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Overview

The primary purpose of otoacoustic emission (OAE) tests is to determine cochlear status, specificallyhair cell function. This information can be used to (1)screen hearing(particularly in neonates, infants,or individuals with developmental disabilities), (2) partially estimate hearing sensitivity within a limited

range, (3) differentiate between the sensory and neural components of sensorineural hearing loss,and (4) test for functional (feigned) hearing loss. The information can be obtained from patients whoare sleeping or even comatose because no behavioral response is required.

The normal cochlea does not just receive sound; it also produces low-intensity sounds called OAEs.These sounds are produced specifically by the cochlea and, most probably, by the cochlear outer haircells as they expand and contract. The presence of cochlear emissions was hypothesized in the1940s on the basis of mathematical models of cochlear nonlinearity. However, OAEs could not bemeasured until the late 1970s, when technology created the extremely sensitive low-noisemicrophones needed to record these responses.

The 4 types of otoacoustic emissions are as follows:

Spontaneous otoacoustic emissions (SOAEs) - Sounds emitted without an acoustic stimulus (ie,

spontaneously) 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 betone-bursts

Distortion product otoacoustic emissions (DPOAEs) - Sounds emitted in response to 2 simultaneoustones of different frequencies

Sustained-frequency otoacoustic emissions (SFOAEs) - Sounds emitted in response to acontinuous tone

An example of multifrequency spontaneous otoacoustic emissions can be seen in the image below.

An example of multifrequency spontaneous otoacoustic emissions (SOAEs)recorded from a 48-year-old woman with normal hearing. The black spikes represent the response above the noise floor.Pure-tone (PT) audiometrymeasures throughout the outer ear, middle ear, cochlea, cranial nerve (CN)VIII, and central auditory system. However, OAEs measure only the peripheral auditory system, whichincludes the outer ear, middle ear, and cochlea. The response only emanates from the cochlea, butthe outer and middle ear must be able to transmit the emitted sound back to the recordingmicrophone. OAE testing often is used as a screening tool to determine the presence or absence ofcochlear function, although analysis can be performed for individual cochlear frequency regions.

OAEs cannot be used to fully describe an individual's auditory thresholds, but they can help questionor validate other threshold measures (eg, in suspected functional [feigned] hearing loss), or they canprovide information about the site of the lesion.

Using current technology, most researchers and clinicians find a correlation between frequency-specific analysis of TOAEs/DPOAEs and cochlear hearing loss. However, at this juncture, thecorrelation cannot fully describe auditory threshold. Naturally, a correlation would not be expected fornoncochlear hearing loss.

Recording

Approach Considerations

Insert a probe with a soft flexible tip in the ear canal to obtain a seal. Use different probes for

neonates and adults; the probes are calibrated differently because of the significant difference in ear

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canal volume. The smaller ear canal results in a higher effective sound pressure level (SPL), thus adifferent probe is used to correct for the difference.

Multiple responses are averaged. All OAEs are analyzed relative to the noise floor; therefore,reduction of physiologic and acoustic ambient noise is critical for good recordings. Because no

behavioral response is required, OAEs can be obtained even from patients who are comatose. For aquiet and cooperative patient, recordings usually require less a few minutes per ear. For anuncooperative or noisy patient, recordings may take significantly longer or may be impossible toobtain on a given visit.

Recording Parameters

For all OAEs, an optimal probe fit is critical. Complete information on recording and interpreting OAEsis beyond the scope of this article; for discussions that are more comprehensive, please see thebibliography.

Spontaneous otoacoustic emissions

This nonevoked response is usually measured in narrow bands (< 30 Hz bandwidth) of frequencies

recorded in the external ear canal. No stimulus is required. Obtain multiple recordings to ensurereplicability and to distinguish the response from the noise floor. SOAE recordings usually span the500-Hz to 7000-Hz frequency range.

Transient otoacoustic emissions

Clicks are the most commonly used stimuli, although tone-burst stimuli may be used. Most commonly,80- to 85-dB SPL stimuli are used clinically. The stimulation rate is less than 60 stimuli per second.TOAEs are generally recorded in the time domain over approximately 20 milliseconds. Alternatingresponses are stored in alternating computer memory banks, A and B. Data that correlate betweenthe 2 memory banks are considered a response. Data that do not correlate are considered noise.When present, TOAEs generally occur at frequencies of 500-4000 Hz. Data in the time domain thenare converted to the frequency domain, usually in octave band analysis.[1]

Distortion product otoacoustic emissions

Stimuli consist of 2 pure tones at 2 frequencies (ie, f1, f2 [f2>f1]) and 2 intensity levels (ie, L1, L2).The relationship between L1-L2 and f1-f2 dictates the frequency response. An f1/f2 ratio yields thegreatest DPOAEs at 1.2 for low and high frequencies and at 1.3 for medium frequencies. To yield anoptimal response, set intensities so that L1 equals or exceeds L2. Lowering the absolute intensity ofthe stimulus renders the DPOAEs more sensitive to abnormality. A setting of 65/55 dB SPL L1/L2 isfrequently used. Responses are usually most robust and recorded at the emitted frequency of 2 f1f2;however, they generally are charted according to f2 because that region approximates the cochlearfrequency region generating the response.

Prerequisites for obtaining otoacoustic emissions

Prerequisities include the following:

Unobstructed outer ear canal

Seal of the ear canal with the probe

Optimal positioning of the probe

Absence of middle ear pathology: Pressure equalization (PE) tubes alone probably will not interferewith results. However, if emissions are absent, results should be interpreted with caution.

Functioning cochlear outer hair cells

A quiescent patient: Excessive movement or vocalization may preclude recording.

Relatively quiet recording environment: A sound booth is not required, but a noisy environment maypreclude accurate recording.

Interpretation

Spontaneous otoacoustic emissions

In general, SOAEs occur in only 40-50% of individuals who have normal hearing. For these adults,the range is about 30-60%; in neonates with normal hearing, the range is approximately 25-80%.

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Poor probe tip placement or poor seal: Most current equipment alerts clinicians to these problems.

Standing waves: Most current equipment alerts clinicians to standing waves.

Cerumen occluding the canal or blocking a probe port

Debris and foreign objects in the outer ear canal

Vernix caseosa in neonates: This is common immediately after birth.

Uncooperative patient: Usually, recordings simply are not obtained.

Pathologic problems that can cause absence of OAEs

Outer ear

Stenosis

External otitis

Cyst

Abnormal middle ear pressureTympanic membrane - Perforation of the eardrum (PE tubes do not necessarily prevent goodrecordings.)

Middle ear

Otosclerosis

Middle ear disarticulation

Cholesteatoma

Cyst

Bilateral otitis media: To record OAEs, the cochlear response must be able to travel efficientlythrough the middle ear and tympanic membrane to the recording microphone in the ear canal. Evenin the presence of normal cochlear function, OAEs generally are absent in the presence of otitismedia. OAE testing is best conducted after the otitis media has cleared. If the patient cannot betested later, when the otitis has cleared, no harm exists in attempting to record OAEs. If OAEs arepresent (as in a very small percentage of patients with otitis media), that information could be useful.If they are absent (as in most patients with otitis media), no conclusions about cochlear function canbe drawn.

Cochlea

Exposure to ototoxic medication or noise exposure (including music): OAE changes may precedethreshold changes in the conventional frequency range.

Any other cochlear pathology

Conditions that do not affect OAEs

CN VIII pathology: If CN VIII pathology also affects the cochlea (eg, vestibular schwannoma thatdecreases cochlear vascular supply), OAEs are affected.

Central auditory disorder

Conditions that elicit abnormal OAEs and normal behavioral thresholds

Tinnitus: OAEs may be abnormal in the frequency region of the tinnitus.

Excessive noise exposure (may cause increase or decrease in amplitude): No clear correlation tonoise-induced threshold changes is noted.[2]

Ototoxicity

Vestibular pathology

Conditions that elicit normal OAEs and abnormal behavioral thresholds

Functional hearing loss

Attention deficits

Autism

Possibly, inner hair cell damage but normal outer hair cells (reported for animals but no humanreports yet)

Auditory neuropathy: This includes central auditory nervous system dysfunction and CN VIII auditorydysfunction.

Auditory Neuropathy

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The advent of otoacoustic emissions (OAE) recordings opened a new area of auditory investigation inauditory neuropathy. Although auditory neuropathy is not a new disorder, OAEs have triggerednumerous new studies. Auditory neuropathy is also more common than previouslythought.[3] Therefore, a more complete listing is provided for this disorder.

Classic auditory neuropathy is characterized by the presence of OAEs or enlarged cochlearmicrophonics, abnormal ABR findings, and, often, absent or abnormal behavioral responses to sound.(OAEs may be absent and an auditory neuropathy still may exist if concomitant cochlear disorder ispresent. Also OAEs may often disappear over time in auditory neuropathy patients.)

ABR abnormalities consistent with auditory neuropathy include absence of all ABR waveforms orprolonged interpeak latencies. A large cochlear microphonic sometimes is observed on the ABRrecordings for these patients. The patient with auditory neuropathy may have any type of audiometricconfiguration, but rising or flat configurations are most common. Often, the patient's word recognitionis disproportionately poor relative to PT thresholds. Listening in noise usually is very difficult. Hearingmay fluctuate. Over time, it may stabilize, improve, or progress to profound hearing loss. If theetiology is known, a more accurate prognosis may frequently be given; however, the disorder can beidiopathic.

The cause of auditory neuropathy sometimes is unknown; however, the following conditions may beassociated with pediatric auditory neuropathy:

Hyperbilirubinemia

Neurodegenerative diseases

Neurometabolic diseases

Demyelinating diseases

Hereditary motor sensory neuropathologies (eg, Charcot-Marie-Tooth diseases with deafness)

Inflammatory neuropathy

Hydrocephalus

Severe and/or pervasive developmental delay

Ischemic-hypoxic neuropathy

Encephalopathy Meningitis

Cerebral palsy

Anatomy and Physiology Underlying Otoacoustic Emissions

Because OAEs may be new to some clinicians, a brief review of the relevant anatomy and physiologyis provided.

When sound is used to elicit an emission, it is transmitted through the outer ear, where the auditorystimulus is converted from an acoustic signal to a mechanical signal at the tympanic membrane and istransmitted through the middle ear ossicles; the stapes footplate moves at the oval window, causing atraveling wave in the fluid-filled cochlea. The cochlear fluid's traveling wave moves the basilarmembrane; each portion of the basilar membrane is maximally sensitive to only a limited frequencyrange. The arrangement is a tonotopic gradient. Regions closest to the oval window are moresensitive to high-frequency stimuli. Regions further away are most sensitive to lower-frequency stimuli.Therefore, for OAEs, the first responses returned and recorded by the probe microphone emanatefrom the highest-frequency cochlear regions because the travel distance is shorter. Responses fromthe lower-frequency regions, closer to the cochlear apex, arrive later.

When the basilar membrane moves, the hair cells are set into motion and an electromechanicalresponse is elicited, while an afferent signal is transmitted and an efferent signal is emitted. Theefferent signal is transmitted back through the auditory pathway, and the signal is measured in theouter ear canal. As described above, the responses from the high-frequency region arrive first,progressively followed by responses from lower-frequency regions.

Outer hair cells are located in the organ of Corti on the basilar membrane. These hair cells are motile;an electrochemical response elicits a motoric response. The 3 rows of outer hair cells have stereocilia

arranged in a W formation. The stereocilia are linked to each other and, therefore, move as a unit.These are the outer hair cells believed to underlie OAE generation.