Asymmetric Optic Disc Edema in Astronauts: A Narrative Review Proposing an Interplay Between Ocular Venous Congestion and Glymphatic Transport.

Spaceflight associated neuro-ocular syndrome (SANS) is a significant ophthalmic complication observed in astronauts during and after long-duration missions, characterized by optic disc edema, globe flattening, choroidal folds, and hyperopic shifts. Unlike papilledema in terrestrial idiopathic intracranial hypertension, optic disc edema in SANS is often asymmetric. The mechanisms underlying this asymmetry remain poorly understood. In this narrative review, we synthesize and critically interpret existing clinical observations, anatomical studies, neuroimaging findings, and experimental evidence, and propose that uneven ocular venous congestion, arising from microgravity-induced cephalad fluid shifts, pre-existing transverse sinus asymmetry, and orbital venous overload, leads to asymmetric optic disc edema by differentially disrupting anterograde ocular glymphatic transport between the eyes. This mechanistic framework highlights the interplay between venous hemodynamics and ocular glymphatic flow as a key factor in SANS pathophysiology. Targeted in-flight monitoring and ground-based analog studies will be essential to rigorously test this hypothesis. To this end, we outline a feasible experimental approach that prospectively integrates preflight cerebral magnetic resonance venography, providing data on transverse sinus dominance, with serial in-flight ophthalmic imaging on the International Space Station. This combined strategy could directly determine whether dural venous sinus anatomy predisposes to uneven ocular venous congestion and asymmetric optic disc edema in microgravity. Insights gained from this work may guide the development of effective countermeasures against SANS and broaden our understanding of ocular fluid dynamics under conditions of altered venous physiology on Earth.