Spaceflight Associated Neuro-ocular Syndrome, or SANS, is a condition involving changes to the eyes and brain of astronauts, particularly on long-duration missions. It is a health consideration for space agencies planning extended missions, such as those to Mars. First described in 2011, SANS involves structural alterations to the eye and optic nerve, leading to shifts in vision. Approximately 70 percent of astronauts on the International Space Station (ISS) have experienced some of these changes, and while it has not caused permanent vision loss, its long-term implications are under investigation.
The Signs and Symptoms of SANS
The signs of SANS include physical changes to the eye and optic nerve. One defining feature is optic disc edema, a swelling of the optic nerve head where it enters the back of the eye. This swelling can be observed in one or both eyes and varies in severity.
Another common finding is the appearance of choroidal and retinal folds, which are wrinkles in the layers at the back of the eye. This is linked to globe flattening, a measurable flattening of the back of the eyeball.
These structural changes lead to changes in an astronaut’s vision. Many experience a hyperopic shift, or farsightedness, making it difficult to see near objects clearly. This shift can be minor or significant enough to require reading glasses, with some changes measured up to +1.75 diopters. While vision is often correctable, the underlying structural alterations can persist for years after returning to Earth.
The Underlying Causes in Microgravity
The leading hypothesis for SANS centers on fluid behavior in microgravity. On Earth, gravity pulls bodily fluids toward the feet, but in space, this pull is absent. This causes a large volume of fluid, including blood and cerebrospinal fluid (CSF), to shift from the lower body to the head in a phenomenon called a cephalad fluid shift.
This fluid redistribution is believed to increase intracranial pressure (ICP). The optic nerve is surrounded by a sheath filled with CSF that is continuous with the brain. Elevated ICP can be transmitted along this sheath, exerting pressure on the optic nerve and eyeball, which is thought to cause the optic disc swelling and globe flattening seen in SANS.
Other factors may contribute to the pressure buildup. Venous congestion in the head and neck could slow the drainage of blood and CSF from the eye. Additionally, the elevated carbon dioxide (CO2) levels in a spacecraft can act as a vasodilator, widening blood vessels and increasing cerebral blood flow, which may add to the pressure inside the skull.
Diagnosis and Monitoring
Diagnosing and tracking SANS requires a monitoring program that starts before flight. Pre-flight exams establish a baseline for each astronaut’s ocular health, including tests for visual acuity and high-resolution imaging of the eye’s internal structures.
During missions, astronauts on the ISS use specialized equipment to monitor for SANS. A primary tool is Optical Coherence Tomography (OCT), a non-invasive test that creates cross-section images of the retina. This allows for the detection of optic disc swelling. Astronauts also use a fundoscope for photographs and ultrasounds to check for globe flattening.
Post-flight monitoring continues on Earth to track recovery. These exams repeat the same tests from before and during the mission. In cases of significant optic disc edema, a lumbar puncture (spinal tap) may be used to measure cerebrospinal fluid pressure. This observation helps researchers understand the long-term effects of SANS.
Countermeasures and Mitigation Strategies
Researchers are developing countermeasures to reduce the severity of SANS, primarily by counteracting the cephalad fluid shift. Most strategies are mechanical and aim to prevent the associated increase in intracranial pressure.
One countermeasure is the use of venous constrictive thigh cuffs. Worn on the upper legs, these cuffs apply pressure to trap fluids in the lower body, preventing them from shifting toward the head. Ground-based studies simulating microgravity have shown these cuffs are effective at this task.
Another prominent countermeasure is the Lower Body Negative Pressure (LBNP) device. This device encloses the lower half of the body and creates a vacuum, which actively pulls fluid toward the legs to mimic the effect of gravity. Both thigh cuffs and LBNP devices have been found to be well-tolerated in long-duration bed rest studies, which are used on Earth to simulate spaceflight. Future research is also exploring other potential strategies, including dietary modifications, to protect astronaut health on long missions.