Anterior ocular globe shift observed in astronauts after long-duration spaceflight

Abstract Background/Objectives Long-duration spaceflight is associated with ophthalmic effects such as optic disc edema, hyperopic shift, ocular globe flattening, and choroidal folds, collectively known as Spaceflight Associated Neuro-ocular Syndrome, or SANS. While the mechanism of SANS remains unknown, modeling of orbital congestion has predicted anterior shift of the eye which has not yet been evaluated. The goal of this study was to develop and apply a non-invasive MRI-based method for quantification of eye shift in long-duration spaceflight astronauts. Subjects/Methods Preflight and postflight 3D T1-weighted MR images of the head were analyzed from 18 astronauts after approximately 6 months of spaceflight, and in 10 terrestrial controls at baseline, 2, 6, and 12-month follow-up. A semi-automated method was developed to quantify 3-dimensional eye movement, in the x, y, and z direction, with respect to the skull at follow-up versus baseline based on ocular globe center-of-mass (eye shift). Association between eye shift and posterior globe flattening was tested with a Pearson correlation. Results Eye shift in the x, y, and z direction (-medial/+lateral, -posterior/+anterior, -inferior/+superior) was 0.34 ± 0.27 mm (p = 0.013), 0.73 ± 0.40 mm (p < 0.0001), -0.21 ± 0.35 mm (p = 0.14), in astronauts compared to -0.10 ± 0.14 mm, 0.12 ± 0.33 mm, and − 0.037 ± 0.35 mm in controls. Correlations between posterior globe flattening and eye shift were not significant in any x, y, or z direction. Conclusions A novel semi-automated MRI-based pipeline was developed to quantify anterior eye shift in astronauts and terrestrial controls. There were statistically significant shifts in eye center of mass in the anterior and lateral directions in astronauts compared to terrestrial controls.