Neuro-ophthalmolmic Sequelae of Sustained Microgravity

Abstract

Space travel has presented novel challenges both technologically and physiologically to the human passenger. Since the first manned spaceflight in 1961, the impact of atmospheric changes and a microgravity environment on the human body have been carefully studied and characterized. The Jules Verne novel “From the Earth to the Moon” published in 1865, nearly 100 years before the first manned mission, raised questions and theories about the weightlessness which would be experienced in space. As terrestrial beings who have evolved in earth’s gravity, the environment of weightlessness induces remarkable multisystem changes which affect the astronauts who venture into space. These changes cover a wide spectrum ranging from transient sensory imbalances and nausea to profound hemodynamic and metabolic alterations, to more chronic anemia, weakening of bones, muscles, and the immune system. Additionally, dramatic and unique ophthalmologic changes are experienced in varying degrees after time spent in a microgravity environment. Findings have included optic disk edema, choroidal folds and cotton wool spots, hyperopic refractive shifts after prolonged spaceflight, and globe flattening with increased CSF signal in the optic nerve sheaths. The retrospective observational report by Mader et al. in 2011 described these findings in a cohort of seven astronauts who had visited the International Space Station (ISS) for a period of 6 months, as well as reviewed and correlated the results of 300 postflight surveys regarding in-flight vision changes. The US National Aeronautics and Space Administration’s (NASA) Space Medicine Division has also evaluated and documented these changes through clinical exam, OCT, MRI, and ultrasound testing of astronauts. As will be discussed, many features of this phenomenon are ophthalmoscopically similar to terrestrial increased intracranial pressure (ICP), and elevated ICP is thought to play at least a partial role. In 2011, NASA held a visual impairment intracranial pressure (VIIP) summit to discuss this problem, and this conference raised significant concerns about longer missions or even visitation to planets with greatly reduced gravity, such as Mars. The pathogenesis of these findings is incompletely understood due to limitations of monitoring and testing during spaceflight. The purpose of this discussion is to review the pathology and discuss possible mechanisms, most importantly the phenomenon of cephalad fluid shifts in a microgravity environment, and to consider possible approaches to investigating adaptive responses. Further research in this area presents practical challenges, such as finding a suitable ground-based research model and/or monitoring ICP during spaceflight, but is necessary to both characterize the physiology as well as to ultimately provide countermeasures for future long-duration space missions.