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.

[Source : as above ]

The research suggests 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.

However, the exact processes which lead to SANS are currently unknown, and are thought to be a multifaceted set of interacting components.

Additionally, finding its mechanism is hampered by the very small number of patients.

Technical details : SANS American Academy of Opthalmology (EyeWiki)

Space flights are now known to lead to a wide range of potentially serious health challenges, including loss in bone density and muscle mass, spaceflight-associated neuro-ocular syndrome (SANS), perturbed immune function, and spaceflight anemia. For full technical details, see the 2024Space Omics and Medical Atlas.

Also see:
Apollo Syndromeplugin-autotooltip__plain plugin-autotooltip_bigApollo Syndrome

A 2016 study into cardiovascular disease (CVD) amongst members of the NASA Apollo missions found that the CVD mortality rate in Apollo lunar astronauts was 4–5 times higher than in non-flight and Low Earth Orbit astronauts.

"[...
Microgravity Immune Suppressionplugin-autotooltip__plain plugin-autotooltip_bigMicrogravity Immune Suppression

"Spaceflight causes alterations in human immunity, a finding which has been well documented immediately following spaceflight. Limited in-flight studies have also confirmed that to some degree immunity is compromised
Spaceflight anemiaplugin-autotooltip__plain plugin-autotooltip_bigSpaceflight anemia

"Anemia in astronauts has been noted since the first space missions, but the mechanisms contributing to anemia in space flight have remained unclear."

Source : Nature Medicine volume 28, pages 59–62 (2022)"

Anomalies in the amount of hemoglobin degradation in astronauts (around 12% higher than normal) was first discovered in the 1980s. The degradation leads to anemia, and tends to take hold in the first 10 days or so of micro-gravity spaceflight. After-effects appear to be…