You are in: eMedicine Specialties > Otolaryngology and Facial Plastic Surgery > AUDIOLOGY Otoacoustic EmissionsArticle Last Updated: May 31, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 9
  Author: Kathleen CM Campbell, PhD, Director of Audiology, Professor, Department of Surgery, Division of Otolaryngology, Southern Illinois University School of Medicine Kathleen C M Campbell is a member of the following medical societies: American Academy of Otolaryngology-Head and Neck Surgery, American Auditory Society, American Tinnitus Association, Association for Research in Otolaryngology, and New York Academy of Sciences Coauthor(s): Ginger Mullin, AuD, Newborn Infant Hearing Program, Division of Specialized Care for Children, State of Illinois Editors: Ted L Tewfik, MD, FRCS(C), Professor, Department of Otolaryngology, Director of Continuing Medical Education of Otolaryngology, McGill University Medical School; Director, Director of Professional Affairs of Otolaryngology, Department of Otolaryngology, Montreal Children's Hospital; Senior Staff, Montreal General Hospital and Royal Victoria Hospital; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Peter S Roland, MD, Professor, Department of Neurological Surgery, Professor and Chairman, Department of Otolaryngology-Head and Neck Surgery, Director of Clinical Center for Auditory, Vestibular and Facial Nerve Disorders, Chief of Pediatric Otology, University of Texas Southwestern Medical Center; Adjunct Professor of Communicative Disorders, School of Human Development, Chief of Medical Services at Callier Center for Communicative Disorders, University of Texas at Dallas; Christopher L Slack, MD, Otolaryngology-Facial Plastic Surgery, Private Practice, Associated Coastal ENT; Medical Director, Treasure Coast Sleep Disorders; Arlen D Meyers, MD, MBA, Professor, Department of Otolaryngology-Head and Neck Surgery, University of Colorado School of Medicine Author and Editor Disclosure Synonyms and related keywords: otoacoustic emissions, otoacoustic emission tests, OAE tests, OAEs, spontaneous otoacoustic emissions, SOAEs, transient otoacoustic emissions, TOAEs, transient evoked otoacoustic emissions, TEOAEs, distortion product otoacoustic emissions, DPOAEs INTRODUCTIONSection 2 of 9
  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, 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 who are 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 hair cells as they expand and contract. The presence 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, when technology created the extremely sensitive low-noise microphones needed to record these responses. The 4 types of otoacoustic emissions are as follows:
Pure-tone (PT) audiometry measures throughout the outer ear, middle ear, cochlea, cranial nerve (CN) VIII, and central 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 often is used as a screening tool to determine the presence or absence of cochlear 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 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 current technology, most researchers and clinicians find a correlation between frequency-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. RECORDINGSection 3 of 9
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 canal volume. The smaller ear canal results in a higher effective sound pressure level (SPL), thus a different 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 a quiet and cooperative patient, recordings usually require less a few minutes per ear. For an uncooperative or noisy patient, recordings may take significantly longer or may be impossible to obtain on a given visit. Recording parametersFor 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 comprehensive, please see the bibliography. Spontaneous otoacoustic emissions This nonevoked response usually is measured in narrow bands ( <30 Hz bandwidth) of frequencies recorded in the external ear canal. No stimulus is required. Obtain multiple recordings to ensure replicability and to distinguish the response from the noise floor. SOAE recordings usually span the 500- 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. Alternating responses are stored in alternating computer memory banks, A and B. Data that correlate between the 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 then are converted to the frequency domain, usually in octave band analysis. 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 the greatest DPOAEs at 1.2 for low and high frequencies and at 1.3 for medium frequencies. To yield an optimal response, set intensities so that L1 equals or exceeds L2. Lowering the absolute intensity of the stimulus renders the DPOAEs more sensitive to abnormality. A setting of 65/55 dB SPL L1/L2 is frequently used. Responses are usually most robust and recorded at the emitted frequency of 2 f1–f2; however, they generally are charted according to f2 because that region approximates the cochlear frequency region generating the response. Prerequisites for obtaining otoacoustic emissions
INTERPRETATIONSection 4 of 9
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%. SOAEs generally are not found in individuals with hearing thresholds worse than 30 dB HL. Therefore, the presence of SOAEs usually is considered a sign of cochlear health, but the absence of SOAEs is not necessarily a sign of abnormality. When present in humans, SOAEs usually occur in the 1000- to 2000-Hz region; amplitudes are between -5 and 15 dB SPL. Some individuals have multifrequency SOAEs over a broader frequency range. SOAEs typically are bilateral rather than unilateral. If unilateral, they are more likely to be present in the right rather than in the left ear. SOAEs occur more often in females than in males (across all ages). Usually, SOAEs are not associated with tinnitus. Because tinnitus often occurs in conjunction with cochlear abnormality, SOAEs usually are absent. SOAEs are seldom used clinically to screen hearing. The absence 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 patient often cannot hear these noises. Such emissions are very uncommon but may coexist with sensory hearing loss. High-level SOAEs are more common in children than in adults. Transient otoacoustic emissions In the clinic, TOAEs commonly are used to screen 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 transient stimuli. Therefore, the stimulus has limited frequency specificity, and the TOAE emanates from a relatively broad cochlear region. However, current analysis techniques allow the response to be separated into various frequency bands for analysis. In general, the presence of a TOAE in a particular frequency band suggests that cochlear sensitivity in that region is approximately 20-40 dB HL or better, depending on the study cited. Most clinicians use the presence of a TOAE in a particular octave band to suggest that hearing sensitivity should be 30 dB HL or better, unless a functional or neural component is present. Distortion product otoacoustic emissions The relative merits of TOAEs and DPOAEs are widely discussed. Essentially, DPOAEs allow greater frequency specificity and can be used to record at higher frequencies than TOAEs. Therefore, DPOAEs may be as particularly useful for early detection of cochlear damage as they are for ototoxicity and noise-induced damage. However, large-scale comparative studies of TOAEs and DPOAEs in these groups of patients currently are lacking. Reliability of DPOAEs is greatest above 1000 Hz. For infant hearing screening, both DPOAEs and TOAEs are used. TOAEs have been used clinically for a longer period and are more established regarding association with behavioral audiometric thresholds. Depending on the methodology employed, DPOAEs often can be recorded in individuals with mild-to-moderate hearing losses for whom TOAEs are absent; however, the accuracy of DPOAEs in estimating actual hearing sensitivity is not fully resolved (research continues in this area). DPOAEs frequently correspond to the audiometric configuration of a cochlear hearing loss, which is helpful in some patients. Sustained-frequency otoacoustic emissions 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 depends on reading a complicated series of ripples in the recording. At present, SFOAEs are not used clinically. FACTORS THAT CAN AFFECT OTOACOUSTIC EMISSIONSSection 5 of 9
Nonpathologic problems that can cause absence of OAEs
Pathologic problems that can cause absence of OAEs Outer ear
Tympanic membrane - Perforation of the eardrum (PE tubes do not necessarily prevent good recordings.) Middle ear
Cochlea
Conditions that do not affect OAEs
Conditions that elicit abnormal OAEs and normal behavioral thresholds
Conditions that elicit normal OAEs and abnormal behavioral thresholds
AUDITORY NEUROPATHYSection 6 of 9
The advent of OAE recordings opened a new area of auditory investigation in auditory neuropathy. Although auditory neuropathy is not a new disorder, OAEs have triggered numerous new studies. Therefore, a more complete listing is provided for this disorder. Classic auditory neuropathy is characterized by the presence of OAEs, 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 is present.) ABR abnormalities consistent with auditory neuropathy include absence of all ABR waveforms or prolonged interpeak latencies. A large cochlear microphonic sometimes is observed on the ABR recordings for these patients. The patient with auditory neuropathy may have any type of audiometric configuration, but rising or flat configurations are most common. Often, the patient's word recognition is disproportionately poor relative to PT thresholds. Listening 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 frequently be given; however, the disorder can be idiopathic. The cause of auditory neuropathy sometimes is unknown; however, the following conditions may be associated with pediatric auditory neuropathy:
ANATOMY AND PHYSIOLOGY UNDERLYING OTOACOUSTIC EMISSIONSSection 7 of 9
Because OAEs may be new to some clinicians, a brief review of the relevant anatomy and physiology is provided. When 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 sensitive to only a limited frequency range. The arrangement is a tonotopic gradient. Regions closest to the oval window are more sensitive 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 emanate from the highest-frequency cochlear regions because the travel distance is shorter. Responses from the lower-frequency regions, closer to the cochlear apex, arrive later. When the basilar membrane moves, the hair cells are set into motion and an electromechanical response is elicited, while an afferent signal is transmitted and an efferent signal is emitted. The efferent 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-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. MULTIMEDIASection 8 of 9
REFERENCESSection 9 of 9
Article Last Updated: May 31, 2006 | ||||||||||||||||||||||||
