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LESSON 133

Auditory neuropathy spectrum disorder

We explain what it means in auditory neuropathy spectrum disorder (ANSD) when OAEs or the cochlear microphonic are present but the ABR is absent or clearly abnormal. We cover the sites of lesion, how to tell the cochlear microphonic from artifact, risk factors, and the mismatch between speech perception and the audiogram.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson we will learn about auditory neuropathy spectrum disorder, or ANSD.
We will learn to recognize this picture, with OAEs and a cochlear microphonic present but no ABR, and look at its risk factors.
Auditory neuropathy spectrum disorder is known by the abbreviation ANSD.
In this condition, the outer hair cells, which act as the cochlear amplifier, mostly work normally.
The problem lies in the inner hair cell, the synapse or the auditory nerve, and the nerve fibers cannot respond to sound in synchrony.
Because the ABR depends on many fibers firing at the same time, the waves disappear or become clearly abnormal when synchrony is lost.
As a result, the person can hear sound but has far more difficulty than expected understanding speech, especially in noise.
The word spectrum stresses that findings range from mild to severe and vary greatly from person to person.
The condition was described in 1996 by Starr and colleagues under the name auditory neuropathy.
But it became clear that the problem was not always in the auditory nerve; sometimes it was in the inner hair cell or the synapse.
So some researchers used the term auditory dyssynchrony, which stresses the loss of synchrony.
In 2008, an expert panel meeting in Italy proposed the name ANSD to reflect the breadth of the findings.
Today most clinical guidelines are based on this name and on the principles set at that meeting.
If the outer hair cells in the organ of Corti are intact, OAEs and the cochlear microphonic can be recorded.
If the lesion is in the inner hair cell or its synapse, it is described as the presynaptic type.
The form linked to the otoferlin gene is the best-known example of this presynaptic group.
If the lesion is in the auditory nerve itself, it is the postsynaptic, or neural, type.
This distinction matters because it can affect the benefit expected from different intervention options.
In both types, the common result is a loss of synchronous firing along the nerve fibers.
Let us gather the typical test findings of ANSD in one table; the first clue is that OAEs are mostly present.
The cochlear microphonic, the electrical response produced by the outer hair cells, is clearly seen in the recording.
In contrast, the ABR shows no wave V even at high levels, or the waves are severely abnormal.
Acoustic reflexes are often absent or clearly elevated.
The audiogram can be at any level from normal to profound, and it can fluctuate over time.
The speech discrimination score is often much worse than the audiogram would suggest.
It should also be kept in mind that OAEs can disappear over time in some people.
The clinical picture often appears at newborn screening or during diagnostic assessment.
In this example, the OAE recording shows a clear response well above the noise floor.
This shows that the outer hair cells and the middle ear are most likely working.
In the same baby's ABR, however, no repeatable wave V can be found, even at high levels.
This dissociation is the key sign that raises suspicion of ANSD.
Most sensorineural losses are cochlear in origin and also affect the outer hair cells.
So in moderate or severe cochlear loss, OAEs are usually absent; in ANSD, by contrast, OAEs are mostly present.
In cochlear loss the ABR threshold is close to the behavioral threshold; in ANSD the ABR cannot be used to estimate hearing level.
In cochlear loss, speech perception usually matches the audiogram; in ANSD it is disproportionately poor.
This comparison shows why it is essential to look at a test battery, not a single test.
The drop in OAE amplitude when noise is given to the opposite ear, contralateral suppression, is usually absent in ANSD.
The ability to follow rapid changes in sound over time, temporal processing, is clearly impaired.
So understanding speech in noise is much harder than in a cochlear loss with a similar audiogram.
In some people, thresholds can fluctuate over time, even with changes in body temperature.
These findings show why the person's difficulty in daily life cannot be explained by the audiogram.
To see the cochlear microphonic, a high-level click is used, for example at 80 to 90 dB nHL.
Separate recordings are made in condensation and rarefaction polarity, and the two are overlaid.
The cochlear microphonic, or CM, follows the stimulus waveform, so it inverts when the polarity changes.
Neural wave V, however, usually does not invert with polarity; only its latency may change slightly.
In ANSD this inverting response can be large and can last for several milliseconds.
But stimulus artifact also inverts with polarity, so the two must be told apart.
With insert earphones, sound travels from the transducer to the ear through a tube in a little under one millisecond.
Thanks to this delay, electrical stimulus artifact can be separated in time from the response coming from the ear.
A control recording is also made with the sound tube clamped or disconnected from the transducer.
When sound does not reach the ear, a true cochlear microphonic disappears, while electrical artifact stays in the recording.
Keeping the transducer and its cables away from the electrode leads also reduces artifact.
A clinical tip: screening with OAEs alone can pass a baby who has ANSD.
This is because OAEs reflect the outer hair cells and middle ear; they do not show neural conduction.
So international guidelines recommend automated ABR screening for babies who stay in neonatal intensive care for more than five days.
If there is concern about speech development or auditory behavior, a past OAE result alone should not be reassuring.
Prematurity is one of the most important known risk factors for ANSD; low birth weight often comes with it.
The risk rises clearly in babies who have stayed a long time in neonatal intensive care.
Hyperbilirubinemia, a raised level of bilirubin in the blood, can damage the auditory pathways, especially at high levels.
Lack of oxygen during birth, or hypoxia, is also among the risk factors.
But ANSD can also occur in babies with no risk factors; in such cases genetic causes come to the fore.
The best-known genetic cause is variants in the OTOF gene, which codes for the protein otoferlin.
Otoferlin is needed for neurotransmitter release at the inner hair cell synapse, so this form is presynaptic.
OTOF-related loss is usually inherited in an autosomal recessive pattern and is present from birth.
Other genes, such as OPA1 and PJVK, have also been linked with an auditory neuropathy picture.
In older children and adults it can occur with neurological diseases such as Charcot-Marie-Tooth disease or Friedreich ataxia.
An absent or very thin auditory nerve, cochlear nerve deficiency, gives similar findings and is shown on MRI.
In this example, the right ear shows a mild to moderate loss that is somewhat greater at the low frequencies.
Bone conduction thresholds are close to air conduction; there is no air-bone gap, so the loss appears sensorineural.
Looking at this audiogram, we might expect the person to understand speech in quiet fairly well.
Yet in this example, the word discrimination score was measured at around thirty percent.
This clear mismatch between the audiogram and speech perception is an important warning sign for ANSD.
Understanding in noise is even more impaired than in quiet.
In ANSD the ABR cannot be used to estimate hearing thresholds, so thresholds are measured as soon as behavioral testing is possible.
The aim is an ear- and frequency-specific audiogram using methods such as visual reinforcement audiometry.
Repeated assessments matter, because in some babies, especially with hyperbilirubinemia, findings can improve over time.
So no permanent conclusion is drawn from a single ABR recording.
Speech and language development is closely monitored; if progress is slower than expected, the team reassesses.
Assessment is teamwork, with ENT, audiology, neurology and genetics working together.
Intervention decisions are based not on the ABR but on behavioral thresholds and functional observations.
Guidelines recommend fitting hearing aids as a trial whose benefit is closely monitored.
Because understanding in noise is so poor, remote microphone systems are a commonly used support.
For some children who do not make enough progress, cochlear implant evaluation is considered.
Implant outcomes have often been reported to be good, especially in the OTOF-related presynaptic form.
For each child, the decision is made by the team on an individual basis.
In ANSD, OAEs or the cochlear microphonic are present, while the ABR is absent or clearly abnormal.
The problem lies in the inner hair cell, the synapse or the auditory nerve; the outer hair cells usually work.
Prematurity, intensive care, hyperbilirubinemia and OTOF variants are the main risk factors.
Speech perception far worse than the audiogram would predict is a typical warning sign.
Test yourself: which combination of findings suggests auditory neuropathy spectrum disorder?
The correct answer is B: OAEs present, ABR absent.
This dissociation arises when neural synchrony is impaired while the outer hair cells are working.
In the next lesson we will see how we interpret test battery results together when separating cochlear and retrocochlear problems.
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