Acute morphological changes in guinea pig cochlea following electrical stimulation. A preliminary scanning electron microscope study

Intratemporal Facial Nerve Anatomy and its Variations in 30 Cases of Cadaveric Temporal Bones

It is important for the ENT surgeon to be familiar with the anatomy of the facial nerve and to prevent iatrogenic injury to nerve as it shows variations in its intratemporal course. Present study was done to delineate the intratemporal course of facial nerve and observe its variations. Thirty wet cadaveric temporal bones were dissected in the temporal bone dissection laboratory in the Department of Otorhinolaryngology and Head Neck Surgery, Government Medical College, Patiala. The length of intratemporal segments of facial nerve, its relationship with important bony landmarks, and the presence of any anomaly or variations in its course were observed. The mean length of labyrinthine, tympanic and mastoid segment was found to be 4.28 ± 0.605 mm, 10.40 ± 1.416 mm and 12.34 ± 0.915 mm respectively in the dissected specimens. The first and second genu angle varied between 50°-90° and 90°-120° respectively. Facial canal dehiscence was present at the level of first genu in 10% of cases and at the level of tympanic segment in 33%. Distance between chorda tympani origin and stylomastoid foramen varied between 4 and 6 mm with mean value of 5.31 ± 0.603 mm. Chorda-facial angle was found to be in the range of 20° to 31° with mean of 25.30° ± 2.90°. The tympanomastoid segment of facial nerve has variations in length and in its relations with various middle ear structures. The facial canal, as it traverses the temporal bone, may display bony dehiscence, variations, and anomalies in its natural course, having its own clinical and surgical significance.

Lateral hump anomaly of pyramid segment of facial nerve in living patients

CONCLUSIONS

A lateral hump anomaly of the pyramid segment of the facial nerve deserves special attention during otologic surgery. Coronary high-resolution CT (HRCT) reconstruction of temporal bone was highly valuable in preoperative detection of the anomaly.

OBJECTIVE

This study aimed to investigate lateral hump anomaly of the pyramid segment of the facial nerve in adult patients and assess the value of coronary HRCT reconstruction of the temporal bone in preoperative detection of the anomaly.

METHODS

We carried out a prospective study in 439 Han Chinese adults who underwent unilateral facial nerve decompression due to Bell's palsy in our department between April 2005 and October 2012, focusing on lateral hump anomaly of the pyramid segment of the facial nerve, detection sensitivity, and accuracy of coronary HRCT reconstruction of the temporal bone.

RESULTS

Lateral hump anomaly of the pyramid segment was observed in 21 cases (4.83%) undergoing surgery. The detection sensitivity and accuracy of coronary HRCT reconstruction were both 100%.

[Clinical anatomy of the temporal bone: taxonomy, topography and important variations]

Detailed knowledge of the complicated anatomy and topography of the temporal bone is a crucial basis for successful ear surgery. In particular, the topographical relations to the essential neighbouring structures (e.g. intratemporal vessels and nerves, neighbouring spaces, especially the endocranium) are of vital importance. Furthermore, typical landmarks play an essential role in topographic orientation. However, this knowledge cannot be acquired from lectures or books, but can only be achieved by independently performing numerous dissections and drill exercises on the temporal bone, once a theoretical understanding of the various structures and regions has been gained on the basis of a clearly defined nomenclature. Furthermore, there are essential anatomical variations which may lead to complicated anatomical conditions. The surgeon must also be familiar with these peculiarities.

Effects of hearing preservation on psychophysical responses to cochlear implant stimulation

Previous studies have shown that residual acoustic hearing supplements cochlear implant function to improve speech recognition in noise as well as perception of music. The current study had two primary objectives. First, we sought to determine how cochlear implantation and electrical stimulation over a time period of 14 to 21 months influence cochlear structures such as hair cells and spiral ganglion neurons. Second, we sought to investigate whether the structures that provide acoustic hearing also affect the perception of electrical stimulation. We compared psychophysical responses to cochlear implant stimulation in two groups of adult guinea pigs. Group I (11 animals) received a cochlear implant in a previously untreated ear, while group II (ten animals) received a cochlear implant in an ear that had been previously infused with neomycin to destroy hearing. Psychophysical thresholds were measured in response to pulse-train and sinusoidal stimuli. Histological analysis of all group I animals and a subset of group II animals was performed. Nine of the 11 group I animals showed survival of the organ of Corti and spiral ganglion neurons adjacent to the electrode array. All group I animals showed survival of these elements in regions apical to the electrode array. Group II animals that were examined histologically showed complete loss of the organ of Corti in regions adjacent and apical to the electrode array and severe spiral ganglion neuron loss, consistent with previous reports for neomycin-treated ears. Behaviorally, group II animals had significantly lower thresholds than group I animals in response to 100 Hz sinusoidal stimuli. However, group I animals had significantly lower thresholds than group II animals in response to pulse-train stimuli (0.02 ms/phase; 156 to 5,000 pps). Additionally, the two groups showed distinct threshold versus pulse rate functions. We hypothesize that the differences in detection thresholds between groups are caused by the electrical activation of the hair cells in group I animals and/or differences between groups in the condition of the spiral ganglion neurons.

Laser stimulation of the auditory nerve

BACKGROUND AND OBJECTIVES

For centuries, electric current has been used to stimulate neurons. Shortcomings of electrical stimulation include the contact between the stimulating electrode and the tissue, and the non-selective stimulation of the tissue. In contrast to electric stimulation, optical radiation can provide spatially selective neural stimulation without tissue contact.

STUDY DESIGN/MATERIALS AND METHODS

Acute in vivo experiments using gerbils were conducted to record optically evoked compound action potentials (CAPs) from the cochlea.

RESULTS

Optical radiation evokes CAPs in normal hearing animals and in deafened animals, in which cochleae lack outer and inner hair cells. Stimulation threshold was measured as 0.018+/-0.003 J/cm(2) (mean+/-SE). Laser radiation could be increased by 30-40 dB until drastic changes were seen in cochlear function. Cochlear response amplitudes to optical radiation were stable over extended stimulation times.

CONCLUSIONS

We have demonstrated that the auditory nerve can be stimulated by optical radiation. One potential clinical use of this technology would be for cochlear implants.

Surgical anatomy of tympano-mastoid segment of facial nerve

OBJECTIVE

Facial nerve is known to have a considerable variations more so in the temporal bone. An otologist with inadequate familiarity with facial nerve usually have a tendency to do incomplete surgery in chronic suppurative otitis media. The present study was conducted to explore the microanatomy of tympanic and mastoid segments of facial nerve.

SETTING

Temporal bone lab.

MATERIALS AND METHODS

This study was conducted at PGIMS Rohtak in 25 wet temporal bones which were dissected under the microscope. Various parameters studied included the length of tympanic and mastoid segments of facial nerve, various anomalies, depth from the cortex and relation to various important structures.

RESULTS

The mean length of tympanic segment was 11.1 mm±0.88 and mastoid segment was 15.4 mm±2.4. The angle at second genu was 95-125° and the facial canal was dehiscent in 12% cases.

CONCLUSIONS

The tympanomastoid segment of facial nerve has variations in length and in its relation with various middle ear structures. Further the nerve, in Indians is also at variance as compared to Japanese and Americans probably because of different racial configuration of the skull.

Desynchronization of electrically evoked auditory-nerve activity by high-frequency pulse trains of long duration

Rubinstein et al. [Hear. Res. 127, 108-118 (1999)] suggested that the neural representation of the waveforms of electric stimuli might be improved by introducing an ongoing, high-rate, desynchronizing pulse train (DPT). A DPT may desynchronize neural responses to electric stimulation in a manner similar to spontaneous activity in a healthy ear. To test this hypothesis, responses of auditory-nerve fibers (ANFs) to 10-min-long electric pulse trains (5 kpps) were recorded from acutely deafened, anesthetized cats. Stimuli were delivered via an intracochlear electrode, and their amplitude was chosen to elicit a response in most ANFs. Responses to pulse trains showed pronounced adaptation during the first 1-2 min, followed by either a sustained response or cessation of spike discharges for the remainder of the stimulus. The adapted discharge rates showed a broad distribution across the ANF population like spontaneous activity. However, a higher proportion of fibers (46%) responded to the DPT at rates below 5 spikes/s than for spontaneous activity, and 12% of the fibers responded at higher rates than any spontaneously active fiber. Interspike interval histograms of sustained responses for some fibers had Poisson-like (exponential) shapes, resembling spontaneous activity, while others exhibited preferred intervals and, occasionally, bursting. Simultaneous recordings from pairs of fibers revealed no evidence of correlated activity, suggesting that the DPT does desynchronize the auditory nerve activity. Overall, these results suggest that responses to an ongoing DPT resemble spontaneous activity in a normal ear for a substantial fraction of the ANFs.

Microsurgical anatomy of the perigeniculate ganglion area as seen from the middle cranial fossa approach

The middle cranial fossa approach is useful for decompressing the perigeniculate ganglion area of the facial nerve in patients with serviceable hearing. The present study was designed to investigate the microsurgical anatomy of the perigeniculate ganglion area of the facial nerve from the point of view of the middle cranial fossa. We dissected 20 human temporal bones under a microscope using a middle fossa approach, and measured the angle between the lines drawn from the malleus head to the vertical crest and from the malleus head to the geniculate ganglion, and the distance from the malleus head to the geniculate ganglion. These were found to be 22.7 degrees +/- 2.2 degrees and 6.5 +/- 0.3 mm, respectively. Detailed knowledge about the microsurgical anatomy of the perigeniculate ganglion area of the facial nerve from the point of view of the middle cranial fossa is imperative for facial nerve decompression by a middle cranial fossa approach.

Frequency specificity of cochlear damage in acute electrical injury: a longitudinal distortion product otoacoustic emission study

HYPOTHESIS

To understand cochlear dysfunction and the recovery pattern of the cochlea after acute electrical injury.

BACKGROUND

The cochlea is believed to be more vulnerable to direct current than to alternating current. However, the damage-and frequency-specific recovery characteristics of the cochlea have not been well described.

METHODS

Baseline distortion product otoacoustic emission measurements were taken via transtympanic electrodes from 19 guinea pigs. A 20-Hz alternating current and a positive direct current, both at a 1,000-microA intensity, were applied to 9 and 10 animals, respectively. The measurements were repeated immediately after the application of current and after 10 days. Comparisons were made for both groups individually in signal-to-noise ratios obtained before and immediately after, immediately and 10 days after, and before and 10 days after the application of electrical current.

RESULTS

Alternating and direct currents caused a significant depression in signal-to-noise ratio immediately after the application. However, 10 days later, the mean signal-to-noise ratio in the animals subjected to alternating current came significantly close to the baseline value, particularly between the frequencies of 2,211 and 3,717 Hz. By contrast, the mean signal-to-noise ratio in the animals subjected to direct current remained significantly depressed throughout all frequencies except for 2,211 Hz.

CONCLUSION

Baseline distortion product otoacoustic emission measurements enabled cochlear function to be monitored in a frequency-specific manner after electrical injury. The functional damages were quantitatively close to each other for both types of currents at a given intensity. Nevertheless, recovery was more apparent in the animals exposed to alternating current than in those exposed to direct current. Recovery was also better in the midfrequency region than in higher frequencies. It is hypothesized that the reason for the difference in recovery in both groups was the net charge left by direct current.

Effects of chronic stimulation on auditory nerve survival in ototoxically deafened animals

For almost 10 years, chronic stimulation has been known to affect spiral ganglion cell (SGC) survival in the deaf ear. However, the reported effects of chronic stimulation vary across preparations and studies. In this review, the effects of chronic stimulation on the deafened auditory periphery are examined, and variables that may impact on the efficacy of chronic stimulation are identified. The effects of deafening on the unstimulated peripheral and central auditory system are also described, as the deafened, unstimulated system is the canvas upon which stimulation-mediated effects are imposed. Discrepancies in the effects of chronic stimulation across studies may be attributable in large part to the combined effects of the deafening method and the post-deafening delay prior to chronic stimulation, which vary across studies. Emphasis is placed on the need to consider the natural progression of SGC loss following deafening in the absence of chronic stimulation, as the rate of SGC loss almost certainly affects both the efficacy of stimulation, and the impact of any delay between deafening and initiation of stimulation. The differences across preparations complicate direct comparison of protective efficacy of stimulation. At the same time, these differences can be used to our advantage, aiding characterization of the effects of different factors on the efficacy of chronic stimulation as a neuroprotective intervention.

Microsurgical anatomy of the perigeniculate ganglion area as seen from a translabyrinthine approach

Detailed anatomic knowledge of the microsurgical anatomy of the perigeniculate ganglion area is essential to probing adjacent to the facial nerve by a translabyrinthine approach. This study was designed to investigate the surgical anatomy of the perigeniculate ganglion area of the facial nerve from a translabyrinthine point of view. We dissected 15 human temporal bones under a microscope, measured the lengths of the tympanic segment and the labyrinthine segment by a middle cranial fossa approach, and measured the angle between the tympanic and labyrinthine segments by a translabyrinthine approach. The distance of the facial nerve from the cochleariform process to the geniculate ganglion was 3.8+/-0.7 mm. The length of the labyrinthine segment of the facial nerve was 4 +/-0.8 mm. The angle between the tympanic and labyrinthine segments from a translabyrinthine point of view was 26 degrees +/-5 degrees. Precise knowledge about the microsurgical anatomy of the perigeniculate ganglion area of the facial nerve from a translabyrinthine viewpoint is imperative for facial nerve decompression by a translabyrinthine approach.

Effects of chronic high-rate electrical stimulation on the cochlea and eighth nerve in the deafened guinea pig

This study was undertaken to examine the effects of chronic high-rate stimulation on the eighth nerve and cochlea. Fifty-four male pigmented guinea pigs were deafened and implanted with single ball electrodes in scala tympani. Four groups of animals received chronic electrical stimulation at a level of 5 microCol/cm2/ph for 1000 h as follows: Group A: 1000 Hz, 100 microseconds/ph duration, 100 microA peak; Group B: 250 Hz, 100 microseconds/ph duration, 100 microA peak; Group C: 2750 Hz, 36 microseconds/ph duration, 250 microA peak; Group D: 250 Hz, 400 microseconds/ph duration, 25 microA peak. Also, two control groups received 20 min stimulation during weekly electrically evoked auditory brainstem response (eABR) measurement (Group E) and about 5 s stimulation (Group F) during a brief eABR 3 day postimplantation and at perfusion. On Day 50, animals were perfused, midmodiolar sections cut and a quantitative assessment of spiral ganglion cells (SGC) performed. All stimulated subjects showed a similar decrease in eABR thresholds and dynamic range over time. No stimulation conditions induced pathology. All stimulation conditions enhanced survival of SGCs compared to unimplanted ears and implanted non-stimulated ears (Group F). There were no statistically significant differences in SGC survival between any stimulated groups, including Group E stimulated once a week. In conclusion, high-rate stimulation, under the conditions of this study, provides no additional risks and the same benefits to SGC survival as low-rate stimulation.

Chronic electrical stimulation of the auditory nerve at high stimulus rates: a physiological and histopathological study

A major factor associated with recent improvements in the clinical performance of cochlear implant patients has been the development of speech-processing strategies based on high stimulation rates. While these processing strategies show clear clinical advantage, we know little of their long-term safety implications. The present study was designed to evaluate the physiological and histopathological effects of long-term intracochlear electrical stimulation using these high rates. Thirteen normal-hearing adult cats were bilaterally implanted with scala tympani electrode arrays and unilaterally stimulated for periods of up to 2100 h using either two pairs of bipolar or three monopolar stimulating electrodes. Stimuli consisted of short duration (25-50 microseconds/phase) charge-balanced biphasic current pulses presented at 1000 pulses per second (pps) per channel for monopolar stimulation, and 2000 pps/channel for bipolar stimulation. The electrodes were shorted between current pulses to minimize any residual direct current, and the pulse trains were presented using a 50% duty cycle (500 ms on; 500 ms off) in order to simulate speech. Both acoustic (ABR) and electrical (EABR) auditory brainstem responses were recorded periodically during the chronic stimulation program. All cochleas showed an increase in the click-evoked ABR threshold following implant surgery; however, recovery to near-normal levels occurred in approximately half of the stimulated cochleas 1 month post-operatively. The use of frequency-specific stimuli indicated that the most extensive hearing loss generally occurred in the high-frequency basal region of the cochlea (12 and 24 kHz) adjacent to the stimulating electrode. However, thresholds at lower frequencies (2, 4 and 8 kHz), appeared at near-normal levels despite long-term electrode implantation and electrical stimulation. Our longitudinal EABR results showed a statistically significant increase in threshold in nearly 40% of the chronically stimulated electrodes evaluated; however, the gradient of the EABR input/output (I/O) function (evoked potential response amplitude versus stimulus current) generally remained quite stable throughout the chronic stimulation period. Histopathological examination of the cochleas showed no statistically significant difference in ganglion cell densities between cochleas using monopolar and bipolar electrode configurations (P = 0.67), and no evidence of cochlear damage caused by high-rate electrical stimulation when compared with control cochleas. Indeed, there was no statistically significant relationship between spiral ganglion cell density and electrical stimulation (P = 0.459), or between the extent of loss of inner (IHC, P = 0.86) or outer (OHC, P = 0.30) hair cells and electrical stimulation. Spiral ganglion cell loss was, however, influenced by the degree of inflammation (P = 0.016) and electrode insertion trauma. These histopathological findings were consistent with the physiological data. Finally, electrode impedance, measured at completion of the chronic stimulation program, showed close correlation with the degree of tissue response adjacent to the electrode array. These results indicated that chronic intracochlear electrical stimulation, using carefully controlled charge-balanced biphasic current pulses at stimulus rates of up to 2000 pps/channel, does not appear to adversely affect residual auditory nerve elements or the cochlea in general. This study provides an important basis for the safe application of improved speech-processing strategies based on high-rate electrical stimulation.

Dimensions of the sinus tympani and its surgical access via a retrofacial approach

The sinus tympani (ST) is a critical anatomic region of the temporal bone. It lies medial to the facial nerve, between the ponticulus and the subiculum, and therefore is not easily visualized by routine surgical approaches to the middle ear and mastoid. This limited access makes the ST a site that is notorious for residual cholesteatoma. An extensive evaluation of the anatomic dimensions of the ST was made from human temporal bones. Three hundred twenty-seven bones were examined at four standardized levels to describe the dimensions and anatomic relationships of the ST with other structures of the temporal bone. The region of the stapedial tendon was found to be the most limited anatomic substructure in the vicinity of the ST. This study demonstrates the feasibility of a retrofacial approach to the ST as an aid in eradication of otherwise hidden disease.

Reduction in excitability of the auditory nerve following electrical stimulation at high stimulus rates

While recent studies have suggested that electrical stimulation of the auditory nerve at high stimulus rates (e.g., 1000 pulses/s) may lead to an improved detection of the fine temporal components in speech among cochlear implant patients, neurophysiological studies have indicated that such stimulation could place metabolic stress on the auditory nerve, which may lead to neural degeneration. To examine this issue we recorded the electrically evoked auditory brainstem response (EABR) of guinea pigs following acute bipolar intracochlear electrical stimulation using charge-balanced biphasic current pulses at stimulus rates varying from 100 to 1000 pulses/s and stimulus intensities ranging from 0.16 to 1.0 microC/phase. Charge density was held constant (approximately 75 microC cm-2 geom/phase) in those experiments. To monitor the recovery in excitability of the auditory nerve following this acute stimulation. EABR thresholds, wave I and III amplitudes and their latencies were determined for periods of up to 12 h following the acute stimulation. Higher stimulus rates and, to a lesser extent, higher intensities led to greater decrements in the post-stimulus EABR amplitude and prolonged the recovery period. While continuous stimulation at 100 pulses/s induced no decrement in the EABR, stimulation at 200 and 400 pulses/s produced an increasingly significant post-stimulus reduction of the EABR amplitude, which showed only partial recovery during the monitoring period. No EABR response could be evoked immediately following stimulation at 1000 pulses/s, using a probe intensity 16-19 dB below the stimulus intensity. However, partial EABR recovery was observed for wave III following stimulation at the lowest stimulus intensity (0.16 microC/phase). These stimulus-induced reductions in the EABR amplitude were also reflected in increased thresholds and latencies. Providing stimulus rate and intensity were held constant, stimulation at different charge densities (37.7, 75.5 and 150.7 microC cm-2 geom/phase) had no influence on the post-stimulus EABR recovery. Significantly, the introduction of a 50% duty cycle into the stimulus pulse train resulted in a more rapid and complete post-stimulus recovery of the EABR compared to continuous stimulation. These data suggest that stimulus rate is a major contributor to the observed reduction in excitability of the electrically stimulated auditory nerve. This reduction may be a result of an activity-induced depletion of neural energy resources required to maintain homeostasis. The present findings have implications for the design of safe speech-processing strategies for use in multichannel cochlear implants.

Stimulation of residual hearing in the cat by pulsatile electrical stimulation of the cochlea

Electrical stimulation of the cochlea may excite residual inner hair cells, either by direct electrical stimulation or through a mechanical event. Hair cell mediated responses of the auditory nerve to electrical stimulation were estimated from forward masking of the compound action potential evoked by an acoustic probe. Masking by a fixed electrical masker peaked for probes equal in frequency to the pulse repetition rate and its second harmonic, suggesting a spatially tuned profile of excitation within the cochlea. Furthermore, the tuning curves for masking of a fixed acoustic probe peaked for masker pulse rates close to the frequency of the probe. A secondary peak of masking was commonly seen for electrical stimulation at one half of the probe frequency, suggesting masking of the probe by the second harmonic of the electrical stimulus. These results suggest that pulsatile stimulation at the base of the cochlea generates a spectrally rich mechanical disturbance in which each component propagates to its place of resonance in the cochlea.

Three-dimensional development of the facial nerve path through the ear region in human embryos

The goal was to determine how the facial nerve path forms normally through the ear region. Five three-dimensional computer reconstructions and eight wax plate-graphic, two-dimensional reconstructions were made from a group of 47 serially sectioned human embryos that were in the blastemal period of ear development, when most of the definitive arrangement is established. The size, shape, and positional changes of the facial nerve relative to the external ear and notochord were studied in both lateral and frontal views. The vertical (dorsoventral) position of the external ear region remains constant but shifts caudally. After formation, the geniculate ganglion assumes a position that gradually becomes rostrodorsal to the external ear. Facial nerve branches form and grow into regions expanding peripherally. The superficial petrosal branch appears to hold the ganglion in position as the horizontal segment of the facial nerve forms when the external ear shifts caudally. Possible growth movements explaining abnormal nerve paths are discussed.

Effects of chronic electrical stimulation on patients using a cochlear prosthesis

Eighteen patients using the Nucleus multichannel cochlear prosthesis underwent annual evaluations for electrical thresholds, dynamic range, and speech recognition abilities for a period of 1 to 5 years. Results revealed no correlation between length of usage of a cochlear implant and electrical thresholds. The dynamic range was initially wider in the patients with open-set speech recognition, but narrowed in subsequent years. There was a correlation between length of deafness and postoperative performance.

Facial nerve monitoring in otologic surgery: clinical indications and intraoperative technique

While identification of the intratemporal portion of the facial nerve is mandatory in most otologic surgical procedures, inadvertent instrumentation, traction, or thermal injury may still result from inaccurate delineation, purposeful avoidance, or false protection of this critical structure. Improved functional preservation of the facial nerve has been achieved in acoustic neuroma surgery through the monitoring of evoked facial electromyographic activity. This technique may also be used during otologic procedures in which facial nerve manipulation is anticipated in the management of recurrent cholesteatoma, temporal bone trauma, congenital deformity, or purposeful access for cochlear implantation. Potential indications for using facial nerve monitoring in contemporary otologic surgery are detailed through illustrative case presentations, and necessary instrumentation and techniques are briefly reviewed. Intraoperative monitoring can assist the surgeon in isolating the facial nerve when chronic inflammation, traumatic injury, or anomalous development has resulted in distortion or absence of microanatomic landmarks.

Effect of electrical stimulation on middle latency response in the guinea pig

A temporary threshold shift (TTS) has been demonstrated in the electrically evoked middle latency response (EMLR) following exposure to moderate levels of continuous electrical stimulation via a cochlear implant. The threshold at which the EMLR was elicited in chronically implanted guinea pigs was elevated by approximately 100% following 30 minutes of moderate intensity (200 microA or more) sinusoidal electrical stimulation of the cochlea. Results obtained under anesthesia varied unacceptably. In awake animals, EMLR thresholds were stable over time and consistent TTSs were observed. The threshold returned to prestimulation levels within 4 hours following termination of the stimulation. The possibility of histopathologic changes and the relevance of these findings in setting safe output levels for cochlear implant processors are discussed.

Effect of electrical pulse shape on AVCN unit responses to cochlear stimulation

Electrical stimulation of the cochlea with a multiple-electrode array is best accomplished using pulsatile instead of continuous stimulation. The optimum shapes of electrical pulses for this purpose are still uncertain due to a lack of knowledge about their stimulation efficiency and requirements of the encoding strategy. We presented an extensive set of charge-balanced, rectangular pulse shapes to the guinea pig cochlea. Durations per phase for these constant-current pulses ranged from 20 microseconds to 900 microseconds with initially positive and initially negative polarities. Spike counts from single units in the anteroventral cochlear nucleus differed significantly for different pulse shapes, as did their initial latencies. Implications for stimulation efficiency and encoding strategies are discussed.

Electrically stimulated increases in cochlear blood flow: I. Frequency and intensity effects

Charge-balanced, sinusoidal current was passed differentially between the apex and round window of the guinea pig cochlea. Cochlear blood flow was measured using a laser Doppler flow monitor. Systemic blood pressure was monitored from a cannula within the common carotid artery. Electrical stimulation increased cochlear blood flow, while systemic blood pressure was unaffected. A cochlear blood flow response parameter, normalized for transient changes in systemic blood pressure, was defined. The magnitude of the response parameter was found to be frequency selective and was also found to be an increasing function of current intensity, with maximum responses obtained with 500 Hz sinusoids. This cochlear blood flow response was not observed in dead animals; was present in preparations paralyzed with gallamine hydrochloride; and was correlated with an increase in cochlear red blood cell velocity, as directly observed by intravital microscopy. These observations imply that electrical stimulation induces a local vasodilation within the temporal bone. The fact that decreased cochlear blood flow was never observed with current injection implies that ischemia is not a likely mechanism of electrically induced tissue damage within the inner ear. The mechanism of this cochlear blood flow response is addressed in a companion report.

Chronic perilymphatic fistula: experimental model in the guinea pig

Chronic perilymphatic fistulas were created in guinea pig cochleas using silicone rubber tubing placed into the scala tympani through the round window. Fistula patency was determined by fluorescein perfusion into cerebral spinal fluid. Fistula were found to be patent in 6 of 6 animals at 7 days and 8 of 13 animals at 28 days. Analysis of ABRs revealed threshold increases of 10 to 15 dB across all frequencies at 1 hour and 7 days. However, thresholds returned to pre-fistula levels by 28 days. Animals with acute fistulas (simple laceration of the round window) had similar threshold increases at 1 hour; however, recovery to baseline levels occurred by day 7. Control animals with intact round windows did not have threshold shifts. Scanning electron microscopy revealed hair cell loss localized to the apical and basal turns of the cochlea. The morphologic changes observed occurred acutely (within 7 days) and were not progressive, despite the presence of a fistula. Hair cell loss or degeneration did not correlate with hearing loss.

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.

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.

Mechanisms of electrically induced damage after cochlear implantation

This report is the fourth in a series of parametric studies designed to evaluate and define conditions that may produce histological damage by means of electrical stimulation from cochlear prostheses. Earlier studies established damage thresholds for both acute (400 microA rms or 70 microC/cm(2)0) and chronic (100 microA rms or 15 to 20 microC/cm(2)0) stimulation with continuous sinusoidal current of 1,000 Hz. In a subsequent study, a tenfold reduction in the stimulation frequency (to 100 Hz) resulted in a 50% reduction in the acute damage threshold (200 microA rms), which was a smaller reduction than anticipated if damage is dependent only on charge density. This finding and the damage patterns observed in the preceding studies suggested that multiple mechanisms are responsible for the sensory and supporting cell degeneration induced by the electrical stimulus. A similar pattern of structural changes has been observed by other investigators after acoustic stimulation of the cochlea, suggesting that common mechanisms may be involved. With electrical stimulation, electrophoretic effects have been implicated; like acoustic trauma, however, mechanical, biochemical, and metabolic processes may also be involved. This investigation was designed to identify and analyze better the damage mechanisms active in acute stimulation. Sixteen normal guinea pigs were implanted and stimulated using an interrupted 1,000-Hz signal at 500 microA for three hours. Duty cycle was reduced to 50% and signal periodicity ranged from 100 to 1,000 ms.(ABSTRACT TRUNCATED AT 250 WORDS)

Histologic evaluation of temporal bones with cochlear implants

Ectopic bone in the scala tympani is characteristic of cochlear otosclerosis and meningitis, which are the two most frequent etiologies of deafness in cochlear implant patients. The ectopic bone in a multiple-electrode implanted bone is probably caused by disease (syphilis) rather than by the electrode or electrical stimulation. The sensory elements being stimulated by intracochlear electrodes probably are the ganglion cells.

The petromastoid canal

The embryology and anatomy of the petromastoid canal is reviewed. This structure may be responsible for the passage of some infections of the middle ear cleft into the posterior fossa. It also transmits important blood vessels to a portion of the bony labyrinth, to the facial canal, and to the mucosa of the mastoid air cell system.

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.