title: Ocular Outcomes Comparison Between 14- and 70-Day Head-Down-Tilt Bed Rest.
abstract: OBJECTIVE
  To compare ocular outcomes in healthy subjects undergoing 14- and/or 70-day head-down-tilt (HDT) bed rest (BR).
  
  
  METHODS
  Participants were selected by using NASA standard screening procedures. Standardized NASA BR conditions were implemented. Subjects maintained a 6° HDT position for 14 and/or 70 consecutive days. Weekly ophthalmologic examinations were performed in the sitting (pre/post-BR only) and HDT positions. Mixed-effects linear models compared pre- and post-HDT BR observations between 14- and 70-day HDT BR in best-corrected visual acuity, spherical equivalent, intraocular pressure (IOP), Spectralis OCT retinal nerve fiber layer thickness, peripapillary and macular retinal thicknesses.
  
  
  RESULTS
  Sixteen and six subjects completed the 14- and 70-day HDT BR studies, respectively. The magnitude of HDT BR-induced changes was not significantly different between the two studies for all outcomes, except the superior (mean pre/post difference of 14- vs. 70-day HDT BR: +4.69 μm versus +11.50 μm), nasal (+4.63 μm versus +11.46 μm), and inferior (+4.34 μm versus +10.08 μm) peripapillary retinal thickness. A +1.42 mm Hg and a +1.79 mm Hg iCare IOP increase from baseline occurred during 14- and 70-day HDT BR, respectively. Modified Amsler grid, red dot test, confrontational visual field, color vision, and stereoscopic fundus photography were unremarkable.
  
  
  CONCLUSIONS
  Seventy-day HDT BR induced greater peripapillary retinal thickening than 14-day HDT BR, suggesting that time may affect the amount of optic disc swelling. Spectralis OCT detected retinal nerve fiber layer thickening post BR, without clinical signs of optic disc edema. A small IOP increase during BR subsided post HDT BR. Such changes may have resulted from BR-induced cephalad fluids shift. The HDT BR duration may be critical for replicating microgravity-related ophthalmologic changes observed in astronauts on ≥6-month spaceflights.
authors: Department of Ophthalmology and Visual Sciences The University of Texas Medical Branch at Galveston, Galveston, Texas, United States.
authors: Giovanni
authors: G
authors: Taibbi
authors: Universities Space Research Association, Division of Space Life Sciences, Houston, Texas, United States.
authors: Ronita L
authors: RL
authors: Cromwell
authors: Universities Space Research Association, Division of Space Life Sciences, Houston, Texas, United States.
authors: Susana B
authors: SB
authors: Zanello
authors: Universities Space Research Association, Division of Space Life Sciences, Houston, Texas, United States.
authors: Patrice O
authors: PO
authors: Yarbough
authors: Universities Space Research Association, Division of Space Life Sciences, Houston, Texas, United States.
authors: Robert J
authors: RJ
authors: Ploutz-Snyder
authors: Department of Ophthalmology and Visual Sciences The University of Texas Medical Branch at Galveston, Galveston, Texas, United States.
authors: Bernard F
authors: BF
authors: Godley
authors: Department of Ophthalmology and Visual Sciences The University of Texas Medical Branch at Galveston, Galveston, Texas, United States.
authors: Gianmarco
authors: G
authors: Vizzeri
mesh_terms: D000328:Adult; D001510:Bed Rest / Q000379:methods*; D005260:Female; D005500:Follow-Up Studies; D018475:Head-Down Tilt / Q000502:physiology*; D006801:Humans; D007429:Intraocular Pressure / Q000502:physiology*; D008297:Male; D009898:Optic Disk / Q000166:cytology; D012165:Retinal Ganglion Cells / Q000166:cytology; D013997:Time Factors; D041623:Tomography, Optical Coherence; D014785:Vision, Ocular / Q000502:physiology*