How is it assessed?

All babies in England are offered a hearing screen routinely within a few weeks of birth as part of the Newborn Hearing Screening Programme (NHSP). For more details about this - see NHSP

Automated Otoacoustic Emission (OAE) screening test

This is the initial test and works on the principle that a healthy cochlea will produce a faint response when stimulated with sound. A small earpiece (containing a speaker and microphone) is placed in the child’s ear. A clicking sound is played and if the cochlea is working properly, the earpiece will pick up the response. This is recorded on a computer.

Automated Auditory Brainstem Response (AABR) screening test

Children who fail the OEA (about 15%) have a second test, the Automated Auditory Brainstem Response (AABR) screening test. This test measures whether sound is being sent from the cochlea and through the auditory nerve to the brain. The audiologist places three small sensors and a set of headphones on the child’s head and in the automated test the results are interpreted by a computer.

Diagnostic Auditory Brainstem Response test (ABR)

Children who fail the AABR are referred for further investigations which are usually carried out at the local audiology clinic. This usually involves a diagnostic ABR where different levels of sound are used and the results are interpreted by a clinician.

If the diagnostic ABR shows a unilateral hearing loss the child is usually monitored. If the diagnostic ABR shows a bilateral permanent hearing loss, the child will be fitted with hearing aids.

Behavioural tests

Older children may have their hearing assessed by behavioural tests. These tests use toys and play as part of the assessment and involve the child listening for a variety of sounds as part of the assessment.

Visual response audiometry (VRA)

Visual response audiometry (VRA) is suitable for children from six months to about two and a half years. Using a machine called an audiometer, sounds of different frequencies and loudness are played through a speaker. When the child hears the sound, they will turn their head when a visual reward is “activated” such as a toy lighting up or a puppet. The test can be conducted through small earphones if the audiologist wants information about each ear separately.

Pure tone audiometry

This is used from about the age of three. Younger children are shown how to move a toy (for example, putting a peg into a board) each time they hear a sound. Older children are asked to respond to sounds by saying yes or pressing a button.

An audiometer generates pure tone signals of frequency 125Hz, 250Hz, 500Hz, 1,2, 4 and 8kHz at variable intensities ranging from –10 dB to +120 dB usually in steps of 5dB.

Signals of decreasing intensity at each frequency are presented to the child tested, from a level the child can hear to find the point at which they fail to hear. The hearing threshold levels are usually plotted on a graph or audiogram, with sound intensity (dB) on the y (vertical) axis and the frequency (Hz) along the x (horizontal) axis. Standard symbols are used to denote right (o) and left (x) ears for air conduction and right ([ or Δ) or left (] or Δ) ears for bone conduction.

The hearing threshold is defined as the quietest sound heard by the child when being tested. A normally hearing child would expect to have a threshold of 20dB or better, and this represents no hearing loss on the audiogram.

It should be noted that audiometry is a subjective test of hearing.

Normal audiogram

Air condition assesses the function of both the conduction (outer and middle ear) and sensorineural (cochlea and auditory nerve) components of the ear. To measure air conduction (AC), the sounds come through either headphones, earphones placed inside the child’s ear or sometimes through a speaker (when the test is known as soundfield audiometry) and the signal passes by air conduction though the outer and middle ear. It is then transmitted to the inner ear, auditory nerve and auditory cortex of the brain.

Bone conduction (BC) assesses the function of the cochlea and auditory nerve. To measure bone conduction, the signal stimulates the cochlea directly by the application of the vibratory stimulus to the skull.

Using these two measures the type of hearing loss can be classified into conductive, sensorineural or mixed type.

Conductive hearing loss

If there is a problem in the external or middle ear (conductive hearing loss) the AC threshold will be higher than the BC threshold because the child will hear better by bone than air conduction. This is called the air bone gap (ABG). The greater the air conduction loss, the greater will be the ABG.

Audiogram showing conductive hearing loss in the left ear

Sensorineural hearing loss

If there is a problem with the cochlea or the auditory nerve, the AC and BC thresholds will be the same.

Audiogram showing sensorineural hearing loss in the right ear

Mixed hearing loss

Mixed hearing loss is a reduction in hearing of both AC and BC.

Audiogram showing mixed hearing loss in the right ear

Speech discrimination tests

Speech discrimination tests check the child’s ability to hear words at different listening levels. The tester asks the child to identify toys or pictures, or to copy words spoken by themselves or from a recording. From this the tester can assess the quietest level at which the child can correctly identify the words used. The test can also be used to assess lip reading and signing skills.

Tympanometry

Tympanometry is not a test of hearing. It is used to check how well the moving parts of the middle ear are working. A small earpiece is held in the ear canal. A pump causes the pressure in the ear canal to change. The eardrum should move freely in and out with the change in pressure. The earpiece measures this by checking the sound reflected by the eardrum. If the eardrum is not moving freely, there is likely to be some fluid or another problem with the middle ear. A build up of fluid is usually due to glue ear.