Effect of head-down tilt on brain water distribution.

Vascular and tissue fluid dynamics in the microgravity of space environments is commonly simulated by head-down tilt (HDT). Previous reports have indicated that intracranial pressure and extracranial vascular pressures increase during acute HDT and may cause cerebral edema. Tissue water changes within the cranium are detectable by T2 magnetic resonance imaging. We obtained T2 images of sagittal slices from five subjects while they were supine and 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 T2 in the subarachnoid cerebrospinal fluid (CSF) compartment and a 11% reduction in the eyes, which implies a reduction of water content; no increase in T2 was observed in other brain regions that have been associated with cerebral edema. These findings suggest that water leaves the CSF and ocular compartments by exudation as a result of increased transmural pressure causing water to leave the cranium via the spinal CSF compartment or the venous circulation.