Amplitude and Phase Changes in Electrocochleographic Real-Time Recordings During Cochlear Implantation and Its Relation to Pre- and Postoperative Hearing

Background: The aim of this study was to relate response patterns of electrocochleography (ECochG) recordings during cochlear implantation to pre- and postoperative hearing. Methods: Thirty subjects with either flat (FA, n = 9) or sloping (SA, n = 21) audiograms before cochlear implantation were prospectively included. Real-time ECochG recordings were conducted via the cochlear implant. The difference curve (DIF) signal of the ECochG recordings was analyzed regarding alteration of the waveform, amplitude changes, and relative phase shifts during insertion. Results: Five subjects (56%) with FA and 13 (62%) with SA exhibited DIF signal drops in the early phase of the insertion. In subjects with FA, alterations of the DIF signal waveform in the early phase of the insertion occurred in 8 subjects (90%), whereas such changes were detectable in only 2 out of 21 subjects (10%) with SA (p < 0.001). DIF signal drops with relative phase shifts of >0.7 radians but without alterations of the waveform occurred in 5 subjects (56%) with FA and 11 (52%) with SA. Such drops were associated with larger postoperative hearing losses than DIF signal drops without phase changes in both groups (FA: 43 versus 20 dB, p = 0.045; SA: 30 versus 14 dB, p = 0.001). Conclusion: Residual cochlear function in basal regions leads to alteration of the DIF signal waveform during insertion, probably not associated with cochlear injury. A decrease of the DIF signal amplitude with a simultaneous relative phase shift but no alteration of the waveform is associated with greater loss of residual hearing independent from the preoperative hearing.