Audiometry involves the testing of hearing. There are various different ways to test hearing depending on the age of the person involved and the type of hearing loss. Note: the information below is a general guide only. The arrangements, and the way tests are performed, may vary between different hospitals. Always follow the instructions given by your doctor or local hospital.
Structure of the ear
The ear is roughly divided into three parts. The external (outer) ear includes the part you can see, called the pinna, and the narrow tube-like structure – the ear canal. At the end of the canal is the eardrum. This separates the external ear from the middle ear. The eardrum is a tightly stretched membrane, a bit like the skin of a drum.
The middle ear is an air-filled compartment. Inside it are the three smallest bones in the body, called the malleus, incus and stapes. These bones are connected to each other. The last in the group, stapes, also makes contact with the internal (inner) ear. The air space of the middle ear connects to the back of the nose by the Eustachian tube.
The inner ear is made up of two components. The cochlea is involved with hearing. The vestibular system helps with balance. The cochlea is a snail-shaped chamber filled with fluid. It is lined with special sensory cells called hair cells. These cells transform sound waves into electrical signals. The cochlea is attached to a nerve that leads to the brain.
The vestibular system is made up of a network of tubes, called the semicircular canals, plus the vestibule. The vestibular system also contains special sensory cells, but here they detect movement instead of sound. Both the cochlea and the vestibular system are connected to a nerve which carries electrical signals to the brain.
How do we hear?
Sound waves are created when air vibrates. To hear, the ear must change sound into electrical signals which the brain can interpret. The pinna (outer part of the ear) funnels sound waves into the ear canal. When sound waves reach the eardrum they cause it to vibrate. Vibrations of the eardrum cause the tiny bones in the middle ear to move too. The last of these bones, stapes, passes on the vibrations to the cochlea. When the cochlea receives the vibrations, the fluid inside it moves. As the fluid moves it causes the special sensory cells to create an electrical signal. This electrical signal is sent to the brain. Special areas in the brain receive these signals and translate them into what we know as sound.
Your ears create electrical signals that represent an extraordinary variety of sounds. For example, the speed at which the eardrum vibrates varies with different types of sound. With low-pitched sounds the eardrum vibrates slowly. With high-pitched sounds it vibrates faster. This means that the special hair cells in the cochlea also vibrate at varying speeds. This causes different signals to be sent to the brain. This is one of the ways we are able to distinguish between a wide range of sounds.
What is audiometry?
Audiometry is the medical word used to mean hearing tests. Audiometry checks whether there are problems with any of the different mechanisms that allow us to hear sound.
What can cause hearing loss?
Damage to any part of the ear can cause a hearing loss. If there is a problem in the ear canal or the middle ear, this causes what is known as a conductive hearing loss. In conductive hearing loss, the movement of sound (conduction) is blocked or does not pass into the inner ear.There are several causes of conductive hearing losses and this may result in a permanent or temporary hearing loss. A common cause in children is the build-up of fluid in the middle ear space, otherwise known as glue ear - for more information see separate leaflet called 'Glue Ear'.
If the cochlea or hearing nerve is not working properly this causes what is known as a sensorineural hearing loss. Usually this means that hair cells in the cochlea are not working properly or there is a problem with the hearing nerve so that some or all of the sounds are not being sent to the brain. Sensorineural hearing losses are usually permanent. They can be mild, moderate, severe or profound and affect one or both ears.
It is also possible for sensorineural and conductive hearing losses to occur together in a mixed hearing loss.
How does audiometry work?
There are various types of tests that can be carried out to check hearing ability. They vary according to who is being tested and why. Babies are obviously not able to say when they have heard a sound, so special methods are used. In children the principles of testing may be the same, but the way in which the tests are carried out may be varied to get the most accurate results. There are also additional tests which help to check how well the middle ear and the brain are working in the hearing pathway.
The most common tests of hearing are described below.
Testing newborn babies
The otoacoustic emissions screen (OAES) is a quick, simple and painless way to screen newborns for hearing loss. A small probe containing a microphone and a mini-loudspeaker is placed in the ear. The loudspeaker makes clicking sounds in the ear. These are passed to the cochlea. If the cochlea is working normally, it responds sending a sound back to the ear canal. This is detected by the microphone. The test is so sensitive that even a slight hearing loss can be detected.
Sometimes, however, this response cannot be detected due to fluid in the middle part of the ear. In this case the result does not necessarily mean the cochlea is not functioning normally. Therefore, this test is sometimes followed up with another type of test called an auditory brainstem response (ABR) test.
In an ABR test a small earphone plays clicks into the baby's ear. If the baby can hear the click, a signal in the hearing nerve can be measured from sensors that are placed on the baby's skin. The loudness of the clicks is set to a particular level. If this does not produce a response, further diagnostic testing will be required.
Both OAE and ABR testing require the child to be asleep as the response to be detected is very small and can be hidden if there is a lot of movement.
Testing in babies and young children
In young children a technique called visual reinforcement audiometry is used. In this test the child hears sounds, usually through speakers in the testing room. When the child hears the sound and turns his/her head towards it he/she is given a reward. Usually this is a visual reward such as the flashing lights of a toy. The person testing the child's hearing continues reinforcing this behaviour with a reward every time the child turns towards a sound. Then the person carrying out the test begins to assess the child's hearing by seeing if he/she responds to different types of sound. By doing this it is possible to find the quietest sound the child can hear.
Different variations of this reward-based test are used as a child gets older and finds it easier to communicate.
Testing in older children and adults
Testing in adults mainly uses a technique called pure tone audiometry. This uses a machine called an audiometer to play a series of tones through headphones. The tones vary in pitch (frequency, measured in hertz) and loudness (intensity, measured in decibels).
The health professional conducting the test will control the volume of a tone and reduce its loudness until you can no longer hear it. Then the tone will get louder until you can hear it again. You signal by raising your hand or pressing a button every time you hear a tone, even if the tone you hear is very faint. The health professional will then repeat the test several times, using a higher-pitched tone each time. Each ear is tested separately.
The results of the test are plotted on a special graph called an audiogram which helps to show the pattern of any hearing loss.
What is audiometry used for?
Audiometry is used in a number of different circumstances including:
- To evaluate possible hearing loss in anyone who has noticed a persistent hearing problem in one or both ears or has had difficulty understanding words in conversation.
- When determining the type and amount of hearing loss (conductive, sensorineural, or both).
- Screening babies and young children for hearing problems that might interfere with their ability to learn, speak, or understand language.
- To screen for hearing loss in people who are repeatedly exposed to loud noises or who are taking certain antibiotics, such as gentamicin.
What should I do to prepare for audiometry?
Usually very little preparation is needed for audiometry. If you are known to have wax in your ears you may have this removed before the test so that it does not interfere with the results.
Let the person doing the test know if you have had, or the child being tested has had, a recent cold or ear infection, as this may interfere with the results.
Are there any side-effects or complications from audiometry?
These are very safe tests; complications arising from these tests are extremely rare.
|Original Author: Dr Tim Kenny||Current Version: Dr Rachel Hoad-Robson|
|Last Checked: 25/03/2011||Document ID: 12710 Version: 2||© EMIS|
Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.