CI Atlas · Audiological Evaluation · Module 09

9ABR — the auditory brainstem response

When a click reaches the ear, a volley of synchronised activity sweeps up the auditory nerve and through the brainstem, and scalp electrodes can catch its wake — a sequence of waves unfolding within ten milliseconds, each a signpost of a different station on the pathway. The auditory brainstem response is tiny, buried in noise, and recoverable only by averaging thousands of sweeps, but it asks for no cooperation at all, which makes it the foundation of testing in babies and the arbiter when behavioural results are in doubt. It can estimate threshold, point to a tumour by the timing of its fifth wave, and — through the curious behaviour of the cochlear microphonic — unmask auditory neuropathy. This module covers what the ABR is, where its waves come from, and how it is read.

TA far-field neural response

The ABR is a far-field potential occurring within ~10 ms of a transient (click or tone burst), under 0.5 µV in amplitude, so it requires averaging thousands of sweeps, recorded between a vertex and an ipsilateral mastoid/earlobe electrode.[2006]

CThe generators of waves I–V

The waves map the ascending pathway: I distal auditory nerve, II proximal nerve, III cochlear nucleus, IV superior olivary complex, V lateral lemniscus terminating in the contralateral inferior colliculus. Wave V is the most robust and the one tracked to threshold.

CThe latency–intensity function

As intensity falls, amplitudes shrink and latencies lengthen — but unevenly: the early waves fade fastest, so near threshold practically only wave V survives (its latency rising from ~5.8 ms at 60 dB SL toward ~8.1 ms near threshold). That behaviour is what threshold-estimation ABR exploits.

CThreshold & neurodiagnostic ABR

Threshold ABR uses tone bursts for frequency-specific thresholds (click ABR mainly reflects the 2–4 kHz region). Neurodiagnostic ABR was the key functional test for vestibular schwannoma — a prolonged interaural wave-V latency or I–V interval — though contralateral wave-V generation complicates simple localisation, so it is read alongside imaging.

CStimulus choices & the cochlear microphonic

Stimulus and recording choices change the waveform — nHL vs peak-equivalent SPL, polarity, repetition rate, and filtering. Crucially, click polarity reveals the cochlear microphonic: a potential that inverts with polarity and persists when neural waves are absent is the electrophysiologic hallmark of auditory neuropathy. The electrically evoked ABR (eABR) through an implant is covered in the Objective Measures chapter; the next module touches pre-operative promontory eABR.

The ABR stimulus is chosen for the question. A click is broadband with excellent neural synchrony — ideal for a quick neurodiagnostic or newborn screen, but it gives no frequency information. A tone-burst trades some synchrony for frequency specificity, letting the audiologist estimate thresholds at 500–4000 Hz and build the audiogram needed to fit an infant. A CE-chirp offsets the cochlear travelling-wave delay so the whole partition fires together, yielding a larger response and faster threshold search. Illustrative; schematic.

Case 10.9 · Only wave V remains

As the click level is lowered during a threshold ABR, the early waves disappear while one later peak persists down toward threshold.

Which wave is it, and why does it matter?

Self-assessment — Module 92 questions

Question 1 · Trainee

Which ABR wave persists nearest threshold and anchors threshold estimation?

Question 2 · Clinician

What is the electrophysiologic hallmark of auditory neuropathy on click ABR?

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