Early hair cell loss in experimental hydrops

The Effect of Endolymphatic Hydrops and Mannitol Dehydration Treatment on Guinea Pigs

Background

Endolymphatic hydrops (EH) is considered as the pathological correlate of Menière’s disease (MD) and cause of hearing loss. The mechanism of EH, remaining unrevealed, poses challenges for formalized clinical trials.

Objective

This study aims to investigate the development of hearing loss, as well as the effect of dehydration treatment on EH animal models.

Methods

In this study, different severity EH animal models were created. The laser Doppler vibrometer (LDV) and auditory brainstem responses (ABR) were used to study the effects of EH and the dehydration effects of mannitol. The LDV was used to measure the vibration of the round window membrane (RWM) reflecting the changes in inner ear impedance. ABR was used to evaluate the hearing changes. Furthermore, tissue section and scanning electron microscopy (SEM) observations were used to analyze the anatomical change to the cochlea and outer hair cells.

Results

The RWM vibrations decreased with the severity of EH, indicating an increase in the cochlear impedance. The dehydration therapy lowered the impedance to restore acoustic transduction in EH 10- and 20-day animal models. Simultaneously, the ABR thresholds increased in EH models and were restored after dehydration. Moreover, a difference in the hearing was found between ABR and LDV results in severe EH animal models, and the dehydration therapy was less effective, indicating a sensorineural hearing loss (SNHL).

Conclusion

Endolymphatic hydrops causes hearing loss by increasing the cochlear impedance in all tested groups, and mannitol dehydration is an effective therapy to restore hearing. However, SNHL occurs for the EH 30-day animal models, limiting the effectiveness of dehydration. Our results suggest the use of dehydrating agents in the early stage of EH.

Auto-inflammatory challenge of the endolymphatic sac--Cochlear damage measured by distortion product oto-acoustic emissions

CONCLUSION

Twenty-five rats were challenged by an immunologic attack of the endolymphatic sac. After 6 months, distortion product oto-acoustic emissions (DPOAE) revealed a dysfunction of the outer hair cells and immunological active cells were observed in the endolymphatic sac. This information could contribute to the understanding of Ménière's disease.

OBJECTIVES

This study investigated if an autoimmune challenge of the endolymphatic sac could affect DPOAE output measurements in rats. Also, a potential autoimmune cell infiltration of the endolymphatic sac was investigated.

METHODS

Eighteen Lewis rats were immunized with a crude endolymphatic sac extract in complete Freund's adjuvant. Seven control animals were injected with Freund's adjuvant in saline. Cochlear damage was estimated by DPOAE dynamics 3 weeks and 6 months after the immunization. Infiltrative cells in the endolymphatic sac were investigated with transmission electron microscopy.

RESULTS

The hearing assessment 6 months after immunization revealed a reduction of the DPOAE, on the full range of frequencies (2-63 kHz) in an average of the mean, of 2 dB ± 1.1 in the immunized group compared to the controls (p < 0.05). The same test showed a 2.5 dB decrease from 2 to 5 kHz (p < 0.01). Immunological active cells were observed in the endolymphatic sac in most of the immunized rats.

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.

Evaluation of the effect of dexamethasone in experimentally induced endolymphatic hydrops in guinea pigs

PURPOSE

The aim of this study was to investigate the audiological and histopathologic effects of dexamethasone in the treatment of experimentally induced endolymphatic hydrops.

MATERIALS AND METHODS

Thirty mature, male guinea pigs weighing 400 +/- 50 g were operated on to induce experimental endolymphatic hydrops in their right ear. Left ear served as control. Subjects were separated into control and dexamethasone groups, with the latter receiving dexamethasone 5 mg/(kg d) intraperitoneally for 10 days. Electrocochleography and auditory brainstem response were applied to all subjects at preoperation, on the second postoperative day and also on the 15th postoperative day in animals that lived for a long time. The histopathologic examination of the inner ear in all animals was done at the end of the study.

RESULTS

The summating potential and the ratio of the summating potential to the action potential measured on the second postoperative day were found to be increased in both groups, but more significantly in the control one. When the left and right ears were compared, significant difference was found in the control group; however, no significant difference was found between the ears in the dexamethasone group. Histopathologic examination revealed varying degrees of hydrops in the control group, but showed only normal findings or minor changes in the dexamethasone group.

CONCLUSIONS

Dexamethasone can prevent the audiological and histopathologic findings of experimentally induced endolymphatic hydrops. However, these results must be supported by clinical and experimental studies designed with a large number of subjects.

Protective effect of edaravone against endolymphatic hydrops

CONCLUSION

Our findings suggest that edaravone prevented the production of reactive oxygen species (ROS). Edaravone also delayed the formation of endolymphatic hydrops in guinea pigs, but had no effect on endolymphatic hydrops.

OBJECTIVE

To analyse the protective effect of a free radical scavenger, edaravone, on endolymphatic hydrops.

MATERIALS AND METHODS

Guinea pigs were subjected to surgical obliteration of the endolymphatic duct (ED). For the detection of ROS, group 1 received intraperitoneal injections of edaravone (3 mg/kg/day) for 2 days, group 2 received edaravone for 2 weeks, group 3 saline for 2 days, and group 4 saline for 2 weeks. ROS production by the organ of Corti and stria vascularis was examined by using dihydrotetramethylrosamine. For the morphological analysis, guinea pigs were divided into five groups, i.e. 2 or 4 weeks after ED obliteration, 2 weeks with edaravone, first or last 2 weeks with edaravone and sacrificed 4 weeks after ED obliteration. Increases in the ratios of the cross-sectional area of scala media were analysed quantitatively to assess the degree of endolymphatic hydrops among the above-mentioned five groups of the hydropic cochlea.

RESULTS

ROS was detected both in the organ of Corti and in the lateral wall of cochleae 2 days after ED obliteration. Edaravone prevented the production of ROS and also attenuated the formation of endolymphatic hydrops in the acute hydrops group.

Meniere's Disease: Prevalence of Contralateral Ear Involvement

OBJECTIVE

Determine the prevalence and time interval for conversion from unilateral to bilateral involvement in Meniere's disease and cochlear hydrops.

STUDY DESIGN AND SETTING

Retrospective chart review in a tertiary otologic referral center.

PATIENTS

232 patients diagnosed with Meniere's Disease (n=186) or cochlear hydrops (n=46) between 1959 and 2001, who visited the clinic over a five-year period between 1997-2001 and have at least 2 audiograms more than 12 months apart.

MAIN OUTCOME MEASURES

Prevalence of cochlear hydrops relative to Meniere's Disease, rate of progression from unilateral to bilateral involvement; interval between unilateral onset of symptoms and bilateral involvement; and rate of progression from cochlear hydrops to Meniere's disease.

RESULTS

Initial diagnosis was Meniere's disease in 71% and cochlear hydrops in 29% of all 950 hydropic patients presenting between 1997 and 2001. In the study sample, Meniere's disease was bilateral at presentation in 11%; an additional 12% (14% of unilaterals) became bilateral during the follow-up period. At presentation, 6.5% of cochlear hydrops patients were bilateral, with another 26% becoming bilateral. Conversion from cochlear hydrops to Meniere's disease occurred in 33% and some of these are included among the bilateral. The average time interval for conversion from unilateral to bilateral Meniere's was 7.6 years (SD=7.0 years).

CONCLUSION

Most otologists are aware of the potential for contralateral ear involvement and conversion from cochlear hydrops to Meniere's disease after diagnosis. These changes are significant, require long-term follow-up for detection, and may necessitate further treatment. Patients should be counseled regarding this potential when interventions are considered, especially with respect to ablative treatments.

Correlation between hearing loss and scala media area in guinea pigs with long-standing endolymphatic hydrops

HYPOTHESIS

Histologic analysis of the hydropic and normal guinea pig cochleae was undertaken to assess a potential correlation between the magnitude of endolymphatic hydrops and hearing loss. It was hypothesized that a greater correlation than previously reported might be found by looking at long-standing endolymphatic hydrops and high-frequency range hearing.

BACKGROUND

Surgically induced endolymphatic hydrops in guinea pigs is the most widely used animal model for the study of human Ménière's Disease and recapitulates both endolymphatic hydrops and progressive sensorineural hearing loss. A strong correlation between the magnitude of hydrops and severity of hearing loss has been reported in the human condition, but not in the animal model.

METHODS

Nine albino guinea pigs were each subjected to surgical obstruction of the endolymphatic sac and duct of the right ear. The left ears remained as internal histologic controls. Hearing was assessed from 2 kHz to 32 kHz by auditory brain stem response testing for 16 to 25 weeks after surgery. Histologic morphometry after the animals were killed was used to quantify both turn-specific and weighted overall hydrops. These measures were correlated with hearing loss in each animal at all tested frequencies.

RESULTS

A statistically significant correlation between the magnitude of hydrops and the severity of hearing loss was observed for 2 kHz and 16 kHz. These frequencies correlated with both turn-specific hydrops and overall hydrops. However, turn-specific hydrops did not reliably correlate with the magnitude of hearing loss at anatomically appropriate frequency ranges. Where such a correlation did exist, it might well have been simply part of an expression of an overall correlation between hydrops and hearing loss.

CONCLUSIONS

There may be a greater correlation between hydrops and hearing loss in guinea pigs with long-standing surgically induced hydrops than has previously been reported in animals with less advanced disease. These findings help to validate continued use of the model for hearing loss related to Ménière's Disease, verify the rationale of treatment modalities aimed at reducing hydrops in the human condition, and may indicate that efforts at reducing hydrops in such patients has benefits toward long-term hearing preservation.

Na+,K+-ATPase and Ca2+-ATPase activities in the cochlear lateral wall following surgical induction of hydrops

Na+,K+-ATPase and Ca2+-ATPase activities have not been studied quantitatively in the cochlea affected by endolymphatic hydrops. The present study was designed to measure quantitatively the Na+,K+-ATPase and Ca2+-ATPase activities in the cochlear lateral wall and the threshold of auditory brainstem response (ABR) for guinea pigs in the early stages (=2 months) of experimentally induced endolymphatic hydrops. A significant negative association was demonstrated between Ca2+-ATPase activity and the change in ABR threshold for hydropic cochleae (P=0.014), but not for control cochleae (P=0.123), although no such significant association was revealed between Na+,K+-ATPase activity and any change in ABR threshold for both hydropic cochleae (P=0.751) and control cochleae (P=0.352). A significant increase in Ca2+-ATPase activity in the cochlear lateral wall was observed for the hydropic ear, in which normal ABR thresholds were maintained, as compared to the control ear. On the contrary, a mild decrease in Ca2+-ATPase activity in the cochlear lateral wall was observed for the hydropic ear, in which ABR thresholds increased significantly. The present findings suggest that alterations of Ca2+-ATPase activity in the cochlear lateral wall may implicate disturbed calcium-homeostasis in the inner ear, resulting in hearing dysfunction in the early stages of experimentally induced endolymphatic hydrops.

Distortion-product otoacoustic emissions and cochlear microphonics: relationships in patients with and without endolymphatic hydrops

OBJECTIVES

Because endolymphatic hydrops causes cochlear malfunction, and both otoacoustic emissions and cochlear microphonics measure specific cochlear activities, some insight into the pathology of Meniere's disease might be gained by using these two test modalities. Specifically, the involvement of cochlear outer hair cells in patients with endolymphatic hydrops may be detected. Furthermore, it is hoped that these two tests might help determine which regions of the cochlea are affected by endolymphatic hydrops, as well as where along the auditory pathway abnormalities are present.

STUDY DESIGN

Data were gathered prospectively on patients presenting to a private, tertiary referral otology/neurotology practice.

METHODS

From February 1999 to April 2000, clinical information was collected on patients presenting with vertigo, hearing loss (HL), sudden HL, fluctuant HL, aural fullness, and/or tinnitus. Data included demographics, diagnosis, pure-tone and speech audiometry, tympanometry, summating potential, action potential, cochlear microphonic, and distortion-product otoacoustic emissions. Descriptive statistics were calculated, and relationships between distortion-product otoacoustic emissions and cochlear microphonics in patients with hydrops (defined as summating potential to action potential ratio > or =0.40) and without hydrops were analyzed.

RESULTS

Distortion-product otoacoustic emissions were present more often and had larger amplitudes at the lower frequencies. No differences were found in the presence of distortion-product otoacoustic emissions across the frequencies for the two groups, but larger mean amplitudes were found for hydropic ears at 7966 Hz. As hearing levels worsened, both hydropic and nonhydropic ears were less likely to have emissions present; however, 18% of hydropic ears had emissions unexpectedly present when the pure-tone thresholds were > or =50 dB. The cochlear microphonic from the hydrops group tended to be smaller, but this was not statistically significant. Analysis of variance showed a small negative correlation between summating potential to action potential ratio and level of emission at 1968 Hz in hydropic ears; otherwise, there was no relationship between the ratio and emissions. The only statistically significant finding when analyzing the relationship between cochlear microphonic and otoacoustic emission was a small positive correlation between level of microphonic and level of emission at 1406 Hz in hydropic ears. No significant relationships were found between hearing thresholds and emissions or microphonics.

CONCLUSIONS

Even though both distortion-product otoacoustic emissions (DPOAEs) and cochlear microphonics (CMs) measure specific cochlear activities, they were not found to be useful for differentiating patients with hydrops from those without. In some patients, however, unexpected distortion-product otoacoustic emissions were present. This may represent localizing information about which regions of the cochlea are being affected by hydrops in these patients. Also, a small positive correlation between the CM and the DPOAE at 1406 Hz was detected in the hydrops group, which may represent the effects of endolymphatic hydrops on the outer hair cell. Future investigations involving hydropic patients with unexpected DPOAEs and studies looking for more DPOAE and CM correlations at frequencies surrounding 1406 Hz are being planned.

Signs of endolymphatic hydrops after perilymphatic perfusion of the guinea pig cochlea with cholera toxin; a pharmacological model of acute endolymphatic hydrops

There are indications that endolymph homeostasis is controlled by intracellular cAMP levels in cells surrounding the scala media. Cholera toxin is a potent stimulator of adenylate cyclase, i.e. it increases cAMP levels. We hypothesized that perilymphatic perfusion of cholera toxin might increase endolymph volume by stimulating adenylate cyclase activity, providing us with a pharmacological model of acute endolymphatic hydrops (EH). Guinea pig cochleas were perfused with artificial perilymph (15 min), with or without cholera toxin (10 microg/ml). The endocochlear potential (EP) was measured during and after perfusion. The summating potential (SP), evoked by 2, 4 and 8 kHz tone bursts, was measured via an apically placed electrode 0, 1, 2, 3 and 4 h after perfusion. Thereafter, the cochleas were fixed to enable measurement of the length of Reissner's membrane, reflecting EH. After perfusion the EP increased significantly over time in the cholera toxin group as compared to the controls. Also, the SP increased gradually at all frequencies in the cholera toxin group. Comparison within animals showed that the increase in SP became significant after 2 h at 4 kHz, after 3 h at 2 kHz and after 4 h at 8 kHz. In the control group the SP did not change significantly. The compound action potential (CAP) amplitude decreased monotonically over time at all frequencies in both the cholera toxin group and the control group, but it decreased faster in the cholera toxin group. Also, the cochlear microphonics amplitude decreased over time at all frequencies in both groups, but the decrease was significant only in the cholera toxin group after 3 h at 2 and 4 kHz. Quantification of the length of Reissner's membrane showed a small but insignificant enlargement in the cholera toxin treated animals compared to controls. These results are in accord with our view that EH is accompanied by an increase in SP and a decrease in CAP. Our results partially confirm previous results of Feldman and Brusilow (Proc. Natl. Acad. Sci. USA (1973) 73, 1761-1764). New aspects in relation to that study are the significantly increased EP and SP. In the classical EH model, based on obstruction of the absorptive function of the endolymphatic sac, increased SPs are accompanied by decreased EPs. In this cholera toxin model of EH, it is unlikely that the endolymphatic sac is involved. Apparently, EH can be based on mechanisms located in the cochlea itself as opposed to mechanisms located in the endolymphatic sac.

Induction of endolymphatic hydrops in the guinea pig by perisaccular deposition of sepharose beads carrying and not carrying immune complexes

We tried to induce endolymphatic hydrops in guinea pig cochleas by unilateral, perisaccular deposition of sepharose beads carrying immune complexes. Controls consisted of the deposition of sepharose beads without immune complexes and the contralateral, untreated ear. The effects of the treatment were studied by light microscopy and electrophysiological recordings of the gross cochlear potentials 1, 2, and 6 weeks after treatment. Each condition included six animals. Analysis of variance of the morphometric data concerning the ears treated with deposition of the beads showed a statistically significant difference (P = 0.04) between the degree of hydrops found for the beads with immune complexes and for those without. The difference between the treated ears and the contralateral untreated ears was significant (P = 0.01) for the beads with immune complexes and not significant (P = 0.8) for those without immune complexes while there was no significant effect of post-treatment time interval. Analysis of variance of the electrophysiological data, collected in response to tone bursts at the apex of the cochlea, showed no significant differences between the results for the beads with and without immune complexes. Therefore these results were pooled. One week after treatment the pooled results for the compound action potential showed a small decrease in amplitude, just significant at 2 kHz, but not at 4 and 8 kHz. This decrease disappeared completely after 6 weeks. The pooled results for the negative summating potential (SP) showed a significant increase in magnitude at all frequencies decreasing with post-treatment interval. The cochlear microphonics did not demonstrate any change in amplitude after treatment. The results indicate that deposition of sepharose beads with immune complexes induces endolymphatic hydrops. Also, deposition of the sepharose beads itself may have induced hydrops together with enhancement of the SP. SP enhancement may be related to the development of endolymphatic hydrops rather than to the presence of hydrops as such. This may be based on pressure build-up while hydrops develops.

Effect of artificial endolymph injection into the cochlear duct on the endocochlear potential

We investigated the effect of acute endolymphatic hydrops on the positive endocochlear potential (+EP) and negative endocochlear potential (-EP). The +EP was measured in guinea pigs during injection (without outlet) and perfusion (with outlet) of artificial endolymph into the cochlear duct. The -EP was measured during anoxia after the injection or the perfusion had finished. Injection of artificial endolymph produced a slight transient increase in the +EP, and a significant decrease in the magnitude of the -EP. Chronic endolymphatic hydrops produces both +EP and -EP decrease. The +EP decrease in chronic endolymphatic hydrops may cause the chronic change of the inner ear. The +EP increase in acute endolymphatic hydrops may be caused by a shift of the basilar membrane. However, the mechanism of the 'transient' +EP increase is not clear. The -EP decrease was not observed in animals whose cochlear duct was perfused with artificial endolymph. Therefore, the artificial endolymph itself did not cause the decrease in magnitude of the -EP. Dysfunction of the hair cells is a possible explanation for the -EP decrease but the mechanism of such a decrease is not clear in the present study. However, the results of this study support the notion that small increases in endolymphatic pressure below the resolution of recent measurements (DeMott and Salt, 1997) can lead directly to a reduction of the -EP during hydrops. The animal model described here can eliminate the chronic effect of hydrops, therefore, this model is useful for investigations into the effect of hydrops itself on the inner ear and the mechanism of hearing loss in Ménière's disease.

Electrocochleographic evaluation of the guinea pig model of endolymphatic hydrops

Electrocochleography (ECochG) was used to evaluate cochlear function in guinea pigs with experimentally induced endolymphatic hydrops (ELH) before and after osmotic dehydration with either glycerol or urea. We surgically induced ELH in the right ears of 9 guinea pigs, while the right ears of 6 guinea pigs received a sham operation. The left ears of the 15 animals constituted the normal group. Eight weeks after surgery, summating potential (SP) and action potential (AP) amplitudes were measured prior to and following the administration of glycerol or urea. The SPs and SP/AP ratios were reduced in all groups, with no significant differences among groups or between dehydrating agents. Some of the hydropic ears, however, did show an increased AP threshold and a recruitment effect. In measurements from 6 additional animals, serum osmolarity increased more with urea than with glycerol. The guinea pig model remains valuable for investigation of ELH, even though it differs in significant respects from ELH in humans.

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.

Vascular permeability of the stria vascularis and morphology of the endolymphatic sac in two types of experimental endolymphatic hydrops

We produced two kinds of experimental endolymphatic hydrops. One was induced by obliteration of the endolymphatic sac (ES), and the other was induced by both systemic keyhole limpet hemocyanin immunization and secondary keyhole limpet hemocyanin challenge into the ES. The vascular permeability of the stria vascularis in acute and chronic phases was compared between both models by light and electron microscopy using the tracer method of horseradish peroxidase. Both models showed that the number of strial capillaries showing horseradish peroxidase leakage was significantly higher in the acute phase than in the chronic phase. In the acute phase, the ES showed acute inflammation in both models. In the chronic phase, extensive fibrosis occurred in the ES obliteration model, whereas the ES appeared to have a normal shape in the immunologically induced model. The mechanism of hydrops formation will be compared between the two models.

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.

Low-frequency modulation of compound action potential in experimental perilymphatic fistula and endolymphatic hydrops

We have tested the hypothesis that the cause of cochlear dysfunction associated with perilymphatic fistula (PLF) is closely related to endolymphatic hydrops (ELH). Using guinea pigs, we studied the tone-burst elicited compound action potential (CAP) and its modulation as caused by a 50 Hz biasing tone in experimental PLF. We compared these results with those of experimental ELH. Following perilymph aspiration through the perforated round window membrane, mild but significant elevations of CAP thresholds at tested frequencies were found. A reduction in the amplitude of cochlear microphonics (CM) for a 50 Hz sine wave appeared to correlate with these CAP threshold changes. However, there were no significant changes in the modulation effect of the 50 Hz biasing tone on the CAP elicited by an 8 kHz tone burst. This finding differed from that in ears with experimental ELH, in which significant reductions of both 50 Hz CM and the degree of CAP modulation were consistently observed. We concluded that it is unlikely that the underlying mechanisms of a modification to the low frequency response of the base of the cochlea following perilymph aspiration is linked to that of experimental ELH.

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.

The influence of noise exposure on endolymphatic hydrops. An experimental study

The influence of noise exposure on endolymphatic hydrops was examined morphologically. Endolymphatic hydrops was produced experimentally in guinea pigs by means of endolymphatic sac obliteration. Approximately 2 months later, the animals were subjected to noise exposure for 2 h and, as a result, the following findings were obtained: Vascular and circulatory disturbances in the 3rd and 4th turns of cochlea, i.e., vasoconstriction-like changes in the upper coiled arterioles and capillaries in the stria vascularis, as well as blood cell sludge. In addition, the outer hair cells in Corti's organ at the same turns disappeared sporadically. No changes were observed in the opposite, control, ear of the same animal which was free from endolymphatic hydrops. On the basis of these results, it is argued that endolymphatic hydropic ears are highly vulnerable to noise exposure.

Transport of HRP through Reissner's membrane in experimental endolymphatic hydrops

Unilateral endolymphatic hydrops was produced in guinea pigs by cauterization of the endolymphatic sac. Measurements of compound action potential (CAP), cochlear microphonics (CM) and negative summating potential (-SP) confirmed endolymphatic hydrops three months after surgery. In both control and hydropic ears, reaction product of HRP was observed only on the perilymphatic surface of the epithelial cells of Reissner's membrane after 10 min perfusion, while it was observed on both the endolymphatic and perilymphatic surfaces after 30 min perfusion. Epithelial tight junctions were not stained and labelled pinocytotic vesicles were observed in the epithelial cells. These findings suggest that the transport of HRP through Reissner's membrane is unchanged in endolymphatic hydrops and that the epithelial junctions are tight regardless of the distension of Reissner's membrane.

Quantification of the relation between electrophysiologic and morphologic changes in experimental endolymphatic hydrops

The degree of hydrops in guinea pig ears that had undergone unilateral endolymphatic sac and duct obliteration was quantified. For 15 guinea pigs from a prior study, the correlation between the degree of hydrops and several electrophysiologic measures of the functional state of the cochlea was determined. A significant correlation between the degree of hydrops and a reduction in the low-frequency cochlear microphonic evoked by a 29-Hz tone was found. No correlation was found between the degree of hydrops and the magnitude of the summating potential evoked by 8-kHz probes. A weak correlation was found between the degree of hydrops and a reduction in the compound action potential evoked by 2-kHz probes. Possible differences in the way the endolymphatic duct obliteration exerts its effect on the different cochlear potentials are discussed.

Modulation of cochlear tuning by low-frequency sound

An intense, low-frequency tone (about 30 Hz) modulates the sensitivity of the inner ear to high-frequency stimulation. This modulation is correlated with the displacement of the basilar membrane. The findings suggest that the modulation may also affect cochlear tuning. We have investigated modulation of cochlear tuning by low-frequency sound in the guinea pig. Applying indirect methods of measurement (narrow-band analysis of compound action potentials and compound-action-potential tuning curves), the results suggest a shift of the excitation pattern along the basilar membrane towards higher-frequency areas. The shift occurred for both scala tympani and scala vestibuli displacement of the cochlear partition. Tuning curves, obtained from single units in the cochlear nerve, show sensitivity loss and a tip shift towards lower frequencies. This was also found for both scala tympani displacement and scala vestibuli displacement. The shift of the tip of the tuning curve towards lower frequencies corresponds to the inferred high-frequency shift of the excitation pattern. The relationship of these phenomena with the pathophysiology of Ménière's disease and with possible active mechanisms in cochlear transduction is discussed.

The Preyer reflex--an easy estimate of hearing function in guinea pigs

Preyer reflex thresholds elicited monaurally by tone bursts from 0.25 to 8 kHz were determined in more than 150 guinea pig ears. Normal reflex thresholds were between 85 and 95 dB SPL in the low and middle frequencies, decreasing to 75 dB in the higher frequencies. The range of measurements at single frequencies usually did not exceed 10 dB. In unselected populations of adult guinea pigs, about half of the animals had increased reflex thresholds--at least unilaterally, in most cases due to middle ear infections. The Preyer reflex showed parallel threshold increase with compound action potential thresholds in conductive loss and recruitment in cochlear hearing loss. Normal Preyer reflex thresholds do not necessarily mean normal hearing, but increased thresholds do indicate hearing impairment. This technique is valuable in the selection and monitoring of animals for otologic experiments.

Cochlear blood flow in endolymphatic hydrops

There is no animal model for Meniere's disease but by obliteration of the endolymphatic duct, endolymphatic hydrops may be achieved in several animal species. In order to measure the cochlear blood flow in ears with endolymphatic hydrops the endolymphatic duct was obliterated in 9 guinea pigs. The blood flow was measured with the microsphere method and the cochlear histology was studied. The regional and total blood flow was determined in the serially sectioned cochleas 2, 4 and 8 months after obliteration of the endolymphatic duct. No change in regional or total cochlear blood flow was observed in the hydropic ears.

Ultrastructure of the organ of Corti in experimental hydrops

The early ultrastructural changes of the organ of Corti in the guinea pig after obliteration of the endolymphatic sac and duct were examined in a time-sequence study. Initial loss of outer hair cells was followed by inner hair cell degeneration, both starting at the apical part of the cochlea. Morphological changes of the sensory cells were found to start at the endolymphatic surface and were characterized by a variety of sterociliary pathologies, distortion of the cuticular plate and a less prominent contrast-staining of glycocalyx. Further degeneration was marked by pathological changes of intracellular organelles and the afferent and efferent nerve endings. Details of the ultrastructural changes in the organ of Corti are discussed with regard to the pathophysiology of experimental hydrops.

Electrocochleographic changes in relation to cochlear histopathology in experimental endolymphatic hydrops

The relation between electrocochleographic and histological changes in experimental endolymphatic hydrops was studied 1, 2, 4, or 8 months after obliteration of the endolymphatic sac. An increase in the compound Action Potential (AP) threshold was found 2, 4, and 8 months after obliteration. This increase was strongly correlated with loss of outer hair cells, nerve fibres and spiral ganglion cells. An enhanced negative Summating Potential (SP) and an enhanced SP-AP ratio were found mainly in animals with an endolymphatic hydrops without further cochlear pathology. A normal or decreased SP and SP-AP ratio was regularly recorded in animals with both an endolymphatic hydrops and a variety of other histopathological changes in the inner ear. An increased second peak (N2) in the AP waveform was recorded from 63% (15/24) of the hydropic ears, equally divided over the four groups. There was no obvious correlation between the increased N2 and other electrophysiological or histological findings.

Cochlear potentials and their modulation by low-frequency sound in early endolymphatic hydrops

Seventeen guinea pigs were unilaterally operated to produce endolymphatic hydrops. After 2 wk (9 animals) or 4 wk (8 animals), extracochlear electrophysiological responses to tone bursts of several frequencies were recorded in both the operated and non-operated ears. In addition, modulation by low-frequency (29 Hz sinusoidal bias) sound of the responses to 8 kHz tonebursts was measured. After the electrophysiological measurements, the animals were killed and examined histologically. Four weeks after the operation, cochlear microphonics in response to a 500 Hz tone burst and to the 29 Hz bias were significantly smaller in the operated ears. The summating potential showed a tendency to be larger in the operated ears. The compound action potential input-output curves for 2 kHz probes showed a small threshold shift accompanied by steep slopes, reminiscent of recruitment. Modulation of summating potentials by the low-frequency bias was smaller on the operated side. In most cochleae an endolymphatic hydrops was observed. Three cochleae showed a collapse of Reissner's membrane.

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

Scanning electron microscopy was employed to investigate hair cell morphology at different stages in the development of experimentally induced hydrops in the guinea pig. A particular form of morpho-pathology, never before described, was identified as characteristic of hydropic cochleas. The pathology was characteristically identified as atrophy of the short and middle stereocilia on the outer hair cells while the inner hair cell stereocilia did not have such a pathology. The atrophy was restricted to the upper cochlear turns in remarkable correspondence with the low/middle frequency sensitivity loss and was detected only at the end of the period of fluctuating thresholds. These stereocilia perturbations appear therefore to be linked with the threshold fluctuations and represent the first evidence for a clear correlation between hair cell morphology and physiology in the experimental model of endolymphatic hydrops. Such a morphopathology might also be expected to occur in cochleas of Menière's patients but may have been overlooked in the past because of the discrete nature of the pathology.

Electrocochleographic study of experimentally induced endolymphatic hydrops

An endolymphatic hydrops was induced in the left ear of each of 32 guinea pigs by obliteration of the endolymphatic sac. Both the hydropic ear and the control ear were examined by electrocochleography 1, 2, 4 or 8 months after obliteration of the sac using electrodes on the apex and near the round window. One month after obliteration, the threshold of the compound action potential (AP) and the increase of the AP amplitude with sound pressure level (SPL) were the same as the results recorded from the control ears despite the presence of a histologically confirmed hydrops. In the 2-month group, small AP threshold differences (10-20 dB) were recorded, increasing up to 10-40 dB in the 4- and 8-month groups. In the latter animals we also found a more rapid increase of the AP amplitude with SPL, a finding suggestive of recruitment. In the 1-month group we recorded an enhanced negative SP for 2- and 4 kHz stimuli when the electrode was placed near the apex. The same was recorded for 4-kHz stimuli in the 2-month group. In the 4- and 8-month groups there was a tendency towards a decrease in the negative SP. The SP-AP ratio recorded from the apical position showed the same pattern as the SP amplitude, an increase after 1 month and a tendency to decrease in the following months. Near the round window there was no significant effect of hydrops on the SP amplitude or on the SP-AP ratio. A deviation in the AP-SP wave form was recorded in 69% (22/32) of the guinea pigs.(ABSTRACT TRUNCATED AT 250 WORDS)