Abstract
Neuro-ophthalmological changes named spaceflight associated neuro-ocular syndrome (SANS) reported after spaceflights are important medical issues. Dry immersion (DI), an analog to microgravity, rapidly induces a centralization of body fluids, immobilization, and hypokinesia similar to that observed during spaceflight. The main objectives of the present study were 2-fold: (1) to assess the neuro-ophthalmological impact during 5 days of DI and (2) to determine the effects of venoconstrictive thigh cuffs (VTC), used as a countermeasure to limit headward fluid shift, on DI-induced ophthalmological adaptations. Eighteen healthy male subjects underwent 5 days of DI with or without VTC countermeasures. The subjects were randomly assigned into two groups of 9: a control and cuffs group. Retinal and optic nerve thickness were assessed with spectral-domain optical coherence tomography (OCT). Optic nerve sheath diameter (ONSD) was measured by ocular ultrasonography and used to assess indirect changes in intracranial pressure (ICP). Intraocular pressure (IOP) was assessed by applanation tonometry. A higher thickness of the retinal nerve fiber layer (RNFL) in the temporal quadrant was observed after DI. ONSD increased significantly during DI and remained higher during the recovery phase. IOP did not significantly change during and after DI. VTC tended to limit the ONSD enlargement but not the higher thickness of an RNFL induced by DI. These findings suggest that 5 days of DI induced significant ophthalmological changes. VTC were found to dampen the ONSD enlargement induced by DI.
Highlights
During the last decade, some studies reported ophthalmic abnormalities in astronauts who spent several months onboard the International Space Station (Lee et al, 2016)
One subject was removed from optical coherence tomography (OCT) data analysis, and 2 other subjects were removed from Optic nerve head (ONH) analysis due to low-quality index
This study shows that 5 days of Dry immersion (DI) induces ophthalmological changes, such as a slight but significant increase in retinal nerve fiber layer thickness (RNFLT) in the temporal quadrant and an enlargement in optic nerve sheath diameter (ONSD)
Summary
Some studies reported ophthalmic abnormalities in astronauts who spent several months onboard the International Space Station (Lee et al, 2016). Recent studies using optical coherence tomography (OCT) have shown some level of optic disc edema in most astronauts when comparing pre-flight and in-flight OCT, suggesting that subclinical SANS involvement may occur in the majority of astronauts (Lee et al, 2020). Some of these changes are similar to those observed in idiopathic intracranial hypertension (Nelson et al, 2014). One previous hypothesis suggested localized events occurring at the level of the intra-orbital optic nerve with the possible implication of elevated intracranial pressure (ICP) (Roberts et al, 2017; Zhang and Hargens, 2018; Mader et al, 2019)
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