During lengthy missions, astronauts can develop spaceflight-associated neuro-ocular syndrome, a condition that causes optic disc edema, chorioretinal folds, globe flattening, choroid thickening, and hyperopic refractive error shift. To compensate, researchers have found that lower body negative pressure shifts fluid toward the feet, offsetting the increase in choroid area and volume.
Researchers believe that loss of hydrostatic gradients in microgravity and a headward shift in vascular and cerebrospinal fluid can cause ocular remodeling. Microgravity lowers intracranial pressure (ICP) similar to the pressure experienced in the supine position on Earth. However, in space, the inability to lower ICP causes mild but persistently high ICP and lower translaminar pressure, contributing to optic remodeling, according to investigators. In some cases, the structural abnormalities associated with space flight, such as choroid thickening and globe flattening, may persist for years after returning to Earth.
Researchers wanted to learn if partial reintroduction of footward fluid shifts during simulated microgravity through lower body negative pressure during sleep would lessen choroid enlargement. They recruited 10 participants with no major cardiovascular, kidney, or ophthalmic disease. Of 10 participants, 5 were women, the mean age was 29±9 years, and the age range was 18 to 55 years. Participants completed 3 days of supine (0°) bed rest with lower body negative pressure and 3 days without 8 hours of lower body negative pressure.
Researchers found that participants experienced increased central venous pressure from the seated to supine position (mean [SD], seated: -2.3 [2.0] vs supine: 6.9 [2.0] mmHg; P <.001), leading to choroid engorgement over 3 days of bed rest (Δ area: +0.09 mm2 [95% CI, 0.04-0.13]; P =.001; Δ volume: +0.37 mm3 [95% CI, 0.19-0.55]; P =.001). Nightly lower body negative pressure resulted in a sustained reduction in supine central venous pressure (mean SD, 5.7±2.2 mm Hg to 1.2 [1.4 mm Hg]; P <.001) and lessened the increase in choroid area (74%) (Δ: 0.02 mm2 [95% -0.02 to 0.06]; P =.01) and volume (53%) (Δ: 0.17 mm3 [95% CI, 0.01-0.34]; P =.05) compared with control participants.
Based on this data, researchers concluded that lower body negative pressure during sleep might effectively reduce spaceflight-associated neuro-ocular syndrome and ocular remodeling during lengthy space missions.
Future research is needed to determine whether lower body negative pressure can prevent spaceflight-associated neuro-ocular syndrome during long-duration bed rest/spaceflight.
Hearon CM, Dias KA, Babu G, et al. Effect of nightly lower body negative pressure on choroid engorgement in a model of spaceflight-associated neuro-ocular syndrome: a randomized crossover trial. JAMA Ophthalmol. Published online December 09, 2021. doi:10.1001/jamaophthalmol.2021.5200.