Research PaperPosture systematically alters ear-canal reflectance and DPOAE properties
Introduction
Noninvasive ear-canal based acoustical measurements have diagnostic potential in the area of neurology to monitor intracranial pressure (ICP) changes. Because the skull is fixed in volume, and its fluid contents are incompressible, changes in cerebral spinal fluid pressure that result from changes in ICP are transmitted to the cochlear fluids. Changes in ICP can be caused by a number of factors, including, head injury, stroke, hydrocephalus, and brain surgery and can lead to worsening brain injury or death by compressing blood vessels supplying the brain or vital brain structures themselves. Current tools used to evaluate ICP objectively (e.g., epidural transducers or intraventricular catheters) are invasive and require direct entry of a probe system through the skull or spine, introducing risks that include infection, intracerebral hemorrhage, and direct brain injury (e.g., Kanter et al., 1985, Maniker et al., 2006, Wolfe and Torbey, 2009, Scheithauer et al., 2009). A noninvasive method for monitoring ICP could eliminate these risks for some patients.
Intracranial pressure changes systematically with postural position (e.g., Chapman et al., 1990). Thus, changing postural position provides a method to induce changes in ICP and study the effects. To this end, it is widely documented that posture affects auditory function, including thresholds, otoacoustic emissions, and middle-ear impedance (for a thorough review see Büki et al., 2000). Thus, the connection between posture and ICP provides a mechanism to study how changes in ICP affect auditory responses and how this relationship might be harnessed to provide a noninvasive means to monitor ICP in some patients.
Wilson (1980) first showed that posture influences otoacoustic emissions, and with this report suggested that the changes might be due to changes in the stiffness of the annular ligament. More recently, a series of publications of both measurements and models from Büki and colleagues demonstrate that low-frequency changes in auditory function with posture are largely a result of changes in middle-ear transmission that result from the changes in ICP associated with changes in posture (Büki et al., 1996, Büki et al., 2000, Büki et al., 2002, de Kleine et al., 2000, de Kleine et al., 2001). Their measurements and models are generally consistent with the following hypothesis. The auditory system is sensitive to changes in ICP because the cochlear aqueduct connects the cerebral spinal fluid to the cochlear fluid; increases in ICP are transferred to increases in intracochlear pressure, which results in outward static displacements of the compliant oval and round windows. These ICP increases are most likely to be detected as reductions in middle-ear transmission that result from an increased stiffness of the annular ligament, which connects the stapes to the oval window (Büki et al., 2000, Büki et al., 2002, Voss et al., 2006), with the effects of increased stiffness most prominent at frequencies below the middle ear’s resonant frequency (i.e., below about 2000 Hz).
Theoretically, different middle-ear transmission measurements could be used to detect ICP changes, including otoacoustic emissions (Büki et al., 1996, Büki et al., 2000, Büki et al., 2002, de Kleine et al., 2000, de Kleine et al., 2001, Frank et al., 2000, Voss et al., 2006), the cochlear-microphonic potential (Büki et al., 2009), changes in middle-ear impedance (Magnano et al., 1994, Liau, 1999), and other related quantities such as reflectance, and changes in displacement patterns of the tympanic membrane (Marchbanks, 1984), which were later shown to be too variable to monitor ICP (Rosingh et al., 1998, Shimbles et al., 2005). An advantage of evoked otoacoustic emissions is that they are affected by two reductions in middle-ear transmission: one in the forward direction as the stimulus and one in the reverse direction as the emission (Voss and Shera, 2004); a limitation is that the emissions may be weak or absent in individuals with a hearing loss. Thus, the potential for monitoring changes in ICP through concomitant changes in middle-ear transmission should be evaluated using multiple measures, and here we quantify how both distortion product otoacoustic emissions (DPOAEs) and reflectance, which is related to impedance measures (e.g., Keefe et al., 1993, Voss and Allen, 1994), are affected by changes in posture, and presumably ICP changes.
The specific goal of this paper is to present measurements of both DPOAE magnitudes and angles and also power reflectance made at the same time at two extreme postures, presumably resulting in ICP changes. Additionally, these measurements were made multiple times on a given subject so that intra-subject variability of these measures could be assessed.
Section snippets
Overview
Measurements of DPOAE magnitudes, DPOAE angles, and power reflectance were made to characterize how posture, and presumably intracranial pressure (ICP), affects these three measures. Measurements were made in the supine (upright) position and a position with the subject tilted at −45° relative to the horizontal. Additionally, the intra-subject variability for all three measures is quantified through repeated measurements across five sessions.
Human subjects
Measurements are reported from the left ears of 12
Middle-ear pressures
The tympanic peak pressure (TPP), assumed equal to the middle-ear pressure (MEP), was measured before each DPOAE and reflectance measurement session. Fig. 1 reports these MEPs at each of the two postural positions. The MEP from 11 of the 12 ears was always within daPa of zero with the subject in the upright position; the exception was Subject 9, whose MEP ranged from −66 to −30 daPa in the upright position. When the subjects were tilted, 5 of these 11 ears remained within daPa of zero for
Summary of results
DPOAE magnitudes, DPOAE angles, and power reflectance all showed systematic changes with posture, and presumably with ICP. The significance of the changes were assessed with two methods: p values computed via (1) a resampling numerical approach for each subject (Fig. 3) and (2) a repeated measures regression model that combined all subjects (Table 1). In both tests, all three measures showed significant changes at multiple frequencies. We note that changes at the lowest frequencies in DPOAE
Acknowledgements
This work was supported by a CAREER award from the National Science Foundation (SEV) and grant R01 DC003687 (CAS) from the NIDCD, National Institutes of Health. We also thank our volunteer subjects and three helpful Hearing Research anonymous reviewers.
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