Otoacoustic Estimate of the Intracranial Pressure

Intracranial pressure (ICP) increase in microgravity is considered as a possible risk factors for astronauts. Non-invasive ICP monitor is needed for present and future space missions in microgravity or reduced gravity conditions. The intracranial and intracochlear fluids are connected by the cochlear aqueduct, hence their pressures are strongly correlated. Monitoring the phase of the null-delay component of Distortion product otoacoustic emissions (DPOAEs) provides an effective ICP diagnostic tool, as demonstrated by the Acoustic Diagnostics experiment on the ISS [1] and in ground experiments [2], which are re-analyzed and discussed here. DPOAE high-frequency resolution spectra are recorded using swept-tones in the 1-6 kHz range. The DPOAE null-delay component generated by nonlinear distortion is unmixed from the total DPOAE response using time-frequency domain filtering [3] with a large SNR improvement. The resulting phase-difference spectra (with respect to the baseline condition in normal gravity) can be averaged over a large frequency range with further SNR increase. The Thevenin calibration of the probe sound sources allows one to independently estimate the acoustic load impedance and the ear canal reflectance, which complement and validate the interpretation of the DPOAE phase change as due to ICP changes. The filtered DPOAE phase difference is capable of detecting the ICP increase associated with postural changes and with microgravity conditions. The information about the middle ear transmission obtained from the ear canal measurements is in principle more directly related to intracochlear pressure changes, but it may be more sensitive to systematic and random errors than the DPOAE phase. A critical issue is related to the insertion depth of the probe, which may be made more reproducible by using personalized earplugs matching the shape of the individual outer ear. An absolute calibration of the specific acquisition system will be obtained in a planned experiment on neurosurgical patients by measuring variable ICP both directly and with the DPOAE technique. The application of state-of-the-art signal acquisition and analysis techniques, based on advanced theoretical knowledge about the mechanisms of the hearing physiology, allows one to develop a sensitive, non-invasive, objective ICP monitor, which may find applications both in space research for monitoring the risk for astronauts, and in the clinic, for monitoring neurosurgical patients in the post- operatory phase.