Hearing Testing and Screening in Young Children

Children with untreated hearing loss, especially if it is permanent and substantial, are likely to have severe difficulty in acquiring language skills. In particular, it is thought that if there is no stimulation of neural pathways involved with hearing and speech within the first 6 to 12 months of life, the ability to develop spoken language may be permanently lost. Sign language can be learned at any age. Language problems can lead to educational difficulties, with associated social and psychological problems.

Early diagnosis of childhood deafness and appropriate intervention, including hearing aids, cochlear implants, education and social support, can substantially improve the outcome.

An audiogram measures the threshold level (the quietest sound) that is heard at different frequencies. This is recorded as the decibel hearing level (dBHL) for that frequency. Hearing loss is classified from mild to profound by the average threshold level over a range of frequencies:

• Mild - 20-39 dBHL (cannot hear whispers)
• Moderate - 40-69 dBHL (cannot hear conversational speech)
• Severe - 70-89 dBHL (cannot hear shouting)
• Profound - 90 dBHL or more (cannot hear sound when it usually becomes painful)

It is not uncommon for children to have a predominantly high frequency hearing loss where the threshold dBHL may be near normal at low frequencies, but their hearing threshold becomes progressively higher with increasing frequencies of sound. This causes inability to distinguish consonants and may present with mispronunciation of words.

In the UK, the prevalence of permanent hearing loss is nearly 2 per 1000 children at age 9-16 years.¹

• Sensorineural hearing loss
  • CongenitalAbout 1 in 1000 children are born with a sensorineural hearing loss sufficient to need hearing support.
    • About 95% are due to lesions within the cochlea. The rest have a problem with the auditory nerve or central connections.
    • About 25% will benefit from cochlear implants.
    • About 90% are born to parents who hear normally.
    • About 50% have other disabilities too.
    In some cases the hearing loss is not apparent at birth but becomes progressive in childhood.
    
    Incidence is 10 times higher in babies admitted to neonatal intensive care.
    Other risk factors include:
    • family history of congenital deafness such as Usher's syndrome. About 30% have a genetic cause and genetic counselling may be appropriate
    • dysmorphic features
    • congenital infections, especially congenital rubella and cytomegalovirus.
    • About 30% have no cause identified.
  • Acquired
    • Meningitis causes up to 1 in 5 cases of profound hearing loss but the incidence is dropping due to HIB and MenC vaccines.
    • Mumps can cause deafness that is usually unilateral. Both this and congenital rubella syndrome may become more frequent if the trend of falling uptake of MMR vaccine is not reversed.
    • Other causes are rare.
• Conductive hearing loss
  • A small number of children have congenital conductive hearing loss caused by rare disorders of the ear canal or middle ear.
  • Glue ear (otitis media with effusion or OME) is very common and typically causes temporary episodes of hearing loss in younger children. About 1 in 10 pre-school children have some degree of glue ear at any one time.
    
    Glue ear can occur in addition to other problems and may aggravate existing hearing loss.

Parents may quickly recognise a baby with severe or profound hearing loss, but moderate hearing loss or high frequency hearing loss may go unnoticed for several years unless formally tested. The improved outcome in children with early detection of hearing loss justifies hearing screening of all children.

Neonatal screening in the UK

Pilot studies using the AOAE test for screening neonates, found it effective at detecting hearing problems early.³ Following the pilots, universal neonatal screening using the AOAE test has been introduced in the UK from 2006 (replacing the distraction test done at 8 months). The AABR test is used for babies who fail the AOAE test.^4,5 Note that both these tests are screening tests only, and a baby who fails them will need further audiological assessment. Most babies referred from screening tests will not be found to have hearing loss on further testing.²

For babies who have had neonatal intensive care, protocols differ from the well baby screening. This is to allow detection of auditory neuropathy, a condition which is rare in well babies, but which can account for approximately 10% of hearing loss in the neonatal intensive care unit. Most neonatal intensive care unit screening programs use AABR for this reason.^2,5

Universal neonatal screening using the AOAE test can achieve coverage of over 90%; the sensitivity of the test is 90% or more, and the specificity is about 95%.⁶

There was some controversy about the relative value of universal and targeted screening. Targeted screening is substantially cheaper per case identified and universal screening has many false positives.⁷ However, targeted screening leaves many cases undetected and is not the preferred technique in the UK. As technology improves it is also possible that AABR will be the test of choice.⁸

Test limitations in neonatal screening²
Both the AOAE and the AABR screen require a quiet baby and a quiet testing environment. Restlessness may result in the test being discontinued. AOAE relies on a functional outer, middle and inner ear, and AABR a functional outer, middle and inner ear, and lower auditory pathway. These screening tests are not designed to detect central hearing impairment (where hearing loss is secondary to dysfunction of the pathways from brainstem to auditory cortex). As both tests are made via the external ear, debris in the canal or middle ear fluid can affect the accuracy of the test. In particular, OAE testing may be affected by amniotic fluid in the ear canal when testing is conducted in the first 48 hours following birth. This may account for some false positive results.

Screening tests for infants and children

• Distraction test - aged 7-8 months The health visitor distraction test is largely of historical interest as it has been replaced by the universal neonatal screening programme using the AOAE test. The neonatal screening programme has greater sensitivity and specificity and is substantially cheaper per case discovered, compared with the distraction test.⁶
• The school entry 'sweep test' This is a modified pure tone audiometry test which checks that the child can hear four frequencies at a set decibel level. It is routine in most areas in the UK. Most children with severe or profound hearing loss will have already been diagnosed. However, occasionally this test picks up a progressive hearing loss. Milder and unilateral cases of sensorineural hearing loss may also be diagnosed for the first time. Also, many cases of glue ear are detected. Although these may not require active treatment, it is useful for teachers and parents to know which children have temporary hearing loss.
• The whispered voice test can also be used to detect hearing impairment in children, but it has not been adequately evaluated as a screening tool.⁹

Who should be referred?

Hearing should be assessed in children newly diagnosed with learning, behavioural, speech and language problems. Also, always take seriously any parental concern about hearing loss in a child, even if a previous hearing test was normal. Because:

• Some hearing disorders, including congenital ones, are progressive and may not manifest for several months or years¹
• The AOAE and AABR tests are sometimes inconclusive.
• Glue ear is common and easily missed.
• High frequency hearing loss may present as 'learning delay'. Many everyday sounds are low frequency, so a child with high frequency hearing loss may respond to these common low frequency sounds. But they may not be able to hear intelligible speech.

Where to refer

Babies with hearing loss found on neonatal screening, should be referred to local support programmes, for a multidisciplinary assessment and hearing support services.^10,3 In the pilot studies, this has worked well and achieved early intervention.³

Other children with suspected hearing loss should be referred promptly to the local audiology service, or according to local protocols. If significant hearing loss is then diagnosed, all children should be referred to a specialist multidisciplinary team.¹⁰