By Marcello Cherchi, MD PhD
For patients
Otoacoustic emissions (OAEs) are a test of part of the hearing apparatus of the inner ear, but different from an audiogram (a “regular” hearing test). OAEs may be helpful in evaluating hearing loss and tinnitus. The test may be performed in a quiet room, or you may be seated in a soundproof booth. The test is not uncomfortable. The audiologist or technician places a small probe in your ear canal. You may or may not hear faint sounds. The test takes a few minutes, and is performed once in each ear. If you have ear wax, this should be removed prior to testing OAEs; otherwise there are no special preparations for the test. There are no specific instructions for after the test.
For clinicians
Overview
The activity of type 2 cochlear hair cells generates vibrations within the cochlea that are transmitted “backwards” through the ear (from the cochlea, through the ossicular chain, to the eardrum) and can be detected by a sensitive probe placed in the external auditory canal — and these are otoacoustic emissions (OAEs). There are several types of OAEs that occur in different circumstances. While OAEs are not a direct test of hearing (in the sense that they do not test type 1 cochlear hair cells), they are regarded as an indirect assessment of cochlear function, and their function/dysfunction correlates (albeit imperfectly) with hearing. Middle ear disease can interfere with assessment of OAEs, so it is reasonable to check tympanometry prior to assessing OAEs. It is reasonable to check OAEs in patients who complain of, or are suspected of having, hearing loss, and in patients with tinnitus. OAEs responses tend to wane with age, and are usually absent over the age of 60 years.
Introduction
Otoacoustic emissions (OAEs), first recognized by David Kemp (Kemp 1978, 1979), are effectively sounds generated by the ear itself, “when the tympanum receives vibrations transmitted backwards through the middle ear from the cochlea” (Kemp 2002). These signals arise from the activity of type (outer) 2 cochlear hair cells. There are several types of OAEs, that are elicited in different ways.
- Spontaneous OAEs.
- Transient evoked OAEs (TEOAEs), elicited in response to click stimuli.
- Distortion product OAEs (DPOAEs), elicited in response to tone stimuli.
The most commonly used OAEs in clinical practice are TEOAEs and DPOAEs.
Kemp is careful to point out that, “OAEs come exclusively from [type 2] outer [cochlear] hair cells which do not themselves activate primary auditory nerve fibres, yet a strong relationship exists between the absence of OAEs and [the presence of] hearing loss,” therefore “an OAE examination is not a hearing test,” however, “OAEs are frequency-specific responses and tend to emerge only in frequency bands where hearing is normal” (Kemp 2002), and thus abnormal OAEs in a particular frequency are clinically regarded as an indirect corroboration of hearing loss at that frequency.
Physiology and neuroanatomy
The apparent function of type 2 cochlear hair cells is to modulate the activity of type 1 cochlear hair cells. In fulfilling this role, the activity of type 2 cochlear hair cells produces sufficient kinetic energy to generate vibration that is transmitted “backwards” (from the cochlea through the ossicular chain to the tympanic membrane) through the ear and is detectable by a sensitive probe in the external auditory canal. There are different patterns of OAEs that occur spontaneously, or in response to particular auditory stimuli.
Equipment needed
OAEs are usually tested with commercially available equipment for the purpose. It is preferable to perform the test in an environment with low ambient noise, ideally in a soundproof audiology booth (Kemp 2002).
How to perform the test
If a soundproof audiology booth is available, it is ideal to seat the patient in the booth when conducting OAEs. The probe is inserted into the ear and the test is run, then repeated with the probe in the other ear.
Since middle ear disease can interfere with assessment of OAEs, it is reasonable to check tympanometry prior to assessing OAEs.
What this test assesses
OAEs assess the function of type 2 cochlear hair cells.
How to interpret the test results
Different software platforms present OAE results differently. If an elicited OAE response exceeds the monitored “noise floor,” then OAEs are said to be “present,” which is normal. If the detected response does not exceed the noise floor, then OAEs are considered to be absent, and are classified as “referred,” which is abnormal.
As discussed earlier, while OAEs are not truly a “hearing test” (in the sense that they do not assess the function of type 1 cochlear hair cells), they are usually regarded as providing indirect information regarding auditory function.
Limitations
OAEs decline with normal healthy aging, and responses are usually absent in individuals over the age of 60 years.
Contraindications
If a patient has otitis externa or an injury of the external auditory canal, then they may not tolerate placement of the OAE probe.
Known middle ear disease, such as a tympanic membrane perforation, will interfere with OAEs and may make the results uninterpretable.
Pitfalls
Middle ear disease can interfere with eliciting OAEs and render the results uninterpretable (Kemp 2002; Zhao et al. 2000).
When is the test indicated
It is reasonable to test OAEs in patients complaining of hearing loss and/or tinnitus.
Diseases that may be diagnosed by this test
OAEs can provide corroborative evidence of hearing loss.
References
Kemp DT (1978) Stimulated acoustic emissions from within the human auditory system. J Acoust Soc Am 64: 1386-91. doi: 10.1121/1.382104
Kemp DT (1979) Evidence of mechanical nonlinearity and frequency selective wave amplification in the cochlea. Arch Otorhinolaryngol 224: 37-45. doi: 10.1007/BF00455222
Kemp DT (2002) Otoacoustic emissions, their origin in cochlear function, and use. Br Med Bull 63: 223-41.
Zhao F, Wada H, Koike T, Stephens D (2000) The influence of middle ear disorders on otoacoustic emissions. Clin Otolaryngol Allied Sci 25: 3-8. doi: 10.1046/j.1365-2273.2000.00312.x
