Interpreting Space’s Message: The Impact on Our Eyes
by Nathaniel Briggs
The vast expanses of the universe have eluded human beings for quite some time. In the past thousand years, we have grown restless in our terrestrial enclosure and have finally decided to embark into the skyward seas of infinity. While aerospace engineers, astrophysicists and astronauts continue to valiantly push the boundaries into unknown territory, other scientists focus their research on how space travel affects the human body, most notably ocular health.
In 2011, NASA hosted the Visual Impairment Intracranial Pressure Summit Report inviting physicians, optometrists and ophthalmologists from around the country to discuss recent findings linking long-term space dwelling to degradation of visual activities, a phenomenon reported by up to sixty percent of NASA astronauts upon returning from various space missions.
The collaboration birthed several hypotheses as to why space travel harms eyesight. It also led to an extensive study the following year. Physicians studied 27 astronauts who spent an average of 108 days in space aboard NASA’s International Space Station or on one of NASA’s space shuttle missions. Magnetic Resonance Imaging (MRI) scans revealed that 33 percent of the astronauts had excess cerebrospinal fluid buildup, also called Visual Impairment Intracranial Pressure (VIIP), around their optic nerve. 22 percent developed flattening in the back of their eyes, and 15 percent had a bulging optic nerve.
The physicians and NASA personnel who attended the conference hypothesized that visual impairment is caused by the zero gravity environment of space which allows fluid normally held by gravity in the legs and feet to build up in the brain. The human body, adapted to the gravity-felt Earth, cannot correct the problem and, over time, excess pressure causes the retina to flatten and the optic nerve to bulge.
There is still much to be understood. In a statement, NASA explained that they still “do not know the etiological mechanisms and contributing risk factors for ocular structural and functional changes seen in-flight and post flight.” At this point, the belief that VIIP is a contributing factor to ocular problems is only a hypothesis, and more research is necessary to validate it.
NASA believes that there are other contributing factors that account for visual impairment in returning astronaut, and increased sodium intake is one of them. Astronauts aboard the International Space Station ingest a daily average of four to five grams of sodium, double what the Mayo Clinic recommends for a healthy diet. Past studies have linked increased sodium retention to elevated intracranial pressure and swelling of other body tissues, a condition called peripheral edema. However, the reliability of these explanations is shaky at best as these studies were carried out on Earth with only female subjects. Replicating the environment of space poses a huge barrier to research on this issue.
Another suggested explanation is elevated levels of homocysteine, a common amino acid in the blood. Homocysteine is involved in the One-Carbon Pathway, one of the chemical processes that make DNA in our bodies. NASA scientists believe that microgravity environments especially affect this pathway. Elevated levels of carbon, which are common in the International Space Station, may also cause vascular permeability, which consequently leads to fluid build-up in the brain.
Yet another proposed factor is the amount of exercise, which has been an important countermeasure in maintaining bone and cardiac health during spaceflight. However, its impact on the VIIP hypothesis is yet to be examined.
Although there have been promising leads into the issue of ocular health in space, a solution is still years away. NASA has teamed with the Russian Federal Space Agency to develop appropriate preventative care and treatment to combat visual impairment. For now, however, ocular health remains a major barrier to space exploration and to the somewhat fantastic but not too unrealistic future of living in space.