Repeatability of High-Frequency Distortion-Product Otoacoustic Emissions in Normal-Hearing Adults

Abstract

Distortion-product otoacoustic emissions (DPOAEs) are repeatable over time at lower frequencies (<or=8 kHz). The purpose of this study was to examine the repeatability of DPOAEs measured with high-frequency stimuli in normal-hearing adult subjects. It was hypothesized that high-frequency DPOAEs would be repeatable over a minimum of 4 wk. DPOAEs were measured in 25 subjects (14 female and 11 male) with normal behavioral thresholds, normal middle ear function, and present acoustic reflexes at 1 kHz evoked by contralateral stimulation. Behavioral thresholds were measured through 16 kHz, using Békèsy tracking. Each subject attended four trials, in which a complete set of data was collected. The four trials were separated by at least one, but no more than 2 wk. At each trial, two different DPOAE paradigms were completed. Frequency and ratio sweeps were used to measure DPOAE levels and calculate DPOAE group delay, respectively. Frequency sweeps were measured with varied stimulus level conditions (L1/L2 = 60/45, 60/50, 70/55, 70/60 dB SPL) and an f2/f1 of 1.2 at discrete f2 frequencies between 2 and 16 kHz. Ratio sweeps were obtained at f2 frequencies of 2, 4, 6, 8, 10, 12, 14, and 16 kHz, with L1 = 60 and L2 = 45 dB SPL, and the ratio (f2/f1) was varied from 1.05 to 1.3. Repeated-measures analysis of variance was performed. Greater variability was found at the higher frequencies (>8 kHz) for DPOAE level measurements. The average DPOAE level differences-between-trials for the higher and lower frequencies for the four different stimulus level conditions was 5.15 (SD = 4.40 dB) and 2.80 (SD = 2.70 dB) dB, respectively. Individual subject analysis revealed that high-frequency DPOAE levels varied no more than 10 dB for 87.5 and 83.1% of young adult subjects for the 70/55 and 60/50 dB SPL stimulus level conditions, respectively. For low frequencies, repeated DPOAE level variations were within +/-10 dB for 98.4 and 96% of young adult subjects for the 70/55 and 60/50 dB SPL stimulus level conditions, respectively. For DPOAE group delay, greater variability was noted at lower frequencies (<or=8 kHz) and at 10 kHz. The average group-delay differences-between-trials for the higher (10 to 16 kHz) and lower (<or=8 kHz) frequencies was 0.22 (SD = 0.20 msec) and 0.28 (SD = 0.24 msec) msec, respectively. Individual subject analysis revealed that DPOAE group-delay values varied no more than 0.70 msec for the majority of young adult subjects (96.5 and 93.5% at high and low frequencies, respectively). Even though the DPOAE level data obtained at frequencies greater than 8 kHz were more variable than at low frequencies, the higher frequencies were found to be repeatable for both paradigms tested. These results encourage the exploration of high-frequency DPOAE measures to be used as an objective test for monitoring ototoxicity in humans. Testing subjects receiving ototoxic therapies is a necessary step in determining if monitoring high-frequency DPOAEs will successfully predict ototoxic effects.