OAE

Introduction

OAE or Otoacoustic Emission Testing is the recording of sounds that the ear produces itself. Otoacoustic emissions were first reported by Kemp in 1978. They appear to be generated by motile cochlear outer hair cells.

Otoacoustic Emissions appear to be part of an "active equilizer" mechanism. Amazingly, the brain can reinforce or diminish incoming sound at particular frequencies, enabling it to adjust the ear for maximum sensitivity for particular frequency patterns. For example, think of "tuning out" some annoying sound, or "listening hard" for a particular sound buried in others.

 
There are 2 types of Otoacoustic Emissions in clinical use:

Transient otoacoustic emissions (TOAEs) or transient evoked otoacoustic emissions (TEOAEs) - Sounds emitted in response to an acoustic stimuli of very short duration; usually clicks but can be tone-bursts
Distortion product otoacoustic emissions (DPOAEs) - Sounds emitted in response to 2 simultaneous tones of different frequencies
While not much relevant to clinical investigation, there are also spontaneous OAEs that occur in about 60% of normal persons. A case is illustrated here.

OAE's can be partially suppressed centrally via the superior olivary complex. Axons of the lateral and medial olivocochlear bundles extend from the superior olive and leave the brainstem as a ventral component to the inferior vestibular nerve. They join the cochlear nerve as Oort's vestibulocochlear anastemosis. Axons of the lateral olivocochlear bundle synapses with afferent neurons from the cochlea. Axons of the medial olivocochlear bundle terminate a the base of cell bodies of the outer hair cells. It is generally believed that the medial efferents counteract the amplifying effects of the outer hair cells. It is probably mediated by acetylcholine. (Bolay et al, 2006).

OAE's are not suppressed by certain inhaled anesthetics (Gungor et al, 2014), and from this observation, it would seem possible that they might be useful to detect damage to the hearing organ during surgery.

Method

OAEs are measured by presenting a series of sounds to the ear through a probe that is inserted in the ear canal. The probe contains a loudspeaker that generates the sounds and a microphone that measures the resulting OAEs that are produced in the cochlea and are transmitted through the middle ear into the outer ear canal. The resulting sound that is picked up by the microphone is digitized and processed using signal averaging methodology.

To obtain an OAE one needs an unobstructed outer ear canal, absence of significant middle ear pathology, and functioning cochlear outer hair cells.

Effect of age
Most tests of the inner ear get worse with age. This is also the case with OAE's. Konrad-Martin et al (2017) repoorted that "Across a wide range of f2's, DPOAE level decreases by 3 to 4 dB from 1 to 13 months of age followed by a more gradual decline of <1 dB/year. An f2 of 6 kHz shows the smallest decrease during the early rapid maturation period. "

Systems:

The elements in the pathway for the OAE include the sound source, the ear drum, ossicular chain, inner ear, and outer hair cells. The same structures transmit sound coming out of the outer hair cells. OAEs can be affected by anything in the chain - -if sound doesn't get in or out -- no OAE. If there is a resonance or filter between the sound source and microphone this will cause altered frequency spectrum of OAEs (Grenner, 2012). Thus, OAEs "reflect" a combination of inner ear and external/middle ear function. Individuals with small ear canals (such as infants) are different than adults due to the difference in external and middle ear size.

Utility

The clinical significance of OAEs is that they only occur in a normal cochlea with normal or near normal hearing. If there is damage to the outer hair cells producing mild hearing loss, then OAEs are not evoked. A rule of thumb is that OAEs are present if hearing is 35 dB or better. Because OAEs are evoked by transient signals that have a wide frequency response, a broad region of the cochlea responds, providing information on the frequency range from 1000 Hz to 4000 Hz. OAEs decline with age.(Gates et al. 2002; Cilento et al. 2003). Our clinical experience with OAEs, is that they do not discriminate well between causes of hearing loss. In other words, we would expect someone with an intact cochlea (including their outer hair cells), perhaps due to an acoustic tumor, to be equally likely to have absent OAEs as someone with cochlear damage due to noise. Kagova and others (2012) have observed that OAEs are generally reduced by acoustic neuromas.

OAEs are appropriate for use in difficult-to-test patients: newborn infants, young children, patients who are attempting to feign a hearing loss (i.e. malingering), and developmentally delayed populations. OAEs primarily provide information about the activity of the cochlea, and do not assess the status of the rest of the auditory pathway, except for crossed responses mediated through the cochlear efferent system.

For the very young group, Trosman et al (2016) suggested that OAE+tymps were more cost effective than audiograms, because of the "questionable reliability" of behavioral audiometry in very young children.

In adults, we feel that OAEs are most helpful in persons who may financially benefit from being diagnosed with hearing loss, and also as a cross-check on audiometry. In our experience, OAEs are very sensitive to noise and age related hearing disturbances.

One would think that OAEs might be a better method of screening for occupational hearing loss, generally caused by noise, than conventional hearing testing. OAEs are based on objective measurements rather than patient responses that are subject to misdirection. Contrary to this common sense hypothesis, Wooles et al (2015) reported that there was "no evidence of a robust relationship" between DPOAE's and pure tone audiometry. We think that this comparison of apples: oranges -- an objective measurement to a subjective measurement in a setting where there is motivation to exaggerate, is fraught with peril due to bias, and that the "jury is still out".

TOAE's are commonly used to screen infant hearing, to validate auditory thresholds obtained via other techniques, and to assess the cochlear contribution to hearing.

DPOAE's are also used for infant screening. They can be obtained in persons in whom TOAE's cannot be obtained, and they can be obtained at higher frequencies than TOAE's (i.e. over 1000 Hz).

OAEs have a special utility in auditory neuropathy. This is a condition, primarily of children, in which hearing is impaired but cochlear function is presumed intact. This is rather rare.

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