Acute effects of changes to the gravitational vector on the eye

Abstract

Intraocular pressure (IOP) initially increases when an individual enters microgravity compared with baseline values when an individual is in a seated position. This has been attributed to a headward fluid shift that increases venous pressures in the head. The change in IOP exceeds changes measured immediately after moving from seated to supine postures on Earth, when a similar fluid shift is produced. Furthermore, central venous and cerebrospinal fluid pressures are at or below supine position levels when measured initially upon entering microgravity, unlike when moving from seated to supine postures on Earth, when these pressures increase. To investigate the effects of altering gravitational forces on the eye, we made ocular measurements on 24 subjects (13 men, 11 women) in the seated, supine, and prone positions in the laboratory, and upon entering microgravity during parabolic flight. IOP in microgravity (16.3 ± 2.7 mmHg) was significantly elevated above values in the seated (11.5 ± 2.0 mmHg) and supine (13.7 ± 3.0 mmHg) positions, and was significantly less than pressure in the prone position (20.3 ± 2.6 mmHg). In all measurements,P< 0.001. Choroidal area was significantly increased in subjects in a microgravity environment (P< 0.007) compared with values from subjects in seated (increase of 0.09 ± 0.1 mm(2)) and supine (increase of 0.06 ± 0.09 mm(2)) positions. IOP results are consistent with the hypothesis that hydrostatic gradients affect IOP, and may explain how IOP can increase beyond supine values in microgravity when central venous and intracranial pressure do not. Understanding gravitational effects on the eye may help develop hypotheses for how microgravity-induced visual changes develop.

Internal jugular pressure increases during parabolic flight

“…The internal jugular veins are a major drainage pathway between the brain and the heart, although the percentage of blood draining through the internal jugular vein in relation to the vertebral veins is position dependent (Manuel Valdueza et al 2000;Cirovic et al 2003). Along with an increased IJVP and venous distension, intraocular pressure also is elevated with parabolic flight (Mader et al 1993;Anderson et al 2016). While this study only reports a finding during acute exposure to weightlessness, increased IJVP during long-duration spaceflight may promote cranial venous and lymphatic congestion chronically.…”

“…; Anderson et al. ). While this study only reports a finding during acute exposure to weightlessness, increased IJVP during long‐duration spaceflight may promote cranial venous and lymphatic congestion chronically.…”

Effect of gravity and microgravity on intracranial pressure

“…Interestingly, IOP is elevated in 0 g relative to the upright posture on Earth (Anderson et al . ), so the effect of microgravity on the pressure gradient at the posterior aspect of the eye (translaminar pressure) may be smaller than expected, yet over prolonged periods of time may be sufficient to cause optic remodelling.…”

“…We speculate here that the absence of diurnal, postural reductions in ICP relative to IOP (Anderson et al . ; Petersen et al . 2015; Eklund et al .…”

The aetiology of spaceflight‐associated neuro‐ocular syndrome might be explained by a neural mechanism regulating intraocular pressure

“…2020) and those of Anderson et al . (2016), we hypothesise that the supine 6° head down tilt (6°HDT) model may not be an appropriate simulation of the microgravity‐induced effects on the choroidal circulation. In contrast to the supine position, the prone 6°HDT position is most likely a better model, as it elevates IOP substantially above the glaucoma threshold.…”