Intraocular pressure and choroidal thickness respond differently to lower body negative pressure during spaceflight
Greenwald SH, Macias BR, Lee SMC, Marshall-Goebel K, Ebert DJ, Liu JHK, Ploutz-Snyder RJ, Alferova IV, Dulchavsky SA, Hargens AR, Stenger MB, and Laurie SS. Intraocular pressure and choroidal thickness respond differently to lower body negative pressure during spaceflight. J Appl Physiol (1985) 2021.
J Appl Physiol (1985)
Spaceflight associated neuro-ocular syndrome (SANS) develops during long-duration (>1 month) spaceflight presumably because of chronic exposure to a headward fluid shift that occurs in weightlessness. We aimed to determine whether reversing this headward fluid shift with acute application of lower body negative pressure (LBNP) can influence outcome measures at the eye. Intraocular pressure (IOP) and subfoveal choroidal thickness were therefore evaluated by tonometry and optical coherence tomography (OCT), respectively, in 14 International Space Station crewmembers before flight in the seated, supine, and 15° head-down tilt (HDT) postures and during spaceflight, without and with application of 25 mmHg LBNP. IOP in the preflight seated posture was 14.4 mmHg (95% CI, 13.5-15.2 mmHg) and spaceflight elevated this value by 1.3 mmHg (95% CI, 0.7-1.8 mmHg, P<0.001). Acute exposure to LBNP during spaceflight reduced IOP to 14.2 mmHg (95% CI, 13.4-15.0 mmHg), which was equivalent to that of the seated posture (P>0.99), indicating that venous fluid redistribution by LBNP can influence ocular outcome variables during spaceflight. Choroidal thickness during spaceflight (374 µm, 95% CI, 325-423 µm) increased by 35 µm (95% CI, 25-45 µm, P<0.001), compared to the preflight seated posture (339 µm, 95% CI, 289-388 µm). Acute use of LBNP during spaceflight did not affect choroidal thickness (381 µm, 95% CI, 331-430 µm, P=0.99). The finding that transmission of reduced venous pressure by LBNP did not decrease choroidal thickness suggests that engorgement of this tissue during spaceflight may reflect changes that are secondary to the chronic cerebral venous congestion associated with spaceflight.
ePub ahead of print