Extra- and Intracochlear Electrocochleography in Cochlear Implant Recipients

Abstract

Objective: To monitor cochlear function by extra- and intracochlear electrocochleography (ECoG) during and after cochlear implantation and thereby to enhance the understanding of changes in cochlear function following cochlear implantation surgery. Methods: ECoG responses to acoustic stimuli of 250, 500 and 1,000 Hz were recorded in 9 cochlear implant recipients with presurgical residual hearing. During surgery extracochlear ECoG recordings were performed before and after insertion of the cochlear implant electrode array. After insertion of the electrode array, intracochlear ECoG recordings were conducted using intracochlear electrode contacts as recording electrodes. Intracochlear ECoG recordings were performed up to 6 months after implantation. ECoG findings were correlated with findings from audiometric tests. Results: Extra- and intracochlear ECoG responses could be recorded in all subjects. Extracochlear ECoG recordings during surgery showed moderate changes. Loss or reduction of the ECoG signal at all three frequencies did not occur during cochlear implantation. During the first week following surgery, conductive hearing loss, due to middle ear effusion, led to a decrease in intracochlear ECoG signal amplitudes. This was not attributable to changes of cochlear function. All persistent reductions in ECoG response magnitude after normalization of the tympanogram occurred during the first week following implantation. Thresholds of ECoG signals were at or below hearing thresholds in all cases. Conclusion: Gross intracochlear trauma during surgery appears to be rare. In the early postoperative phase the ability to assess cochlear status by ECoG recordings was limited due to the regular occurrence of middle ear effusion. Still, intracochlear ECoG along with tympanogram recordings suggests that any changes of low-frequency cochlear function occur mainly during the first week after cochlear implantation. ECoG seems to be a promising tool to objectively assess changes in cochlear function in cochlear implant recipients and may allow further insight into the mechanisms underlying the loss of residual hearing.