whereas patients with pure-tone thresholds larger than 60 dB exhibit no OAEs at all. In the intermediate range (30- to 60-dB thresholds) two categories of patients are distinguished: patients with relatively large emissions and patients without measurable emissions. These findings suggest the presence of various stages in the pathophysiological mechanism involved in Ménière's disease. Additionally, Ménière's patients with contralateral ears with normal thresholds have significantly smaller emissions than normal-hearing adults. This observation could represent a very early manifestation of bilateral Ménière's disease, which cannot be detected by other diagnostic methods.

Immunoreactivity for taurine in the cochlea: its abundance in supporting cells

Taurine is the second most abundant free amino acid in the brain where its osmoregulatory function is well established. Taurine-deprived kittens show retinal pathology leading to blindness. In the inner ear, taurine has been reported to be the most abundant free amino acid although its role in inner ear function is not known. Immunohistochemistry was employed here to investigate the localisation of taurine in normal cochleae of the guinea pig compared with two different conditions: experimentally induced endolymphatic hydrops and after oral administration of glycerol. In normal cochleae, by light microscopy, taurine-like immunoreaction was never observed in the sensory outer hair cells and appeared absent from the inner hair cells. In contrast taurine-like immunolabeling was found to be present in all supporting tissue with the striking exception of the tectorial membrane and the outer pillar cell which had no or little taurine immunoreactivity respectively. In early experimental endolymphatic hydrops, the distribution of taurine-like immunoreactivity appeared similar to that observed for normal cochleae. In long-term hydrops, degenerated outer hair cells were replaced by the swelling of the phalangeal process of the Deiters' cells which became highly immunoreactive to taurine. After glycerol administration, the tectorial membrane became more tightly bound to the apical surface of the sensory hair cells and distinctly immunoreactive to taurine. The localisation of taurine in the organ of Corti shown here is consistent with taurine being involved in the maintenance of osmotic equilibrium in the normal and perhaps also in the restructuration of the pathological organ of Corti.

Ultrastructural changes in the hydropic cochlea of the guinea-pig

Transmission electron microscopy of the cochlear organ of Corti in experimental endolymphatic hydrops revealed two principal features. Starting 1 month after induction of hydrops, osmiophilic inclusions thought to represent lipofuscin accumulation were frequently observed in the subcuticular cytoplasm of the outer hair cells along the length of the cochlea. Starting 3 months after induction of hydrops the efferent terminals on the outer hair cells appeared to be vacuolated. These data suggest that oxidative insult is likely to contribute to the pathology associated with endolymphatic hydrops and thus that free radical scavengers might be useful in the treatment of Menière's disease patients. In addition the early modification of the efferent innervation of the hydropic cochlea might underlie the known hypersensitivity to various insults, including noise stimulation, glycerol administration and hypoxia.

Effect of acoustic overstimulation on the hydropic ear

The effect of acoustic overstimulation (2 kHz pure tone) on the compound action potential (CAP) threshold was investigated at frequencies ranging from 2 to 16 kHz using albino guinea pigs, both normal and with experimentally induced endolymphatic hydrops. The hydropic ears were less susceptible to acoustic overstimulation than the normal ears. As the CAP threshold was raised, the frequency exhibiting the greatest CAP threshold shift increased in both animal groups. The tendency was more noticeable in the hydropic ears than in the normal ears. These results are discussed from the aspect of cochlear hydrodynamics.

Degenerative changes in the organ of Corti and lateral cochlear wall in experimental endolymphatic hydrops and human Menière's disease

The pathogenesis of sensorineural hearing loss in Menière's disease and experimental endolymphatic hydrops is not fully understood. At the light microscopic level, there is poor correlation between the histopathology and loss of sensitivity and speech discrimination. The results of electron microscopic investigation of histopathology and alterations in immunoreactivity in the organ of Corti and lateral cochlear wall in the hydropic guinea pig are presented. Loss of outer and inner hair cells and spiral ganglion cells, particularly in the apical turn was evident by light microscopy. By electron microscopy, further evidence of degeneration was detected in the cuticular plate of outer hair cells, neural endings of both inner and outer hair cells, myelinated dendritic fibers, spiral ganglion cells, and types I and II fibrocytes of the lateral cochlear wall. There was a marked decrease in immunoreactivity for a variety of enzymes, calcium binding proteins, structural proteins, and integral membrane proteins of gap junctions, particularly among type I and type II fibrocytes of the lateral cochlear wall. The evidence suggests that dysfunction and degeneration of hair cells, afferent neurons and fibrocytes of the lateral cochlear wall are involved in the pathogenesis of hearing loss in endolymphatic hydrops.

Alterations of auditory nerve responses by hypoxia in normal and hydropic ears of awake guinea pigs

Total interruption of blood or oxygen supply to the inner ear produces very rapid and drastic effects, whereas moderate decreases can be well tolerated by normal ears. In experimental endolymphatic hydrops some moderate alterations of cochlear vasculature have been described which might affect cochlear adaptation to moderate blood or oxygen deficiencies. In order to test this hypothesis an hypoxia at 5% oxygen was imposed for 30 min in normal and hydropic ears of awake guinea pigs and cochlear function was monitored with an electrode at the round window. Electrophysiological recordings used measures of compound action potential (CAP) amplitudes evoked by high-intensity tones, and of CAP thresholds. In normal ears hypoxia induced threshold elevations at all frequencies and decreases of CAP amplitude only for high frequencies. Hydropic ears presented similar or smaller threshold elevations but showed CAP amplitude decreases extending to lower frequencies and showed a much slower recovery both for CAP thresholds and amplitudes. The data indicate that hypoxia had different effects on auditory nerve responses evoked by high versus low intensity tones. The deleterious effects of hypoxia were increased in hydropic ears. Hypoxia-induced alterations were measured twice at one week intervals during which an anti-ischemic drug was administered to the animals; some beneficial effects of the drug treatment were observed on normal but not on hydropic ears.

The effect of nimodipine on cochlear potentials and Na+/K(+)-ATPase activity in normal and hydropic cochleas of the albino guinea pig

In experimental endolymphatic hydrops (EEH) a decrease in the endocochlear potential (EP) has been reported and is thought to be due to decreased activity of the enzyme Na+/K(+)-ATPase in the stria vascularis. By stimulating Na+/K(+)-ATPase, the EP, and thereby cochlear function as a whole, might be restored. On the other hand, stimulation of stria vascularis Na+/K(+)-ATPase might result in excessive production of endolymph and thus produce or augment hydrops. In this study we have investigated the effect of intraperitoneally applied nimodipine on cochlear potentials and on Na+/K(+)-ATPase activity in the stria vascularis, both in normal cochleas (control) and in cochleas with EEH. Nimodipine is an L-type Ca(2+)-channel blocking agent with Na+/K(+)-ATPase stimulating properties at concentrations as low as 1.5 nM. The compound action potential (CAP), evoked by 2,4 and 8 kHz tone bursts was found to be depressed in the EEH ears with and without nimodipine treatment, and in the nimodipine treated control ears. Statistical analysis (ANOVA) showed that the effects of EEH and nimodipine on the CAP were additive. The negative summating potential (SP), measured extracochlearly at the apex, in response to 4 and 8 kHz tone bursts was significantly enhanced in the EEH ears. Nimodipine treatment did not affect the SP, neither in the control, nor in the EEH ears. Cytochemically, Na+/K(+)-ATPase activity appeared to be decreased in the oedematous stria vascularis of hydropic cochleas. No effect of nimodipine on Na+/K(+)-ATPase activity could be established ultracytochemically, neither in the controls nor in the EEH ears. In the lower turns of some of the nimodipine treated control cochleas a mild hydrops was seen during light-microscopic evaluation. Although it was not possible to prove a stimulatory effect of nimodipine on the enzyme Na+/K(+)-ATPase cytochemically, the finding of mild endolymphatic hydrops in nimodipine treated control ears suggests (a history of) increased endolymph production. This hydrops might be responsible for the depression of the CAP in the nimodipine treated ears.

Time course of endolymph volume increase in experimental hydrops measured in vivo with an ionic volume marker

A new method has been developed to measure the cross-sectional area (CSA) of scala media in the living cochlea. The method has some advantages over histological methods, in which tissues may shrink or move during processing. In the present study, scala media CSA was measured in the second turn of guinea-pig cochleas in which endolymphatic hydrops was induced surgically. The area measurement method used an iontophoretic injection of a volume marker into scala media, during which the concentration of marker in endolymph was monitored with an ion-selective microelectrode. The measured marker concentration was inversely proportional to the CSA of endolymph. The marker we used was the anion arsenic hexafluoride (AsF6-), which was almost ideal for the purpose as it was retained well in endolymph. Area was measured in normal animals and in hydropic animals at times from 4 days to 16 weeks after endolymphatic duct obstruction. The results showed that hydrops develops within days of ablation of the endolymphatic duct. The degree of hydrops was compared with electrophysiological measures of function, including the endocochlear potential, action potential thresholds and the amplitudes of the cochlear microphonic, summating potential and action potentials. In the initial stages of hydrops development, electrophysiological changes were small. In contrast, there were marked functional changes between 8 and 16 weeks, when endolymph volume was no longer increasing. If the same is true for dysfunction in the ears of patients with Ménière's Disease, then it may not be possible to restore normal function simply by alleviating the hydrops.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional changes associated with experimentally induced endolymphatic hydrops

A guinea pig model with surgically induced endolymphatic hydrops of the inner ear has been developed and studied over the past thirty years. The aim of such studies is to obtain insight into physiological processes associated with endolymphatic hydrops in man and in particular in Menière's disease where endolymphatic hydrops is systematically encountered at post-mortem examination of the temporal bones. The present review attempts to draw together the data pertaining to functional modifications of inner ear function in the animal model. For simplicity the data are categorised under five main titles: electrochemical modifications, electrophysiological modifications, pressure and hydrops, sensitivity to other insults and vestibular dysfunction. One of the most striking observations that can be made is that the data originating from different authors are very variable. There is, however, some evidence suggesting that the evolution of the auditory dysfunction could be considered as consisting of a series of different phases. This kind of information could serve as a basic framework for future research on the animal model.

Glycocalyx heterogeneity in normal and hydropic cochleas of the guinea pig

The endo- and perilymphatic glycocalyx of the cochlear epithelia were investigated ultrastructurally in normal and hydropic cochleas using the electron-dense markers cationized ferritin and colloidal thorium. In the normal cochleas all epithelial surfaces showed reactivity with both markers. Using cationized ferritin, no regional differences in reactivity could be demonstrated. With colloidal thorium, however, the apical membranes of the outer hair cells reacted more intensely than either the basolateral membranes or the membranes of the supporting cells of the organ of Corti. Cationized ferritin reactivity was not affected by digestion with either neuraminidase or hyaluronidase. In contrast, colloidal thorium reactivity of the endolymphatic surfaces was greatly reduced after neuraminidase digestion. Reactivity of the cell membranes lining the perilymphatic compartments of the cochlea was less affected by neuraminidase digestion, except for the basolateral membranes of the sensory and supporting cells of the organ of Corti, which demonstrated a greatly reduced reactivity. These findings indicate that the glycocalyx of the epithelial surfaces lining the endolymphatic compartment has a high content of sialic acid. The differences that are observed in normal cochleas in regard to colloidal thorium reactivity between the apical membranes of the outer hair cells and the membranes of the Deiters' cells, could not be demonstrated in hydropic cochleas. This probably contributes to the early functional changes in outer hair cells observed in experimental hydrops.

Transiently evoked otoacoustic emissions in patients with Menière's disease

Transiently evoked otoacoustic emissions (TEOAEs) were stimulated using clicks or 1-kHz tone bursts in both ears of 31 patients with unilateral Meniere's disease. Using click stimuli, responses were present in 29/31 of the non-Meniere's ears and in 26/31 of the Meniere's ears. Stimulation using 1-kHz tone bursts produced responses in 30/31 of the non-Meniere's ears and in 28/31 of the Meniere's ears. Audiometric configuration strongly influenced the presence of TEOAEs in the affected ears. In all but 2 ears with average hearing sensitivity (0.5, 1, 2 kHz) poorer than 25 dB HL, responses were present only when accompanied by at least one pure-tone threshold better than 30 dB HL from 0.75 to 2 kHz. In the opposite ears, all of which had essentially normal hearing, responses were approximately 5 dB lower in level and contained fewer spectral peaks in comparison with results obtained from a normative database. Neither age nor pure-tone results could account for these differences. Thus, TEOAEs obtained from patients with Meniere's disease manifest features that may be considered as atypical either in comparison to results from ears with normal hearing or from ears with relatively flat sensorineural hearing losses due to other etiologies.

Contribution of increased endolymphatic pressure to hearing loss in experimental hydrops

After the induction of experimental hydrops there follows a strict sequence of compound action potential (CAP) audiogram changes. Within days a low-frequency loss (below 8 kHz) is detected; within weeks a very high-frequency loss (above 8 kHz) develops; and finally within months the 8-kHz region is also affected. Following the application of excess pressure, presumably to the endolymphatic spaces, via a cannula placed in the endolymphatic duct, a sequence of CAP audiogram changes occurs that is not similar to that observed with hydrops. There is first a very high-frequency loss (above 4 kHz), then a very low-frequency loss (below 4 kHz), and finally the 4 kHz region is also affected and thresholds for all frequencies become even more raised. The data thus suggest that a slow accumulation of endolymph as induced by blockage of the endolymphatic duct does not produce the same disorder as a rapid onset of a high pressure applied at the base of the cochlea. In addition the data suggest that raised endolymphatic pressure is not likely to be significant in early stages of hydrops, but could contribute to the later high-frequency sensitivity losses.

Atrophy of outer hair cell stereocilia and hearing loss in hydropic cochleae

We have earlier described selective atrophy of short and middle stereocilia on outer hair cells of the three upper cochlear turns in hydropic cochleae of guinea pigs. The present study describes sequential early stages of stereocilia degeneration leading to this specific atrophy. Comparison of the morpho-pathology with the ultimate CAP audiograms taken before sacrifice indicated a close association between the low frequency hearing loss and this atrophy of stereocilia. The atrophy appeared to be associated first with the short and then the middle stereocilia of the 2nd and 3rd rows of outer hair cells between 0.5 kHz and 2 kHz and with time included the 1st row of all outer hair cells of the upper cochlear turns down to the 8 kHz region.

Old theme and new reflections: hearing impairment associated with endolymphatic hydrops

The cause(s) of the hearing impairment associated with Menière's disease are not understood but are undoubtedly associated with the inner ear endolymphatic hydrops. Two major hypotheses have been proposed and widely received: endolymphatic overpressure followed by leaky membranes and subsequently the mixing of high K+ endolymph with perilymph. Our recent data on an experimental model of endolymphatic hydrops have provided grounds for renewed reflections on the pathology. Indeed our data might be interpreted without involving either of the above hypotheses and suggests that the symptoms of Menière's disease might be accounted for by a flow of perilymph from scala vestibuli towards scala tympani with the mixing of the two perilymphs which are similar but not identical in composition. The higher K+ concentration arriving from the scala vestibuli into the scala tympani at the apex of the cochlea via the helicotrema is likely to be toxic to hair cell and auditory nerve fiber function. The mixing of the two perilymphs could result in deterioration of low frequency sensitivity, provoke low frequency tinnitus and in the long term cause spiral ganglion cell degeneration at the apex of the cochlea. The feeling of fullness in the ear might be the result of the decreasing perilymph volume in the scala vestibuli which could give rise to inner ear conductive losses. The patency of the cochlear aqueduct might play a role in determining the high risk group of individuals likely to manifest the symptoms of endolymphatic hydrops.

Hypersensitivity of hydropic ears, at frequencies with normal thresholds, to temporary threshold shifts

We have shown that experimentally-induced hydrops in the guinea pig systematically provokes an early low frequency (6.4 kHz and below) fluctuant hearing loss. The present study was aimed at investigating one aspect of the functioning of that part of the audiogram with normal CAP thresholds (mid and high frequencies) in early hydrops. Temporary threshold shifts (TTS) as a function of pure tone stimulation level (8 kHz, 1 min 75-95 dB SPL), were investigated. The TTS was measured as a function of pure-tone exposure level. The different points on the input/output curve were determined on sequential days in order to allow full recuperation from the previous stimulation. The group of hydropic ears (N = 5) proved to be more sensitive by about 6 dB than the group of control ears (N = 10). The data indicate that whilst short-term endolymphatic hydrops can be characterised by a selective low frequency loss other auditory deficits exist throughout the cochlea and become manifest at supraliminal stimulus levels.

Mechanisms of salicylate ototoxicity

The ototoxic effects of salicylates, reversible hearing loss and tinnitus, are well documented. However, the pharmacological mechanisms underlying these changes in cochlear function are not well understood. The studies reported here were an investigation of the site and mechanism of salicylate ototoxicity through an examination of its effects on ionic, neural and mechanical aspects of cochlear transduction. Salicylate administration produced an intensity dependent reduction of the AP and SP, with the predominant effects occurring at low stimulus levels. In direct contrast, a significant increase was observed for corresponding CM responses, independent of stimulus intensity. Salicylates also reduced the magnitude of efferent induced shifts in the AP, CM and EP. Cochlear mechanics were altered as evidenced by the reduction in two-tone distortion products, electrically evoked emissions, and electrophonic APs. These changes in cochlear function are attributed to a salicylate mediated increase in the membrane conductance of the outer hair cells. This change in membrane permeability interferes with the reverse transduction process, effectively reducing the gain of the cochlear amplifier. Results of single unit recordings suggest parallels between salicylate intoxication and noise trauma, which are discussed with regard to potential mechanisms of tinnitus generation.

Alterations of CAP audiogram by increased endolymphatic pressure and its relation to hydrops

Most current theories regarding the inner ear pathology of Menières disease assume that there is an augmentation of the endolymphatic pressure due to the presence of hydrops. In this study normal hearing pigmented guinea pigs were employed to investigate the effect of increased endolymphatic pressure on the compound action potential (CAP) audiogram. All animals were implanted with an electrode on the round window and the CAP audiogram was determined prior to further surgery. The endolymphatic canal was then visualised by a posterior fossa intra-dural surgical approach. A hole was pierced in the canal and a cannula inserted. The CAP audiogram was again determined before, and at frequent intervals after, the application of hydrostatic pressure (0.5-1 cm Hg). A similar sequence of CAP sensitivity losses was observed within 2 h for 0.5 cm Hg or 15 min for 1 cm Hg. There was at first a very high frequency loss, followed by a very low frequency loss and finally a mid frequency sensitivity loss rendered the audiogram flat and lying around 50 dB sound pressure level. Given that the first characteristic index for experimental hydrops is a low frequency loss the present data suggest that an increase in endolymphatic pressure, as in these experiments, is likely to be a rather late pathological feature of hydrops. Indeed we have shown that a high frequency loss develops at a second phase during the evolution of hydrops.

Distortion products in early stage experimental hydrops in the guinea pig

The swelling of the endolymphatic spaces in experimental hydrops is likely to induce mechanical changes all along the cochlea. The selective atrophy of short and middle stereocilia on the outer hair cell above the first cochlear turn is expected to alter micromechanics and transduction. Two tone stimuli were employed in the present study to investigate cochlear distortion products in hydropic ears. Despite a substantial low frequency sensitivity loss on the CAP audiogram, 2f1-f2 distortion products recorded in the RWCM and the CAP were similar in hydropic ears and normal control ears which reflect the probable common origin of these responses--the base of the cochlea where no threshold shifts can be detected. On the other hand this distortion product in the otoacoustic emissions was considerably reduced in hydropic cochleas when the primary tones lay within the pathological part of the audiogram. The reduction of CDT emissions cannot be accounted for by hair cell loss since at the early stage of hydrops described here there is only discrete hair cell loss which is restricted to the apex of the cochlea. On the other hand the reduction in the amplitude of the CDT might reflect disturbance in hair cell function and/or cochlear mechanics above the first cochlear turn.

Long-term treatment with chlorthalidone reduces experimental hydrops but does not prevent the hearing loss

Long-term treatment of guinea pigs with the diuretics chlorthalidone and acetazolamide, following the experimental obstruction of the endolymphatic duct, was assessed using chronically implanted round window cochlear electrodes. The diuretic chlorthalidone appeared to curb the progressive low-frequency sensitivity loss during the first 4 weeks following surgery, as compared with animals receiving the diuretic acetazolamide or no treatment. However, this apparent beneficial effect decreased after 4 weeks and was not apparent at 14 weeks post-induction. On the other hand, morphological control at the end of 14 weeks confirmed a marked reduction in hydrops in the chlorthalidone-treated animals. The data clearly demonstrate a dissociation between hydrops and the development of hearing loss and suggest that the augmenting endolymph volume is only one of several contributing factors to the deteriorating auditory function in experimental hydrops.

Asymmetry of evoked rotatory nystagmus in the guinea pig after experimental induction of endolymphatic hydrops

We have previously shown by chronic implantation of round window electrodes, that after the experimental induction of endolymphatic hydrops by surgical blockade of the endolymphatic canal, in the guinea pig, there is a low/mid-frequency fluctuant hearing loss, followed by a very high frequency loss and after a few months the audiogram is flat. This evolution reproduces exactly that observed in Meniere's patients. The aim of this study was to investigate the evolution of one aspect of vestibular function in parallel with audiogram changes. The nystagmic responses in the dark were tested every month during the 4 months which followed hydrops induction. There was considerable variation in the number of evoked saccades for different control animals and even between different recordings for the same control animal. However, in general, the number of saccades to right and left rotation was symmetrical for control animals, whereas for hydropic animals there was a period, within the first 2 months post-operation, when there was a reduction of saccades to the left (operated side) with sometimes an increase to the right. This asymmetry which occurred in the period of fluctuant hearing losses did not however appear synchronized with audiogram fluctuations. The nystagmic responses tended to become symmetrical over the 4 months post-operation. Oral administration of glycerol to animals whose nystagmus was asymmetric brought the vestibular response towards symmetry in 50% of the cases. Although episodes of vertigo, as known in Meniere's disease, were never observed in operated guinea pigs, these data indicate that they have some functional vestibular disturbance.

Evolution of recruitment at different frequencies during the development of endolymphatic hydrops in the guinea pig

The study of human temporal bones has identified endolymphatic hydrops as a common feature of several diseases. In particular it is systematically found in those bones removed from patients with premortem Menière's disease. Menière's disease is known to induce sensorineural pathology with recruitment, which changes with the evolution of the hearing loss, and is suspected to induce a cochlear conductive loss by a possible increase in static pressure of endolymph. Amplitude/intensity functions of sensorineural responses can reflect recruitment and/or conduction loss. Experimentally induced hydrops in animals provokes cochlear physiological alterations, some of which closely resemble certain features of Menière's disease. In the present study using a guinea-pig animal model, we have examined amplitude/intensity functions at the round window for cochlear microphonics (RWCM), summating potentials (RWSP) and action potentials (CAP) at different stages of hearing loss in experimentally induced hydrops. During the period of fluctuating thresholds there was reduction of maximal RWCM amplitude, no change in RWSP and recruitment on the CAP. At a later stage when the audiogram was flat and fluctuations were no longer seen, RWCM remained unchanged. At this time RWSP could show recruitment while CAP amplitudes at all intensities were reduced, indicating either a cochlear conductive loss and/or a general depression of neural activity.

Independent fluctuations of the round-window summating potential and compound action potential following the surgical induction of endolymphatic hydrops in the guinea pig

The diagnosis of Menière's disease is classically based on the triad of symptoms including fluctuating hearing loss, tinnitus and vertigo. Modifications to the electrocochleographic response have been searched as a possible help in the diagnosis. Various authors have reported a tendency for an increased ratio of the summating potential (SP) to action potential (AP) which is generally thought to be due to an enhanced SP. However, the large variability between patients has precluded any clear-cut conclusion. This dispersion of data might represent real individual differences or might be attributed to unavoidable technical variations, such as electrode placement and/or precise control of stimulus levels. As an attempt to answer this issue, we employed an animal model of experimental hydrops in which these difficulties can be overcome by chronic implantation of round-window electrodes and carefully controlling the stimulus level. In the present study, the SP and AP were monitored over several months for different frequencies and different intensities. In the early period of fluctuating thresholds at low and mid frequencies, AP amplitude varied in loose correspondence with the fluctuating audiogram. The SP amplitude also varied apparently not associated with AP or threshold changes and no consistent increase in SP was observed. At a later stage when all thresholds were elevated both SP and AP diminished. In normal ears increases in the stimulus intensity induce an augmentation of SP/AP. In hydropic ears, at the period of fluctuating thresholds, the SP/AP growth curve was at first similar to that of controls but later became very variable for different animals, but in general much larger than normal.(ABSTRACT TRUNCATED AT 250 WORDS)