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. A core component of the cochlea is the Organ of Corti, the sensory orga ...

are present and functional, but sound information is not transmitted sufficiently by the

auditory nerve The cochlear nerve (also auditory nerve or acoustic nerve) is one of two parts of the vestibulocochlear nerve, a cranial nerve present in amniotes, the other part being the vestibular nerve. The cochlear nerve carries auditory sensory information ...

to the brain. Hearing loss with AN can range from normal hearing sensitivity to profound hearing loss. A

neuropathy Peripheral neuropathy, often shortened to neuropathy, is a general term describing disease affecting the peripheral nerves, meaning nerves beyond the brain and spinal cord. Damage to peripheral nerves may impair sensation, movement, gland, or o ...

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 Hypoxia means a lower than normal level of oxygen, and may refer to: Reduced or insufficient oxygen * Hypoxia (environmental), abnormally low oxygen content of the specific environment * Hypoxia (medical), abnormally low level of oxygen in the t ...

low birth weight Low birth weight (LBW) is defined by the World Health Organization as a birth weight of an infant of or less, regardless of gestational age. Infants born with LBW have added health risks which require close management, often in a neonatal intensi ...

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 between 2 ...

hyperbilirubinemia Bilirubin (BR) (Latin for "red bile") is a red-orange compound that occurs in the normal catabolic pathway that breaks down heme in vertebrates. This catabolism is a necessary process in the body's clearance of waste products that arise from the ...

jaundice Jaundice, also known as icterus, is a yellowish or 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 abnormal heme me ...

, 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), is an auditory evoked potential extracted from ongoing electrical activity in the brain and recorded via electrodes placed on the scalp. The measured r ...

and

otoacoustic emissions An otoacoustic emission (OAE) is a sound that is generated from within the inner ear. Having been predicted by Austrian astrophysicist Thomas Gold in 1948, its existence was first demonstrated experimentally by British physicist David Kemp in 1978 ...

. 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 or FA) is an autosomal-recessive genetic disease that causes difficulty walking, a loss of sensation in the arms and legs, and impaired speech that worsens over time. Symptoms generally start between 5 and 20 year ...

. 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 really is no characteristic presentation on the audiogram. When diagnosing someone with auditory neuropathy, there is no characteristic level of functioning either. People can present 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 su ...

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. In most parts of Australia, hearing screening via AABR 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 , '' -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 employing vario ...

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