Submacular Choroid Thickness Increases during Long-Duration Spaceflight

Abstract

Purpose : Optic disc edema, choroidal/retinal folds, globe flattening, cotton wool spot, and/or refractive error changes ≥0.75D have been observed in 24 astronauts during or after long-duration spaceflight. A leading hypothesis is that the headward fluid shift that occurs during spaceflight contributes to these ocular findings, but how this fluid shift affects posterior ocular structure is unknown. We characterized submacular choroidal thickness, intraocular pressure (IOP), and axial length before and during long-duration spaceflight.

Methods : Eight astronauts were studied preflight while seated and ~10-20 min after assuming supine posture, and ~140 days into spaceflight. We imaged the choroid with optical coherence tomography (OCT, Spectralis) using a single B-scan aligned through the optic nerve and fovea (OS, central fixation). Bruch’s membrane was automatically segmented (Heidelberg Eye Explorer, 1.9.10) and two independent observers manually segmented the choroid-scleral border. Submacular choroid thickness was calculated as the average thickness over a 6 mm distance centered under the fovea. The difference in thickness measures between the two observers for all 24 images was <5%, and the average of their measures was used for statistical comparisons. IOP (Icare Pro during seated and supine; TonoPen during spaceflight) (OS) and axial length (ultrasound; OD) were measured in triplicate following OCT imaging. We used repeated-measures mixed effects linear regression modeling (STATA, v14.1) to calculate means and 95% confidence intervals.

Results : Choroid thickness was not different between upright and supine postures before spaceflight (310.8±55.9 vs 305.1±55.9 µm, respectively p=0.685), but increased significantly during spaceflight (356.6±55.9 µm vs upright, p=0.001). Upright, supine, and spaceflight measures of IOP (14.6±1.6 mmHg, 15.0±1.6 mmHg, 15.2±1.6 mmHg) and axial length (2.31±0.05 cm, 2.31±0.05 cm, 2.31±0.05 cm) did not differ across time.

Conclusions : Acute posture changes on Earth do not significantly affect submacular choroid thickness in these 8 subjects, but prolonged exposure to spaceflight leads to choroidal engorgement without changing IOP or axial length. Future research will investigate if relationships exist between choroid thickness changes during spaceflight and development of ocular structural changes, including optic disc edema and choroidal/retinal folds.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.