When testing children’s hearing, it is important that an appropriate method is used based on the child’s age, ability and interest, and one that will give the information that is required. Some tests require children to be alert, awake, co-operative and happy to “play” with the Audiologist (Behavioural Tests). Others require the child to be quiet and still or asleep (Objective Tests).
The Audiology Department offers a range of tests for children attending the hospital.
Subjective Hearing Tests
Behavioural Observation Audiometry (BOA)
This is a test sometimes used for babies up to the age of 6 months. At this age babies may not be developmentally ready to turn to look for sounds. This test is not often used as it has limited reliability but it can be helpful sometimes to monitor your baby’s responses to sounds. It can be done on also be a useful test for baby’s with hearing aids to monitor the comfort of the sounds. Sounds are presented via speakers or using toys and the baby is observed for a “response”. A response to sound at this age may include quieting, eye widening, startle, etc.
The distraction test is ideally used for babies who are developmentally six to eight months old (although it can be used from six months to 24 months of age by experienced testers), and assesses the ability of the baby to hear a sound then turn to locate it.
The sounds are produced to the right or left behind the baby out of their field of view. Various sound stimuli are used such as rattles, voice, drum and warble tones. The loudness of the sounds produced are varied to try and establish the minimum level that the baby responds to.
Visual reinforced audiometry (VRA)
This test is usually suitable for infants from six months to two years old. This is a powerful technique that can be used to determine responses of each ear independently to a range of sounds of different pitches.
The child is seated, usually on a parents lap, between two TV screens with a loud speaker below each one. A sound is played from one of the speakers on the left or right. If the child turns to the sound they are shown a visual reward in the form of an interesting animation on the TV screen.
The sounds and pictures are presented a number of the times until the baby or child has learned to turn to look for a picture on the screen every time a sound is played. Sometimes babies and children need encouragement to learn to do this. This process is called conditioning and some babies may not be ready to learn this process in which case an alternative test may be used, such as the distraction test described above.
For babies and children that have been “conditioned” a series of sounds are then played at reducing loudness to obtain a minimal level that the baby/child responds to. Each time the child turns to a sound they are visually rewarded with one of the animations.
In order to test each ear separately the sounds may be made through inserts into the ears.
The audiologist may play with your child using a carefully selected group of toys that your child is familiar with. The toy test involves checking how well your child can identify the different toys when the audiologist asks for the them using a quiet voice.
Play audiometry is suitable for pre-school children of 20 months to five years old (developmentally). The child must be able to understand simple instructions and commands.
A game is played in which the child must perform an action when a sound is heard. This could take the form of putting toy people in a boat or building a tower from blocks. The complexity of the task can be modified to be age appropriate.
The child wears headphones or small foam earphones during the test. Sounds at various pitches are presented to the child and gradually reduced in loudness. Each time they hear the sound they must perform the action until they can no longer hear it.
The point at which they stop performing the task is taken to be the hearing threshold at the particular pitch being tested. The process is repeated for a range of pitches in each ear.
Pure tone audiometry (PTA)
Children of school age upwards are generally able to manage this test. Tones of varying pitch and loudness are played through headphones or small foam earphones. The child must press a button or tap a stick on the table for as long as they can hear a sound.
The loudness of the sounds are reduced until the child stops responding, this is the threshold of hearing. Up to six different pitches from 250Hz to 8KHz are tested in each ear allowing the threshold of hearing to be plotted across the speech range creating an audiogram.
Objective Hearing Tests
Otoacoustic emissions (OAE)
Sound is presented into the ear canal via a soft ear tip. The sound wave passes through the ear drum and middle ear then into the inner ear (cochlea). This stimulates some of the hair cells in the cochlea causing them to twitch in response creating a sound or echo.
The echo passes back through the middle ear and ear drum where it is detected by a microphone situated in ear tip. This signal is stored on a computer and plotted on a graph.
The results from this test can give an indication of how well the outer hair cells are working. The outer hair cells are able to increase the sensitivity of the cochlea when functioning normally by up to a factor of 1000 times. An absence of or damage to these cells will significantly reduce hearing thresholds.
Computer analysis of the echo from these cells can indicate the level of function present in the cochlear under test.
Auditory Brainstem Response (ABR)
Clicks or tones of varying loudness levels are presented using ear tips placed just inside the ear. Sensors are attached behind each ear and one to the upper forehead. The sound in the ear canal passes to the cochlea which converts the sound waves into tiny electrical signals which pass along the auditory nerve to the brain.
These tiny electrical signals are collected by the sensors, amplified, and passed to a computer for processing. The test is painless. It is usually performed in babies during normal sleep although occasionally toddlers require sedation for this test. This is to make sure that the baby or child is still and quiet during the test as any muscle movements will interfere with the signals being measured
Presenting progressively quieter sounds enables an estimate of hearing threshold to be obtained.
Middle Ear Tests
This test measures the stiffness of the ear drum by applying a positive and negative pressure in the ear canal. Maximum mobility of the eardrum will occur when the pressure in the canal equals the pressure in the middle ear. Negative pressure on the eardrum or reduced mobility may be early indications of middle ear disease.
If fluid builds up behind the ear drum (glue ear) the eardrum will become less elastic thus reducing the sensitivity of sound detection. This is known as conductive hearing loss.