Intraoperative electrocochleography of endolymphatic hydrops surgery using clicks and tone bursts

Intraoperative Electrocochleography in Subjects Affected by Vestibular Schwannoma and Ménière's Disease: Comparison of Results

OBJECTIVES

Histologic reports of temporal bones of ears with vestibular schwannomas (VSs) have indicated findings of endolymphatic hydrops (ELH) in some cases. The main goal of this investigation was to test ears with VSs to determine if they exhibit electrophysiological characteristics similar to those of ears expected to experience ELH.

DESIGN

Fifty-three subjects with surgically confirmed VS aged ≥18 and with normal middle ear status were included in this study. In addition, a second group of adult subjects (n = 24) undergoing labyrinthectomy (n = 6) or endolymphatic sac decompression and shunt (ELS) placement (n = 18) for poorly controlled vestibular symptoms associated with Meniere's disease (MD) participated in this research. Intraoperative electrocochleography (ECochG) from the round window was performed using tone burst stimuli. Audiometric testing and word recognition scores (WRS) were performed preoperatively. ECochG amplitudes, cochlear microphonic/auditory nerve neurophonic (ANN) in the form of the "ongoing" response and summation potential, were analyzed and compared between the two groups of subjects. In addition, to evaluate any effect of auditory nerve function, the auditory nerve score was calculated for each subject. Pure-tone averages were obtained using the average air conduction thresholds at 0.5, 1, and 2 kHz while WRS was assessed using Northwestern University Auditory Test No. 6 word lists.

RESULTS

In the VS group the average pure-tone averages and WRS were 59.6 dB HL and 44.8%, respectively, while in the MD group they were 52.3 dB HL and 73.8%. ECochG findings in both groups revealed a reduced trend in amplitude of the ongoing response with increased stimulus frequency. The summation potential amplitudes of subjects with VS were found to be less negative than the MD subjects for nearly all test frequencies. Finally, the VS group exhibited poorer amounts of auditory nerve function compared to the MD group.

CONCLUSIONS

The current findings suggest cochlear pathology (e.g., hair cell loss) in both groups but do not support the hypothesis that VSs cause ELH.

Hair cell and neural contributions to the cochlear summating potential

The cochlear summating potential (SP) to a tone is a baseline shift that persists for the duration of the burst. It is often considered the most enigmatic of cochlear potentials because its magnitude and polarity vary across frequency and level and its origins are uncertain. In this study, we used pharmacology to isolate sources of the SP originating from the gerbil cochlea. Animals either had the full complement of outer and inner hair cells (OHCs and IHCs) and an intact auditory nerve or had systemic treatment with furosemide and kanamycin (FK) to remove the outer hair cells. Responses to tone bursts were recorded from the round window before and after the neurotoxin kainic acid (KA) was applied. IHC responses were then isolated from the post-KA responses in FK animals, neural responses were isolated from the subtraction of post-KA from pre-KA responses in NH animals, and OHC responses were isolated by subtraction of post-KA responses in FK animals from post-KA responses in normal hearing (NH) animals. All three sources contributed to the SP; OHCs with a negative polarity and IHCs and the auditory nerve with positive polarity. Thus the recorded SP in NH animals is a sum of contributions from different sources, contributing to the variety of magnitudes and polarities seen across frequency and intensity. When this information was applied to observations of the SP recorded from the round window in human cochlear implant subjects, a strong neural contribution to the SP was confirmed in humans as well as gerbils. NEW & NOTEWORTHY Of the various potentials produced by the cochlea, the summating potential (SP) is typically described as the most enigmatic. Using combinations of ototoxins and neurotoxins, we show contributions to the SP from the auditory nerve and from inner and outer hair cells, which differ in polarity and vary in size across frequency and level. This complexity of sources helps to explain the enigmatic nature of the SP.

Electrocochleography in Retrosigmoid Vestibular Nerve Section for Intractable Vertigo Caused by Meniere's Disease

Interest in electrocochleography has increased in recent years because of the discovery of an elevated summating potential to action potential amplitude ratio (SP/AP ratio) in patients with endolymphatic hydrops caused by Meniere's disease or perilymph fistula. It was the purpose of this investigation to determine whether the intraoperative SP/AP ratio will decrease after vestibular nerve section in patients with intractable Meniere's disease. Fourteen patients with medically intractable classic Meniere's disease underwent retrosigmoid vestibular nerve section. Intraoperative transtympanic electrocochleography was performed with alternating click stimuli presented at 95 dB HL. In all patients the SP/AP ratio was recorded before the skin incision (“baseline” condition) and after the dura was closed (“closing” condition). Statistical analysis was applied to the recorded data. In 11 (79%) patients, the SP/AP ratio was found to be elevated above 0.30 in the baseline state. In 13 (93%) patients, the SP/AP ratio decreased more than 25% after the nerve was sectioned. These results were highly statistically significant (p < 0.001). We conclude that the SP/AP ratio does decrease in patients with Meniere's disease after undergoing retrosigmoid vestibular nerve section and offer a possible explanation. (Otolaryngol Head Neck Surg 1997;116:593–6.)

Transitory effect on endolymphatic hydrops of the intratympanic steroids for Ménière's disease

OBJECTIVES/HYPOTHESIS

This study aimed to evaluate the changes in electrocochleography (EcohG) measurements after intratympanic (IT) dexamethasone therapy and to correlate them with the long-term effects on the control of vertigo.

STUDY DESIGN

Prospective outcomes research.

METHODS

This study included 62 patients with unilateral Ménière's Disease (MD) refractory to medical therapy for at least 1 year. Each patient was treated with a fixed protocol of three consecutive weekly injections of a commercial 4 mg/mL dexamethasone preparation. The 1995 American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS) criteria for reporting treatment outcomes for MD were used. Electrocochleography (EcohG) measurements were performed 1 month before and 1 and 12 months after IT steroid therapy. Caloric test and vestibular evoked myogenic potential (VEMPs) were performed before the IT treatment. The summating potential/action potential (SP/AP) ratio was measured before and after the IT treatment. A Kaplan-Meier analysis was used to evaluate the control of vertigo over a 2-year period.

RESULTS

Complete vertigo control (class A) was achieved in 26 patients (41.9%) at the 12-month follow-up and in 12 patients (19.3%) at the 24-month follow-up. A significant reduction (P < 0.01) in the SP/AP ratio after the IT steroid treatment was observed in the first-month determination, but no significant differences were found when the initial and 12-month determination were compared.

CONCLUSIONS

IT dexamethasone provides an alternative treatment for patients with Ménière's Disease. A transitory reduction of the endolymphatic hydrops is detected by the EcohG 1 month after treatment. The hydrops levels returned to their initial values in the 1-year EcohG follow-up.

LEVEL OF EVIDENCE

2b.

The use of electrocochleography to monitor the response of Ménière's disease patients to intratympanic steroids

CONCLUSIONS

Intratympanic (IT) dexamethasone provides an alternative for patients with Ménière's disease (MD). A reduction of the endolymphatic hydrops is detected by the EcohG 1 month after the treatment.

OBJECTIVE

The use of intratympanic corticoid injections for MD has become popular due to the lack of reported adverse effects, but the mechanism of action is not well established. This study aimed to evaluate the changes in electrocochleography (EcohG) measurements during IT dexamethasone therapy.

METHODS

This study included 53 patients with unilateral MD refractory to medical therapy for at least 1 year. Each patient was treated with a fixed protocol of three consecutive weekly injections of a commercial 4 mg/ml dexamethasone preparation. EcohG measurements were performed 1 month before and 1 month after IT steroid therapy. The SP/AP ratio was measured before and after the IT treatment. A Kaplan-Meier analysis was used to evaluate the control of vertigo over a 2-year period.

RESULTS

Complete vertigo control (class A) was achieved in 22 patients (41.5%) at the 12-month follow-up and 8 patients (15.1%) at the 24-month follow-up. A significant reduction (p < 0.01) in the SP/AP ratio after the IT steroid treatment was observed.

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.

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.