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LESSON 123

Distortion Product Otoacoustic Emissions (DPOAE)

Year 2
Spring semester
Electrophysiological Tests I
Otoacoustic emissions
Duration: 11:30

In DPOAE measurement, two primary tones are presented and the 2f1-f2 product generated by the cochlea is recorded. In this lesson we look at the protocol with f2/f1 = 1.22 and L1/L2 = 65/55 dB SPL, where the product is generated in the cochlea, how to read the DP-gram together with the noise floor, and how DPOAE compares with TEOAE.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we will learn about the measurement of distortion product otoacoustic emissions.
We will see how the two primary tones are chosen, the two f1 minus f2 product, and how to read the DP-gram.
In distortion product otoacoustic emission, or DPOAE, measurement, two pure tones are delivered into the ear canal at the same time.
These tones are called the primary tones.
The lower frequency tone is called f1, and the higher frequency tone f2.
A healthy cochlea is not linear, so while processing the two tones it produces sounds at new frequencies, called distortion products.
In humans, the largest product is at the frequency two f1 minus f2, and this is the one measured in the clinic.
This nonlinearity depends on the active function of the outer hair cells; when they are damaged, the product shrinks or disappears.
The product is much weaker than the primary tones; in a normal ear, it is usually about fifty to sixty decibels lower.
Let us work through an example: f2 is set at two thousand hertz.
When the ratio of f2 to f1 is one point two two, f1 is about one thousand six hundred forty hertz.
Two f1 minus f2 then comes to about one thousand two hundred eighty hertz; the product is looked for at this frequency.
So the product appears below both primary tones, at a lower frequency.
However, the result is usually plotted not by the product frequency but by the f2 frequency.
This is because the main generation site is near the f2 place in the cochlea, where the vibrations of the two tones overlap.
The two tones create two traveling waves in the cochlea; the peak of f2 is near the base, and that of f1 a little closer to the apex.
The region where the two waves overlap lies just next to the f2 place, and the product is mainly generated here.
The product also travels toward its own frequency place in the cochlea, and a second component can be reflected from there.
This two-source structure sometimes causes a fine structure in the DP-gram that ripples with frequency.
For clinical interpretation, the practical rule is simple: each point on the DP-gram roughly reflects the outer hair cells in the f2 region.
So the DP-gram can be read like a frequency-specific map of cochlear function.
In the most widely used protocol, the ratio of f2 to f1 is set at one point two two.
This ratio became common because, at moderate levels in the human ear, it usually gives the largest product.
The primary tone levels are written L1 and L2; the most common choice is L1 at sixty five and L2 at fifty five dB SPL.
So the lower frequency tone, f1, is presented ten decibels stronger than f2.
Keeping L1 above L2 gives a more balanced overlap of the two tones at the f2 place, and a larger product.
Measurement is usually made at several points per octave, with f2 between about one and eight kilohertz.
Levels are given in dB SPL, and the device sets them by measuring in the ear canal.
Before measurement, the ear canal is checked with otoscopy; if possible, tympanometry is also done.
The probe contains two separate loudspeakers for the two primary tones and one microphone; it is inserted so that no air leaks.
The device first measures the levels of the tones in the ear canal and adjusts them to the target levels.
The measurement moves from frequency to frequency; many devices start high and record the product and the noise at each point.
When enough noise reduction is reached at a point, or a set time has passed, the device moves on to the next frequency.
At the end, the product level, the noise floor and the pass criterion are evaluated together for each frequency.
DPOAE results are shown on a graph called a DP-gram.
The horizontal axis shows the f2 frequency, and the vertical axis shows the product level in dB SPL.
The gray area shows the background noise at the product frequency for each point, that is, the noise floor.
How far the product rises above the noise floor determines whether the response is real.
In this example, at two kilohertz, the product is about twenty one decibels above the noise.
A commonly used criterion is that the product must exceed the noise floor by at least six decibels.
At low frequencies, especially below one thousand hertz, the noise floor is high and the measurement is less reliable.
A product above the noise floor shows that a response is present, but whether its level is normal is a separate question.
This is why many devices draw a normal range, obtained from people with normal hearing, over the DP-gram.
The large-sample study published by Gorga and colleagues in nineteen ninety seven is one of the main sources of clinical norms.
If the product is below the normal range, reduced outer hair cell function in that region can be considered.
Norms depend on the protocol and the device; norms obtained with different L1 and L2 cannot replace one another.
The result is always interpreted together with the audiogram and tympanometry.
In this example, an adult's DP-gram shows a clear response at low and mid frequencies.
After three kilohertz the product falls, and above four kilohertz it drops into the noise floor.
The person's audiogram also shows a moderate loss at four kilohertz and above.
DPOAEs can be recorded, though at low levels, in hearing losses up to about forty five to fifty dB HL.
This frequency-specific information makes DPOAEs valuable for ototoxicity monitoring and in noise-induced hearing loss.
However, DPOAE is not a threshold test either; hearing threshold is not calculated from the product level.
A DP-gram is measured at a single stimulus level, but the product can also be studied as the stimulus level changes.
The graph drawn by raising the level step by step at a given f2 frequency is called the DPOAE input-output function.
In a normal ear, the product appears at low levels and its growth slows at high levels; this reflects cochlear compression.
With outer hair cell damage, the product appears only at higher stimulus levels.
This measurement is used mostly in research and in detailed diagnostic assessment.
Transient-evoked otoacoustic emissions, or TEOAEs, assess a wide frequency region in a single measurement with a click stimulus.
The response is most reliably recorded between about one and four kilohertz.
It is usually lost once a cochlear loss exceeds about thirty to forty dB HL, which shows its sensitivity to mild losses.
Because it is fast and simple, it is widely used in newborn screening.
DPOAE, on the other hand, assesses one specific frequency region, the f2 place, in each measurement.
It can extend to high frequencies, to eight kilohertz and above.
It can also be recorded in more severe losses; responses can be seen up to about forty five to fifty dB HL.
The two tests are not rivals but complements; one or both are chosen according to the clinical question.
The frequency-specific nature of DPOAE makes it stand out in several clinical areas.
In patients receiving ototoxic drugs, a decline in high frequency responses can be an early warning.
It is also being studied for monitoring outer hair cell effects in workers exposed to noise.
It provides a cross-check when behavioral testing is not reliable.
It is also used as an alternative to TEOAE in newborn screening; we will see the details in the next lesson.
A clinical tip: on a DP-gram, an absent product and a product that could not be measured are not the same thing.
If the noise floor is high, even a small product that is present can stay below the noise.
In that case, record the result as could not be assessed due to noise rather than no response, and repeat when the setting is quieter.
Missing responses at low frequencies in particular are often caused by noise.
In DPOAE, two primary tones are presented, and the largest product is measured at the frequency two f1 minus f2.
In the common protocol, the ratio of f2 to f1 is one point two two, and the levels are sixty five and fifty five dB SPL.
The DP-gram shows the product level by f2 frequency, and the response is compared with the noise floor.
A commonly used criterion is that the product exceeds the noise floor by at least six decibels.
Test yourself: f1 is one thousand six hundred forty and f2 two thousand hertz. At what frequency is the two f1 minus f2 product?
The correct answer is B: about one thousand two hundred eighty hertz.
Two times 1640 is 3280; subtracting 2000 gives about 1280 hertz.
In the next lesson, we will look at the clinical uses of otoacoustic emissions, from screening to ototoxicity monitoring.
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Prerequisites

LESSON 122 Transient-Evoked Otoacoustic Emissions (TEOAE)

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