Auditory neuropathy spectrum disorder

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How ANSD affects the hearing system

In some cases, the apparent hearing loss is not related to ear malfunctioning, but to alterations in the peripheral nervous system. One of such cases is auditory neuropathy, a condition that alters the process of sound interpretation, causing incorrect understanding of words.

What is auditory neuropathy?

In patients with auditory neuropathy, sounds enter the inner ear normally but signal transmission from the inner ear to the brain is compromised. At the root of this pathology is a form of degeneration or injury of the cochlea or peripheral nervous system, i.e. the nerves and hair cells that transmit information to the central nervous system.The practical consequence is an i nterference in the temporal coding of sound stimuli, which are conducted along the auditory nerve constantly 'delayed', overlapping with signals immediately preceding or following them and thus causing sounds that are difficult to distinguish. This pathology accounts for 10% of all cases of deafness.

Why is ANSD called a spectrum disorder?

Auditory neuropathy is often a disorder that is present from birth. Since it occurs before the patient learns to communicate, the severe impairment of speech comprehension has consequences for language development, which usually develops late.

This is why this pathology is included among the autistic spectrum disorders, i.e. those neurodevelopmental disorders that primarily involve language, communication and social interaction. 

Typical symptoms of ANSD

The symptoms of auditory neuropathy can be very varied. Individuals affected by this disorder may undergo audiometric examinations that seemingly align with the results of those with typical hearing. Despite accurately perceiving sound intensity, these individuals often struggle to comprehend speech, perceiving it as nonsensical, leading to an incomplete understanding of spoken words. In other cases, however, auditory neuropathy can result in varying degrees of hearing loss, ranging from mild to profound.

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What Causes ANSD?

The definite causes of auditory neuropathy are still unknown. It is estimated that 40% of cases are triggered by a genetic predisposition. A number of situations have been found that seem to determine the onset symptoms of this disease, including damage to the hair cells (those that convert sound into electrical impulses before transmitting them to the brain), lesions of the auditory nerve, and an altered connection between hair cells and the auditory nerve.

Although a cause/effect correlation has not been proven, there are several risk factors for neuropathy in children, including jaundice, prematurity, low birth weight, hypoxia.

How is ANSD Diagnosed?

A correct diagnosis of auditory neuropathy requires a variable combination of different tests, depending on the specific case. Usually, the auditory potential test ( ABR) is combined with the otoacoustic emission test ( OAE), which indicates whether or not the outer hair cells, the cells responsible for the reception of sound, are functioning properly.

Auditory brainstem response test

One of the main methods of diagnosing a hearing deficit is the Auditory brainstem response test (known as ABR), which is used to analyse the sensitivity, quality and response times of all peripheral sensory nerves, including the auditory nerve.

This diagnostic examination can also be performed on newborns, as it does not require the patient's cooperation. This examination is carried out by placing electrodes on the central lobes and having them wear a headset through which sound impulses are transmitted to which an electrical signal corresponds.

If the test reveals a sensorineural hearing loss, the specialist will then have to carry out further tests to understand its origins and establish a treatment. 

Otoacoustic emissions test

The otoacoustic emissions test (OAE) is used to assess the proper functioning of the outer hair cells, which are responsible for the reception of sound. For individuals experiencing auditory neuropathy, alterations in the auditory potential test (ABR) results are noticeable.

However, the presence of normal otoacoustic emissions (OAEs) suggests that the issue doesn't stem from damaged or non-functional hair cells but instead resides within the peripheral nervous system.

Tympanometry and stapedial reflex testing

Tympanometry is an examination composed of two tests: the first is the tympanogram, which consists of analysing the elasticity of the tympanic membrane and the degree of movement of the inner ossicles. In other words, it measures the resistance opposed by the middle ear to the passage of the sound wave.

The second test is the stapedial reflex, which provides information on the contractions of a very small muscle located in the inner ear, called the stapedium.

The absence or presence of the stapedial reflex is a fundamental index for the diagnosis of various pathologies, including sensorineural hearing loss.

Behavioural audiometry

In children, the timely recognition of a hearing defecit is crucial for timely intervention. Behavioral audiometry encompasses a range of techniques designed to subjectively evaluate the hearing abilities of young patients by observing reflexive or voluntary changes in their behavior in response to auditory stimuli.

The methods employed adapt to the child's age and level of cooperation, but the overarching goal remains constant: to quantitatively assess the child's auditory threshold, defined as the minimum intensity of sound stimulus the patient can distinguish.

Imaging studies

Diagnostic imaging plays a crucial role in pinpointing the lesions responsible for acoustic neuropathy, predominantly found in the cochlea or central nervous system. This examination, together with the evaluation of alterations in acoustic potentials - not justified by the degree of hearing loss - can help diagose a neuropathy; other instruments or clinical observations cannot be conclusive due to the wide variety of pathologies involved.

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How to treat ANSD?

At the moment, researchers are still seeking a specific therapy for neuropathy. As far as implants and other recent technologies are concerned, hearing care professionals have differing opinions on the benefits and, unfortunately, no tests are currently available to help determine the best treatment.

On the other hand, many cases of auditory neuropathy are treated with cochlear implants, which consist of one part placed behind the ear and another completely implanted under the skin. The positive results obtained are comparable with typical cases of profound deafness. That said, hearing rehabilitation must be assessed on a case-by-case basis, depending on the pathophysiology in question and the degree of severity of the deafness.

Frequently asked questions about ANSD

Does ANSD ever get better or worse?

Some children with neuropathy from birth improve and start to hear and speak within one or two years. Others remain stable, while some worsen and show signs of decreased functionality of even the outer hair cells. In adults with neuropathy, hearing may remain stable, fluctuate, or progressively worsen, depending on the underlying factors. 

What are the roles of the hair cells?

Hair cells, which are divided into inner and outer, are part of the Organs of the Corti inside our ear and are sensory receptors, i.e., those responsible for converting acoustic information into nerve impulses. When we are born, hair cells are present in large numbers but cannot regenerate. Their damage, therefore, leads to a varying degree of reduction in hearing capacity with consequent alteration in the perception of sounds and comprehension of speech. We speak in this case of sensorineural hearing loss.

How to communicate with child?

Childhood auditory neuropathy is one of the most frequent neurosensory deficits that can limit a person's life. Speaking and socialising with a deaf child relies mainly on sign language. In addition to this, lip movements can also help to understand speech; this is why it is important that school staff and classmates articulate words well and emphasise mouth and tongue movements. Cooperation from everyone and proactive awareness is therefore necessary. 

Can auditory neuropathy go away?

Currently, due to the nature of the disorder, it is not possible to restore the damaged hair cells or peripheral nerves. In recent years, however, research is being carried out with stem cells, which would replace the damaged ones and use them auditory receptors. For the time being, these experiments are carried out on animals and it will probably take years before they can be used to treat deafness in humans.
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