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°, 0°, −30°, and −45° to the horizontal). At these positions, it is expected that ICP varied from about 0 (90°) to 22 mm Hg (−45°). DPOAE magnitudes were measured for a set of frequencies 750\u3cf 2\u3c4000, with f 2/f 1=1.2. Results For the low-frequency range of 750≤f 2≤1500, the differences in DPOAE magnitude between upright and −45° were highly significant (all p\u3c0.01), and above 1500 Hz there were minimal differences between magnitudes at 90° versus −45°. There were no significant differences in the DPOAE magnitudes with subjects at 90° and 0° 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

3D imaging and modeling of the middle and inner ear

The bones of the middle ear are the smallest bones in the body and are among the most complicated functionally. They are located within the temporal bone – rendering them difficult to access and study. An accurate 3D model can offer an excellent illustration of the complex spatial relationships between the ossicles and the nerves and muscles with which they intertwine. The overall objective was to create an educational module for learning the anatomy of the outer, middle and inner ear from MRI data. Such a teaching tool will provide surgeons, radiologists and audiologists with a detailed self-guided tour of ear anatomy. MRI images of the auditory canal were acquired using a 9 Tesla MR scanner. The acquired images were reformatted along obliquely oriented axes to obtain the desired orientation relative to anatomical planes. An automated segmentation algorithm was applied to the MRI data to separate the cochlea, auditory nerve and semi-circular canals in the inner ear. Semi-automated segmentation was used to separate the middle ear bones. This was necessary in order to detach the malleus from the incus and the tympanic membrane from the malleus, as the boundaries between these structures were not sufficiently distinct in the data. Each structure became an independent object to facilitate its interactive manipulation. Different angles of view of the 3D structures were rendered illustrating the anatomic pathway starting at the tympanic membrane, through the middle ear bones, to the semi-circular canals, cochlea and auditory nerve in the inner ear