Posture-induced changes in distortion-product otoacoustic emissions and the potential for noninvasive monitoring of changes in intracranial pressure.

INTRODUCTION: Intracranial pressure (ICP) monitoring is currently an invasive procedure that requires access to the intracranial space through an opening in the skull. Noninvasive monitoring of ICP via the auditory system is theoretically possible because changes in ICP transfer to the inner ear through connections between the cerebral spinal fluid and the cochlear fluids. In particular, low-frequency distortion-product otoacoustic emissions (DPOAEs), measured noninvasively in the external ear canal, have magnitudes that depend on ICP. Postural changes in healthy humans cause systematic changes in ICP. Here, we quantify the effects of postural changes, and presumably ICP changes, on DPOAE magnitudes.
METHODS: DPOAE magnitudes were measured on seven normal-hearing, healthy subjects at four postural positions on a tilting table (angles 90 degrees , 0 degrees , - 30 degrees , and - 45 degrees to the horizontal). At these positions, it is expected that ICP varied from about 0 (90 degrees ) to 22 mm Hg ( - 45 degrees ). DPOAE magnitudes were measured for a set of frequencies 750 < f2 < 4000, with f2/f1 = 1.2.
RESULTS: For the low-frequency range of 750 <or=f2<or= 1500, the differences in DPOAE magnitude between upright and - 45 degrees were highly significant (all p < 0.01), and above 1500 Hz there were minimal differences between magnitudes at 90 degrees versus - 45 degrees. There were no significant differences in the DPOAE magnitudes with subjects at 90 degrees and 0 degrees postures.
CONCLUSIONS: Changes in ICP can be detected using the auditory-based measurement of DPOAEs. In particular, changes are largest at low frequencies. Although this approach does not allow for absolute measurement of ICP, it appears that measurement of DPOAEs may be a useful means of noninvasively monitoring ICP.