LESSON 129

Neurodiagnostic ABR

Name: Neurodiagnostic ABR
Uploaded: 2026-11-04T04:00:00+03:00
Duration: 12 min 23 s
Description: We explain how the auditory brainstem response (ABR) is used when a retrocochlear lesion is suspected. With examples, we cover interpeak latencies, the interaural wave V latency difference (IT5), correction for high-frequency hearing loss, typical patterns, and the sensitivity limits of ABR compared with MRI.

We explain how the auditory brainstem response (ABR) is used when a retrocochlear lesion is suspected. With examples, we cover interpeak latencies, the interaural wave V latency difference (IT5), correction for high-frequency hearing loss, typical patterns, and the sensitivity limits of ABR compared with MRI.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn how the auditory brainstem response, or ABR, is used to investigate retrocochlear problems.
With examples, we will cover the latency criteria we look at, the interaural wave V difference, and the limits of the method.
Retrocochlear means beyond the cochlea and covers the pathway from the auditory nerve to the brainstem.
A problem in this region can slow the signal in the brainstem or cause it to lose synchrony.
The most classic example is vestibular schwannoma, a benign tumor that grows in the internal auditory canal.
Multiple sclerosis, vascular problems affecting the brainstem, and other tumors can also disrupt the ABR.
The ABR measures how fast and how regularly the signal is conducted; imaging methods show the structure itself.
So neurodiagnostic ABR is a screening and supporting test; the diagnosis is made by physician assessment and imaging.
The internal auditory canal is a narrow bony channel through which the hearing, balance and facial nerves pass together.
Vestibular schwannoma usually arises from the Schwann cells in the sheath of the superior or inferior vestibular nerve.
As the tumor grows, it compresses the neighboring cochlear nerve; nerve conduction slows and synchrony breaks down.
The labyrinthine artery that supplies the cochlea can also be affected, so cochlear findings are sometimes present as well.
The facial nerve, which runs through the same canal, can be affected by large tumors, but facial palsy is rare early on.
A current audiogram should be available before neurodiagnostic ABR, because cochlear hearing loss affects latencies.
The stimulus is a click or broadband chirp at high intensity, for example 80 to 90 dB nHL, that is, normalized hearing level.
To see the waves clearly, the stimulus rate is kept low, for example about 11 to 21 per second.
At least two recordings are obtained in each condition to confirm that the waves are repeatable.
Both ears are tested with the same stimulus, the same intensity and the same settings, so the ears can be compared.
If there is a clear asymmetry between the ears, masking noise may be given to the opposite ear to prevent cross-hearing.
Some protocols also add recordings at higher rates, because impaired conduction can become more obvious at high rates.
The first criterion is wave V latency at high intensity; in adults, the upper limit is usually around 6 milliseconds.
An I-III interval above about 2.5 milliseconds suggests delayed conduction between the auditory nerve and the lower pons.
A III-V interval above about 2.4 milliseconds may point to a delay in the upper part of the brainstem.
An I-V interval above about 4.4 to 4.5 milliseconds is the most widely used delay criterion.
An interaural wave V difference above about 0.3 to 0.4 milliseconds is also considered abnormal.
Finally, absent waves that hearing level cannot explain, or an unrepeatable recording, are also important findings.
The limits in the table are approximate; each clinic should use norms obtained with its own device and protocol.
On screen, the dashed line shows a normal recording and the solid line an example recording.
In this example, wave I is at a normal latency; so the first step of conduction from the cochlea to the nerve starts on time.
Wave V, however, arrives clearly later than in the normal recording.
As a result, the I-V interval is prolonged to about 4.8 milliseconds and exceeds the normal limit.
This pattern suggests that the delay lies not in the cochlea but in nerve and brainstem conduction, and needs further assessment.
The interaural wave V latency difference, or IT5, is the difference between the wave V latencies of the two ears at the same intensity.
In this example, wave V in the right ear is seen at 5.6 milliseconds.
In the left ear, at the same intensity, wave V arrives at 6.1 milliseconds.
The difference is 0.5 milliseconds.
A difference above about 0.3 to 0.4 milliseconds is usually considered abnormal; this example is above that limit.
The strength of this criterion is that it compares the patient with their own other ear; factors like age and sex affect both ears.
However, this comparison is most reliable when the cochlear hearing levels of the two ears are close to each other.
A cochlear loss in the high frequencies can delay wave V, because it affects the region the click stimulates most strongly.
In an asymmetric high-frequency loss like this example, the interaural difference may also come from the cochlear difference.
In a common correction from Selters and Brackmann, 0.1 ms is subtracted for every 10 dB of 4000 Hz loss above 50 dB.
In this example, the left 4000 Hz threshold is 70 decibels, so a correction of about 0.2 milliseconds is made.
Corrections are rough estimates; if wave I is visible, the I-V interval is more reliable because the cochlea affects it much less.
If wave I is weak, the ear canal electrode we saw in earlier lessons can make wave I more prominent.
In retrocochlear lesions, the most common pattern is prolonged interpeak intervals, especially the I-III and I-V intervals.
In unilateral lesions, the interaural wave V difference is often the first finding to stand out.
Sometimes only wave I is seen and the later waves are lost; this suggests the signal breaks down in the more central part of the nerve.
When hearing thresholds are good enough to record an ABR, the absence of any wave is also a strong warning sign.
Recordings obtained in the same condition that do not match each other can also point to a loss of synchrony.
A wave V clearly smaller than wave I is considered an additional, supporting finding.
A clinical tip: always check the audiogram before interpreting absent waves.
If there is a severe loss in the high frequencies, the inability to record a click ABR may also be cochlear in origin.
In contrast, the absence of any repeatable wave in an ear with fairly good hearing is much more significant for a retrocochlear problem.
State this distinction clearly in the report; the reading physician judges the weight of the finding from it.
In studies, conventional ABR was abnormal in the great majority of large vestibular schwannomas.
But in small tumors confined to the internal auditory canal, especially those under one centimeter, sensitivity drops clearly.
A small tumor can grow without affecting some of the nerve fibers and may not change the click response.
So a normal ABR result does not definitively rule out a retrocochlear lesion.
Today, the gold standard for retrocochlear assessment is magnetic resonance imaging, or MRI, often with contrast.
ABR remains valuable for people who cannot have an MRI or for whom it is unsuitable, and when functional information is needed.
MRI directly shows the presence, size and location of a tumor; it can detect lesions of even a few millimeters.
ABR measures function rather than structure; it is noninvasive, relatively low cost, and tells us how the patient's auditory pathway works.
These two methods do not replace each other; they complement each other.
Advanced techniques such as stacked ABR have been developed to increase sensitivity to small tumors, but they are not widely used.
The physician decides which imaging is done and when; the audiologist's role is to present the findings clearly.
In diseases that affect myelin, such as multiple sclerosis, the III-V and I-V intervals in particular may lengthen and waves may degrade.
Strokes or hemorrhages affecting the brainstem can also delay specific waves, depending on the site of the lesion.
Other tumors in the cerebellopontine angle, the region between the cerebellum and the pons, can give similar findings.
In these cases, the ABR can be abnormal even with normal hearing thresholds, because the ABR measures conduction, not threshold.
In some cases, serial ABR recordings are also used to follow the course of the disease.
For each ear, the report lists the absolute latencies of waves I, III and V and the interpeak intervals.
The interaural wave V difference is stated, along with any correction that was used.
The result is described, according to the clinic's norms, as normal or as abnormal for retrocochlear involvement.
With an abnormal finding, the audiologist's role is to refer the patient to a physician for further assessment.
The report does not diagnose; instead of saying there is a tumor, it states the findings may be consistent with a retrocochlear process.
Neurodiagnostic ABR uses a high-intensity click to assess auditory nerve and brainstem conduction.
Prolonged interpeak intervals and an interaural wave V difference above about 0.3 to 0.4 milliseconds are the key criteria.
High-frequency cochlear loss can delay wave V, so the audiogram must always be interpreted alongside it.
ABR is not sensitive to small tumors; the decision on imaging belongs to the physician.
Test yourself: wave V is 5.6 milliseconds on the right and 6.1 on the left. How is this difference interpreted?
The correct answer is B: it is abnormal and needs further assessment.
The difference is 0.5 milliseconds, above the usually accepted limit; before interpreting, check the audiograms of both ears.
In the next lesson, we will see how we use ABR for frequency-specific threshold estimation in infants.
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Prerequisites

LESSON 128 Clicks, tone bursts and chirps