Intraoperative electrically evoked vestibular potentials in humans

Vestibular-Evoked Cerebral Potentials

The human vestibular cortex has mostly been approached using functional magnetic resonance imaging and positron emission tomography combined with artificial stimulation of the vestibular receptors or nerve. Few studies have used electroencephalography and benefited from its high temporal resolution to describe the spatiotemporal dynamics of vestibular information processing from the first milliseconds following vestibular stimulation. Evoked potentials (EPs) are largely used to describe neural processing of other sensory signals, but they remain poorly developed and standardized in vestibular neuroscience and neuro-otology. Yet, vestibular EPs of brainstem, cerebellar, and cortical origin have been reported as early as the 1960s. This review article summarizes and compares results from studies that have used a large range of vestibular stimulation, including natural vestibular stimulation on rotating chairs and motion platforms, as well as artificial vestibular stimulation (e.g., sounds, impulsive acceleration stimulation, galvanic stimulation). These studies identified vestibular EPs with short latency (<20 ms), middle latency (from 20 to 50 ms), and late latency (>50 ms). Analysis of the generators (source analysis) of these responses offers new insights into the neuroimaging of the vestibular system. Generators were consistently found in the parieto-insular and temporo-parietal junction-the core of the vestibular cortex-as well as in the prefrontal and frontal areas, superior parietal, and temporal areas. We discuss the relevance of vestibular EPs for basic research and clinical neuroscience and highlight their limitations.

Acoustically evoked, short latency negative response in children with sensorineural hearing loss

Introduction:

The auditory brainstem response consists of fast and slow waves. The acoustically evoked, short latency negative response is a large, negative deflection with a latency of 3 milliseconds which has been reported in patients with profound hearing loss. It may be of vestibular, particularly saccular, origin, as is the vestibular evoked myogenic potential.

Purpose:

To assess the presence of acoustically evoked, short latency negative responses in children with severe to profound sensorineural hearing loss.

Materials and methods:

Twenty-three children (46 ears) with sensorineural hearing loss underwent audiological evaluation and auditory brainstem response, vestibular evoked myogenic potential and caloric testing.

Results:

An acoustically evoked, short latency negative response was present in 30.43 per cent of ears and absent in 69.57 per cent. Vestibular evoked myogenic potentials were recorded in all ears in the former group, but in only 53.13 per cent in the latter group. Caloric testing was normal in 82.6 per cent of the total ears tested.

Conclusion:

The presence of an acoustically evoked, short latency negative response is dependent not on residual hearing but on normal saccular function. This response can be measured in patients who cannot contract their neck muscles.

Saccular origin of acoustically evoked short latency negative response

OBJECTIVES

To determine the origin of the acoustically evoked short latency negative response (ASNR), a peculiar V-shaped response at about 3 to 4 milliseconds found during auditory brainstem response recordings. Previous reports demonstrated that the ASNR is present only in ears with profound hearing loss under intense stimulation. It has already been excluded as any kind of artifact. The individuals with ASNR had good vestibular function in spite of their poor hearing, suggesting a relation between the ASNR and the vestibular system. The saccule and vestibular nuclei are hypothesized to be the sense organ and the generator of the response, respectively. The current study tested the saccular function for ears with ASNR and searched for ASNR in ears with a functionless cochlea.

STUDY DESIGN

Prospective study.

SETTING

Academic tertiary referral center.

PATIENTS

Twenty patients with bilateral profound hearing loss, aged 6 to 62 years, including 16 cochlear implant recipients. Twelve healthy participants with normal hearing, aged 23 to 30 years, served as the control group.

INTERVENTIONS

Recordings of vestibular evoked myogenic potential (VEMP) and auditory brainstem responses.

MAIN OUTCOME MEASURES

The presence or absence of ASNR in ears with cochlear implants. The presence or absence and threshold of VEMP in ears with ASNR versus ears without ASNR.

RESULTS

ASNR was recorded by sound stimulation from three ears with unaided cochlear implants, a model of functionless cochlea. VEMP was evoked by sound stimulation to all the nine ASNR ears without threshold difference from normal control ( > 0.05), implying normal saccular function for the ASNR ears. For the ears with profound hearing loss and absence of ASNR, about two thirds were considered to have saccular afunction because of absence of VEMP. The other third displayed VEMPs with higher threshold than normal control ( < 0.01), indicating saccular hypofunction. Furthermore, ASNR and VEMP were elicited from an ear diagnosed with semicircular canal hypofunction.

CONCLUSIONS

It is clear that the presence of ASNR is dependent not on residual hearing but on normal saccular function. On the basis of these results, the authors believe ASNR to be saccular in origin.

Electrically evoked auditory potentials: comparison between transtympanic promontory and round-window stimulations

OBJECTIVE

Electrically evoked auditory potential (eEAP) techniques have been developed in order to assess electrical excitability of auditory nervous pathways in deaf patients before cochlear implantation. The aim of this study was to compare eEAPs recorded after a transtympanic promontory stimulation with those obtained after a round-window stimulation during a surgical approach.

DESIGN

eEAPs were recorded after promontory and round-window stimulation in 10 deaf subjects (i.e. 14 ears) who were candidates for cochlear implantation.

RESULTS

eEAPs were successfully recorded in 13 out of 14 (93%) and 11 out of 14 (79%) ears after promontory stimulation and round-window stimulation, respectively. Similar waveforms, latencies, thresholds and eV amplitudes were observed in both cases.

CONCLUSIONS

Both tests provide objective and useful information for the patient selection process before cochlear implantation. Transtympanic stimulation should be preferred in clinical practice as it is less invasive than round-window stimulation and does not require surgical exposure.

Electrical and physiological changes during short-term and chronic electrical stimulation of the normal cochlea

In the electrical stimulation (ES) of auditory pathways, the type of stimulus and the electrode/tissue interface are critical parameters for the safety and efficacy of the protocol. In this study the influence of alternate pulses, applied between round window and vertex electrodes in chronically implanted guinea pigs, and maintained during 1 and 25 daily periods of 2 h (short-term and long-term experiments, respectively), was investigated. ES consisted of monophasic current pulses of +/- 70 microA and 300 (micro)s in duration at a rate of 167/s, with alternate polarity. Compound Action Potential (CAP) audiograms, amplitudes and latencies of click-evoked CAPs, amplitudes and latencies of electrically-evoked auditory responses (EARs), and electrode impedances, were measured periodically outside or during the ES periods. Short-term ES induced no change in CAP thresholds, amplitude and latency in response to clicks at 80 dB above normal threshold, but induced a slight latency increase and amplitude decrease of the EAR, correlated with an exponential decay of the electrode impedance. On a long-term basis, CAP audiograms and latencies did not change significantly, whereas CAP amplitudes and electrode impedances increased, in correlation with each other. In control guinea pigs receiving no ES, the same CAP amplitude and impedance changes were observed over the same long-term period. The EAR and CAP changes can be explained by a variation of the electrical impedance of the electrode/tissue interface. This is possibly due to a change in electrolytes around the electrode under the influence of the ES for the short-term variation, and to an electrode encapsulation by fibrous tissue independent of the ES for the long-term change. In itself, and under the conditions of this experiment, the ES demonstrated no adverse effects on the auditory function and can be safely used for inner-ear exploration.

Electric response audiometry in young children before cochlear implantation: a short latency component

OBJECTIVE

Evaluation of a short latency component (SLC) observed in profoundly deaf young children during recordings of the auditory brain stem response (ABR) before cochlear implantation.

DESIGN

Seventy young children (ages 2 to 11 yr) were investigated as part of their routine audiologic and diagnostic assessment, before cochlear implantation. The ABR was evoked using click stimuli and tone pips (500 Hz and 1 kHz) at intensity levels up to 105 dB nHL. The incidence of the SLC arising at a latency of 3 msec with high level click stimuli, in addition to residual ABR waves, was assessed.

RESULTS

An SLC was observed in a total of 18 of the 70 children investigated. It was present in 15 out of 31 congenitally deaf children (48%) compared with only 2 out of 33 children deafened after meningitis (6%). A chi-squared test employing a 2 x 2 contingency table shows that this dependency on etiology of deafness is highly statistically significant (p < 0.001). The presence of the SLC in the congenitally deaf children is related to age at the time of the Electric Response Audiometry test and is more likely to be observed in the younger child (p < 0.01).

CONCLUSIONS

Interpretation of the ABR in profoundly deaf children should take into consideration the possible presence of the SLC. An evoked potential arising from stimulation of the vestibular system, particularly the vestibular nuclei, is proposed as a likely origin for this component. Damage to sensory cells in the vestibular portion of the labyrinth might explain why an SLC is seen rarely in cases of deafness after meningitis.

Ménière's disease

There now exists a vast literature on Ménière's disease. In this review, we examine closely the foundations of some of the current concepts regarding various aspects of the disorder, including definitions, clinical features, natural history, pathology, pathogenesis, pathophysiology, diagnosis and treatment. We will highlight areas where the current state of knowledge is incomplete and also suggest some avenues for further research.

Electrophysiologic identification of the cochlear nerve fibers during cerebello-pontine angle surgery

To facilitate identification and preservation of the auditory nerve during cerebello-pontine angle surgery, bipolar recording of cochlear nerve compound action potentials (CNAPs) was performed. Two silver wires insulated with teflon up to the exposed ends were utilized as electrodes. They were twisted together, the distance between the two tips being 1 mm or less. Rarefaction polarity clicks (31/s) ranging from the psychoacoustical threshold to 120 dB pe SPL were used as stimuli. The investigation was performed in three groups of patients. The first group consisted of 9 patients submitted to vestibular neurectomy and 4 patients operated on by microvascular decompression of the eighth nerve. The second group comprised 8 patients with acoustic tumors smaller than 2 mm and serviceable hearing. Postoperative audiometric results in the subjects in the second group were compared with those obtained in well-matched homogeneous controls consisting of patients with acoustic neuroma operated on without the aid of CNAP recording. Bipolar recording from the eighth nerve was extremely selective, a good response being obtained only when positioning the electrode on the cochlear portion of the eighth nerve. During removal of the acoustic neuroma, repeated bipolar probing of the tumor and eighth nerve facilitated the task of distinguishing the cochlear nerve from other nervous structures and from the tumor, and contributed to preserving hearing in most patients.