Otoacoustic Emissions: Quick Reference

OAEs were hypothesized in the 1940s from cochlear nonlinearity models (Thomas Gold, 1948).
Measurable only after sensitive low-noise microphones were developed in the late 1970s.
David Kemp discovered otoacoustic emissions in 1978.

Sounds generated in the inner ear (cochlea) that travel through the middle ear into the ear canal and are measurable by a microphone.
They reach the ear canal because cochlear activity causes eardrum vibration, producing audible air in the ear canal.

In controlled studies with cochlear outer hair cell (OHC) lesions, OAEs disappear.
OAEs reflect OHC health: healthy OHCs → robust OAEs; damaged OHCs → reduced/absent OAEs.
Lesions outside the cochlear OHCs do not abolish OAEs if OHCs remain intact (3 rows of outer hair cells).

Spontaneous OAEs are highly prevalent in newborns, indicating that the cochlear function is generally intact.
- These emissions can serve as early indicators for hearing screening and can help identify newborns who are at risk for hearing loss.
DPOAEs are present in nearly all normal-hearing newborns and children.
In newborns/children, OAEs are higher in level than in adults: approximate neonatal levels around $20\ \mathrm{dB\ SPL}$ vs adults around $10\ \mathrm{dB\ SPL}$ .
DPOAE levels peak midway through the first year of life and gradually decrease with age but stay higher than adult levels during childhood.

Normal OAE amplitudes indicate that the corresponding portion of the cochlea’s outer hair cells is intact.
Normal OAEs imply normal middle-ear function for the tested frequency range.
Decreased or absent OAEs indicate cochlear dysfunction involving outer hair cells.

Proper probe fit is essential to reduce room noise and maintain stimulus level, preventing reductions in OAE level.

Good: probe tip appropriately sized, securely seated, deep enough in the ear canal, snug seal, stable.
Bad: loose fitting, inadequate seal, pressure on canal walls.
Recording time can be reduced by not testing low frequencies.

Common causes of unsuccessful recordings:
- Improper probe fit
- Fluid/debris in the ear (conductive elements)
Infants: test during sleep; entertain to keep calm; keep probe cable out of reach; calm environment to minimize background noise.

Simple non-invasive test to determine cochlear status, specifically hair-cell function.
Applications:
- Newborn hearing screening
- School screenings
- Site-of-lesion testing (cochlear vs retrocochlear)
- Monitoring ototoxic drugs’ effect on cochlear function
- Partial estimation of hearing sensitivity within a limited range
- Cross-check principle

In difficult-to-test children, OAEs can indicate the need for further intervention.
Normal OAEs with normal tympanograms and acoustic reflexes generally rule out peripheral hearing loss.
Absent or abnormal OAEs with a normal tympanogram indicate the need for further evaluation.

Distortion Product OAEs arise from two simultaneously presented primary tones, f1 and f2, with different frequencies.
The most robust DP occurs at 2f1-f2.
Usable frequency range: 500Hz to 8000Hz

Spontaneous OAEs (SOAEs)
Evoked OAEs:
- Transient Evoked OAEs (TEOAEs)
- Distortion Product OAEs (DPOAEs)
Sustained Frequency OAEs (SFOAEs) [mentioned as a type in the overview]

Used clinically to screen infant hearing, validate behavioral/electrophysiologic thresholds, and assess cochlear function relative to lesion site.
Evoked by very short stimuli (e.g., click or tone burst).
Almost always absent when hearing loss exceeds $30\ \mathrm{dB\ HL}$ .

OAEs can be categorized as: present and within normal region; present but abnormal; absent.
Diagnostic notes often reference: signal-to-noise ratio thresholds and band-specific criteria.

Stimulus: a click; Response is analyzed across frequency bands.
Key interpretation criteria include:
- Stimulus level around $83.5\ \mathrm{dB\ peSPL}$ (example from data)
- SNR per frequency band: typically require SNR > $3\ \mathrm{dB}$ in multiple bands for a pass
- Overall TEOAE pass/fail decision
- Many time samples may be rejected in a noisy test; note the effect on validity
Neonatal screening interpretation typically considers:
- PASS if robust OAE response across bands; REJ if weak or absent

Robust response across a broad frequency range supports normal cochlear function for those frequencies.
Weak or absent responses in certain bands suggest frequency-specific cochlear dysfunction.

DP response strength and noise floor per band determine normal vs abnormal status.
A normal DP across the tested bands supports normal cochlear status within those frequencies.
Absent or markedly reduced DPOAEs indicate cochlear dysfunction at corresponding frequencies.

If feasible, record both TE OAE and DPOAE (use diagnostic code to reflect both exams).
DPOAEs and TE OAEs provide different information and may target different cochlear mechanisms; together offer a more comprehensive cochlear health assessment.
If DPOAEs are not detectable at standard stimulus levels of $65-55\ \mathrm{dB\ SPL}$ , try higher levels such as $75-75\ \mathrm{dB\ SPL}$ to reveal non-profound hearing loss when present.
Sources: Audiology Today article on updated approaches; related literature on reflection vs distortion OAEs.

OAEs are generated in the cochlea but must pass through the middle ear to the ear canal.
Otitis media or negative middle-ear pressure can disrupt transmission, altering OAEs.
Severe otitis media can affect tympanic membrane mobility; OAEs may be absent.
Negative middle-ear pressure usually reduces OAE levels but may not eliminate them unless there is mild SNHL.
Patent PE tubes or TM perforations can cause variable OAEs, though measurable OAEs may still be obtained in some infants.

OAEs provide important information about the integrity of outer hair cells and assist in differential diagnosis of auditory neuropathy spectrum disorder and sensorineural hearing loss.
(Gorga et al., 2000)