Summating potential and water balance in Meniere's disease

Assessment Tools for Use in Patients with Ménière Disease: An Update

A number of electrophysiological tests have been proposed for the initial diagnostic assessment or for the follow-up phase of patients affected by Ménière disease. The most common are: (i) vestibular evoked myogenic potentials (VEMPs); (ii) electrocochleography (ECochG); and (iii) otoacoustic emissions (OAEs). This paper presents the latest clinical developments with these 3 testing modalities. The PubMed, Embase, and Cinahl databases were searched from 2006 to December 2016. Full-text articles were obtained in cases where the title, abstract, or key words suggested that the study may be eligible for this review. The medical subject heading (MeSH) terms included the following: Ménière, hearing threshold, vestibule, otoacoustic emissions, inner ear, ECochG, VEMPs. There were 368 identified papers, out of which 87 were eligible for inclusion. Overall the data in the literature are still limited and the recommended procedures have not reached an international consensus. From the available data, one can conclude that none of the electrophysiological tests could be considered as pathognomonic, for the diagnosis of Ménière disease: presently, the tests could be mostly used in a supportive role to the clinical diagnosis. Hopefully, in the future, improved technology in electrophysiological testing could contribute to the development of better strategies for the diagnosis of Ménière disease.

Tone Burst Electrocochleography for the Diagnosis of Clinically Certain Meniere's Disease

The technique of transtympanic electrocochleography was initially developed as an objective hearing threshold test by Eggermont. Gibson et al. (1977) claimed that an enlarged direct current component of the action potential (AP) called the summating potential (SP) is an indication of endolymphatic hydrops, later confirmed by Coates who proposed an SP/AP ratio measure. This led to numerous publications using diagnostic ratios of 0.33-0.35. The insensitivity led to an eventual disenchantment with the test as a reliable objective test for Meniere's disease. It was further confused by audiologists employing remote canal or ear drum electrodes which give a response about one-fourth of the magnitude obtained by an electrode in contact with the cochlea. Subsequently Gibson stated that an SP/AP ratio of <0.5 is not diagnostic for hydrops. He then showed that a tone burst stimulus gave the test a significantly higher sensitivity and specificity, which has been supported by others. On MRI inner ear imaging with gadolinium hydrops can be seen, but the quality of images and what is seen may vary according to brand of scanner, settings, mode of gadolinium administration, and the possibility that gadolinium entry may favor the vestibule. Transtympanic tone burst electrocochleography is to date the simplest, cheapest and most sensitive technique for detecting cochlear endolymphatic hydrops to confirm a diagnosis of Meniere's disease.

Ups and Downs in 75 Years of Electrocochleography

Before 1964, electrocochleography (ECochG) was a surgical procedure carried out in the operating theatre. Currently, the newest application is also an intra-operative one, often carried out in conjunction with cochlear implant surgery. Starting in 1967, the recording methods became either minimal- or not-invasive, i.e., trans-tympanic (TT) or extra tympanic (ET), and included extensive studies of the arguments pro and con. I will review several valuable applications of ECochG, from a historical point of view, but covering all 75 years if applicable. The main topics will be: (1) comparing human and animal cochlear electrophysiology; (2) the use in objective audiometry involving tone pip stimulation-currently mostly pre cochlear implantation but otherwise replaced by auditory brainstem response (ABR) recordings; (3) attempts to diagnose Ménière's disease and the role of the summating potential (SP); (4) early use in diagnosing vestibular schwannomas-now taken over by ABR screening and MRI confirmation; (5) relating human electrophysiology to the effects of genes as in auditory neuropathy; and (6) intracochlear recording using the cochlear implant electrodes. The last two applications are the most recently added ones. The "historical aspects" of this review article will highlight the founding years prior to 1980 when relevant. A survey of articles on Pubmed shows several ups and downs in the clinical interest as reflected in the publication counts over the last 75 years.

A mouse model validates the utility of electrocochleography in verifying endolymphatic hydrops

Endolymphatic hydrops (ELH) is a disorder of the inner ear that causes tinnitus, vertigo, and hearing loss. An elevated ratio of the summating potential (SP) to the action potential (AP) measured by electrocochleography has long been considered to be the electrophysiological correlate of ELH-related clinical conditions, such as Meniere's disease, but in vivo confirmation and correlation between an elevated SP/AP ratio and ELH has not yet been possible. Confirming this relationship will be important to show that elevated SP/AP ratio is indeed diagnostic of ELH. Here, we sought to confirm that an elevated SP/AP ratio is associated with ELH and test the hypothesis that severity of ELH and hearing loss would also correlate with the SP/AP ratio in vivo using the Phex(Hyp-Duk)/Y mouse model of postnatal ELH. In addition, we describe a minimally invasive approach for electrocochleography in mice. Auditory brainstem responses and electrocochleography data were collected from controls and Phex(Hyp-Duk)/Y mutants at postnatal day 21 and the mice (all male) were euthanized immediately for cochlear histology. Our results show that (1) the SP/AP ratio was significantly elevated in mice with histological ELH compared to controls, (2) the SP/AP ratio was not correlated with the severity of histological ELH or hearing loss, and (3) the severity of hearing loss correlated with the severity of histological ELH. Our study demonstrates that an elevated SP/AP ratio is diagnostic of ELH and that the severity of hearing loss is a better predictor of the severity of ELH than is the SP/AP ratio.

The effects of high stimulus rate on the electrocochleogram in normal-hearing subjects

The use of high stimulus rates has the potential to improve the electrocochleogram's (ECochG) sensitivity and specificity in endolymphatic hydrops and Menière's disease, but is currently hindered by the absence of an acceptable normative database. In response, this study recorded click-evoked ECochG tracings from 51 normal-hearing subjects (102 ears), between 18 and 60 years of age, at 7.1, 51.1, 101.1 and 151.1 clicks/s using a tympanic membrane electrode. As stimulus rate increased, various statistically significant (p < 0.05) changes were observed. In general, summating potential (SP) latency and amplitude and action potential (AP) latency increased, SP/AP amplitude ratio and waveform width increased but then plateaued, and AP amplitude fluctuated. While providing the largest contribution to a high-stimulus-rate ECochG normative database published to date, potential clinical limitations were identified and a possible solution proposed.

Postural control and glycerol test in Ménière's disease

Ménière's disease is a clinical disorder, characterized by fluctuating hearing loss, recurrent spontaneous episodic vertigo, tinnitus and aural fullness, which may be defined as the idiopathic syndrome of endolymphatic hydrops. The most important test for diagnosis of Ménière's disease is the glycerol test. This is a simple and rapid method and several authors have confirmed its efficiency for identifying endolymphatic hydrops. This test provides information on the cochlear response to the osmotic changes produced by glycerol in the inner ear, whereas modifications in the vestibular labyrinth are usually not evaluated. The aim of this study was to evaluate the effects of glycerol on postural control during attacks of Ménière's disease, and to correlate this data with data on cochlear function. After the glycerol test, an improvement in postural control was recorded in 70% of patients, with all patients reporting a recovery of vertigo. The impairment of postural control during endolymphatic hydrops could be related to a pressure increase in the labyrinth, which interferes with the normal dynamics of the endolymph, and a rapid functional recovery could occur during an osmotic depletion. Dynamic posturography improves the sensitivity of the glycerol test and may therefore be useful in the diagnosis and staging of Ménière's disease.

Rate influences on tone burst summating potential amplitude in electrocochleography: clinical(a) and experimental(b) data

Electrocochleographic recordings of action and summating potentials are widely used in the electrophysiological assessment of endolymphatic hydrops (ELH). Increased amplitudes of the summating potential (SP) in response to tone burst stimuli are indicative of positive ELH. This study reports the effect of repetition rate of tone burst stimulation on the SP amplitude. Using transtympanic electrocochleography (ECochG), the SP in response to 1 kHz tone bursts was recorded in both a Ménière and a non-Ménière population. Absolute values of the SP were systematically higher in the Ménière group. Moreover, in the Ménière and non-Ménière groups, the response amplitudes of the SP at a repetition rate of 8.4 tone bursts/s were only 66 and 32%, respectively, of the maximal response amplitude which was obtained at the rate of 37.4 tone bursts/s. Additionally, in normal guinea pigs chronically implanted with a round window electrode, the SP was recorded to 0.5-16 kHz tone burst stimulations presented at 100 dB SPL with the same different repetition rates. Similar enhancement of the SP amplitude was observed from 8.4 to 37.4 stimuli/s, whatever the frequency. This effect is interpreted as an increased asymmetry of vibration of the cochlear partition, whose mechanical operating point would not return to the normal resting position at high repetition rates, since it is permanently shifted in ELH.

The prognostic value of electrocochleography in severely hearing-impaired infants

This paper presents a longitudinal evaluation of electrocochleographic assessment in severely hearing-impaired infants. Electrophysiological data were obtained by transtympanic electrocochleography to tone-burst stimuli at octave frequencies of 500 to 8000 Hz at the age of 0-6 years in a group of 126 subjects. The results are compared with auditory thresholds determined at school age in the same children by means of pure-tone audiometry. Cochlear microphonics could be recorded in virtually all ears, although the majority of subjects had hearing losses of 90 dB and more. Compound action potentials (CAPs) showed waveforms varying from normal to a wide range of abnormalities. Audiometric thresholds correlated generally well with the compound action potential (CAP) thresholds obtained in infancy. The error in the predicted audiometric thresholds is between 15 and 20 dB, as compared with 11 dB reported for more moderate hearing losses. It is shown that, in spite of the high stimulus levels used, substantial frequency-specific threshold information is retained. Occasional large discrepancies in thresholds were often associated with markedly abnormal response waveforms. Among the many cases in which no ABR could be elicited, 68 per cent produced detectable electrocochleographic responses in the 1000-4000 Hz range. It is concluded that electrocochleography is a valuable method for the assessment of residual hearing in infants suspected of having a severe hearing impairment.

Inner ear pressure in Menière's disease and fluctuating hearing loss determined by tympanic membrane displacement analysis

Menière's disease and fluctuating hearing loss are related to labyrinthine fluid pressure variations. The development of a new indirect method of analysis of the tympanic membrane displacement during the stapedial reflex, using the Marchbanks Measurements System (MMS 10), allows us to study inner ear fluid pressure during these pathological conditions. In this study, measurements with this method were made in four groups of patients: a control group with normal hearing (n=7), stable sensorineural hearing loss (n=9), fluctuating hearing loss (n=8), and Menière's disease (n=25). Results show, first, a good relationship between the recording of negative curves, suggesting a high pressure, and the acute episodes of fluctuating hearing loss; and, secondly, in the case of Menière's disease two types of situation: positive and negative curves suggesting normal and high pressures, respectively.

Sensitivity and specificity of transtympanic electrocochleography in Meniere's disease

Transtympanic electrocochleography (TT ECoG) was conducted in 61 patients (61 ears) with the clinical diagnosis of Meniere's disease and 15 patients (21 ears) with cochlear hearing loss of other aetiologies to assess the ability of the method to separate different cochlear disorders. The click-evoked summation potential/action potential (SP/AP) ratio and the 1 and 2 kHz burst-evoked SP amplitudes were evaluated. Based on the TT ECoG results from 13 normal hearing subjects (15 ears) the upper limit of each variable was determined, above which a value was considered pathological, i.e. indicative of endolymphatic hydrops. The results showed that the sensitivity of the SP/AP ratio was 62% and specificity 95%. When evaluating the combinations of TT ECoG parameters with logistic regression, SP/AP ratio and 1 kHz burst-evoked SP amplitude were shown to increase the discriminating ability of TT ECoG significantly. Inclusion of the 1 kHz burst-evoked SP amplitudes increased sensitivity to 82%, without changing specificity. Inclusion of the 2 kHz burst had no effect on sensitivity or specificity.

The furosemide test and vestibular status in Meniere's disease

A total of 62 ears of patients with typical Meniere's disease was examined by the furosemide test to detect endolymphatic hydrops. In 95% of the normal control group, the per cent change in the maximum velocity of the slow phase of caloric nystagmus (MVS) after injection of furosemide was under 10%. Therefore, a positive furosemide test was defined as a change in MVS of more than 10%. Thirty-five (56%) of the 62 ears with typical Meniere's disease showed a positive furosemide test. When the affected ears were divided into two groups according to vestibular symptoms, only 11 (38%) of 29 inactive ears were positive while 24 (73%) of 33 active ears were positive. There was a significant difference in the positive rate of the furosemide test between the ears with clinically inactive and active vestibular disease. The per cent canal paresis (CP%) was determined to assess canal excitability and a CP%> 25% was defined as canal paresis. There was no significant difference in the furosemide test positive rate between ears with canal paresis and ears with a normal CP%, although the former tended to show a greater MVS change. The response to the furosemide test showed no relationship to the results of pure tone audiometry. In conclusion, the furosemide test appears to indicate the vestibular status in various stages of Meniere'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.

A review of electrocochleography: instrumentation settings and meta-analysis of criteria for diagnosis of endolymphatic hydrops

This paper reviews the literature on instrumentation settings used for transtympanic (TT-ECOG) an extratympanic electrocochleography (ET-ECOG). There is wide variation regard to the applied stimulus, the settings and interpretation of ECOG results. For most physicians, the presence of endolymphatic hydrops is indicated either by the summating/action potential (SP/AP) ratio for click stimuli or the SP amplitude after tone bursts. Different upper limits have been proposed to discriminate hydropic from non-hydropic ears. Based upon a meta-analysis and classification criteria obtained from multivariate statistics, the authors propose that an SP/AP ratio with click stimulation > 0.35 using TT-ECOG, or > 0.42 using ET-ECOG, is indicative of hydrops. With tone burst stimulation set at a repetition rate of 30-40 stimuli per second, a SP of < -2 microV using TT-ECOG for at least one frequency within the range 0.5-8.0 kHz is considered pathologic.

Short term effects of induced middle ear pressure changes on the electrocochleogram in Menière's disease

Electrocochleographic recordings were made in 17 patients with Menièr's disease before and immediately after exposure to pressure applications in their middle ear. The variables of the response to clicks such as the SP/AP ratio and the width of the complex as well as the amplitudes of the SP in response to long tone bursts were evaluated. Improvement of electrocochleographic recordings, characterized by normalization of the waveforms and reduction of the SP, was found to be significant. The results indicate a mechanoelectric change in the hydroptic cochlea registered directly after exposure to middle ear pressure changes.

Frequency characteristics of summating potential in Menière's disease

SP iso-response functions in 9 patients with Menière's disease were examined for differences in the properties between +SP and -SP. The iso-response functions of +SP and -SP exhibited a large difference. The iso-response function for +SP was centered at high frequencies (around 9 kHz in most cases) with fairly steep slopes on both sides towards both higher and lower frequencies, whereas -SP showed a low-pass function. The tendency was found for patients with normal hearing at high frequencies to have a larger Q10dB than patients with a hearing loss. The difference in properties between +SP and -SP suggests the possibility that +SP may provide different information on the pathophysiology of Ménière's disease from that obtained from -SP.

Summating-potential/action-potential ratio in normal ears: effects of dehydration

INTRODUCTION

Studies using electrocochleography (ECoG) in patients with Meniere's disease and in patients with perilymph fistula have demonstrated abnormally large summating-potential (SP) components compared with the amplitude of the action-potential (AP). The possible influence of normal physiological variation, methodological differences, and test-retest variations are often difficult to interpret. The purpose of this study is to specify what constitutes a clinically important change in the SP/AP ratio in normal hearing subjects under the condition of dehydration induced by urea.

MATERIALS AND METHODS

Baseline ECoG was performed on 20 normal hearing volunteer subjects after an 8 hour fast. Urea, 20 g, was administered orally, following which recordings were made at 60 and 90 minutes. The majority of subjects were found to have maximal SP/AP changes at 60 minutes post dehydration; therefore, only these results were included in the final analysis. Subsets of data were statistically evaluated.

RESULTS

In all ears but one, the preingestion SP/AP ratio was less than or equal to 0.37. Male and female data were shown to comprise separate populations both before and following dehydration. The 95% upper limit for baseline SP/AP ratio was 0.39 for males and was 0.25 for females. The gender-related difference remained after dehydration. The mean male SP/AP ratio showed a statistically significant but slight decrease whereas the female SP/AP ratio was unaltered.

CONCLUSION

These data demonstrate a statistically significant gender-related difference in SP/AP ratio in normals. The upper limit of normal is 0.39 for males and 0.25 for females. A basis for the gender difference is puzzling and requires further investigation. Dehydration produces a statistically significant reduction in SP/AP ratio in men. This change was not observed in women.

The role of summating potential in the diagnosis and management of Menière's disease

To assess the role of a negative summating potential (-SP) in the clinical diagnosis and management of Menière's disease, the relationship of -SP abnormality to clinical symptoms, hearing level and caloric test results was examined in 70 patients with unilateral Meniere's disease. The click-induced -SP and action potential (AP) were recorded by extratympanic electrocochleography. When SP/AP ratio exceeded 0.43, the -SP was considered to be abnormal. No significant relationship was found between abnormal -SP and clinical symptoms (the duration of cochlear and vestibular symptoms, recent hearing fluctuation and recent vertigo attack). Ears with abnormal -SP had a significantly worse hearing loss at high frequencies (2-8 kHz) than ears with normal -SP, whereas there was no significant difference in hearing loss at low frequencies (0.25-1 kHz) between both ears. No significant relation was found between abnormal -SP and abnormal unilateral weakness in caloric test. The results indicate that the click-induced -SP reflects the pathophysiology of Menière's disease basically in the cochlear partition at high frequencies.

Tympanic electrocochleography: normal and abnormal patterns of response

Electrocochleography has been widely used in human and animal studies of endolymphatic hydrops. A variety of response patterns have been reported in normal and hydropic ears. Recent clinical studies have focused almost exclusively on the amplitude ratio of the summating potential (SP) and action potential (AP) derived from alternating polarity click responses. In this report normal response patterns are described with a tympanic membrane electrode employing condensation, rarefaction and alternating polarity clicks and tone burst stimulation. A variety of response abnormalities are described in patients with suspected endolymphatic hydrops. The exclusive use of alternating polarity clicks is not adequate to reveal the nature of these abnormalities.

A clinical study of electrocochleography in Menière's disease

Electrocochleography (ECochG) of 168 ears with definite Meniere's disease was studied. The results were compared with recordings from 29 normal ears and 444 ears with other types of sensorineural hearing loss. It is shown that the SP/AP ratio is much more useful than SP amplitude for detecting endolymphatic hydrops. There was little difference in the mean values of SP/AP from those proposed by many other investigators; the mean values all being near 0.25. From 0.30 to 0.40 of SP/AP was considered adequate as the upper limit. Following intravenous administration of glycerol, a significant decrease in SP/AP was found in 21 Meniere's ears. However, there was no change of SP/AP after oral administration of glycerol and isosorbide. This difference may be due to the fact that a larger oral than intravenous administration is necessary to affect SP/AP. Although a postoperative decrease of 10% or more in SP/AP was observed in 5 patients, 10 patients followed up for 2 years or more after surgery did not show a statistically significant change of SP/AP and pure tone threshold. ECochG should be repeatedly recorded to be useful as a monitor for the presence of hydrops. It is discussed whether biochemical or mechanical changes in endolymph may cause a decrease in SP/AP after the use of dehydrating agents and endolymphatic sac surgery.

Summating potential/action potential ratio in perilymph fistula

A prospective study of electrocochleography in patients with clinically suspected perilymph fistula was undertaken to determine its predictive value in that disorder. One hundred forty-four patients suspected of having perilymph fistula had electrocochleography performed--34 of these (39 ears) had exploratory tympanotomy. Of the 19 ears with normal preoperative summating potential/action potential (SP/AP) ratio, ten had perilymph fistula identified at the time of surgery. Of the 20 ears with abnormal SP/AP ratio, 16 had perilymph fistula confirmed at exploration, 18 had resolution of symptoms after oval window and round membrane grafting, and only one ear had postoperative persistence of the abnormal SP/AP ratio. This study suggests that an abnormal SP/AP ratio is not only predictive of endolymphatic hydrops, but also of perilymph fistula (both problems of inner ear fluid imbalance). This study also suggests that, while abnormal SP/AP ratio is fairly specific for inner ear fluid imbalance, it is not sensitive.

Electrocochleographic changes in endolymphatic hydrops induced by type II collagen immunization through the stylomastoid foramen

Changes in action potential (AP) and summating potential (SP) were investigated in guinea pigs immunized with type II collagen through the stylomastoid foramen. Endolymphatic hydrops could be induced in four of 11 guinea pigs. The striking feature of the electrocochleographic waveform in guinea pigs with endolymphatic hydrops was the negative SP recording in response to high frequency tone bursts. Furthermore, abnormal changes in AP were observed in three of four hydropic guinea pigs. Morphologic study of the cochleas in these three guinea pigs with light microscopy revealed spiral ganglion cell degeneration in addition to endolymphatic hydrops and almost normal sensory hair cells. These results suggest that guinea pigs with hydrops as produced by our procedure can serve as a useful model of Meniere's disease, that autoimmune response may play an important role in the etiopathogenesis of Meniere's disease, and that spiral ganglion cell degeneration together with endolymphatic hydrops seems to contribute to abnormal changes in AP.

Change in the summating potential and action potential during the fluctuation of hearing in Ménière's disease

Summating potential (SP) and action potential (AP) were monitored by extratympanic electrocochleography (ECoG) during the fluctuation of hearing in 8 patients with Ménière's disease. The relationship of SP and AP parameters to pure-tone hearing threshold level was examined. The correlational analysis revealed the following: AP amplitude decreases with increase in the hearing threshold level at higher frequencies (2-8 kHz), whereas it alters independently of the change in the hearing threshold level at lower frequencies (0.25-1 kHz). -SP amplitude is unchanged irrespective of changes in the hearing threshold level at any frequency. The ratio of -SP to AP amplitude increases with increase in the hearing threshold level at higher frequencies (2-8 kHz), whereas it alters independently of the change in the hearing threshold level at lower frequencies (0.25-1 kHz).

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)

Auditory brainstem responses and extratympanic electrocochleography. A threshold comparison in children

Electrocochleography (ECoG) and auditory brainstem response (ABR) have been recorded simultaneously in 23 children referred for threshold evaluation. The ECoG electrode is an extratympanic silver ball (Life-Tech) whereas the ABR is recorded with Ag-AgCl surface electrodes. Of the 30 ears investigated according to this protocol, 11 had no response with either technique, and in a further 11 no difference was found between ECoG and ABR thresholds. In the remaining 8 ears, ABR threshold was lower in 6 and the ECoG in 2. We conclude that the extra time needed for cleansing the ear canal and inserting the electrode does not justify the use of ECoG in threshold evaluations, since the ABR alone has the necessary sensitivity.

Frequency selectivity: reliability of electrocochleographic measures with iso-intensity masking

In a group of 12 normal or hearing-impaired subjects, frequency selectivity was determined psychoacoustically and electrophysiologically with the fast ECochG iso-intensity masking procedure, using a 4 kHz test tone and simultaneous pure-tone masking. Psychoacoustic curves measured twice for each subject were reproducible and showed a broadening related to hearing loss. The curves determined with ECochG repeated two or three times for each subject varied greatly for half of the subjects and their widths were not related to hearing loss. ECochG measures were reproducible in the cases where the amplitude of the response was larger. The data set out the conditions and limits of this ECochG procedure for clinical use.

Glycerol-induced changes in the cochlear responses of the guinea pig hydropic ear

Pigmented guinea pigs were chronically implanted with bilateral round-window electrodes. Endolymphatic hydrops was induced by obstruction of the endolymphatic duct on one side. At 1-5 months post-surgery, glycerol was orally administered to each animal and the acute effect on cochlear and eighth nerve responses was investigated. Doses from half to four times the clinical dose of glycerol failed to improve the CAP audiogram in operated ears and were often found to raise the thresholds. Glycerol was found to diminish the amplitude of the summating potential as well as that of the compound action potential. Similar effects were observed for the higher doses in the control ears, although no effect was seen when doses used were equal to or less than the clinical dose. Our data suggest that the cochlear conditions in the guinea pig model of hydrops differ notably from those seen in patients with Ménière's disease.

Rapidly fluctuating thresholds at the onset of experimentally-induced hydrops in the guinea pig

Pigmented guinea pigs were chronically implanted bilaterally with a platinum electrode on each round window. After recovery the endolymphatic sac was destroyed and the duct blocked on one side only; the other side was employed as a control. The round window response thresholds on both sides were recorded several times per week over a three month period. There were three main results. A sensitivity loss of up to 20 dB was observed for frequencies between 250 Hz and 6.4 kHz within two weeks post-op. At the end of three months the threshold elevation for these frequencies was as much as 50 dB. On the other hand the thresholds for frequencies between 8 and 16 kHz remained within 10 dB of their pre-operative value for at least two months. The thresholds fluctuated with a shift of as much as 25 dB within 24 h. The threshold elevation was associated with a decrease in the latency, at threshold, of the round window AP response which at frequencies between 250 Hz and 6.4 kHz was as short as that for 8 kHz. This observation suggested that it was the base, only, of the cochlea which responded. The present study has indicated that experimentally induced endolymphatic hydrops in the guinea pig mimics well the progressive and fluctuating hearing loss characteristic of Ménière's disease.