Ocular changes over 60 min in supine and prone postures

Abstract

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.

Characterization of mouse ocular response to a 35-day spaceflight mission: Evidence of blood-retinal barrier disruption and ocular adaptations

“…Studies have shown that changes in the gravitational environment can have pronounced transient and sustained effects on the eye 24 . Microgravity-induced vision changes may result from the interaction of multiple physiological factors, including changes in IOP, fluid shifts, ocular geometry and cardiovascular alterations 25–27 . Similarly, increases in IOP have been documented in ground-based models of spaceflight, such as head-down tilt 28 .…”

Long‐duration spaceflight alters estimated intracranial pressure and cerebral blood velocity

“…lack of normal circadian variation in ICP (Lawley et al 2017), or upward brain shift (Shinojima et al 2018) other than pathologically elevated ICP. It is also possible that there are other features of long-duration spaceflight that are more specific to hydrostatic gradients within eye itself that could be responsible, independently of ICP as hypothesized by Buckey et al (Anderson et al 2017). It should be emphasized that both astronauts with SANS signs showed increases in mean MCAv and heart rate and decreases in haemoglobin concentration after spaceflight, similar to the changes of the group averages of the 11 astronauts.…”

“…(Anderson et al . 2017). It should be emphasized that both astronauts with SANS signs showed increases in mean MCAv and heart rate and decreases in haemoglobin concentration after spaceflight, similar to the changes of the group averages of the 11 astronauts.…”

Intraocular pressure and cardiovascular alterations investigated in artificial gravity as a countermeasure to spaceflight associated neuro-ocular syndrome

“…On the testing day, subjects experienced each of the four experimental conditions in a randomized order: 1) standing, 2) supine, 3) AGEC, and 4) AG2G. In each condition, participants first acclimatized for 10 min in the posture before taking measurements, to allow the cardiovascular system and IOP to equilibrate (1). Subjects remained stationary, feet shoulder width apart, hands at their side.…”

“…These short-term effects may not be indicative of ocular status once the eye has reached steady state under AG exposure. A study by Anderson et al (1) found IOP has a time constant of 5.3 min, reaching steady-state pressure in~10 min when going from the seated to the prone position. Furthermore, the Chung study used long-radius centrifugation, whereas current investigations for AG to mitigate SANS have been proposed as short-radius centrifugation, where head-to-toe hydrostatic gradients are larger (14,21).…”