Daily Generation of a Footward Fluid Shift Attenuates Ocular Changes Associated with Head-Down Tilt Bedrest.

Astronauts have presented with a constellation of visual changes referred to as spaceflight-associated neuro-ocular syndrome (SANS). However, early markers of microgravity-induced optic remodeling have not been fully identified nor have countermeasures been developed. In order to identify early markers of SANS, we studied 10 subjects with optical coherence tomography and ultrasound when upright and supine, and again after 24 hours of 6-degree head down tilt (HDT) bedrest. Upon acute transition from upright to supine, choroid area (2.24±0.53 to 2.28±0.52 mm2, p=0.001) and volume (9.51±2.08 to 9.73±2.08 mm3, p=0.002) increased. After 24 hours of HDT bedrest, subfoveal choroidal thickness (372±93 to 381±95 µm, p=0.02) , choroid area (2.25±0.52 to 2.33±0.54 mm2, p=0.08) and volume (9.64±2.03 to 9.82±2.08 mm3, p=0.08) increased relative to the supine position. Subsequently, 7 subjects spent 3 days in -6 deg HDT bedrest to assess whether low-level lower body negative pressure (LBNP) could prevent the observed choroidal engorgement during bedrest. Maintaining the -6 deg HDT position for 3 days caused choroid area (Δ0.11 mm2, p=0.05) and volume (Δ0.45 mm3, p=0.003) to increase. When participants also spent 8 hours daily under -20mmHg LBNP, choroid volume still increased, but substantially (40%) less than in the control trial (Δ0.27 mm3, p=0.05). Moreover, the increase in choroid area was diminished (Δ0.03 mm2, p=0.13), indicating that low-level LBNP attenuates the choroid expansion associated with 3 days of -6 deg HDT bedrest. These data suggest that low-level LBNP may be an effective countermeasure for SANS.