Visual impairment due to intracranial pressure

Spaceflight-associated neuro-ocular syndrome (SANS), previously known as Spaceflight-induced visual impairment, is hypothesized to be a result of increased intracranial pressure (ICP). The study of visual changes and ICP in astronauts on long-duration flights is a relatively recent topic of interest to space medicine professionals. Although reported signs and symptoms have not appeared to be severe enough to cause blindness in the near term, long term consequences of chronically elevated intracranial pressure are unknown. NASA has reported that fifteen long-duration male astronauts (45–55 years of age) have experienced confirmed visual and anatomical changes during or after long-duration flights. Optic disc edema, globe flattening, choroidal folds, hyperopic shifts and an increased intracranial pressure have been documented in these astronauts. Some individuals experienced transient changes post-flight while others have reported persistent changes with varying degrees of severity. Although the exact cause is not known, it is suspected that microgravity-induced cephalad fluid shift and comparable physiological changes play a significant role in these changes. Other contributing factors may include pockets of increased carbon dioxide (CO2) and an increase in sodium intake. It seems unlikely that resistive or aerobic exercise are contributing factors, but they may be potential countermeasures to reduce intraocular pressure (IOP) or ICP in-flight. Although a definitive cause (or set of causes) for the symptoms outlined in the Existing Long-Duration Flight Occurrences section is unknown, it is thought that venous congestion in the brain brought about by cephalad fluid shifts may be a unifying pathologic mechanism. Additionally, a recent study reports changes in cerebrospinal fluid (CSF) hydrodynamics and increased diffusivity around the optic nerve under simulated microgravity conditions which may contribute to ocular changes in spaceflight.