The immediate functional and structural consequences of head-down tilt on the cranial venous vasculature and brain temperature.

Cephalad fluid shift is strongly implicated in the brain-structural and craniovascular disturbances observed after long-duration spaceflight. It is part of what has been termed the cranial microgravity response (CMR). However, the physiological mechanisms underlying these changes remain unclear. We employed a ~2-h, -10° head-down tilt (HDT) paradigm to investigate the immediate effects of cephalad fluid shift on cranial venous outflow, cephalic venous vasculature, and brain temperature. Twenty-one adults (26.6 ± 2.4 years) underwent four MRI sessions combining 4D-flow measurements, susceptibility-weighed imaging, Time-of-Flight angiography, and MR Spectroscopy. HDT resulted in a significant increase in internal jugular vein cross-sectional area (p < 0.001), decreased jugular vein flow velocity (p < 0.001) and normalized mean flowrate (p < 0.001), a volumetric expansion of the main cervical arteries (p < 0.001), and a reduction in superior sagittal sinus flow velocity (p = 0.03). We also found a significant increase in global brain temperature after HDT (0.33°C ± 0.28°C; p < 0.001). These findings provide converging evidence for compromised cranial venous outflow in simulated microgravity, suggesting an upstream impact on the cerebral venous vasculature and thermal homeostasis of the head. Our setup now offers a comprehensive and fast-turnaround framework for the testing of countermeasures against the CMR.