Subfoveal Choroidal Thickness and Foveal Retinal Thickness During Head-Down Tilt

Introduction: To reveal subtle morphological changes in the eye during simulated microgravity for spaceflights, we measured subfoveal choroidal thickness and foveal retinal thickness during 10 degrees head-down tilt (HDT). We hypothesized that elevated ophthalmic vein pressure during simulated microgravity increases subfoveal choroidal thickness via enlargement of the choroidal vasculature and greater choroidal blood volume. Methods: The right eyes of nine healthy subjects (seven men, two women) were examined. Subfoveal choroidal thickness and foveal retinal thickness were measured using spectral domain-optical coherence tomography in the sitting position, and after 15 and 30 min of 10 degrees HOT. Intraocular pressure was also measured. Results: Mean subfoveal choroidal thickness (+/- SEM) increased from 300 +/- 31 mu m in the sitting position to 315 +/- 31 mu m with 15-min HDT, and 333 +/- 31 mu m with 30-min HDT. However, no change in foveal retinal thickness was observed (228 +/- 9 mu m in the sitting position, 228 +/- 10 mu m with 15-min HDT and 228 +/- 9 mu m with 30-min HOT). Intraocular pressure increased from 14 +/- 1 mmHg in the sitting position to 21 +/- 2 mmHg with 30-min HOT (54 +/- 6%, N = 5). Discussion: Subfoyeal choroidal thickness and intraocular pressure were increased by HOT during simulated microgravity, although no change in foveal retinal thickness was observed.