As a part of the audiological test battery, otoacoustic emissions can help to differentiate between auditory pathologies and provide useful information for the management of hearing impaired patients.

As reviewed earlier, OAEs have revealed that most cochlear threshold elevations involve a loss in mechanical responsiveness of the basilar membrane to sound vibration. We had no way of knowing this before. Cochlear hearing losses up to around 40dB may be solely due to poor outer haircell performance. The corresponding depression of cochlear travelling wave development and the degradation of the sound image would be adequate to account for the loss in hearing sensitivity. Of course a complementary type of cochlear loss must exist in which the travelling wave develops normally but inner haircells fail to translate the excitory image into neural code. Clinical research is needed to clarify this potential dichotomy. Some hydrops patients do exhibit OAEs with elevated audiometric threshold but most threshold elevations result in absent OAEs.

The logic for the incorporation of OAEs into the audiological test battery is easy to work out once the scope of each test is clearly defined. The pure tone audiogram tests the whole auditory system but includes unwanted central and psychological factors. The ABR tests the auditory periphery and neural pathways as far as the brain stem. OAEs test only the peripheral system - including the organ of Corti - up to the point of excitation of the inner hair cells but not the cells themselves. Tympanometry tests the system up to the cochlea.

When interpreting OAE data it should always be remembered that the cochlea is a frequency specific organ. OAEs - whether obtained by DPOAE or TEOAE - should be considered on a frequency by frequency basis. For example, a patient with normal hearing up to say 2kHz then a precipitous loss will still show OAEs - but only to stimuli containing components in the normal threshold range. Clicks contain all frequencies so will excite an OAE in such a patient - but the OAE response will not include frequency components from within the hearing loss range. This is the meaning of frequency specificity.

In general if there is a hearing problem and there are no other indications, it makes sense for an OAE examination to be the first objective test performed. It is fast and helps confirm normal middle ear and cochlear function. In all except newborns an absent OAE should be followed by tympanometry. Absent OAEs with a normal tympanogram usually indicate a cochlear dysfunction but this can sometimes be quite minor. The click stimulus intensity normally used for TEOAEs is around 55dB normal sensation level, but this still provides high sensitivity to losses as small as 15-20dB and even to subclinical factors 4kHz in adults. DPOAEs elicited with stimuli greater than 60dBspl are less sensitive to cochlear dysfunction. 25-30dB loss is needed to abolish the DPOAEs but this sensitivity is maintained up to higher frequencies. A two stage clinical OAE test is recommended - TEOAEs followed by DPOAEs. OAE testing cannot determine auditory threshold. ABR testing is needed to estimate threshold if audiometry is not possible. If OAEs are strongly present with substantial threshold elevation this can indicate a retrocochlear loss, an inner haircell loss, or an inorganic loss.

Auditory nerve pathology often coexists with absent OAEs so that the demonstration of a cochlear component of a hearing loss by OAEs cannot be used to exclude retrocochlear pathology. However, the presence of OAE with retrocochlear pathology indicates an intact cochlea which may indicate a policy of cochlear preservation during surgery.

In a minority of congenital hearing losses cochlear function remains intact. The provision of amplification to an intact cochlea has to be seriously reconsidered. OAE examination should always precede hearing aid fitment of infants, especially if not used in the identification process. It has been found valuable to re-examine very young and handicapped hearing aid users with OAEs to identify those who actually have normal cochlear mechanical function.

Tympanometry primarily examines the stiffness of the eardrum using low frequency tones. OAEs on the other hand require normal middle ear function from 1kHz to 6kHz. OAEs therefore provide evidence of normal middle ear function, strongly biased towards the transmission properties of the middle ear at speech frequencies rather than its sound reflection properties. This additional information is of course available only where the cochlea is known to be normal. The quality and integrity of surgical reconstruction of the middle ear could be assessed using OAEs.

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.