title: Ocular outcomes evaluation in a 14-day head-down bed rest study.
abstract: BACKGROUND
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Ophthalmology and Visual Sciences, The University of Texas Medical Branch, Galveston, TX, USA.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Taibbi
authors.forename: Ronita L
authors.initials: RL
authors.lastname: Cromwell
authors.forename: Susana B
authors.initials: SB
authors.lastname: Zanello
authors.forename: Patrice O
authors.initials: PO
authors.lastname: Yarbough
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Ploutz-Snyder
authors.forename: Bernard F
authors.initials: BF
authors.lastname: Godley
authors.forename: Gianmarco
authors.initials: G
authors.lastname: Vizzeri
mesh_terms: D000328:Adult; D001510:Bed Rest; D005260:Female; D018475:Head-Down Tilt; D006801:Humans; D007429:Intraocular Pressure; D008297:Male; D008875:Middle Aged; D041623:Tomography, Optical Coherence; D014065:Tonometry, Ocular; D014787:Vision Tests; D014785:Vision, Ocular; D014792:Visual Acuity