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LESSON 121

Otoacoustic emissions

Year 2
Spring semester
Electrophysiological Tests I
Otoacoustic emissions
Duration: 10:39

Otoacoustic emissions (OAEs) are very faint sounds produced by the active function of the cochlear outer hair cells and recorded in the ear canal. In this lesson we cover Kemp's discovery in 1978, spontaneous and evoked OAE types, the link with outer hair cells, and the effect of the middle ear on OAE measurement.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will get to know otoacoustic emissions, the sounds produced by the cochlea itself.
We will talk about the types of emissions, their link with the outer hair cells and the effect of the middle ear on measurement.
Otoacoustic emissions, or OAEs, are very faint sounds produced by the cochlea itself.
These sounds travel back through the middle ear and can be recorded with a sensitive microphone placed in the ear canal.
Otoacoustic emissions were first recorded and published in 1978 by the British researcher David Kemp.
Before this discovery, in 1948, Thomas Gold had proposed that there must be an active amplification process in the cochlea.
Kemp's finding showed that the cochlea is not just a passive receiver of sound, but actively amplifies it.
This discovery soon became a clinical test that also changed newborn hearing screening.
OAE recording entered the clinic as an objective method giving fast, noninvasive information about how the outer hair cells work.
The source of OAEs is the outer hair cells in the organ of Corti.
When stimulated, outer hair cells can rapidly change length; this is called electromotility, and it depends on a protein called prestin.
This movement amplifies basilar membrane vibration, especially at low intensities; this process is called the cochlear amplifier.
The inner hair cells, by contrast, are the true sensory cells and pass information to the auditory nerve.
A by-product of the active movement of the outer hair cells is sound energy reflected back into the ear canal; this is the OAE we measure.
So OAEs give direct information about the function of the outer hair cells.
Noise, ageing and ototoxic drugs are among the factors that often affect the outer hair cells first.
Spontaneous otoacoustic emissions, or SOAEs, are narrowband sounds recorded in the ear canal with no stimulus at all.
SOAEs are not present in everyone with normal hearing, so their absence means little on its own and their clinical use is limited.
Among evoked emissions, transient-evoked OAEs, or TEOAEs, are recorded in response to a brief stimulus such as a click.
Distortion product OAEs, or DPOAEs, arise in response to two tones of different frequencies given at the same time.
These two types are the OAE tests most widely used in the clinic today.
Stimulus frequency OAEs, or SFOAEs, measured in response to a single tone, are used mostly in research.
In clinical use, the choice among these types depends on the clinical question and the age group being tested.
In TEOAE recording, brief click stimuli are delivered into the ear canal.
A few milliseconds after the stimulus, the faint response returning from the cochlea is recorded.
The response is measured by averaging many stimuli, which reduces random noise.
How far the response rises above the noise floor, the signal-to-noise ratio, determines how reliable the result is.
Because different frequencies come from different regions of the cochlea, the response can also be analyzed in frequency bands.
We will see the detailed pass criteria in the next lesson.
In DPOAE recording, two tones are delivered into the ear canal at the same time; they are called f1 and f2.
A healthy cochlea produces sound at new frequencies not present in the two tones; the largest is the product at two f1 minus f2.
This product arises because the cochlea works in a nonlinear way.
In standard protocols, the ratio of f2 to f1 is set at about one point two two.
The level of the product is compared with the noise floor and repeated at different frequencies to build a graph.
Because the response arises in the cochlea near the f2 region, the result is usually plotted by f2 frequency.
Recording an OAE in a frequency region shows that the outer hair cells in that region are largely working.
When a cochlear loss exceeds about thirty to forty dB HL, TEOAEs are usually absent; DPOAEs may persist up to forty five to fifty dB HL.
So the presence of OAEs suggests there is no significant cochlear loss in that region.
But OAE is not a threshold test; it does not directly tell us the hearing threshold or the degree of loss.
OAEs do not assess the inner hair cells, the auditory nerve or the central pathways.
For example, in auditory neuropathy spectrum disorder, OAEs may be present while the auditory brainstem response, or ABR, is very abnormal.
So an OAE result becomes meaningful when it is assessed together with the audiogram and other tests.
In OAE measurement, sound passes through the middle ear twice: the stimulus inward, and the emission outward.
Because the emission is so faint, even a small loss in outward transmission can make it unmeasurable.
Middle ear fluid, clearly negative pressure or a perforation can reduce OAEs or abolish them completely.
In that case, absent OAEs may reflect a blockage in the conductive pathway rather than a cochlear problem.
Wax in the ear canal, vernix in a newborn, or a poorly seated probe can spoil the result in the same way.
This is why OAEs are interpreted together with tympanometry and otoscopy.
Here is an important tip from the field.
When OAEs are absent, do not immediately interpret this as cochlear loss.
First look at the tympanogram; clearly negative pressure or a type B curve can by itself wipe out OAEs.
Then check otoscopy and whether the probe is seated correctly in the ear canal.
Room or patient noise can also spoil the recording; repeating the test while the child is calm often changes the result.
These simple checks can prevent an unnecessary refer result and needless worry for the family.
Because OAEs are so faint, the test is done in a quiet room while the patient is calm.
The probe is placed in the ear canal with a good seal; devices often check probe fit before measuring.
The stimulus level in the ear canal is monitored in every measurement and adjusted if needed.
The test usually takes less than a few minutes per ear.
Screening devices often give the result as pass, or as needing further testing.
With diagnostic devices, response levels and the noise floor are examined in detail by frequency.
Because they are fast, objective and need no response from the patient, OAEs are widely used in newborn hearing screening.
In patients receiving ototoxic drugs, they are also valuable for detecting outer hair cell damage early.
They are a cross-check tool in children whose behavioral test results are not reliable.
Together with brainstem responses, they help in recognizing auditory neuropathy spectrum disorder.
When functional hearing loss is suspected, the presence of OAEs despite a reported severe loss is an important finding.
We will cover each of these uses in detail in later lessons.
In short, OAEs are a window onto the function of the outer hair cells.
TEOAEs and DPOAEs can give separate information for different frequency regions.
The measurement is objective, quick, and needs no response from the patient.
But OAEs do not measure the hearing threshold; a normal OAE does not always mean normal hearing.
Nor do they give information about auditory nerve or brainstem function.
On their own, OAEs cannot tell whether absent emissions are due to the middle ear or the cochlea; that needs a test battery.
OAEs are faint sounds produced by the cochlea and recorded in the ear canal; they were first recorded by Kemp.
Their source is the outer hair cells, part of the cochlear amplifier.
Spontaneous emissions have limited clinical value; TEOAEs and DPOAEs are the most used in the clinic.
Because the emission is recorded after passing through the middle ear, the result is read with tympanometry and otoscopy.
Test yourself: the tympanogram is type B and no TEOAE was obtained. What is the best interpretation?
The correct answer is B: a middle ear problem may have blocked the OAEs.
Because the emission passes through the middle ear, fluid there can wipe out OAEs even when the cochlea is normal.
In the next lesson we will examine TEOAE measurement, the click stimulus and the pass criteria in detail.
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