What could be simpler than testing an infant’s hearing with an insert-earphone! It takes only a few seconds to record the transient otoacoustic emissions in a quiet office from a typical newborn who has clean ear canals and a well drained middle ear. If conditions are not ideal it can take longer - but 5 minutes is an exceptionally long time for an experienced OAE screener to test a newborn - and it would usually mean that the newborn was not ready to be tested.

Transient OAE technology is generally preferred for screening at this time because the instrumentation provides very fast feedback to the screener on general probe fit, noise and test outcome. DPOAEs can also be used effectively. TEOAE screening has the advantage of testing a wide range of frequencies individually yet simultaneously giving a continuous panorama of cochlear function with frequency. Around 100 universal screening programmes in the USA currently use TEOAEs. A 1996 survey by the National Center for Hearing Assessment and Management (NCHAM) showed referral rate of less than 5%. The reportedly very high sensitivity of the technique for universal screening has not been challenged, despite many hundreds of thousands of TEOAE screenings starting with the Rhode Island Hearing Assessment Project in 1989.

There is an acknowledged learning curve for newborn screening with OAEs. Initial attempts at newborn screening with OAEs can be disappointing unless a few important guidelines are followed. Firstly, probe fitting is paramount. Inspect the ear and select a suitable size of tip. Straighten the ear canal by gently pulling the pinna. Insert the probe firmly and deeply. This opens up the ear canal and excludes external noise contamination. The room need not be audiometrically quiet but continuous background noise should be avoided. Observe the noise received by a suspended probe relative to the instrument’s noise artifact rejection range. If the background noise level exceeds 50dBA don’t attempt newborn screening.

Expect to see some indication of an OAE response in about ten seconds with a newborn. If not - and if both the baby and room are quiet - then assume that the probe insertion has not fully opened up the ear canal or that the disposable tip has become clogged with debris. If having dealt with this there is still no OAE, assume that there is retained fluid still to be cleared from the middle ear, and retest the baby later. Babies tested on the first day of life more often fail to show an OAE as a result of mild ear contamination - but they usually pass on retest hours later. When OAEs do appear, continue collecting data until the required level of confirmation is achieved. The screener must be trained to judge the technical adequacy of a measurement and to recognise the need for a repeat test. Technical adequacy includes the achievement of the specific statistical and signal-to-noise targets needed to validate OAE responses. Instrument signal processing and automation greatly assist in this process.

It is the responsibility of the audiologist or physician to set an appropriate pass/refer criteria for the baby. The previous gold standard for hearing screening - the ABR - accepted a proven wave V response of normal latency to the selected level of click stimulation as sufficient proof of normal auditory function. By the same standard, any technically valid OAE response within the speech range in response to a click stimulus could be reasonably accepted as proof of adequate cochlear function. In practice, most screening programs set a more stringent criteria than this. It is common to require OAE responses to be 3 or 6dB above the noise in 2, 3 or more half octave bands between 1 and 4kHz. This exceeds the stringency of the ABR test, and results in a higher refer rate. However the widespread acceptance of multifrequency pass criteria for OAEs must be taken to indicate an underlying dissatisfaction with the non-frequency specific nature of screening ABR. It remains to be seen if such stringent multifrequency OAE pass criteria persist and yield tangible benefits.

OAE screening has proven very effective in the detection of hearing impairment in newborns, even though the neural pathway is not being assessed. Failure to show an OAE is probably the single most important risk factor for hearing impairment but other risk factors should never be ignored. Any risk of neurological significance means an ABR test must also be conducted. To date, among the hundreds of thousands of OAE screenings monitored by NCHAM the incidence of late onset hearing losses missed by OAE screening appears to be very low - around 1% of the hearing impaired population. OAEs appear to be ideal for the first stage of universal screening programs.

This article has been extracted from the publication ‘Understanding & Using Otoacoustic Emissions’, written by Professor David Kemp and published by Otodynamics, and is reproduced with the author’s permission. Copyright remains with the author.