The effect of microgravity exposure in space puts the eyes of astronauts under pressure, which can result in spaceflight-associated neuro-ocular syndrome (SANS), a progressive flattening of the back of the eyeball, swelling of the optic nerve, development of retinal and choroidal folds and vision impairment. Astronauts suffering from SANS became more farsighted, making it difficult for them to read and conduct experiments. The longer the duration in space, the more affected the eyes of astronauts. The effects can be long term and even result in permanent changes. Seventy percent of astronauts on long duration space missions experienced some degree of swelling in the back of the eye, according to NASA.
Microgravity fluid shifts redistribute venous blood toward the head and ocular orbit and cerebrospinal fluid to the brain. Our bodies need gravity to keep blood and cerebrospinal fluid from shifting toward the head, eye and brain. On earth the pressure on our brain increases when we lie down but is relieved when we stand. Standing allows gravity to pull the body fluids down, a process known as unloading. Due to the lack of gravitational pull experienced in space, astronauts in effect never stand up so never unload to relieve the pressure on the brain. The role of intracranial pressure (ICP) and translaminar pressure differences between ICP and intraocular pressure (IOP) in the development of SANS is under investigation.
Lawley et al. suggests that microgravity prevents the normal lowering of intracranial pressure (ICP) when in an upright posture. His findings suggest that on earth, the human brain is protected by the daily circadian cycles in regional ICPs. The absence of diurnal, postural reductions in ICP compared to IOP in microgravity creates a persistently lower pressure gradient at the posterior aspect of the eye that may result in optic remodeling. (Lawley, J. S. et al. Effect of gravity and microgravity on intracranial pressure. J. Physiol. 595, 2115–2127 2017)
With longer spaceflights planned such as a return to the moon and a mission to Mars, astronauts’ bodies will experience microgravity conditions for up to 2.5 years. Vision impairment is a mission-critical problem, and since the problem is the lack of gravitational pull countermeasures are under research to introduce gravity to the bodies of astronauts on long duration space missions. One proposed countermeasure under development is a special sleeping bag. The sleeping bag will use vacuum suction to create a pressure difference drawing fluids toward the feet, relieving the buildup in the brain that puts pressure on the eyeball.
As we celebrate our adventurous astronaut heroes and marvel at their courage, we must also acknowledge that they do so at risk to their body, brain and sight. Their bodies are living laboratories whose study will help scientists develop countermeasures to make space travel safer for humans.
• Deborah Kotob
Pro to Pro Director