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Advanced Issues in Otoacoustic Emissions Thierry Morlet, Ph.D. Outline Cochlear Physiology OAE definition, Types and Characteristics Interpretation of OAEs OAE measurements OAEs variation in Clinical Populations Clinical Applications of OAEs Organ of Corti 12,000 Outer Hair Cells Scala Vestibuli 4,000 Inner Hair Cells Scala Tympani Hair Cells Tallest stereocilia are embedded into The tectorial membrane The IHC stereocilia can be considered freestanding in the subtectorial space Innervation 20 cochlear afferents nerve endings per IHC L. Andrade and B. Kachar, NIDCD/NIH Afferent Innervation The vast majority of the afferent auditory nerve fibers form one-to-one connections with a single IHC after entering the cochlea A minority (about 5%) of the afferent neurons cross the organ of Corti and connect with groups of OHC The afferent innervation of OHC and IHC has little, if any, impact on OAEs Efferent Innervation: The olivocochlear System Cochlear Potentials Endocochlear potential Cochlear microphonic Summating potential Compound action potential Otoacoustic emissions OHC Electromotility There is a constant drive of potassium from the endolymph into the OHC. When stimulated, the stereocila will bend which opens more ion channels into the OHC and lead to their depolarization. The depolarization is the trigger for the activation of electromotility. Hyperpolarization causes OHC elongation Depolarization causes OHC contraction Cochlear Active Mechanism Dr Ashmore, 1987 Amplification/Fine Frequency Discrimination Otoacoustic Emissions First discovery by Kemp (1978) Discovery of motile characteristics of OHCs: Brownell et al., 1985 The origin of OAEs is ascribed to processes associated with the mechanical motion of the OHCs, and OAEs are thought to be modulated by the efferent auditory pathways via the olivocochlear system. OAEs appear sensitive to subtle changes in cochlear function that are not revealed in the octave interval behavioral audiogram. OAEs have high test-retest reliability, which contributes to their clinical utility. The OAE Story (by Dr Kemp) Nobel Prize winner George von Bekesy first explained how sound created travelling waves on the basilar membrane in the 1940s. The travelling wave separated frequency components in the cochlea but the degree of frequency separation seen by Bekesy in human ears post mortem was quite poor. In contrast, recordings made in auditory nerve fibers themselves showed that the healthy cochlea somehow managed to achieve sharp frequency division. As early as 1948, Thomas Gold argued that to achieve simultaneously both high sensitivity and high frequency selectivity there must be a biological vibration amplifier. As in primitive radio receivers, this extra energy could be applied as positive feedback to the travelling wave to overcome the natural viscous loss of energy. Gold explained his ideas to von Bekesy but neither he nor any other auditory researcher took Gold’s ideas seriously. The OAE Story “In July of 1977 the crucial experiment was performed. I placed a miniature microphone salvaged from a hearing aid over the opening of my ear canal and then closed the ear canal with silicone putty to keep the ear’s sound in. I fed the output of the microphone into a hetrodyne analyser - an instrument which allows you to tune in to a very narrow frequency band (10Hz) at any frequency. Via a headphone I then applied a single pure tone at a frequency and level that I knew would give rise to a clear aural distortion tone as the external tone combined with the (supposedly) internal oscillation. I knew the frequency of the combination tone exactly by applying the formulate 2f1-f2 to the external tone (f1) and the internal tone (f2) and I accurately tuned the analyser to this frequency. Immediately there was a reading on the dial! As I changed the frequency of the external tone, the frequency of the combination tone in the ear canal changed exactly as the formula predicted. The internal tone was therefore physically present - not a neural phantom - and so should it also be detectable. I turned off the stimulus tone and tuned the analyser to the frequency of the supposed internal tone. It was there. A signal of 85dBSPL at 1253Hz was continuously present in my left ear canal. And another at 1760Hz. What’s more, they were still there the next day at exactly the same frequency - within 1 or 2Hz. Such stability was difficult to obtain even with high quality electronic equipment in those days - and to find it in a biological system was hard to believe. But the evidence was incontrovertible.” The OAE Story “In 1977, it seemed that, as in 1948, the true significance of these experiments for auditory science might be lost due to entrenched thinking and misunderstandings about the highly technical acoustic experiments. To help overcome this, one final experiment was performed. The reasoning was that if sound energy reverberated inside the cochlea as it did in a large room, then applying a short click to the ear would, like a clap in a room, resulting an echo. The hetrodyne analyser was replaced by a physiological signal averager and the pure tone stimulus was replaced with a click. Sure enough, the ear gave an evoked response to the click – a long complex emission of sound lasting 16 milliseconds and more. It was like nothing seen before from the auditory system. It was a cochlea echo.” The OAE Story The first scientific presentation of acoustic emissions was in April 1978 at a meeting of the British Society of Audiology in Keele University. The new discovery was received with great skepticism, not least because the concept of waves travelling in reverse in the cochlea contradicted firmly held views at the time. Many physiologists also doubted the early evidence that OAEs came from the cochlea. Types of OAEs After their discovery, the otoacoustic emissions (OAEs) were classified on the basis of the nature of the stimuli used to evoke them. 2 main categories: – Spontaneous otoacoustic emissions (SOAEs) are recorded in the absence of external stimuli. – Evoked OAEs: OAEs recorded with stimuli. Classification of OAEs Among the OAEs generated by a stimulus, we distinguish 3 categories: – Distortion product otoacoustic emissions (DPOAEs): OAEs evoked using tonal pairs. They appear at frequencies consistent with intermodulation distortion of the 2 stimulus tones. – Transient (or click-evoked) otoacoustic emissions (TEOAEs, CEOAEs): OAEs generated with a very brief stimulus (click). A subcategory is the toneburst OAEs (TBOAEs): OAEs generated by a very brief sound that contains less frequencies than a click (tone-burst). These TBOAEs are mostly used in research and have less of a clinical application. – Stimulus frequency otoacoustic emissions (SFOAEs): evoked by a single tonal stimulus. The frequency of the SFOAE is identical to that of the stimulus. Spontaneous Otoacoustic Emissions (SOAEs) Occur without external stimuli Transient Otoacoustic Emissions (TEOAEs) Elicited by brief pulses (clicks or tonebursts or even amplitude modulated tones, etc.). Distortion Product Otoacoustic Emissions (DPOAEs) Recorded in response to pairs of tones Classification of OAEs The classification of OAEs based on the sound (or absence of sound) used to generate them is still in use today. Another way of classifying the OAEs has emerged more recently. OAEs can be classified based on their mechanisms of generation (according to Kemp and his colleagues): – Wave-fixed OAEs: when their generators move or translate along with the envelope of the travelling waves of the stimuli. – Place-fixed OAEs: when their generators are fixed in place along the basilar membrane. Classification of OAEs A more formal model has been developed by Shera and Guinan (1999). In this model, OAEs are generated by 2 mechanisms: – Reflection (roughly similar to place-fixed OAEs) – Distortion (roughly similar to wave-fixed OAEs) OAEs recorded in the ear canal could also be due to the 2 different mechanisms (mixed OAEs) when contributions of each mechanism are present. Transient Evoked OAEs TEOAEs Equivalent to click-evoked OAEs (or CEOAEs) Recorded with clicks of very short duration (80 microseconds). Depending on the intensity of the click, 2 different protocols are available: – Non linear – Linear The first few milliseconds of the post-stimulus window are discarded before transforming the data to the frequency domain. The level of the TEOAEs in comparison to the noise floor is analyzed by frequency bands. Absent TEOAEs TEOAEs Linear Recording from a preterm neonate Quickscreen recording from a preterm neonate TEOAEs elicited by Tone Bursts 2 kHz 4 kHz Characteristics of TEOAEs The prevalence of TEOAEs is close to 100% in normally hearing adults. Highest amplitude from 1000-2000 Hz in adults; more higher frequency components in infants. Noise tends to be concentrated in the lower frequencies Show a unique “signature” in individuals, based on characteristics of ear. Higher amplitude in females than in males. Higher amplitude in many individuals in right than left ears. Amplitude decreases with age. Neonatal subjects have significantly stronger emissions than adults and under normal conditions a lower average of stimuli is sufficient to obtain a good response. Age and Otoacoustic Emissions • Highest amplitude in infants and children than in adults. • More high frequency components in infants. Higher response from 10002000 Hz in adults. • Criteria to interpret absence/presence of OAEs essentially similar in newborns, infants, children and adults (i.e., signal to noise ratio). • However, the normative data used to decide if OAE amplitude at specific frequencies is normal or abnormal are different. TEOAEs and Hearing TEOAEs are usually not present when the average pure tone threshold is higher than 35 to 40 dB HL. TEOAEs cannot predict the auditory threshold. The reason is the fact that auditory perception involves other parts of the auditory system than just cochlear mechanisms. Nevertheless when: – The external and middle ear functions are normal. – There are no inner hair cells and/or retrocochlear hearing complications The audiometric outcomes and the information from the TEOAEs are in agreement. Distortion Product OAEs The DPOAEs are recorded by stimulating the cochlea simultaenously with 2 pure tones. The cochlea generates additional tonal signals at frequencies arithmetically related to those of the stimulus tones. The pure tones which stimulate the cochlea are called primaries and they are assigned as F1 and F2 and their corresponding amplitudes are assigned as L1 and L2. The most prominent and mostly used in clinical practice is the cubic difference distortion product denoted as 2F1 - F2. The healthy cochlea generates other DPOAEs: 3f1-2f2, 2f2-f1, 3f2-2f1, etc. The primaries should have frequencies which are close to one another. The ratio of the F2 / F1 frequencies is called frequency ratio. The choice of the frequency ratio has an effect on the amplitude of the DPOAEs at different tested frequencies. Most prominent DPOAE for FR between 1.20 and 1.22. DPOAEs Two main types of DPOAE protocols: – Primaries with equal intensities (L1 = L2), for example 70-70 dB SPL. – Unequal primary intensities (L1 > L2), for example 65-55 dB SPL. The latter can identify better cases with hearing impairment and they are used in most screening programs. When asymmetrical DPOAE protocols are used, the intermodulation components are generated close to the F2 primary tone. Therefore the DPOAE information is referenced to F2. When symmetrical protocols are used the DPOAE information is referenced to the geometric mean, which is defined a the square root of F1 * F2. DPOAEs There are two ways to present the DPOAE information: – In the DP-gram modality we measure the 2F1 - F2 amplitudes at various F2 frequencies, having fixed the stimulus intensities, for example F1=65 dB and F2=55 dB SPL. – In the Input -Output (IO) modality, we measure the 2F1 - F2 at a fixed F2 frequency, varying the primary stimulus levels. For screening applications a small number of frequencies is tested, such as 2.0, 3.0 and 4.0 kHz (referenced to F2). For ototoxicity- monitoring applications the bandwidth of the measurements extends up to 8-10 kHz (referenced to F2). DPOAEs DPOAEs DP-gram: measure the 2F1 - F2 amplitudes at various F2 frequencies, having fixed the stimulus intensities, for example F1=65 dB and F2=55 dB SPL. Input/Output Function Input -Output (IO) modality: measure 2F1 - F2 at a fixed F2 frequency, varying the primary stimulus levels. DPOAE Microstructure Number of points per octave depends on time available and desired frequency resolution. DPOAE Characteristics The prevalence of DPOAEs is close to 100% in normal adult ears. Highest amplitude at 1000-2000 Hz. Responses from the left and right ears are often correlated (that is, they are very similar). For normal subjects women have higher amplitude DPOAEs. Aging process has an effect on DPOAE responses by lowering the DPOAE amplitude and narrowing the DPOAE response spectrum ( i.e. responses at higher frequencies are gradually diminishing). DPOAE amplitude is dependent on frequency relationship between the two tones. DPOAE amplitude is dependent on intensity relationship between the two tones. DPOAE Characteristics The relationship between DPOAEs and Audiometric outcomes (mainly with the Pure Tone Audiometry) has been also a debating issue from the early days of otoacoustic emissions. While it is known that hearing impairments higher than 30 to 40 dB HL cause a significance decrease of the TEOAE responses, the same it is not valid for DPOAE recordings. Due to the efficiency of the DPOAE stimulation schemes it is possible to record responses even from cases presenting hearing losses as high as 50 dB HL. When: – The external and middle ear functions are normal. – There are no retrocochlear hearing complications The audiometric outcomes and the information from the DPOAEs are in agreement. Spontaneous OAE SOAEs are low level tonal signals that can be recorded in the external auditory meatus in the absence of any stimulus. SOAE are recorded using the same equipment as other types of OAEs. The same sensitive microphone is required. SOAEs can be recorded at any time, even during sleep (babies). SOAEs SOAE Characteristics SOAE Number 8 6 4 2 0 The prevalence of SOAEs has been shown to be higher in newborns and infants than in adults. Newborns Adults SOAE Frequency Higher frequency in newborns and infants than in adults. 0.5 2.5 4.5 Gender Prevalence The SOAEs demonstrate a gender-prevalence, in that they are significantly more prevalent in women than men and with higher numbers of SOAEs in females than in males. Right Ear Advantage There is a higher prevalence of SOAEs and a greater SOAE number in the right ear than in the left ear. SOAE presence is suggested to be linked to higher auditory sensitivity. Vincent van Gogh. Self-Portrait with Bandaged Ear 1889; Oil on canvas; Courtauld Institute Galleries, London TEOAEs and SOAEs SOAEs are an indicator of strong and robust TEOAEs. SOAE Characteristics The prevalence of SOAEs was first estimated at less than 40%. The prevalence is now estimated at approximately 80%, with more SOAEs recorded in newborns and infants than in adults. The increase in prevalence is mostly due to the improvement in instrumentation leading to lower noise floors. Low level SOAEs can now be unrevealed. In a laboratory setting, it is now expected to record SOAEs in almost all normal-hearing young adults. SOAEs are very stable with time and do not fluctuate much in frequency, making them a good indicator of trauma that can occur in one ear, as with other types of OAEs. However, the amplitude of each SOAE can fluctuate by as much as 10 dB depending on the time of recording. SOAE Characteristics SOAEs are though to reflect the activity of the cochlear amplifier. Absence of SOAEs does not signify that the subject has a hearing loss. However any insult to the cochlear amplification process will adversely affect the SOAEs. In fact, there seems to be a significant correlation between the global presence of SOAEs and good hearing sensitivity. There is no known correlation between SOAE number and audiological assessment. Many studies suggest that SOAEs are markers of cochlear damage. The functional relevance of the presence of SOAEs is far from clear. Two Different Types of SOAEs? To reconcile the presence of SOAEs in subjects with high auditory sensitivity and SOAEs due to cochlear damage (noise for example), some hypothesized that they may represent 2 distinct classes of SOAEs. – SOAEs recorded in normal-hearing individuals are the result of the active cochlear mechanisms and a phenomenon of global resonance in the cochlea. – Isolated, high-level SOAEs associated with cochlear damage could be a result of artificial punctate boundaries produced along the cochlear partition, which act as barriers or reflectors creating SOAEs. Because SOAEs are not present in 100% of normalhearing adults, their clinical use remains uncertain at the present time. SOAEs are commonly not recorded in clinical settings, except in specific situations (tinnitus, etc.). Interpretation of OAEs Interpretation of OAEs Stimulus stability: 80 or 90% or greater Overall amplitude: Above noise – S/N of 3 to 6 dB used – Absolute amplitude varies widely TEOAE reproducibility – Overall or by frequency bands – Adults: 80% or greater – Infants: 50-60% or greater acceptable Main criteria: Presence or absence of OAEs. OAEs can be absent with functioning Outer Hair Cells. No correlation between OAE amplitude and ability to hear. Interpretation of OAEs Main criteria: Presence or absence of OAEs. OAEs can be absent with functioning Outer Hair Cells. No correlation between OAE amplitude and ability to hear. Misconception: DPOAEs are more frequency sensitive than TEOAEs. This misconception has been generated by the familiar audiological representation of the DPgram. In reality, both types of OAEs provide frequency specific information for a particular cochlear segment. PASS FAIL ? ? Pass? Interpretation of OAEs: what are YOUR criteria? Diagnostic OAEs Obtain a high degree of frequency specificity on the function of the Outer Hair Cells. For DPOAEs: at least 5-8 frequencies per octave over the range of 500 to 8000 Hz. TEOAEs: are usually covering all available frequency bands from 0 to 5000 Hz. The test will define: – Normal amplitude (within a frequency region) – Present OAEs but with abnormal amplitudes – OAEs not present Diagnostic OAEs Verify that the recording parameters were adequate: – Stimulus intensity closed to the target level – Noise within the normal range – OAEs above the noise floor (6 dB) for all frequencies or only some frequencies? – TEOAE reproducibility above 90% – OAE amplitude within normal limits (if not, OAEs could be absent or present but abnormal) OAE Measurement Status of the external and middle ears Fitting of the probe Stimulus characteristics and stability Noise – Physiologic – Environment It is easy to misinterpret OAE results if these factors are not controlled properly. Identify the presence of a response. Determine whether criteria for an acceptable response have been met. Interpret the amplitude and frequency characteristics of the response. Screening/Diagnostic Equipment Screening: – Usually only display a “pass/refer” outcome. Diagnostic: – Display the stimulus, OAEs and noise levels – Number of artifacts – Artifact rejection limit Other significant differences: – TEOAE and DPOAE frequency and intensity ranges usually much smaller for a screening than a diagnostic system. – Results may not be stored with a screening system. OAE Probe Size and shape do matter: – will it be easy to fit the probe in a variety of ear canals? – Will the probe be comfortable for the patient? Choose an OAE probe with a design to safeguard against easy entry of cerumen or debris. Choose an OAE probe that is easy to clean. Calibration Standards for calibration of OAE equipment are not well established. It is important to understand the calibration method used with the equipment in obtaining norms. Normative data may be built-in the system. How were they obtain? When starting using a new piece of equipment, obtaining your own norms on a subset of patients in your specific environment would be ideal. Calibration Proper calibration is important to ensure that the desired stimulus is being presented to the patient’s ear. Proper calibration also ensures that the recorded signal is a reasonable approximation of the physical signal that was present in the ear canal. Proper (and consistent) calibration allows us to compare a patient’s result over time with confidence. Proper calibration also allow us to compare a patient’s result with those published. Recording Considerations Equipment: – – – – Calibration of the probe. Try the probe every day. Clean the probe. Visual inspection before and after every procedure is important. – Check noise level during recording. Probe Fitting Probe fit is assessed with an analysis of the response to a broadband signal presented in the ear canal. Specific features will detect a leak in the fitting of the probe, a blocked probe or other anomalous conditions. Usually the equipment doesn’t allow the recording to proceed if the fitting is not appropriate (check your manual). Probe Fitting The cable of the probe should be directed 45° towards the top of the head. Experience has shown that this angle is not feasible with neonatal subjects, where a wider angle is used. The position of the probe into the external auditory canal is a crucial component of the proper conduction of the test especially in newborns. Deeper insertion of the probe tip within the ear canal and tighter probe tip coupling within the ear canal will decrease the ambient noise. Experience is key! Probe fitting A standard click stimulus is applied and the sound in the ear canal is displayed as a waveform and spectrum so that the operator can adjust the fit of the probe and ensure proper stimulation for performing correctly the test. Probe Fit Although the probe is unlikely to move in older children and adults during the recording as long as the probe was well fit at first and the patient is still, it is a likely scenario in young infants. Visual inspection of the probe and of the stimulus and noise values should be done periodically during the recording. Clinical devices usually give an indication if the stimulus characteristics or the noise level are changing during the recording. Recording Considerations Environment: – Depends on the age of the subject (sound treated booth, quiet room, noisy nursery….). – Will define the setting of the recording: noise level considered as adequate in a booth setting will not be in a nursery. – OAE amplitude is higher in newborns than in adults; therefore, a higher level of noise will still allow OAE recordings in neonates – except for low frequencies. – Night Time? Subject: – Relaxed, comfortable – OAE can be recorded from sleeping or sedated children. Noise High noise level in the ear canal affects the OAE recording and can obscure partially or totally the response. Low frequency noise (below 1-1.5 kHz) is inevitable, because it is linked to body functions. By looking at normative data, it is possible to have an idea of an “acceptable” noise level. Noise levels will however vary considerably depending of factors linked to the equipment, the patient population and the test setting. Minimizing Noise Level Turn off equipment not in use in the test setting. Ask colleagues and family members that the recording will start and to stay quiet. Ensure quiet environment outside the testing room. Adequate probe and tip size. Tight coupling between the tip and the ear canal with probe deeply inserted. Position the tested ear away from running equipment (computer fan, general ventilation, etc.) Replicate OAE recordings for some frequencies if they are obscured by noise if possible. If patient is old enough, instruct to remain still and quiet (no chewing). For younger infants, maintain their attention in one direction to avoid motion. Always monitor the noise level during the recording: if it changes suddenly, something is going wrong! Stop criteria The stop-criteria for the test vary. Can be set up for a specific protocol / clinical population. The majority of programs use a pre-specified number of sweeps for each subject category (neonates, NICU residents, children, young adults, adults). Can be adjusted manually in some circumstances if needed. Stop Criteria It is also common to stop the recording when a variable reaches a specific value. Several factors to take into account: – – – – OAE amplitude level Noise floor level Signal-to-noise ratio Maximum recording time Examples – when the overall TEOAE reproducibility exceeds 75%. – when the S/N ratio at certain frequencies is above a pre-determined level (6 dB). – when the DPOAE level reaches a predetermined level (10 dB SPL). – when the noise floor reaches a predetermined level (-20 dB SPL). General rule: reduce the testing time to a minimum (define optimal recording conditions). With a screening equipment, the outcome are preset. However, several changes can be made if needed (know your screening machine). Artifact Rejection OAEs in the ear canal are easily contaminated by ambient and physiologic noise. Clinical equipment incorporates built-in functions to recognize and reject artifacts. – The ambient noise that is not with the frequency range of interest can be eliminated by filtering. – Very large level signals can be rejected automatically. This option is usually controllable. The threshold for rejection will vary depending on the environment (NICU, sound proof room) and the patient (quiet child, moving infant). Ideally, the rejection limit will be set up to avoid including too much noise while minimizing the duration of the test. Outer Ear Plays a crucial role in the stimulus delivery and otoacoustic emission recording Any debris or excess of cerumen will influence the OAE recording The ear canal acoustics influence the stimuli used to generate the OAEs. Importance of calibration (clinical systems do it automatically) A variety of pathologic and nonpathologic conditions of the external ear can affect the OAE recording Influence of External Ear Conditions Visual inspection is important when possible. Removal of any debris, foreign objects, excessive cerumen, etc. will not only change the OAE results but also improve the patient’s comfort. Cerumen or debris – Occlusion of stimulus and/or microphone ports in the OAE probe. – Blockage of stimulus energy delivery or return from the probe to the tympanic membrane. Otoscopic inspection is not necessary if patient was just seen for an otologic inspection and in most newborns (unlikely to have pathology, foreign objects or cerumen). Middle Ear Both the stimuli and the OAE have to travel into and from the cochlea through the middle ear The middle ear condition is thus very important in OAE recording Middle ear dysfunction will lead to either OAE reduced in amplitude or OAE not detectable at all in the external ear canal Middle ear dysfunction SHOULD be ruled out before concluding that OAE abnormalities are secondary to cochlear dysfunction Influence of Outer Ear and Middle Ear Conditions TM perforation Eustachian tube dysfunction Negative or positive pressure Middle-ear effusion Ventilation Tubes. – OAEs can be recorded in presence of tubes, but not in 100% of ears. Always eliminate the possibility of external or middle ear problems when interpreting abnormal OAE results. Influence of Middle Ear Function When OAEs are abnormal (partially or totally in terms of frequency), the middle ear function must be evaluated. The status of the Outer Hair Cells can only be inferred if the middle ear status is known and middle ear pathology ruled out. Inversely, normal OAE findings (normal amplitude for the entire frequency range) argue strongly for normal middle ear function (no need for middle ear evaluation, notably in newborns). Ventilating Tubes OAEs can be recorded in presence of tubes, but not in 100% of ears. Fritsch et al., 2002: 81% of ears (out of 385) showed TEOAEs postoperatively. Insufficient stimulus energy at 4 kHz. Owens et al., 1993: Ears with ventilating tubes exhibited DPOAE amplitude lower than amplitudes from healthy ears, but higher than those of the untreated diseased ears. New Trend Combined OAE and tympanometry devices (using the same probe). OAE Analysis Not difficult if OAEs (TEOAEs and DPOAEs) were first recorded with good technique under optimal testing conditions (appropriate protocol, quiet setting, quiet patient). The story is different if OAE testing was done in a neonatal intensive care unit with a restless infant. In that case, not only the data recording is more difficult, but the interpretation of the test is also more complicated. Clinical application of OAEs and Protocols Cross-Check Principle Drs Jerger and Hayes, 1976: “no audiologic test result should be accepted until it is confirmed by an independent measure”. Principle was first based on behavioral audiometry, immittance measures and ABR. The principle now incorporate the OAEs. Triage Every new patient should be tested first with: – Tympanometry (high frequency probe tone in infants) – Middle ear muscle reflex – OAEs Newborn Hearing Screening Goal: Screen all types of Hearing Loss Outer hair cells Inner hair cells Auditory nerve INSERM, Montpelier Promenade ‘round the cochlea Karl White, Ph.D. Utah State University Newborn Hearing Screening Is it necessary to evaluate the hearing status of all newborns (NICU, full-term babies)? How can we evaluate their hearing? – Behavioral testing – ABRs – OAEs Are some protocols more useful than others? Which equipment is the best? High Risk Infants Karl White, Ph.D. Utah State University Which OAEs for a good screening? For screening purposes both TEOAEs and DPOAEs convey the same information . Estimates at the frequencies of 2.0, 3.0 and 4.0 kHz have been established as good descriptors of healthy peripheral functioning. SOAEs: are not a useful tool as they are not present in 100% of ears. OAEs are frequency-specific. OAEs are not modified by sleep stages. Possibility to record OAEs in less than a minute How to judge if the acquired OAE response is normal (screening criteria) There is still no consensus on the screening criteria or their corresponding values. Experience has shown that it is better to divide the evaluation of the responses into two categories: PASS or REFER. For the TEOAEs the majority of the UNHS programs utilize the values of the S/N ratio as indicators, or the TEOAE reproducibility at the frequencies of 2.0, 3.0, and 4.0 kHz. For a PASS the reproducibility at ALL three frequencies should be higher than 75% and the S/N ratios higher than 6 dB. For the DPOAEs the screening criteria are protocol dependent (65-55 is considered the default option). Usually the DPOAE S/N values are estimated at 2.0, 3.0 and 4.0 kHz and if ALL three are higher than 6 dB the case is assigned as a PASS. Reducing Referral Rates To avoid high referral rates various methods have been used, including: Maintaining an experienced group of screening personnel (Spivak et al., 2000). Reducing ambient noise (Rhoades et al.,. 1999). Delaying age of testing as neonates older than 24 hours are less likely to have occluding vernix caseosa (Maxon et al., 1997). Combined OAE and ABR screening (Prieve and Stevens, 2000). Limitations of OAE Screening OAEs are sensitive to outer hair cell dysfunction. OAEs can be reliably recorded in neonates in response to stimuli in the frequency range above 1500 Hz. The OAE is known to be sensitive to outer ear canal obstruction and middle ear effusion and therefore, temporary conductive dysfunction can cause a "refer" outcome in the presence of normal cochlear function. Hearing screening in the neonatal period cannot identify acquired or progressive hearing loss occurring subsequently (also true for ABR screening). Limitations to Newborn Screening Some infants with hearing loss will pass the newborn hearing screening. Both ABR and OAE technology can show false-negative findings, depending on whether hearing loss exists in configurations that include normal hearing for one or more frequencies in the target range. These would include isolated low-frequency (i.e., below 1000 Hz) hearing loss or steeply sloping high-frequency (i.e., above 2000 Hz) hearing loss. Auditory Neuropathy and Newborn Hearing Screening If only OAEs are used as an initial screener, 10% of children with HL who have normal OAEs also may have serious auditory synchrony problems. Similarly, if only alternating polarity or single polarity ABR is used as an initial screener, approximately 10% of children with flat or abnormal ABRs will have normal OAEs and may misdiagnosed. Children with ANSD are currently being found more frequently because of the proliferation of newborn ABRbased hearing screening programs. Genetic Hearing Loss OAEs can detect early cochlear dysfunction (site specific). OAEs can detect subclinical auditory dysfunction before it becomes apparent on the pure tone audiogram. Early evidence of OAE abnormalities could help predict which family member(s) is likely to develop hearing loss in the future. OAEs are very useful for regular monitoring of auditory dysfunction in individuals at risk for hearing loss. DPOAEs in Usher Carrier and Matched Control Subject From Hood et al., 1999 TEOAE Amplitude in Cx26 Carriers and Matched Control Subjects Cx 26 (n=15) Control (n=15) 18 16 DPOAE Amplitude in dB 14 12 10 8 6 4 2 0 -2 -4 Right Left Ear From Hood et al., 2001 Noise Induced Hearing Loss Noise causes changes in the function of the inner ear which results in hearing loss: Noise Induced Hearing Loss (NIHL). Noise can also produce tinnitus. NIHL depends on duration of exposure and intensity. Noise Induced Hearing Loss can be – Temporary: temporary threshold shift (TTS) – Permanent: threshold stabilize at an elevated value: permanent threshold shift (PTS) When NIHL is temporary, recovery occurs after hours, days or weeks depending on initial severity. Noise-induced hearing loss (NIHL) is a major health problem because opportunities for overexposure abound, and exposures that damage hearing are not necessarily painful or even annoying (music). Kujawa and Liberman, 2009 Managing Musicians: The Use of Otoacoustic Emissions in Monitoring Acoustic Trauma and Counseling Young woman with fifteen years of noise exposure to classical music Acoustic Trauma Similar audiogram with a much more robust transient OAE tracing of a young woman of the same age with no history of noise exposure. Counseling Musicians The most effective demonstration is to personalize the effect of noise by testing a musician's emissions before and after a loud concert thus demonstrating to them their own temporary loss of outer hair cell function. DP-gram of a young woman with normal hearing. Emissions the morning after a concert that was reported to be "very loud". Note that the reduction in DP amplitude is most different between 3 and 4 KHz where noise has its biggest effect. Ototoxicity Two main classes of drugs that can cause permanent hearing loss: – Aminoglycoside antibiotics – Platinum-based chemotherapeutic agents Chronic generation of reactive oxygen species (ROS) Drugs: Antioxidants and ROS scavengers Problem: Systemic administration of otoprotective agent may interact with the drug and inactivate it. Cochlear targeted drug delivery is extremely difficult: – Transtympanic injections – Direct intracochlear application Monitoring Ototoxicity OAEs: – Highly sensitive to OHC dysfunction which are very often affected first in ototoxicity. – Earlier detection than behavioral audiometry – Can be performed in young and sick patients DPOAEs are superior to TEOAEs because of their extended high frequency limit. However, if time allows, both TEOAEs and DPOAEs should be performed (they represent 2 different mechanisms of generation). One limitation is the higher incidence of otitis media of patients receiving ototoxic medications. Some changes in OAEs might not be visible right away. Follow-up recordings should be schedule even months after administration of the drug. Tinnitus Could be related to cochlear dysfunction associated with either aging or noise exposure. Damage to outer and inner hair cells can lead to disruption in the normal resting activity of afferent auditory neurons. Subsequently, reorganization of the function of higher auditory centers may lead to tinnitus. OAE abnormalities are a common finding in patients with tinnitus. But keep in mind that OAEs can also be normal in some instances. Pseudohypacusis OAEs are useful when an individual will not or cannot voluntarily perform behavioral audiometry. OAEs allow to resolve conflicts in findings (but only after ruling out technical explanations, non pathologic factors (patient didn’t understand the task) and pathologic factors (IHC or neural dysfunction). Pediatric patients with history of emotional trauma and/or abuse are an example of patients at risk for malingering. OAEs and Neural Hearing Loss OAEs may or may not be present in patients with space-occupying lesions affecting the VIIIth nerve and/or caudal brainstem. Some patients with such neural lesions demonstrate an absence of OAEs, most likely due to the restriction of blood flow to the cochlea, which limits the oxygen and nutrients needed by the cochlea. In addition to the effects on blood supply, neural lesions may destroy cochlear fibers by pressure, atrophy, or invasion and contribute to biomechanical degradation of the fluids of the inner ear, thus affecting hair cell and other cochlear processes. OAEs are usually present in cases of auditory neuropathy. Summary: Clinical Applications of OAEs OAE advantages: – – – – – – – – Relatively short test time. Ear specific audiologic information. Frequency specific information. Assesses cochlear function specifically. Physiologic, not dependent on behavioral responses. Can be recorded from sleeping and sedated children. Monitoring ototoxicity: many ototoxic drugs exert their effect on OHC function. Noise-induced HL: OHCs are extremely vulnerable to sound over-stimulation and are the first to be affected amongst the hair cells. – OAEs can detect cochlear dysfunction before it is evident by pure-tone audiometry. – Valuable contribution to the “cross-check principle.” Summary: Clinical Applications of OAEs Pitfalls: – Presence dependent on normal middle-ear function – Need quiet test environment. OAEs are affected by ambient and physiological noise which can limit use in lower frequency ranges. – Present in individuals with mild cochlear hearing loss, neural losses. – The fact that a subject has acceptable OAE responses at the tested frequencies (PASS) does not IMPLY automatically that the subject CAN HEAR.