Intracochlear electrocochleography during cochlear implantation.

OBJECTIVE: Electrophysiologic responses to acoustic stimuli are present in nearly all cochlear implant recipients when measured at the round window (RW). Intracochlear recording sites might provide an even larger signal and improve the sensitivity and the potential clinical utility of electrocochleography (ECoG). Thus, the goal of this study is to compare RW to intracochlear recording sites and to determine if such recordings can be used to monitor cochlear function during insertion of a cochlear implant. METHODS: Intraoperative ECoG recordings were obtained in subjects receiving a cochlear implant from the RW and from just inside scala tympani (n = 26). Stimuli were tones at high levels (80-100 dB HL). Further recordings were obtained during insertions of a temporary lateral cochlear wall electrode (n = 8). Response magnitudes were determined as the sum of the first and second harmonics amplitudes. RESULTS: All subjects had measurable extracochlear responses at the RW. Twenty cases (78%) showed a larger intracochlear response, compared with three (11%) that had a smaller response and three that were unchanged. On average, signal amplitudes increased with increasing electrode insertion depths, with the largest increase between 15 and 20 mm from the RW. CONCLUSION: ECoG to acoustic stimuli via an intracochlear electrode is feasible in standard cochlear implant recipients. The increased signal can improve the speed and efficiency of data collection. The growth of response magnitudes with deeper intrascalar electrode positions could be explained by closer proximity or favorable geometry with respect to residual apical signal generators. Reductions in magnitude may represent unfavorable geometry or cochlear trauma.

Round window electrocochleography just before cochlear implantation: relationship to word recognition outcomes in adults.

HYPOTHESES: Electrocochleography (ECoG) to acoustic stimuli can differentiate relative degrees of cochlear responsiveness across the population of cochlear implant recipients. The magnitude of the ongoing portion of the ECoG, which includes both hair cell and neural contributions, will correlate with speech outcomes as measured by results on CNC word score tests. BACKGROUND: Postoperative speech outcomes with cochlear implants vary from almost no benefit to near normal comprehension. A factor expected to have a high predictive value is the degree of neural survival. However, speech performance with the implant does not correlate with the number and distribution of surviving ganglion cells when measured postmortem. We will investigate whether ECoG can provide an estimate of cochlear function that helps predict postoperative speech outcomes. METHODS: An electrode was placed at the round window of the ear about to be implanted during implant surgery. Tone bursts were delivered through an insert earphone. Subjects included children (n = 52, 1-18 yr) and postlingually hearing impaired adults (n = 32). Word scores at 6 months were available from 21 adult subjects. RESULTS: Significant responses to sound were recorded from almost all subjects (80/84 or 95%). The ECoG magnitudes spanned more than 50 dB in both children and adults. The distributions of ECoG magnitudes and frequencies were similar between children and adults. The correlation between the ECoG magnitude and word score accounted for 47% of the variance. CONCLUSION: ECoGs with high signal-to-noise ratios can be recorded from almost all implant candidates, including both adult and pediatric populations. In postlingual adults, the ECoG magnitude is more predictive of implant outcomes than other nonsurgical variables such as duration of deafness or degree of residual hearing.

Correlation of early auditory potentials and intracochlear electrode insertion properties: an animal model featuring near real-time monitoring.

OBJECTIVE: The goal of this work was to assess electrophysiologic response changes to acoustic stimuli as an intracochlear electrode impacted cochlear structures in an animal model of hearing preservation cochlear implantation. The ultimate goal is to develop efficient procedures for assessing the status of cochlear physiology for intraoperative use. METHODS: Sixteen gerbils and 18 ears were tested. A rigid electrode was inserted through a basal turn cochleostomy and directed toward the basilar membrane/osseous spiral lamina complex. We recorded acoustically evoked early auditory potentials including cochlear microphonics (CMs) and compound action potentials (CAPs) to a short stimulation sequence consisting of one stimulus frequency and intensity as the electrode was advanced. A microendoscope was used to visualize the electrode insertion progress and to identify the site of electrode impact. After each experiment, the site of intracochlear trauma was confirmed using whole mount preparations. RESULTS: Electrophysiologic changes correlated well with the degree and location of trauma. We observed four distinct patterns. In addition, the endoscope in conjunction with the short recording sequence allowed for the detection of response changes that were reversible when the electrode was retracted. These cases were associated with less than full-thickness damage on histology. CONCLUSION: The short recording sequence to obtain acoustically evoked intracochlear potentials and the microendoscope allowed us to detect various levels of cochlear trauma including minor and reversible damage. Recordings of this type are potentially available using current implant technology. Future improvements in the measurements can be expected to improve the efficiency of the recording paradigm to produce a clinically useful tool.

Intracochlear recordings of electrophysiological parameters indicating cochlear damage.

OBJECTIVE: : The pathophysiologic mechanisms resulting in hearing loss during electrode implantation are largely unknown. To better understand the functional implications of electrode implantation, we recorded the effects of cochlear damage on acoustically evoked intracochlear measurements using normal-hearing gerbils. METHODS: : A metal electrode was placed on the surface of the round window, and recordings of the cochlear microphonic (CM) and compound action potential (CAP) were made in response to stimulation with tone-bursts at various frequencies in 1-octave intervals and at intensities of 15 to 72 dB sound pressure level. The electrode was then advanced incrementally, with CM and CAP measurements obtained at each step. These data were compared with data obtained at the round window, and the electrode was withdrawn when a significant change was observed. After electrophysiological analysis, the cochlea was examined histologically. RESULTS: : Results show that on electrode insertion, loss of amplitude in the CM and CAP occurs after damage to cochlear structures. Loss of activity was typically first apparent in the CAP rather than the CM. CONCLUSION: : These results suggest that a reduction of the CAP can be an early marker of interaction of the electrode with cochlear structures. Such measurements are potentially available with slight modifications to current cochlear implant technology.