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.
One latency feature is informative in itself: because electrical stimulation bypasses the cochlear traveling wave and the hair-cell–nerve synapse, the electrical waves III and V occur about 1–1.5 ms earlier than their acoustic-ABR counterparts. And unlike the acoustic ABR, eABR latency changes little as stimulus level rises — it is mainly amplitude that grows.[1990, 1994]
Biphasic current pulses at ~10–80 Hz, pulse width 25–400 µs, alternating polarity; 500–2000 sweeps averaged. Montage: non-inverting at the vertex/high forehead (Cz), inverting at the contralateral mastoid/earlobe. A ~10 ms window with the first ~1.0 ms blocked to reject the stimulus artifact, bandpass ~100–3000 Hz, artifact rejection around ±15 µV.[1994]
The eABR is a small, slow-to-acquire response: wave eV peaks at roughly 1–2 µV at high levels and only ~0.25 µV near threshold, so 1000–2000 sweeps must be averaged (against 50–100 for the ECAP). It is recordable in about 71–95% of recipients, and at high stimulation levels forward masking has essentially recovered by 4–6 ms (Abbas and Brown). Figures as reported by Hughes (2013).[2013]
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]
The same electrical question can be asked beforea device is placed, by stimulating the promontory or round window through a transtympanic electrode (a large-surface “golf-club” tip gives a more consistent round-window contact) and recording the eABR. A robust transtympanic eABR predicts an outcome at least as good as a cochlear sensory loss, whereas an absent or abnormal one — together with the round-window electrocochleography recorded alongside it — flags poorer prospects, most usefully in auditory neuropathy and abnormal cochleovestibular anatomy. It is not used to choose the ear routinely, but to gauge likely benefit in the hard cases.[2007]
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.
Its everyday role has shrunk since intracochlear telemetry and the ECAP arrived: the ECAP is faster, needs no separate evoked-potential system or sedation, and answers most nerve-response questions. The eABR is now mainly reserved for implants without telemetry, or when the ECAP cannot be recorded (for example a heavily ossified cochlea) — and for the central-integrity questions above, where its reach beyond the nerve to the brainstem is exactly the point. It is a poor predictor of T and upper levels and is not used to set them.[2020]
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.
CBinaural interaction — matching bilateral implants
With two implants a further objective question arises: are the two ears stimulating matching places in the cochlea? The binaural interaction component (BIC) of the eABR speaks to it. Recorded as the difference between the summed monaural responses and the true binaural response — BIC = (left + right) − binaural — it reflects genuine central binaural processing rather than two independent ears, and it is largest when the interaural electrode pair is well matched. That makes it an objective handle for matching electrodes across the two devices.[2013]
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?