The cochlear nerve in various forms of deafness

Survey of selective electrode deactivation attitudes and practices by cochlear implant audiologists

OBJECTIVES

The purpose of this study was to explore clinician attitudes regarding selective electrode deactivation and to investigate the primary methodology used to identify poorly encoded electrodes, deactivate identified electrodes, and measure outcomes.

METHODS

An online survey consisting of 32 questions was administered to certified clinical and research cochlear implant (CI) audiologists. Questions asked participants about their demographic information, device programming patterns, and attitudes regarding selective electrode deactivation.

RESULTS

Fifty-four audiologists completed the survey. When asked whether they believed selectively deactivating poorly encoded electrodes could improve speech perception outcomes, 43% of respondents selected 'Probably Yes,' 39% selected 'Definitely Yes,' and 18% selected 'Might or Might Not.' Of those who reported deactivating electrodes as part of CI programming, various methodology was reported to identify and deactivate poorly encoding electrodes and evaluate effectiveness of deactivation. General reasons against deactivation were also reported.

DISCUSSION

CI audiologists generally believed selective electrode deactivation could be used to improve speech perception outcomes for patients; however, few reported implementing selective electrode deactivation in practice. Among those who do perform selective electrode deactivation, the reported methodology was highly variable.

CONCLUSION

These findings support the need for clinical practice guidelines to assist audiologists in performing selective electrode deactivation.

Evaluating and Comparing Behavioural and Electrophysiological Estimates of Neural Health in Cochlear Implant Users

Variations in neural health along the cochlea can degrade the spectral and temporal representation of sounds conveyed by cochlear implants (CIs). We evaluated and compared one electrophysiological measure and two behavioural measures that have been proposed as estimates of neural health patterns, in order to explore the extent to which the different measures provide converging and consistent neural health estimates. All measures were obtained from the same 11 users of the Cochlear Corporation CI. The two behavioural measures were multipulse integration (MPI) and the polarity effect (PE), both measured on each of seven electrodes per subject. MPI was measured as the difference between thresholds at 80 pps and 1000 pps, and PE as the difference in thresholds between cathodic- and anodic-centred quadraphasic (QP) 80-pps pulse trains. It has been proposed that good neural health corresponds to a large MPI and to a large negative PE (lower thresholds for cathodic than anodic pulses). The electrophysiological measure was the effect of interphase gap (IPG) on the offset of the ECAP amplitude growth function (AGF), which has been correlated with spiral ganglion neuron density in guinea pigs. This 'IPG offset' was obtained on the same subset of electrodes used for the behavioural measures. Despite high test-retest reliability, there were no significant correlations between the neural health estimates for either within-subject comparisons across the electrode array, or between-subject comparisons of the means. A phenomenological model of a population of spiral ganglion neurons was then used to investigate physiological mechanisms that might underlie the different neural health estimates. The combined experimental and modelling results provide evidence that PE, MPI and IPG offset may reflect different characteristics of the electrode-neural interface.

Interpreting the Effect of Stimulus Parameters on the Electrically Evoked Compound Action Potential and on Neural Health Estimates

Variations in the condition of the neural population along the length of the cochlea can degrade the spectral and temporal representation of sounds conveyed by CIs, thereby limiting speech perception. One measurement that has been proposed as an estimate of neural survival (the number of remaining functional neurons) or neural health (the health of those remaining neurons) is the effect of stimulation parameters, such as the interphase gap (IPG), on the amplitude growth function (AGF) of the electrically evoked compound action potential (ECAP). The extent to which such measures reflect neural factors, rather than non-neural factors (e.g. electrode orientation, electrode-modiolus distance, and impedance), depends crucially upon how the AGF data are analysed. However, there is currently no consensus in the literature for the correct method to interpret changes in the ECAP AGF due to changes in stimulation parameters. We present a simple theoretical model for the effect of IPG on ECAP AGFs, along with a re-analysis of both animal and human data that measured the IPG effect. Both the theoretical model and the re-analysis of the animal data suggest that the IPG effect on ECAP AGF slope (IPG slope effect), measured using either a linear or logarithmic input-output scale, does not successfully control for the effects of non-neural factors. Both the model and the data suggest that the appropriate method to estimate neural health is by measuring the IPG offset effect, defined as the dB offset between the linear portions of ECAP AGFs for two stimuli differing only in IPG.

Pictorial review of MRI/CT Scan in congenital temporal bone anomalies, in patients for cochlear implant

High-resolution CT scan (HRCT) and MRI are routinely performed prior to cochlear implant surgery. These modalities help assess the status of the inner ear structures. A few patients have significant anomalies, which need to be assessed and understood in detail. We present a pictorial essay of these anomalies and described our HRCT and MRI techniques in patients being imaged prior to surgery.

Favorable Outcome of Cochlear Implant in VIIIth Nerve Deficiency

OBJECTIVE

To report on the outcomes of cochlear implantation (CI) in a child with cochleovestibular nerves (CVN) hypoplasia.

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center, University hospital.

PATIENTS

An 18-month-old child with profound bilateral congenital hearing loss and bilateral hypoplasia of the CVN at imaging.

INTERVENTION

Left CI at age 29 months with a Nucleus Contour device (Cochlear Ltd., Lane Cove, New South Wales, Australia) after unsatisfactory results of hearing aid use for 10 months.

MAIN OUTCOME MEASURES

Speech perception tests, behavioral observation, electrophysiologic tests, and cognitive evaluation.

RESULTS

Although the child scores poorly in every perceptive category with the CI alone, the device greatly enhances his speech understanding with the hearing aid in the opposite ear. In the bimodal condition, his words and sentences identification, recognition, and comprehension far exceed the monaural figures. The Meaningful Auditory Integration Scale (MAIS) tests reaches a score of 26/40, and the MacArthur's questionnaires confirm the improvement of language production and comprehension. These results became noticeable after 5 to 6 months and continued to improve up to the 10th month. The child's cognitive scores and overall performance competences greatly benefit from the CI, with the mental age overcoming the chronological age.

CONCLUSION

We can confirm the chance of achieving satisfactory results by CI even when the imaging of CVN is doubtful and the electrophysiological tests are disappointing. In our experience, a CI in Type IIb dysplasia of the CVN is a feasible option, provided that the candidate shows some responses at aided audiogram and at least minimal signs of language development. Adequate counseling is necessary for these children because the expected outcome is somewhat lower than that of their deaf peers with normal appearance of the nerves.

Auditory Neuropathy Characteristics in Children with Cochlear Nerve Deficiency

OBJECTIVE

To describe a group of children exhibiting electrophysiologic responses characteristic of auditory neuropathy (AN) who were subsequently identified as having absent or small cochlear nerves (i.e., cochlear nerve deficiency).

DESIGN

A retrospective review of the clinical records, audiological testing results, and magnetic resonance imaging (MRI) studies. Fifty-one of 65 children with AN characteristics on auditory brain stem response (ABR) testing had MRI available for review. Nine (18%) of these 51 children with ABR characteristic of AN have been identified as having small (N = 2; 4%) or absent (N = 7; 14%) cochlear nerves on MRI.

RESULTS

Of the nine children with cochlear nerve deficiency, five (56%) were affected unilaterally and four (44%) bilaterally. Eight of nine presented after failing a newborn infant hearing screening, whereas one presented at 3 yr of age. On diagnostic ABR testing, all 9 children (9 of 13 affected ears; 69%) had evidence of a cochlear microphonic (CM) and absent neural responses in at least one ear. In the unilateral cases, AN characteristics were detected in all affected ears. In bilateral cases, at least one of the ears in each child demonstrated the AN phenotype, whereas the contralateral ear had no CM identified. Only one ear with cochlear nerve deficiency had present otoacoustic emissions as measured by distortion-product otoacoustic emissions. In children with appropriate available behavioral testing results, all ears without cochlear nerves were identified as having a profound hearing loss. Only 4 (31%) of the 13 ears with cochlear nerve deficiency had a small internal auditory canal on MRI.

CONCLUSIONS

Children with cochlear nerve deficiency can present with electrophysiologic evidence of AN. These children frequently refer on newborn screening examinations that use ABR-based testing methods. Similar to other causes of AN, diagnostic ABR testing will show a CM with absent neural responses. Given that 9 (18%) of 51 children with available MRI and electrophysiologic characteristics of AN in our program have been identified as having cochlear nerve deficiency makes this a relatively common diagnosis. These findings suggest that MRI is indicated for all children diagnosed with AN. Moreover, electrophysiologic evidence of unilateral AN in association with a profound hearing loss should make the clinician highly suspicious for this problem. Although children with cochlear nerve deficiency who have a small nerve may benefit from cochlear implantation or amplification, these interventions are obviously contraindicated in children with completely absent cochlear nerves.

Electrical stimulation of the auditory nerve. I. Correlation of physiological responses with cochlear status

The purpose of the present study was to evaluate evoked potential and single fibre responses to biphasic current pulses in animals with varying degrees of cochlear pathology, and to correlate any differences in the physiological response with status of the auditory nerve. Six cats, whose cochleae ranged from normal to a severe neural loss (< 5% spiral ganglion survival), were used. Morphology of the electrically evoked auditory brainstem response (EABR) was similar across all animals, although electrophonic responses were only observed from the normal animal. In animals with extensive neural pathology, EABR thresholds were elevated and response amplitudes throughout the dynamic range were moderately reduced. Analysis of single VIIIth nerve fibre responses were based on 207 neurons. Spontaneous discharge rates among fibres depended on hearing status, with the majority of fibres recorded from deafened animals exhibiting little or no spontaneous activity. Electrical stimulation produced a monotonic increase in discharge rate, and a systematic reduction in response latency and temporal jitter as a function of stimulus intensity for all fibres examined. Short-duration current pulses elicited a highly synchronous response (latency < 0.7 ms), with a less well synchronized response sometimes present (0.7-1.1 ms). There were, however, a number of significant differences between responses from normal and deafened cochleae. Electrophonic activity was only present in recordings from the normal animal, while mean threshold, dynamic range and latency of the direct electrical response varied with cochlear pathology. Differences in the ability of fibres to follow high stimulation rates were also observed; while neurons from the normal cochlea were capable of 100% entrainment at high rates (600-800 pulses per second (pps)), fibres recorded from deafened animals were often not capable of such entrainment at rates above 400 pps. Finally, a number of fibres in deafened animals showed evidence of 'bursting', in which responses rapidly alternated between high entrainment and periods of complete inactivity. This bursting pattern was presumably associated with degenerating auditory nerve fibres, since it was not recorded from the normal animal. The present study has shown that the pathological response of the cochlea following a sensorineural hearing loss can lead to a number of significant changes in the patterns of neural activity evoked via electrical stimulation. Knowledge of the extent of these changes have important implications for the clinical application of cochlear implants.

Multicentre evaluation of the temporal bones obtained from a patient with suspected Menière's disease

A multicentre study of the inner ears of an 88-year-old patient with vertiginous spells and severe hearing loss in the left ear was performed, employing regular and block surface preparations, light and electron microscopy with qualitative and quantitative evaluation of the cochlear and vestibular nerves. There was severe hydrops of the left cochlea and saccule. Reissner's membrane extended into the vestibule and herniated into the perilymphatic space of the non-ampullated end of the horizontal canal. Furthermore, the short canal connecting the posterior ampulla with the utricle had a small, exceedingly thin balloon-like expansion. Only slight hydrops limited to the cochlea was found in the right ear. Sensorineural degeneration was much more pronounced in the left cochlea than in the right. The number of cochlear and vestibular nerve fibres was greatly reduced in the left ear where more fibres with degenerative changes were present. In both specimens the number of myelinated nerve fibres in osseous spiral lamina was smaller than that in the cochlear nerve in the internal auditory canal. Changes occurred in the endolymphatic sacs but were considered non-specific. In this case severe, apparently progressive hydrops and sensorineural degeneration, characteristic of Menière's disease, were associated with atypical onset of clinical symptoms at a late age.

Auditory nerve of the normal and jaundiced rat. I. Spontaneous discharge rate and cochlear nerve histology

Hyperbilirubinemia is a major problem in neonatal intensive care. Hearing impairment is one of its sequelae. Although lesions of the central auditory pathways are known to be associated with this disorder in both humans and homozygous Gunn rats, the presence of cochler pathology is still controversial. The purpose of this study was to examine the functional integrity of the peripheral auditory system in the Gunn rat. The Gunn rat is a mutant of the Wistar strain with congenital deficiency of the liver enzyme uridine diphosphoglucuronyl transferase which is essential for bilirubin conjugation. This deficiency is inherited as an autosomal recessive trait, with the homozygous animals (jj) showing evidence of bilirubin encephalopathy. The heterozygotes (Jj) have 50% enzyme deficiency and are not jaundiced. The Long-Evans rat served as a control. The approach was to study the discharge characteristics fo single auditory nerve fibers using standard procedures in a closed and calibrated sound system. Various response measurements which would reveal pathological processes in the cochlea were analyzed. In this study, spontaneous discharge rate distribution and interspike interval statistics derived from Gunn rat auditory nerve recordings were found to be within the normal range, and cochlear nerve histology showed no evidence of neuropathy.

Analysis of the human auditory nerve

In human temporal bones of patients with normal hearing or sensory neural deafness, the cochlear neurons were quantitatively and qualitatively evaluated at the level of the osseous spiral lamina, the spiral ganglion and the cochlear nerve. We found from 32,000 to 31,000 myelinated nerve fibres in the cochlear nerve of normal hearing individuals and any lower number in cases of sensory neural deafness. There was in general a good correspondence between the counted numbers of the myelinated nerve fibres in the osseous spiral lamina, the spiral ganglion cells and the myelinated nerve fibres in the cochlear nerve in the inner acoustic meatus. The diameter of the peripheral axons of the type I neurons are about half the diameter of the central axons. The average diameter of the central axons is 2.5 millimicrons with a narrow distribution in children, but an increasingly larger range of fiber calibers with increasing age (0.5 to 7 microns in the 40 to 50 year age group adults).

Cochlear nerve topography and fiber spectrum in the pigmented mouse

The topographical and cytological features of the pigmented mouse (CBA/CBA) cochlear nerve were analyzed. The cochlear nerve is very short and is approximately 0.2-0.3 mm in its length. The entire cochlear nerve and a portion of the ventral cochlear nucleus are located within the internal acoustic meatus, and are closely surrounded by bone. Approximately 10,000 nerve fibers are present, of which only about 3% are unmyelinated. The distribution of the axon diameters in the myelinated nerves is close to unimodal.