Eye-brain axis in microgravity and its implications for Spaceflight Associated Neuro-ocular Syndrome

Abstract

Long-duration human spaceflight can lead to changes in both the eye and the brain, which have been referred to as Spaceflight Associated Neuro-ocular Syndrome (SANS). These changes may manifest as a constellation of symptoms, which can include optic disc edema, optic nerve sheath distension, choroidal folds, globe flattening, hyperopic shift, and cotton wool spots. Although the underpinning mechanisms for SANS are not yet known, contributors may include intracranial interstitial fluid accumulation following microgravity induced headward fluid shift. Development and validation of SANS countermeasures contribute to our understanding of etiology and accelerate new technology including exercise modalities, Lower Body Negative Pressure suits, venous thigh cuffs, and Impedance Threshold Devices. However, significant knowledge gaps remain including biomarkers, a full set of countermeasures and/or treatment regimes, and finally reliable ground based analogs to accelerate the research. This review from the European Space Agency SANS expert group summarizes past research and current knowledge on SANS, potential countermeasures, and key knowledge gaps, to further our understanding, prevention, and treatment of SANS both during human spaceflight and future extraterrestrial surface exploration.

Fig. 1. Eyeand brain examinations and possible countermeasures in space and analogs contributing to the understanding of SANS.

It is crucial to adopt a comprehensive perspective that integrates eye and brain observations and analysis when approaching SANS. Optical coherence tomography (OCT) and magnetic resonance imaging (MRI) are performed together with tonometry to measure intraocular pressure (IOP), the main working horses in detecting eye and brain changes. A –6° head-down tilt bedrest and dry immersion are excellent analogs to simulate the effects of microgravity on the human body. Artificial gravity and lower body negative pressure (LBNP) are both investigated aspotential countermeasures to mitigate the effects of fluid shift and increased cerebral blood flow. Credit: Figure by Johanne AG Petersen, images modified from ESA- and DLR-web portal.

Fig. 2

Optic Nerve Sheath Diameter measurement by ultrasound in Dry Immersion (Credit: MEDES).