Comparison of cochlear histopathology following two implant designs for use in scala tympani

Surgery for the Safe Insertion and Reinsertion of the Banded Electrode Array

Adhering to the surgical technique outlined in the protocol for the Nucleus implant has resulted in over 100 patients worldwide obtaining significant benefit from multichannel stimulation. A detailed analysis of the results in 40 patients shows that it improves their awareness of environmental sounds and their abilities in understanding running speech when combined with lipreading. In addition, one third to one half of the patients also understand significant amounts of running speech without lipreading and some can have interactive conversations over the telephone. It is clear that any insertion trauma is not significant, which is confirmed by the excellent clinical results.

Pneumococcal Middle Ear Infection and Cochlear Implantation

A limited study for the experimental induction of pneumococcal otitis media is presented. It is a useful model to study the effects of otitis media in the implanted and nonimplanted cochlea of the cat. Pneumococcal otitis media caused minor pathological changes in two nonimplanted cochleas and more widespread changes together with significant loss of neural elements in two implanted cochleas. However, the small number of animals used in this study did not allow us to distinguish between the effects of electrode insertion trauma, infection, or the combination of both.

Surgical findings and auditory performance after cochlear implant revision surgery

The objective of this study was to review cochlear reimplantation outcomes in the tertiary hospital and analyze whether facts such as type of failure, surgical findings, or etiology of deafness have an influence. A retrospective study including 38 patients who underwent cochlear implant revision surgery in a tertiary center is performed. Auditory outcomes (pure tone audiometry, % disyllabic words) along with etiology of deafness, type of complication, issues with insertion, and cochlear findings are included. Complication rate is 2.7 %. Technical failure rate is 57.9 % (50 % hard failure and 50 % soft failure), and medical failure (device infection or extrusion, migration, wound, or flap complication) is seen in 42.1 % of the cases. Management of cochlear implant complications and revision surgery is increasing due to a growing number of implantees. Cases that require explantation and reimplantation of the cochlear implant are safe procedures, where the depth of insertion and speech perception results are equal or higher in most cases. Nevertheless, there must be an increasing effort on using minimally traumatic electrode arrays and surgical techniques to improve currently obtained results.

Surgical implications of perimodiolar cochlear implant electrode design: avoiding intracochlear damage and scala vestibuli insertion

OBJECTIVE

To review the mechanisms and nature of intracochlear damage associated with cochlear implant electrode array insertion, in particular, the various perimodiolar electrode designs. Make recommendations regarding surgical techniques for the Nucleus Contour electrode to ensure correct position and minimal insertion trauma.

BACKGROUND

The potential advantages of increased modiolar proximity of intracochlear multichannel electrode arrays are a reduction in stimulation thresholds, an increase in dynamic range and more localized neural excitation. This may improve speech perception and reduce power consumption. These advantages may be negated if increased intracochlear damage results from the method used to position the electrodes close to the modiolus.

METHOD

A review of the University of Melbourne Department of Otolaryngology experience with temporal bone safety studies using the Nucleus standard straight electrode array and a variety of perimodiolar electrode array designs; comparison with temporal bone insertion studies from other centres and postmortem histopathology studies reported in the literature. Review of our initial clinical experience using the Nucleus Contour electrode array.

RESULTS

The nature of intracochlear damage resulting from electrode insertion trauma ranges from minor, localized, spiral ligament tear to diffuse organ of Corti disruption and osseous spiral lamina fracture. The type of damage depends on the mechanical characteristics of the electrode array, the stiffness, curvature and size of the electrode in relation to the scala, and the surgical technique. The narrow, flexible, straight arrays are the least traumatic. Pre-curved or stiffer arrays are associated with an incidence of basilar membrane perforation. The cochleostomy must be correctly sited in relation to the round window to ensure scala tympani insertion. A cochleostomy anterior to the round window rather than inferior may lead to scala media or scala vestibuli insertion.

CONCLUSION

Proximity of electrodes to the modiolus can be achieved without intracochlear damage provided the electrode array is a free fit within the scala, of appropriate size and shape, and accurate scala tympani insertion is performed.

Effects of inner ear trauma on the risk of pneumococcal meningitis

OBJECTIVE

To examine the risk of pneumococcal meningitis in healthy rats that received a severe surgical trauma to the modiolus and osseous spiral lamina or the standard insertion technique for acute cochlear implantation.

DESIGN

Interventional animal studies.

SUBJECTS

Fifty-four otologically normal adult Hooded-Wistar rats.

INTERVENTIONS

Fifty-four rats (18 of which received a cochleostomy alone; 18, a cochleostomy and acute cochlear implantation using standard surgical techniques; and 18, a cochleostomy followed by severe inner ear trauma) were infected 4 weeks after surgery with Streptococcus pneumoniae via 3 different routes (hematogenous, middle ear, and inner ear) to represent all potential routes of bacterial infection from the upper respiratory tract to the meninges in cochlear implant recipients with meningitis.

RESULTS

Severe trauma to the osseous spiral lamina and modiolus increased the risk of pneumococcal meningitis when the bacteria were given via the middle or inner ear (Fisher exact test, P<.05). However, the risk of meningitis did not change when the bacteria were given via the hematogenous route. Acute electrode insertion did not alter the risk of subsequent pneumococcal meningitis for any route of infection.

CONCLUSIONS

Severe inner ear surgical trauma to the osseous spiral lamina and modiolus can increase the risk of pneumococcal meningitis. Therefore, every effort should be made to ensure that cochlear implant design and insertion technique cause minimal trauma to the bony structures of the inner ear to reduce the risk of pneumococcal meningitis.

Vestibular effects of cochlear implantation

OBJECTIVES/HYPOTHESIS

Cochlear implantation (CI) carries with it the potential risk for vestibular system insult or stimulation with resultant dysfunction. As candidate profiles continue to evolve and with the recent development of bilateral CI, understanding the significance of this risk takes on an increasing importance.

STUDY DESIGN

Between 1997 to 2001, a prospective observational study was carried out in a tertiary care medical center to assess the effects of unilateral CI on the vestibular system.

METHODS

Assessment was performed using the dizziness handicap inventory (DHI), vestibulo-ocular reflex (VOR) testing using both alternate bithermal caloric irrigations (ENG) and rotational chair-generated sinusoidal harmonic accelerations (SHA), and computerized dynamic platform posturography (CDP) at preoperative, 1-month, 4-month, 1-year and 2-year postimplantation visits. CI was carried out without respect to the preoperative vestibular function test results.

RESULTS

Specifically, 86 patients were entered into the study after informed consent. For the group as a whole, pair wise comparisons revealed few significant differences between preoperative and postoperative values for VOR testing (ENG and SHA) at any of the follow-up intervals. Likewise, DHI testing was also unchanged except for significant reductions (improvements) in the emotional subcategory scores at both the 4-month and 1-year intervals. CDP results demonstrated substantial improvements in postural sway in the vestibular conditions (5 and 6) as well as composite scores with the device "off" and "on" at the 1-month, 4-month, 1-year, and 2-year intervals. Device activation appeared to improve postural stability in some conditions. Excluding those patients with preoperative areflexic or hyporeflexic responses in the implanted ear (total [warm + cool] caloric response <or= 15 deg/s), substantial reductions (>or=21 deg/s maximum slow phase velocity) in total caloric response were observed for 8 (29%) patients at the 4-month interval. These persisted throughout the study period. These changes were accompanied by significant low frequency phase changes on SHA testing confirming a VOR insult. Of interest, no significant changes were detected in the DHI or CDP, and there were no effects of age, sex, device manufacturer, or etiology of hearing loss (HL) for these patients.

CONCLUSIONS

Unilateral CI rarely results in significant adverse effects on the vestibular system as measured by the DHI, ENG, SHA, and CDP. On the contrary, patients that underwent CI experienced significant improvements in the objective measures of postural stability as measured by CDP. Device activation in music appeared to have an additional positive effect on postural stability during CDP testing. Although VOR testing demonstrated some decreases in response, patients did not suffer from disabling vestibular effects following CI. The mechanism underlying these findings remains speculative. These findings should be considered in counseling patients about CI.

Interaction of spiral ganglion neuron processes with alloplastic materials in vitro(1)

The cochlear implant (CI) involves the introduction of alloplastic materials into the cochlea. While current implants interact with cochlear neurons at a distance, direct interactions between spiral ganglion (SG) neurites and implants could be fostered by appropriate treatment with neurotrophic factors. The interactions of fibroblasts and osteoblasts with alloplastic materials have been well studied in vitro and in vivo. However, interactions of inner ear neurons with such alloplastic materials have yet to be described. To investigate survival and growth behavior of SG neurons on different materials, SG explants from post-natal day 5 rat SG were cultured for 72 h in the presence of neurotrophin-3 (10 ng/ml) on titanium, gold, stainless steel, platinum, silicone and plastic surfaces that had been coated with laminin and poly-L-lysine. Neurite outgrowth was investigated after immunohistological staining for neurofilament, by image analysis to determine neurite extension and directional changes. Neurite morphology and adhesion to the alloplastic material were also evaluated by scanning electron microscopy (SEM). On titanium, SG neurites reached the highest extent of outgrowth, with an average length of 662 microm and a mean of 31 neurites per explant, compared to 568 microm and 21 neurites on gold, 574 microm and 24 neurites on stainless steel, 509 microm and 16 neurites on platinum, 281 microm and 12 neurites on silicone and 483 microm and 31 neurites on plastic. SEM revealed details of adhesion of neurites and interaction with non-neuronal cells. The results of this study indicate that the growth of SG neurons in vitro is strongly influenced by alloplastic materials, with titanium exhibiting the highest degree of biocompatibility with respect to neurite extension. The knowledge of neurite interaction with different alloplastic materials is of clinical interest, as development in CI technology leads to closer contact of implanted electrodes with surviving inner ear neurons.

The contour electrode array: safety study and initial patient trials of a new perimodiolar design

OBJECTIVE

The aim of these studies was to investigate the insertion properties and safety of a new intracochlear perimodiolar electrode array design (Contour).

BACKGROUND

An electrode array positioned close to the neural elements could be expected to reduce stimulation thresholds and might potentially reduce channel interaction.

METHODS

Two sequential studies were conducted. In study 1, the Contour electrode array was inserted in 12 human temporal bones. After cochlear surface preparation, the position of the array was noted and the basilar membrane was examined for insertion damage. On the basis of the outcome of this temporal bone study, study 2 investigated the Contour array, mounted on a Nucleus CI-24 M device and implanted in three adult patients.

RESULTS

Study I showed that in 10 temporal bones, the Contour array was positioned close to the modiolus, and the basilar membrane was intact. In the two remaining bones, the arrays had pierced the basilar membrane and were positioned in the scala vestibuli apical to the penetration. Statistical analysis showed an equivalent probability of insertion-induced damage of the two array designs. In study 2, image analysis indicated that the Contour electrodes were positioned closer to the modiolus than the standard Nucleus straight array. Lower T and C levels, but higher impedance values, were recorded from electrodes close to the modiolus. Initial speech perception data showed that all patients gained useful open-set speech perception, two patients achieving scores of 100% on sentence material 3 months postoperatively.

CONCLUSIONS

The temporal bone studies showed the Contour electrode array to be generally positioned closer to the modiolus than the standard Nucleus straight array, and to have an equivalent probability of causing insertion-induced damage.

Strategies to improve electrode positioning and safety in cochlear implants

An injection-molded internal supporting rib has been produced to control the flexibility of silicone rubber encapsulated electrodes designed to electrically stimulate the auditory nerve in human subjects with severe to profound hearing loss. The rib molding dies, and molds for silicone rubber encapsulation of the electrode, were designed and machined using AutoCad and MasterCam software packages in a PC environment. After molding, the prototype plastic ribs were iteratively modified based on observations of the performance of the rib/silicone composite insert in a clear plastic model of the human scala tympani cavity. The rib-based electrodes were reliably inserted farther into these models, required less insertion force and were positioned closer to the target auditory neural elements than currently available cochlear implant electrodes. With further design improvements the injection-molded rib may also function to accurately support metal stimulating contacts and wire leads during assembly to significantly increase the manufacturing efficiency of these devices. This method to reliably control the mechanical properties of miniature implantable devices with multiple electrical leads may be valuable in other areas of biomedical device design.

Long-term measures of electrode impedance and auditory thresholds for the Ineraid cochlear implant

Measures of electrode impedance and of detection thresholds for electrical stimuli were extracted from the records of patients implanted with the Ineraid cochlear prosthesis. An analysis of impedance measures, obtained at 1, 12, 24, and 36 months after surgery, demonstrated (a) a significant decrease in impedance over the first year for electrodes that carried current and (b) significant increases in impedance at 24 and 36 months for electrodes that did not carry current. An analysis of detection thresholds, obtained at the same times as the impedance measures, demonstrated that averaged thresholds for the current-carrying electrodes varied no more than 0.5 dB over the 3-year period. These results support the conclusion that stimulation with the Ineraid device does not produce deleterious changes in the electrodes or in the target neural tissue.

Cochlear pathology following chronic electrical stimulation of the auditory nerve. I: Normal hearing kittens

The present study examines the histopathological effects of long-term intracochlear electrical stimulation in young normal hearing animals. Eight-week old kittens were implanted with scala tympani electrode arrays and stimulated for periods of up to 1500 h using charge balanced biphasic current pulses at charge densities in the range 21-52 microC cm-2 geom. per phase. Both click and electrically evoked auditory brainstem responses were periodically recorded to monitor the status of the hair cell and spiral ganglion cell populations. In addition, the impedance of the stimulating electrodes was measured daily to monitor their electrical characteristics during chronic implantation. Histopathological examination of the cochleas showed no evidence of stimulus induced damage to cochlear structures when compared with implanted, unstimulated control cochleas. Indeed, there was no statistically significant difference in the ganglion cell density adjacent to the stimulating electrodes when compared with a similar population in implanted control cochleas. In addition, hair cell loss, which was restricted to regions adjacent to the electrode array, was not influenced by the degree of electrical stimulation. These histopathological findings were consistent with the evoked potential recordings. Finally, electrode impedance data correlated well with the degree of tissue growth observed within the scala tympani. The present findings indicate that the young mammalian cochlea is no more susceptible to cochlear pathology following chronic implantation and electrical stimulation than is the adult.

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.

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

The aim of these experiments was to investigate the use of titanium implants for anchorage of stimulating electrodes or other clinical or experimental devices in the bony wall of the cochlea. Twenty-six cylindrical titanium fixtures, 0.6 mm in diameter, were inserted into holes drilled in the otic capsule in 8 ears in 5 nonhuman primates and then examined for stability after periods of 2 months to 2 years. Following sacrifice, the bone-metal interfaces were examined microscopically. Fourteen of the implants were firmly fixed in the bone, 6 were loosely fixed and 6 came out. Poor fixation was associated with infection in the middle ear. In uninfected ears, 90% of the implants were stable. The implants were not osseointegrated in the classic sense, but in stable implants, direct bone contact covering 5 to 60% of the titanium oxide surface of the implant shaft was observed.

Computer-generated three-dimensional reconstruction of the cochlea

Computer-generated three-dimensional reconstructions of the nerve fibers from the organ of Corti to the spiral ganglion were used to determine the optimum maximal length of the cochlear implant electrode. The spiral ganglion within the modiolus is much shorter than the organ of Corti. The spiral ganglion has 1 3/4 turns and reaches no higher than the middle of the second turn of the organ of Corti, which has 2 3/4 turns. The spiral ganglion is concentric and basal with respect to the organ of Corti. The spiral ganglion dendrites within the osseous spiral lamina of the basal turn project radially, nearly perpendicular to the central axis of the modiolus. Upon entering the modiolus, they turn basally at an angle of approximately 120 degrees. The projection of dendrites within the osseous spiral lamina became increasingly oblique as the ganglion extended apically. The organization of the cochlear nerve results from the spiraling of the ganglion. These findings are in agreement with previous reports. Implications of these findings and their possible relevance to the optimum length of the cochlear implant electrode are discussed with reference to cochlear damage resulting from longer electrodes.

Structural effects of short term and chronic extracochlear electrical stimulation on the guinea pig spiral organ

To assess the effects of extracochlear electrical stimulation on cochlear structure, guinea pigs were implanted and stimulated with single middle ear electrodes either at round window or promontory sites, and their cochleae examined by transmission electron microscopy. Implanted but unstimulated, or unimplanted control animals were examined in the same way. Alternating current stimulation at the promontory for 2 h at 150 Hz, 500 microA, caused outer hair cell efferent endings to become dense and vacuolated, but no hair cells were damaged. With direct current stimulation at 500 microA for 2 h the basal regions of the stimulated cochlea were badly damaged and many outer hair cells lysed. Long term (up to 1200 h) round window stimulation at 100 or 141 Hz, 15-91 microA rms, did not cause cell death or inner hair cell damage, but basal outer hair cells and their efferent endings were badly affected in both ipsilateral and contralateral cochleae. The compound action potential of the auditory evoked response to broad band click stimuli was not altered by chronic electrical stimulation. It is concluded that chronic stimulation with the parameters used does not threaten cochlear survival, and it is proposed that the bilateral structural changes induced by chronic stimulation are caused by excessive activation of the cochlear efferent pathways.

Electrical stimulation of the guinea pig cochlea

As a result of practical considerations, histopathologic findings of the temporal bone in humans with cochlear prosthesis implants have been limited. This project attempts to better define safe parameters of electrical stimulation of the inner ear and compare the safe limits of intracochlear vs. extracochlear stimulation sites. Guinea pigs were implanted with single electrodes either on the promontory or in the scala tympani and were stimulated relative to a remote indifferent for 12 hours distributed over a 4-week period. Electrical auditory brainstem evoked responses (EABRs) were tested before and after each of four 3-hour stimulation sessions. Six weeks after implantation, the animals were killed, and their cochleas were examined under the scanning electron microscope. Intracochlear electrodes exhibited thresholds for damage well below one half of that found for most extracochlear stimulation sites. The function-relating damage threshold (in amperes) to frequency of intracochlear stimulation is represented by two straight lines, with an intercept of 1 kHz. The low-frequency limb exhibited a slope of 3 to 4 dB/octave, whereas the high-frequency limb exhibited a slope of 9 to 10 dB/octave. Extracochlear results were too variable to permit speculation. Changes in EABRs were only variably related to histopathologic findings.

Effect of experimentally induced otitis media on cochlear implants

Cat cochleas implanted with scala tympani prostheses were investigated histologically after inoculating the bullae with a suspension of group A streptococci. The prosthesis was passed through the round window membrane in one ear. In the other the prosthesis bypassed the round window via an opening anteroinferior to the round window niche. Before death, horseradish peroxidase was administered as a tracer for possible pathways of infection. Results showed that group A streptococci were pathogenic to the cat and caused inflammation in the bulla. The unimplanted round window membrane and the seals around the electrode entry points prevented infection from entering the cochlea. The seals around electrodes inserted either through the round window membrane or an opening drilled anteroinferior to the niche were equally effective. The horseradish peroxidase tracer studies showed, however, that a gap existed between the electrode and membranous seal, and this could be a potentially vulnerable site under certain conditions. Drilling an anteroinferior opening into the cochlea resulted in bony sequestra entering the cochlea. This can be avoided by blue-lining the opening and removing bone with picks before making an opening through the endosteum.

Pathology as it relates to ear surgery. VI. Cochlear implantation

The surgical anatomy and pathology of the cochlea have been reviewed in relation to cochlear implant surgery. Animal experimentation, as well as human temporal bone studies, have shown that the implant electrodes were well tolerated by the cochlea. The possible chemical and mechanical trauma induced by the electrodes can be avoided by better choice of shape, size, length and material of the implants. Long-term electrical stimulation did not seem to cause any deleterious effects on the neuronal population of the cochlea. In the present state of the art, cochlear implantation seems justified in well chosen cases.

Brainstem auditory pathway degeneration associated with chronic cochlear implants in the monkey

The form and pattern of first-order and transsynaptic degeneration in the central auditory pathway was studied in monkeys following inner ear stimulation by a cochlear implant. Multielectrode, scala tympani, and modiolar systems were implanted; in some cases, neomycin was perfused into the cochlea to destroy the organ of Corti at the time of implantation. The monkeys were maintained chronically for 5 to 120 weeks, then the cochleas and brainstems were examined histologically. The extent of spiral ganglion cell loss across animals showed variability, reflecting the different procedures and devices used. The degree and distribution of spiral ganglion cell loss was related to the degree and distribution of neural degeneration seen in the cochlear nucleus in all cases. Peripheral damage progressed toward the cochlear apex as survival time increased, and this progression was reflected in the cochlear nucleus by a ventrolateral shift in the locus of degeneration over time. In addition, evidence for transneuronal degeneration was seen at the superior olive, the lateral lemniscus and the inferior colliculus. Our findings indicate that several factors inherent in the use of a cochlear prosthesis, i.e., insertion trauma, host reaction, and/or electrical stimulation, may be associated with a long-term, continuing process of central degeneration visible at several levels of the auditory system.

Structural alteration of hair cells in the contralateral ear resulting from extracochlear electrical stimulation

Chronic electrical stimulation of the auditory nerve in patients with profound sensori-neural deafness is becoming increasingly routine. Therefore, it is important to understand more about the long-term consequences of this procedure. Hitherto, structural studies in animals after electrocochlear stimulation have concentrated on the stimulated cochlea. Here we have examined the effects of unilateral extracochlear electrical stimulation on the spiral organ of both the ipsilateral and contralateral ears of the mature guinea pig, and have found alterations in the structure of the outer hair cells and their efferent nerve terminals in the contralateral as well as the ipsilateral cochlea. This is the first evidence for a structural influence of efferent activity on the cochlea. Although the importance of the efferent system, consisting of the crossed and uncrossed olivo-cochlear bundles, is well established in providing central control of the sensory pathways, its exact role in hearing is incompletely understood. However, it is known that the outer hair cells and their efferent innervation are important in their contribution to inner hair cell responses and in modulating the micromechanics of the whole cochlea. These efferent functions now appear to be related to an important part of cochlear morphology, and are also relevant to our understanding of cochlear neurobiology, normal development and the management of hearing disability in both adult and child.

Banded intracochlear electrode array: evaluation of insertion trauma in human temporal bones

A banded free-fit scala tympani array was inserted into the basal turn of nine human cochleas to evaluate the trauma produced by the procedure. These nine cochleas, together with five nonimplanted controls, were serially sectioned and examined microscopically for damage to the membranous labyrinth, in particular the spiral ligament, the basilar and Reissner's membranes, the stria vascularis, and the osseous spiral lamina. The severity and location of any trauma along the cochlear spiral were recorded. The results indicate that the insertion of the banded scala tympani array resulted in minimal mechanical damage, occurring primarily to a localized region of the spiral ligament. This would not result in significant neural degeneration, and therefore would not compromise the efficacy of the multichannel cochlear prosthesis.

Cochlear neuronal populations in developmental defects of the inner ear. Implications for cochlear implantation

A histological study was made to determine the cochlear neuronal populations of 20 human ears having hearing loss caused by developmental defects. The neuronal populations ranged from 7677 in an ear with Mondini dysplasia to 30 753 in an ear with DiGeorge's syndrome, the norm for young human subjects being 35 000 neurons. The length of the cochlear (spiral) ganglion varied from 7.3 mm to 14.8 mm, the norm for human subjects being 12 mm. The sensorineural hearing losses in all cases were attributable to malformation or degeneration of the sensorineural structures. The hearing loss was moderate in one case of Alport's syndrome, severe in one case of Usher's syndrome and in one case of severe Mondini dysplasia; it was profound in one case of maternal rubella, one of congenital deafness of unknown cause, another case of severe Mondini dysplasia and one of Down's syndrome (Trisomy 21). One case of mild Mondini dysplasia and one of DiGeorge's syndrome were known to have normal hearing. In 4 other cases hearing status was absent.

Cochlear prostheses: stimulation-induced damage

The effects of 4 weekly, three-hour exposures to continuous sinusoidal (l kHz) electrical stimulation of the inner ear at various current levels were assessed in the chronically implanted guinea pig. With scala tympani stimulation, histopathological damage, including new bone growth, was observed for currents at and above 100 microA rms. No changes were observed in similarly implanted, but not stimulated cochleas. At equal current levels, less damage was found in subjects stimulated via electrodes placed on the round window and promontory, as compared to the scala tympani. Consistent reversible changes in threshold and suprathreshold features of the electrically evoked auditory brainstem response (EABR) were found. The magnitude of EABR change was directly related to exposure stimulus current level and to cochlear stimulation site. Suprathreshold features of the EABR were more sensitive to continuous stimulation exposures than threshold measures. Reversible EABR changes were found in the presence and absence of stimulation-induced histopathology.

Cochlear implant effects on the spiral ganglion

To evaluate the effects of chronic intrascalar implants on spiral ganglion cells, we studied 15 monkey cochleas that had had implants for periods ranging from 1 to 28 months. All cases exhibited loss or ongoing degeneration of cells. Cell loss typically was greater in the basal turn, where the implants were located, than at more apical locations. Increasing apical damage was a function of postimplant survival time. Preimplant local treatment of the inner ear with neomycin did not influence the loss of spiral ganglion cells. Osteoneogenesis occurred in the majority of cases, appearing in the basal turn with occasional extension into middle and apical turns. Cell loss was inconsistently associated with new bone formation. Electrical stimulation had no obvious influence on cell survival.

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.

Brainstem histopathology following chronic scala tympani implantation in monkeys

Degeneration in brainstem auditory nuclei was studied in monkeys following chronic implantation with scala tympani multielectrode systems. Nauta and Fink/Heimer-stained material from animals with survival times to 16 months were studied. The density and distribution of degeneration in the cochlear nuclei were consistent with observed patterns of spiral ganglion cell degeneration. Transneuronal degeneration was seen up to the level of the inferior colliculus. The density and form of the degeneration material in animals of varying survival times was consistent with the interpretation that a process of continuing degeneration occurred in these implanted ears.