8Intraoperative ECochG
The other objective measures interrogate the implant after it is in. This one watches the cochlea as the electrode goes in. By recording the cochlea's own acoustically-evoked response through the implant during insertion, the surgeon gets a real-time signal of cochlear health — a drop in amplitude can warn of trauma while the array is still moving and there is still time to act. It is the bridge between this atlas and electrocochleography proper, and the objective measure most directly tied to preserving residual hearing.
FTWhy monitor insertion
Modern cochlear implantation increasingly aims to preserve residual low-frequency hearing — for electric-acoustic stimulation, and because an atraumatic insertion protects the cochlea generally. The threat to that hearing is mechanical trauma during electrode insertion: basilar-membrane contact, scalar translocation, or excessive force. Intraoperative ECochG provides a live, objective readout of cochlear function during the very manoeuvre that endangers it.[2015, 2015]
This is the same physiology as clinical electrocochleography — the cochlear potentials evoked by acoustic stimulation — but recorded from inside the cochlea by the implant rather than from the promontory or ear canal. For the full account of the underlying signals, see the companion ECochG Atlas; this module focuses on the CI-specific, intraoperative use.
TRecording through the implant
A loud acoustic stimulus — typically a low-frequency tone burst (e.g. 250–500 Hz) — is delivered to the ear via an insert phone while the implant's most apical available electrode records the cochlear response. As the array advances, the response is sampled repeatedly, generating a running trace of amplitude against insertion depth/time. Because the recording electrode is intracochlear and moving with the array, the signal is large and exquisitely sensitive to the local cochlear environment. Modern systems acquire this response in real time directly through the implant, no separate recording hardware in the surgical field.[2015, 2017]
ECochG is also informative pre-operatively. Recording from the round window just before insertion captures the residual cochlear response, and the size of that response carries prognostic information — larger preoperative responses are associated with better post-operative word-recognition outcomes, reflecting surviving cochlear and neural substrate.[2014]
TCThe signals
The acoustically-evoked intracochlear response is a composite of the same components seen in electrocochleography:
| Component | Generator | Intraoperative relevance |
|---|---|---|
| CM — cochlear microphonic | Outer hair cells (receptor current) | The dominant monitored signal; follows the stimulus waveform and is highly sensitive to apical hair-cell health. |
| ANN — auditory nerve neurophonic | Phase-locked auditory nerve firing | Neural contribution riding with the CM; separable by polarity analysis. |
| SP — summating potential | Hair-cell / dendritic DC shift | A DC component through the stimulus; part of the composite response. |
| CAP — compound action potential | Synchronous auditory nerve onset | Onset neural response; the acoustic analogue of the ECAP. |
In practice the cochlear microphonic amplitude at the stimulus frequency is the workhorse monitored signal during insertion, because it is large and tracks apical cochlear health in real time.
CInsertion patterns
Plotting CM amplitude against insertion progress yields a small number of characteristic trajectories. Step the array into the cochlear spiral below and watch the trace track alongside it; switch patterns to compare the response types and see when the ≥30% drop event fires.[2019, 2022]
- Growth— CM amplitude rises smoothly as the array advances apically, with no drop > 30% from the running maximum. The cochlea is undisturbed; this pattern carries the smallest postoperative hearing loss.
- Fluctuating — amplitude oscillates with transient drops and partial recoveries. Amplitude alone is hard to interpret here, which is what multifeature/automated analysis aims to resolve.[2026]
- Total loss — the alarm pattern: an abrupt fall without recovery. A ≥30% drop from the running maximum triggers the trial intervention — pause, withdraw ~2 mm, reposition, resume — suggesting basilar-membrane contact, translocation, or trauma.
Single-amplitude drop thresholds are a useful first alarm but are imperfect: not every drop reflects trauma, and not all trauma produces a clean drop. Newer work moves toward multi-feature analysis — combining amplitude, phase, and the different signal components, and integrating with impedance and electrode-position data — to improve specificity. The amplitude trace is the established signal; richer analysis is the direction of travel.
CEvidence & how to respond
Intraoperative ECochG is one of the better-evidenced CI objective measures: intracochlear ECochG correlates with hearing-preservation outcomes, and ECochG-informed surgical technique is associated with better residual-hearing preservation than insertion without monitoring. The strongest evidence is a randomised clinical trial in which ECochG-triggered intervention — acting on a ≥30% amplitude drop — significantly improved residual-hearing preservation versus standard insertion; the headline for the CI surgeon is that a real-time cochlear signal makes atraumatic insertion an actively guided process rather than a blind one.[2022, 2016, 2015, 2015]
The appropriate response to an alarm-pattern drop is surgical, not electronic: pause insertion, consider partial withdrawal and re-advance, reassess trajectory and speed. The value of the measure is entirely in that it arrives while the array is still moving.
What is the appropriate intraoperative response?
Intraoperative ECochG during CI surgery primarily aims to:
The workhorse signal monitored during insertion is the:
An abrupt ~30–40% drop in CM amplitude during insertion should prompt: