Otoacoustic emissions, or OAE tests as they are popularly referred to, are conducted to check cochlear status, especially hair cell functioning. This test comes in handy to screen hearing of infants, neonates, and people with development disabilities. The test also helps establish hearing sensitivity. Additionally, an OAE test can help distinguish between neural and sensory hearing loss, and determine feigned or functional hearing loss.
A well functioning cochlea not only receives sound, but also emits sounds of low-intensity. These sounds are called otoacoustic emissions (OAEs), and this is what the test aims to capture. The cochlea produces these sounds, along with its outer hair cells, as they respond to the sound stimulus by expanding and contracting. The actual measurement of these sounds was only calculated in the 1970s with the advent of technology that created extremely sensitive microphones that could record low-intensity responses.
There are four types of OAEs believed to exist:
The peripheral auditory system consisting of the cochlea, outer ear, and middle ear is measured using an OAE. Even though the cochlea emits the responses or sounds, in a well functioning ear, the middle and outer ear is capable of transmitting the response sounds to the microphone recording the sounds. So, an OAE test is a method used to check the functioning of the cochlea. However, more analysis will be required to gauge individual cochlea frequency areas. Also, an OAE is not a tool to comprehensively determine your auditory functioning; it can, however, help reaffirm or question the functioning, and help gain insight in case of a lesion.
A probe is inserted into the ear canal by a doctor with the aim of getting a seal. This tip of the probe is flexible and soft. The probe used will vary for adults and neonates. This is done keeping in mind the difference in the volume of the ear canal. A larger effective pressure of sound is produced by the ear canal which is smaller.
An OAE is measured keeping in mind the ambient noise. Since this is entirely an acoustic recording, it does not require any kind of behavioral response. It is, however, necessary that the patient remains as still as possible and is quiet. Hence, the time taken to record emissions per ear will depend on the ambient noise and patient co-operation. Barring an SOAE, the other 3 OAEs require stimulus to obtain emissions.
In case of transient evoked otoacoustic emissions, the most used stimuli are clicks, and the rate of stimulation is typically lower than 60 per second (stimuli). Usually, transient evoked otoacoustic emissions are used to screen the hearing of infants. Since TOAEs are recorded as a response to short stimuli, the stimuli have limited specificity of frequency, and the transient emissions come from a broader cochlear region.
The presence of TOAEs in a frequency band indicates that the cochlear sensitivity of that area should be around 20 to 40 dB HL or more. So, the presence of TOAE indicates that the individual's hearing sensitivity is 30 dB HL or thereabouts.
In certain cases, the patient could exhibit normal behavior, yet display abnormal OAEs. They include ototoxicity, excessive exposure to noise and vestibular pathology when the emission is in the frequency region of the tinnitus.
There are also instances when a person could have a normal OAE range but elicit abnormal behavior. This happens in cases of attention deficits, autism, functional hearing loss, probable inner hair cell damage, or even central auditory nervous system dysfunction.