Threshold ABR
Frequency-specific objective hearing assessment — the standard test for any patient whose behavioural responses cannot be trusted.
What threshold ABR is
The lowest stimulus intensity at which a reproducible wave V is obtained, expressed in dB nHL, with a laboratory-specific correction to estimate behavioural threshold in dB eHL[16]. Tone-burst stimuli at 500, 1000, 2000, and 4000 Hz give a frequency-specific picture; click ABR alone summarises a broad mid- to high-frequency band.
Search strategy
Start at a clearly suprathreshold intensity — typically 80 dB nHL — and drop in 20 dB steps until wave V disappears. Then bracket back up in 10 dB steps. Replicate each level. The lowest intensity at which wave V is reliably present in two independent runs is the threshold[17].
| Stimulus | Adult normal threshold |
|---|---|
| Click | ≤ 20 dB nHL |
| 500 Hz tone burst | ≤ 30 dB nHL |
| 1000 Hz tone burst | ≤ 25 dB nHL |
| 2000 Hz tone burst | ≤ 25 dB nHL |
| 4000 Hz tone burst | ≤ 25 dB nHL |
The L–I function as a pattern
Plotting wave V latency against intensity gives the latency–intensity function. A normal curve has a slope of 0.03 ms/dB to threshold (~20 dB nHL). A parallel rightward shift means cochlear loss — every intensity needs more dB to reach the same latency. A steeper slope, with threshold not greatly elevated, suggests retrocochlear pathology[3].
Air- vs bone-conducted threshold ABR
Air-conducted threshold ABR alone cannot distinguish conductive from sensorineural loss. Bone-conducted threshold ABR is the counter-test: in pure conductive loss the BC threshold is normal while AC threshold is elevated, and the difference quantifies the air–bone gap[18]. Maximum output of a bone oscillator is limited (~50–55 dB nHL); large losses cannot be distinguished from mixed losses on BC alone.
Chirp stimuli
Chirps compensate the cochlear travelling-wave delay so basal and apical contributions arrive synchronously at the eighth nerve. Wave V is larger, threshold estimation is faster — useful in time-limited paediatric protocols[8].
Functional (non-organic) hearing loss
Pseudohypacusis — discordance between behavioural threshold and objective measures — is uncommon but important in medico-legal and paediatric contexts. ABR threshold consistent with normal cochlear function while behavioural threshold is elevated is the textbook signature[19]. Causes range from voluntary malingering to unrecognised somatic-symptom presentations; the test choice is forensic, the management is sensitive.
Limits
Tone-burst ABR underestimates true behavioural threshold by 5–15 dB in the mid frequencies, more at 500 Hz where the response is inherently smaller. The dB nHL → dB eHL conversion is laboratory-specific and depends on transducer, polarity, and protocol.