Otoacoustic Estimate of Astronauts' Intracranial Pressure Changes During Spaceflight

Abstract

Purpose: To investigate the potential correlation between prolonged exposure to microgravity on the International Space Station and increased intracranial fluid pressure, which is considered a risk factor for the astronauts' vision, and to explore the feasibility of using distortion product otoacoustic emissions as a non-invasive in-flight monitor for intracranial pressure changes.

Methods: Distortion product otoacoustic emission phase measurements were taken from both ears of five astronauts pre-flight, in-flight, and post-flight. These measurements served as indirect indicators of intracranial pressure changes, given their high sensitivity to middle ear transmission alterations. The baseline pre-flight ground measurements were taken in the seated upright position.

Results: In-flight measurements revealed a significant systematic increase in otoacoustic phase, indicating elevated intracranial pressure during spaceflight compared to seated upright pre-flight ground baseline. Noteworthy, in two astronauts, strong agreement was also observed between the time course of the phase changes measured in the two ears during and after the mission. Reproducibility and stability of the probe placement in the ear canal were recognized as a critical issue.

Conclusions: The study suggests that distortion product otoacoustic emissions hold promise as a non-invasive tool for monitoring intracranial pressure changes in astronauts during space missions. Pre-flight measurements in different body postures and probe fitting strategies based on the individual ear morphology are needed to validate and refine this approach.

Keywords: Astronaut physiology; Microgravity; Middle ear transmission; Otoacoustic emissions.

Fig. 1

Left: high-resolution ZL DPOAE phase difference, with respect to pre-flight baseline phase, in the f_DP = 1–4 kHz range. Solid lines of thickness increasing with time identify the five in-flight sessions. The first and second post-flight BDCs are plotted as dotted and dashed lines, respectively. Right: time course of the Φ₁₄ change during and after the ISS mission (shaded area)

Fig. 2

Φ₁₄ change with respect to the seated baseline as a function of time relative to the launch day L, normalized to the mission duration, for the four subjects A, B, C, and D, with available pre-flight data. In subject E, with unavailable pre-flight data, the 2nd post-flight BDC phase was used as reference, and the individual data are not shown for privacy reasons

Fig. 3

Violin plots summarizing the Φ₁₄ change, with respect to the baseline value, in five-time intervals during and after the ISS mission. The 50 in-flight data are sorted in three-time intervals within the mission duration, the first sixty flight days (FD) after the day of launch, the next sixty days, and the days on the ISS after FD120, including, respectively, 20, 14, and 16 measurements. The first post-flight measurement was taken within the second week after the day of return to Earth, and the second post-flight measurement 4–7 months after that. The day of the post-flight measurement RD is measured from the day of the return to Earth. As the distribution is strongly non-gaussian, the median (pentagrams) is also shown along with the average (squares), although none is representative of the distributions. The points from ear A left are marked with a filled black circle

Fig. 4

Violin and swarm plots summarizing the distribution of the Φ₁₄ change w/r to the baseline, in the other BDC 1G sessions (crosses) and in spaceflight 0G (circles) groups, for all astronauts. The medians of the two distributions are indicated by a pentagram