Development of cochlear-wall implants for electrical stimulation of the auditory nerve

Preservation of auditory brainstem response thresholds after cochleostomy and titanium microactuator implantation in the lateral wall of cat scala tympani

HYPOTHESIS

The safety of implanting a titanium microactuator into the lateral wall of cat scala tympani was assessed by comparing preoperative and postoperative auditory brainstem response (ABR) thresholds for 1 to 3 months.

BACKGROUND

The safety of directly stimulating cochlear perilymph with an implantable hearing system requires maintaining preoperative hearing levels. This cat study is an essential step in the development of the next generation of fully implantable hearing devices for humans.

METHODS

Following GLP surgical standards, a 1-mm cochleostomy was drilled into the lateral wall of the scala tympani, and a nonfunctioning titanium anchor/microactuator assembly was inserted in 8 cats. The scala media was damaged in the 1 cat. ABR thresholds with click and 4- and 8-kHz stimuli were measured preoperatively and compared with postoperative thresholds at 1, 2, and 3 months. Nonimplanted ear thresholds were also measured to establish statistical significance for threshold shifts (>28.4 dB). Two audiologists independently interpreted thresholds.

RESULTS

Postoperatively, 7 cats implanted in the scala tympani demonstrated no significant ABR threshold shift for click stimulus; one shifted ABR thresholds to 4- and 8-kHz stimuli. The eighth cat, with surgical damage to the scala media, maintained stable click threshold but had a significant shift to 4- and 8-kHz stimuli.

CONCLUSION

This cat study provides no evidence of worsening hearing thresholds after fenestration of the scala tympani and insertion of a titanium anchor/microactuator, provided there is no surgical trauma to the scala media and the implanted device is securely anchored in the cochleostomy. These 2 issues have been resolved in the development of a fully implantable hearing system for humans. The long-term hearing stability (combined with histologic studies) reaffirm that the microactuator is well tolerated by the cat cochlea.

Consistent phosphenes generated by electrical microstimulation of the visual thalamus. An experimental approach for thalamic visual neuroprostheses

Most work on visual prostheses has centered on developing retinal or cortical devices. However, when retinal implants are not feasible, neuroprostheses could be implanted in the lateral geniculate nucleus (LGN) of the thalamus, the intermediate relay station of visual information from the retina to the visual cortex (V1). The objective of the present study was to determine the types of artificial stimuli that when delivered to the visual thalamus can generate reliable responses of the cortical neurons similar to those obtained when the eye perceives a visual image. Visual stimuli {S(i)} were presented to one eye of an experimental animal and both, the thalamic {RTh(i)} and cortical responses {RV1(i)} to such stimuli were recorded. Electrical patterns {RTh(i)*} resembling {RTh(i)} were then injected into the visual thalamus to obtain cortical responses {RV1(i)*} similar to {RV1(i)}. Visually- and electrically generated V1 responses were compared.

RESULTS

During the course of this work we: (i) characterized the response of V1 neurons to visual stimuli according to response magnitude, duration, spiking rate, and the distribution of interspike intervals; (ii) experimentally tested the dependence of V1 responses on stimulation parameters such as intensity, frequency, duration, etc., and determined the ranges of these parameters generating the desired cortical activity; (iii) identified similarities between responses of V1 useful to compare the naturally and artificially generated neuronal activity of V1; and (iv) by modifying the stimulation parameters, we generated artificial V1 responses similar to those elicited by visual stimuli. Generation of predictable and consistent phosphenes by means of artificial stimulation of the LGN is important for the feasibility of visual prostheses. Here we proved that electrical stimuli to the LGN can generate V1 neural responses that resemble those elicited by natural visual stimuli.

Implantation of the lateral cochlear wall for auditory nerve stimulation

Although cochlear implants now regularly achieve gratifying results, traditional intrascalar implants have certain limitations. Extraluminal implants may offset some of these problems by accessing neurons subserving a wider tonotopic range, avoiding intracochlear insertion trauma, and offering alternatives when cochlear obliteration is present. We have investigated the utility of a lateral cochlear wall implant in a normal-hearing cat model with implants at the middle and basal turns, and found successful activation of the auditory nerve at thresholds of 28.1 and 40.6 microA, respectively. No adventitial stimulation of the facial nerve was noted within the dynamic range. Maximum responsiveness was observed with implants of the middle turn of the cochlea, an area that is not reliably approached with current intrascalar implants. These observations support and extend prior observations of the feasibility of extraluminal stimulation of the auditory nerve.

Effects of electrode configuration on threshold functions for electrical stimulation of the cochlea

Psychophysical detection threshold vs frequency functions for sinusoidal electrical stimulation of the deafened cochlea were measured in 18 nonhuman primate subjects. Functions for monopolar or widely-spaced ( > 2.5 mm) bipolar stimulation were lower and usually had steeper slopes than those for more narrowly-spaced ( < 2.0 mm) bipolar stimulation. In 56% of the cases the difference between thresholds for narrowly-spaced bipolar of monopolar stimulation was greater for low frequency stimuli (63 or 100 Hz) than for high frequency stimuli (800 or 1,000 Hz) by 5 dB or more. Two cases were compared in more detail using pulsatile stimuli. For sinusoidal stimuli, one of these cases showed a moderate frequency dependent effect of electrode configuration and the other did not. The case with the frequency dependent effect of electrode configuration for sinusoids also showed a phase-duration dependent effect of electrode configuration for detection of single biphasic pulses: strength-duration curves (detection threshold in decibels vs pulse duration in ms/phase) were steeper for monopolar stimulation than for narrowly-spaced (0.7 mm) bipolar stimulation. This effect was not seen in the case that showed little or no frequency dependence in the effect of electrode configuration for sinusoidal stimuli. Slopes of threshold vs pulse rate functions where pulse duration was held constant at 2 ms/phase were not affected by electrode configuration in either subject.

High-resolution computed tomography of the temporal bone of the Macaca mulatta monkey

High-resolution computed tomography (CT) slices of the temporal bone of the Macaca mulatta monkey were performed and key anatomical landmarks were identified and labelled. The scans provided detailed anatomical definition of the monkey temporal bone and were considered a useful basis for studying the anatomy of a primate model.

Cochlear wall titanium implants for auditory nerve stimulation

Genetically deaf dalmatian dogs and ototoxically deafened macaque monkeys were implanted with electrodes housed in cochlear wall titanium implants to assess long-term stability, tolerance, and performance. Short-term human implantation, followed by trials of stimulation, was performed in 4 unilaterally deaf patients. In the dog experiments, cochlear wall electrode stimulation produced consistent electrophysiologic thresholds that were higher, by approximately 6 dB, than those obtained with bipolar scala tympani stimulation. Clinical testing revealed electrically evoked middle latency response, auditory brain stem response, and/or behavioral detection responses in 3 of 4 patients, at levels below those for facial nerve activation and pain sensation. Electrode place discrimination studies, with controls for loudness cues, revealed near-perfect discrimination in a monkey subject. Eleven of the 12 animal implants were found to be rigidly fixed in the cochlear bone, with direct contract between bone and implant over 8% to 23% of the implant surface for the 6 implants examined in detail. These results suggest that long-term fixation of titanium cochlear wall implants occurs by virtue of intimate implant-bone contact in restricted areas. This approach to prosthetic stimulation demonstrates encouraging performance characteristics in achieving auditory activation.

Bone repair in the otic capsule of the rabbit

The repair of 1 mm drill hole lesions in rabbit long bones and otic capsules was studied in undecalcified sections. Following surgery, bone formation was monitored during 12 weeks by intravital sequential time labeling with bone-seeking fluorochromes. In diaphyseal bone, initial repair by trabecular woven bone was accompanied by a transient remodeling response in primary repair bone and the surrounding compacta. In comparison, capsular defects showed a slightly deficient initial repair by woven bone which, once formed, persisted unchanged with no subsequent osteonal remodeling. In the surrounding capsule, the remodeling response was subtle and its duration much shorter. Moreover, the centrifugal distribution of bone remodeling units around inner ear spaces was preserved, suggesting that capsular repair is affected by the same morphogenetic field that controls perilabyrinthine growth, modeling and remodeling. The significance of these findings is briefly discussed in relation to otosclerosis and cochlear implant strategies.

Electrode placements for cochlear implants: a review

Of the variety of different sites that have been proposed and used for cochlear implants, only two are now seeing widespread use. For single-channel electrodes, it appears that a placement at the round window, or just inside it, leads to no disadvantages (and perhaps some advantages) over deeper insertions. For multichannel implants, it appears that intra-cochlear electrodes are, generally speaking, preferable to extra-cochlear ones in a variety of ways, especially as the early fears about the dangers of intra-cochlear placement now seem exaggerated. One way in which extra-cochlear approaches may play an important accessory role in multichannel intra-cochlear implants is in allowing access to the apical end of the cochlea where residual nerve survival is likely to be best. Special considerations may apply for groups of patients who until recently have been implanted relatively infrequently--adults with significant residual hearing and children.

Inner ear implants for experimental electrical stimulation of auditory nerve arrays

Electrode arrays chronically implanted in the inner ear are gaining increased use for experimental studies of the auditory nervous system, as well as for studies related to development of improved auditory prostheses. Commercially available electrode arrays are designed for human use and thus may be unsuitable for experimental studies, particularly in small animals. This paper describes a simple, inexpensive method for making custom electrode arrays in a variety of configurations, suitable for animals ranging from small rodents to non-human primates.