Auditory neuropathy

Auditory neuropathy is a variety of hearing loss in which the outer hair cells within the cochlea are present and functional, but sound information is not faithfully transmitted to the auditory nerve and brain properly.

A neuropathy simply refers to a disease of the peripheral nerve or nerves.

"Hearing" Simplified

Infobox Anatomy
Name = Eardrum


Caption = Anatomy of the human ear. |Sound's pathway from environment to cortex involves many levels of processing. Sounds produced by a source travel through the air and are funneled into the ear canal and to the ear drum membrane. Vibrations of the ear drum membrane are transmitted through the ossicles to the inner ear- the cochlea. The motion of the ossicles induce waves of vibrations that disturb the fluid within the cochlea. The cochlea contains the hearing organ, the Organ of Corti, which is stimulated by these vibrations. Outer hair cells and inner hair cells are both structures embedded in the Organ of Corti. Outer hair cells add energy to the small fluid vibrations within the cochlea and induce movement of the nearby inner hair cells. Movement of the inner hair cells create electrical potentials which code vibrations as neural impulses, the language of the brain. Successful transmission of sound vibrations from the peripheral to central structures results in sound awareness.

Possible Sites of Lesion

Infobox_neuron
neuron_name = Hair cell
image_neuron = Gray931.png align="left"
caption_neuron = Three rows of Outer Hair Cells in the Organ of Corti, one row of Inner Hair Cells.
GraySubject = 232
GrayPage = 1057Diagnostic test results from subjects with auditory neuropathy localize the disruption of sound information to one or more of three probable places: 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 cite journal|last=Starr
first=A.
coauthors=Picton, T.W.; Sininger, Y.; Hood, L.J.; Berlin, C.I.
year=1996
title=Auditory Neuropathy
journal=Brain.
volume=119
pages=741–753
doi=10.1093/brain/119.3.741
pmid=8673487] .

Diagnosing Auditory Neuropathy

This is possible through a battery of tests. Two tests are always included in this battery: the auditory brainstem response and otoacoustic emissions.

Auditory neuropathy is diagnosed when a person is found to have present Otoacoustic Emissions in combination with absent or abnormal Auditory Brainstem Response. This is counterintuitive to an audiologist, as present Otoacoustic Emissions are typically associated with normal hearing while an absent Auditory Brainstem Response would suggest significant hearing loss cite journal|last=Stapells
first=D.R.
coauthors=Oates, P.
year=1997
title= Estimation of the pure-tone audiogram by the auditory brainstem response: a review
journal= Audiology & Neurotology.
volume=2
pages=257–280.] .

Neither Auditory Brainstem Response nor Otoacoustic Emissions are true tests of hearing. They test the structural integrity of the ear-brain connection.

The Auditory Brainstem Response is widely used as a reasonable predictor of hearing ability and is one of the two major screening tools used by hospitals and clinics to test the hearing of newborn babies (Otoacoustic Emissions are the other). Auditory neuropathy presents a case where the Auditory Brainstem Response cannot be used to reliably predict a person’s audiogram. Despite an absent or distorted Auditory Brainstem Response, individuals with Auditory Neuropathy can display a cortical awareness of sound and sometimes demonstrate some degree of usable hearing.

Sometimes people with Auditory Neuropathy are subsequently diagnosed with diseases such as Charcot-Marie-Tooth disease and Friedreich's ataxia. In these cases, Auditory Neuropathy may be a symptom of the more global effects these diseases rather than an isolated neuropathy of the ear-brain connection. Diseases such as these are known to clinically manifest with absent or distorted ABRs cite journal|last=Butinar
first=D.
coauthors= Starr, A.; Vatovec, J.
year=2000
title= Brainstem auditory evoked potentials and cochlear microphonics in the HMSN family with auditory neuropathy
journal= Pflugers Archiv: European journal of physiology
volume=439
pages=204–205.
doi= 10.1007/s004240000146] cite journal|last=DePablos
first=C.
coauthors= Berciano, J.; Calleja, J.
year=1991
title= Brain-stem auditory evoked potentials and blink reflex in Friedreich's ataxia
journal= Journal of Neurology
volume=238
pages=212–216
doi= 10.1007/BF00314783] , a result of axonal demyelination, neural degeneration, or neuronal death within the peripheral nervous system, including the auditory nerve.

Residual Auditory Function

There is no characteristic audiogram or degree of hearing loss associated with Auditory Neuropathy. Evidence from several studies indicate that various individuals have demonstrated a wide array of audiometrically charted hearing losses, including permanent or fluctuating, unilateral or bilateral, stable or progressive, progressively worsening or progressively improving, as well as spontaneous recovery cite journal|last=Rance
first=G.
coauthors=Aud, D.
year=2005
title=Auditory Neuropathy/Dys-synchrony and Its Perceptual Consequences
journal=Trends in Amplification
volume=9
pages=1–43
doi=10.1177/108471380500900102
pmid=15920648] .

Hearing aids are usually not beneficial to individuals with Auditory Neuropathy. There are subjective reports in the Auditory Neuropathy literature from adults with Auditory Neuropathy who report that they have awareness of people speaking, but their discrimination of the specific words isn’t there. (See the external link for an interesting simulation of how auditory neuropathy might sound). Sometimes hearing aids and other assistive listening devices help an individual with Auditory Neuropathy compensate for listening difficulty by increasing the loudness of speech over competing noise cite journal|last=Doyle
first=K.J.
coauthors=Sininger, Y.; Starr, A.
year=1998
title=Auditory Neuropathy in Childhood
journal=The Laryngoscope
volume=108
pages=1374–1377
doi=10.1097/00005537-199809000-00022] cite jouranl|last=Rance
first=G.
coauthors=Beer, D.E.; Cone-Wesson, B.; et al.
year=1999
title=Clinical Findings for a Group of Infants and Young Children with Auditory Neuropathy.
journal=Ear & Hearing
volume=20
pages=238-252] , but the majority of people with Auditory Neuropathy find that hearing aids do not improve their understanding of speech. Hearing aids are not designed to treat the underlying problem of Auditory Neuropathy --the dys-synchronous neural activity of the ear-brain connection. Hearing aids provide amplification of sound signals, they do not fix temporal distortion of the sound signal.

People with Auditory Neuropathy are sometimes mis-diagnosed as having Auditory Processing Disorder; however, a test of the Acoustic reflex (which is almost always absent or elevated for people with Auditory Neuropathy cite journal|last=Berlin
first=C.I.
coauthors=Hood, L.J.; Morlet, T.; Wilensky, D.; St. John, P.; Montgomery, E.; Thibodaux, M.
year=2005
title=Absent or elevated Middle Ear Muscle Reflexes in the presence of Normal Otoacoustic Emissions: A Universal finding in 136 cases of Auditory Neuropathy/Dys-synchrony
journal=Journal American Academy of Audiology
volume=16
pages=546–553
doi=10.3766/jaaa.16.8.3] ) one can get a quick index of whether to follow up with an Auditory Brainstem Response to differentiate Auditory Processing Disorder from Auditory Neuropathy. Auditory Neuropathy patients will have absent or severely dys-synchronous Auditory Brainstem Response while Auditory Processing Disorder patients will show normal Auditory Brainstem Response both to a positive polarity (condensation) and negative polarity (rarefaction) clicks cite journal|last=Berlin
first=C.I.
coauthors=Bordelon, J.; St. John, P.; Wilensky, D.; Hurley, A.; Kluka, E.; Hood, L.J.
year=1998
title=Reversing Click Polarity may uncover Auditory Neuropathy in Infants
journal=Ear & Hearing
volume=19
pages=37
doi=10.1097/00003446-199802000-00002] .

Some people with AN are helped by cochlear implants. Conceptually, if Auditory Neuropathy is a result of inner hair cell damage or an improper synapse across the junction to the auditory nerve, success with a cochlear implant would be related to the fact that the implant's electrodes bypass those elements and stimulate the nerve directly. There are reports of unfavorable outcomes of cochlear implantation in patients with Auditory Neuropathy cite journal|last=Rance
first=G.
coauthors=Beer, D.E.; Cone-Wesson, B.; et al.
year=1999
title=Clinical Findings for a Group of Infants and Young Children with Auditory Neuropathy
journal=Ear & Hearing
volume=20
pages=238–252
doi=10.1097/00003446-199906000-00006] , leading to skepticism of implantation as a viable treatment for the disorder. However, more recent accounts of successful implantation of children with Auditory Neuropathy exist cite journal|last=Shallop
first=J.K.
coauthors=Peterson, A; Facer, G.W.; et al.
year=2001
title=Cochlear Implants in Five Cases of Auditory Neuropathy: Postoperative Findings and Progress
journal=The Laryngoscope
volume=111
pages=555–562
doi=10.1097/00005537-200104000-00001] cite journal|last=Mason
first=J.C.
coauthors=De Michele, A.; Stevens, C.; et al.
year=2003
title=Cochlear Implantation in Patients With Auditory Neuropathy of Varied Etiologies
journal=The Laryngoscope
volume=113
pages=45–49
doi=10.1097/00005537-200301000-00009] , as well as very encouraging information about subsequent improvements in their speech perception abilities cite journal|last=Zeng
first=F.G.
coauthors=Liu, S.
year=2006
title=Speech perception in individuals with auditory neuropathy
journal=Journal of Speech, Language, and Hearing Research
volume=49
pages=367–380] .

Conclusion

Even with Universal Newborn Hearing Screenings (UNHS) being mandated in 37 states in the USA, sometimes children with hearing loss remain undetected at birth. Auditory neuropathy is a prime example, as most UNHS programs utilize either the Auditory Brainstem Response test or the Otoacoustic Emissions test as a screening tool. Rarely are the two used in combination due to the staggering cost necessary to implement such a program. This is an unfortunate reality for babies with Auditory Neuropathy- it is the distinct combination of present Otoacoustic Emissions and absent Auditory Brainstem Response that usually send up the initial red flags. However, parental suspicion of a hearing loss continues to be a trustworthy screening tool in the unfortunate event that Auditory Neuropathy would go undetected at birth. If a parent suspects a hearing loss, that is sufficient reason to seek professional assessment of the child by an audiologist. The sophisticated family physician will recognize the need to refer to an audiologist if a parent expresses concern about hearing loss or speech delays. An audiologist is a specialist in the domain of hearing assessment, hearing loss, and aural rehabilitation. The audiologist can be extremely useful in the provision of information needed to guide newly diagnosed families.

ee also

*Sensorineural hearing impairment
*Auditory processing disorder

References

External links

* [http://www.medschool.lsuhsc.edu/Otorhinolaryngology/deafness_article1.asp Auditory neuropathy: What is it and what can we do about it?] from the LSU Medical School
* [http://auditoryneuropathy.com/ANindex.html Auditory Neuropathy Information site]
* [http://www.dizziness-and-balance.com/disorders/hearing/aud_neuropathy.html Auditory Neuropathy by Timothy C. Hain, MD]
* [http://www.ucihs.uci.edu/hesp/Simulations/simulationsmain.htm A simulation of what individuals with Auditory Neuropathy may perceive as speech]