June - October 2015: Why WAI (Wideband Acoustic Immitance)??

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Category: Guest editorial

Last Updated on Friday, 04 December 2015 23:50

Written by Judi Lapsley Miller Ph.D

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About the Guest-Editor

This email address is being protected from spambots. You need JavaScript enabled to view it. is a senior scientist at Mimosa Acoustics, Illinois, USA, where she works remotely from Wellington, New Zealand. She obtained her PhD in Psychology in 1999 from Victoria University of Wellington on the psychophysics of human hearing. Her background is in psychophysics, otoacoustic emissions, and hearing conservation.

Dr Lapsley-Miller specializes in translational hearing research; bridging the gap between new technologies forged in research laboratories and clinicians wanting state-of-the-art easy-to-use tests for their patients.

Why WAI ????

Wideband acoustic immittance (WAI) provides a new window on the middle ear, by showing us how the middle ear is working over a wide frequency range (0.2 to 8 kHz). Information about middle-ear status helps us make more nuanced interpretations of inner-ear status seen through otoacoustic emission (OAE) testing. This is especially true in universal newborn hearing screening programs. For no extra effort, WAI also provides improvements in stimulus calibration that can increase OAE validity and reliability.

• WAI terminology is confusing – here’s a handy look-up guide

To make an OAE measurement, an acoustic stimulus is played into the ear canal. This stimulus propagates through the middle ear to the inner ear. Within the inner ear, OAEs are evoked by the stimulus and propagate back out through the middle ear into the ear canal where they are measured by a sensitive microphone. We cannot get a clear view of inner-ear status with OAEs without considering middle-ear status too.

For instance, if there is middle-ear dysfunction such as negative middle-ear pressure or fluid in the middle-ear space, sound propagation can decrease. This is readily apparent when viewing middle-ear reflectance, which is a quantity derived from a WAI measurement. To understand what middle-ear reflectance represents, consider the sound power measured in the ear canal. It is a superposition of two pressure waves: the incident pressure due to the sound emitted by the probe, which travels towards the tympanic membrane; and the reflected pressure wave, which is primarily the sound reflected by the tympanic membrane, ossicles, and cochlea. Reflectance is the percentage of acoustic power reflected, relative to the incident acoustic power, and is plotted as a function of frequency. In a normal ear, reflectance is high below 1 kHz, low from 1-4 kHz, and high around 4-6 kHz. For an example, see the green line in the top panel of Figure 1. When reflectance is high, more acoustic power is reflected from the tympanic membrane and other structures back into the ear canal. When reflectance is low, more acoustic power propagates through the middle ear. The reflectance pattern over frequency can assist with differential diagnoses.

Figure 1. Reflectance and TEOAEs before (green) and after (pink) a Toynbee manoeuver that changed TPP from 0 daPa to ‑135 daPa. The black spectrum is the TEOAE noise level.

OAE measurements get hit with a double-whammy when middle-ear reflectance is higher than normal. First, the middle-ear attenuates the OAE stimulus as it propagates through the middle ear into the inner ear. This can decrease the effective OAE stimulus level. (Typically OAEs increase with increasing stimulus level, so a lower effective stimulus level decreases the OAE magnitude.) Second, the already-diminished OAE is attenuated further as it propagates back out through the stiffened middle ear. The measured OAE is smaller than it would have been had the middle ear been healthy. The resulting OAE can be lowered so much that you get a false-positive for sensorineural hearing loss. Or if you are tracking changes in OAEs over time, say in a hearing-conservation program or when monitoring for ototoxicity, you may see a false-positive decrease in OAE level.

In Figure 1, you can see the difference in a transient-evoked OAE (TEOAE) when there is negative middle-ear pressure (case example from the study reported in Marshall et al. (2015)). In this plot there are two measurements: one done with the participant’s middle-ear at ambient pressure (green line, tympanometric peak pressure 0 daPa) and one after a Toynbee manoeuver (pink line, tympanometric peak pressure -135 daPa), where the participant held their nose and swallowed. The top plot shows reflectance and the bottom plot shows the TEOAE spectrum. When the ear is pressurized, more acoustic power is reflected, especially at lower frequencies. The impact is seen in the TEOAE spectrum with a noticeable decrease in TEOAE amplitude, especially around 1 kHz where the biggest change in reflectance occurred.

As well as middle-ear conditions, reflectance can also be high if the probe is partially blocked or is pressed up against the side of the ear canal. Poorly fitting probes are also apparent, with acoustic leaks showing up as low reflectance at very low frequencies. Making a WAI measurement after fitting the probe is a great way to see if there is a good probe fit prior to making an OAE measurement.

The goal of universal newborn hearing screening (UNHS) programs is to detect babies who have sensorineural hearing loss so they can benefit from early intervention. UNHS programs provide a Pass or Refer result from either OAE or ABR tests. These screening tests are not diagnostic, but are used to determine referrals for more extensive diagnostic follow-ups.

It has long been best-practice in UNHS programs to rescreen babies who get a Refer result to reduce false-positives for diagnostic referrals. This rescreening is usually done after a delay, because testing within 24 hours of birth is much more likely to produce a refer result than testing after 24 hours (and preferably 36 hours). The majority of these false-positive referrals are from transient middle-ear dysfunction from the birth process (e.g., amniotic fluid, mesenchyme, and meconium in the middle-ear space), which clears within the first few days of life. Hunter et al. (2010) showed why rescreening OAEs after a delay was often successful – middle-ear reflectance tends to decrease over time, presumably as the middle ear clears, allowing more sound to propagate into the inner ear and back.

This transient middle-ear dysfunction is not reliably picked up with tympanometry in newborns. Hunter et al. (2010) and Sanford et al. (2009) both showed that WAI reflectance was vastly superior to tympanometry in predicting which ears would show low OAE levels due to middle-ear dysfunction. Adding WAI to OAE screening can potentially identify those babies most in need of diagnostic follow-up, help determine the best time for repeat screening, and reduce false-alarm referrals.

When using WAI with OAEs in testing newborns, a pass/refer result can be assigned to each test, giving four possible outcomes. Each outcome is illustrated in Figure 2 with real examples from the Hunter et al. (2010) study. This study showed that the reflectance around 2 kHz was the best predictor for whether the DPOAE test passed (DPOAE levels at 3 or 4 out of 4 frequencies are normal) or not (DPOAEs at 2 or more out of 4 frequencies are abnormally low). So although the spectrum is plotted from 1 to 6 kHz, pay particular attention to the 2 kHz region in the reflectance plot. Reflectance below 1 kHz is not plotted because in babies this region is often noisy and therefore is less diagnostic (Hunter et al., 2010).

Also plotted are two normative regions in gray. For the WAI plot, the gray norm represents an ambiguous region. Reflectance below this region (especially at 2 kHz), was associated with DPOAE pass results. Reflectance above this region (especially at 2 kHz) was associated with DPOAE refer results. Similarly, for the DPOAE plot, the gray norm also represents an ambiguous region (from the Boystown norms (Gorga et al., 1997)). DPOAEs above this region are associated with normal hearing. DPOAEs below this region are associated with abnormal hearing.

A complication with interpreting WAI and DPOAEs is there is not a one-to-one relationship between reflectance frequency and DPOAEs frequency, in part because the DPOAE is first generated by two tones at different frequencies (F1 and F2) and is measured at a third (2F1-F2), which is then plotted against F2. So for a DPOAE plotted at 2 kHz, the frequencies involved are 1.3, 1.7, and 2.0 kHz. Reflectance at all three frequencies may affect the resulting DPOAE measurement.

Figure 2. Four outcomes are possible when using WAI with DPOAEs in a newborn hearing screening program. The WAI plots show power reflectance (green line, %) and the normative ambiguous region (gray region, %). The DPOAE plots show DPOAE amplitude (green bar, dB SPL), the noise floor (black bar, dB SPL), and the Boystown 90% ambiguous region (gray region, dB SPL), where DPOAEs below the region are considered refer results. For the screening protocol used in this study, if 3 or 4 out of 4 DPOAE frequencies get a pass result, the overall result is a pass (left plots). If 2 or more out of 4 DPOAE frequencies get a refer result or are noisy, the overall result is a DPOAE refer (right plots). We can reduce these referrals by considering the WAI result. If the WAI result is elevated, the DPOAE refer is probably due to middle-ear dysfunction (top right). However, the possibility of underlying sensorineural hearing loss cannot be excluded, so repeat screening is needed. If the WAI result is normal, the DPOAE refer needs diagnostic follow-up for possible sensorineural hearing loss (bottom right). Sometimes the WAI result will be elevated but the DPOAEs are so strong, they are able to overcome the reduction in middle-ear transmission (bottom left).

How could WAI be used in UNHS programs? Specific guidance is still in development, but potentially WAI can be used to enable smarter timing for rescreenings and follow-ups.  For instance, in the examples in Figure 2, the following courses of action may be appropriate:

1. Normal reflectance – normal DPOAEs across all frequencies (top left). Screening passed and no rescreening or follow-up is needed.
2. Elevated reflectance – low DPOAEs: possible middle-ear fluid (top right). Wait a few hours and rescreen to see if reflectance is lower and DPOAEs pass. Refer for diagnostic follow-up if DPOAEs do not pass on rescreening. Chances are reflectance will decrease as the middle-ear clears and the true DPOAE status will be more clearly revealed.  Elevated reflectance and low DPOAEs is commonly seen in newborn hearing screening programs, and causes undue worry for parents and an increased workload due to unnecessary diagnostic follow-ups. With WAI + DPOAEs, testers can immediately see if there is middle-ear dysfunction and can reassure parents that this is common and not of concern.
3. Normal reflectance – low DPOAEs (bottom right). This ear is a priority for diagnostic follow-up because it may be permanent sensorineural hearing loss.  Rescreening is optional because we have eliminated the usual reason for DPOAE false-alarms – high middle-ear reflectance from transient middle-ear dysfunction. Any rescreening can occur immediately because the WAI results show the middle ear is not impeding sound propagation into the inner ear.
4. Elevated reflectance – normal DPOAEs (bottom left). The DPOAEs are strong enough to overcome what is possibly a probe blockage or transient middle-ear dysfunction. Check for probe or ear canal blockage, or a collapsed ear canal, and then retest reflectance.  Since DPOAEs passed, rescreening is optional because an outer or middle-ear condition is not typically a reason for referral.

Although tympanometry is more reliable in older infants, children, and adults, the same principles for newborns apply for interpreting WAI + OAE tests in these age groups. In these older age-groups; however, high reflectance is also cause for follow-up for middle-ear dysfunction like otitis media.

It is especially convenient to have a clinical device that can measure WAI and OAEs with the same probe fit and without the need for pressurization. This provides many opportunities for differential diagnoses. With Mimosa Acoustics OtoStat system, both WAI and DPOAE results are achieved within a minute and are displayed on the same screen. A further benefit to using WAI is that the WAI measurement itself can be used to calibrate the DPOAE stimuli. This clever trick means the combined WAI + DPOAE test takes no more time than the DPOAE test on its own. Saving time is crucial when testing uncooperative patients like infants and small children. Once the WAI test is started, it automatically goes on to the DPOAE test without further button pressing.

Enhanced calibration using WAI

WAI allows for a new type of calibration: forward pressure level (FPL) calibration. FPL calibration is perhaps the most significant advancement in calibration techniques in recent years.

You may have noticed that above 4 kHz (and in some ears even above 2 kHz) that calibration and the resulting OAE levels are unreliable (Siegel, 1994). This can be due to standing wave nulls. The larger the distance between the probe tip and the tympanic membrane (referred to as the residual ear canal), the lower the standing-wave null frequency. In a null, it is not possible to accurately set the OAE stimulus level using normal in-the-ear calibration. The calibrated level could be higher or lower than intended depending on how the system adjusts its levels. This difference can be up to 20 dB! (Siegel, 1994). This is a particular problem when monitoring an ear over time, because if the probe depth is different at each measurement, the null frequency changes. This can change the actual OAE stimulus level, and therefore evoke a higher- or lower-level OAE. Changes in OAEs could be merely due to probe placement and not inner-ear dysfunction.

So what does FPL calibration do? Remember earlier how the sound pressure in the ear canal is a superposition of two waves? When doing a regular in-the-ear (ITE) calibration the pressure is measured at the microphone, which can be some distance away from the tympanic membrane, and is the combination of both the forward and backward - traveling pressure wave.

Because the WAI measurement separates out the forward and backward components of the pressure, it allows us to set the level for just the forward-going (incident) part of the pressure in the ear canal – the part propagating into the middle and inner ear – and ignore the backward-going (reflected) component. This circumvents the problematic standing-wave nulls allowing for accurate levels throughout the entire frequency range. This is only possible with a WAI system where the probe is calibrated with known impedance (Allen, 1986). The resulting FPL stimulus level is still measured in dB SPL.

Figure 3 shows an example of DPOAEs measured in an adult ear using a standard screening protocol. The top plot shows the in-the-ear calibrations and the bottom plot shows the resulting DPOAE levels. The measurement in pink used regular in-the-ear calibration to set the DPOAE stimulus levels, where the pressure is measured at the microphone-end of the ear canal.  The dip at 4 kHz is characteristic of a standing wave null. This dip disappears when FPL calibration is used instead (green line). When setting the levels for the DPOAE measurement, the regular in-the-ear calibration overestimated the level needed around 4 kHz and underestimated below 2 and above 4 kHz.  

The resulting DPOAE levels were quite different. In this example, the effect is particularly noticeable at 4 kHz and above. At 8 kHz, the DPOAE measured with FPL calibration was more than 7 dB SPL higher.

FPL calibration not only increases the validity and reliability of OAE measurements, but also pure-tone audiometry (Withnell et al., 2009; Withnell et al., 2014). It is currently only available to researchers, but is soon to be released for clinical use.

Figure 3. Two DPOAE measurements with the same probe position in the ear canal. The first measurement uses regular in-the-ear calibration (pink) where the pressure is measured at the probe-microphone end of the sealed ear canal. The second measurement uses the forward going component (green), which estimates the pressure at the tympanic membrane end of the ear canal. The resulting DPOAE levels differ, especially at 6 kHz. Black bars represent the noise level; the gray region represents the Boystown 90% ambiguous norm.

Why use WAI with OAE testing? By providing a view of the middle-ear while doing inner-ear testing with OAEs, using the same equipment, inner-ear status can be more accurately determined in a quick and convenient way. By providing a more accurate calibration, result validity and reliability is increased. WAI + OAE systems are set to revolutionize OAE testing.

Mimosa Acoustics would like to thank the OAE Portal for this opportunity to describe how WAI can benefit OAE testing. We would also like to thank :

Lisa Hunter (PhD, FAAA, Scientific Director, Audiology, Cincinnati Children's Hospital, and Associate Professor at University of Cincinnati) for her helpful feedback on UNHS programs ;

Linton Miller for editing.

Allen, J. B. (1986). "Measurement of eardrum acoustic impedance," in Peripheral auditory mechanisms, edited by J. B. Allen, J. L. Hall, A. E. Hubbard, S. T. Neely, and A. Tubis (Springer-Verlag, New York), pp. 44-51.

Gorga, M. P., Neely, S. T., Ohlrich, B., Hoover, B., Redner, J., and Peters, J. (1997). "From laboratory to clinic: a large scale study of distortion product otoacoustic emissions in ears with normal hearing and ears with hearing loss," Ear Hear. 18, 440-455.

Hunter, L. L., Feeney, M. P., Lapsley Miller, J. A., Jeng, P. S., and Bohning, S. (2010). "Wideband Reflectance in Newborns: Normative Regions and Relationship to Hearing-Screening Results," Ear Hear. 31, 599-610.

Marshall, L., Lapsley Miller, J. A., and Reed, C. M. (2015). Evaluating otoacoustic emission shifts due to middle-ear pressure with tympanometry and wideband acoustic immittance. (Poster presented at the 42nd Annual Scientific and Technology Conference of the American Auditory Society, Scottsdale, AZ), Mar 5-7. https://www.researchgate.net/publication/273001208

Sanford, C. A., Keefe, D. H., Liu, Y. W., Fitzpatrick, D., McCreery, R. W., Lewis, D. E., and Gorga, M. P. (2009). "Sound-conduction effects on distortion-product otoacoustic emission screening outcomes in newborn infants: test performance of wideband acoustic transfer functions and 1-kHz tympanometry," Ear Hear. 30, 635-652.

Siegel, J. H. (1994). "Ear-canal standing waves and high-frequency sound calibration using otoacoustic emission probes," J. Acoust. Soc. Am. 95, 2589-2597.

Withnell, R. H., Jeng, P. S., Parent, P., and Levitt, H. (2014). "The clinical utility of expressing hearing thresholds in terms of the forward-going sound pressure wave," Int. J. Audiol. 53, 522-530.

Withnell, R. H., Jeng, P. S., Waldvogel, K., Morgenstein, K., and Allen, J. B. (2009). "An in situ calibration for hearing thresholds," J. Acoust. Soc. Am. 125, 1605-1611.

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