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LESSON 131

Latency-intensity and bone conduction

We explain how the latency-intensity function of the auditory brainstem response (ABR) changes in conductive, cochlear and retrocochlear hearing loss, and how bone-conduction ABR helps identify the type of loss. With examples, we cover vibrator placement, technical limits and interpretation in infants.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn the latency-intensity function and bone-conduction testing in the auditory brainstem response, or ABR.
We will see how these two tools help us tell conductive, cochlear and retrocochlear patterns apart.
The latency-intensity function, or LIF, is a graph showing how wave V latency changes with intensity.
Intensity is on the horizontal axis and wave V latency on the vertical axis; as intensity decreases, latency increases.
In a normal-hearing adult, with a click at 80 dB nHL, that is, normalized hearing level, wave V appears at about 5.6 milliseconds.
At low intensities near threshold, latency increases to about 7.5 to 8 milliseconds.
Since we already record at different intensities during the threshold search, this graph needs no extra test.
Each clinic draws the normal range obtained with its own device and stimulus on this graph for comparison.
At high intensity, a click also strongly stimulates the fast-responding base of the cochlea, and the response comes early.
As intensity decreases, the response starts to come from more sensitive regions of the cochlea, closer to the apex.
Because the traveling wave reaches these regions later, the total latency increases.
In addition, at low intensity, triggering at the level of the hair cell and synapse also happens slightly later.
Cochlear damage changes this balance, so the shape of the curve gives us a clue about the site of the loss.
Now let us compare four example curves on the same graph; the blue curve shows a normal-hearing ear.
The orange curve represents a conductive loss; it keeps the shape of the normal curve but shifts to the right, toward higher intensities.
This is because a problem in the outer or middle ear reduces the sound reaching the cochlea by the same amount at every intensity.
The green curve shows a cochlear loss; the threshold is raised, but as intensity increases, latency quickly approaches normal.
This steep slope is also called a recruitment pattern, because it resembles the rapid growth of loudness in a damaged cochlea.
The red dashed curve represents a retrocochlear pattern; latency is longer than normal at every intensity.
These curves are examples; in real recordings, patterns can overlap, and mixed losses produce intermediate shapes.
In a conductive loss, the sound reaching the cochlea is weaker, so all waves, including wave I, are delayed.
But because conduction in the nerve and brainstem is intact, the I-V interval stays normal.
The latency-intensity curve shifts to the right, parallel to the normal curve.
The amount of shift gives a rough idea of the size of the air-bone gap, but bone-conduction ABR is needed for an exact measure.
In infants, middle ear fluid is one of the most common causes of this pattern, so tympanometry results are considered alongside.
In a cochlear, that is, sensorineural, loss, the ABR threshold rises in line with the degree of loss.
At intensities near threshold, wave V latency is relatively long.
But as intensity increases, latency shortens quickly and reaches near-normal values at high intensities.
As the example audiogram on screen shows, the situation changes somewhat if there is a marked loss in the high frequencies.
Because the region that contributes most to the click response is damaged, wave V may be slightly delayed even at high intensity.
In the retrocochlear pattern, wave V latency is longer than the normal range at every intensity, and the curve looks shifted upward.
If wave I is visible, the I-V interval is seen to be prolonged; this shows the delay lies in brainstem conduction.
The waves may also become smaller, broader or lose their repeatability.
This pattern is assessed together with the neurodiagnostic ABR criteria from the previous lessons and requires referral to a physician.
In a young infant, it is not possible to obtain behavioral bone-conduction thresholds.
When air-conduction ABR thresholds come out raised, we need to know whether the loss is conductive or sensorineural.
Bone-conduction ABR delivers sound directly to the skull with a bone vibrator, largely bypassing the outer and middle ear.
Together, air- and bone-conduction ABR thresholds reveal an estimated air-bone gap in the infant.
This distinction directly affects whether the infant is referred first for medical assessment or for early hearing aid fitting.
Bone-conduction ABR uses a bone vibrator similar to the one we use in audiometry.
In infants, the vibrator is placed on the temporal bone, near the upper back part of the pinna, without touching the pinna.
The vibrator is held with steady, adequate force, either by an elastic band or by the tester's hand.
A change in force can change the stimulus intensity, so it is important that it stays the same throughout the recording.
The reference electrode should not sit under the vibrator, so a site such as the earlobe or the nape can be chosen.
In infants, placing the vibrator on the temporal bone rather than the forehead usually gives larger responses.
A bone vibrator cannot produce high intensities without distortion; the maximum level is usually limited to about 50 to 55 dB nHL.
Because the vibrator radiates a strong electromagnetic field, a large stimulus artifact can appear in the recording.
Alternating polarity and keeping the vibrator cable away from the electrode leads reduce this artifact.
In bone-conduction ABR, latencies and thresholds differ from air conduction, so they are interpreted only with bone-conduction norms.
In infants, bone-conduction thresholds can be better than in adults, especially at low frequencies, which calls for age-specific norms.
Some infant ABR protocols recommend looking for bone-conduction responses at a few levels.
In one protocol, a response to a 500 Hz bone-conducted tone burst at 20 dB nHL suggests near-normal cochlear sensitivity.
In the same protocol, this level is 30 dB nHL for 2000 Hz.
These values vary with the device, stimulus and protocol used; they are given here only as examples.
If air-conduction thresholds are raised but bone-conduction responses are present at these levels, a conductive component is likely.
In bone conduction, the skull vibrates as a whole, so the vibrator can stimulate both cochleas at once.
In infants, the skull bones are not yet fully fused, so interaural attenuation for bone conduction is greater than in adults.
Also, in a two-channel recording, wave V is usually earlier and larger in the ipsilateral channel, on the side of the responding cochlea.
In the contralateral channel, wave V is smaller and slightly later; this asymmetry hints at which side the response comes from.
If these clues are not enough, masking noise is given to the opposite ear to confirm the source of the response.
Now let us bring the interpretation together with two purely example infants.
In example A, the 2000 Hz air-conduction ABR threshold is 50 dB nHL, but there is a clear bone-conduction response at 30 dB nHL.
This picture suggests that cochlear sensitivity is relatively preserved and the loss is largely conductive; tympanometry is added to this.
In example B, with the same air-conduction threshold, there is no bone-conduction response at 30 dB nHL.
In this case, the loss is thought to have a sensorineural component; a definite assessment needs the full test battery and a medical exam.
A clinical tip: the most common error in bone-conduction ABR is the vibrator slipping or its force changing.
Hold the vibrator with the same force throughout the recording, without letting it touch the pinna.
Keep the vibrator cable away from the electrode leads; this clearly reduces stimulus artifact.
If you are not sure which cochlea the response comes from, do not hesitate to give masking noise to the opposite ear.
The latency-intensity function shows how wave V latency increases as intensity decreases.
In a conductive loss, the curve shifts to the right in parallel, and interpeak intervals stay normal.
In a cochlear loss the slope is steep; in the retrocochlear pattern, latency is long at every intensity.
Bone-conduction ABR helps tell whether an infant's loss is conductive or sensorineural.
Test yourself: all waves, including wave I, are delayed, and the I-V interval is normal. What is the most likely pattern?
The correct answer is A: a conductive pattern.
The delay comes from less sound reaching the cochlea; since brainstem conduction is intact, the I-V interval is normal.
In the next lesson, we will look at the artifacts that disrupt ABR recordings and how we can fix them.
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Prerequisites

LESSON 130 Threshold estimation with ABR

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