This case series evaluated whether a second exposure to long-duration spaceflight (~6 months on the ISS) or its analog (head-down tilt bed rest, HDTBR, 30–60 days) worsens optic disc edema (measured by peripapillary total retinal thickness, TRT) or brain lateral ventricular volume (LVV) compared to the first exposure, in 7 astronauts and 5 HDTBR participants.
A second exposure did not significantly increase the magnitude of change in TRT (spaceflight: mean difference −5.6 µm, 95% CI −15 to 3.7 µm, P=.23; HDTBR: mean difference 3.1 µm, 95% CI −3.3 to 9.5 µm, P=.33) or lateral ventricular volume (spaceflight: mean difference 0.1 mL, 95% CI −0.9 to 1.1 mL, P=.78).
- Very small sample (7 astronauts, 5 HDTBR participants), limiting statistical power to detect modest cumulative effects. - Only one repeat exposure was assessed; whether effects become additive over three or more missions remains unknown. - HDTBR MRI data were available for only 3 of 5 participants.
A single repeat spaceflight mission or HDTBR campaign does not appear to amplify SANS-related ocular or brain structural changes beyond the first exposure. Space medicine teams can use this as baseline evidence when counseling astronauts about multi-mission risk, while noting that long-term functional consequences of cumulative exposures remain unresolved.
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