Ocular Outcomes Comparison Between 14- and 70-Day Head-Down-Tilt Bed Rest

Abstract

PurposeTo compare ocular outcomes in healthy subjects undergoing 14- and/or 70-day head-down-tilt (HDT) bed rest (BR).MethodsParticipants 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.ResultsSixteen 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.ConclusionsSeventy-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.

Astronaut ophthalmic syndrome

“…Studies of fluid shifts in relation to cardiovascular and ocular examination during long-duration spaceflight are currently underway with ISS astronauts. Ophthalmic issues have been assessed in head-down tilt (HDT) bed rest, an often used analog of weightlessness, with some subclinical findings but none as overt as those observed after actual spaceflight (3,(58)(59)(60). Diuresis occurs in HDT bed rest but not during spaceflight, and total body water decreases during HDT bed rest but is unchanged during spaceflight (56).…”

Separate and Combined Effects of Hypoxia and Horizontal Bed Rest on Retinal Blood Vessel Diameters

“…5 Ocular effects during bed rest are now being studied more systematically, with a recent investigation showing that a 70-day bed rest induced greater peripapillary retinal thickening than a 14-day bed rest. 6 The present study was part of a larger research program investigating the effect of the anticipated environments in future planetary habitats, 7 which will be hypobaric and hypoxic, on physiological systems. The purpose of establishing a hypobaric hypoxic environment in space habitats is primarily to eliminate the need for long decompression profiles requiring oxygen breathing prior to extravehicular activities.…”

Ophthalmological Evaluation of Integrated Resistance and Aerobic Training During 70-Day Bed Rest

“…25, 29 Proposed mechanisms to explain the IOP elevation during head-down-tilt include increased episcleral venous pressure and choroidal engorgement with expansion against the sclera, respectively. 4, 15 In our study, we found no differences in iCare IOP elevation between controls and exercisers (+1.38 mmHg and +1.63 mmHg increase from baseline, respectively).…”

“…Mixed-effect restricted maximum likelihood regression models have recently shown the importance of the time spent in head-down-tilt on the magnitude of peripapillary thickness changes induced by bed rest. 29 Thus, it is expected that longer bed rest durations would result in greater cpRNFL thickening.…”

“…However, previous research has shown peripapillary retinal thickening and increased circumpapillary retinal nerve fiber layer (cpRNFL) thickness in healthy individuals who spent 70 consecutive days in the 6° head-down-tilt position. 29 It is conceivable that interventions with the potential to mitigate the cephalad fluids shift, such as lower body physical exercise, may also attenuate the magnitude of posture-induced ocular changes. Therefore, the purpose of the present investigation was to evaluate the ophthalmological effects of integrated resistance and aerobic training (iRAT) during 70 days of bed rest.…”