Cochlear pathology with chronically implanted scala tympani electrodes.

In summary, these results with chronic implantation of three types of scala tympani electrode indicate the critical importance of two factors in reducing the risk of additional damage to cochlear structure by implantation surgery: (1) The shape and mechanical characteristics of the array must be precisely controlled such that insertion can be performed with an acceptably low incidence of trauma; and (2) the specific electrode materials and fabrication procedures must be demonstrated to be highly biocompatible in in vivo animal control studies. It appears that the neuronal elements of a prior normal cochlea (in the cat) can withstand chronic implantation of scala tympani electrodes (for at least 1 year) if these two prerequisites are met. The effects of chronic electrical stimulation with these arrays at current levels appropriate for the operation of such devices in patients is not known at present.

Chronic electrical stimulation of auditory nerve in cat: Physiological and histological results.

Cats were implanted with two-channel scala tympani bipolar electrode arrays consisting of four PtIr wires in a molded silicone rubber carrier. The electrically evoked auditory brainstem response (ABR) was recorded to monitor the physiological response to biphasic pulsatile stimulation in these chronic preparations. Baseline data were collected over a 1-6 month period. Animals were then subjected to a long period of continuous high level stimulation delivered through a system designed to insure delivery of charge-balanced biphasic waveforms. Subsequent changes in physiological response were interpreted as indicating electrically induced damage to the cochlea. Localized loss of hair cells and growth of connective tissue resulted from the implantation of scala tympani inserts. Electrically evoked ABR responses were not altered by the long-term presence of the electrode, nor by the presence of intervening connective tissue. Physiological manifestations of stimulus-induced change appeared only after hundreds of hours of continuous stimulation. Apparent functional damage was not suspended or reversed with cessation of stimulation, but rather continued for several hundred hours after the stimulation was terminated. Deterioration of physiological response was accompanied by two deleterious histological changes: (a) bone growth within the scala tympani; and (b) loss of nerve fibers and spiral ganglion cells. Both of these changes were restricted to an area corresponding to the implant intracochlear location and were most marked in the region adjacent to the chronically stimulated electrode pair. In cases where stimuli were not charge balanced or surgical trauma was incurred, bone growth was most extensive and nerve damage most pervasive. The data from cases stimulated at lower levels of charge density, i.e. 20-40 muC/cm2, suggest that these may be more feasible levels for safe chronic electrical stimulation in scala tympani.

Uptake and transport of horseradish peroxidase by cochlear spiral ganglion neurons.

Cochlear spiral ganglion neurons in the cat have been classified as type I and type II. The type II neurons are of special interest since they are reported to provide the afferent innervation to three-quarters of the cochlear hair cells, and also because of a recent assertion that they do not project to the brain. In the present study the neuronal marker horseradish peroxidase (HRP) was utilized to re-examine these issues. HRP was injected into the intranuclear cochlear nerve root of adult cats. 18 to 48 h post-injection HRP reaction product was observed in both type I and type II neurons. The majority of labeled cells were characterized by granular reaction product, while a smaller number of cells were also diffusely filled with HRP, resulting in intense staining of the cell soma and its processes. These data indicate that the central axons of type II neurons are constituents of the cochlear nerve root at the level of the brainstem and thus do project centrally. In the organ of Corti HRP-filled fibers to both inner and outer hair cells were observed and demonstrated in electron microscopy to be afferent neurons. This intra-axonal accumulation of the HRP by outer hair cell afferents constitutes direct evidence of the functional projection of these neurons to the cochlear nucleus.

Deaf animal models for studies of a multichannel cochlear prosthesis.

Pathological alterations of the cochlea were studied in three different deaf animal (cat) populations. The ototoxic drug neomycin sulfate, was administered in one experimental series by direct infusion into the cochlear perilymph; a second group was given a series of intramuscular injections of the drug; and in a third experiment a mechanical lesion was made in the basilar membrane of the basal turn and the animals subsequently deafened by systemic neomycin. Hearing losses were tracked by monitoring thresholds of auditory brainstem responses to click stimulation. These deaf cat preparations fairly efficiently model pathologies recorded in man and are highly predictable over an acceptable time frame. Such preparations are of practical value for experiments involving intracochlear electrical stimulation (e.g., with model cochlear prosthesis electrodes).

Some practical considerations in development of multichannel scala tympani prostheses.

There are several basic issues regarding the safety and feasibility of implanting multichannel scala tympani cochlear prostheses in humans. Physiological and technical studies have been designed to resolve some of these questions. Results of physiological investigations demonstrate that (1) nerve viability is not affected by the presence (without activation) of a multichannel array; (2) use of a highly specified bipolar electrode configuration permits discrete and predictable stimulation of sectors of the auditory nerve array, and (3) some deleterious effects (i.e., nerve damage and calcification products) may result when stimulus parameters (intensity, duration and waveform symmetry) are not well controlled over long periods of stimulation. Results of technical investigations regarding the specifications (materials and design) of multi-electrode arrays and engineering studies regarding hardware and software for safe and efficient stimulators for humans have provided devices with which 2 patients could be implanted and tested psychophysically. Some preliminary results of testing with these provide data regarding threshold, loudness discrimination and pitch perception.