What is the Spaceflight-Associated Neuro-ocular Syndrome (SANS)?

Here on Earth, if you google ‘SANS’, the first link that comes up is SANS.org, an organization dedicated to information security training and certification.

But if you talk to medical professionals in the aerospace industry, SANS takes on a completely different meaning…

Here is how it all started…

Back in April 2005, John Lynch Phillips, NASA astronaut, along with his fellow crew-members Sergei Krikalev and Roberto Vittori, departed to the International Space Station (ISS) aboard Soyuz TMA-6 for what would be Expedition 11 to the ISS.

About halfway through the mission, Philips started noticing a worrisome deterioration in his distance vision. When he looked out the window towards Earth, instead of seeing the sharp, beautiful marble-like image of our planet, his vision was fuzzy and out of focus.

Philips believed that the condition would go away after his return to Earth, but his post-flight physical exam painted a different picture. His vision had gone from 20/20 to 20/100 in just six months.

Multiple MRIs, retinal scans, neurological tests and a spinal tap later, the tests showed that not only had his vision changed, but his eyes had suffered anatomical changes too.  The optic nerves were inflamed and the backs of his eyes had gotten flatter and had developed choroidal folds – parallel grooves on the interior surface of the eye opposite the lens.

Philips’ vision never returned to 20/20, and his case became the first widely recognized of Spaceflight-Associated Neuro-ocular Syndrome (SANS), prompting NASA to initiate SANS-related testing on astronauts in 2008, starting with Expedition 18 to the ISS.

Since then, the pre-flight, in-flight and post-flight eye exams for astronauts have become increasingly more sophisticated, as researchers have sought to identify the cause or causes of an array of visual impairments reported by a large number of astronauts – close to 30 percent of the American space shuttle crewmembers and 60 percent of the ISS astronauts.  Efforts to collect medical data on astronauts before, during and after the return from spaceflight has resulted in a large body of information on the changes in eye structure and functions that develop, allowing researches to propose and test hypotheses regarding the root causes for such changes.

So – what is the Space-Associated Neuro-ocular Syndrome (SANS)?

SANS is a constellation of vision health changes noted in a subset of the astronauts and space crew engaged on long-duration (i.e., greater than two weeks) space missions. NASA’s observations and studies conducted on specific astronauts suggest that living in a micro-gravity environment induces ocular changes, including disc edema (swelling of the optic nerve disc, where the optic nerve enters the eye – see Figure 1), globe flattening (the posterior part of the eye is pushed out and loses its curvature), choroidal folds (parallel lines in the vascular tissue forming the choroid), retinal folds (wrinkles in the retina usually caused by optic disc distension or by the thickening of the sclera), cotton wool spots (small, yellow-white lesions in the back of the eye, often associated with hypertension).

Figure 1: “Medical gallery of Blausen Medical 2014”. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436.

Of these, the flattening of the globe can, in turn, lead to farsightedness-type refractive error shifts, where the eye cannot focus light properly on the retina because the eye is “too short” and the images appear blurry. Interestingly, right eyes show more significant changes in vision acuity and structural than left eyes. Researchers have not yet found an explanation for this discrepancy.

These days, many of the SANS-related tests are performed by astronauts themselves on board the ISS. The medical instrumentation on the ISS has evolved over time, allowing the space station to become a true laboratory for performing studies and investigations into conditions that affect human health while in space. Currently, the ISS is equipped with ocular health test[i] instruments such as:

  • Ocular ultrasound: A handheld ocular ultrasound device is used to identify changes in the eye anatomy – such as the flattening of the back of the eye – and to measure changes in the size of the optic nerve sheath and the thickness of blood vessels in the choroid (see Figure 1). The ISS crew members are trained in using the ocular ultrasound by positioning the probe over the closed eyelid (using water as the lubrication medium rather than gel) and collecting ultrasound images.  With an ultrasound, one can see the optic disc and optic nerve swelling.
  • Fundoscopy:  Fundoscopy is performed on each eye, after dilation drops have been applied to the eye to allow for a more complete view of the entire retina.  The procedure is guided remotely, by a specialist on Earth, and is used to take images of the fundus of the eye, i.e., the back of the inside of the eye, including the retina and optic nerve. When the optic disc appears distended and pushed forward into the eye, that is usually an indication that the astronaut suffers from SANS caused by high intra-cranial pressure (ICP).
  • Tonometry:  Tonometry is a procedure performed to determine the intraocular pressure, i.e., the fluid pressure inside the eye. The measurement is done using a handheld tonometer, which is essentially a pressure gauge, by tapping the center of the eyeball (very carefully!) about ten times.
  • Visual Acuity/Amsler Grid: Astronauts on the ISS are expected to perform both vision acuity tests and Amsler Grid tests. Both rely on the subject’s characterization of the clarity of the images viewed and are carried out for each eye independently. For vision acuity tests, the crew member reads off a chart positioned 16 inches away (for near-vision), and from a computer screen positioned 10 feet away (for distance vision). Amsler Grid testing measures the integrity of the retina; the subject sits 16 inches from the grid, with one eye covered, and the other eye fixed on the dot in the center of the grid. If any of the grid lines appear distorted or missing, this is indicative of a change in the retina.
  • Optical coherence tomography (OCT): OCT uses imaging techniques to detect choroidal and retinal thickening and measure the height of the distension of the optic disc tissue to detect swelling. The SPECTRALIS OCT2 Module installed on the ISS is able to identify and render high-fidelity images for two layers of the retina, the neurosensory, light-sensitive layer and the retinal pigment epithelium (the cell layer that lies between the neurosensory retina and the choroid).

Figure 2: NASA astronaut Steve Swanson covers his left eye while performing Visual Acuity and Amsler Grid testing with his right eye using the Acuity Pro software. NASA Image.

The vision tests performed by astronauts during spaceflight are complemented by a suite of tests aimed at measuring the intracranial pressure and its impacts on the cardiovascular system, such as:

  • Transcranial Doppler (TCD) Ultrasound: This type of ultrasound is used to measure the speed of the blood flow in the basal arteries of the brain and assess blood flow responses to changes in blood pressure, changes in end-tidal CO2 (the amount of carbon dioxide detected at the end of an exhalation), or cognitive and motor activation. In-flight TCD is performed with the subject holding still in the same position, and expiring CO2 through a nasal prong. 
  • Cardiac Ultrasound: This test uses the ultrasound equipment on the ISS to examine the blood flow through the carotid arteries and determine changes in blood vessels and the heart.

The in-flight tests are accompanied by more extensive ocular tests taken before and after the flight, for comparison purposes.

Possible Causes of SANS

After 40 years of studies and observations, the reasons why SANS occurs are still unclear. Researchers have hypothesized that the shift of fluids towards the head due to weightlessness may be the root cause of the problem, resulting in an increase in the intra-cranial pressure (ICP) in comparison to the pressure of the fluid inside the – the intro-ocular pressure (IOP), the difference in pressure effectively pushing against the back of the eye and affecting the retinal tissue.

Other hypotheses consider related factors such as the rise in the cerebrospinal fluid (CSF) pressure within the optic nerve sheath, the increase in the volume of blood in the upper body due to the lack of gravity, and the decrease in vascular compliance observed – where veins in the upper body become “stiffer” when the blood pressure rises.

Challenges to Long-Term Space Travel

While the causes behind SANS are still being investigated, it is clear that the visual acuity and eye morphology problems in astronauts pose a serious challenge to NASA’s and other space agencies’ long-term space exploration plans.

A trip to Mars takes approximately 7 months, assuming the travel takes place when Mars is at its closest point to Earth (55 million km from Earth). Once on Mars, the space travelers would have to wait for almost another year and a half, until Earth’s and Mars’ orbits bring the two planets once again into relative proximity.

With research showing that most eyesight degradation issues manifest in the first couple of months of flight, a journey of such duration may render most flight participants visually impaired before they even arrive on Mars – and certainly before it is time for them to return to Earth.

Possible Countermeasures

With so much at stake, research centers around the world are looking for preventative measures and possible countermeasures to SANS.  While detecting the signs of SANS has been made easier by the array of in-flight and post-flight tests available, preventing and/or treating it is another matter altogether. Astronauts traveling to the ISS are equipped with corrective lenses (should their vision degrade while in orbit) and are expected to adhere to a daily aerobic workout routine.   According to NASA Human Research Program (HRP) researchers, reducing the amount of sodium in the astronauts’ diet while in space, together with a reduction of the CO2 levels in the space station may also alleviate SANS symptoms.

Apart from these basic measures, three promising technologies could help keep astronauts’ vision sharp during long-term missions in space:

SkinSuit: Designed by Dr. James Waldie, aerospace engineer and senior research associate at RMIT University in Melbourne, Australia, and developed in collaboration with scientists from the Massachusetts Institute of Technology, Kings College London and the European Space Agency – the SkinSuit was first tested in ground trials and on parabolic flights. The real-life test was done in 2015, when Denmark’s first astronaut, Andreas Mogensen, wore the SkinSuit on the ISS to test its effectiveness . The SkinSuit is a form-fitting, flexible suit with built-in elastic fibers that simulate the effect of gravity by gradually increasing the pressure on the body from the shoulders to the feet. The suit provides sensory and neuromuscular stimuli by applying pressure on the soles of the feet and by exerting compression along the length of the body. While the SkinSuit was designed primarily to prevent spine elongation and alleviate the low back pain felt by astronauts in-flight, its gravity loading and simulation effects may also have a positive effect in reducing or delaying the onset of SANS.

Figure 3: ESA astronaut Thomas Pesquet testing Skinsuit in weightlessness on a parabolic flight.  Image courtesy of CNES/Novespace, 2014.

Lower Body Pressure Negative (LBNP) Suit: Improving on the design of the original “Chibis pants”, researcher Lonnie Petersen, PhD, with the University of California, San Diego, together with Alan Hargens – University of California and Benjamin Levine – University of Texas Southwestern Medical Center at Dallas, have developed and tested a fully mobile gravity suit comprised of flexible pressurized-pants and attached vest. The suit looks much like a regular pair of ski-pants and simulates the effects of gravity through the application of low-level lower body negative pressure. Additionally, the suit induces a ground reaction force at the bottom of the feet and a mechanical load along the entire body axis, to more closely resemble the pressure placed on the human body by Earth’s gravity. (Read more about it in our June 2019 edition of Space Health News.)

Pressure-Regulating Goggles: Researchers at the NASA Johnson Space Center have performed a small-scale study to examine how the combination of exercise and artificially-increased intra-ocular pressure (through the wear of regular swimming goggles) affects the relationship between the intra-cranial fluid pressure and the intra-ocular fluid pressure. The results were promising as the use of swimming goggles was found to be associated with a small increase in pressure in the eye. While further research and testing on the ISS are still necessary to prove their long-term efficacy, goggles may offer the promise of a safe and inexpensive countermeasure to the negative effects on the eye structure and vision observed in astronauts during extensive space flights. (Read more about it in our June 2019 edition of Space Health News.)


  1. This is another well written article that addresses the concerns of prolonged space exploration. As a fellow space buff I really appreciate the topics this site tackles because there are so many challenges that must be overcome as we reach for the stars!

    Liked by 1 person

  2. Very interesting article. It sounds like, not all astronauts are affected by SANS. It appears, it accelerates something that would happen with age? Astronaut Kelley spent a year in Space, how did it affect his eyesight?


    1. Hi Christina, this is a great question. Scott Kelly’s eyesight was indeed affected (he has to wear glasses now), and in the “twin study” done after his return to Earth, the research found evidence of carotid artery wall thickening and elevated levels of the protein AQP2 in him – which are linked to the vision deterioration.


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