Intraocular pressure and choroidal changes during a simulated microgravity and attempted countermeasure: relevance to long-term space travel

Abstract

Purpose : Astronauts experience vision changes during and after long-duration space flight. The hypothesis is that spaceflight-induced headward/cephalad fluid shift alters the intraocular pressure (IOP) and the choroid, affecting visual acuity. Therefore, we evaluated the use of venoconstrictive thigh cuffs (VTC) as a potential countermeasure to cephalad fluid shift-induced effects on IOP and the choroid in a simulated microgravity environment.

Methods : In this IRB-approved prospective comparative study, 20 healthy participants were subjected to a tilt protocol for 10 min in each posture: seated, supine, 15° head-down tilt (HDT), and 15° HDT with bilateral VTC with 60 mm Hg (HDTC) as a countermeasure. IOP was measured using a pneumotonometer and EDI-OCT scans of the choroid were acquired, in each position. Various choroidal parameters at the sub-fovea including choroidal thickness (CT), choroidal area (CA), luminal area (LA) and choroidal vascularity index (CVI: the proportion of LA to CA) were computed. A mixed-effects linear regression model with repeated measures was used to test for significant IOP and choroidal changes.

Results : Results are expressed as mean ± standard error. IOP significantly (p = 0.004 to < 0.0001) increased when subjects moved from seated (16.7 ± 3.2 mm Hg) to supine (18.8 ± 2.5 mm Hg) and to HDT (20.3 ± 2.5 mm Hg), and subsequently decreased (p = 0.02) in HDTC (18.3 ± 2.5 mm Hg). CT significantly (p = 0.04 to < 0.0001) thickened from seated (286 ± 94.9 µm) to supine (307 ± 97.4 µm) and to HDT (314.9 ± 97.4 µm), and subsequently became thinner (p < 0.0001) with HDTC (283.4 ± 87.6 µm). Both CA and LA significantly (p < 0.001) increased in HDT (CA: 0.59 ± 0.05 mm²; LA: 0.38 ± 0.03 mm²) from seated (CA: 0.54 ± 0.05 mm²; LA: 0.35 ± 0.13 mm²) and supine (CA: 0.54 ± 0.05 mm²; LA: 0.36 ± 0.03 mm²) positions, followed by a significant (p < 0.001) decrease in HDTC (CA: 0.53 ± 0.20 mm²; LA: 0.35 ± 0.03 mm²). There were no significant differences in LA between seated, supine and HDTC. The CVI was not significantly altered between the postures.

Conclusions : HDTC with 60 mm Hg for 10 min mitigates the IOP and choroidal changes during HDT. We propose a model using VTC that could potentially be used to inhibit the cephalad fluid shift, and consequently ameliorate vision changes among astronauts during long-haul space missions.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.