Menière's disease: pathology and manifestations

A possible mechanism of the formation of endolymphatic hydrops and its associated inner ear disorders

The pathology of Meniere's disease (MD) is well established to be endolymphatic hydrops. However, the mechanism underlying deafness and vertigo of MD or idiopathic endolymphatic hydrops is still unknown. In order to evaluate the pathogenesis of deafness and vertigo in MD, it seems to be rational to investigate the interrelationship between hydrops and inner ear disorders using animals with experimentally-induced endolymphatic hydrops. In spite of intense efforts by many researchers, the mechanism of vertiginous attack has been unexplained, because animals with experimental hydrops usually did not show vertiginous attack. Recently, there are two reports to succeed to evoke vertiginous attack in animals with experimental hydrops. In the present paper were first surveyed past proposals about underlying mechanism of the development of hydrops and inner ear disorders associated with hydrops, and were discussed the pathogenetic mechanism of vertiginous attack in hydrops. In conclusion, abrupt development of hydrops was thought to play a pivotal role in the onset of vertiginous seizure.

Recent and Frequent Vertigo Attacks Produce Negative Findings on Furosemide-Loading Vestibular Evoked Myogenic Potential Testing in Meniere's Disease

Objective: The peak-to-peak amplitude of the p13-n23 wave in cervical vestibular evoked myogenic potential can increase after furosemide administration in patients with Meniere's disease [furosemide-loading VEMP (FVEMP) testing]. The examination is used to test for the presence of endolymphatic hydrops; we investigated factors that may influence the results.

Methods: Forty-two subjects (23 males and 19 females, aged 24–70 years) with unilateral definite Meniere's disease who underwent FVEMP testing were retrospectively studied. Possible factors associated with the results of FVEMP testing were studied using logistic regression analysis.

Results: Ages, sex, affected side, stage, disease duration, and mean hearing level of pure tone audiometry did not influence the results of FVEMP testing in the univariate analysis (p > 0.05). Number of days since the last vertigo attack [odds ratio (OR): 1.07, p = 0.031] and frequency of vertigo attacks per month (OR: 0.42, p = 0.003) were significantly associated with the results of testing. Multivariate analysis showed that both days since the last vertigo attack < 7 (OR: 0.13, p = 0.04) and frequency of vertigo attacks per month ≥ 2 (OR: 0.06, p = 0.004) were risk factors for negative results on FVEMP testing.

Conclusion: This study found that recent and frequent vertigo attacks produced negative findings on FVEMP testing in Meniere's disease. This apparently irrational finding can be explained by the consequences of membranous labyrinth rupture during vertigo attacks, where the altered saccular resonance due to EH cannot be recovered by furosemide administration because of the dissolving dehydration effect that occurs through communication between the endolymphatic and perilymphatic spaces. In addition, the impairment of sensory cells that is caused by endolymph and perilymph mixing upon rupture does not improve upon furosemide administration. FVEMP testing results may provide us with pathophysiological information regarding the membranous labyrinth.

Multiple indices of the 'bounce' phenomenon obtained from the same human ears

Loud low-frequency sounds can induce temporary oscillatory changes in cochlear sensitivity, which have been termed the 'bounce' phenomenon. The origin of these sensitivity changes has been attributed to slow fluctuations in cochlear homeostasis, causing changes in the operating points of the outer hair cell mechano-electrical and electro-mechanical transducers. Here, we acquired three objective and subjective measures resulting in a comprehensive dataset of the bounce phenomenon in each of 22 normal-hearing human subjects. We analysed the level and phase of cubic and quadratic distortion product otoacoustic emissions and the auditory thresholds before and after presentation of a low-frequency stimulus (30 Hz sine wave, 120 dB SPL, 90 s) as a function of time. In addition, the perceived loudness of temporary, tinnitus-like sensations occurring in all subjects after cessation of the low-frequency stimulus was tracked over time. The majority of the subjects (70 %) showed a significant, biphasic change of quadratic, but not cubic, distortion product otoacoustic emissions of about 3-4 dB. Eighty-six percent of the tested subjects showed significant alterations of hearing thresholds after low-frequency stimulation. Four different types of threshold changes were observed, namely monophasic desensitisations (the majority of cases), monophasic sensitisations, biphasic alterations with initial sensitisation and biphasic alterations with initial desensitisation. The similar duration of the three bounce phenomenon measures indicates a common origin. The current findings are consistent with the hypothesis that slow oscillations of homeostatic control mechanisms and associated operating point shifts within the cochlea are the source of the bounce phenomenon.

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.

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.

Pathology and pathophysiology of Meniere's disease

Histopathologic study of the human temporal bone entails microscopic examination and analysis of a series of histologic sections. This is currently the most effective method for observing the pathologic conditions of MD by examining the entire inner ear. Complete temporal bone histopathology cannot be replaced by either clinical pathologic study of small biopsy specimens obtained during surgery, or experimental animal studies that can create endolymphatic hydrops but not create MD. We believe that the histopathologic findings together with clinical information on MD is valuable in enhancing our understanding of the pathophysiology of the inner ear in MD. For example, a hypoplastic VA and ES in MD seem to indicate that there may be congenital predisposing factors in the development of MD. The exact pathologic findings characteristic of MD remains unclear, however. Many of the temporal bone specimens were obtained years after patients were diagnosed with MD and those specimens were involved with moderate postmortem changes. For these reasons, further collection of temporal bone specimens with fewer postmortem changes, obtained within a shorter premortem time period between occurrence of the disease and the time of the patients' death, and from patients with a well-characterized clinical history of MD, is imperative. Contemporary temporal bone studies now include in situ hybridization histochemistry or polymerase chain reaction (PCR) analysis for protein, enzymes, or viral antigens that can be directed at specimens from patients with MD [54,55]. It is hoped that in the near future such advanced research studies with human temporal bone histology sections will support and enhance the significant contribution of temporal bone histopathology to clinical otology.

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.

Potassium-induced release of an endogenous toxic activity for outer hair cells and auditory neurons in the cochlea: a new pathophysiological mechanism in Menière's disease?

In Menière's disease, the increase of extracellular potassium concentration in the perilymph is thought to play a key role in determining the progressive loss of cochlear hair cells. In this paper, we describe a serum-free culture preparation of hair cells from 5 day-old rat and report the release by the cochlea, in response to an increase of extracellular potassium concentration, of a cytotoxic activity active on hair cells and auditory neurons. The toxic activity is associated with low molecular weight (less than 10,000 Dalton) molecule(s) as revealed by ultrafiltration. Morphological studies performed on the organ of Corti incubated during 24 h in the presence of the cochlea-derived toxic activity (CTA), show that this factor is toxic for hair cells and not for supporting or surrounding cells. The release of CTA occurs both in the spiral ganglion and in the organ of Corti. We suggest that this cochlea-derived toxic activity may play an important role in the pathophysiology of the hearing loss that occurs during the progression of Menière's disease.

Bilateral endolymphatic hydrops in Menière's disease: review of temporal bone autopsies

The wide range in incidence rates reported for bilateral Meniere's disease results mainly from the lack of established clinical criteria for bilaterality, and from follow-ups of varying duration. In order to help determine a reasonable bilateral involvement rate, we examined histologic reports of temporal bones with endolymphatic hydrops. We reviewed the literature on temporal bone autopsies from 1938 to 1988, and selected 72 cases for histologic examination. Of the 72 cases, 67 (93%) showed endolymphatic hydrops, 2 (2.8%) showed collapse, and 3 (4.2%) showed membranous labyrinths of normal appearance. Twenty of the 67 endolymphatic hydrops cases (29.9%) displayed bilateral involvement, suggesting a rate of bilaterality in Meniere's disease of approximately 30%.

Ultrastructural findings in a case of Menière's disease

The temporal bones of an individual with documented unilateral Meniere's disease were prepared for light and electron microscopy. A morphometric analysis was performed on hair cells, spiral ganglion cells, dendritic fibers in the osseous spiral lamina, afferent and efferent endings, and afferent synaptic contacts. In the ear with Meniere's disease, we found hair cell damage, including disruption of the cuticular bodies and basalward displacement of some outer hair cells. There was no significant difference in the number of hair cells or spiral ganglion cells on the two sides. There was a significant decrease, however, in the number of afferent nerve endings and afferent synapses at the base of both inner and outer hair cells in the ear with Meniere's disease as compared to the contralateral ear.

Investigations into the cause of canal paresis in Menière's disease

Since the literature has a paucity of documented lesions of the vestibular hair cells and neurons in Menière's disease, the cause of canal paresis remains unexplained. A clinicopathological correlation was sought and demonstrated between ampullary distortion of lateral canal and reduced caloric response. The findings in this investigation make a strong case for ampullary distortion disrupting the tenuous but vital cupulary attachment resulting in canal paresis. This concept is attractive because it explains an important clinical event in Menière's disease on the basis of the only consistent histopathological finding, namely, membrane distension.

Fistulae in the membranous labyrinth

Fistulization at various parts of the membranous labyrinth has been currently in use as one of the treatments for the symptoms of Meniere's disease. The present report concerns our experience with membranous fistulae in experimental animal ears with and without manifestation of endolymphatic hydrops. The materials presented are mostly new; however, our previous data are also incorporated in order to bring this issue into better perspective. The first part describes the nonproduction aspect of endolymphatic hydrops in the rat, chinchilla, and squirrel monkey after obliteration of the endolymphatic duct in relation to the occurrence of spontaneous fistulae. The second part describes surgical fistulization of the vestibular membranous labyrinth and cochlear duct in order to prevent or control development of endolymphatic hydrops in the guinea pig, the species in which hydrops can be produced consistently. These results are tabulated and analyzed in terms of the pathological consequences that can be expected in performing fistulization of the membranous labyrinths in animals as well as in humans.

Endolymphatic hydrops in the rabbit: auditory brainstem responses and cochlear morphology

A rabbit model of endolymphatic hydrops was studied using detailed functional and cytohistologic methods. Immediately following surgical destruction of the endolymphatic sac and the distal portion of the duct, measures of the evoked auditory brainstem response (ABR) revealed mild to profound losses specific to low- and high-frequency test stimuli while responses to mid-frequency signals remained unchanged for the majority of animals. Rabbits exhibited varying degrees of vestibular upset involving both overt behavior and reduced responses to caloric stimulation. Histologic processing of the plastic embedded cochleae demonstrated distended Reissner's membranes along with extensive damage to apical and basal turn sensory cells and myelinated afferent nerve fibers while the middle portion of the cochlear duct remained relatively unaltered. An atypical pattern of hair cell lesions involving a greater loss for inner than for outer hair cells was identified at the interface between damaged apical sensorineural elements and the normal appearing organ of Corti of the middle turns.

Pathophysiology of the Meniere attack: facts and theories

A pathophysiological interpretation is discussed of the clinical findings that in one out of three cases at the onset of Meniere's disease the cochlear and vestibular symptoms start simultaneously and that during the course of Meniere's disease most attacks show both cochlear and vestibular symptoms. It is concluded that the Meniere attack is not caused by a slowly expanding endolymphatic system that finally ruptures or leaks, but that the pathogenesis of the attack is different from that of the underlying endolymphatic hydrops.

Membranous ruptures in Meniere's disease: existence, location, and incidence

Whether or not attacks of Meniere's disease are related to ruptures in the membranous labyrinth is still a point of contention. To help clarify this point, we used light microscopy to study the membranous labyrinth in 14 temporal bones with hydrops from patients with Meniere's disease, four bones with hydrops from patients without Meniere's disease, and 11 normal bones. Findings suggest that ruptures are specific to bones with hydrops from patients with Meniere's disease and that they occur more frequently in Reissner's membrane than in the vestibular membranes.

Utricular and saccular volumetry in human temporal bones

Using serially sectioned human temporal bones, endolymphatic volumes of utriculus and sacculus were measured with the aid of a computer. The mean of the utricular volume was 8.187 mm3, and the saccular volume, 2.096 mm3. The former was 3.9 X the latter. The surface area of the macula utriculi was 3.271 mm2, and the macula sacculi was 2.188 mm2. The former was 1.5 X the latter. The study using pathologic temporal bones (Meniere's disease, otosclerosis, and otitis media chronica) showed that the utricular volume could indeed increase, and the saccular membrane was found to be more fragile than the utricular membrane in those diseased conditions.

Cochlear nerve fiber populations in 25 patients with profound deafness due to Menière's disease or sudden deafness

Surgical biopsy specimens of the cochlear nerve from 19 patients with Menière's disease and six patients with sudden deafness were studied. The total number of myelinated nerve fibres in the transverse fascicular area of each nerve were counted and the results were correlated with the clinical and preoperative audiological data. The total number of cochlear neurons in patients with Menière's disease ranged from 5,760 to 27,722 and in patients with sudden deafness from 8,500 to 22,714. The long duration of the disease or absent speech discrimination appeared not to be connected with lower numbers of neurons than preserved speech discrimination or short history of the disease.

Menière's disease and endolymphatic hydrops: clinical-histopathological correlations

The clinical-histopathological correlation between Menière's disease and endolymphatic hydrops was done to explain, as much as possible, causes of symptoms of Menière's disease. Twenty-three temporal bones with endolymphatic hydrops from 17 patients were reviewed and clinical and histopathological findings were correlated. Histopathological examination revealed frequent, severe deformities in the labyrinthine walls and permanent changes in Reissner's membranes. Evidence of rupture was difficult to assess. Of 21 ears of patients with the clinical diagnosis of Menière's disease, 93% had endolymphatic hydrops. A statistical correlation between increased area of the cochlear duct and hearing loss was found. Some correlation was also found between frequency of vertigo and results of electronystagmography with histopathological findings. Consequently, the mechanical effect of endolymphatic hydrops seems to have greater significance in the production of symptoms of Menière's disease than the biochemical effect of ruptures. In a review of ten unusual cases from this series of 17 patients, traumatic neuromas or remnants of vestibular structures were found after an incomplete labyrinthectomy. Histopathological findings of four patients who had undergone endolymphatic subarachnoid shunt surgery are described.

[Unilateral deafness (author's transl)]

Unilateral conductive deafness has a readily determined aetiology in most cases. In contrast, unilateral sensorineural hearing loss requires more refined and extensive investigation. The most frequent causes of unilateral sensorineural hearing loss in our patients were sudden deafness, Menière's disease, cranio-vertebral dysplasia and cerebellopontine angle tumors. Early diagnosis of acoustic neuroma or other lesions of the internal auditory meatus or cerebellopontine angle requires special attention. The definitive diagnosis of these tumors often demands intensive clinical investigation but a high degree of suspicion may be entertained following modern routine audiometry, vestibular function testing and radiological examination. The importance of early diagnosis of these lesions is stressed with regard to the mortality rate for larger tumors and to the preservation of facial nerve function. The reliability of different diagnostic investigations is documented.

Transtympanic electrocochleography in patients with syphilis and hearing loss

Transtympanic electrocochleography was carried out on 18 syphilitic patients (30 ears were tested) most of whom were suffering from the late onset congenital form of the disease. A diphasic action potential with a large negative summating potential on the descending limb was found in 77.7% of ears; the cochlear microphonic potential was always of small amplitude. While these findings are not pathognomonic of syphilis, they are characteristic and may be explained on pathological grounds.

Current status of surgical decompression and drainage procedures upon the endolymphatic system

At the present time, neither the etiologic factors nor the pathophysiologic mechanisms that lead to the development of endolymphatic hydrops or Ménière's disease are known; however, the anatomic-pathologic alterations in the labyrinth that are produced by the disease are well-documented. Undoubtedly, what is not known is responsible for the controversial status and questionable benefit that surgical decompression and drainage procedures have upon the endolymphatic system.

Unilateral endolymphatic hydrops and associated abnormalities

A case of unilateral endolymphatic hydrops is presented in which several significant histopathological findings are observed in the affected inner ear and are absent in the opposite ear. Moderate endolymphatic hydrops is present in all cochlear turns on the involved side. Atrophic changes in the cochlear and vestibular end-organs, partial collapse and infolding of Reissner's and the saccular membranes, and collapse of the posterior canal membrane are also seen. There is also marked narrowing of the proximal rugose portion of the endolymphatic sac, associated with a flattened, epithelial lining, dense fibrotic connective tissue, brown pigment deposition, and poor vascularity in a relatively narrowed and straightened vestibular aqueduct. Fibrosis, loss of vascularity, and brown pigment deposition are also observed in the bony channels surrounding the vestibular aqueduct.