Posture systematically alters ear-canal reflectance and DPOAE properties

Abstract

Several studies have demonstrated that the auditory system is sensitive to changes in posture, presumably through changes in intracranial pressure (ICP) that in turn alter the intracochlear pressure, which affects the stiffness of the middle-ear system. This observation has led to efforts to develop an ear-canal based noninvasive diagnostic measure for monitoring ICP, which is currently monitored invasively via access through the skull or spine. Here, we demonstrate the effects of postural changes, and presumably ICP changes, on distortion product otoacoustic emissions (DPOAE) magnitude, DPOAE angle, and power reflectance. Measurements were made on 12 normal-hearing subjects in two postural positions: upright at 90 degrees and tilted at −45 degrees to the horizontal. Measurements on each subject were repeated five times across five separate measurement sessions. All three measures showed significant changes (p < 0.001) between upright and tilted for frequencies between 500 and 2000 Hz, and DPOAE angle changes were significant at all measured frequencies (500-4000 Hz). Intrasubject variability, assessed via standard deviations for each subject's multiple measurements, were generally smaller in the upright position relative to the tilted position.

Intracranial Pressure Modulates Distortion Product Otoacoustic Emissions

“…The artifact rejection algorithm and the noise-floor calculation with HearID were used as described elsewhere. 10 Each DPOAE measurement took 3 to 5 minutes, depending on the signal-to-noise ratio for a given measurement. Tympanometry and DPOAE measurements were performed twice in each subject, concurrently with the ICP measurements at opening and closing pressures.…”

“…DPOAE data that had magnitudes within 6 dB of the noise floor were considered within the noise for analysis. 10 …”

“…9,10 Specifically, in the upright position (190°, presumed lower ICP) compared with the head-down tilt position (245°to horizontal, presumed higher ICP), the DPOAE magnitudes increased and phase angles decreased with lower ICP for specific frequency ranges.…”

“…7,8 Therefore, pressure changes in the intracranial compartment are transmitted to the cochlea and can influence auditory function. 9,10 Increases in ICP likely alter the middle-ear compliance via an increased stiffness of the annular ligament that connects the stapes to the oval window; such increased stiffness attenuates low frequency (,2000 Hz) middle-ear sound transmission from the ear canal to the cochlea. [11][12][13] Thus, auditory measurements of otoacoustic emissions that depend on middle-ear function would theoretically be influenced by changes in ICP.…”

“…10 More specifically, we were interested in determining which DPOAE measures (ie, magnitude and phase angle) are most sensitive to ICP changes and what degree of ICP change would alter these DPOAE responses.…”