Electrode Selection and Speech Understanding in Patients With Auditory Brainstem Implants

Assessing temporal responsiveness of primary stimulated neurons in auditory brainstem and cochlear implant users

The reasons why clinical outcomes with auditory brainstem implants (ABIs) are generally poorer than with cochlear implants (CIs) are still somewhat elusive. Prior work has focused on differences in processing of spectral information due to possibly poorer tonotopic representation and higher channel interaction with ABIs than with CIs. In contrast, this study examines the hypothesis that a potential contributing reason for poor speech perception in ABI users may be the relative lack of temporal responsiveness of the primary neurons that are stimulated by the ABI. The cochlear nucleus, the site of ABI stimulation, consists of different neuron types, most of which have much more complex responses than the auditory nerve neurons stimulated by a CI. Temporal responsiveness of primary stimulated neurons was assessed in a group of ABI and CI users by measuring recovery of electrically evoked compound action potentials (ECAPs) from single-pulse forward masking. Slower ECAP recovery tended to be associated with poorer hearing outcomes in both groups. ABI subjects with the longest recovery time had no speech understanding or even no hearing sensation with their ABI device; speech perception for the one CI outlier with long ECAP recovery time was well below average. To the extent that ECAP recovery measures reveal temporal properties of the primary neurons that receive direct stimulation form neural prosthesis devices, they may provide a physiological underpinning for clinical outcomes of auditory implants. ECAP recovery measures may be used to determine which portions of the cochlear nucleus to stimulate, and possibly allow us to enhance the stimulation paradigms.

The Electrically Evoked Auditory Change Complex Evoked by Temporal Gaps Using Cochlear Implants or Auditory Brainstem Implants in Children With Cochlear Nerve Deficiency

OBJECTIVES

This study aimed to (1) establish the feasibility of measuring the electrically evoked auditory change complex (eACC) in response to temporal gaps in children with cochlear nerve deficiency (CND) who are using cochlear implants (CIs) and/or auditory brainstem implants (ABIs); and (2) explore the association between neural encoding of, and perceptual sensitivity to, temporal gaps in these patients.

DESIGN

Study participants included 5 children (S1 to S5) ranging in age from 3.8 to 8.2 years (mean: 6.3 years) at the time of testing. All subjects were unilaterally implanted with a Nucleus 24M ABI due to CND. For each subject, two or more stimulating electrodes of the ABI were tested. S2, S3, and S5 previously received a CI in the contralateral ear. For these 3 subjects, at least two stimulating electrodes of their CIs were also tested. For electrophysiological measures, the stimulus was an 800-msec biphasic pulse train delivered to individual electrodes at the maximum comfortable level (C level). The electrically evoked responses, including the onset response and the eACC, were measured for two stimulation conditions. In the standard condition, the 800-msec pulse train was delivered uninterrupted to individual stimulating electrodes. In the gapped condition, a temporal gap was inserted into the pulse train after 400 msec of stimulation. Gap durations tested in this study ranged from 2 up to 128 msec. The shortest gap that could reliably evoke the eACC was defined as the objective gap detection threshold (GDT). For behavioral GDT measures, the stimulus was a 500-msec biphasic pulse train presented at the C level. The behavioral GDT was measured for individual stimulating electrodes using a one-interval, two-alternative forced-choice procedure.

RESULTS

The eACCs to temporal gaps were recorded successfully in all subjects for at least one stimulating electrode using either the ABI or the CI. Objective GDTs showed intersubject variations, as well as variations across stimulating electrodes of the ABI or the CI within each subject. Behavioral GDTs were measured for one ABI electrode in S2 and for multiple ABI and CI electrodes in S5. All other subjects could not complete the task. S5 showed smaller behavioral GDTs for CI electrodes than those measured for ABI electrodes. One CI and two ABI electrodes in S5 showed comparable objective and behavioral GDTs. In contrast, one CI and two ABI electrodes in S5 and one ABI electrode in S2 showed measurable behavioral GDTs but no identifiable eACCs.

CONCLUSIONS

The eACCs to temporal gaps were recorded in children with CND using either ABIs or CIs. Both objective and behavioral GDTs showed inter- and intrasubject variations. Consistency between results of eACC recordings and psychophysical measures of GDT was observed for some but not all ABI or CI electrodes in these subjects.

Auditory Brainstem Implantation in Neurofibromatosis Type 2: Experience From the Manchester Programme

OBJECTIVE

To describe the experience of auditory brainstem implantation (ABI) in patients with Neurofibromatosis type 2 (NF2).

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral centre.

PATIENTS

Implanted with a Cochlear ABI22 or ABI24M between 1994 and 2009 because of NF2 disease.

INTERVENTION(S)

Rehabilitative.

MAIN OUTCOME MEASURE(S)

Surgical complication rate; audiological outcomes.

RESULTS

There were 50 primary ABI insertions in 49 patients, including 16 inserted at the time of first side tumor removal as a sleeper, and two revision repositionings which failed to improve outcome. Postoperatively three patients had cerebrospinal fluid leaks which did not require reoperation, one patient had meningitis, and eleven patients suffered either temporary or permanent lower cranial nerve dysfunction. Twenty-nine patients became full time users; a further 12 patients became non-users. Three patients died while their device was inactive. Five patients retain serviceable contralateral hearing. Audiological open set testing of users showed means of: environmental sounds discrimination 51%; phoneme discrimination: with ABI alone 22%/lip reading (LR) 45%/ABI with LR 65%; sentence testing: with ABI alone 13%/LR 19%/ABI with LR 54%.

CONCLUSIONS

The majority of patients with NF2 implanted with an ABI find the device a useful aid to communication in conjunction with LR and in recognizing common environmental sounds. A small proportion gain open set discrimination. Almost a third of patients may end up as non-users. There is probably an increased risk of postoperative lower cranial nerve dysfunction so careful preoperative assessment is advised.

Effect of Pulse Rate and Polarity on the Sensitivity of Auditory Brainstem and Cochlear Implant Users to Electrical Stimulation

To further understand the response of the human brainstem to electrical stimulation, a series of experiments compared the effect of pulse rate and polarity on detection thresholds between auditory brainstem implant (ABI) and cochlear implant (CI) patients. Experiment 1 showed that for 400-ms pulse trains, ABI users’ thresholds dropped by about 2 dB as pulse rate was increased from 71 to 500 pps, but only by an average of 0.6 dB as rate was increased further to 3500 pps. This latter decrease was much smaller than the 7.7-dB observed for CI users. A similar result was obtained for pulse trains with a 40-ms duration. Furthermore, experiment 2 showed that the threshold difference between 500- and 3500-pps pulse trains remained much smaller for ABI than for CI users, even for durations as short as 2 ms, indicating the effect of a fast-acting mechanism. Experiment 3 showed that ABI users’ thresholds were lower for alternating-polarity than for fixed-polarity pulse trains, and that this difference was greater at 3500 pps than at 500 pps, consistent with the effect of pulse rate on ABI users’ thresholds being influenced by charge interactions between successive biphasic pulses. Experiment 4 compared thresholds and loudness between trains of asymmetric pulses of opposite polarity, in monopolar mode, and showed that in both cases less current was needed when the anodic, rather than the cathodic, current was concentrated into a short time interval. This finding is similar to that previously observed for CI users and is consistent with ABI users being more sensitive to anodic than cathodic current. We argue that our results constrain potential explanations for the differences in the perception of electrical stimulation by CI and ABI users, and have potential implications for future ABI stimulation strategies.