Spaceflight Associated Neuro-Ocular Syndrome in astronauts—the ICP hypothesis

Space exploration is an extraordinary endeavour with the promise of fulfilling the innate human drive to explore and understand a limitless environment. Human anatomy that has evolved over many millennia to seamlessly acclimate to the terrestrial world of Earth is exposed to a new environment, one that it has not had a chance to adapt to on an evolutionary basis. Spaceflight Associated Neuro-Ocular Syndrome has surfaced as a consequence of long duration spaceflight in a number of Astronauts. Changes in the ocular globe, retina, optic nerve, and brain occur as a result of long duration spaceflight and produce short term and long term potentially deleterious effects. Papilledema, cotton wool spots and changes in the optic nerve and globe are well documented, as well as changes in intracranial pressure documented with a lumbar puncture in affected astronauts that persists for a number of days after return to earth. Elevated intracranial pressure that occurs as a result of an inability to return venous blood to the heart in a microgravity or zero gravity environment is one very plausible explanation for the changes that occur and can explain a number of them. A challenging consequence of space exploration on physiology is based on the human body’s fluid adaptation to a 1G environment, as exposure to microgravity predisposes astronauts to cephalad fluid shifts that result in microvascular dysregulation and can produce a number of ocular and central nervous system changes that can persist even after return to Earth. Here we discuss the leading hypotheses describing Space-Induced Intracranial Hypertension (SIIH) and the associated phenomenon, Spaceflight Associated Neuro-ocular Syndrome (SANS). In this chapter we present, analyze, and discuss a number of studies that model SANS within the confines of Earth gravity, and potential mechanisms that likely are ongoing that produce the phenomenon. The goal of presenting this work is to gain a better understanding of the pathophysiology in order to equip astronauts to adapt to microgravity, thereby alleviating these deleterious consequences of spaceflight. Attaining a better understanding is the cause of the syndrome is essential to guide treatment and prophylaxis. It will be critical as space flight expands to months and even years, and humankind reaches out to other planets, solar systems, and perhaps one day, even galaxies.