Non-invasive monitoring of intracranial pressure changes: utilizing otoacoustic emissions.

Introduction: Invasive techniques for objectively measuring intracranial pressure (ICP) pose significant risks including infection, intracerebral hemorrhage, and brain injury (Maniker et al., 2006; Wolfe and Torbey, 2009; Scheithauer et al., 2009). Otoacoustic emissions (OAEs) provide non-invasive indicators of ICP changes, as the OAE phase reflects middle ear transmission, which is influenced by ICP via its connection to intracochlear pressure. A calibration of the technique is based on the ICP and OAE phase data collected by Buki et al. (1996) in patients undergoing controlled ICP changes during neurosurgery. Forward pressure level (FPL) calibration provides additional insight into middle ear transmission, complementing and validating OAE phase measurements. Material and methods: Five International Space Station (ISS) normal-hearing astronauts were tested for distortion product otoacoustic emissions (DPOAEs) pre-flight, in microgravi-ty in-flight conditions, and post-flight. Ground experiments were conducted involving 20 young volunteers, who were DPOAE tested in different body postures. The DPOAE response was time-frequency filtered (Moleti et al., 2012) to unmix the distortion and reflection components. The FPL calibration data provided, as a byproduct, a direct estimate of the load impedance measured in the ear canal, which is related to the middle ear reflectance. Results: Systematic changes associated with microgravity and postural changes were observed for all considered physical quantities, with the DPOAE phase generally yielding the best results. Although physical quantities such as reflectance and load impedance are more directly related to ICP changes than the OAE phase, their numerical evaluation may be more difficult, because it involves ratios between complex quantities that are very sensitive to phase uncertainties. Conclusions: DPOAE measurements with FPL calibration and component unmixing provide effective non-invasive indicators of ICP changes. Future research directions include applications in Glaucoma patients and absolute calibration in neural surgery environments for several types of clinical population such as hydrocephalus and intracranial tumor. [ABSTRACT FROM AUTHOR]