In this section we explore the structure and function of the ear. How is hearing tested and management of hearing loss.
Structure and Function of the Ear
- Auricle: This is the outer part of the ear situated on either side of the head.
- Ear canal: This canal is lined by skin and passes from the hole in the auricle through cartilage and bone to the tympanic membrane (ear drum).
- Tympanic membrane: This is the thin transparent piece of skin which sits at the end of the ear canal
- This is a small, air-filled, bony chamber, that contains three small bones connecting the tympanic membrane to the inner ear.
This consists of two parts:
- Cochlea: This spiral shaped, fluid-filled structure has an elastic membrane along its entire length. Thousands of hair cells are aligned in 4 rows on top of the membrane. They are activated when the hairs on their upper end bend with resonating vibration. This “bending” is limited to the places where sound is resonating with the membrane. Each of the thousands of hair cells is connected to its own nerve and the brain is able to differentiate sounds based on where and which hair cells are triggered from the membrane’s resonance. This association is called “tonotopic coding of sound”.
- Semi circular canals: These 3 interconnecting tubes are also filled with fluid.
- Vestibulo-cochlear nerve: This nerve is connected to both the cochlea and the semi circular canals. Each nerve fibre in the cochlear part of this nerve is attached to just one hair cell. This nerve conducts messages to the hearing centre in the brain.
When a sound is present it sets up vibrations in the ear which pass through the ear canal to the tympanic membrane and the three small bones in the middle ear. Together these structures increase the force of the vibrations entering the cochlea. The hair cells bend with this resonating vibration and stimulate its attached nerve fibre. The nerve fibres then pass information about the sound to the part of the brain that deals with hearing.
The Usher Syndrome Ear
The affected genes that cause Usher Syndrome are also responsible for a number of proteins that are found in the hair cells of the inner ear. These proteins are required to ensure healthy growth of the hair cells. The hair cells are called stereocilia and are responsible for converting the vibrational waves of sound into the electrical signals sent to the brain. The lack of these proteins causes the majority of the stereocilia to be underdeveloped and non functional, therefore unable to convert the sound to electrical signals. When these electrical signals aren’t able to get through to the auditory nerve and then onto the brain, it results in deafness in Usher syndrome patients.
How hearing is tested in children
Newborn Infant Hearing Screening
All States and Territories in Australia have universal neonatal hearing screening programs. These programs screen the hearing of all newborn babies in their first few weeks of life. They aim to find out as early as possible whether a baby has a hearing loss. The screen is performed by trained hearing screeners using valid and reliable technology and is usually carried out at the baby’s bedside in hospital while they are asleep.
Auditory Brainstem Response (ABR)
The ABR is the most common test given to babies during the screen. Sensors are placed on the baby’s forehead and behind the ears with some sticky gel (see image). Clicking sounds are then played through an earpiece placed inside the baby’s ear while they are asleep. Brain responses to these sounds are recorded. These brain responses provide information about the softest level of sound the baby can hear. This information can help confirm whether further testing or further interventions (such as hearing aids) or special education may be recommended.
For older children, behavioural tests are used to determine a child’s response to sound. Each ear is tested separately with the ear not being tested covered with a soundproof earpiece. A series of sounds are produced by a machine and relayed to the ear being tested. The sounds vary according to their pitch or frequency (cycles/second) and loudness (decibel level). The loudness of the sound detected by the child is recorded for several frequency levels between 250 and 8000 cycles per second (see below images).
Hearing Tests Educational Video
Hearing Tests for Cochlear Implants Educational Video
Management of Hearing Loss
It is important to understand that each child has different needs when it comes to communication and these needs may change over time. The right approach to communication is one that works best for the individual child and their family. In consultation with a range of different support agencies and specialists, families can make informed decisions about the communication needs of their child. ‘Choices’ from Australian Hearing is a great resource.
Progressive audiological assessment
Neonatal screening for hearing loss takes place in all Australian states. Tests to monitor the progress of any detected hearing loss will be necessary to determine the appropriate time to provide further intervention.
Early fitting of
Consultation with an ENT surgeon and audiologist will be important when deciding whether your child will benefit from using hearing aids. Even though hearing aids will not provide enough sound for some children to acquire speech due to their degree of hearing loss, at the very least evidence suggests that early fitting of hearing aids helps to keep the auditory nerve healthy while investigating other intervention options.
Auslan is the sign language of the Australian Deaf community. It can be accessed through many Early Intervention and Education settings across Australia. It can be learnt in conjunction with, or exclusive of assisted hearing devices.