Electrophysiologic consequences of flexible electrode insertions in gerbils with noise-induced hearing loss.

HYPOTHESIS: Flexible electrode interaction with intracochlear structures in a noise-damaged region of the cochlea can lead to measureable electrophysiologic changes. BACKGROUND: An emerging goal in cochlear implantation is preservation of residual hearing subsequently allowing for combined electric and acoustic stimulation (EAS). However, residual hearing is at least partially lost in most patients as a result of electrode insertion. A gerbil model was used to examine changes to acoustically evoked cochlear potentials during simulated cochlear implantation. METHODS: Gerbils were partially deafened by noise exposure to mimic residual hearing in human cochlear implant candidates. After 1 month, round window and intracochlear recordings during flexible electrode insertion were made in response to 1 kHz tone burst stimuli at 80 dB SPL. After the insertion, the cochleas were histologically examined for hair cell loss because of the noise exposure and trauma because of the electrode insertion. RESULTS: Anatomic damage from the flexible electrode was not observable in most cases. However, insertions caused response declines that were, on average, greater than the controls, although some losses were similar to the controls. The CM was more sensitive than the CAP for detecting cochlear disturbance. CONCLUSION: Because response reductions occurred in the absence of anatomic damage, disturbances in the fluid at the base appear to affect responses from the apex. The losses were less than in previous experiments where the basilar membrane was penetrated.

Influence of an intraoperative perilymph gusher on cochlear implant performance in children with labyrinthine malformations.

OBJECTIVES: To assess the effect of an intraoperative perilymph fluid gusher during cochlear implantation on speech perception abilities in pediatric patients with labyrinthine anomalies. SETTING: Tertiary care academic referral center. METHODS: Seventy subjects with labyrinthine malformations who received a cochlear implant were identified in our pediatric cochlear implant database. In 30 cases, an intraoperative perilymph fluid gusher was encountered during surgery. Fifteen children with GJB2 positive hereditary hearing loss served as controls. Multiple speech perception measures were obtained with the cochlear implant. The best score for each subject over time was determined as a speech reception index in quiet. This index was compared among groups and malformation types. RESULTS: The speech reception index in quiet demonstrated overall good performance scores of cochlear implantation in children with incomplete partitioning/enlarged vestibular aqueduct type malformations. Children with hypoplastic malformations, on the other hand, showed variable outcomes with many children demonstrating only limited long-term speech discrimination abilities. The presence or absence of a perilymph gusher did not significantly influence results after cochlear implantation. CONCLUSION: This report documents the variable outcomes of pediatric cochlear implantation in children with inner ear malformations. More importantly, anatomic parameters, such as the classification of the anomaly and the presence of a viable cochlear nerve, seem to influence performance measures substantially. The presence of a perilymph gusher did not influence outcomes in both hypoplastic and incomplete partitioning/enlarged vestibular aqueduct type malformations. LEVEL OF EVIDENCE: 2b Individual retrospective cohort study.

Detection of intracochlear damage with cochlear implantation in a gerbil model of hearing loss.

HYPOTHESIS: Cochlear trauma due to electrode insertion can be detected in acoustic responses to low frequencies in an animal model with a hearing condition similar to patients using electroacoustic stimulation. BACKGROUND: Clinical evidence suggests that intracochlear damage during cochlear implantation negatively affects residual hearing. Recently, we demonstrated the usefulness of acoustically evoked potentials to detect cochlear trauma in normal-hearing gerbils. Here, gerbils with noise-induced hearing loss were used to investigate the effects of remote trauma on residual hearing. METHODS: Gerbils underwent high-pass (4-kHz cutoff) noise exposure to produce sloping hearing loss. After 1 month of recovery, each animal's hearing loss was determined from auditory brainstem responses and baseline intracochlear recording of the cochlear microphonic and compound action potential (CAP) obtained at the round window. Subsequently, electrode insertions were performed to produce basal trauma, whereas the acoustically generated potentials to a 1-kHz tone-burst were recorded after each step of electrode advancement. Hair cell counts were made to characterize the noise damage, and cochlear whole mounts were used to identify cochlear trauma due to the electrode. RESULTS: The noise exposure paradigm produced a pattern of hair cell, auditory brainstem response, and intracochlear potential losses that closely mimicked that of electrical and acoustic stimulation patients. Trauma in the basal turn, in the 15- to 30-kHz portion of the deafened region, remote from preserved hair cells, induced a decline in intracochlear acoustic responses to the hearing preserved frequency of 1 kHz. CONCLUSION: The results indicate that a recording algorithm based on physiological markers to low-frequency acoustic stimuli can identify cochlear trauma during implantation. Future work will focus on translating these results for use with current cochlear implant technology in humans.