Otoacoustic emissions (OAE) test: how it detects outer hair cell function

What otoacoustic emissions are

When a healthy cochlea processes sound, the outer hair cells (OHCs) in the inner ear do not just receive signals; they also produce tiny sounds of their own as a byproduct of their mechanical activity. These sounds, called otoacoustic emissions (OAEs), travel back through the middle ear and can be detected in the ear canal with a sensitive microphone.

The discovery that the ear emits sound earned British physicist David Kemp the recognition of a field-changing finding in 1978. The practical implication is straightforward: if you can measure OAEs, you have direct evidence that at least some outer hair cells are functioning.

Why outer hair cells matter

The cochlea contains two main types of sensory cell: inner hair cells and outer hair cells. Inner hair cells are the primary signal transducers, converting mechanical vibration into electrical nerve signals sent to the brain. Outer hair cells amplify and sharpen the mechanical motion of the basilar membrane, adding roughly 40 to 60 decibels of gain and increasing frequency selectivity.

Outer hair cells are far more vulnerable than inner hair cells. Noise exposure, ototoxic medications, and age-related changes preferentially damage OHCs, often before any change appears on a standard audiogram. The NIDCD notes that standard pure-tone audiometry only detects hearing loss once a threshold shift of 25 dB HL or more has occurred. OAE testing can reveal OHC dysfunction before that threshold is crossed, which is why it is sometimes called a pre-threshold screening tool.

Two main types of OAE test

Transient-evoked OAEs (TEOAEs)

TEOAEs are triggered by brief broadband clicks or tone bursts. The cochlea responds across a wide frequency range, and the resulting emissions are recorded over the 5 to 20 milliseconds following each click. TEOAEs are present in virtually all ears with hearing thresholds better than about 30 dB HL, making them sensitive screens for significant OHC dysfunction. They are widely used in newborn hearing screening programs.

Distortion product OAEs (DPOAEs)

DPOAEs are generated by presenting two pure tones at slightly different frequencies (F1 and F2) simultaneously. The nonlinear mechanics of healthy outer hair cells produce a third tone at a mathematically predictable frequency, typically 2F1 minus F2. By varying F1 and F2 across frequency pairs, clinicians can construct a frequency-specific picture of OHC function across the cochlea. DPOAEs are particularly useful for ototoxicity monitoring, where tracking OHC status at specific frequencies over time matters more than a simple pass-fail result.

How the test is performed

The audiologist places a small probe snugly into the ear canal. The probe houses a speaker (or two speakers for DPOAEs) and a very sensitive microphone. The room must be quiet because background noise can mask the low-amplitude cochlear emissions. The patient sits still; no button press or verbal response is required.

Emissions are recorded and averaged over multiple stimulus repetitions to separate them from background noise. Software then compares the emission amplitude to the noise floor. A response that rises above the noise floor by a set threshold (commonly 6 dB or more) is considered present.

The whole test usually takes five to ten minutes per ear, though DPOAE frequency sweeps can extend that somewhat.

OAEs in newborn hearing screening

OAE testing became the cornerstone of universal newborn hearing screening programs beginning in the 1990s, following NIDCD-supported research showing that unidentified hearing loss in infancy delays language development. The CDC Early Hearing Detection and Intervention (EHDI) program recommends that all newborns be screened by one month of age, diagnosed by three months if a concern is found, and enrolled in early intervention by six months.

In most hospitals, the bedside OAE screen takes three to five minutes and produces an automated pass or refer result. A refer result does not confirm hearing loss; it means a follow-up evaluation is needed. Approximately one to two percent of newborns require follow-up, and most turn out to have normal hearing on comprehensive testing.

OAEs and the limits of the test

OAE testing has important blind spots. The test assesses outer hair cell function only. It does not evaluate inner hair cells, the auditory nerve, or any part of the central auditory pathway. A condition called auditory neuropathy spectrum disorder (ANSD) illustrates this: OAEs can be present and even robust while the auditory nerve fails to transmit signals reliably, producing significant functional hearing difficulty that OAE testing alone would miss.

Middle-ear status also affects OAE results. Fluid, negative pressure, or any disruption to sound conduction through the middle ear will reduce or eliminate OAEs even if the cochlea is intact. For this reason, OAE testing is usually preceded by tympanometry to confirm the middle ear is clear.

OAEs in ototoxicity monitoring

Several medication classes damage cochlear hair cells as a side effect. Platinum-based chemotherapy agents (especially cisplatin) and aminoglycoside antibiotics (such as gentamicin) are among the most significant. NIOSH and clinical audiology guidelines recommend baseline and serial hearing monitoring for patients receiving these drugs, often using both pure-tone audiometry and DPOAE measures.

DPOAEs can detect high-frequency OHC loss before it appears on the standard audiogram (which typically tests up to 8000 Hz), particularly when extended high-frequency audiometry or high-frequency DPOAE protocols are used. Catching early changes allows medical teams to weigh continuing versus adjusting treatment.

When OAE testing is ordered alongside other tests

OAE results are almost always interpreted alongside a standard audiogram and, when appropriate, auditory brainstem response testing. A full picture may require knowing whether the cochlea is responding, whether the auditory nerve is transmitting, and what thresholds are at each frequency. No single test answers all three questions.

If symptoms persist or change, see an audiologist or physician.