Auditory neuropathy spectrum disorder (ANSD) is a type of hearing impairment caused by sounds not travelling to the brain effectively.
The ear consists of three parts, the outer ear, the middle ear and the inner ear. Sound waves enter the ear canal and cause the eardrum to vibrate. The sound then passes through the middle ear via the three small bones of hearing (ossicles) on to the inner ear, which is filled with fluid.
The movement of the fluid in the cochlea stimulates the hair cells inside it to trigger a nerve impulse, which is carried to the brain by the auditory nerve. The brain then interprets these nerve impulses as sound.
ANSD occurs due to damage to the hair cells inside the inner ear, abnormal connections between the hair cells and auditory nerve, damage to the auditory nerve itself or a combination of all three. This means that sounds are not transmitted to the brain through the auditory nerve as they should, causing hearing loss and difficulty interpreting sounds.
ANSD is a ‘spectrum disorder’, which means that the symptoms experienced vary from mild to severe. They may also vary over time, improving on some days or worsening on others.
What causes auditory neuropathy spectrum disorder?
Doctors are not sure exactly what causes auditory neuropathy spectrum disorder (ANSD) but think it may be linked to premature birth and/or low birthweight. Some cases seem to run in families so there may also be a genetic component, that is, a faulty gene is passed on from parent to child. ANSD also appears alongside other conditions, such as Friedrich’s Ataxia.
What are the signs and symptoms of auditory neuropathy spectrum disorder?
Children with auditory neuropathy spectrum disorder (ANSD) hear sounds differently, so all sounds may sound similar – like a television that has lost its signal – or they may sound distorted. This causes problems understanding speech, which in turn, leads to difficulty developing speech.
There is also a degree of hearing loss, from mild to severe. ANSD or another type of hearing loss is often suspected when a child does not start to babble or make sounds at the expected age.
How is auditory neuropathy spectrum disorder diagnosed?
The usual newborn hearing tests carried out soon after birth may not pick up auditory neuropathy spectrum disorder (ANSD) as the symptoms are so variable. Specialised hearing tests are a more reliable method of diagnosing ANSD although they cannot usually identify the severity of symptoms, only that they are present. The two main tests used to diagnose ANSD are otoacoustic emissions and auditory brainstem response (ABR) tests.
The oto-acoustic emissions test involves placing a small earpiece in the ear canal for about one minute. Clicking sounds are played through the earpiece and the response from the cochlea (inner ear) is recorded.
The auditory brainstem response (ABR) test involves placing small stickers which have special sensors on the skin to detect the electrical signal sent by the hearing nerves. Sounds are played through headphones, earphones or through a sound conductor placed behind the ear. This test shows the response of the hearing pathway up to the lower part of the brain.
The results of the tests will usually show that while a child may respond to sound, the hair cells in the inner ear and the auditory nerve are not responding. Often these tests are repeated as the child grows older as the auditory nerve may not be fully developed when a baby is born prematurely.
How is auditory neuropathy spectrum disorder treated?
Various types of hearing aids can be helpful in improving sound transmission to the brain, understanding speech and therefore encourage speech development. These can be regular hearing aids, a FM system that transmits sound from a microphone to the ear or cochlear implants. In addition to hearing aids, communication aids may be suggested as well. This can include signing using Makaton® or British Sign Language and/or lip reading. Your doctor will discuss which options are best suited to your child.
Children with ANSD will require regular check-ups and repeated tests to monitor symptoms.
What happens next?
The level of hearing loss seems to change, with some children improving over time but some getting worse. With the support of hearing and communication aids, children can develop and attend mainstream school with support.