It has been almost 25 years since Kemp first described the otoacoustic emission (OAE).1 However, it is only within the last decade or so that this simple measure of outer hair cell function has been used consistently for screening and differential diagnosis of hearing loss.
Because of the ease and objective nature of the measure, it is perfectly suited to the assessment of infant ears and has become an integral part of the test battery for diagnosis of hearing loss in children referred from newborn hearing screening programs. Many states and organizations have prescribed otoacoustic emissions as a necessary element of an appropriate diagnostic protocol for follow-up testing of such infants for the purposes of determining the type and degree of hearing loss in each ear. Specifically, those recommending OAEs in infant audiologic diagnostic protocols include the Joint Committee on Infant Hearing (JCIH), comprising the American Academy of Audiology, American Speech-Language-Hearing Association, American Academy of Pediatrics, American Academy of Otolaryngology-Head and Neck Surgery, Directors of Speech and Hearing Programs in State Health and Welfare Agencies, and the Council on the Education of the Deaf.2 The Audiology Clinical Practice Guidelines on Pediatric Audiologic Assessment produced by the Joint Audiology Committee (American Academy of Audiology, American Speech-Language-Hearing Association and Department of Veterans Affairs) also recommends OAEs as an important element in pediatric diagnostic assessment,3 as do essentially all states that have issued diagnostic protocol guidelines, including Texas, Colorado, New York, and California.
The primary goal of this article is to discuss the importance of including the OAE in any complete diagnostic audiology protocol for assessment of infants and toddlers, and also to emphasize the appropriate interpretation of the OAEs within a test battery. Appropriate application of the test battery will determine how each test can and should be interpreted. Details regarding the physiologic basis of OAEs and appropriate measurement parameters for infants and children can be found elsewhere.4–6
PHYSIOLOGIC BASIS OF OAEs
The inner hair cell (IHC) is the sensory cell that sends information regarding the presence of sound to the central nervous system. The outer hair cells (OHC) provide local amplification of basilar membrane motion facilitating very sensitive hearing and exquisite frequency tuning. The OHC has electro-motile properties that allow it to expand and contract in response to stimulation.7,8 The consequence of this motility is to amplify the motion of the basilar membrane at the specific location (frequency) of the OHC making the IHCs in the same region 30 to 40 dB more sensitive. Otoacoustic emissions are believed to be by-products of this added energy, which is propagated in reverse through the ossicular chain and tympanic membrane to create a sound wave in the external ear canal.9
OAEs are purely mechanical (pre-neural) events that require no innervation.10,11 Consequently, they are perfect tools to aid in differential diagnosis between cochlear disorders and neural dysfunction. Their presence is an easily measured test for hair cell normality.
HEARING LOSS PREDICTION WITH OAEs
In general, OHCs are more vulnerable to insult from noise, disease, or ototoxic medications than IHCs. When OHCs are missing in isolation, only a mild or moderate hearing loss will result. As the insult becomes more severe, IHC loss will result. When the hearing loss is moderate or greater, the degree and frequency region of hearing loss generally reflects the degree and cochlear region of IHC loss.
OAEs are generated only by OHC.12 Evidence of OHC dysfunction, manifested as absent OAEs, provides no information on the status of the IHC. If OAEs (and presumably OHCs) are normal, it is reasonable to assume that the IHCs are functioning as well. In this instance, one could assume that the hearing is better than about 30 dB.
One can assume this unless—and this is a big unless—there is an auditory system disorder beyond the cochlea that disrupts hearing. It is possible to have bilateral auditory nerve dysfunction, known as auditory neuropathy, in which disease of the peripheral portion of the nerve or synapse will disrupt hearing even in the presence of completely normal cochlear function.13 For this reason, it is not possible to predict hearing levels from OAEs alone, without some measure that shows that the auditory nerve is functioning. This can be accomplished with auditory brainstem response testing or even acoustic reflex (auditory stimulation of the middle ear muscle reflex).
Clinically, there are two methods of measuring otoacoustic emissions. A transient OAE (TEOAE) is measured using a single stimulus, usually a click, though tone bursts are sometimes used as well. Following each transient presentation, a broad-band, time-dispersed response is recorded in the time domain. Many responses are averaged as in an evoked potential measurement allowing a small signal, ranging from about −5 to 35 dB SPL, to be detected as distinct from background noise.
The other type of OAE is the distortion-product OAE (DPOAE), which is measured in response to two primary tones with certain frequencies (f1 and f2) and levels (L1 and L2). These tones produce distortion in the human cochlea in the form of other tones, the largest of which occurs at a frequency related to the two primary tones as 2f1-f2. The distortion is optimal in human infants and adults when the tones are spaced so that f2 is 1.22 times f1 and the tone levels should be spaced (L1-L2) by 10–15 dB.14 For a thorough review of clinical applications of both types of OAEs see Prieve and Fitzgerald.15
Although OAEs are thought of as objective, physiologic responses, interpretation of response presence or absence is not always straightforward. Routines for response detection vary with the type of response.
Both types of OAE can be measured in frequency-specific regions, which is helpful for audiogram prediction in patients. Regions of normal and abnormal OHC function can be predicted by patterns of OAE response. There are currently no universally accepted methods for determining when an OAE is present and clearly discernible from background noise. OAE equipment will provide measures of “OAE amplitude,” “background noise,” and often “signal-to-noise ratio,” which is a comparison of the two. TEOAE recording systems generally provide a “reproducibility” index as well, which essentially determines how well the response will reproduce when measured twice. All this information must be considered when determining if a response is present.
Kemp et al.16 recommended a minimum of 50% reproducibility for determining response presence, while Prieve et al.15 found 70% to be a reasonable expectation, along with an overall minimum amplitude (wide band) of 6 dB SPL. For narrow frequency bands, amplitude of 3 dB above background noise may give reasonable assurance of a TEOAE response for that frequency region alone.17
Likewise, standard values for presence and absence of DPOAEs have not been established. Comparison of the DPOAE level to the background noise is critical.18 It is not clear how large the speech-to-noise ratio (SNR) must be to be considered significantly greater than 0. A minimum of a 3-dB DPOAE level above background noise and a minimum actual DP amplitude of 0 dB SPL are reasonable guidelines for response presence (Caroline Abdala, personal communication). When a low-amplitude (3 dB SNR) response is seen at an isolated frequency, it may be suspicious. A conservative approach would be to expect a DP/noise of 6 dB as evidence of the presence of a true response.
A word of caution is necessary regarding maximum stimulus level for any OAE measure. The mechanism of OAE generation is a non-linear phenomenon that should occur only in response to low-level stimuli and then will saturate. To avoid measuring artifacts, it is important not to use high-level stimuli. Transient stimuli should be limited to about 86 dB SPL and the stimulus levels for DPOAE measures in the clinic should not exceed 65 and 55 dB SPL, or 65 and 50 dB SPL for the f1 and f2 tones.
INCORPORATING OAEs INTO A DIAGNOSTIC TEST PROTOCOL
The battery of tests for assessment of infants and young children suspected of hearing loss should include: (1) frequency-specific ABR (see Stapells in this issue), (2) some measure of middle ear function (either bone-conduction ABR, immittance measures, or both), and (3) otoacoustic emissions. Age-appropriate behavioral observation and parent report of auditory behaviors should also be used.
In general, the threshold of the frequency-specific ABR should be close to the hearing threshold, of the same frequency, that would be obtained if the child could give accurate, behavioral threshold responses. The otoacoustic emission in a given frequency region will be present if the hearing threshold in that region is better than about 30 to 40 dB and absent if the threshold is higher. It is important that these measures agree and no one is used in isolation to predict hearing thresholds.
An infant with a moderate-to-mild, low-frequency, sensory hearing loss, for example, would show ABR tone-burst thresholds close to true hearing thresholds and would probably show good OAEs only in the frequency region of mild loss or normal sensitivity. This child would show normal middle ear immittance measures and have bone- and air-conduction ABR thresholds intertwined.
As a rule, an infant with a severe-to-profound sensory loss would show no OAEs at any frequency, have normal middle ear immittance measures, and have ABR thresholds either at the limits of the equipment output or at very high levels corresponding to behavioral thresholds. However, there are exceptions to this rule.
- Compromised mobility of the middle ear: In certain situations, the interpretation of OAE results may be different from that just described. Because the OAE is low-amplitude and must travel through the middle ear to produce a pressure wave in the ear canal, even small obstructions to mobility in the tympanic membrane and middle ear can obliterate an OAE, regardless of the condition of the OHCs. If an OAE is absent and middle ear mobility is not completely normal, one should not assume that the absence of OAEs is evidence of hair cell dysfunction.
- ❖Auditory neuropathy or similar dysfunction of the auditory nerve or low brainstem: Dysfunctions of this type will affect the interpretation of hearing status in the presence of OAEs. A classic sign of auditory neuropathy (AN) is an ABR indicating no neural activity but with a clearly present cochlear microphonic (recorded activity in the range of 1 to 5 ms that changes polarity with like changes in the stimulus). An example of an ABR with cochlear microphonic can be seen in Figure 1.
The ABR from a patient with AN will be absent or severely abnormal to any level stimuli, regardless of how much hearing the patient may have. The patient with AN will usually have a normal OAE. The TEOAE elicited from the patient with the ABR shown in Figure 1 was present from 1000–5000 Hz with overall level of 13.5 and 15.6 dB SPL and reproducibility of 64% and 87% in the right and left ears, respectively. This same patient was found to have the audiogram seen in Figure 2, which demonstrates an important principle. The degree of actual hearing loss on the pure-tone audiogram cannot be predicted by either ABR threshold or OAE presence when AN exists.
Another classic sign of AN is an absent acoustic middle ear muscle reflex in combination with mild hearing loss or with present OAEs. Both the acoustic reflex and the ABR require auditory nerve activity that is appropriately time-locked to the eliciting stimulus. This precise timing disappears with AN.
An additional complication exists when the patient has AN and the OAE is absent. This may be secondary to middle ear status, as previously mentioned. In addition, many patients with AN have been shown to lose their OAEs over time for unknown reasons.19,20 Consequently, the absence of an OAE does not rule out AN. A patient with a clear cochlear microphonic and no ABR should be suspected of having AN. A patient with hearing thresholds that are much better than the “no response” on ABR or acoustic reflex testing would predict should also be suspected of having AN, regardless of the status of the OAE.
Determining the type and degree of hearing loss in infants and toddlers is feasible and can be reliable if a test battery approach is applied. That test battery is not complete without the use of OAEs. A summary of the type of response patterns that should be seen based on type of hearing loss is found in Table 1.
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