INTRODUCTION: A number of ophthalmic abnormalities, including optic disc edema, have been reported in several astronauts involved in long-duration spaceflights. An increased understanding of factors contributing to this syndrome, initially designated visual impairment and intracranial pressure syndrome and recently renamed spaceflight-associated neuro-ocular syndrome, has become a high priority for ESA and NASA, especially in view of future long-duration missions, including trips to Mars. The underlying pathophysiological mechanisms of this syndrome are still not well understood. In the present paper, we propose that optic disc edema in astronauts may occur, at least in part, as a result of retention of interstitial fluid in distended paravascular spaces at the prelaminar region of the optic nerve head. Preflight, in-flight, and postflight analysis of the optic nerve head and surrounding structures by optical coherence tomography in long-duration International Space Station crewmembers could provide important structural information in this respect.Wostyn P, De Winne F, Stern C, De Deyn PP. Dilated prelaminar paravascular spaces as a possible mechanism for optic disc edema in astronauts. Aerosp Med Hum Perform. 2018; 89(12):1089-1091.