There are many ways to measure the way the auditory system is functioning.
For adults and children, the most common test of hearing ability is called pure tone audiometry where you listen to a range of beeps and whistles (called pure tones) and indicate when you can hear them, usually by pressing a button. The loudness of each tone is reduced until you can just hear the tone. The softest sound you can hear (your hearing threshold) is then marked on a graph called an audiogram.
When hearing is measured with pure tones presented through headphones, the measurement is called air conduction. The sounds go via the air, down the ear canal, through the middle ear and to the very delicate organ of hearing in the inner ear – the cochlea.
The sensitivity of the cochlea can also be tested by placing a small vibrator on the mastoid bone behind the ear, and again measuring the softest sounds that can be heard. Sounds presented this way travel though the bones of the skull to the cochlea and the hearing nerves, bypassing the middle ear. This type of testing is called bone conduction.
The air conduction and bone conduction hearing levels on the audiogram can tell us a lot about where the hearing problem is.
There are two main types of hearing testing for babies and young children - behavioural testing and objective (electrophysiological) testing. Both have advantages and it is useful to have results from both types of tests.
Behavioural testing is where children do something to let us know that they have heard the sound. This has the advantage of telling us the complete picture about the function of the auditory pathway.
Behavioural Observation Audiometry
For children under six months we can use Behavioural Observation Audiometry (BOA) to test their hearing. In this type of test, the child’s behavioural responses to sound are assessed. Behavioural responses may include startling to loud noises, stirring from sleep in response to a sound, stopping sucking when a sound is heard or trying to look to the sound. An assortment of noisemakers, such as crunching cellophane, tiny bells, chimes and bicycle hooters, are used and most can be classified as low, mid or high-frequency sounds. The loudness of the sound is measured with a sound level meter. Although hearing levels cannot be determined exactly, an experienced audiologist can obtain a great deal of information from this procedure about the severity of a hearing loss and the child’s ability to detect different sound frequencies. This procedure tests both ears at once as the child does not wear headphones.
Once the child has good head control and can localise sound (turn to where a sound is coming from), a more advanced procedure can be used.
Visually Reinforced Orientation Audiometry
Visually Reinforced Orientation Audiometry (VROA) involves the child turning towards a loudspeaker whenever a sound is presented. When the child looks to the speaker, a puppet or some other visual reward is given. Children soon learn that when they hear the sound, the puppet is there if they look around. From this procedure, accurate hearing thresholds can be obtained. As VROA is usually performed without headphones, individual ear information is not available. However, individual ear results can be obtained if the child will wear head phones. Most children enjoy this game and soon learn its rules.
VROA is used from when the child is about six months until the time their concentration span is long enough to move on to play audiometry (usually two-and-a-half to three-and-a-half years).
Play audiometry works the same way as pure tone audiometry described earlier except, on hearing a tone, the child puts a marble in a marble race, presses a computer keyboard to make something happen, or puts a piece in a puzzle. Making a “game” of the tests keeps the child’s interest and enables a larger number of thresholds to be obtained. This style of testing is sometimes called stimulus-response testing where the pure tone is the stimulus and the child’s action (putting a marble in a race) is the response. As the child wears headphones for this test, individual information from both ears is obtained.
Objective testing does not require the child to participate. The objective tests commonly used are:
Auditory brainstem response (ABR)
The Auditory Brainstem Response (ABR) (sometimes known as BERA) procedure measures the activity of various parts of the nerve pathway from the ear to the brain when a sound is administered. Electrodes (small metal disks) are attached to the child’s head to record electrical energy that occurs in the auditory pathway in response to sound. The child wears headphones and this allows recording of the auditory system’s response to specific frequencies and different intensities. This can usually be done as the child sleeps or lies quietly, but sometimes a light sedation is needed for older children who are inclined to wriggle! Tone Burst ABR can provide information about the detection of information at different frequencies. It measures the electrical signal being carried to the brain by the auditory nerve. Each ear can be tested separately. Young babies usually sleep through the procedure and it does not cause any pain or discomfort.
Otoacoustic emissions (OAEs)
The Otoacoustic Emissions (OAEs) procedure is fairly quick and involves a small probe (or plug) being placed in the ear. The child then hears a series of “clicks” or tones through the probe. OAEs measure whether or not the outer hair cells in the cochlea are functioning normally in response to sound. OAEs test each ear individually but cannot give information about the degree of hearing loss. If a hearing loss greater than 40dB is present, no emissions will be measured. OAEs are used mainly as a way of finding out whether or not the hair cells in the inner ear are functioning properly. If a child has OAEs they are likely to have near normal hearing in that ear or at the frequencies where OAEs are measured. The exception to this is where a child has Auditory Neuropathy and the OAEs do not predict the hearing ability.
A small rubber tip is placed in the ear and a little air is pumped into the outer ear canal. If there is a problem in the middle ear it will usually show up on this test.
For example, if there is very little movement of the eardrum, it may indicate there is fluid behind the drum as a result of a middle ear infection. Often the results of typmanometry indicate the location of the blockage that is causing the hearing loss and whether medical treatment may help.
In Electrocochleography (ECochG), a very fine electrode is placed through the eardrum into the middle ear and rested against the cochlea. The electrode can then pick up the tiny electrical signals generated in the cochlea in response to sound. The electrical activity measured in ECochG supplies information about the functioning of the cochlea and the start of the nerve pathway up to the brain. In children, this test is performed in hospital under anaesthetic. This allows the measurement of hearing at different frequencies.
Auditory Steady State Response (ASSR)
Auditory Steady State Response (ASSR) results are recorded in the same way we measure ABR. That is, electrodes are placed on the child’s head to record electrical energy that occurs in the auditory pathway in response to sound. This allows recording of the auditory system’s response to specific frequencies and different intensities. It is more accurate for severe-to-profound hearing losses than mild-to-moderate losses. (ASSR is sometimes known as steady state evoked potentials - SSEP)
There are other objective assessments which are not used as frequently but which may be suggested by your ENT Specialist.
When assessing children, a test battery is used. A test battery is a number of different tests to give different types of information about a child’s hearing ability. An example of a test battery for a young baby may be:
Doing a variety of tests gives the audiologist and the parent a more thorough picture of the child’s hearing ability. The younger a child is, the longer it can take to obtain al the necessary information about the hearing loss.
Disclaimer: The information contained on this website is not intended as a substitute for independent professional advice.
14-Nov-2015 10:54 AM (AEST)