Otoacoustic Emissions

Otoacoustic Emissions

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The primary purpose of otoacoustic emission (OAE) tests is to determine cochlear status, specifically hair cell function. This information can be used to (1) screen hearing (particularly in neonates, inf, or individuals with developmtal disabilities), (2) partially estimate hearing ssitivity within a range, (3) differtiate betwe the ssory and neural componts of ssorineural hearing loss, and (4) test for functional (feigned) hearing loss. The information can be obtained from patits who are sleeping or ev comatose because no behavioral response is required.

The normal cochlea does not just receive sound; it also produces low-intsity sounds called OAEs. These sounds are produced specifically by the cochlea and, most probably, by the cochlear outer hair cells as they expand and contract. The presce of cochlear emissions was hypothesized in the 1940s on the basis of mathematical models of cochlear nonlinearity. However, OAEs could not be measured until the late 1970s, wh extremely ssitive low-noise microphones needed to record these responses became available.

The four types of OEAs are as follows:

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

Transit otoacoustic emissions (TOAEs) or transit evoked otoacoustic emissions (TEOAEs) – Sounds emitted in response to an acoustic stimuli of very short duration; usually clicks but can be tone-bursts

Distortion product otoacoustic emissions (DPOAEs) – Sounds emitted in response to 2 simultaneous tones of differt frequcies

Sustained-frequcy otoacoustic emissions (SFOAEs) – Sounds emitted in response to a continuous tone

An example of multifrequcy spontaneous otoacoustic emissions can be se in the im below.

Pure-tone (PT) audiometry measures throughout the outer ear, middle ear, cochlea, cranial nerve (CN) VIII, and ctral auditory system. However, OAEs measure only the peripheral auditory system, which includes the outer ear, middle ear, and cochlea. The response only emanates from the cochlea, but the outer and middle ear must be able to transmit the emitted sound back to the recording microphone. OAE testing oft is used as a screing tool to determine the presce or ance of cochlear function, although analysis can be performed for individual cochlear frequcy regions. OAEs cannot be used to fully describe an individual’s auditory thresholds, but they can help question or validate other threshold measures (eg, in suspected functional [feigned] hearing loss), or they can provide information about the site of the lesion.

Using currt technology, most researchers and clinicians find a correlation betwe frequcy-specific analysis of TOAEs/DPOAEs and cochlear hearing loss. However, at this juncture, the correlation cannot fully describe auditory threshold. Naturally, a correlation would not be expected for noncochlear hearing loss.

Insert a probe with a soft flexible tip in the ear canal to obtain a seal. Use differt probes for neonates and adults; the probes are calibrated differtly because of the significant differce in ear canal volume. The smaller ear canal results in a higher effective sound pressure level (SPL), thus a differt probe is used to correct for the differce.

Multiple responses are averd. All OAEs are analyzed relative to the noise floor; therefore, reduction of physiologic and acoustic ambit noise is critical for good recordings. Because no behavioral response is required, OAEs can be obtained ev from patits who are comatose. For a quiet and cooperative patit, recordings usually require less a few minutes per ear. For an uncooperative or noisy patit, recordings may take significantly longer or may be impossible to obtain on a giv visit.

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

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 gerally recorded in the time domain over approximately 20 milliseconds. Alternating responses are stored in alternating computer memory banks, A and B. Data that correlate betwe the 2 memory banks are considered a response. Data that do not correlate are considered noise. Wh prest, TOAEs gerally occur at frequcies of 500-4000 Hz. Data in the time domain th are converted to the frequcy domain, usually in octave band analysis. [1]

Stimuli consist of 2 pure tones at 2 frequcies (ie, f1, f2 [f2>f1]) and 2 intsity levels (ie, L1, L2). The relationship betwe L1-L2 and f1-f2 dictates the frequcy response. An f1/f2 ratio yields the grest DPOAEs at 1.2 for low and high frequcies and at 1.3 for medium frequcies. To yield an optimal response, set intsities so that L1 equals or exceeds L2. Lowering the absolute intsity of the stimulus rders the DPOAEs more ssitive to abnormality. A setting of 65/55 dB SPL L1/L2 is frequtly used. Responses are usually most robust and recorded at the emitted frequcy of 2 f1–f2; however, they gerally are charted according to f2 because that region approximates the cochlear frequcy region gerating the response. [2]

Prerequisities include the following:

Unobstructed outer ear canal

Seal of the ear canal with the probe

Optimal positioning of the probe

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

Functioning cochlear outer hair cells

A quiesct patit: Excessive movemt or vocalization may preclude recording.

Relatively quiet recording vironmt: A sound booth is not required, but a noisy vironmt may preclude accurate recording.

In geral, 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%. SOAEs gerally are not found in individuals with hearing thresholds worse than 30 dB HL. Therefore, the presce of SOAEs usually is considered a sign of cochlear health, but the ance of SOAEs is not necessarily a sign of abnormality.

Wh prest in humans, SOAEs usually occur in the 1000- to 2000-Hz region; amplitudes are betwe -5 and 15 dB SPL. Some individuals have multifrequcy SOAEs over a broader frequcy range.

SOAEs typically are bilateral rather than unilateral. If unilateral, they are more likely to be prest in the right rather than in the left ear. SOAEs occur more oft in females than in males (across all s).

Usually, SOAEs are not associated with tinnitus. Because tinnitus oft occurs in conjunction with cochlear abnormality, SOAEs usually are ant. SOAEs are seldom used clinically to scre hearing. The ance of SOAEs does not imply abnormal auditory function, as indicated above.

High-level SOAEs may occur. These emissions can be heard by others. Objective tinnitus is usually a misnomer because the patit oft cannot hear these noises. Such emissions are very uncommon but may coexist with ssory hearing loss. High-level SOAEs are more common in childr than in adults.

In the clinic, TOAEs commonly are used to scre infant hearing, to validate behavioral or electrophysiologic auditory thresholds, and to assess cochlear function relative to the site of the lesion. By definition, TOAEs are recorded only in response to very short or transit stimuli. Therefore, the stimulus has frequcy specificity, and the TOAE emanates from a relatively broad cochlear region. However, currt analysis techniques allow the response to be separated into various frequcy bands for analysis.

In geral, the presce of a TOAE in a particular frequcy band suggests that cochlear ssitivity in that region is approximately 20-40 dB HL or better, depding on the study cited. Most clinicians use the presce of a TOAE in a particular octave band to suggest that hearing ssitivity should be 30 dB HL or better, unless a functional or neural compont is prest.

The relative merits of TOAEs and DPOAEs are widely discussed. Esstially, DPOAEs allow grer frequcy specificity and can be used to record at higher frequcies than TOAEs. Therefore, DPOAEs may be as particularly useful for early detection of cochlear dam as they are for ototoxicity and noise-induced dam. However, large-scale comparative studies of TOAEs and DPOAEs in these groups of patits currtly are lacking. Reliability of DPOAEs is grest above 1000 Hz.

For infant hearing screing, both DPOAEs and TOAEs are used. TOAEs have be used clinically for a longer period and are more established regarding association with behavioral audiometric thresholds.

Depding on the methodology employed, DPOAEs oft can be recorded in individuals with mild-to-moderate hearing losses for whom TOAEs are ant; however, the accuracy of DPOAEs in estimating actual hearing ssitivity is not fully resolved (research continues in this area). DPOAEs frequtly correspond to the audiometric configuration of a cochlear hearing loss, which is helpful in some patits.

A retrospective study by Wooles et al indicated that DPOAEs would not be a useful replacemt for pure tone audiometry in longitudinal screing for auditory deficits related to occupational noise exposure. In an analysis of 16 brickyard workers, the study found that, although the pure tone audiometry threshold was elevated in 13 of the workers, sev of the workers had substantial DPOAEs at the frequcies associated with these thresholds. [3]

SFOAEs are responses recorded to a continuous tone. Because the stimulus and the emission overlap in the ear canal, the recording microphone detects both. Therefore, interpretation depds on reading a complicated series of ripples in the recording. At prest, SFOAEs are not used clinically.

These include the following:

Poor probe tip placemt or poor seal: Most currt equipmt alerts clinicians to these problems.

Standing waves: Most currt equipmt alerts clinicians to standing waves.

Cerum 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 patit: Usually, recordings simply are not obtained

A study by Abdala et al indicated that DPOAEs have a grer rate of decline with than do SFOAEs, with the investigators finding evidce that -related increases in intracochlear roughness may lead to the relative preservation of SFOAE levels. [4]

Outer ear

Stosis

External otitis

Cyst

Abnormal middle ear pressure

Tympanic membrane – Perforation of the eardrum (PE tubes do not necessarily prevt good recordings.)

Middle ear

Otosclerosis

Middle ear disarticulation

Cholestoma

Cyst

Bilateral otitis media: To record OAEs, the cochlear response must be able to travel efficitly through the middle ear and tympanic membrane to the recording microphone in the ear canal. Ev in the presce of normal cochlear function, OAEs gerally are ant in the presce of otitis media. OAE testing is best conducted after the otitis media has ed. If the patit cannot be tested later, wh the otitis has ed, no harm exists in attempting to record OAEs. If OAEs are prest (as in a very small perct of patits with otitis media), that information could be useful. If they are ant (as in most patits with otitis media), no conclusions about cochlear function can be drawn.

Cochlea

Exposure to ototoxic medication or noise exposure (including music): OAE changes may precede threshold changes in the convtional frequcy range.

Any other cochlear pathology

See the below:

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

Ctral auditory disorder

See the below:

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

Excessive noise exposure (may cause increase or decrease in amplitude): No correlation to noise-induced threshold changes is noted. [5]

Ototoxicity

Vestibular pathology

See the below:

Functional hearing loss

Atttion deficits

Autism

Possibly, inner hair cell dam but normal outer hair cells (reported for animals but no human reports yet)

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

The advt of otoacoustic emissions (OAE) recordings oped a new area of auditory investigation in auditory neuropathy. Although auditory neuropathy is not a new disorder, OAEs have triggered numerous new studies. Auditory neuropathy is also more common than previously thought. [6] Therefore, a more complete ing is provided for this disorder.

Classic auditory neuropathy is characterized by the presce of OAEs or larged cochlear microphonics, abnormal auditory brainstem response (ABR) findings, and, oft, ant or abnormal behavioral responses to sound. (OAEs may be ant and an auditory neuropathy still may exist if concomitant cochlear disorder is prest. Also OAEs may oft disappear over time in auditory neuropathy patits.)

ABR abnormalities consistt with auditory neuropathy include ance of all ABR waveforms or prolonged interpeak latcies. A large cochlear microphonic sometimes is orved on the ABR recordings for these patits. The patit with auditory neuropathy may have any type of audiometric configuration, but rising or flat configurations are most common. Oft, the patit’s word recognition is disproportionately poor relative to PT thresholds. ing in noise usually is very difficult. Hearing may fluctuate. Over time, it may stabilize, improve, or progress to profound hearing loss. If the etiology is known, a more accurate prognosis may frequtly be giv; however, the disorder can be idiopathic.

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

Hyperbilirubinemia

Neurodegerative diseases

Neurometabolic diseases

Demyelinating diseases

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

Inflammatory neuropathy

Hydrocephalus

Severe and/or pervasive developmtal delay

Ischemic-hypoxic neuropathy

cephalopathy

Mingitis

Cerebral palsy

A study by Narne et al suggested that TOAE variations can aid in understanding the pathophysiology of auditory neuropathy. In the study, of 15 persons with an auditory neuropathy spectrum disorder and 22 individuals with normal hearing, wavelet analysis in narrow-band frequcy regions indicated that TOAEs in the auditory neuropathy group had a higher amplitude and slightly shorter latcy than did TOAEs in the group with normal hearing, at low frequcies and midfrequcies. These differces, according to the investigators, could be ascribed to effert system dam in auditory neuropathy. [7]

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

Wh sound is used to elicit an emission, it is transmitted through the outer ear, where the auditory stimulus is converted from an acoustic signal to a mechanical signal at the tympanic membrane and is transmitted through the middle ear ossicles; the stapes footplate moves at the oval window, causing a traveling wave in the fluid-filled cochlea. The cochlear fluid’s traveling wave moves the basilar membrane; each portion of the basilar membrane is maximally ssitive to only a frequcy range. The arrangemt is a tonotopic gradit. Regions closest to the oval window are more ssitive to high-frequcy stimuli. Regions further away are most ssitive to lower-frequcy stimuli. Therefore, for OAEs, the first responses returned and recorded by the probe microphone emanate from the highest-frequcy cochlear regions because the travel distance is shorter. Responses from the lower-frequcy regions, closer to the cochlear apex, arrive later.

Wh the basilar membrane moves, the hair cells are set into motion and an electromechanical response is elicited, while an affert signal is transmitted and an effert signal is emitted. The effert signal is transmitted back through the auditory pathway, and the signal is measured in the outer ear canal. As described above, the responses from the high-frequcy region arrive first, progressively followed by responses from lower-frequcy regions.

Outer hair cells are located in the organ of Corti on the basilar membrane. These hair cells are motile; an electro 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 geration.

McPherson B, Li SF, Shi BX, Tang JL, Wong BY. Neonatal hearing screing: evaluation of tone-burst and click-evoked otoacoustic emission test criteria. Ear Hear. 2006 Jun. 27(3):256-62. [Medline].

Peters L, Wilson WJ, Kathard H. Towards the preferred stimulus parameters for distortion product otoacoustic emissions in adults: A preliminary study. S Afr J Commun Disord. 2018 Jul 16. 65 (1):e1-e10. [Medline].

Wooles N, Mulheran M, Bray P, Brewster M, Banerjee AR. Comparison of distortion product otoacoustic emissions and pure tone audiometry in occupational screing for auditory deficit due to noise exposure. J Laryngol Otol. 2015 Dec. 129 (12):1174-81. [Medline].

Abdala C, Ortmann AJ, Shera CA. Reflection- and Distortion-Source Otoacoustic Emissions: Evidce for Increased Irregularity in the Human Cochlea During Aging. J Assoc Res Otolaryngol. 2018 Jul 2. [Medline].

Helleman HW, Dreschler WA. Overall versus individual changes for otoacoustic emissions and audiometry in a noise-exposed cohort. Int J Audiol. 2012 May. 51(5):362-72. [Medline].

Dowley AC, Whitehouse WP, Mason SM, Cope Y, Grant J, Gibbin KP. Auditory neuropathy: unexpectedly common in a screed newborn population. Dev Med Child Neurol. 2009 Aug. 51(8):642-6. [Medline].

Narne VK, Prabhu PP, Chatni S. Time-frequcy analysis of transit evoked-otoacoustic emissions in individuals with auditory neuropathy spectrum disorder. Hear Res. 2014 Jul. 313:1-8. [Medline].

Kathle C M Campbell, PhD Distinguished Scholar and Professor, Departmt of Microbiology, Immunology and Cell Biology, Southern Illinois University School of Medicine

Kathle C M Campbell, PhD is a member of the following societies: American Academy of Audiology, American Academy of Otolaryngology-Head and Neck Surgery, American Auditory Society, American Speech-Language-Hearing Association, Association for Research in Otolaryngology, International Society of Audiology

Disclosure: Nothing to disclose.

Ginger Mullin, AuD Illinois EHDI Program, Illinois Departmt of Public Health

Disclosure: Nothing to disclose.

Francisco Talavera, PharmD, PhD Adjunct Assistant Professor, University of Nebraska Cter College of Pharmacy; Editor-in-Chief, Medscape Drug Referce

Disclosure: Received salary from Medscape for employmt. for: Medscape.

Peter S Roland, MD Professor, Departmt of Neurological Surgery, Professor and Chairman, Departmt of Otolaryngology-Head and Neck Surgery, Director, Clinical Cter for Auditory, Vestibular, and Facial Nerve Disorders, Chief of Otology, University of Texas Southwestern Cter; Chief of Otology, Childr’s Cter of Dallas; Presidt of Staff, Parkland Memorial Hospital; Adjunct Professor of Communicative Disorders, School of Behavioral and Brain Scices, Chief of Service, Callier Cter for Communicative Disorders, University of Texas School of Human Developmt

Peter S Roland, MD is a member of the following societies: Alpha Omega Alpha, American Academy of Otolaryngic Allergy, American Academy of Otolaryngology-Head and Neck Surgery, American Auditory Society, American Neurotology Society, American Otological Society, North American Skull Base Society, Society of University Otolaryngologists-Head and Neck Surgeons, The Triological Society

Disclosure: Received honoraria from Alcon Labs for consulting; Received honoraria from Advanced Bionics for board membership; Received honoraria from Cochlear Corp for board membership; Received travel gr from Med El Corp for consulting.

Arl D Meyers, MD, MBA Professor of Otolaryngology, Dtistry, and gineering, University of Colorado School of Medicine

Arl D Meyers, MD, MBA is a member of the following societies: American Academy of Facial Plastic and Reconstructive Surgery, American Academy of Otolaryngology-Head and Neck Surgery, American Head and Neck Society

Disclosure: Serve(d) as a director, officer, partner, employee, advisor, consultant or trustee for: Cerescan;RxRevu;Cliexa;Preacute Population Health Manmt;The Physicians Edge
Received income in an amount equal to or grer than $250 from: The Physicians Edge, Cliexa
Received stock from RxRevu; Received ownership interest from Cerescan for consulting; for: Rxblockchain;Bridge Health.

Ted L Tewfik, MD Professor of Otolaryngology-Head and Neck Surgery, Professor of Surgery, McGill University Faculty of Medicine; Sior Staff, Montreal Childr’s Hospital, Montreal Geral Hospital, and Royal Victoria Hospital

Ted L Tewfik, MD is a member of the following societies: American Society of Pediatric Otolaryngology, Canadian Society of Otolaryngology-Head & Neck Surgery

Disclosure: Nothing to disclose.

Otoacoustic Emissions


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