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
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
Diagnostic 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.

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.

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, 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.

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, 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 ) 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.

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, 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, as well as very encouraging information about subsequent improvements in their speech perception abilities.

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.