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.