New research designed to open eyes on space travel and microgravity

In space, during long-duration flight, astronauts experience changes to their bodies—including globe flattening of their eyes—due to prolonged weightlessness in space.

Bone density drops. Muscles lose strength. And body fluids, instead of shifting generally down toward the feet under the force of gravity, shift up toward the head. This fluid shift can set the stage for the production of ocular anatomic anomalies that may lead to chronic visual acuity changes during and after long-duration space flight. Those anomalies might include disc edema, globe flattening, choroidal folds, and nerve-fiber-layer thickening.

The future of long-duration space flights—to Mars, for instance—could rely on how the human eye adjusts to living in space. Two recent studies focus on why changes occur in the eye. The desired result of the research is that a solution can be found to preserve or improve the vision of these space-traveling astronauts.

Effect of space travel on the eyes

For years, researchers have been studying physical and visual changes in the eyes that occur during space flight. At the 2016 Optometry's Meeting^® in Boston, space pioneer and keynote speaker Buzz Aldrin mentioned that NASA was studying the effect of long stays in space on astronauts' eyes.

"In 2011, a report from the Space Medicine Division of the National Aeronautics and Space Administration proposed that elevated intrasheath cerebrospinal fluid pressure (CSF) within the orbit was a possible mechanism that might explain optic disc edema, globe flattening and other findings reported in astronauts during and after long-duration space flight," says C. Robert Gibson, O.D., who provides eye care at NASA's Flight Medicine Optometric Clinic at the Johnson Space Center in Houston, Texas.

Dr. Gibson also is listed as a co-investigator/collaborator on NASA's current research on microgravity's effect on the visual, vascular and central nervous systems.

He adds, "This elevation in CSF pressure within the optic nerve sheath is thought to be caused by a rise in intracranial pressure transferred down the optic nerve sheaths from the brain and/or by the sequestration of fluid within the nerve sheaths as a result of localized events occurring at the orbital level with or without a rise in intracranial pressure. Regardless of specific etiology, elevated intrasheath CSF pressure is thought to cause the subarachnoid compartment within the orbit to exert an anterior force that indents the posterior sclera resulting in posterior globe flattening, choroidal folding and axial shortening. This ocular shortening appears to be the main culprit responsible for the visual changes."

Dr. Gibson is a contributor to a new study, "Persistent Asymmetric Optic Disc Swelling After Long-Duration Space Flight: Implications for Pathogenesis," published Dec. 5, 2016, in the Journal of Neuro-Ophthalmology. It is a case study of ocular changes in an astronaut, 45, during and after six months on the International Space Station (ISS).

Anatomic changes

In the study, the astronaut's eyes were examined pre-mission, during the mission and post-mission using ground and ISS-based fundus photography, ultrasound and optical coherence tomography (OCT).

"We documented asymmetric choroidal expansion in flight that largely resolved by 30 days post-flight, asymmetric disc swelling observed in flight that persisted for over 180 days post-flight, asymmetric optic disc morphologic changes documented in flight by OCT that persisted for 630 days post-flight, and asymmetric globe flattening that began in flight and continued 660 days post-flight," the authors write. "Interestingly, lumbar puncture opening pressures obtained at 7 and 365 days post-mission were 22 and 16 cm H20, respectively. These pressures are not high enough to cause or maintain globe flattening.

"The persistent asymmetric findings noted above, coupled with these lumbar puncture opening pressures, suggest that prolonged microgravity exposure may have produced asymmetric pressure changes within the perioptic subarachnoid space with no significant concurrent rise in intracranial pressure," the authors conclude. "Thus, local compartmentalization of CSF within the orbital optic nerve sheath is likely responsible for the changes observed in this astronaut." Additional research on the impact of long-duration space flight on eyes was presented Nov. 28 at the Radiological Society of North America annual meeting. This research also examined the impact of cerebrospinal fluid on ocular changes during space flight. The research was presented by lead author Noam Alperin, Ph.D., of the University of Miami - Miller School of Medicine.

Dr. Gibson reviewed the results presented by Dr. Alperin. In Dr. Alperin's study, researchers performed high-resolution orbit and brain MRI scans before and after space flights on seven long-duration mission ISS astronauts versus nine short-duration space shuttle astronauts. Dr. Alperin's research further confirmed the link between CSF changes and globe flattening.

"Although the precise mechanism is unknown, our published research studies and Dr. Alperin's recent report strongly suggest that increased optic nerve sheath CSF pressure and volume changes are responsible for the disc edema, globe flattening and hyperopic shifts documented in astronauts during and after long-duration space flight," Dr Gibson says.

What doctors of optometry should know

An important takeaway from both studies is—unless a solution can be found—astronauts will continue to report blurred vision and anatomic changes to their eyes during long-duration space flights.

"These changes will undoubtedly result in visual acuity anomalies in some space travelers that, while correctable, may be unpredictable in magnitude," Dr. Gibson says. "As space travel becomes more commonplace and available to larger numbers of people, it would be appropriate for optometrists to be familiar with the possible impacts of space travel on the visual acuity of these space travelers."

He adds, "It appears that clinical optic disc edema, as documented by microscopic slit lamp exam, may last for months post-mission and OCT evidence may last much longer. So there is a gradual resolution of disc edema, but it can be very slow and vary from person to person. Other anatomic changes appear to be more persistent and perhaps permanent in some astronauts."