Journal
AVIATION SPACE AND ENVIRONMENTAL MEDICINE
Volume 85, Issue 10, Pages 983-992Publisher
AEROSPACE MEDICAL ASSOC
DOI: 10.3357/ASEM.4055.2014
Keywords
microgravity; intraocular pressure; retinal nerve fiber layer; optimal alpha
Funding
- NASA Flight Analogs Project [516724.03.04.01]
- NIH/NCRR [1UL1RR029876-01]
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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 mu m), temporal-inferior (+1.58 mu m) and nasal-inferior RNFLT (+3.48 mu m). 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 degrees HDTBR determined a small nonprogressive 1013 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.
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