Dose-dependent reduction of optic nerve sheath diameter during graded lower body negative pressure in healthy adults of both sexes.

BACKGROUND: Optic nerve sheath diameter (ONSD) measured by ultrasound provides a non-invasive surrogate of intracranial pressure. Although lower body negative pressure (LBNP) is widely used to simulate central hypovolemia, the dose-response relationship between graded LBNP and ONSD, and possible sex differences therein, are incompletely characterized. METHODS: Forty-four healthy young adults (22 female, 22 male) underwent graded LBNP ( - 10 to - 40 mmHg) in the supine position; valid ONSD data were available in 37 participants (18 female, 19 male) who constituted the analytical sample. ONSD was measured from transbulbar B-mode ultrasound images 3 mm posterior to the globe, with three representative frames averaged per condition. Relative changes from individual supine baseline were analyzed using a linear mixed-effects model with state, sex, and their interaction as fixed effects. RESULTS: A progressive and statistically significant reduction in ONSD was observed at every LBNP level in both sexes. Female participants showed reductions of approximately - 4.4 % (LBNP10, p = 0.001), - 9.1 % (LBNP20, p < 0.001), - 13.2 % (LBNP30, p < 0.001) and - 15.9 % (LBNP40, p < 0.001) relative to supine baseline. Male-specific contrasts showed comparable reductions (LBNP10: - 3.0 % , p = 0.022; LBNP20: - 7.9 % , p < 0.001; LBNP30: - 10.1 % , p < 0.001; LBNP40: - 13.4 % , p < 0.001). The main effect of sex was not significant (p = 1.00), and no state × sex interaction reached significance (all p > 0.1). ONSD returned to baseline during recovery (female: p = 0.173; male: p = 0.059). CONCLUSION: Graded LBNP induces a progressive, dose-dependent reduction in ONSD that is detectable already at mild LBNP ( - 10 mmHg) and reaches approximately - 15 % at - 40 mmHg. No statistically significant sex-related differences in the ONSD response to graded LBNP were detected in the present study. These findings support the use of transbulbar ONSD ultrasound as a sensitive non-invasive marker consistent with intracranial-pressure reductions during simulated central hypovolemia. By linking a systemic cardiovascular perturbation to a cerebro-ocular pressure readout, the study offers a network-physiology view of central-hypovolemia responses relevant to spaceflight and orthostatic-stress research.