Ocular outcomes evaluation in a 14-day head-down bed rest study

Abstract

Introduction: We evaluated ocular outcomes in a 14-d head-down tilt (HDT) bed rest (BR) study designed to simulate the effects of microgravity on the human body.

Methods: Healthy subjects were selected using NASA standard screening procedures. Standardized NASA BR conditions were implemented (e.g., strict sleep-wake cycle, standardized diet, 24-hour-a-day BR, continuous video monitoring). Subjects maintained a 6° HDT position for 14 consecutive days. Weekly ophthalmological examinations were performed in the sitting (pre/post-BR) and HDT (in-bed phase) positions. Equivalency tests with optimal-alpha techniques evaluated pre/post-BR differences in best-corrected visual acuity (BCVA), spherical equivalent, intraocular pressure (IOP), Spectral-domain OCT retinal nerve fiber layer thickness (RNFLT), optic disc and macular parameters.

Results: 16 subjects (12 men and 4 women) were enrolled. Nearly all ocular outcomes were within our predefined clinically relevant thresholds following HDTBR, except near BCVA (pre/post-BR mean difference: -0.06 logMAR), spherical equivalent (-0.30 D), Tonopen XL IOP (+3.03 mmHg) and Spectralis OCT average (+1.14 μm), temporal-inferior (+1.58 μm) and nasal-inferior RNFLT (+3.48 μm). Modified Amsler grid, red dot test, confrontational visual field, and color vision were within normal limits throughout. No changes were detected on stereoscopic color fundus photography.

Discussion: A few functional and structural changes were detected after 14-d HDTBR, notably an improved BCVA possibly due to learning effect and RNFL thickening without signs of optic disc edema. In general, 6° HDTBR determined a small nonprogressive IOP elevation, which returned to baseline levels post-BR. Further studies with different BR duration and/or tilt angle are warranted to investigate microgravity-induced ophthalmological changes.

Fig 1

Intraocular pressure (IOP) plots of the study subjects. In each plot, the solid vertical lines identify the pre-bed rest (left), in-bed (center) and post-bed rest (right) phases, respectively. The dashed horizontal lines correspond to the baseline IOP of the right and the left eye, respectively, calculated as the mean of the two pre-bed rest visits. BR, bed rest.

Fig 2

Intraocular pressure plots for iCare (A) and Goldmann applanation tonometer (B). Error bars represent the 95% confidence interval of the mean. In (A), the two vertical lines identify the pre-bed rest (left), in-bed (center) and post-bed rest (right) phases, respectively. BR, bed rest.