(G-256) Posterior Optic Globe Flattening in Long Duration Spaceflight Astronauts and Healthy Controls

A subset of long-duration spaceflight (LDSF) astronauts experience reductions in visual acuity in what is known as spaceflight-associated neuro-ocular syndrome (SANS). SANS is characterized by hyperopic shift, cotton wool spots, choroidal folds, optic disc edema, and posterior globe flattening. A quantitative and automated method was developed to quantify the degree of globe flattening in LDSF astronauts by calculating posterior optic globe volume displacement using MRI collected before and after spaceflight. Additionally, these methods were applied to healthy subjects in a longitudinal study to characterize the limitations in the method and understand what changes in posterior optic globe volume occur naturally over time. These findings can help to better understand the underlying causes of SANS and can be used to relate other physiological findings.

T2-weighted MRIs were collected from 10 LDSF astronauts before and after approximately 6 months of spaceflight. The same imaging protocol was also used to collect images from 10 healthy controls at baseline and 2-, 6-, and 12-month follow-up timepoints. A technique for quantifying posterior globe volume displacement was applied to both cohorts. The optic globe was radially resliced in one-degree increments about an axis connecting the lens center and the optic nerve head. Thresholding was applied for each slice to segment the optic globe and used to create a 3D reconstruction of the globe. The reconstructions from the follow-up timepoints were registered to their baseline counterparts and volume displacement of the posterior globe was calculated for a region within a 4mm radius of the optic nerve head. Statistical methods were applied to the control cohort over a 1-year duration to quantify measurement repeatability and natural variation over time.

Posterior optic globe volume displacement in the healthy controls was -1.92±4.99 mm³, 4.50±3.65 mm³, and -0.61±6.41 mm³ at 2,6, and12 months, respectively. Volume displacement after ~6 months of spaceflight was 11.09±11.30 mm³. These results suggest that volume displacement exceeding 10.2 mm³ cannot be attributed to naturally occurring variation alone and thus, the displacement observed in a subset of astronauts is attributable to microgravity exposure. Notably, the astronaut with the highest degree of displacement was also characterized as having SANS. The methods described here allow for detailed quantification of structural changes in the optic globe that will improve our understanding of SANS and other terrestrial cerebrospinal fluid disorders.