Impacts of Simulated Weightlessness by Dry Immersion on Optic Nerve Sheath Diameter and Cerebral Autoregulation.

Marc Kermorgant,Dina N Arvanitis,Marc-Antoine Custaud,Anne Pavy-Le Traon,Nathalie Nasr,Marek Czosnyka,Jean-Michel Senard,Florian Leca,Thomas Geeraerts

doi:10.3389/fphys.2017.00780

Abstract

Dry immersion (DI) is used to simulate weightlessness. We investigated in healthy volunteers if DI induces changes in ONSD, as a surrogate marker of intracranial pressure (ICP) and how these changes could affect cerebral autoregulation (CA). Changes in ICP were indirectly measured by changes in optic nerve sheath diameter (ONSD). 12 healthy male volunteers underwent 3 days of DI. ONSD was indirectly assessed by ocular ultrasonography. Cerebral blood flow velocity (CBFV) of the middle cerebral artery was gauged using transcranial Doppler ultrasonography. CA was evaluated by two methods: (1) transfer function analysis was calculated to determine the relationship between mean CBFV and mean arterial blood pressure (ABP) and (2) correlation index Mxa between mean CBFV and mean ABP.ONSD increased significantly during the first day, the third day and the first day of recovery of DI (P < 0.001).DI induced a reduction in Mxa index (P < 0.001) and an elevation in phase shift in low frequency bandwidth (P < 0.05). After DI, Mxa and coherence were strongly correlated with ONSD (P < 0.05) but not before DI. These results indicate that 3 days of DI induces significant changes in ONSD most likely reflecting an increase in ICP. CA was improved but also negatively correlated with ONSD suggesting that a persistent elevation ICP favors poor CA recovery after simulated microgravity.

Highlights

Due to the difficulties to perform in-flight experiments in addition to restricted opportunities of spaceflight, models are used on Earth to simulate the effects of microgravity
We noted no significant changes either in systolic blood pressure (SBP) or diastolic blood pressure (DBP) whereas heart rate (HR) increased after Dry immersion (DI) (P < 0.001)
Our results indicate that DI: (1) could induce an optic nerve sheath diameter (ONSD) enlargement similar to those observed in intracranial hypertension, (2) DI could improve cerebral autoregulation (CA), and (3) a poorer recovery of ONSD would be related to a less improvement in CA in supine position

Summary

Introduction

Due to the difficulties to perform in-flight experiments in addition to restricted opportunities of spaceflight, models are used on Earth to simulate the effects of microgravity. Exposure to real or simulated microgravity induces a redistribution of body fluids toward the upper part of the body (Charles and Lathers, 1991) This cranial redistribution observed in astronauts after exposure to microgravity is likely responsible for the elevated intracranial pressure (ICP) reported after longduration flights (Nelson et al, 2014). Visual impairment has been reported in some astronauts exposed to long duration spaceflights Some changes such an increase in ONSD, posterior globe flattening and optic nerve protrusion suggest a potential intracranial hypertension (Kramer et al, 2012), resulting in the Vision Impairment and Intracranial Pressure syndrome (Nelson et al, 2014).studies are currently performed to examine and understand the effect of exposure to microgravity on ICP. A study performed on astronauts in a 1 and 2-week spaceflight showed an improved CA with a decreased low frequency (LF) gain (Iwasaki et al, 2007)

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