New research published in the IEEE Open Journal of Engineering in Medicine and Biology is revealing startling findings about the impact of space missions on astronauts’ vision.
First, let’s get some background on vision and space.
Changes in the eye and overall vision during spaceflight are one of the most immediate physiological changes of traveling out of the Earth’s gravity and into microgravity (low levels of gravity).
- These ocular changes are known as spaceflight-associated neuro-ocular syndrome (SANS)
The reason for this: Microgravity, in which weightlessness causes blood and cerebrospinal fluid (CSF) to travel toward the head (known as a “water-hammer effect”).
- The stats: 40-70% of astronauts traveling to the International Space Station (ISS) are reported to experience some amount of swelling in the back of the eye.
And how has this been investigated so far?
Decades of research into SANS and the impact of long-duration space travel have uncovered a wide range of ocular changes, including:
- Unilateral or bilateral optic disc edema
- Globe flattening
- Choroidal and retinal folds
- Hyperopic refractive error shifts
- Retinal ischemia focal areas (cotton wool spots)
These changes—coupled with nutritional, metabolic, and genetic components—are now known to play key roles in contributing to an increased risk for developing SANS.
- See here for other recent investigations conducted by the National Aeronautics and Space Administration (NASA) aboard the ISS on the ocular impact of space flight.
Now to this new research.
A research team from Maisonneuve Rosemont Hospital Research Center in Quebec, Canada worked with the Canadian Space Agency (CSA) and NASA investigators to analyze a total of 26 eyes from 13 astronauts (average age: 48±9 years; 31% female) representing space agencies in the United States, Europe, Japan, and Canada.
About these participants: All were part of long-duration space missions on the ISS, with 38.4% (eight crew members) on their first mission during this study.
- Their mission durations: Ranged from 170 to 371 days
What measurements were taken?
Three key ocular parameters:
- Ocular rigidity (OR)
- A biomarker used to determine ocular pathology that describes IOP changes produced by an adjustment in ocular volume
- Measured via optical coherence tomography (OCT) with a unique video module for advanced image quality of the ocular vasculature
- A biomarker used to determine ocular pathology that describes IOP changes produced by an adjustment in ocular volume
- IOP
- Measured via tonometry
- Ocular pulse amplitude (OPA)
- The change in IOP between diastole and systole
- Measured via tonometry
- The change in IOP between diastole and systole
And the results?
Important to note: The study results did not specify when baseline measurements were taken.
However: All post-flight measurements were taken one to 30 days after landing, with:
- A significant mean reduction of 33% in OR from baseline (p = 0.04)
- Note: Only 17 eyes (nine astronauts) were analyzed for this (see why)
- An 11% decrease in IOP following exposure to microgravity (from baseline)
- Mean IOP: 16.0±2 mmHg (before) and 14.2±2 mmHg (after) (p = 0.004)
- A 25% decrease in OPA (p < 0.005) from baseline
Plus: Multivariate analysis was performed to determine the potential effect of sex, age, mission experience, and mission duration on OR and IOP.
- However, no significant correlation was found between these variables and characteristics, and no difference was noted between the days after landing.
Interesting … was reduced OPA and a lower OR a surprising finding?
The investigators noted that this instance has been observed in prior research and can be “explained by the fact that during the pulse, if the scleral wall is less stiff, the globe will expand more in response to the increased blood volume, and consequently the IOP will rise less.”
In other words: This finding isn’t necessarily an anomaly.
Got it. And what ocular symptoms accompanied these changes?
Such manifestations included:
- Reduced eye size
- Altered focal field
- Optic nerve edema
- Retinal folds
Let’s talk choroidal thickness.
First: A normal choroidal thickness (CT) in healthy individuals is typically anywhere from 250 to 400 μm (depending on age, sex, etc.).
The study results: While baseline CT in eight participants was < 400 mm, five participants had a CT of > 400 mm.
- Interestingly: This did not correlate with age, gender, or mission experience.
About the excluded participants in this: They presented a non-visible interface between the choroid and the sclera, while one eye (of one participant) had a low-quality scan.
What did investigators say about these long-term changes?
The study authors noted that when choroidal expansion occurs during weightlessness, this “may lead to stretching of the scleral collagen.”
- However: Once an individual returns to gravity (Earth), this expanded choroidal volume would then return to normal—though “the scleral mechanical properties remain altered, wrestling in lower OR,” they stated.
See here for a list of limitations associated with this study.
Circle back to that “water hammer” effect from earlier.
The authors also stated that—as evidenced in prior research—pulsatile changes (blood pulsations) during microgravity conditions could create the water-hammer effect associated with SANS.
Relating that back to this study: “It is possible that a similar water hammer effect could also lead to tissue remodeling in the eye, effecting scleral stiffness,” they reasoned.
So should these ocular effects be a cause for concern among astronauts?
Not necessarily—particularly if a space mission doesn’t extend beyond 6 to 12 months.
Despite a vast majority (80%) of participants developing at least one ocular symptom, all eyes returned to normal following their return to Earth.
- Also take note: Corrective eyewear was, for the most part, largely beneficial in correcting these ocular changes.
And what’s the key takeaway from this?
To put it plainly (per the authors): Long-term space missions significantly alter ocular biomechanics and have the potential to become biomarkers of SANS disease progression.
Study investigator Santiago Costantino, MD, an ophthalmologist at the University of Montreal in Quebec, Canada, concluded that these observed changes could “help identify at-risk astronauts before they develop serious eye problems during long-duration missions."