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LESSON 122

Transient-Evoked Otoacoustic Emissions (TEOAE)

Year 2
Spring semester
Electrophysiological Tests I
Otoacoustic emissions
Duration: 10:50

TEOAE is the broadband response of the cochlea to a click stimulus. In this lesson we look at probe fit, the nonlinear click protocol, reproducibility with the A and B buffers, signal-to-noise ratio in frequency bands, sample pass rules, and the relationship between TEOAE and hearing loss.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we will learn about the measurement of transient-evoked otoacoustic emissions in detail.
We will talk about the click stimulus, the nonlinear protocol, and the reproducibility and signal-to-noise ratio criteria.
A transient-evoked otoacoustic emission, or TEOAE, is the cochlear response to a brief stimulus.
The stimulus is usually a click; because a click is very short, it stimulates a wide frequency band at once.
So many regions of the cochlea respond together, and the response is recorded as a mix of different frequencies.
The response begins a few milliseconds after the stimulus and is recorded within a window of about twenty milliseconds.
High frequency components come from the base of the cochlea, so they arrive earlier; low frequency components arrive later.
Recall from the previous lesson: the response comes from the outer hair cells, and it is recorded after passing through the middle ear.
The probe used in otoacoustic emission, or OAE, measurement is a small tip placed in the ear canal.
It contains a miniature loudspeaker that delivers the stimulus and a sensitive microphone that records the response.
The probe tip must seal the ear canal well; ambient noise should not get in, and low frequencies should not leak out.
At the start of the measurement, the device measures the ear canal's reaction to a brief stimulus and assesses the probe fit.
The stimulus waveform in the ear canal should be clean and brief; prolonged ringing suggests that the probe is not seated well.
The click level is usually set at about eighty decibels peak equivalent sound pressure level.
The basic problem in TEOAE recording is that the stimulus echo in the ear canal is very large, while the emission is very small.
In the nonlinear protocol, clicks are presented in groups of four: three clicks have the same polarity.
The fourth click has the opposite polarity and is three times stronger than the others.
When the four responses are added, the ear canal and probe echo, which follows the stimulus linearly, largely cancels out.
The cochlear emission, however, does not grow linearly; as the stimulus gets stronger, the response grows less and nears saturation.
So part of the emission is preserved in the sum and can be separated from the stimulus artifact.
A linear protocol gives a larger response but makes the artifact hard to remove; this is why the nonlinear protocol is common in the clinic.
Because the emission is so small, a single response is lost in the noise; so many responses are averaged.
The device distributes the responses in turn into two separate memories, the A and B buffers.
This gives two independent averages, recorded from the same ear over the same period.
A true emission looks the same in both averages, while random noise differs between them.
The difference between the two averages is used as an estimate of the noise, and their sum as an estimate of the response.
Very noisy sweeps are excluded from the average by artifact rejection.
Reproducibility is a percentage that shows how closely the A and B averages match each other.
If the two curves almost overlap, reproducibility is high, which points to a true response.
Because random noise differs between the two buffers, reproducibility stays low when only noise is recorded.
A common criterion for overall reproducibility is about seventy percent or more.
Some protocols use lower limits, such as fifty to sixty percent; the criterion depends on the institution's protocol.
Reproducibility can also be calculated separately in each frequency band.
A high percentage alone is not enough; we also look at how much larger the response is than the noise.
The second key criterion is the signal-to-noise ratio, or SNR.
The device splits the response into frequency bands and compares the response level with the noise level in each band.
The bands are usually centered around one, one and a half, two, three and four kilohertz.
In this example, in the two kilohertz band, the response is about fourteen decibels above the noise.
Common criteria require the response in a band to be at least three to six decibels larger than the noise.
Noise is higher in bands at and below one thousand hertz, so the low bands often cannot be assessed reliably.
The absolute level of the response is also noted; a very low-level response is interpreted with care, even when noise is low.
Protocols combine these criteria into a decision rule; here is a sample rule.
First, the probe fit is checked, and that the stimulus stayed stable throughout the measurement.
If stability is low, the baby may have moved or the probe may have shifted.
Next, overall reproducibility is checked.
In each band, the response must exceed the noise by a set amount.
For example, if the criterion is met in at least three of the five bands, the result can be counted as a pass.
These numbers are examples; each center should set a written criterion based on its own device and sources.
TEOAE does not measure hearing threshold, but it gives rough information about hearing level.
When a cochlear hearing loss exceeds about thirty to forty dB HL, TEOAE is usually absent.
In this example, hearing is normal at low and mid frequencies, and there is a moderate loss above two kilohertz.
The TEOAE response is present in the one and two kilohertz bands and absent in the three and four kilohertz bands.
The band information gives a clue about where the loss may be, but no threshold estimate is made.
A normal TEOAE does not fully rule out a mild hearing loss, so the result is interpreted together with the audiogram.
In newborns, TEOAE amplitude is usually clearly larger than in adults.
In the newborn, the response is also richer toward high frequencies; the small ear canal volume is one of the reasons.
These features make TEOAE a suitable test for newborn screening.
In infants, however, sucking, movement and breathing sounds can make measurement harder.
In adults, response amplitude may decrease with age; part of this decrease is due to accompanying hearing loss.
Adults can usually sit quietly, so controlling noise is easier.
Ambient noise raises the noise floor and masks weak responses.
In infants, sucking, crying and movement increase the number of rejected sweeps and lengthen the measurement.
Cerumen in the ear canal, or vernix in the newborn, can reduce the transmission of the stimulus and the response.
If the probe opening rests against the ear canal wall, or the tip is blocked, the recording is spoiled.
Fluid or marked negative pressure in the middle ear can abolish the response even when the cochlea is normal.
A refer result given without considering these factors leads to unnecessary referrals.
A tip from practice: when TEOAE is absent, do not call it a refer right away.
Remove the probe, check the tip for cerumen or fluid, and change the tip if needed.
Reinsert the probe and check that the stimulus waveform is clean.
A repeat measurement when the baby is asleep or calm often gives a different result.
When reporting a TEOAE result, the protocol and the stimulus level used are written down.
Overall reproducibility, stimulus stability and the signal-to-noise ratio of each band are recorded.
The result is summarized separately for each ear, according to the pass criterion used.
Noise during the measurement, the baby's state and any tympanometry result are also noted.
Screening devices often give the result only as pass or refer; diagnostic assessment needs detailed data.
TEOAE is the broadband response of the cochlea to a click, recorded in a window of about twenty milliseconds.
The nonlinear protocol, with three same and one inverted click, largely cancels the ear canal echo.
The result is evaluated by the reproducibility of the A and B buffers and the signal-to-noise ratio in the bands.
When a cochlear hearing loss exceeds about thirty to forty dB HL, TEOAE is usually absent.
Test yourself: In a TEOAE recording, which measure shows how closely the A and B averages match?
The correct answer is B: reproducibility.
A true emission looks the same in two independent averages, while noise differs; this similarity shows the response is reliable.
In the next lesson, we will look at distortion product otoacoustic emissions, which are measured with two tones.
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Prerequisites

LESSON 121 Otoacoustic emissions

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