Spaceflight Associated Neuro-ocular Syndrome (SANS) is a medical condition made up of a constellation of symptoms affecting at least 34 percent of American astronauts who have flown International Space Station (ISS) missions. SANS is defined primarily by visual acuity deficits and anatomical changes to eye structures. Exposure to head-down tilt bed rest, a well-accepted analog to induce cardiovascular adaptations similar to space flight, has not resulted in findings representative of SANS that have been observed in some astronauts participating in long-duration ISS missions. Factors other than the cephalad fluid shift observed during space flight and bed rest have been hypothesized to contribute to the development of SANS but have not been tested.
APPROACH:
This bed rest study sought to investigate two factors, age and diet, with regard to vascular compliance and ocular measures. Specifically, seven younger men and four older men participated in a 6° head-down bed rest study under standard conditions, with the exception that a higher than normal sodium intake was prescribed to match the intake of ISS astronauts. Dietary sodium intake in this study was approximately 40% higher than in previous NASA bed rest studies.
Plasma volume, vascular measures (area and diameter of head and neck vessels among other ultrasound measures), and ocular measures (intraocular pressure, optical coherence tomography, and ocular ultrasound) were collected on all subjects. Non-invasive internal jugular venous pressure was collected using ultrasound.
RESULTS:
For the majority of cardiovascular and ocular variables studied, there was no significant effect of 14 days of bed rest. The bed rest-induced decrease in plasma volume did not appear to differ from plasma volume losses that occurred during previous NASA bed rest studies using standard dietary practices. Arterial diameters and flow were not different from pre- to post-bed rest at rest during the whole body tilting protocol (-20, -10, 0, +10, +20 degrees of tilt). intraocular pressure responded to posture changes, but did not change pre- to post-bed rest. Similarly, axial length (via ultrasound) and retinal nerve fiber layer (via Optical Coherence Tomography) were not different after bed rest. In contrast, internal jugular vein area (via ultrasound) was lower after bed rest in the head-down tilt postures, suggesting a decrease in venous compliance. This effect was transient and recovered to pre-bed rest levels after three days of normal ambulation. Unfortunately, the investigators were unable to recruit a sufficient number of older subjects to test the interactions between aging and bed rest on the parameters studied. These results suggest that a high salt diet more closely matching that of astronauts during space flight does not increase the likelihood of developing signs of VIIP alone after 14 days of head-down tilt bed rest.