Stimulus & Recording Technique
A reproducible ABR depends on small choices made before the patient walks in: stimulus, electrodes, filters, rate, polarity. Most failed recordings are technical, not biological.
Stimulus
The 100 µs rectangular click is the diagnostic workhorse. Its broad-band spectrum excites a wide cochlear region synchronously, producing the largest, most reproducible waveform[3]. Tone bursts are used when frequency specificity matters — typically at 500, 1000, 2000, and 4000 Hz with Blackman envelopes (2-1-2 or 2-2-2 cycles) — for threshold estimation. Chirp stimuli compensate for the cochlear travelling-wave delay and yield larger wave V amplitudes than the click[8].
Intensity reference
Stimulus intensity is reported in dB nHL — normalised hearing level, referenced to the behavioural threshold of normal-hearing listeners for the same stimulus. 0 dB nHL ≈ 30 dB peSPL for a click; the offset is stimulus- and equipment-dependent. Each laboratory establishes its own dB eHL correction (estimated behavioural threshold from ABR threshold).
Polarity
Alternating polarity is the default for diagnostic ABR because it cancels the stimulus artefact and the cochlear microphonic. When ANSD is suspected, the alternating run is exactly wrong — it cancels the CM that confirms the diagnosis[9]. Run rarefaction and condensation separately and look for a CM that inverts cleanly with polarity.
Rate
11.1/s is conventional — odd to avoid 50/60 Hz line-noise harmonic entrainment, and slow enough to preserve interpeak intervals[10]. Faster rates (30–90/s) shorten recording time but degrade IPLs; they are used in threshold searching, not in diagnostic interpretation. Stressing the response with high rates can unmask demyelination.
Electrode montage
Two-channel ipsilateral montage: Cz non-inverting, A1 or A2 inverting (mastoid or earlobe, same side as the stimulus ear), Fpz ground. Impedance below 5 kΩ at every electrode is non-negotiable. The first move when traces look noisy is to re-prep the skin and re-check impedance.
Filters and averaging
Band-pass 100–3000 Hz is standard for click ABR. Tone-burst ABR extends the low cut to 30 Hz to capture lower-frequency energy. Typical averaged sweep counts are 1500–2000 per run, replicated to confirm reproducibility. Artefact rejection at ±25 µV catches most myogenic contamination.
Insert earphones
Insert earphones are preferred to supra-aural cans for two reasons: interaural attenuation is higher (~70 dB vs ~40 dB), and the ~0.9 ms acoustic delay in the foam tube separates stimulus artefact from wave I — recovering wave I that supra-aural recordings often lose.
Patient state
ABR is robust to sleep and most general anaesthetic agents, with small dose-dependent latency shifts[11]. Adults are tested awake but relaxed; infants and young children are tested during natural sleep, with sedation reserved for failures of the natural attempt.