Increased intracranial pressure (ICP) in prolonged microgravity conditions is considered one of the main risk factors for the astronauts’ health, as it can cause serious damage to vision, summarized under the definition of space flight-associated neuro-ocular syndrome (SANS). Direct, highly invasive methods are not usable to measure ICP in orbit, so identifying new non-invasive and reliable approaches becomes of fundamental importance. One of the most promising indirect methods is based on the measurement of the phase of distortion product otoacoustic emissions (DPOAE) as an indicator of changes in intracranial pressure during spaceflight.

Surprisingly, the human ear is not only sensitive to acoustic signals, but it is also able to generate and emit them, in the form of otoacoustic emissions (OAE). DPOAEs are distortion products OAE generated non-linearly within the sensory organ of hearing, the cochlea, and measurable with a microphone inserted into the ear canal. The phase of these signals depends on the stiffness of the middle ear, which in turn is influenced by changes in intracochlear fluid pressure, which is directly connected to ICP. Consequently, measuring the DPOAE phase allows one to indirectly evaluate, in addition to the functionality of the auditory system, also the increase in intracranial pressure, through a rapid, objective and non-invasive test.

The association between OAE phase and ICP has already been demonstrated in several studies conducted on Earth at different postures, and in transient microgravity conditions during the free-fall phases of parabolic flight. A calibration of the method, which allows to convert the increase in phase into an estimate of the increase in ICP, has been obtained from simultaneous measurements of OAE and ICP, varied under controlled conditions in neurosurgical patients. On the ISS, the only study that assessed ICP using OAE found no significant phase change in orbit compared to the seated posture in terrestrial gravity, and a decrease in phase compared to the supine posture.

In this context, the Acoustic Diagnostics experiment fits in, funded by ASI and led by Prof. Arturo Moleti of the Department of Physics of the University of Rome Tor Vergata, in collaboration with Altec, Campus Biomedico, INAIL, CNR and Sapienza University, and recently published in the peer-reviewed Journal of the Association for Research in Otolaryngology.

The study, conducted on board the ISS from 2019 to 2022, demonstrated for the first time a significant increase in the DPOAE phase during spaceflight compared to measurements taken on Earth, interpretable as an increase in ICP. In particular, DPOAE were recorded in five astronauts before, during and after their long-term stay (6-8 months) on board the ISS. The data measured in orbit were then compared with those recorded in the seated posture on Earth, before and after the flight. In addition to an average increase in ICP, a different sensitivity in the interindividual response was observed. In some subjects, in fact, the increase in ICP in flight estimated independently in the two ears was reproducible and significantly higher than that observed when moving from a sitting to a supine position, on Earth. In other subjects the data from the two ears were less correlated, suggesting a criticality related to the reproducibility of the probe insertion in the ear canal.

Although in principle the measurement is easy to understand and interpret, a great effort was necessary to develop a sufficiently sensitive technology like that used in the Acoustic Diagnostics experiment, also based on advanced signal analysis techniques. The study was therefore of fundamental importance both for the new technology used, but above all because it demonstrated how the measurement of the DPOAE phase represents a promising non-invasive tool for monitoring changes in astronauts' ICP during long-term space missions. This monitoring technique, if made available to a larger population of astronauts, could help to better understand not only how human physiology is affected by microgravity but also how to monitor and prevent some of the damage observed in astronauts at the neuro-ocular level.