Approximately 60% of the crew members of the International Space Station have reported altered visual acuity after long-duration exposure to micro-gravity.
Astronauts on long-duration spaceflight missions may develop changes in ocular structure and function, which can persist for years after the return to normal gravity. Chronic exposure to elevated intracranial pressure during spaceflight is hypothesized to be a contributing factor, however, the etiologic causes remain unknown.
Source: Radiology, Vol. 295, No. 3
A 2020 study (link above) which used MRI techniques to scan the brains of 11 astronauts, found that they had physical distortions of their pituitary glands, and other brain volume changes - some still persisting 1 year after landing.
Long-duration space-flight was associated with increased pituitary deformation, augmented aqueductal cerebrospinal fluid (CSF) hydrodynamics, and expansion of summated brain and CSF volumes.
The research team suggest that similar physical deformation effects might be responsible for the adverse changes in astronauts' visual system, which can persist for years after return to Earth.
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