Some astronauts are returning from long-duration spaceflight with structural ocular and visual changes. We investigated both the transient and sustained effects of changes in the direction of the gravity vector acting on the eye using changes in body posture. Intraocular pressure (IOP; measured by Perkins tonometer), ocular geometry (axial length, corneal thickness, and aqueous depth-noncontact biometer), and the choroid (volume and subfoveal thickness optical coherence tomography) were measured in 10 subjects (5 males and 5 females). Measures were taken over the course of 60 min and analyzed with repeated-measures analysis of covariance to assess the effects of posture and time. In the supine position, choroidal volume increased significantly with time (average value at <5 min = 8.8 ± 2.3 mm, 60 min = 9.0 ± 2.4 mm, = 0.03). In the prone position, IOP and axial length increased with time (IOP at<5 min 15 ± 2.7 mmHg, 60 min = 19.8 ± 4.1 mmHg, < 0.0001; axial length at <5 min = 24.29 ± 0.77 mm, 60 min = 24.31 ± 0.76 mm, = 0.002). Each increased exponentially, with time constants of 5.3 and 14 min, respectively. Prone corneal thickness also increased with time (<5 min = 528 ± 35 μm, 60 min = 537 ± 35 μm, < 0.001). Aqueous depth was shortened in the prone position (baseline = 3.22 ± 0.31 mm, 60 min = 3.18 ± 0.32 mm, < 0.0001) but did not change with time. The data show that changes in the gravity vector have pronounced transient and sustained effects on the geometry and physiology of the eye. We show that gravity has pronounced transient and sustained effects on the eye by making detailed ocular measurements over 60 min in the supine and prone postures. These data inform our understanding of how gravitational forces can affect ocular structures, which is essential for hypothesizing how ocular changes could occur with microgravity exposure.

Astronauts have presented with a constellation of visual changes referred to as spaceflight-associated neuro-ocular syndrome (SANS). However, early markers of microgravity-induced optic remodeling have not been fully identified nor have countermeasures been developed. In order to identify early markers of SANS, we studied 10 subjects with optical coherence tomography and ultrasound when upright and supine, and again after 24 hours of 6-degree head down tilt (HDT) bedrest. Upon acute transition from upright to supine, choroid area (2.24±0.53 to 2.28±0.52 mm2, p=0.001) and volume (9.51±2.08 to 9.73±2.08 mm3, p=0.002) increased. After 24 hours of HDT bedrest, subfoveal choroidal thickness (372±93 to 381±95 µm, p=0.02) , choroid area (2.25±0.52 to 2.33±0.54 mm2, p=0.08) and volume (9.64±2.03 to 9.82±2.08 mm3, p=0.08) increased relative to the supine position. Subsequently, 7 subjects spent 3 days in -6 deg HDT bedrest to assess whether low-level lower body negative pressure (LBNP) could prevent the observed choroidal engorgement during bedrest. Maintaining the -6 deg HDT position for 3 days caused choroid area (Δ0.11 mm2, p=0.05) and volume (Δ0.45 mm3, p=0.003) to increase. When participants also spent 8 hours daily under -20mmHg LBNP, choroid volume still increased, but substantially (40%) less than in the control trial (Δ0.27 mm3, p=0.05). Moreover, the increase in choroid area was diminished (Δ0.03 mm2, p=0.13), indicating that low-level LBNP attenuates the choroid expansion associated with 3 days of -6 deg HDT bedrest. These data suggest that low-level LBNP may be an effective countermeasure for SANS.

IMPORTANCE Astronauts returning from long-duration spaceflight experience ocular remodeling related to cephalad fluid shifts induced by microgravity. It is hypothesized that the absence of diurnal reductions in intracranial pressure in microgravity creates a low but persistent pressure gradient at the posterior aspect of the eye, which results in ocular remodeling and space-associated neuro-ocular syndrome (SANS) over many months.OBJECTIVE To determine whether partial reintroduction of footward fluid shifts during simulated microgravity via lower body negative pressure (LBNP) during sleep attenuates choroid engorgement, an early marker of ocular remodeling related to SANS. DESIGN, SETTING, AND PARTICIPANTSBetween May 2019 and February 2020, participants with no major cardiovascular, kidney, or ophthalmic disease completed 3 days of supine (0°) bed rest with and 3 days without 8 hours of nightly LBNP in a randomized, crossover design. This single-center investigation took place at the UT Southwestern Medical Center. All analyses were conducted blinded to condition and time point. INTERVENTIONS Eight hours of nightly LBNP (−20 mm Hg) vs no LBNP.MAIN OUTCOMES AND MEASURES The primary outcome was the change in choroid area and volume after 3 days of bed rest measured by optical coherence tomography. RESULTSOf 10 participants, 5 were female, the mean (SD) age was 29 (9) years, and the age range was 18 to 55 years. Central venous pressure increased from the seated to supine position (mean [SD], seated: −2.3 [2.0] vs supine: 6.9 [2.0] mm Hg; P < .001), leading to choroid engorgement over 3 days of bed rest (Δ area: +0.09 mm 2 [95% CI, 0.04-0.13]; P = .001; Δ volume: +0.37 mm 3 [95% CI, 0.19-0.55]; P = .001). Nightly LBNP caused 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 attenuated the increase in choroid area (74%) (Δ: 0.02 mm 2 [95% −0.02 to 0.06]; P = .01) and volume (53%) (Δ: 0.17 mm 3 [95% CI, 0.01-0.34]; P = .05) compared with control.CONCLUSIONS AND RELEVANCE Nightly LBNP reinstated a footward fluid shift and mitigated the increase in choroid area and volume. LBNP during sleep may be an effective countermeasure for ocular remodeling and SANS during long-duration space missions.

We developed an elective course titled Medicine in Extreme Environments (MEE) at the University of Texas Southwestern Medical Center for firstand second-year medical students. This course covered physiology, research, clinical practice, and career guidance regarding the fields of wilderness, space, hyperbaric, combat, and exercise medicine. The primary aim was to generate interest in and awareness of these seldom covered fields of medicine by exposing medical students to these disciplines during their preclinical years. A postcourse questionnaire was implemented to investigate whether the MEE course increased awareness of, interest in, and knowledge in the fields of medicine included in the curriculum. Through 2 iterations of the class, a total of 67 students enrolled in the course, and 38 students completed the questionnaire. After course completion, 95% felt they better understood the work and lifestyle of the fields covered, 100% learned more about concepts of each field, and 74% agreed that the elective influenced the direction of their future careers to include some part of the fields emphasized. Although only a limited number of students enrolled in this course, these initial findings suggest that the MEE curriculum may have some utility in promoting awareness of and interest in these medical disciplines among students who attend the course. With continued student and faculty support, this course will likely be continued annually at our institution. We believe that certain aspects of this course may be useful in helping develop similar courses at other medical schools.