Early evoked potentials in patients with acoustic neuroma

A novel intraoperative continuous monitoring method combining dorsal cochlear nucleus action potentials monitoring with auditory nerve test system

Highly accurate real-time cochlear nerve monitoring to preserve cochlear nerve function is essential for simultaneous cochlear implantation and ipsilateral vestibular schwannoma resection. In the present study, we developed a novel real-time monitoring system that combines dorsal cochlear nucleus action potential monitoring with intracochlear stimulating electrodes (Auditory Nerve Test System, ANTS). We used this system for a case with vestibular schwannoma resection via the translabyrinthine approach. The monitoring system developed in this study detected highly reliable evoked potentials from the cochlear nerve every two seconds continuously during tumor resection. Near-total tumor resection was achieved, and cochlear implantation was performed successfully after confirming the preservation of cochlear nerve function in a case. The patient's hearing was well compensated by cochlear implantation after surgery. Our novel method continuously achieved real-time monitoring of the cochlear nerve every two seconds during vestibular schwannoma resection. The usefulness of this monitoring system for simultaneous tumor resection and cochlear implantation was demonstrated in the present case. The system developed in this study is compatible with continuous facial nerve monitoring. This highly accurate and novel monitoring method will broaden the number of candidates for this type of surgery in the future.

Audiologic characterization using clinical physiological measures: Normative data from macaque monkeys

Clinical auditory physiological measures (e.g., auditory brainstem responses, ABRs, and distortion product otoacoustic emissions, DPOAEs) provide diagnostic specificity for differentially diagnosing overt hearing impairments, but they remain limited in their ability to detect specific sites of lesion and subtle levels of cochlear damage. Studies in animal models may hold the key to improve differential diagnosis due to the ability to induce tightly controlled and histologically verifiable subclinical cochlear pathologies. Here, we present a normative set of traditional and clinically novel physiological measures using ABRs and DPOAEs measured in a large cohort of male macaque monkeys. Given the high similarities between macaque and human auditory anatomy, physiology, and susceptibility to hearing damage, this normative data set will serve as a crucial baseline to investigate novel physiological measures to improve diagnostics. DPOAE amplitudes were robust at f2 = 1.22, L1/L2 = 65/55, increased with frequency up to 10 kHz, and exhibited high test re-test reliability. DPOAE thresholds were lowest from 2-10 kHz and highest < 2 kHz. ABRs with a standard clinical electrode montage (vertex-to-mastoid, VM) produced Waves I-IV with a less frequently observed Wave-I, and lower thresholds. ABRs with a vertex-to-tympanic membrane (VT) electrode montage produced a more robust Wave-I, but absent Waves II-IV and higher thresholds. Further study with the VM montage revealed amplitudes that increased with stimulus level and were largest in response to click stimuli, with Wave-II showing the largest ABR amplitude, followed by -IV and -I, with high inter- and intra-subject variability. ABR wave latencies decreased with stimulus level and frequency. When stimulus presentation rate increased or stimuli were presented in close temporal proximity, ABR amplitude decreased, and latency increased. These findings expand upon existing literature of normative clinical physiological data in nonhuman primates and lay the groundwork for future studies investigating the effects of noise-induced pathologies in macaques.

The evaluation of tinnitus and auditory brainstem response in benign paroxysmal positional vertigo accompanied by tinnitus

PURPOSE

The aim of this study is to investigate auditory brainstem response (ABR) in patients with benign paroxysmal positional vertigo (BPPV) accompanied by tinnitus and to suggest possible interpretative hypotheses.

METHODS

Ninety individuals were included in the study. Individuals were separated into three groups: patients reporting tinnitus with BPPV (Group I), patients with BPPV (Group II), and a control group. The ABR test was applied at a low and at a high rate.

RESULTS

For patients reporting tinnitus with BPPV, tinnitus was found to be localized in the ear affected by BPPV. Tinnitus disappeared after therapeutic interventions in 23 individuals with tinnitus. The difference between the Wave V  latency at high rate and Wave V  latency at a low rate in the affected ears of all individuals with BPPV (Groups I and II) was significantly long. In the affected ears of all BPPV patients, at a high rate of ABR, the absolute latency of the Wave III was found to be significantly longer than for the control group.

CONCLUSIONS

Individuals with BPPV showed prolonged latencies in affected ears in a high rate of ABR without the effect of tinnitus. High rate of ABR in individuals with BPPV can be used to obtain preliminary information in cases where ischemia in the auditory pathways is suspected in BPPV formation.

Auditory neuropathy in a patient with hemochromatosis

OBJECTIVE

To evaluate the auditory function of an individual with genetically confirmed hemochromatosis.

METHODS

A 57 year old male with mildly impaired sound detection thresholds underwent a range of behavioural, electroacoustic and electrophysiologic assessments. These included the recording of otoacoustic emissions and auditory brainstem responses, measurement of monaural temporal resolution and evaluation of binaural speech processing. Findings for this patient were subsequently compared with those of 80 healthy controls with similar audiometric thresholds.

RESULTS

The patient showed the three cardinal features of auditory neuropathy, presenting with evidence of normal cochlear outer hair cell function, disrupted neural activity in the auditory nerve/brainstem and impaired temporal processing. His functional hearing ability (speech perception) was significantly affected and suggested a reduced capacity to use localization cues to segregate signals in the presence of background noise.

CONCLUSION

We present the first case of an individual with hemochromatosis and auditory neuropathy. The findings for this patient highlight the need for careful evaluation of auditory function in individuals with the disorder.

Pathophysiological mechanisms and functional hearing consequences of auditory neuropathy

The effects of inner ear abnormality on audibility have been explored since the early 20th century when sound detection measures were first used to define and quantify 'hearing loss'. The development in the 1970s of objective measures of cochlear hair cell function (cochlear microphonics, otoacoustic emissions, summating potentials) and auditory nerve/brainstem activity (auditory brainstem responses) have made it possible to distinguish both synaptic and auditory nerve disorders from sensory receptor loss. This distinction is critically important when considering aetiology and management. In this review we address the clinical and pathophysiological features of auditory neuropathy that distinguish site(s) of dysfunction. We describe the diagnostic criteria for: (i) presynaptic disorders affecting inner hair cells and ribbon synapses; (ii) postsynaptic disorders affecting unmyelinated auditory nerve dendrites; (iii) postsynaptic disorders affecting auditory ganglion cells and their myelinated axons and dendrites; and (iv) central neural pathway disorders affecting the auditory brainstem. We review data and principles to identify treatment options for affected patients and explore their benefits as a function of site of lesion.

The use of QSD (q-sequence deconvolution) to recover superposed, transient evoked-responses

OBJECTIVE

We describe q-sequence deconvolution (QSD), a new data acquisition/analysis method for evoked-responses that solves the problem of waveform distortion at high stimulus repetition-rates, due to response overlap. QSD can increase the sensitivity of clinically useful evoked-responses because it is well known that high stimulus repetition-rates are better for detecting pathophysiology.

METHODS

QSD is applicable to a variety of experimental conditions. Because some QSD-parameters must be chosen by the experimenter, the underlying principles and assumptions of the method are described in detail. The theoretical and mathematical bases of the QSD method are also described, including some equivalent computational formulations.

RESULTS

QSD was applied to recordings of the human auditory brainstem response (ABR) at stimulus repetition-rates that overlapped the responses. The transient ABR was recovered at all rates tested (highest 160/s), and showed systematic changes with stimulus repetition-rate within a single subject.

CONCLUSIONS

QSD offers a new method of recovering brain evoked-response activity having a duration longer than the time between stimuli.

SIGNIFICANCE

The use of this new technique for analysis of evoked responses will permit examination of brain activation patterns across a broad range of stimulus repetition-rates, some never before studied. Such studies will improve the sensitivity of evoked-responses for the detection of brain pathophysiology. New measures of brain activity may be discovered using QSD. The method also permits the recovery of the transient brain waveforms that overlap to form 'steady-state' waveforms. An additional benefit of the QSD method is that repetition-rate can be isolated as a variable, independent of other stimulus characteristics, even if the response is a nonlinear function of rate.

Age-related changes in auditory evoked potentials of gerbils. III. Low-frequency responses and repetition rate effects

The auditory brainstem response (ABR) was recorded non-invasively from Mongolian gerbils ranging in age from 6 to 36 months. The ABR was elicited using gaussian tone bursts at octave intervals from 1 to 16 kHz. Responses were bandpass filtered from 30 to 300 Hz (LF-ABR; low-frequency component) and from 300 to 3000 Hz (HF-ABR; high-frequency component). In Experiment A, the thresholds of the two components (HF- and LF-ABR) were compared in 6- and 36-month subjects. The LF-ABR varied more with age than did the HF-ABR, particularly at stimulus frequencies of 2 kHz and above. As shown previously for the HF-ABR, the latencies of the LF-ABR increased as a function of hearing loss in aged gerbils whereas amplitudes of the LF-ABR were reduced in all aged gerbils, regardless of age-related threshold elevation. In Experiment B, tone bursts were presented at rates of 11-91/s to gerbils aged 6, 18, 30, and 36 months. Increased repetition rate resulted in an increase in the latency of both the HF- and LF-ABR, but to the same degree in each age group. Similarly, the interpeak intervals of the HF-ABR increased as a function of repetition rate in all subjects to the same degree. Increased age and increased repetition rate both resulted in significant reductions in ABR amplitudes, but rate did not interact with age. The data suggest that (1) the LF-ABR may be more sensitive to aging than is the HF-ABR and (2) there are no age-related changes in the HF- or LF-ABR which are dependent upon the repetition rate.

Audiological findings following head trauma

Traumatic head injury is the principal cause of death and serious physical disability in adolescents and young adult. Hearing loss is a common sequela of head trauma. The results of pure tone and auditory brain-stem response (ABR) assessment of minor head injury revealed that 20 per cent of these cases showed impairment of hearing including both conductive and sensorineural hearing loss mostly in the high frequencies loss. ABR findings showed a significant difference of absolute wave V latency and interwave (III-V) latencies at repetition rate 11.1 click/sec., and a highly significant difference at a repetition rate of 51.1 click/sec., compared with the control group. In severe head injury, conductive hearing loss was found in only one case, in which a longitudinal fracture of the temporal bone was identified radiologically. No ABR could be recorded at a repetition rate of 11.1 click/sec. in either ear of this case; this patient died a few days later. Three cases were found with a mixed sensorineural hearing loss; one of them showed a longitudinal fracture radiologically. ABR findings in two cases showed prolongation of wave V and I-V interwave latencies, and the third case showed absence of ABR recording. In comatose patients, an ABR grading system is a more sensitive index of brainstem dysfunction than the simple division of normal versus abnormal ABR waves and interwave latencies. Presence or prolongation of wave V and interwave latency I-V even in one ear is of good prognostic value in the comatosed patient.

Auditory brainstem evoked potentials in sudden deafness

Auditory Brainstem Evoked Potentials (ABEP) were recorded from 53 adult subjects suffering from sudden deafness at the time of examination. The onset of sudden deafness was up to three years prior to the study. ABEP were recorded in response to 75 dBHL clicks presented at rates of 10/sec and 40/sec. ABEP peak latencies as well as interpeak latency differences, and also the effect of increasing stimulus rate, were determined for each patient and compared with the clinical symptoms. The psychoacoustic and ABEP detection thresholds were also compared. A highly significant correlation was observed between ABEP detection threshold and the psychoacoustically determined hearing threshold, and the differences between them were found to be insignificant. The results of this study indicate a central component in sudden deafness. This impairment manifests itself in abnormal effects of increasing stimulus rate on ABEP, and may be related to reduced central synaptic efficacy.

Three-channel Lissajous' trajectory of human auditory brain-stem evoked potentials. III. Effects of click rate

Three-channel Lissajous' trajectories (3-CLT) of the human auditory brain-stem evoked potentials (ABEPs) were recorded from 14 adult subjects using click rates of 10, 55 and 80/sec. The 3-CLTs were analysed and described in terms of their constituent planar segments and their trajectory amplitudes at each stimulus rate. Increasing stimulus rate resulted in an increase of planar segment duration which was more pronounced for segments 'a' and 'e', an increase in apex latency which was more pronounced the later the component and a decrease in planar segment size and peak trajectory amplitude which was more pronounced the earlier the component. These findings support the involvement of synaptic efficacy changes in the effects of stimulus rate on ABEP. The results are explained by overlapping convergence and divergence in the ascending auditory pathway. These results support the notion that the principal generator of each component is activated by the principal generator of the previous component, with some temporal overlap of their activities. Such temporal overlap may be minimized by using low intensity high rate stimuli.

Diagnostic value of auditory brain stem responses (ABR) in differential diagnosis between cochlear deafness and acoustic neuromas

The diagnostic value of ABR for the differential diagnosis between cochlear deafness (CD) and acoustic neuromas (AN) was investigated. The following ABR parameters against 80 dB click stimulation was compared between CD and AN; (1) incidence of presence of each wave, (2) wave I latency, (3) wave III latency, (4) wave V latency, (5) I-III interpeak latency, (6) I-V interpeak latency, (7) right and left latency difference of wave III, (8) right and left latency difference of wave V, and (9) ABR pattern. It was concluded that a battery of 6 latency parameters (3-8) is very useful for the differential diagnosis between CD and AN and therefore it can be expected to be used as a powerful tool for finding out the small size AN.

Intraoperative monitoring of brain-stem auditory evoked potentials

Intraoperative brain-stem auditory evoked potentials (BAEP's) were monitored in 46 patients undergoing intracranial surgery for a variety of pathological conditions to determine whether this technique was capable of providing useful information to the operating surgeon. Intraoperative BAEP's were unchanged throughout surgery in 34 patients (74%), and these individuals had no postoperative hearing deficits. Four patients (9%) developed an abrupt ipsilateral loss of all waveform components beyond Wave I and had postoperative evidence of a pronounced hearing loss in the affected ear. An additional patient demonstrated BAEP loss contralateral to the side of surgery, and this was associated with subsequent signs of severe brain-stem dysfunction. Seven patients (15%) developed intraoperative delays of BAEP waveform latency values, but maintained recognizable waveforms beyond Wave I. Postoperatively, their hearing was either normal or mildly impaired, and there were no indications of other brain-stem abnormalities. This group represents the individuals who may have been benefited by evoked potential monitoring, since corrective surgical measures were taken when latency delays were observed. Intraoperative BAEP's can be reliably and routinely recorded in an operating room environment. They provide a good predictor of postoperative auditory status, and may have prevented permanent neurological deficits in a small segment of patients by alerting the surgeon to potentially reversible abnormalities.

Evoked potentials in the clinical neurosciences

The use of evoked potentials for the evaluation of disorders of the nervous system has become a most valuable aid to the neurosurgeon and neurologist, often providing information of critical value without recourse to invasive techniques. In order to employ these techniques, it is helpful to understand the principles of evoked potential electrogenesis and the methodology used for analysis of evoked potential clinical data. This article is aimed at providing the clinical neurosurgeon with this type of information and with a review of current clinical applications in this rapidly developing field.

Prognostic implications of early multimodality evoked potentials in severely head-injured patients. A prospective study

Results of multimodality evoked potential (MEP) studies recorded from 100 comatose patients soon after severe head injury were analyzed prospectively, using a previously established grading system, to assess the prognostic value of MEP's with respect to patient outcome, to evaluate the effect of clinically relevant sequelae of head injury on the prediction of outcome by MEP's and to describe time to clinical recovery as a function of initial MEP grade. Graded MEP's when recorded in the first few days after head injury, could predict patient outcome at 1 year with approximately 80% accuracy. Exclusion from the analysis of patients who died from causes unrelated to the brain and those with severe systemic complications that occurred after the evoked potentials were recorded improved the accuracy of outcome prediction to nearly 100%. The presence of a mass lesion requiring surgery reduces the probability if good to moderate outcome for a given MEP grade by approximately 25% to 40% from the seen in patients without mass lesions. The clinical outcome predicted shortly after head injury by MEP grades may not be realized for many months. Patients with mild MEP abnormality (Grade I or II) generally reach their outcome by 3 to 6 months, whereas those with more severe deficits (Grade III) may not show improvement for at least 1 year.

Auditory brain stem evoked potentials: clinical promise of increasing stimulus rate

Auditory brain stem evoked potentials (ABEPs) were recorded from 10 adults and 10 children who where neurologically and audiometrically normal. ABEPs were recorded in response to 75 dB HL clicks presented at rates of 10/sec and 50/sec. Normative values were calculated for amplitude and latency, as well as for inter-peak amplitude ratio and a variety of inter-peak latency differences and interaural differences at the two stimulus presentation rates. Normative values of the effect of increasing stimulus rate were calculated as well. Measures of changes in ABEPs between stimulus rates of 50/sec and 10/sec were the only derived measures that were significantly different between our adult and child normal populations. In addition, 50 patients with various conditions affecting the brain stem were examined. Increasing stimulus presentation rate had a significant effect on detection of abnormality in ABEPs from the patients examined. Measures of changes in ABEPs between stimulus rates of 50/sec and 10/sec seemed to be sensitive to a subset of abnormalities in our patient population. The case histories of the patients indicate that the subset may be impaired synaptic function. Measures of the effect of rate on ABEPs may complement the traditional measures that are primarily sensitive to white matter lesions.

Evaluation and graduation of brain stem auditory evoked potentials in patients with neurological diseases

Based on normative data from 40 subjects, criteria for evaluating brain stem auditory evoked potentials have been elaborated. This system contained six grades from normal to diffusely abnormal, and was used in evaluating recordings from 142 patients, including 38 patients with definite and 48 with probable and possible multiple sclerosis, 18 patients with brain stem infarctions, six patients with other organic brain stem lesions, six patients with a postcommotional syndrome and 26 patients without any brain stem involvement. A statistically significant difference was found between normal persons and patients without evidence of brain stem lesions versus patients with such evidence, as well as between subgroups of slightly and severely affected patients. The system was found useful as a way of describing recordings, in company groups of patients and in a follow up of a few patients.

Reconstruction of the audiogram using brain stem responses and high-pass noise masking

Contributions to the brain stem electrical responses (BSER) presumably initiated from specific frequency regions of the cochlea with center frequencies similar to the major audiometric frequencies (0.5, 1, 2, 4, and 8 kHz) are derived by the application of a high-pass noise masking technique utilizing click stimuli. In normal hearing subjects, these derived narrow-band responses from the midfrequency regions (4, 2, and 1 kHz) can be recognized at click levels as low as 10 dB HL. For the frequency regions around 8 kHz and 0.5 kHz, these derived responses can be discerned at click levels of 30 dB HL and higher. When one uses the lowest click level at which these derived responses can be obtained from a given frequency region, the differences between a patient with a hearing loss and a normal hearing subject correlate well with the amount of hearing loss (air conduction) recorded by conventional pure tone audiometry. Use of the high-pass noise masking technique to reconstruct the audiogram may be of great potential value in assessing young children and other individuals who cannot or will not respond to conventional audiometry.

Effects of interstimulus interval on latency of the brainstem auditory evoked potential

Peak latency of the inferior collicular response (wave V) of the brainstem auditory evoked potential (BAEP) increases with decreasing interstimulus interval (ISI). The present research investigated relative peak shifts of major components of the BAEP in an attempt to identify the source of the observed shift in wave V. Peripheral as well as central transmission times (CTT) were computed for 9 young adults between an ISI of 100 msec and an ISI of 10 msec. The results indicate that shifts in wave V latency with changing ISI are due to a combination of central and peripheral processes. The magnitude of the shift observed for wave V was approximately 0.50 msec. This increase in latency was accounted for by an average 0.29 msec increase in peripheral transmission time, a 0.14 msec increase in CTT between waves I and III, and a 0.07 msec increase in CTT between waves III and V. The results suggest a dual process underlying latency changes in wave V of the BAEP as a function of decreasing ISI. These are peripheral adaptation or fatigue and central processes that may include short-term habituation.

Auditory brain-stem responses in comatose patients: relationship with brain-stem reflexes and levels of coma

Auditory brain-stem responses (BSR) were recorded in 20 comatose patients in whom the level of brain-stem dysfunction was defined by clinical assessment of brain-stem reflexes and posture. No BSR abnormalities were found in the 10 cases with cortico-subcortical or diencephalic levels. The other 10 patients showed a clear relationship between alteration of the different components of the BSR and the clinical levels of brain-stem dysfunction caused by the rostro-caudal evolution. Alteration of wave P5 seems related to a midbrain dysfunction, of P3 to a pontine dysfunction and of P1 or P2 to a lower brain-stem dysfunction.

On the use of click-evoked electric brainstem responses in audiological diagnosis. I. The variability of the normal response

The normal click-evoked BSER was studied in a total of 28 young (aged 12 to 40) normally hearing subjects in regard to wave latencies and amplitude ratios, considered to be (besides wave replicability) the characteristics of major interest from a clinical point of view. Latency means and standard deviations (S.D.), and peak-to-peak amplitude ratios between wave V on the one hand and wave I and III, respectively, on the other were determined in a sample of 20 subjects using unfiltered clicks at 20 to 90 dB SL. The latency averages were compared with those of other reports. Latency S.D.s were found to be generally less than 0.25 ms at levels above 50 dB SL. Medians of amplitude ratios (AV/AI and AV/AIII) were found to be significantly greater than unity. No significant latency differences concerning waves I, III, and V could be established on stimulation with condensation, rarefaction, and alternating clicks in 10 subjects at 80 and 60 dB SL, nor did the peak-to-peak amplitude predominance of wave V change with click polarity. With half-octave filtered clicks (2 and 4 kHz), latencies were found to increase significantly (and the responses to become less precise) at 80 and 60 dB SL in a sample of 12 subjects. Changing the interstimulus interval (ISI) from 80 to 60 to 40 ms (at 80 and 60 dB SL) hardly made any difference in regard to I, III, and V latencies, or wave V amplitude supremacy. Finally, a satisfactory test-retest reliability was established in a sample of 6 subjects, the intersession interval being 6 months.