Current evidence suggests that NASA's spaceflight-associated neuro-opthalmological syndrome (SANS/VIIP) risk is related to an elevation in intracranial pressure (ICP) during spaceflight, either consequent to or aggravated by cephalad fluid shifts in microgravity. From recent data, SANS occurs in >50% of astronauts, to varying degrees, and can lead to long term visual changes. Although its cause is unknown, its importance is high enough to motivate studies of potential countermeasures. The key objectives of this project were therefore: (1) to test and help validate two commercial devices as mechanical countermeasures for cephalad fluid shifts to potentially treat elevated ICP, (2) identify any potential adverse consequences during use or following release of such countermeasures, and (3) optimize deployment procedures for such countermeasures.
The Russians currently use Braslet—an elastic thigh band—to help sequester blood in the legs and alleviate symptoms resulting from cephalad fluid shifts. While promising, this device has not been tested as a SANS countermeasure. Lower body negative pressure (LBNP) is an alternative approach, which draws fluid into the legs using vacuum mechanism. Both have drawbacks, however. Braslet devices are custom-built, difficult to obtain, and have limited calibration options. LBNP is typically bulky and hence could only be used at limited times. As an alternative to Braslet, we tested the Kaatsu thigh cuff system. This commercially available system is designed for enhanced muscle training on Earth. In addition, we investigated use of a LymphaPress compression garment configured to provide a vascular resistance for fluid return from the lower body (as opposed to enhanced fluid return for which the device was designed for clinically). In Experiments 1 and 2 we conducted tests using both countermeasures (at different inflation pressures) in healthy subjects undergoing -6 degrees head-down tilt (HDT). We characterized cerebral blood volume and flow, intraocular pressure, structural eye parameters, and cerebral vascular parameter changes associated with application, maintenance, and following release of each countermeasure. In Experiment 3, we tested the Kaatsu system in neurointensive care unit patients with invasive ICP devices implanted to monitor and treat elevated ICP.
Together, the data from these studies suggested that—at the chosen inflation pressures--neither countermeasure exhibited significant potential as a treatment for, or mitigator of, cephalad fluid shifts and elevated ICP.