Stress electrocochleography

Electrocochleographic recordings after changes of middle-ear and intracranial pressure were studied in a group of otologically normal subjects and in patients with suspected Ménière's disease. Electrocochleography performed under these conditions was called stress electrocochleography. It was useful to distinguish among Ménière's disease, benign positional vertigo, and a round-window fistula. Changes of middle-ear pressure, whether positive or negative, always resulted in a rise of the SP/AP ratio. Raised intracranial pressure after Trendelenburg positioning showed fluctuations of the SP/AP ratio in both normal subjects and symptomatic patients. However, in normal subjects, a congruent pattern of these fluctuations was apparent, whereas symptomatic patients exhibited a discordant pattern. The discordant pattern, typical for Ménière's disease, showed an increase of the SP voltage, whereas the AP voltage decreased. In benign positional vertigo, the SP voltage decreased, and the AP voltage increased, once during Trendelenburg positioning and again at the end of the test when starting position was resumed. During raised intracranial pressure after the Queckenstedt maneuver, round-window fistulas showed an increase of the SP voltage, whereas the AP voltage decreased considerably. At our clinic, stress electrocochleography has become an important neurootological test in the differential diagnosis of Ménière's disease.     

Unusual electrocochleography cases

The electrocochleographic findings of 3 unusual cases from about 350 performed on patients with various hearing loss are presented; they demonstrate the value of electrocochleography as a clinical tool in otological investigation. The first case was an electrocochleographically detected acute impairment of the first neuron, the second a hereditary hearing loss with endolymphatic hydrops, and the last shows the effect of an endolymphatic shunt on a hydropic hearing loss.     

Salt-load electrocochleography.

OBJECTIVE: To introduce a new protocol for diagnostic electrocochleography using a pretest oral salt load to improve test sensitivity in patients with suspected inner ear fluid imbalance. STUDY DESIGN: A retrospective review of patients who reported vertigo that, by medical history, was suggestive of an inner ear fluid imbalance was preformed. The patients received a complete audiovestibular evaluation that included a baseline electrocochleogram. Despite the incapacitating nature of their vertigo, there were no symptoms or electrophysiologic abnormalities that would isolate an etiologic ear. After the baseline studies, the patients received 4 g of sodium chloride daily for 3 days before repeat electrocochleography. A control group of 13 healthy volunteers with normal baseline electrocochleography and pure tone audiometry was tested under similar conditions. SETTING: This study was conducted at an ambulatory care clinic associated with a tertiary referral medical center. INTERVENTION: Electrocochleography was performed using alternating polarity clicks presented at a rate of 9.7/sec at 95 dB nHL by an extratympanic TIPtrode electrode and recorded with a Nicolet Spirit (Nicolet Instrument Corp., Madison, WI, U.S.A.). Responses were averaged for 1000 sweeps using a 10-msec time base with bandpass filtering from 5 to 1500 Hz. A summating potential/action potential (SP/AP) ratio of 0.37 was considered the upper limit of normal. MAIN OUTCOME MEASURES: Enhancement in the SP/AP ratio from a normal baseline value to > 0.37 after oral salt loading was indicative of a positive test. RESULTS: None of the ears from control subjects had a positive salt load electrocochleogram, and one or both ears in 38% of the patients in the study group with normal baseline SP/AP ratios and symptoms of inner ear fluid imbalance converted to abnormal. The mean SP/AP ratio of the control group for the conditions before and after salt-load was not statistically different (p = 0.48), although the difference in the mean SP/AP ratio in the study group after salt loading was statistically significant (p = 1.329 x 10(-5)). CONCLUSIONS: A group of patients who reported vertigo with no localizing abnormalities had a statistically significant increase in the mean SP/AP ratio after ingestion of a large quantity of sodium chloride. A modest percentage had elevation of the SP/AP ratio above the upper limit of normal for our audiovestibular lab. The localization of a "salt-senstitive" ear could assist the clinician in the management of these difficult problems with long-term medical therapy or surgical treatment when alternative measures fail.     

Anaesthesia for electrocochleography

Until recently Ketamine has been the anaesthetic most commonly used for electrocochleography. It was believed that the use of a conventional anaesthetic machine would produce artifacts in the recordings but experience has shown that this is not the case; in fact quieter tracings are obtained due to the abolition of muscle potentials. Ketamine is preferred for children under 3 years old but the others receive a general inhalation anaesthetic. The intramuscular route is chosen for the administration of Ketamine in order to minimize the occurrence of hallucinations in the recovery period. Consistent results have been obtained with a dosage related to the metabolic rate of the child. One milligram of Ketamine is administered for each 35 cm² of surface area. General inhalation anaesthesia is given by a semiopen circuit and an oral endotracheal tube is used. Induction is with methohexitone or nitrous oxide and halothane, and maintenance is also with nitrous oxide and halothane. The main drawback is pollution of the atmosphere of the sound-proofed testing room with halothane. This has been overcome by ducting the expired gases outside the room using an Enderby valve.     

Ultrasonic electrocochleography in guinea pig

In order to obtain information about how ultrasonic stimuli are perceived (USP) in man, guinea pig cochleae were stimulated by bone conduction with frequencies (98.8 and 143.5 kHz) above the normal auditory field of this animal. The cochlear potentials recorded consisted of CM, SP and AP originating from the basal turn of the cochlea, and were found to be influenced by asphyxia, administration of ethacrynic acid, hypothermia and change of interstimulus interval. In addition, in kanamycin-treated animals the mean AP amplitude decreased to about one fourth of the normal value, and the mean AP latency increased significantly. These findings suggest that there is no special sense organ for the detection of USP but that such sounds activate hair cells in the basal turn of the cochlea.     

The selective effects of central masking on brain stem potentials

The effect of contralateral white noise stimulation on ipsilaterally presented click evoked brain stem potentials was examined. Continuous contralateral white noise does not influence the brain stem response components but pulsed white noise simultaneously presented with the clicks produced a central masking effect which was observed as an amplitude reduction confined to Wave V. Our findings confirm those of Zwislocki (1971) that pulsed noise is a more effective central masker than continuous noise. A computer addition of the responses to pulsed white noise and clicks recorded independently revealed no reduction in Wave V amplitude but the response to the two stimuli presented simultaneously showed a specific reduction in Wave V amplitude. No change in the latency of Wave V was observed to suggest any 'cross-over' masking of the click stimulus. Patients with total unilateral deafness did not exhibit this phenomenon. The effect of a time delay of the masker to one ear in relation to the click presented to the other ear reduced the amplitude of Wave V less than when the two stimuli were presented simultaneously. The reduction in the amplitude, being specific to Wave V, suggests that the effect is central and that the particular locus for this aspect of central masking is at the level of the inferior colliculus.     

Intrasubject test-retest variability in clinical electrocochleography

Seventeen healthy volunteers without a history of auditory or vestibular difficulty and with normal screening audiometry had bilateral (34 ears) electrocochleography performed repeatedly at 1-week intervals. Each subject had from four to seven electrocochleograms performed on each ear. Measurements were made in the external auditory canal using a gold-foil “TIPtrode.” Amplitude and latency for the summating potential (SP) and action potential (AP) were measured and SP/AP ratios were calculated. Averages and standard deviations for amplitude, latency, and SP/AP ratios were computed. The average SP/AP ratio was 0.22 with a standard deviation of 0.06. The range was 0.04 to 0.50. The average difference between the highest and lowest SP/AP ratio for a given individual was 0.155 (i.e., the range 0.03 to 0.31) with a standard deviation of 0.07. These normative data are useful in assessing the results of dehydration electrocochleography and in establishing how much test-retest variation is required to suggest pathology.     

Anaesthesia for electrocochleography.

Until recently Ketamine has been the anaesthetic most commonly used for electrocochleography. It was believed that the use of a conventional anaesthetic machine would produce artifacts in the recordings but experience has shown that this is not the case; in fact quieter tracings are obtained due to the abolition of muscle potentials. Ketamine is preferred for children under 3 years old but the others receive a general inhalation anaesthetic. The intramuscular route is chosen for the administration of Ketamine in order to minimize the occurrence of hallucinations in the recovery period. Consistent results have been obtained with a dosage related to the metabolic rate of the child. One milligram of Ketamine is administered for each 35 cm2 of surface area. General inhalation anaesthesia is given by a semiopen circuit and an oral endotracheal tube is used. Induction is with methohexitone or nitrous oxide and halothane, and maintenance is also with nitrous oxide and halothane. The main drawback is pollution of the atmosphere of the sound-proofed testing room with halothane. This has been overcome by ducting the expired gases outside the room using an Enderby valve.     

Frequency selectivity and comodulation masking release in adults and in 6-year-old children

Frequency selectivity and comodulation masking release (CMR) for a 1000-Hz signal frequency were examined in 6-year-old children and adults. An abbreviated measure of frequency selectivity was also conducted for a 500-Hz signal. Frequency selectivity was measured using a notched-noise masking method, and CMR was measured using narrow bands of noise whose amplitude envelopes were either uncorrelated or correlated. There were 6 listeners in each age group. No differences were observed between the adults and children for either auditory measure. Similarly, no differences were observed in the ability to detect a pure-tone signal in a relatively wideband noise masker. When the masking noise was narrowband, however, the masked thresholds of the children were higher than those of the adults. Two characteristics that distinguish narrowband noise from wideband noise are: (1) narrowband noise has a pitch quality corresponding to its center frequency, whereas wideband noise does not have definite pitch; (2) the intensity fluctuations are relatively greater in narrowband noise than in wideband noise. This may suggest that 6-year-old children have a reduced ability to detect signals in noise backgrounds where the signal has perceptual qualities similar to the noise, or in noise backgrounds having a high degree of fluctuation.     

Amplitude effects on electrocochleography outcomes

OBJECTIVE: Electrocochleography measures in humans were examined to determine factors affecting results. STUDY DESIGN: Retrospective study. METHODS: Analysis was performed on data from 479 patients seen from 1998 to 2002 who underwent electrocochleography with a tympanic membrane electrode. Measures from click evoked electrocochleography (the summating potential to action potential ratio [SP/AP]) were compared with tone evoked electrocochleography (the SP amplitude). RESULTS: Findings indicate a strong correlation between click and tone measures if the AP from click measures was over 1 microV (P <.001), but no significant correlation between these measures if the AP was under 1 microV. Results were also analyzed with respect to patient hearing sensitivity and final diagnosis. CONCLUSION: Tympanic membrane electrocochleography is more efficacious when the AP from click stimulation is over 1 microV.     

Transtympanic electrocochleography during glycerol dehydration

Thirteen patients with Meniere's disorder were tested using transtympanic electrocochleography during glycerol dehydration. The most common finding was a decrease in the negative summating potential and this appeared to be a more sensitive indicator of changes occurring in the cochlea than pure tone audiometry and speech discrimination. A pathophysiological explanation for this observation is offered and the possibility of using this decrease in the negative summating potential as a pointer to the prognosis of endolymphatic sac surgery is discussed.     

Reliability of cochlear microphonics in clinical electrocochleography

A study was made on the reliability of cochlear microphonics (CM) recorded in clinical electrocochleography (ECochG) by the transtympanic electrode technique. In normal-hearing subjects, the CM shift increased as the frequency was lowered at the same sound pressure, and as the sound pressure was lowered at the same frequency. The CM shift for the frequencies from 8 to 0.5 kHz was greater as the sound pressure was lowered. The above results suggest that clinical ECochG CM responses may well reflect the potential of the cochlea.     

Clinical electrocochleography in Menière's disease

Transtympanic electrocochleography (ECochG) was performed on 137 definite Menière ears, 29 normal ears and 383 ears with other diseases. The average SP/AP amplitude ratio (SP/AP) was significantly larger than that of normal subjects in the following four disease groups whose causes are thought to be endolymphatic hydrops: 1) definite Menière's disease, 2) suspected Menière's disease, 3) delayed endolymphatic hydrops and 4) labyrinthine syphilis. The SP amplitude could not provide any information for a differential diagnosis of endolymphatic hydrops from normal subjects nor from other sensorineural deafness . The upper 99% confidence limit of normal SP/AP using a 100dBSPL click was 0.37 for 29 subjects. In patients with Menière's disease the relationship between SP/AP and clinical data such as pure tone threshold, audiogram pattern, results of glycerol test and duration of illness from the first vertiginous attack to the test were investigated. In patients with severe hearing loss, the number of those showing SP/AP of greater than 0.37 was much higher than in patients with less severe hearing loss. There were no relationship, however, between SP/AP and duration of illness, or audiogram pattern, or glycerol test. ECochG was recorded before and after intravenous administration of 50 gr of glycerol (10% solution, 500ml), and a significant decrease of SP/AP was found in 21 Menière ears. However, there were no change of SP/AP after oral administration of 50 gr of glycerol (50% solution, 100ml) and isosorbide (70% solution, 2.0gr/kg of body weight). After endolymphatic sac surgery a decrease of SP/AP of 10% or more was found in 5 ears, an increase of SP/AP in 3 ears, and no change of SP/AP in 3 ears. In the patients showing no change of SP/AP after the surgery, numerical values of vertigo by AAOHNS were higher than in patients showing an increase or a decrease of SP/AP. The results demonstrate the usefulness of ECochG in diagnosis of endolymphatic hydrops and indicate a possibility of monitoring the state of hydrops after administration of glycerol and endolymphatic sac surgery.     

Bone-conduction electrocochleography: clinical applications

Cases are presented which show the clinical utility of recording an electrocochleographic response to bone-conducted stimuli. The procedure is fraught with problems of acoustic control and artifact generation, but has distinct although limited values in clarifying masking dilemmas in patients with bilateral hearing loss.     

Abnormal positive potentials in round window electrocochleography

OBJECTIVE: To describe an atypical waveform, termed an abnormal positive potential (APP), on round window electrocochleograms (RW ECochG) of children and to relate its occurrence to clinical history. STUDY DESIGN: APPs were identified prospectively, and a retrospective analysis was made of these patients' clinical histories, audiograms, and auditory outcomes (hearing aid, cochlear implant, or nonauditory communication) SETTING: Tertiary referral teaching hospital, day surgery and clinics. PATIENTS: All 431 children <110 months of age suspected of a severe to profound hearing loss who underwent RW ECochG from January 1993 to August 1997. INTERVENTION: Diagnostic RW ECochG for auditory threshold estimation. MAIN OUTCOME MEASURE: The presence on the RW ECochG of the APP: an early positive potential in the absence of a compound action potential (CAP). RESULTS: An APP was observed in 34 children. The APP was most marked in response to clicks and 8-kHz tones. The APP click threshold averaged 70 dB hearing loss. The brainstem evoked potential of these children showed an absence of waves, or a broad positive wave with no subsequent waves. Twenty-nine of 30 behavioral audiograms obtained were indicative of severe to profound hearing loss. Auditory outcomes were available from 26 children; 45% of them derived no help from a hearing aid, and 8 children received a cochlear implant. Clinical factors frequently associated with APP were prematurity in combination with kemicterus or hypoxia. CONCLUSIONS: APP thresholds were lower than neural thresholds or behavioral thresholds. Children with APP need close follow-up, because half of those studied needed nonauditory strategies to develop effective communication.     

Stapedial electromyograms recorded by electrocochleography

The stapedial reflex (SR) can be obtained by the impedance method only when the middle ear is intact. In order to examine the function of the stapedius muscle in diseased ears, the recording of the stapedial electromyogram by an electrocochleographic (ECoG) method was considered. The SR was recorded simultaneously by the impedance method in ten normal subjects for comparison. The outputs of ECoG were averaged by a signal processor up to 200 times under a longer sweep time, and a large biphasic wave with a latency of about 11 ms was obtained on the ECoG records. The amplitude increase of the biphasic wave caused by raising the intensity of the acoustic stimulation paralleled that of the SR. This means that the biphasic wave obtained by ECoG with a latency of about 11 ms originated from the stapedius muscle.