Auditory neuropathy spectrum disorder (ANSD)

What is ANSD

Auditory neuropathy spectrum disorder (ANSD) is a relatively rare form of sensorineural deafness (deafness in the inner ear). ANSD affects about one to two babies in every 10,000 born and occurs in about 1 in 10 children with sensorineural deafness.

ANSD occurs when sounds are received normally by the cochlea (inner ear) but become disrupted as they travel to the brain.

Children with ANSD are likely to have greater difficulty understanding speech and distinguishing one sound from another, especially when there’s background noise. The difficulty they experience is more than would be expected for a child with a similar level of hearing. Whilst some children with ANSD will have exceptionally poor speech recognition for their level of hearing, there may be others who will hear in a similar way to another child with ‘typical’ sensorineural deafness.

A similar experience would be using a mobile phone when the reception is poor and the sounds they hear are distorted.

Typical features of ANSD

• hearing that may vary from normal hearing thresholds to profound deafness
• hearing that may change over time: it may improve, deteriorate (be progressive), or fluctuate on a day-to-day basis
• difficulty understanding speech, especially with background noise
• speech recognition that is worse than predicted for the child’s level of hearing
• difficulty in hearing rapid changes in speech (difficulty following conversation)
• fluctuations in hearing related to body temperature (for example, a fever) has been found to affect a few children with ANSD

Why is ANSD called a spectrum disorder?

ANSD is called a spectrum disorder because if affects children in different ways, with symptoms ranging from mild to severe.

How ANSD affects the hearing system

ANSD is a type of sensorineural deafness. Sensorineural deafness occurs when the inner ear (cochlea) or hearing nerve is not working as well as it should.

The cochlea contains thousands of sound sensitive ‘inner’ and ‘outer’ hair cells. The outer hair cells help amplify the sound vibrations which travel from the middle ear to the inner ear. The inner hair cells then convert these vibrations into electrical signals which then travel along the auditory nerve as impulses to the brain. The brain then interprets these impulses as sound. Find out more about how the ear works.

With ANSD, the outer hair cells appear to function normally. ANSD can affect one or more of the following parts of your hearing system:

• the inner hair cells
• the synaptic junction of the inner hair cell and the junction of the auditory nerve
• dys-synchrony of the auditory nerve causing disruption to the signal as it travels along the nerve to the brain
• underdeveloped or absent auditory (hearing) nerve
• impaired temporal resolution (the ability to perceive rapid changes in auditory signals over time)
• auditory neurons in the spiral ganglion
• brainstem auditory nuclei.

ANSD usually affects both ears (bilateral) but can also affect one ear only (unilateral).

Causes of ANSD

ANSD happens more frequently in babies who have spent time in neonatal intensive care units (NICU) and is particularly common in babies born extremely prematurely (less than 26 weeks gestation).

Premature babies are more at risk of having ANSD because they are prone to:

• infections
• severe jaundice (hyperbilirubinaemia)
• lack of oxygen or a period of dependency on a ventilator
• some drugs used to treat neonatal infections are known to be ototoxic (meaning they can cause damage to the inner ear), which could cause deafness and ANSD.

Other possible causes may include:

• Genetics
  ANSD may be genetic (inherited), even when there is no family history of deafness. The Otoferlin gene, necessary for normal hearing, is affected in people with ANSD, and this can be inherited. There also appears to be an association between the Connexin 26 gene (one of the most common causes of congenital sensorineural deafness) and ANSD.
  
  
• Pre-natal infections
  ANSD may also be associated with certain infections during the pregnancy such as Cytomegalovirus (CMV).
  
  
• Post-natal infections
  ANSD has also been associated with infections due to various viruses such as measles, mumps and meningitis.
  
  
• Neurological conditions
  ANSD may develop later in childhood for the first time. This is rarer, but may be associated with neurological conditions, for example Charcot-Marie-Tooth Disease and Friedreich’s Ataxia.

The reason why children have ANSD is not always known, but there is lots of research happening in this area. For more information on the medical and genetic tests used to try and identify the cause of deafness, read the information on our genetic counselling pages and our booklet, Understanding your child’s hearing tests (PDF).

Testing for ANSD

ANSD is characterised by absent or abnormal auditory brainstem responses (ABR) in the presence of functioning outer hair cells.

There are several types of hearing tests that are used in combination to diagnose ANSD. These tests are usually carried out for babies who fail their newborn hearing screen or who are considered at risk for ANSD.

You can find out more about what each of the following tests involves in the hearing tests section of our website.

Auditory brainstem response (ABR) test

The ABR test measures whether sound is being sent from the cochlea (inner ear) and through the auditory (hearing) nerve to the brain. A child with ANSD will have an abnormal or absent ABR. This may be due to:

• damage to or absence of the inner hair cells of the cochlea
• the inner hair cells not working together and sending a dyssynchronous (disordered) message to the nerves
• damage to or poor development of the auditory nerve.

Some babies, particularly premature babies, may have an underdeveloped auditory nerve, so the ABR test should be carried out again once the baby’s hearing system has had time to develop.

Otoacoustic emissions (OAE) test

Otoacoustic emissions (OAEs) are sounds that are generated from the outer hair cells in a healthy cochlea. OAE testing provides an indication as to how well the outer hair cells of the cochlea are working. With ANSD, the outer hair cells are not affected and so the OAE test results will be normal.

OAEs may disappear over time, even for children with normal hearing; the reasons for this are not yet fully understood.

Cochlear microphonic (CM) test

Like the OAE test, a CM test also measures how well the cochlea’s outer hair cells are working. CM testing is carried out as part of the tests to diagnose ANSD.

Tympanometry and stapedial (middle ear) reflex testing

This test checks how well the moving parts of the middle ear are working. The middle ear muscles contract in response to loud sounds, and this reflex can be measured as part of the hearing assessment. Where there is a problem with the hearing nerve, as with ANSD, the stapedial reflexes are often absent or elevated.

Behavioural audiometry

Behavioural hearing tests are used for children older than five or six months. These tests use toys and play as part of the hearing assessment and involve your child listening for a variety of sounds as part of a game. These tests will tell your audiologist more about what your child actually hears, processes and responds to.

Imaging studies

Imaging studies, such as an MRI, can be used to examine the anatomy of the inner ear and auditory nerve to see if there are any defects or abnormalities.

Diagnosing ANSD

Diagnosis in babies

Babies who have spent time in neonatal intensive care units (NICU) may be diagnosed soon after birth because both OAE and ABR testing is carried out as part of their routine hearing screening.

Babies who are born well and have not spent any time in special care or NICU usually have just OAE testing. OAE testing alone will not identify ANSD, so if you have concerns about your baby’s hearing or speech development, you should raise this with your child’s doctor. They may refer you to an audiologist for further testing.

Delayed maturation/myelination

Some premature babies are diagnosed with ANSD but show improvement in their hearing over time.

This may happen when a very premature baby is born before the myelination of their auditory nerve has completed. Myelin is an insulating layer that forms around nerves, including the nerves in the hearing system. This myelin sheath allows nerve impulses to transmit quickly and efficiently along the nerve cells. Myelination of the auditory nerve happens at about 26–29 weeks gestation before birth.

If myelination is not complete, a baby’s ABR test will not give an accurate recording of the function of the hearing nerve. As the nerve matures, some of these babies may show improvement in their hearing over time. This spontaneous improvement has been seen in hearing thresholds up to around two years of age.

It is therefore essential to monitor babies and young children with ANSD closely to distinguish ANSD from delayed maturation in premature babies where possible.

Diagnosis in children

Otoacoustic emissions may reduce or disappear altogether with time in children with ANSD.

If your child has ANSD, the ABR test is unable indicate the level at which they respond to sound. So, as your child grows older, their audiologist will be able to get more information about their hearing using behavioural hearing tests.

How to communicate with your child

Children with ANSD need support at an early age to help them develop language and communication skills. Here are some ways you can help your child communicate:

• Use visual support
  Your child is likely to benefit from visual support (such as cued speech or sign language) together with spoken language, especially in their early years. This will help to ease your child’s frustration at not being able to communicate when they are very young.
  
  
• Consider using hearing technology
  Many children with ANSD also benefit from hearing technology, such as hearing aids or cochlear implants (see technology for children with ANSD).
  
  
• Reduce background noise
  Background noise is particularly a problem for children with ANSD. Try to reduce background noise as much as possible when talking with your child, for example, by turning off the TV or radio. You can also try using soft furnishings, as well as wall and floor coverings to reduce echoes. Find out more about improving your child’s listening environment.

Most children with ANSD do go on to develop good spoken language. However, some children with ANSD, even those wearing good-quality hearing aids, will not be able to hear enough speech sounds for them to be able to develop speech themselves.

For further information on supporting communication development and the different approaches available, visit the communication section of our website.

When a child has unilateral ANSD (affecting just one ear)

Children with unilateral ANSD should be monitored closely to ensure that it is not affecting their language, communication, educational and social development.

If a child has very poor speech discrimination abilities in the ear with ANSD, then they are unlikely to benefit from using a hearing aid but may find a personal radio aid or soundfield system useful. There are some situations where an older child will choose to wear an earplug in the ear with ANSD because they find sound quality on that side can be very distracting.

Technology for children with ANSD

Hearing aids

It is very difficult to predict whether a child will benefit from wearing a hearing aid. About 50% of children with ANSD benefit from some form of amplification, whilst others find hearing aids unhelpful as they are unable to overcome the distortion caused by ANSD. In addition, although hearing aids improve sound audibility, they do not resolve the problem of understanding speech that a child with ANSD may experience.

Because ABR thresholds are unreliable in children with ANSD, hearing aids are fitted based on behavioural hearing tests instead. This means your child’s hearing aid fitting may be delayed until they are old enough and developmentally ready to undertake a behavioural hearing test.

Some parents and audiologists choose to wait to fit hearing aids until their child’s hearing threshold levels are stable and consistent.

The professionals working with your child will want to regularly check your child’s hearing levels when they are wearing their hearing aids, as well as check their understanding of speech and their speech development.

Cochlear implants

If your child has an intact and developed auditory nerve, but has not benefited from using hearing aids, then you could consider having them assessed for a cochlear implant.

Cochlear implants can overcome the problems of dys-synchrony in the auditory nerve because they give direct electrical stimulation to the hearing nerves. Therefore, children with ANSD may be offered a cochlear implant even if they have hearing thesholds better than the usual criteria for needing one.

Children with ANSD usually get similar levels of benefit from cochlear implants as children with typical severe profound sensorineural deafness.

Auditory brainstem implants (ABI)

Children born with very thin or absent auditory nerves may be considered for an auditory brainstem implant.

ABIs are still a very new development. Working in a similar way to cochlear implants, an ABI stimulates the auditory brainstem directly, bypassing the ear and auditory nerve to provide a sensation of hearing.

An ABI does not provide the same benefit or sound as that perceived by a cochlear implant. However, ABIs can help aid lip-reading and provide other clues used in spoken communication (for example, the rhythm and speed of speech).

Other technology

There is a wide range of listening and alerting devices that are beneficial for children with ANSD.

Pre-school children and school-age children often benefit from using a radio aid system. This is when the teacher or parent wears a microphone (transmitter) and the child picks up the voice directly with a receiver, hearing the teacher’s voice through an ear-piece or directly through their hearing aids or cochlear implants.

Soundfield systems, where the teacher’s voice is fed through a loudspeaker, may also be helpful in the classroom. These systems help to raise the speaker’s voice to a comfortable listening level above the background noise.

See further information about radio aids and soundfield systems.