When I first heard the term “Otoacoustic Emissions” back in varsity, I must admit, I felt quite uneasy. I remember thinking: “Emissions? Otoacoustic? Like something being sent out the ear? Something like sound? I thought all the ear does is hear sounds, not make them?” Little did I know how intriguing and interesting OAEs would turn out to be.
I clearly remember being taught that an OAE is “an echo the cochlear makes in response to a sound”, and for years that was my understanding of what an OAE was. Nevertheless, I have come to learn that an OAE is so much more than an “echo”. OAEs are typically elicited by sound stimuli but may also be spontaneously emitted. As we all know by now, to record an OAE a stimulus is presented into the ear and the sound (travelling wave) moves through middle ear and reaches the hair cells in the cochlear. Here, the travelling wave causes vibration which results in the movement of these hair cells in the Organ of Corti. This movement sparks a neural impulse and becomes part of a forward travelling wave to CNVIII. But wait, there’s more…(This is where it gets interesting folks), the same anatomical movement that sends the travelling wave to the brain also creates secondary movement and reflections within the liquid (endolymph) of the cochlea. These secondary vibrations flow towards the middle ear and tympanic membrane and can be identified in the ear canal. The vibrations are converted back into sound waves and recorded as a response. This my friend is an OAE (not just an ‘echo’ huh?)
I’m not going to dive too deep into what an OAE is, but more into how OAEs can be used for screening. Let’s talk about OAE’s for screening: The what, when, how and why.
We’ve spoken briefly about what an OAE is, but the what I’m talking about now is more like: What does/doesn’t an OAE tell you.
Basically, an OAE provides information about outer hair cell (OHC) functioning. As a response to vibration caused by auditory stimuli, the OHCs contract and expand. This spring-like movement occurs most robustly at the specific spot on the basilar membrane where the exact frequency being heard is captured to be sent to the brain for processing. The tonotopic arrangement of the cochlear means that certain OHCs on certain areas of the cochlear respond to certain frequencies, i.e. they are frequency specific.
However, the inner hair cells (IHC) still play a role. They are responsible for afferent action / taking sound to the brain. Only IHCs send info to the central nervous system (CNS). If the brain only got info from the IHC’s, there would be very little frequency selectivity. In other words, the robust movement of the OHC allows the correct, corresponding inner hair cell to send a strong, frequency-specific signal to the brain saying “I am hearing a high pitch sound”.
Movement, this is important to remember. An OAE gives us information about the movement of the OHCs. Without this movement, energy is not created, and the processing of sound is affected. More specifically, sound loudness and clarity diminishes. The movement amplifies and sharpens the peak of the incoming sound which is then sent to the brain via the IHCs. With complete OHC dysfunction patients may present with a hearing loss of 60-70dB and speech presented above their threshold still sounds distorted and unclear.
Therefore, what an OAE does tell you is the function of OHCs and their ability to amplify and sharpen the travelling wave (sound) of different frequencies.
What an OAE doesn’t tell you is the functioning of the inner hair cells or the auditory nerve. Furthermore, it does not specify the degree of hearing loss. If a hearing loss is more than 30/40dB then OAEs will not be present. OAEs do not provide specific anatomical information about which part of the Organ of Corti has been damaged. For example, OAEs can be absent with normal OHC functioning but problems with stria vascularis or an absence of stereocilia.
Most commonly, OAE screening is used for newborn screening due to the ability to examine cochlear status without behavioural responses.
OAE screening is also used in the difficult to test population due to developmental difficulties or functional hearing loss.
OAE screenings are done when there’s very little time (because it is a quick test) and there are only certain frequencies that need to be checked. It can also be done for industrial or military screening.
OAE screenings are very useful for school screenings as it is quick in identifying children with possible hearing difficulties who can be referred for a full diagnostic assessment. Early identification lowers the risk of speech and language delay, cognitive delay, and ultimately slow learning which affects social integration and long-term success in life.
OAEs can also be used to monitor cochlea function, especially in patients who are at risk of cochlea damage. OAEs are able to identify deterioration in OHC integrity before most other tests available. So for example, in patients who are exposed to excessive noise or ototoxic medication, conducting an OAE and picking up early damage and preventative measures can be put in place to ensure no further deterioration.
OAEs are performed using a probe with 3 holes. Two of the holes are the speakers that send the sound to the structures of the ear, and one of the holes is the microphone that records the sound moving back out the ear.
Make sure the probe fit is appropriate, i.e. the tip is the proper size, properly seated on the probe, the probe is properly inserted in the ear and is secure and stable without being held in place.
OAEs are very sensitive to noise. Therefore, an OAE should be performed in quiet conditions with very little background noise. It would therefore be useful if the patient sleeps during the test.
Why are OAEs so sensitive to noise? Well, as previously mentioned, an OAE response is due to a result of a fraction of the travelling wave moving back through the middle ear and tympanic membrane. When this happens, some energy is lost due to a small oval window sending vibrations to a larger tympanic membrane, making the recording very small and easily affected by external noise. #physics.
It is also important to rule out any middle ear disorders before performing an OAE, as this will affect the results.
The primary use of an OAE is to determine cochlear status, specifically hair cell function. Since most hearing impairments include OHC damage, this information can be used to identify hearing impairment without behavioural responses, saving time and money and streamlining the referral and helping in the decision making about further testing.
If you’re currently a student, I hope this post has helped you better understand OAEs and the use of OAEs as a screening tool. If you’re a practicing audiologist, I hope this post has affirmed your understanding of OAEs as a screening tool and possibly inspired you to learn more about OAEs and its intricacies.
Once upon a time, in a village far from here there lived two little girls named EMMA & AUDREY. It was a very special day today – the first day of Grade 1. They were as scared as they were excited.
