Serial Monitoring of Hearing With Otoacoustic Emissions: Effects of Forward Pressure Level Calibration

To use otoacoustic emissions (OAEs) for the serial monitoring of hearing or the tracking of hearing loss, test-retest reliability must be known, and normal OAE level variation must be established. Past studies have defined the reliability of distortion-product OAEs (DPOAEs) evoked with stimuli calibrated in a conventional manner (in-the-ear, dB sound pressure level), which can produce inaccuracies due to ear-canal standing waves. In the present study, we make within-subject comparisons of test reliability for conventionally measured 2 f1 - f2 DPOAEs compared with DPOAEs evoked by stimulus tones calibrated in dB forward pressure level (dB FPL) to mitigate the effect of standing waves. We also define the test-retest reliability of the stimulus-frequency OAE (SFOAE).

Conventional DPOAEs evoked by discrete tones calibrated in dB SPL as well as DPOAEs and SFOAEs evoked by swept tones calibrated in dB FPL were measured in the same ear of 20 adult subjects every 2 mo over the course of 1 yr. OAE test reliability was assessed for each OAE paradigm by calculating absolute level differences, standard errors of the measurement, and 95% normative shift limits. Comparisons in reliability were made between conventional DPOAEs and those using dB FPL calibration and between DPOAEs and SFOAEs, both evoked by stimuli calibrated in dB FPL.

Results indicate that overall DPOAEs evoked by swept tones calibrated in dB FPL have significantly better test-retest reliability than do conventional DPOAEs. Swept-tone DPOAEs also showed better reliability than swept-tone SFOAEs at mid-to-high frequencies, though at low frequencies test-to-test variation was comparable. In a subgroup of 5 subjects showing mild declines in audiometric hearing during the study, we found that DPOAEs and SFOAEs evoked by stimuli calibrated in dB FPL detected most instances of audiometric decline while conventional DPOAEs did not.

FPL-based calibration of stimulus level reduces test-to-test variation in DPOAE level compared with conventional in-the-ear SPL methods. By creating a tighter distribution of natural or typical OAE level fluctuations on repeated testing, forward pressure calibration appears to improve the serial monitoring of OAEs and could provide earlier detection of slight hearing changes. Our preliminary look at this question should be further explored with a large group of individuals showing confirmed changes in hearing over time.