Spaceflight-Associated Neuro-ocular Syndrome (SANS), often called “space blindness,” describes a range of vision changes astronauts experience during and after missions. These changes affect eye structure and visual function. SANS presents a significant challenge for human space exploration, particularly as mission durations lengthen. Understanding this syndrome is important for space medicine, impacting future long-duration journeys.
What Causes Vision Changes in Space?
The primary cause of vision changes in space is microgravity-induced fluid shifts within the body. On Earth, gravity pulls fluids, such as blood and cerebrospinal fluid (CSF), towards the lower body. In microgravity, fluids redistribute towards the head and upper body.
This cephalad fluid shift increases intracranial pressure (ICP), the pressure within the skull. The elevated pressure inside the brain transfers down the optic nerve sheaths, which surround the optic nerves, to the eye. This increased pressure on the optic nerve and surrounding structures is a primary hypothesis for SANS.
Research indicates that CSF, which cushions the brain and spinal cord, drains less effectively in microgravity, contributing to increased pressure around the optic nerve. This impaired drainage leads to fluid accumulation within the optic nerve sheath. Other contributing factors include elevated carbon dioxide levels in spacecraft and variations in ocular rigidity.
How Spaceflight Impacts Vision
Astronauts experiencing SANS develop visual symptoms and eye changes. Blurred vision, particularly near vision, is a common complaint, with some astronauts reporting a need for stronger reading glasses as their mission progresses. This change in focus is often described as a hyperopic shift, meaning they become more farsighted.
Physical changes in the eye are also well-documented. Optic disc swelling, also known as papilledema, occurs when the optic nerve head, where the nerve connects to the retina, becomes swollen due to increased pressure. Other observable signs include choroidal folds, which are wrinkles in the choroid layer of the eye, and cotton wool spots, which are areas of nerve fiber damage on the retina.
Imaging studies have revealed changes in the shape of the eye, such as posterior globe flattening, where the back of the eyeball becomes less curved. The optic nerve sheath can also show distension and tortuosity, indicating fluid accumulation around the nerve. While these changes can vary in severity and may resolve after returning to Earth, some, like choroidal folds and globe flattening, can persist for years post-flight.
Addressing Vision Issues in Space
Addressing SANS involves pre-flight screening and in-flight monitoring. Astronauts undergo comprehensive eye exams before and after missions, including advanced imaging techniques like optical coherence tomography (OCT) and magnetic resonance imaging (MRI) to detect structural changes in the eye and brain. In-flight monitoring tools, such as specialized visual acuity tests and orbital ultrasounds, help track changes during the mission.
Researchers investigate countermeasures and treatments to mitigate SANS. One promising mechanical intervention is lower body negative pressure (LBNP), which involves a device that applies suction to the lower body to draw fluids away from the head, mimicking Earth’s gravity. Studies are also exploring the use of venous constrictive thigh cuffs to passively trap fluids in the lower extremities.
Beyond mechanical solutions, other approaches include specific exercise regimens designed to promote fluid redistribution and maintain normal circulation, as well as nutritional interventions. Some research is examining the role of certain supplements and even pharmaceutical options to reduce fluid pressure or protect ocular tissues. Resolving SANS is a significant hurdle for NASA and other space agencies, as effective countermeasures are needed to ensure astronaut health and mission success for future long-duration voyages, such as those planned for the Moon and Mars.