Ultrasound-based Assessment of Jugular Vein Alterations in Response to Simulated Microgravity and Countermeasures: A Systematic Review

During spaceflight, assessment of jugular vein (JV) alterations is critical as it reflects systemic responses to fluid redistribution and carries a risk of thrombosis, a potentially fatal condition if undetected. Imaging options during spaceflight are restricted, making ultrasound the only feasible modality. However, available data on JV alterations are from small astronaut cohorts (typically n <12), limiting statistical power and robust validation. As a result, ground-based microgravity simulation studies are required to characterise microgravity-induced physiological changes, including JV alterations. This systematic review synthesised ultrasound-based findings on JV alterations across simulated microgravity models, including the effects of countermeasure techniques intended to mitigate these simulated microgravity-induced changes. The findings were compared with reported spaceflight JV measurements to examine similarities in venous responses. This review further reports variability in ultrasound systems, probes, and imaging methods, highlighting their implications for measurement consistency and translational relevance to true microgravity. Of 102 records identified, 21 studies were included in the review and four simulated microgravity models were identified: head-down tilt (n = 9), head-down bed rest (n = 6), dry immersion (n = 3) and parabolic flight (n = 3). Nine studies tested countermeasures, including thigh cuffs (n = 4), lower body negative pressure (n = 3) and exercise (n = 2). Across models, JV cross-sectional area, volume and pressure increased with greater intensity or longer exposure to simulation, indicating progressive venous distension. Lower body negative pressure was most effective at mitigating JV changes, thigh cuffs provided transient benefit and exercise showed limited mitigation of venous enlargement. Comparisons with spaceflight data showed that simulated microgravity reproduces the direction but often exaggerates the magnitude of venous changes, particularly in short-term models. Methodological variability and male-dominant cohorts limit the translational validity of findings to spaceflight ultrasound assessment, highlighting the need for standardised protocols and larger, sex-balanced cohorts in future simulated microgravity studies. From the countermeasure evidence identified in this review, future studies should further optimise and validate approaches that mitigate JV distension, minimise venous stasis and reduce the risk of thrombosis in simulated microgravity and, by extension, spaceflight.