Contribution of increased endolymphatic pressure to hearing loss in experimental hydrops

On the Relationship Between Menière's Disease and Endolymphatic Hydrops

: The relationship between Menière's disease and endolymphatic hydrops is ambiguous. On the one hand, the existence of cases of endolymphatic hydrops lacking the classic symptoms of Menière's disease has prompted the assertion that endolymphatic hydrops alone is insufficient to cause symptoms and drives the hypothesis that endolymphatic hydrops is a mere epiphenomenon. Yet, on the other hand, there is considerable evidence suggesting a relationship between the mechanical pressure effects of endolymphatic hydrops and resultant disordered auditory physiology and symptomatology. A critical appraisal of this topic is undertaken, including a review of key histopathologic data chiefly responsible for the epiphenomenon hypothesis. Overall, a case is made that A) the preponderance of available evidence suggests endolymphatic hydrops is likely responsible for some of the auditory symptoms of Menière's disease, particularly those that can be modulated by mechanical manipulation of the basilar membrane and cochlear microphonic; B) Menière's disease can be reasonably considered part of a larger spectrum of hydropic inner ear disease that also includes some cases that lack vertigo. C) The relationship with endolymphatic hydrops sufficiently robust to consider its presence a hallmark defining feature of Menière's disease and a sensible target for diagnostic detection.

Effects of a Nitric Oxide Synthase Type II Inhibitor on Compound Action Potential Thresholds in Experimental Endolymphatic Hydrops

HYPOTHESIS

Nitric oxide (NO) is likely to be synthesized by nitric oxide synthase Type II (NOS II) action and may partake in the origin of changes of compound action potential (CAP) threshold observed in guinea pigs with induced endolymphatic hydrops. This study aimed to assess the action of a NOS II inhibitor on CAP thresholds in these experimental samples.

BACKGROUND

In guinea pigs with experimental endolymphatic hydrops, there are lesions on the cochlea and progressive increase of CAP threshold. NOS II was found in the cochlea of this animal model, and it was inferred that NO can contribute by such alterations.

METHODS

The animals were divided into two groups, in which eight received an intake of a NOS II inhibitor, aminoguanidine, and another eight served as a control group. During 16 weeks, CAP thresholds at 1,000, 2,000, 4,000 and 6,000 on electrocochleography were compared between the groups.

RESULTS

The group that had an intake of aminoguanidine showed a lower increase on CAP thresholds at 2,000 (p < 0.05) and 6,000 Hz (p < 0.05) at the 12th postoperative week, and at 1,000 (p < 0.05), 2,000 (p < 0.001), 4,000 (p < 0.001), > and 6,000 Hz (p < 0.001) at the 16th week.

CONCLUSION

We conclude that NOS II inhibitor reduced the elevation of CAP thresholds in experimentally induced endolymphatic hydrops.

Experimental endolymphatic hydrops under action of a type II nitric oxide synthase inhibitor: otoacoustic emissions evaluation and electrocochleography

In experimental endolymphatic hydrops distortion-products otoacoustic emission (dpoae) amplitudes decrease and there is elevation on electrocochleographic thresholds. Some authors found type ii nitric oxide synthase (nos ii) expression in hydropic cochleas and they suggest nitric oxide (no) may be involved in endolymphatic hydrops pathogenesis. The aim of this study was to evaluate the action of a nos ii inhibitor on dpoae and electrocochleography in experimental endolymphatic hydrops. Material and methods: endolymphatic hydrops was induced in 16 guinea pigs by obliterating the endolymphatic duct and sac in the right ear. They were divided in two groups: eigth guinea pigs under the action of aminoguanidine, a nos ii inhibitor and eigth control guinea pigs. We compared dpoae amplitudes at geometric means of frequencies 1062, 2187, 4375 and 7000 hz, compound action potential threshold at 1000, 2000, 4000 and 6000 hz and summating potential to action potential (sp/ap) ratio between the groups during the postoperative observation period of 16 weeks. Results: there were no significant changes in the dpoae amplitudes and in the sp/ap ratio. The group that received aminoguanidine had a lower degree of threshold increase at 2000 (p<0.05) And 6000 hz (p<0.05) In 12th postoperative week and at 1000 (p<0.05), 2000 (P<0.001), 4000 (P<0.001) And 6000 hz (p<0.001) At 16th postoperative week. Conclusions: nos ii inhibitor decreased the electrocochleography threshold elevation on experimental endolymphatic hydrops.

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.

The peaked audiometric configuration in Meniere's disease: disease related?

A retrospective analysis of audiograms was completed for persons from 3 different patient groups. These 3 groups were (1) persons with unilateral Meniere's disease, (2) persons with unilateral acoustic tumor, and (3) persons from the general clinical population of an audiology clinic in a major medical center. As Paparella, McDermott, Luiz, and de Sousa (1982) also report, the most common audiometric configuration in the Meniere's disease group was peaked, a condition in which poorer hearing is reported in the low and high frequencies, and an island of better hearing sensitivity occurs at either 1.0 or 2.0 kHz. Some of the peaked audiograms from persons in our Meniere's disease group could be a result of a low-frequency hearing loss caused by Meniere's disease combined with a high-frequency hearing loss due to aging or other environmental factors. However, 27% (13/48) of the peaked audiograms in ears with Meniere's disease were found in persons with no hearing loss in the high frequencies of their nondiseased ear. Thus, the peaked audiometric configuration is associated with Meniere's disease, as suggested by research results from animals with experimentally induced endolymphatic hydrops. Peaked audiometric configurations were also observed in roughly 9% of the general clinical population and in 12.5% of ears of persons with acoustic tumors. A rule--based on the audiometric configurations that maximized identification of patients with Meniere's disease--using the general clinical population as a control did not fare as well when the tumor group was used as a control. This finding provides further evidence that peaked audiograms, although common to Meniere's disease, are not pathognomonic of this disease. Furthermore, these results indicate that the diagnosis of Meniere's disease from audiometric profiles is risky, at best.

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.

Auditory and vestibular function in experimental hydrops

Endolymphatic hydrops is the characteristic morphopathology observed at postmortem examination of temporal bones of Meniere's disease. Surgical induction of endolymphatic hydrops in the guinea pig provides a reproducible animal model for the investigation of some effects of endolymphatic hydrops on inner ear function. Episodic and incapacitating vertigo are classic features of Meniere's disease, although very limited vestibular dysfunction has ever been described for the animal model. On the other hand, the auditory deficits peculiar to Meniere's disease, including fluctuant sensitivity losses and a peak audiogram, are also observed at different stages in the development of experimental hydrops. In future studies the model is likely to provide a means of investigating osmoregulatory function of the inner ear.

Immediate and long-term effects of daily glycerol administration on guinea pig hydropic ears

Oral administration of glycerol at 0.5 g/kg and 1.0 g/kg body weight in the guinea pig with experimentally induced hydrops provoked no obvious modifications of the CAP audiogram within a 3-h recording session. Daily administration of glycerol at the same dose level during a 4-month period did not modify the deterioration of CAP thresholds although some limited beneficial effect was detected for some animals after 8 weeks of treatment.

Osmotically induced pressure difference in the cochlea and its effect on cochlear potentials

The electrophysiological effects observed during scala tympani displacements in low-frequency biasing experiments, an increase of the summating potential (SP) together with a decrease of the compound action potential (CAP), correlate well with the effects found in guinea pigs with evoked endolymphatic hydrops. This contributes to the hypothesis that displacement of the basilar membrane underlies the changes found in endolymphatic hydrops. A major difference between both experimental situations is that in low-frequency biasing the basilar membrane is continuously moving, whereas in hydrops the hypothesized displacement would be static. To evaluate the importance of this difference, experiments were performed which attempted to evoke a static displacement of the basilar membrane by perfusing the perilymphatic spaces with perfusates of various osmolalities. Perfusion with hypotonic perfusate (183 mOsm/kg) increased the SP and decreased the CAP (4 kHz stimulation) whereas perfusion with a hypertonic perfusate (397 mOsm/kg) decreased both these potentials. The cochlear microphonics were hardly affected. These data demonstrate that both experimental situations (biasing, i.e. dynamic displacement and osmotic pressure, i.e. static displacement) cause similar changes in the SP and the CAP and the data support the hypothesis that basilar membrane displacement towards scala tympani is an important contributing factor to the electrophysiologic changes in endolymphatic hydrops.

Experimental endolymphatic hydrops and glycerol. Electrophysiologic study

The object of the present study was to evaluate electrophysiologically the therapeutic effect of glycerol on the cochlear function of guinea pigs in which experimental hydrops had been surgically induced. Fifteen albino guinea pigs were used. Each animal was chronically implanted to follow the evolution of hearing. The experimental protocol considered 3 groups of guinea pigs (5 animals each). Group 1 received no drug treatment, while groups 2 and 3 were given glycerol orally at 0.75 and 0.50 g/kg of body weight once a day for 4 months. The audiograms of the animals treated with glycerol showed a statistically significant hearing improvement compared with the control group. Clinically, our results seem to support the view of those who favor the use of glycerol for the medical treatment of Meniere's disease at an early stage.

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.