Delayed Effect of Active Pressure Treatment on Endolymphatic Hydrops.

OBJECTIVE: To identify eventual correlations between the effect of low-pressure treatment and endolymphatic hydrops in Ménière patients. MATERIAL AND METHODS: The study group consisted of subjects affected by definite Ménière disease (2015) and a severe degree of disability, who received a ventilation tube with or without a low-pressure treatment before undergoing a surgical procedure (vestibular neurectomy). After the placement of the ventilation tube, the subjects were either left alone with the tube or received 1 month of self-administered low-pressure therapy with a portable device. In all subjects, an electrocochleography (ECochG) was performed and specific questionnaires - Dizziness Handicap Inventory (DHI) and Functional Scale Level (FSL) - were completed before starting either arm of treatment, at the end of treatment, and then 3 and 6 months later. RESULTS: All selected subjects presented with an ECochG pattern that was indicative of endolymphatic hydrops before starting either treatment. At the end of pressure treatment, 80% showed symptomatic improvement while maintaining the hydropic ECochG pattern. At the 3-month control stage, the hydropic pattern resulted normalized (<0.5) in all the improved subjects. CONCLUSIONS: Although 1 month of low-pressure treatment provided a positive symptomatological outcome, normalization of the hydropic ECochG parameters occurred only at a later time. Therefore, it is possible to assume that endolymphatic hydrops could be concurrent with a non-symptomatic stage of Ménière disease, and that the anti-hydropic effect of the low-pressure treatment, if any, would present with a certain delay after its completion.

Intraoperative observation of changes in cochlear nerve action potentials during exposure to electromagnetic fields generated by mobile phones.

BACKGROUND: The rapid spread of devices generating electromagnetic fields (EMF) has raised concerns as to the possible effects of this technology on humans. The auditory system is the neural organ most frequently and directly exposed to electromagnetic activity owing to the daily use of mobile phones. In recent publications, a possible correlation between mobile phone usage and central nervous system tumours has been detected. Very recently a deterioration in otoacoustic emissions and in the auditory middle latency responses after intensive and long-term magnetic field exposure in humans has been demonstrated. METHODS: To determine with objective observations if exposure to mobile phone EMF affects acoustically evoked cochlear nerve compound action potentials, seven patients suffering from Ménière's disease and undergoing retrosigmoid vestibular neurectomy were exposed to the effects of mobile phone placed over the craniotomy for 5 min. RESULTS: All patients showed a substantial decrease in amplitude and a significant increase in latency of cochlear nerve compound action potentials during the 5 min of exposure to EMF. These changes lasted for a period of around 5 min after exposure. DISCUSSION: The possibility that EMF can produce relatively long-lasting effects on cochlear nerve conduction is discussed and analysed in light of contrasting previous literature obtained under non-surgical conditions. Limitations of this novel approach, including the effects of the anaesthetics, craniotomy and surgical procedure, are presented in detail.

Characteristics of tone burst-evoked myogenic potentials in the sternocleidomastoid muscles.

HYPOTHESIS: Optimum stimulus parameters for tone burst-evoked myogenic responses can be defined. These optimized responses will be similar to those evoked by clicks in the same subjects. BACKGROUND: Loud tones give rise to myogenic responses in the anterior neck muscles, similar to click-evoked potentials, and are likely to be saccular in origin. METHODS: Tone burst-evoked and click-evoked myogenic potentials were measured from the sternocleidomastoid muscles of 12 normal subjects (6 men, 6 women) during tonic activation. The effects of tone burst frequency and duration were systematically investigated. Thresholds were measured and compared with click thresholds for the same subjects. Patients with specific lesions were studied using both stimuli. RESULTS: Tone burst-evoked responses showed frequency tuning, with the largest reflex amplitudes at either 500 Hz or 1 kHz. As the stimulus duration was increased, using a constant repetition rate, there was an increase in the reflex amplitudes followed by a decline. The overall optimum stimulus duration was 7 milliseconds. The mean tone burst threshold was 114.4-dB sound pressure level. Stimulus thresholds for click-evoked and tone burst-evoked responses were significantly correlated. Tone burst-evoked and click-evoked responses were present after stimulation of the affected ears of subjects with profound sensorineural hearing loss. Four subjects who had previously undergone vestibular neurectomy had an absence of click and tone burst-evoked responses on the side of the lesion, confirming their vestibular dependence. CONCLUSION: Tone burst-evoked myogenic responses are similar to click-evoked responses but require lower absolute stimulus intensities. To be certain of an optimum response, a stimulus duration of 7 milliseconds, an adequate intensity, and frequencies of both 500 Hz and 1 kHz should be used.

Vestibular projections in the human cortex.

There is considerable evidence from studies on cats and monkeys that several cortical areas such as area 2v at the tip of the intraparietal sulcus, area 3av in the sulcus centralis, the parietoinsular vestibular cortex adjacent to the posterior insula (PIVC) and area 7 in the inferior parietal lobule are involved in the processing of vestibular information. Microelectrode recordings from these areas have shown that: (1) most of these cortical neurons are connected trisynaptically to the labyrinthine endorgans and (2) they receive converging vestibular, visual and somatosensory inputs. These data suggest that a multimodal cortical system is involved in postural and gaze control. In humans, recent positron emission tomography (PET) scans and functional magnetic resonance imaging (fMRI) studies have largely confirmed these data. However, because of the limited temporal resolution of these two methods, the minimum time of arrival of labyrinthine inputs from the vestibular hair cells to these cortical areas has not yet been determined. In this study, we used the evoked potential method to attempt to answer this question. Due to its excellent temporal resolution, this method is ideal for the investigation of the tri- or polysynaptic nature of the vestibulocortical pathways. Eleven volunteer patients, who underwent a vestibular neurectomy due to intractable Meniere's disease (MD) or acoustic neurinoma resection, were included in this experiment. Patients were anesthetized and the vestibular nerve was electrically stimulated. The evoked potentials were recorded by 30 subcutaneous active electrodes located on the scalp. The brain electrical source imaging (BESA) program (version 2.0, 1995) was used to calculate dipole sources. The latency period for the activation of five distinct cortical zones, including the prefrontal and/or the frontal lobe, the ipsilateral temporoparietal cortex, the anterior portion of the supplementary motor area (SMA) and the contralateral parietal cortex, was 6 ms. The short latency period recorded for each of these areas indicates that several trisynaptic pathways, passing through the vestibular nuclei and the thalamic neurons, link the primary vestibular afferents to the cortex. We suggest that all these areas, including the prefrontal area, process egomotion information and may be involved in planning motor synergies to counteract loss of equilibrium.

Evidence that the ginkgo biloba extract, EGb 761, neither accelerates nor enhances the rapid compensation of the static symptoms of unilateral vestibular deafferentation in guinea pig.

The concentrated Ginkgo biloba extract, EGb 761, has previously been reported to enhance and accelerate vestibular compensation following unilateral vestibular deafferentation (UVD), in particular, compensation of the dynamic postural symptoms such as locomotor dysequilibrium. However, many of these studies have not included a complete analysis of the static symptoms of UVD, such as spontaneous nystagmus (SN), yaw head tilt (YHT), and roll head tilt (RHT), nor have they included a dose-response analysis or vehicle controls for EGb 761. The aim of the present study was to examine the effects of the EGb 761 extract on static vestibular compensation in guinea pig, using a dose-response analysis and both vehicle and saline controls. Analysis of variance showed that there was a significant decrease in SN frequency (P < 0.05) and a significant change in the rate of SN compensation (P < 0.05), using 3 i.p. injections of EGb 761 (25, 50, or 100 mg/kg), or vehicle, or saline, at 0, 25, and 40 h post-UVD. However, post-hoc testing revealed that this was due entirely to significant differences between the saline and vehicle groups at 35, 40, and 50 h post-UVD (P < 0.05 in all cases) and between the saline and the 100 mg/kg and 25 mg/kg EGb 761 groups at 35 and 50 h post-UVD, respectively (P < 0.05 for both comparisons); there were no significant differences between the vehicle and drug groups at any time. YHT and RHT were not significantly different between the drug, saline, and vehicle groups. In a second set of experiments, the 50 and 100 mg/kg EGb 761 i.p. injection frequencies were doubled. However, once again, neither SN nor YHT were significantly different between the EGb 761 groups and the vehicle controls. These results suggest that 1) EGb 761 does not significantly enhance or accelerate compensation of the static symptoms of UVD in guinea pig and 2) the EGb 761 vehicle may exert some effects on its own. Therefore, EGb 761 may be of limited use in the treatment of acute vestibular dysfunction in humans.

Vestibulocollic reflexes evoked by short-duration galvanic stimulation in man.

1. Vestibular-dependent responses in leg muscles following transmastoid galvanic stimulation have been well characterized. Here we describe the properties of vestibulocollic responses evoked by transmastoid galvanic stimulation. 2. In twelve healthy human subjects we examined the averaged responses in unrectified sternocleidomastoid (SCM) EMG evoked by transmastoid stimulation using current pulses of 4 mA intensity and 2 ms duration. In ten subjects we also examined the effects of unilateral vestibular stimulation with the indifferent electrode at the vertex. In further experiments we studied the effects of different levels of background muscle activation, head position, current intensity and current duration. We compared these responses with click-evoked vestibulocollic responses in SCM. 3. A clearly defined biphasic response, beginning with a surface positivity, was recorded in the SCM ipsilateral to the side of cathode placement in all subjects. We refer to this as the p13/n23 [g] (galvanic) response, given the close similarity, in terms of waveform and latencies, to the previously described click-evoked p13/n23 vestibulocollic response. The amplitude of this response was linearly related to background muscle activation, current intensity and current duration, but independent of head position. Unilateral galvanic stimulation revealed the p13/n23 [g] response to be solely generated by the cathode. 4. A biphasic response beginning with a surface negativity (n12/p20 [g]) contralateral to the cathode was seen in all subjects and was generated by both the cathode contralaterally and the anode ipsilaterally. 5. Both the p13/n23 [g] and n12/p20 [g] potentials were abolished by selective vestibular nerve section and unaffected by severe sensorineural deafness. 6. We conclude that galvanic stimulation evokes short-latency vestibulocollic reflexes. These vestibulocollic reflexes have properties that are distinct from those described for galvanic-evoked vestibular reflexes in leg muscles, and which may be related to their differing physiological roles.

Nonsurgical factors predictive of postoperative hearing for patients with vestibular schwannoma.

OBJECTIVE: The purposes of the study were to determine whether preoperative cochlear reserve as measured by evoked otoacoustic emissions (EOAE) as well as other hearing variables often associated with hearing preservation are correlated with hearing preservation after tumor removal and to determine whether any hearing variables are independent of tumor size as a predictor of hearing preservation. STUDY DESIGN: Preoperative audiologic data for 104 patients having vestibular nerve schwannomas removed via a retrosigmoid surgical approach were reviewed and subjected to factor analysis. SETTING: All patients were seen at the Mayo Clinic, Rochester, Minnesota. PATIENTS: The patient sample was divided into two groups based on hearing thresholds after surgery. Group I consisted of 73 ears without hearing preservation. The remaining 31 ears, group II, had preserved hearing (defined as average postoperative pure-tone thresholds < or = 85-dB HL for 0.5, 1, 2, and 3 kHz). MAIN OUTCOME MEASURES: Variables not predictive of hearing preservation were age, gender, tumor laterality, and cochlear reserve (EOAE). Variables predictive of hearing preservation were small tumor size, pure-tone hearing sensitivity, speech reception thresholds, word recognition scores, integrity of cochlear nerve (acoustic reflex thresholds, and auditory brain stem response [ABR] waveforms). RESULTS: A multivariate logistic regression analysis showed that only word recognition scores at 40-dB sensation level were independent of tumor size as a predictor of hearing preservation.

Medial olivocochlear system stabilizes active cochlear micromechanical properties in humans.

To investigate the involvement of the medial olivocochlear system (MOCS) in outer hair cell (OHC) motility stabilization, evoked otoacoustic emissions (EOAEs) were recorded in 20 normal-hearing subjects and in eight vestibular-neurotomized subjects, successively in the presence and absence of low-intensity contralateral acoustic stimulation. Intrasubject EOAE amplitude variability was assessed as the standard deviation computed over several successive recordings. In normal-hearing subjects, a significantly lower EOAE amplitude variability with contralateral acoustic stimulation (CAS) was observed in subjects in whom the CAS induced the greatest EOAE amplitude reduction. This result could not be attributed to the EOAE amplitude reduction itself, since variability was otherwise found to increase when EOAE amplitude decreased. Moreover, statistically significant correlations between EOAE amplitude attenuation and EOAE amplitude variability under CAS were observed. In the eight subjects operated for vestibular neurotomy, no such effect was found. Being sectioned in vestibular-neurotomized subjects, the MOCS can no longer exert its effects. These results strongly support the notion that MOCS activity, as induced by CAS, elicits a reduction in EOAE amplitude variability in normal-hearing subjects. This finding and some of its possible implications for understanding the role of the MOCS in hearing in humans are discussed.

Effect of olivocochlear bundle section on evoked otoacoustic emissions recorded using maximum length sequences.

Presenting clicks according to maximum length sequences (MLS) enables transient evoked otoacoustic emissions (TEOAE) to be recorded at very high stimulation rates. As the click rate is increased from 40 clicks/s up to a maximum rate of 5000 clicks/s there is a reduction in TEOAE amplitude that reaches an approximate asymptote at 1500 clicks/s. One hypothesis put forward to explain this MLS 'rate effect' is that ipsilateral efferent activity is involved. To test this hypothesis TEOAEs were recorded from both ears of five patients who had undergone a unilateral vestibular nerve section--a surgical procedure which also entails sectioning the olivocochlear bundle. TEOAEs were recorded conventionally at 40 clicks/s and using MLS stimulation at 5000 clicks/s. Increasing the rate from 40 to 5000 clicks/s was found to reduce the amplitude of the TEOAEs by equivalent amounts in ears ipsilateral and contralateral to a vestibular nerve section as well as in the ears of normal-hearing adults. Since an ear ipsilateral to a vestibular nerve section should have no efferent innervation the hypothesis that efferent activity is the major mechanism involved in the MLS rate effect is rejected. Instead, the possibility that intracochlear processes are the underlying mechanism will now be investigated.

On the role of the olivocochlear bundle in hearing: 16 case studies.

Earlier we presented data (Scharf et al. (1994) Hear. Res. 75, 11-26) from a young patient (S.B.) who had undergone a vestibular neurotomy, during which the olivocochlear bundle (OCB) was severed. Those data are complemented by measurements on 15 other patients-some like S.B. with normal audiometric thresholds, none with a loss greater than 35 dB at experimental frequencies. Comparisons of performance for the same ear before and after surgery or between the operated and healthy ears do not provide evidence that the lack of OCB input impairs the following psychoacoustical functions: (1) detection of tonal signals, (2) intensity discrimination, (3) frequency selectivity, (4) loudness adaptation, (5) frequency discrimination within a tonal series, (6) in-head lateralization. Data on single-tone frequency discrimination are equivocal. These mostly negative results apply to listening both in the quiet and, where relevant, in noise. The only clear change in hearing after a vestibular neurotomy is that most patients detect signals at unexpected frequencies better than before. This change suggests an impaired ability to focus attention in the frequency domain. Although limited in scope, our finding that human hearing without OCB input is essentially normal agrees with much of the relevant literature on animal behavior and with the patients' self-reports.

Pharmacological activity of the Ginkgo biloba extract (EGb 761) on equilibrium function recovery in the unilateral vestibular neurectomized cat.

Locomotor balance recovery after unilateral vestibular neurectomy has been found strongly accelerated in the cat when the animals received a postoperative treatment with Ginkgo biloba Extract (EGb 761:50 mg/kg/d, i.p.), a result due to the improvement of plasticity mechanisms involved in vestibular compensation. The aim of this study was to determine which of the two main biochemical components (terpenes vs. flavonoids) contained in the extract was the most active in the recovery process, to test the influence of the route of administration, and to look for dose-dependent effects. Experiments were performed in six experimental groups of cats that were compared with each other and with three control groups. Comparisons were done on the recovery profile and time course of equilibrium function restoration, as quantified by the rotating beam test. Four experimental groups were treated with the standardized extract EGb 761 given orally (p.o.:2 groups; 40 mg and 80 mg/kg) or intraperitoneally (i.p.: 2 groups; 50 mg and 25 mg/kg), whereas the two others received only a special extract that did not contain the terpenes (i.p. administration: 25 mg and 10 mg/kg). Treatment was always given until complete recovery of locomotor balance function. The control groups received either no treatment (untreated cats), an oral vehicle (placebo cats), or a sham i.p. injection (sham cats). Results showed that locomotor balance recovery was significantly improved in all the experimental groups as compared to the control groups of cats, which recovered similarly and more slowly. Efficacy of the special extract without the terpenes was comparable to that of the total extract, indicating that the nonterpenic fraction was the most active biochemical constituent in this experimental model of central nervous system (CNS) plasticity. Pharmacological activity of the extract was also significantly better when given i.p. as compared to the p.o. route of administration, and dose-dependent effects were evidenced with the i.p. administration of the special extract without the terpenes, with a lower efficacy for the lowest dose (10 mg/kg). These data confirm that EGb 761 treatment serves as useful therapy in supporting brain functional recovery in this animal model of vestibular compensation and lead to a more precise understanding of the biochemical component that is active in this recovery process.

Effects of contralateral white noise stimulation on transitory evoked otoacoustic emissions in patients with acoustic neuroma.

Transitory evoked otoacoustic emissions are normal phenomena observed in most persons with hearing levels greater than 35 dB. Further, masking of the contralateral ear produces amplitude reductions in the transitory evoked otoacoustic emissions. We have undertaken a study of transitory evoked otoacoustic emissions in 20 patients with acoustic neuroma. All patients were assessed for transitory evoked otoacoustic emissions bilaterally, with and without contralateral masking with white band noise at 40, 50, and 60 dB. We found that transitory evoked otoacoustic emissions were present in 30% of ears with tumor and that the presence of transitory evoked otoacoustic emissions is associated with improved preoperative hearing levels, but that tumor size is not associated with the presence or absence of transitory evoked otoacoustic emissions. The amplitude of transitory evoked otoacoustic emissions from ears with tumor, when present, is decreased when compared with normal ears of normal patients. Further, with contralateral masking little of the amplitude reduction observed in normal patients is observed in the ears with acoustic neuroma. However, with masking of the contralateral ear, the ear without tumor demonstrated significantly greater amplitude reductions than normal ears from normal patients (p = 0.0006). Pertinent anatomy and possible explanations for these findings are discussed.

Vestibular evoked myogenic potentials in the sternomastoid muscle are not of lateral canal origin.

We studied vestibular evoked myogenic potentials in 6 patients after unilateral vestibular neurectomy and in 22 patients after unilateral vestibular neuritis and unilateral absent caloric responses. We found that the ipsilesional vestibular evoked potentials were abolished in every patient after unilateral vestibular neurectomy. In vestibular neuritis patients we found that the ipsilesional vestibular evoked potentials were absent in some but not in all cases. These findings confirm that the p13-n23 potential is of vestibular origin and also show that it is not of lateral canal origin.

Effects of olivocochlear bundle section on otoacoustic emissions in humans: efferent effects in comparison with control subjects.

The effects of contralateral acoustic stimulation on evoked otoacoustic emissions (OAE) were examined in three subject groups in order that the impact of efferent olivocochlear bundle section (as a consequence of vestibular neurectomy) could be compared with normal findings, and with a control surgical population. Results demonstrated that the inhibitory effect of contralateral noise on OAE amplitude was absent from the cochlea with severed efferent fibers. These findings appear to be independent of acoustic reflex activity, as suppression was absent despite normal reflexes. Inter-aural suppression of emissions recorded from the patients' intact cochleae act as a control and show a clear reduction in amplitude during contralateral stimulation in a frequency specific pattern consistent with normal findings. Patients who had undergone a similar surgical approach for vascular decompression of the VIIIth nerve without vestibular nerve section, were studied in order to assess the impact of retrolabyrinthine surgery on inter-aural suppression. Inhibition of OAE amplitude was maintained in all control cases in both the operated and intact sides, and was consistent with suppression observed in normal subjects, suggesting that the surgical procedures had not disturbed inter-aural suppression of otoacoustic emissions. It is concluded that the olivocochlear efferent system, when activated by low level contralateral acoustic stimulation, has an inhibitory role in controlling the cellular mechanisms responsible for the generation of otoacoustic emissions in humans. OAE techniques in conjunction with contralateral acoustic stimulation may thus prove to be of value in providing a rapid and non-invasive clinical test of efferent function and offer a means of investigating the functional significance of the efferent auditory system in humans.

Click-evoked responses from the exposed intracranial portion of the eighth nerve during vestibular nerve section: bipolar and monopolar recordings.

We compare the click-evoked compound action potentials from the exposed intracranial portion of the eighth nerve using bipolar and monopolar recording electrodes in patients undergoing vestibular nerve section. It is assumed that a bipolar recording electrode will only record propagated neural activity in the auditory nerve, whereas a monopolar recording electrode may in addition record electrical activity that is conducted passively to the recording site. The results of the present study confirm that the earliest detectable propagated neural activity in the intracranial portion of the auditory nerve occurs with a latency that is close to that of peak II of the brain-stem auditory evoked potentials, and the results also confirm that the late components in the click-evoked compound action potentials that have been demonstrated previously using the monopolar recording technique represent propagated neural activity in the auditory nerve. The results also indicate that the responses that are recorded by a bipolar recording electrode, when the small tips of which are placed on the eighth nerve when it is relatively dry, represent only small populations of nerve fibers. Even when an attempt is made to align the two tips of a bipolar electrode with the course of the auditory nerve, this type of electrode may record from different populations of nerve fibers.

Tympanic and transtympanic electrocochleography in acoustic neuroma and vestibular nerve section surgery.

Eighth nerve action potential (AP) amplitudes and latencies and cochlear microphonic (CM) amplitudes were compared using tympanic and transtympanic electrocochleography (ECOG) in two patient groups. Tympanic ECOG was performed with a wick electrode placed on the tympanic membrane (TM). Transtympanic ECOG was performed with a needle electrode placed on the promontory of the anesthetized patient. Eighteen subjects were tested by tympanic ECOG as part of a preoperative assessment for either acoustic neuroma removal or transection of the vestibular portion of the eighth cranial nerve. Surgery occurred within 1 week of the preoperative evaluation. Intraoperative auditory monitoring was performed using transtympanic ECOG. Baseline recordings were compared to the preoperative tympanic ECOG data. Stimuli were condensation and rarefaction clicks and tone bursts, presented by an insert earphone. As expected, the two methods resulted in essentially identical response latencies and large amplitude differences, although the response amplitudes were extremely variable. The AP amplitude and the CM amplitude did not increase by the same factor with the transtympanic (TT) electrode compared to the tympanic electrode. On comparison of preoperative and intraoperative response amplitudes with regard to stimulus polarity, tympanic electrocochleography appears to be a useful method of gathering preliminary information on the status of the patient's auditory system. In this study, tympanic ECOG was found to have some predictive value when trying to ascertain the best intraoperative monitoring situation.

[The status of conditioned reflex activity in cats after unilateral vestibular neurotomy]. / Issledovanie sostoianiia uslovnoreflektornoi deiatel'nosti u koshek posle odnostronnei vestibuliarnoi neirotomii.

In chronic experiments on 6 cats the influence was studied of unilateral vestibular neurotomy on conditioned, oculographic and electrocardiographic reactions. In operated animals appeared sharply expressed posetonic and oculomotor disturbances, lowered general functional brain state, what was manifested in an increase of specific weight of slow spindle-shaped rhythmics and lowering of the conditioned activity level. Against the background of the lowered functional brain state interhemispheric asymmetry developed with relative predominance of the contralateral hemisphere, what was reflected in electrocorticographic manifestations and disturbance of conditioned spatial differentiations. Significance is grounded of the appearing interhemispheric asymmetry in the development of disturbances of spatial analysis in operated animals.