Ocular Changes Over 60 Minutes in Supine and Prone Postures

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-Perkins tonometer), ocular geometry (axial length, corneal thickness, and aqueous depth-noncontact biometer), and the choroidal (volume and subfoveal thickness optical coherence tomography) was measured in 10 subjects (5M, 5F). Measures were taken over the course of 60 minutes and analyzed with repeated measures ANCOVA to assess the effects of posture and time. In the supine position, choroidal volume increased significantly with time (average value at < 5 minutes = 8.8±2.3 mm³, 60 minutes = 9.0±2.4 mm³, p = 0.03). In the prone position, IOP and axial length increased with time (IOP at < 5 minutes 15±2.7 mmHg, 60 minutes = 19.8±4.1 mmHg, p < 0.0001; Axial length at < 5 minutes = 24.29±0.77 mm, 60 minutes = 24.31±0.76 mm, p = 0.002). Each increased exponentially, with time constants of 5.3 minutes and 14 minutes respectively. Prone corneal thickness also increased with time (< 5 minutes = 528±35 μm, 60 minutes = 537±35 μm ³, p < 0.001). Aqueous depth was shortened in the prone position (baseline = 3.22±0.31 mm, 60 minutes = 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.

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