CI Atlas · Audiological Evaluation · Module 08

8Otoacoustic emissions

The cochlea does not merely receive sound — it makes it. The outer hair cells that sharpen and amplify the travelling wave leak a little energy back out, and a sensitive microphone in the ear canal can record those faint echoes. Their presence is a near-instant, cooperation-free sign that the cochlear amplifier is working and the middle ear is clear, which is why otoacoustic emissions became the workhorse of newborn screening. But they answer a yes/no question, not a how-much one — they vanish once a loss passes a modest depth — and their greatest diagnostic value comes from a disagreement: emissions present while the brainstem response is absent, the unmistakable signature of auditory neuropathy. This module covers what OAEs measure, what they cannot, and the cross-check that makes them indispensable.

TEchoes of the cochlear amplifier

OAEs are low-level sounds generated by outer-hair-cell motility — the cochlear amplifier (Chapter 2) — and recorded in the ear canal. Their presence implies functioning outer hair cells and a clear middle ear; their absence is non-specific.

CTEOAEs, DPOAEs & the DP-gram

Two clinical classes: transient-evoked OAEs (to clicks) and distortion-product OAEs (to two tones, f1/f2). DPOAEs give a frequency-by-frequency DP-gram of outer-hair-cell function — a map of where the amplifier still works.

CA screen, not a threshold

OAEs are essentially absent once thresholds exceed ~30–35 dB HL and of normal amplitude only when hearing is better than ~20 dB — so they give a pass/refer screen but no threshold estimate. They are the fast, cooperation-free workhorse of newborn screening, but a middle-ear effusion abolishes them and causes a false refer, and chronic hearing-aid use can secondarily abolish previously present emissions. In sloping losses, emissions preserved in the lows may steer the surgeon toward a shorter array to protect the apical amplifier.[2020]

CThe OAE–ABR cross-check

The signature finding: present OAEs (or cochlear microphonic) with an absent or grossly abnormal ABR is auditory neuropathy spectrum disorder — the outer hair cells work while neural synchrony fails (Starr et al.). It is the clearest example of disagreement being the diagnosis, and it matters because these ears, including OTOF cases, often do well with an implant that bypasses the failed synapse.[1996]

Two independent measures of the same pathway make a powerful cross-check. OAEs (and the cochlear microphonic) read outer-hair-cell function; the ABR reads neural synchrony. When OAEs are present but the ABR is absent, the cochlea's amplifier works while the nerve cannot fire in time — auditory neuropathy spectrum disorder (Starr et al.). It is the classic example of disagreement being the diagnosis, and it matters because these ears, including OTOF cases, often do well with an implant that bypasses the failed synapse. Schematic.

Otoacoustic emissions are a by-product of the cochlear amplifier: healthy outer hair cells actively boost the travelling wave by some 40–60 dB, sharpening tuning, and a fraction of that energy is emitted back out the ear as a measurable echo. So present emissions are a quick objective sign of working OHCs (cochlear loss < ~30–40 dB), and absent emissions mean the amplifier is gone. The diagnostic power is in the cross-check: present OAEs with absent neural responses is the fingerprint of auditory neuropathy — the cochlea works, the signal is not relayed. Schematic.

Case 10.8 · The newborn who passed, then failed

A newborn passes the OAE screen but later shows profound deafness on diagnostic ABR, with a cochlear microphonic that inverts with stimulus polarity.

What is the diagnosis?

Self-assessment — Module 82 questions

Question 1 · Foundation

What do otoacoustic emissions tell you?

Question 2 · Clinician

What does present OAE with an absent ABR indicate?

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