Ophthalmic changes and increased intracranial pressure associated with long duration spaceflight: An emerging understanding

For many years, there have been anecdotal reports of vision changes by astronauts following short and long-duration spaceflight. Much of this was attributed to hyperopic shifts related to the age of the flying population. However, it has recently been recognized that vision changes are actually quite common in astronauts and are associated with a constellation of findings including elevated intracranial pressure, optic disc edema, globe flattening, optic nerve sheath thickening, hyperopic shifts and retinal changes. With advanced imaging modalities available on the ground along with the fidelity of in-flight diagnostic capabilities previously unavailable, information on this newly recognized syndrome is accumulating. As of this writing, 11 cases of visual impairment experienced by astronauts during missions on-board the International Space Station (ISS) have been documented and studied. Although the exact mechanisms of the vision changes are unknown, it is hypothesized that increased intracranial pressure (ICP) is a contributing factor.Microgravity is the dominant cause of many physiological changes during spaceflight and is thought to contribute significantly to the observed ophthalmic changes. However, several secondary factors that could contribute to increased ICP and vision changes in spaceflight have been proposed. Possible contributors include microgravity-induced cephalad fluid shift, venous obstruction due to microgravity-induced anatomical shifts, high levels of spacecraft cabin carbon dioxide, heavy resistive exercise, and high sodium diet. Individual susceptibility to visual impairment is not fully understood, though a demographic of affected astronauts is emerging.This paper describes the current understanding of this newly recognized syndrome, presents data from 11 individual cases, and discusses details of potential contributing factors. The occurrence of visual changes in long duration missions in microgravity is one of the most significant clinical issues to date for the human spaceflight community, and a comprehensive understanding of the issue at whole is critical to ensure safe space exploration in the future.