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.
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.
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.
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.
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 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.
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.
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.