Space travel is one of humanity’s most incredible achievements—but it comes with unique challenges, especially for the human body. While most people know about muscle atrophy and bone density loss in space, few realize that astronauts also experience significant changes in their vision. In fact, over half of astronauts on long-duration missions report visual problems—some of which persist even after they return to Earth.

So, what exactly happens to the eyes in space? And what can these changes teach us about ocular health on Earth?

What is SANS?

The term SANS (Spaceflight-Associated Neuro-ocular Syndrome) was coined by NASA to describe the collection of eye-related issues astronauts face during extended space missions.

Common symptoms include:

• Blurred or reduced near vision

• Changes in refractive error (usually hyperopic shift)

• Optic disc edema (swelling)

• Globe flattening (altered eye shape)

• Choroidal and retinal folds

• Cotton wool spots and vascular changes

These symptoms can appear gradually during spaceflight and sometimes persist for months post-mission.

Why Do These Vision Changes Happen?

1. Fluid Shift in Microgravity

• Flattening of the posterior globe

• Increased pressure around the optic nerve

• Disc edema and visual distortion

2. Increased Intracranial Pressure (ICP)

Although astronauts don’t develop the same kind of elevated ICP seen in typical medical conditions, the space-induced rise in pressure may still compress the optic nerve and alter cerebrospinal fluid (CSF) dynamics, affecting vision.

3. Structural Changes in the Eye

MRI and OCT imaging have shown that astronauts often develop:

• Optic nerve sheath distention

• Choroidal thickening

• Folds in the retina or choroid These structural changes can lead to temporary or even lasting visual symptoms.

What Research is Being Done?

NASA is actively investigating SANS to ensure astronaut health on future missions—especially for longer journeys like Mars exploration. Key research areas include:

• OCT imaging in space to monitor retinal and optic nerve changes in real time

• Portable visual acuity and refraction tools for in-flight assessment

• Fluid redistribution countermeasures, such as lower-body negative pressure suits

• Artificial gravity studies to offset microgravity effects

Understanding how microgravity affects the eye can also help us uncover new insights into glaucoma, intracranial hypertension, and other neuro-ocular diseases back on Earth.

The Vision-Brain Connection

Vision issues in space highlight the delicate relationship between the eye and the brain. Changes in CSF pressure, vascular flow, and even hormonal alterations all contribute to visual disturbances. Studying these effects may reveal new clues about neuro-ophthalmology, particularly in conditions where brain pressure or fluid regulation is a factor.

Eye Protection in Space

Aside from SANS, astronauts face other ocular risks:

• UV radiation exposure from solar rays

• Dry eye from reduced blink rate in low-humidity environments

• Debris in microgravity (floating particles can irritate or injure the eye)

To combat this, space agencies provide:

• Specialized UV-protective visors and eyewear

• Lubricating eye drops

• Closed-loop air filtration to minimize contaminants

What the Future Holds

As space missions become longer and more common—especially with commercial spaceflight and lunar colonization on the horizon—understanding and managing vision changes will be vital. Innovations in tele-ophthalmology, automated eye diagnostics, and portable OCT technology are already being tested for off-Earth use.

Lessons for Earth

Studying eye changes in space doesn’t just benefit astronauts—it can help patients on Earth:

• Insights into fluid regulation may inform treatment for glaucoma and idiopathic intracranial hypertension

• Understanding optic nerve changes can aid in managing neuro-ocular diseases

• Portable vision tools developed for astronauts could improve remote eye care access on Earth

Final Thoughts

The human eye is incredibly adaptable, but space pushes it to the limits. As we prepare for the next generation of space exploration, maintaining ocular health will be just as crucial as muscle strength and cardiovascular fitness.

Every time we send someone into orbit, we’re not just exploring space—we’re learning more about how our bodies work, adapt, and survive in extreme environments. And the eyes, it turns out, are among our best teachers.