Test-retest reliability of distortion-product thresholds compared to behavioral auditory thresholds
2021-04-28

When referred to baseline measures, serial monitoring of pure-tone behavioral thresholds and distortion-product otoacoustic emissions (DPOAEs) can be used to detect the progression of cochlear damage. Semi-logarithmic DPOAE input-output (I/O) functions enable the computation of estimated distortion-product thresholds (EDPTs) by means of linear regression, a metric that provides a quantitative estimate of hearing loss due to cochlear-amplifier degradation. DPOAE wave interference and a suboptimal choice of stimulus levels limit the accuracy of EDPTs. This work identifies the test-retest reliability of EDPTs derived from short-pulse DPOAE level maps (EDPT<sub>LM</sub>), a method that circumvents limitations associated with both wave interference and suboptimal choice of stimulus levels. The test-retest reliability was compared to that of EDPTs derived from semi-logarithmic I/O functions (EDPT<sub>I/O</sub>) and that of behavioral thresholds acquired with pure-tone audiometry (PTA) and modified Békésy tracking audiometry (TA) to provide a foundation for identifying and interpreting significant threshold shifts. The DPOAE-based auditory thresholds (EDPT<sub>LM</sub> and EDPT<sub>I/O</sub>) and behavioral thresholds (PTA and TA) were recorded seven times within three months at 14 frequencies with f<sub>2</sub> = 1-14 kHz in 20 ears from ten subjects with normal hearing (4PTA<sub>0.5-4kHz</sub> < 20 dB HL). To obtain EDPT<sub>LM</sub>, short-pulse DPOAEs were recorded using 21 L<sub>1</sub>,L<sub>2</sub> pairs. Reconstruction of DPOAE growth behavior as a function of L<sub>1</sub> and L<sub>2</sub> using nonlinear curve fitting enabled the derivation of EDPT<sub>LM</sub> for each frequency. Test-retest reliability was determined using three different approaches: 1) centered thresholds, 2) average threshold differences, and 3) average absolute threshold differences, between each possible test session (N = 21). Test-retest reliability based on centered thresholds and average threshold differences showed no statistically significant difference between EDPT<sub>LM</sub>, EDPT<sub>I/O</sub>, PTA, and TA for the pooled analysis incorporating all stimulus frequencies. Average absolute threshold differences presented small but significant differences in test-retest reliability with median values of 3.00 dB for PTA, 3.20 dB for TA, 3.34 dB for EDPT<sub>LM</sub>, and 3.51 dB for EDPT<sub>I/O</sub>. A considerable frequency dependence of test-retest reliability was found; namely, the highest test-retest reliability was for EDPT<sub>LM</sub> at f<sub>2</sub> = 11 - 14 kHz. Otherwise, at lower frequencies, the highest test-retest reliability was for TA at f<sub>2</sub> =1 - 2 kHz. Overall, the test-retest reliability of EDPT<sub>LM</sub> was better than that of EDPT<sub>I/O</sub> and was similar to that for behavioral thresholds. Hence, deriving EDPT<sub>LM</sub> from individual level maps is a promising and sensitive method for objectively monitoring the state of the cochlea. Furthermore, the detection of an equidirectional threshold change at a single frequency in both EDPT<sub>LM</sub> and TA might allow reducing the threshold shift as indication of a follow-up examination from the clinical standard of 10 dB down to 5 dB. This stricter indicator might be beneficial when monitoring cochlear damage, for example ototoxicity, in the presence of (remnant) cochlear amplification at baseline.