Overview
Auditory neuropathy is a variety of hearing loss. In this condition the outer hair cells within the cochlea are present and functional, but sound information is not properly transmitted to the auditory nerve and the brain. This condition is also known as Auditory Neuropathy/Auditory Dys-synchrony (AN/AD) and Auditory Neuropathy Spectrum Disorder (ANSD).
A neuropathy usually refers to a disease of the peripheral nerve or nerves. Auditory nerve itself is not always affected in Auditory Neuropathy Spectrum Disorders.
Based on clinical testing, the disruption in the stream of sound information has been localized to one (or more) of 3 locations:
• inner hair cells of the cochlea
• synapse between the inner hair cells
• auditory nerve, or a lesion of the ascending auditory nerve
What exactly is auditory neuropathy?
Auditory neuropathy (AN) is a hearing disorder in which sound enters the inner ear, but the transmission of signal from the inner ear to the brain is impaired. It can affect patients of all ages. The exact number of patients that are affected by AN is not known. The condition affects a relatively small percentage of patients with deafness or hearing-impairment.
Sufferers of auditory neuropathy may have normal hearing or hearing loss (from mild to severe). Patients have poor speech-perception abilities and overall trouble understanding speech clearly.
Sometimes speech perception is worse than would be predicted by the degree of hearing loss. Patients with AN may be able to hear sounds, but would still have difficulty recognizing spoken word. Sounds may fade in and out for these individuals and seem out of sync.
Causes
Auditory neuropathy ethiology is not yet fully understood. Scientists believe the condition probably has more than one cause. In some cases it involves damage to the inner hair cells. These are specialized sensory cells in the inner ear and they transmit sound information through the nervous system and to the brain.
Other causes may include:
• faulty connections between inner hair cells and the nerve going from inner ear to brain
• nerve damage
• combination of these
Outer hair cells (the hair cells adjacent to and more numerous than the inner hair cells) are usually more prone to damage than inner hair cells. In most cases outer hair cells seem to function normally in patients with auditory neuropathy.
Outer hair cells help amplify sound vibrations entering the inner ear from the middle ear. When hearing is normal, the inner hair cells convert the vibrations into electrical signals. These travel as nerve impulses to the brain and the impulses are interpreted as sound by the brain.
Risk factors
Several factors have been linked to AN in children. However, a clear cause and effect relationship has not been proven. Children diagnosed with auditory neuropathy experience certain health complications and problems as newborns.
Such problems include, but are not limited to:
• jaundice
• premature birth
• low birth weight
• inadequate oxygen supply to the unborn
Some drugs that have been used to treat medical complications in pregnant women or newborns may damage the inner hair cells in the baby’s ears, causing auditory neuropathy.
In some cases auditory neuropathy runs in families and genetic factors may be involved. Some patients with auditory neuropathy have neurological disorders causing problems outside of the hearing system (i.e. Charcot-Marie-Tooth syndrome, Friedreich’s ataxia).
Diagnose
Health professionals (otolaryngologists, pediatricians audiologists) use a combination of methods to diagnose auditory neuropathy. These include various tests of auditory brainstem response (ABR) as well as otoacoustic emissions (OAE).
ABR test monitors the brain wave activity in response to sound. This method is using electrodes placed on the patients’s head and ears. OAE tests use small sensitive microphones inserted in the ear canal. These devices are used to monitor the faint sounds that are produced by the outer hair cells in response to stimulation done by a series of clicks. ABR and OAE testing are painless. They can be used for newborn babies and infants and for children and adults as well.
Treatment
Researchers are still seeking effective treatments for patients with auditory neuropathy. Doctors and professionals in the hearing field often differ in their opinions about the potential benefits of methods such as hearing aids, cochlear implants as well as other advanced technologies for patients with diagnosed auditory neuropathy. Hearing aids and personal listening devices like frequency modulation (FM) systems are helpful for some patients with auditory neuropathy.
Cochlear implants can also help some patients with auditory neuropathy.
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