The escape of retrobulbar cerebrospinal fluid in the astronaut's eye: mission impossible?

Ophthalmic abnormalities including unilateral and bilateral optic disc edema, optic nerve sheath distention, globe flattening, choroidal folds, and hyperopic shifts have been observed in astronauts during and after long-duration spaceflight. An increased understanding of factors contributing to this syndrome, termed spaceflight-associated neuro-ocular syndrome, is currently a top priority for the ESA and NASA, especially since this medical obstacle could impact the visual health of astronauts as well as the success of future missions, including continued trips to the International Space Station, a return to the moon, or a future human mission to Mars. Currently, the exact mechanisms causing this neuro-ocular syndrome are not fully understood. In the present paper, we propose a hypothetical framework by which optic disc edema in astronauts may result, at least partly, from the forcing of perioptic cerebrospinal fluid into the optic nerve and optic disc along perivascular spaces surrounding the central retinal vessels, related to long-standing microgravity fluid shifts and variations in optic nerve sheath anatomy and compliance. Although this hypothesis remains speculative at the present time, future research in this area of investigation could not only provide exciting new insights into the mechanisms underlying microgravity-induced optic disc swelling but also offer opportunities to develop countermeasure strategies.