Spaceflight-associated neuro-ocular syndrome (SANS), previously known as Spaceflight-induced visual impairment, is hypothesized to be a result of increased intracranial pressure (ICP), although, experiments directly measuring ICP in parabolic flight have shown ICP to be in normal physiological ranges during acute weightless exposure. 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.
Causes and current studies
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. As part of the effort to elucidate the cause(s), NASA has initiated an enhanced occupational monitoring program for all mission astronauts with special attention to signs and symptoms related to ICP.
Similar findings have been reported among Russian cosmonauts who flew long-duration missions on Mir. The findings were published by Mayasnikov and Stepanova in 2008.
Animal research from the Russian Bion-M1 mission indicates duress of the cerebral arteries may induce reduced blood flow, thereby contributing to impaired vision.
On 2 November 2017, scientists reported that significant changes in the position and structure of the brain have been found in astronauts who have taken trips in space, based on magnetic resonance imaging (MRI) studies. Astronauts who took longer space trips were associated with greater brain changes.
CO2
Carbon dioxide (CO2) is a natural product of metabolism. People typically exhale around 200mL of CO2 per minute at rest and over 4.0 L at peak exercise levels. In a closed environment, CO2 levels can quickly rise and can be expected to a certain degree in an environment such as the International Space Station (ISS). Nominal CO2 concentrations on Earth are approximately 0.23 mmHg while nominal CO2 levels aboard the ISS are up to 20 times that at 2.3 to 5.3 mmHg. Those astronauts who experienced VIIP symptoms were not exposed to CO2 levels in excess of 5 mmHg.
Ventilation and heart rate increase as CO2 rise. Hypercapnia also stimulates vasodilation of cerebral blood vessels, increased cerebral blood flow and elevated ICP presumably leading to headache, visual disturbance and other central nervous system (CNS) symptoms. CO2 is a known potent vasodilator and an increase in cerebral perfusion pressure will increase the CSF production by about 4%.
Since air movement is reduced in microgravity, local pockets of increased CO2 concentrations may form. Without proper ventilation, CO2 concentrations ppCO2 could rise above 9mmHg within 10 minutes around a sleeping astronaut's mouth and chin. More data is needed to fully understand the individual and environmental factors that contribute to CO2-related symptoms in microgravity.
