Auditory neuropathy (AN) is a hearing disorder in which the outer hair cells of the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...

are present and functional, but sound information is not transmitted sufficiently by the auditory nerve to the brain. The cause may be several dysfunctions of the inner hair cells of the cochlea or spiral ganglion neuron levels. Hearing loss with AN can range from normal hearing sensitivity to profound hearing loss. A

neuropathy Peripheral neuropathy, often shortened to neuropathy, refers to damage or disease affecting the nerves. Damage to nerves may impair sensation, movement, gland function, and/or organ function depending on which nerve fibers are affected. Neuropa ...

usually refers to a disease of the peripheral nerve or nerves, but the auditory nerve itself is not always affected in auditory neuropathy spectrum disorders. Prevalence in the population is relatively unknown. Neonates with high risk factors for hearing loss have a prevalence of up to 40% (Vignesh, Jaya, & Muraleedharan 2016). These high-risk factors are: hypoxia, low birth weight,

premature birth Preterm birth, also known as premature birth, is the birth of a baby at fewer than 37 weeks gestational age, as opposed to full-term delivery at approximately 40 weeks. Extreme preterm is less than 28 weeks, very early preterm birth is betwee ...

, hyperbilirubinemia,

jaundice Jaundice, also known as icterus, is a yellowish or, less frequently, greenish pigmentation of the skin and sclera due to high bilirubin levels. Jaundice in adults is typically a sign indicating the presence of underlying diseases involving ...

, and

aminoglycoside Aminoglycoside is a medicinal and bacteriologic category of traditional Gram-negative antibacterial medications that inhibit protein synthesis and contain as a portion of the molecule an amino-modified glycoside (sugar). The term can also refer ...

antibiotic treatments (NIDCD, 2018).

Possible sites of lesion

Based on clinical testing of subjects with auditory neuropathy, the disruption in the stream of sound information has been localized to one or more of three probable locations: the inner hair cells of the cochlea, the synapse between the inner hair cells and the auditory nerve, or a lesion of the ascending auditory nerve itself.

Diagnosing auditory neuropathy

Diagnosis is possible after a test battery, that must necessarily include the following: the

auditory brainstem response The auditory brainstem response (ABR), also called brainstem evoked response audiometry (BERA) or brainstem auditory evoked potentials (BAEPs) or brainstem auditory evoked responses (BAERs) is an auditory evoked potential extracted from ongoing ...

and otoacoustic emissions. Auditory brainstem response should be tested with both polarities (helps in identifying cochlear microphonics). Auditory neuropathy can be diagnosed with a battery of tests including otoacoustic emissions (OAE), auditory brainstem response (ABR), and acoustic reflexes. The classic AN paradigm would include present OAEs indicating normal outer hair cell function, absent or abnormal ABR with presence of the cochlear microphonic, and absent acoustic reflexes. Other tests would include pure-tone and speech audiometry. AN patients can have a range of hearing thresholds with difficulty in speech perception. Patients with auditory neuropathy spectrum disorders have to date never been shown to have normal middle ear muscle reflexes at 95 dB HL or less despite having normal otoacoustic emissions. Auditory neuropathy can occur spontaneously, or in combination with diseases like Charcot-Marie-Tooth disease and

Friedreich's ataxia Friedreich's ataxia (FRDA) is a rare, inherited, autosomal recessive neurodegenerative disorder that primarily affects the nervous system, causing progressive damage to the spinal cord, peripheral nerves, and cerebellum, leading to impaired ...

. AN can have either congenital or acquired causes. AN can be due to genetic factors in syndromic, non-syndromic, and mitochondrial related patterns. Approximately 40% of AN cases are estimated to have a genetic cause. It appears that regardless of the audiometric pattern (hearing thresholds) or of their function on traditional speech testing in quiet the vast majority of those affected have very poor hearing in background noise situations.

Residual auditory function

When testing the auditory system, there is no characteristic presentation on the audiogram. When diagnosing a patient with auditory neuropathy, there is no characteristic level of functioning either. Patients can present with relatively little dysfunction other than problems of hearing speech in noise, or can present as completely deaf and gaining no useful information from auditory signals.

Hearing aids A hearing aid is a device designed to improve hearing by making sound audible to a person with hearing loss. Hearing aids are classified as medical devices in most countries, and regulated by the respective regulations. Small audio amplifiers ...

are sometimes prescribed, with mixed success. FM systems in combination with hearing aids or cochlear implants could increase success of these amplification devices. Some people with auditory neuropathy obtain

cochlear implants A cochlear implant (CI) is a surgically implanted neuroprosthesis that provides a person who has moderate-to-profound sensorineural hearing loss with sound perception. With the help of therapy, cochlear implants may allow for improved speech unde ...

, also with mixed success.

Screening

Universal newborn hearing screenings are mandated in a majority of the United States. These screenings include OAE testing – otoacoustic emissions, which are sounds generated from the cochlea transmitted across the middle ear to the external ear canal, where they can be recorded. It assesses functionality of the cochlea, but not the auditory nerve. An ABR (auditory brainstem response) is required to assess that the sound is successfully transmitted through the nerve to the brain. In most parts of Australia, hearing screening via ABR testing is mandated, meaning that essentially all congenital (i.e., not those related to later onset degenerative disorders) auditory neuropathy cases should be diagnosed at birth. Auditory neuropathy is sometimes difficult to catch right away, even with these precautions in place. Parental suspicion of a hearing loss is a trustworthy screening tool for hearing loss, too; if it is suspected, that is sufficient reason to seek a hearing evaluation from an

audiologist Audiology (from Latin 'to hear'; and from Greek branch of learning , ''-logia'') is a branch of science that studies hearing, balance, and related disorders. Audiologists treat those with hearing loss and proactively prevent related damage. By ...

References

External links

Auditory neuropathy: What is it and what can we do about it?
from the LSU Medical School

*https://www.nidcd.nih.gov/health/auditory-neuropathy *https://www.audiology.org/tags/auditory-neuropathy-spectrum-disorder-ansd *http://www.asha.org/auditory-neuropathy *Auditory Neuropathy. (2018, March 16). Retrieved from https://www.nidcd.nih.gov/health/auditory- neuropathy *Interacoustics. (2016). Cochlear Microphonics CM. Retrieved from https://www.interacoustics.com/guides/test/abr-tests/cochlear-microphonics-cm {{DEFAULTSORT:Auditory Neuropathy Audiology Deafness Hearing loss