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 mm³, 60 min = 9.0 ± 2.4 mm³, P = 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, P < 0.0001; axial length at <5 min = 24.29 ± 0.77 mm, 60 min = 24.31 ± 0.76 mm, P = 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 μm³, P < 0.001). Aqueous depth was shortened in the prone position (baseline = 3.22 ± 0.31 mm, 60 min = 3.18 ± 0.32 mm, P < 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.NEW & NOTEWORTHY 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.

Keywords: choroidal volume; intraocular pressure; ocular geometry; visual impairment and intracranial pressure.

Fig. 1.

The geometry of the ocular globe was measured with a laser optical biometer to determine axial length (distance from A to E), lens thickness (B to C), aqueous depth (C to D), and cornea thickness (D to E). [Eye image credit Montfolio, used with permission.]

Fig. 2.

Choroidal volume scan using optical coherence tomography. A total of 25 single line scans were taken centered on the fovea (top). Each line scan was manually segmented to determine the top and bottom of the choroid layer (middle). The choroidal volume calculation was restricted to a 6-mm diameter circular region centered on the fovea (bottom).

Fig. 3.

The change of intraocular pressure (IOP), aqueous depth (AD), axial length (AL), corneal thickness (CO), choroidal volume (CHV), and subfoveal choroid thickness (SFCT) from seated baseline over an hour in the supine and prone postures. The mean and standard deviation of the difference from baseline are shown. *Measure changed significantly with time in the posture. †Measurements in the postures were significantly different in supine and prone positions. A repeated-measures analysis of covariance with baseline as the covariate was used to assess changes (n = 10 subjects for all tests). [Eye image credit Montfolio, used with permission.]