6Electrically-evoked ABR (eABR)
The ECAP confirms the auditory nerve fires near the electrode. But firing at the cochlea is not the same as a signal arriving at the brainstem — and for some of the hardest CI decisions, that brainstem question is the whole point. The electrically-evoked ABR follows the volley further up the pathway, to wave eV in the rostral brainstem. It is slower to record and harder to read than the ECAP, but it answers questions the ECAP cannot: is there a functioning auditory nerve at all, and will electrical hearing reach the brain?
FWhat the eABR is
The electrically-evoked auditory brainstem response (eABR) is the brainstem auditory response evoked by electrical stimulation through the implant, recorded from scalp electrodes just as an acoustic ABR is. Where the ECAP samples the distal nerve within the first half-millisecond, the eABR samples the propagation of that volley through the auditory nerve and brainstem over the first few milliseconds.[2002, 2004]
The practical consequence: the eABR tests a longer stretch of the pathway. A present eABR confirms not just that the nerve fires locally, but that the signal travels centrally to the brainstem — exactly the question that matters when nerve integrity is in doubt.
TRecording technique
The eABR uses scalp electrodes in a montage similar to the acoustic ABR (vertex to mastoid/earlobe), with the cochlear implant providing the stimulus instead of an insert earphone. The dominant practical challenge is, again, stimulus artifact: the electrical pulse produces a large transient that can obscure the early waves. Mitigations include alternating polarity, careful electrode montage, and — because eV is relatively late — focusing on the robust later wave rather than the artifact-contaminated early ones.[2004]
- Stimulus. Biphasic current pulses delivered on chosen intracochlear electrodes, level varied to find a threshold.
- Averaging. Many sweeps averaged to extract the scalp-recorded response, as with acoustic ABR.
- State. Best recorded with the patient still or asleep/sedated, since myogenic noise degrades the small response — one reason it is often done under the same anaesthetic as surgery in children.
TWave eV & morphology
The clinical readout of the eABR is wave eV — the electrical analogue of acoustic ABR wave V, generated in the rostral brainstem (lateral lemniscus / inferior colliculus region). It is the most robust and reliably identified peak; the earlier electrical waves (the analogues of waves I–III) are often lost under stimulus artifact.[2002]
The presence of a replicable wave eV, and the lowest level at which it persists (the eABR threshold), are the usual outputs. Waveform morphology and eV latency add information about the integrity and synchrony of central transmission.
TCWhen to use it — the questions only eABR answers
The eABR is not a routine fitting tool like the ECAP; it is reserved for situations where the question is auditory nerve and brainstem integrity:
| Scenario | What the eABR contributes |
|---|---|
| Auditory neuropathy (ANSD) | Whether electrical stimulation can produce synchronous central activity when acoustic responses are dys-synchronous — supporting CI candidacy and predicting benefit. |
| Cochlear nerve deficiency / hypoplasia | Whether a thin or questionable nerve on MRI can actually carry an electrical signal centrally — informing the CI-vs-ABI decision. |
| ABI candidacy / outcome workup | Where a CI fails to produce a central response, an absent eABR supports moving to an auditory brainstem implant. |
| Difficult / non-responsive recipients | Objective confirmation that the pathway conducts centrally when behavioural and ECAP findings are ambiguous. |
In children especially, the eABR can be recorded under the surgical anaesthetic, giving an early integrity check at the moment of implantation; serial recordings then document activity-dependent maturation of the brainstem pathways once the device is in use.[2004, 2003]
The candidacy logic is sharpest at the extreme: a child with no detectable response on acoustic ABR is not thereby a poor implant candidate — many such children have an auditory nerve that responds well to electrical stimulation, and cochlear implantation is frequently indicated. An absent acoustic ABR is a reason to ask the electrical question, not to abandon the implant.[2015]
CInterpretation & limits
A clear, replicable wave eV is strong evidence of a functioning nerve-to-brainstem pathway. But interpretation demands care: a present eABR is reassuring, whereas an absent eABR is harder to act on — it may reflect a genuinely non-conducting pathway, but also severe artifact contamination, sub-threshold stimulation, technical failure, or a very dys-synchronous but not absent nerve. As elsewhere, the eABR informs a clinical judgement built from imaging, behavioural progress, and the rest of the objective battery — it is not a solitary verdict.
Think of the two as nested. The ECAP asks: does the nerve fire at the cochlea? The eABR asks: does that firing reach the brainstem? You generally only need the eABR when the ECAP answer is ambiguous or when the central question itself is the issue — ANSD, nerve hypoplasia, ABI candidacy. Most routine recipients never need an eABR; the difficult ones may hinge on it.
How does the eABR result inform the decision?
Compared with the ECAP, the eABR primarily adds information about:
The most robust and clinically used peak of the eABR is:
In which scenario is the eABR most valuable?