Hearing loss - auditory neuropathy

Auditory neuropathy is a rare form of hearing loss caused by disruption of the nerve impulses travelling from the inner ear to the brain. What triggers it is often unknown, and there is no cure. Both ears are usually affected, and the hearing loss ranges from mild to severe. It is sometimes referred to as a form of neural or nerve deafness.

In some cases, the affected person can hear but has difficulty understanding spoken words, particularly in noisy environments. Hearing and understanding speech may be improved by the use of hearing aids or cochlear implants.

The symptoms vary from one person to the next and can fluctuate, so a person may experience both improvements and deteriorations in their hearing. The long-term outlook varies too: the hearing of someone diagnosed with auditory neuropathy may improve, deteriorate or remain unchanged, and there is no way to predict the outcome.

Symptoms#

Symptoms of auditory neuropathy can include:

• Mild to severe hearing loss
• Sounds fading in and out
• Difficulty understanding spoken words (poor speech perception)
• Normal hearing but with poor speech perception
• Worse speech perception in noisy environments

Many of these symptoms can also be caused by more common types of hearing loss.

How we hear#

The different structures involved in hearing include the outer, middle and inner ear:

• The outer ear is the part you can see. It funnels sound into the middle ear.
• The middle ear is separated from the outer ear by the eardrum, which moves in response to sound waves. This movement is amplified by tiny bones in the middle ear.
• The amplified sound waves are picked up by a small spiral-shaped organ called the cochlea in the inner ear. Tiny hair-like nerve endings in the cochlea sense the vibration and turn it into electrical impulses.
• These impulses are sent by the cochlear nerve along the eighth cranial nerve to the auditory pathways in the brain stem. The brain stem sits at the base of the skull and acts as a ‘bridge’ between the brain and the spinal cord.
• The brain interprets the electrical impulses and the sound is ‘heard’.

What causes auditory neuropathy?#

When someone has auditory neuropathy, some of the structures within the ear may be working perfectly, but the information from the ear is distorted on its way to the brain. The exact cause is not known, but researchers suspect that several factors may be involved, including damage to:

• The hair cells in the inner ear
• The connections between the hair cells and the cochlear nerve
• The cochlear nerve and the eighth cranial nerve
• The auditory pathways in the brain stem

Anyone can be affected, but known risk factors for auditory neuropathy in children include:

• Health problems before birth, such as reduced oxygen supply to the fetus
• Particular medications
• Health problems at birth, such as premature birth, jaundice or low birth weight
• Other neurological disorders, such as Charcot-Marie-Tooth disease or Friedreich’s ataxia
• A genetic component – some children with auditory neuropathy have similarly affected family members

Diagnosis#

Auditory neuropathy is diagnosed using a number of tests, including:

• Auditory brainstem response (ABR) – electrodes on the ears and head detect brain wave patterns when the person is exposed to a series of sounds. A person with auditory neuropathy typically has little or no response.
• Otoacoustic emissions (OAE) – a tiny microphone placed in the ear canal checks whether the cochlear hair cells are working by picking up the faint sounds they make in response to noise. A person with auditory neuropathy typically has some properly functioning hair cells.
• Other hearing tests, such as speech recognition. A person with auditory neuropathy generally performs poorly when the test is given with background noise.
• Other tests, to rule out conditions that may produce similar symptoms, such as multiple sclerosis.

Treatment#

There is no cure for auditory neuropathy. Because the condition is so variable, some treatments work in some cases and not in others. There is no test to predict whether a particular treatment will help, so the person and their hearing specialist may have to experiment to find out. Treatment options include:

• Cochlear implant – a surgically implanted device that stimulates the nerves of the inner ear and has been shown to help many people improve their ability to understand speech.
• Frequency modulation (FM) systems – a portable receiver and headset that amplifies sounds without the need for wiring.
• Hearing aids – to amplify all sounds. Generally, adults with auditory neuropathy do not find hearing aids particularly helpful.

Communication#

There are differing views on how best to teach children with auditory neuropathy to communicate. One approach advocates teaching the child sign language; another suggests fitting the child with devices such as hearing aids and supporting them to learn to listen and speak. Some professionals believe a combination is best, because there is no way of knowing whether the child’s hearing will improve, stay the same or deteriorate. Exposure to spoken language is important in case the child’s hearing improves.

For adults who already have spoken language skills, learning to use supplemental cues such as lip reading may help.

Key points#

• Auditory neuropathy is a rare hearing loss caused by disruption of nerve impulses travelling from the inner ear to the brain. Its cause is often unknown and there is no cure.
• Symptoms vary from person to person and can fluctuate.
• Symptoms can include worse speech perception in noisy environments and sounds fading in and out.
• Many symptoms can also be caused by more common types of hearing loss.
• The condition is variable, so some treatments work in some cases but not others.

Where to get help#