Long-haul space missions reshape the optic nerve head

Spaceflight-associated neuro-ocular syndrome is a newly described condition characterized by hyperopic shift, disc edema, globe flattening, choroidal folds and cotton wool spots following space travel and prolonged exposure to microgravity. Using OCT, the authors describe changes in the optic nerve head and surrounding tissue in astronauts after a 6-month mission.

Study design

The authors monitored 15 astronauts who underwent OCT imaging before and after a 6-month long mission to the International Space Station. They quantified morphological changes in various ocular structures, and compared findings from the astronauts and 43 healthy controls.

Outcomes

After returning from their mission, investigators noted increases in total retinal thickness and retinal nerve fiber layer thickness, which were most pronounced in the region closest to the optic nerve head rim margin. There were no changes in choroidal thickness.

Perhaps the most interesting finding was that the Bruch membrane opening (BMO) was recessed in the astronauts before embarking on missions compared with controls, presumably due to prior spaceflight. The BMO further deepened after the space mission (median change of -9.9µm).

Approximately 27% of the astronaut eyes had choroidal folds on their preflight OCTs, which increased to 60% after the space mission.

Limitations

The main limitation of this study was that the OCT images were obtained an average of 5 days after returning to Earth rather than during space flight. Therefore, some of these changes could be diminished or rebound compensatory changes might have occurred, which could explain the paradoxical BMO orientation. Because the astronaut data were confidential, it was not possible to know which astronauts had undergone space travel prior to the current trip. Lastly, lumbar puncture opening pressures were not available for these astronauts and therefore an association between opening pressure and OCT changes could not be determined.

Clinical significance

The findings here add more evidence that long-term space missions have a negative impact on astronaut’s eyes. The changes that were noted in the BMO is the opposite of what occurs in terrestrial papilledema, where the raised intracranial pressure causes a forward bowing of the BMO upward toward the vitreous body. The authors speculate that the downward BMO deflection in astronauts may be due to the return to Earth’s gravity resulting in rapid resumption of hydrostatic drainage and resultant fluid shifts.

Another observation is the predominance of choroidal folds in astronauts, which suggests a different pathophysiology of extraterrestrial versus terrestrial papilledema.

Future and ongoing studies include OCT imaging during space flight, which will provide more insight into the changes that are occurring from microgravity during space travel. This is an important area of research as the U.S. prepares for even longer-duration spaceflights to the International Space Station, moon, asteroid belt or Mars.