Effect of head-down tilt on brain water distribution

Vascular and tissue fluid dynamics in the microgravity of space environment s is commonly simulated by head-down tilt (HDT). Previous reports have indi cated that intracranial pressure and extracranial vascular pressures increa se during acute HDT and may cause cerebral edema. Tissue water changes with in the cranium are detectable by T-2 magnetic resonance imaging. We obtaine d T-2 images of sagittal slices from five subjects while they were supine a nd during -13 degrees HDT using a 1.5-Tesla whole-body magnet. The analysis of difference images demonstrated that HDT leads to a 21% reduction of T-2 in the subarachnoid cerebrospinal fluid (CSF) compartment and a 11% reduct ion in the eyes, which implies a reduction of water content; no increase in T-2 was observed in other brain regions that have been associated with cer ebral edema. These findings suggest that water leaves the CSF and ocular co mpartments by exudation as a result of increased transmural pressure causin g water to leave the cranium via the spinal CSF compartment or the venous c irculation.