Objectives: (1) To correlate simultaneously recorded intra-and extracochlear electrocochleography (ECochG) signals during electrode insertion into the cochlea, (2) to track changes in the ECochG signal during insertion and removal of an electrode, and (3) to correlate the findings with the preoperative residual hearing. We hypothesized that intracochlear ECochG recordings show signal changes not reflected in simultaneous extracochlear ECochG recordings. Design: During cochlear implantation in human cochlear implant recipients, a short, slim, custom-made electrode was inserted and removed in a stepwise manner. At each step, ECochG recordings were simultaneously recorded by an extracochlear electrode near the round window and via the inserted electrode. The acoustic stimulus was a 500 Hz tone burst at 110 to 130 dB SPL. Results: The mean amplitude difference between intra-and extracochlear ongoing ECochG responses was 14 dB (range 9 to 24 dB; n = 10) at the beginning of insertion. Intracochlear ECochG responses were larger in all cases. Extracochlear ECochG responses remained stable while intracochlear recordings showed large variations regarding amplitude and phase during the electrode array insertion. Intracochlear signal changes during insertion were reversible with retraction of the electrode. There were only weak to moderate (rs = 0.006 to 0.4), nonsignificant correlations of residual preoperative hearing with maximum amplitudes and amplitude changes during electrode insertion and removal in intracochlear recordings. Conclusions: Signals in intracochlear ECochG recordings are reliably larger than ECochG signals recorded simultaneously from an extracochlear location. Intracochlear ECochG recordings show reversible amplitude and phase changes during insertion, not reflected in simultaneous extracochlear ECochG recordings. Such changes are most likely due to the movement of the recording electrode in relation to the signal generators. Residual high-frequency hearing is associated with larger ECochG signal amplitudes. Modeling of expected intracochlear ECochG changes during electrode insertions may allow detection of cochlear trauma in the future.

The use of electrocochleography (ECochG) for providing real-time feedback of cochlear function during cochlear implantation is receiving increased attention for preventing cochlear trauma and preserving residual hearing. Although various studies investigated the relationship between intra-operative ECochG measurements and surgical outcomes in recent years, the limited interpretability of ECochG response changes leads to conflicting study results and prevents the adoption of this method for clinical use. Specifically, the movement of the recording electrode with respect to the different signal generators in intracochlear recordings makes the interpretation of signal changes with respect to cochlear trauma difficult. Here, we demonstrate that comparison of ECochG signals recorded simultaneously from intracochlear locations and from a fixed extracochlear location can potentially allow a differentiation between traumatic and atraumatic signal changes in intracochlear recordings. We measured ECochG responses to 500 Hz tone bursts with alternating starting phases during cochlear implant insertions in six human cochlear implant recipients. Our results show that an amplitude decrease with associated near 180° phase shift and harmonic distortions in the intracochlear difference curve during the first half of insertion was not accompanied by a decrease in the extracochlear difference curve’s amplitude ( n = 1), while late amplitude decreases in intracochlear difference curves (near full insertion, n = 2) did correspond to extracochlear amplitude decreases. These findings suggest a role for phase shifts, harmonic distortions, and recording location in interpreting intracochlear ECochG responses.

Objective: To assess the prevalence and implications of phase changes in extracochlear electrocochleography (ECochG) recordings during cochlear implantation. Materials and Methods: Extracochlear ECochG recordings were performed before and after insertion of the cochlear implant (CI) electrode by a recording electrode placed on the promontory. Acoustic stimuli were tone bursts at 250, 500, 750, and 1,000 Hz. The pure tone average (PTA) was determined before and approximately 4 weeks after surgery.