4Listening to the Nerve: Intraoperative ECAP/NRT
Within seconds of seating the array, the implant can stimulate one electrode and use its neighbours to record the auditory nerve's own answer. The electrically-evoked compound action potential is the surgeon's first proof that the nerve is alive, listening, and reachable across the array.
FWhat the ECAP is, and why we record it in theatre
The electrically-evoked compound action potential (ECAP) is the synchronous firing of thousands of spiral ganglion neurons in response to a single biphasic pulse, captured by the implant itself on a non-stimulating electrode — no scalp electrodes, no behavioural response needed. Each manufacturer brands the same measurement differently: NRT (Neural Response Telemetry, Cochlear), NRI (Neural Response Imaging, Advanced Bionics) and ART (Auditory Response Telemetry, MED-EL). The underlying physiology — forward-masking or alternating-polarity artifact subtraction to unmask the ~0.2-0.4 ms N1-P2 wave — is covered in the Objective Measures chapter; here the point is its INTRAOPERATIVE use under anaesthesia. Recorded with the patient asleep, the ECAP is unaffected by attention or cooperation, which is exactly why it is so valuable in the very young child who can give no behavioural feedback.[2020][2019]
CA starting MAP and a per-electrode threshold profile
Sweeping the ECAP threshold across the array yields a per-electrode threshold PROFILE — the shape (which electrodes need more current) that can seed the very first MAP before any behavioural levels exist. ECAP thresholds correlate with, but do not equal, behavioural T and C levels; they typically sit between threshold and comfort and predict the SHAPE of the map better than absolute levels. Intraoperative ECAP thresholds are systematically higher and shift over the first weeks (one series found intraoperative thresholds ~15 current-level units above later values), so they orient the first fitting rather than fix it. In prelingual children, NRT/ECAP thresholds significantly predict behavioural T and C levels and are a recognised objective scaffold for the first map when combined with behavioural observation.[2019][2000][2020]
TThe single biphasic pulse at a slow rate
Intraoperative ECAP uses a single biphasic probe pulse delivered at a slow rate (tens of Hz), not the rapid continuous stimulation of everyday listening — the goal is a clean isolated nerve response, not loudness. Because the probe is slow and brief, the measure is comfortable physiologically and does not require the high charge of a behavioural C-level; it tests the interface, not the percept. Recording electrode choice (typically an adjacent intracochlear contact), gain and the masker-probe or polarity-subtraction scheme determine artifact rejection and the visible N1-P2 morphology.[2020][2000]
CRecordable in ~95% — and the value of when it is not
A measurable ECAP is obtained in roughly 95% of recipients; a present response confirms the device, the array-nerve interface and the nerve's responsiveness in one objective step. Failures cluster in cochlear nerve deficiency (too few neurons to summate) and in ossified or malformed cochleae — exactly the ears where eABR (next module) adds the most. A flat, absent or sharply reduced intraoperative ECAP is an EARLY WARNING: it should prompt a check of electrode position and impedances, consideration of cochlear nerve status, and tempered counselling — not necessarily abandonment. Because it is non-behavioural, the intraoperative ECAP is the first objective datapoint a family receives on the day of surgery, before activation.[2015][2020][2019]
Which intraoperative measure most directly confirms that the auditory nerve responds to electrical stimulation through the implant?
In approximately what proportion of recipients is an intraoperative ECAP recordable?
A flat or absent intraoperative ECAP is MOST associated with which conditions?
How should intraoperative ECAP thresholds be used for the first MAP?