Otoacoustic Emissions (OAEs). An ever-evolving, fast, easy to obtain, minimally-invasive screening test, used for screening for hearing loss in early life. OAE tests play an important role in monitoring ototoxicity and noise-induced hearing loss (NIHL) because OAEs can detect outer hair cell dysfunction earlier than a pure tone audiogram can. Otoacoustic Emissions Otoacoustic Emissions
Pathophysiology: OAE are the sounds of low intensity produced by the outer hair cells of a normal cochlea and recorded from the external auditory canal with the help of a sensitive microphone placed in the external ear canal. The sound produced by the outer hair cells travels in a reverse direction: outer hair cells → basilar membrane → perilymph → oval window → ossicles → tympanic membrane → ear canal.
OAEs are present in healthy outer hair cells, thus helps to test the functional status of the cochlea. OAE’s are absent in 50% of normal individuals, lesions of the cochlea, middle ear disorders (as sound travelling in the reverse direction cannot be picked up) and when the hearing loss exceeds 30 dB.
Types of OAEs:
(i) Spontaneous OAE’s are seen in the persons with normal hearing or when hearing loss < 30 dB.
(ii) Evoked OAE’s. Depending on the sound stimulus used, evoked OAE may be:
- Transient evoked OAEs (TEOAEs): A series of click stimuli are presented at 80–85 dB SPL (sound pressure level) and the response is recorded. TEOAEs can be recorded from 500–4000Hz but are more sensitive for hearing loss at 500 and 1000 Hz.
- Distortion product OAEs (DPOAEs): Two continuous tones of moderate-intensity, e.g. 55 and 65dB SPL, are presented to the cochlea at the same time to produce distortion. They are used to test hearing in the range of 1000–8000 Hz but have a better clinical performance for hearing loss at 4000 Hz.
Uses of OAE’s
- OAEs is used to distinguish between deafness caused by cochlear and retro-cochlear lesions. OAEs are absent in cochlear lesions, e.g. ototoxic sensorineural hearing loss.
- DPOAEs may be used to monitor ototoxicity effects earlier than pure tone audiometry.
- It may also be used in non-cooperative or mentally unwell patients, in non-organic hearing loss after giving sedation. Sedation does not affect OAEs.
- It is advisable to do tympanometry with OAEs because the middle ear pathology may interfere with the recording of OAEs. There are chances that the clinicians may misinterpret the absence of OAEs as a sensorineural hearing loss if they are unaware of the existence of middle ear pathology.
- OAEs may not diagnose the auditory neuropathy in the neonates. Auditory neuropathy is a hearing disorder characterized by abnormal or absent auditory brainstem response in the presence of normal outer hair cell function. Therefore, AABR is the best method in the NICU/SCBU population to detect auditory neuropathy.
OAE test results are affected by the ambient noise level and the patient’s internal noise level (e.g. breathing or body movements. Therefore, the results of OAE tests should be interpreted in conjunction with pure tone audiometry, tympanometry etc.
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Dr. Rahul Kumar Bagla
MS & Fellow Rhinoplasty & Facial Plastic Surgery.
Associate Professor & Head
GIMS, Greater Noida, India