ECochG Atlas · Module 04

4Normal waves

What a normal click-evoked ECochG looks like, how the latency-intensity function (LIF) separates conductive, cochlear, and retrocochlear patterns, and how click rate affects the AP. This is the foundation for everything in the disease modules — without a clear sense of normal, the diagnostic ratios that follow have no anchor.

FReading a normal trace

A normal click-evoked ECochG recorded from a tympanic electrode at 90 dB nHL contains three components in sequence: the cochlear microphonic (CM) riding the stimulus, the summating potential (SP) as a small positive shoulder, and the action potential (AP) as a large downward N1 peak around 1.5–2.0 ms.[1981] The SP/AP amplitude ratio in normal subjects sits well below the Ferraro 0.40 cutoff used for hydrops diagnosis — typically 0.15–0.25 from tiptrode and TM recordings, slightly lower from transtympanic.[2017]

The N1 latency depends mostly on stimulus intensity, with modest contributions from rate and electrode-site geometry. Wave I of the auditory brainstem response is the same neural event as N1, recorded further from the generator — Module 13 covers the overlap and the integrated test battery.

TLatency-intensity function — interactive

The latency-intensity function (LIF) plots AP latency against stimulus intensity and is the principal diagnostic tool for separating conductive, cochlear, and retrocochlear patterns of hearing loss when ECochG is used alongside ABR. Toggle the four diagnostic patterns below to see how each departs from normal.

Normal. Reference latency-intensity function; shortens ~0.030 ms/dB.

Fig 4.1Latency-intensity functions for the four canonical diagnostic patterns, overlaid on the Picton ±2 SD normative band.[1981] Conductive loss produces a parallel rightward shift (latency at every intensity equivalent to a lower normal-ear intensity). Cochlear loss with recruitment produces a steeper slope near threshold that rolls over to normal latency at high intensities. Retrocochlear pathology (schwannoma is the canonical example) keeps the latency prolonged even at the highest intensities, since the conduction delay is independent of cochlear input.

TRate effects on the AP

Click rate has predictable effects on the AP: amplitude falls and latency lengthens as rate increases above the clinical default of 11 Hz. The mechanism is auditory-nerve fibre refractoriness — the same fibres can't fire as synchronously when packed tighter together in time. Yagi and Burkard (2016) characterised this systematically with continuous loop averaging deconvolution (CLAD), which allows averaging at rates up to 300 Hz without overlap artifact.[2016]

Click rate11 Hz

AP latency1.60 ms

AP amplitude100% of baseline

Fig 4.2Click-rate effect on the AP. The dashed reference trace shows the clinical baseline at 11 Hz. As rate increases, amplitude falls (towards ~30% of baseline at 300 Hz) and latency lengthens (~0.15 ms per doubling of rate above 11 Hz). The slope of the rate-amplitude function is itself diagnostic in some pathologies — auditory neuropathy spectrum disorder shows disproportionate amplitude loss at high rates compared with normal cochleae, reflecting the de-synchronisation that defines the condition.

FBaseline checklist

Before interpreting any clinical ECochG, verify:

Check	Normal range	What a deviation means
AP latency at 90 dB nHL	1.5–2.0 ms	Prolongation: retrocochlear or severe cochlear loss; shortening: implausible — recheck electrode and stimulus.
AP amplitude (tiptrode)	0.5–2.0 µV at 90 dB nHL	Reduced AP: severe cochlear loss, ANSD (with intact CM), or recording issue.
SP/AP amplitude ratio	≤ 0.40 (Ferraro), ≤ 0.34 (Adams)	Elevated: hydrops (Module 5), third-window pathology (Module 6).
CM presence	Visible during stimulus; inverts with polarity	Preserved CM with absent AP is the classical ANSD signature (Module 7).
Interaural latency difference	< 0.20 ms	Larger differences raise concern for unilateral retrocochlear pathology.

A note on the "normal" concept

ECochG normative data are electrode-site-dependent — a 1.5 µV AP from a tiptrode is robust; the same value from a transtympanic electrode is suspiciously small. Each lab should maintain its own normative range against its own equipment, electrode type, and patient population. Published cutoffs (Ferraro 0.40, Adams 0.34, Liberman 0.35–0.60) refer to ratios, which are largely electrode-independent — but absolute amplitudes are not.

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