As human spaceflight moves beyond low Earth orbit and commercial missions expand, ophthalmology is being pulled into a new frontier: how to protect the eye in microgravity. A new expert consensus published in Eye sets out a framework for diagnosing and managing spaceflight-associated neuro-ocular syndrome (SANS), a condition affecting the eye, optic nerve, and potentially the brain during long-duration spaceflight.
SANS was first described in 2011, when astronauts returning from missions aboard the International Space Station were found to have optic disc edema, choroidal folds, globe flattening, cotton wool spots, retinal nerve fiber layer thickening, and hyperopic refractive shifts. The new consensus, led by Jessie Gew, Yousef F. Hyder, and colleagues, brings together NASA SANS specialists and experts in terrestrial cerebrospinal fluid disorders to harmonize case identification and guide clinical decision-making.
The scale of the problem is substantial. At least one feature of SANS has been reported in 81 percent of NASA astronauts assessed after long-duration spaceflight. Early changes appear to be common in microgravity, while severity seems to increase with longer missions and repeated exposure.
For astronauts, the most immediate clinical risks are hyperopic shift, central visual distortion, and visual field changes such as enlarged blind spot. While “space anticipation glasses” are already routinely deployed to address refractive shifts for astronauts, the consensus emphasizes that SANS is more than simply a refractive inconvenience. Some structural changes, including globe flattening and axial length alteration, have persisted for years after return to Earth, although no permanent vision loss or cognitive decline has yet been documented for these long-duration spaceflights.
The pathophysiology remains unresolved. Microgravity causes cephalad fluid shift, with approximately two liters of fluid redistributed upward from the lower body. Proposed mechanisms include elevated or altered intracranial pressure dynamics, impaired venous return, venous overload choroidopathy, cerebrospinal fluid sequestration around the optic nerve, altered glymphatic drainage, carbon dioxide exposure, anthropometric risk factors, and genetic variation in one-carbon metabolism. The study authors note that while SANS carries some diagnostic resemblance to pseudotumor cerebri (including bilateral disc edema), “there is no terrestrial disease that is a perfect model of SANS.”
OCT is central to the proposed diagnostic approach. The consensus identifies an average increase of 20 µm in peripapillary total retinal thickness as the earliest indication of optic disc edema, while a 55 µm increase or Frisén grade 1 edema marks a clinically concerning threshold. The classification also includes chorioretinal folds, posterior globe flattening, and refractive shift, with higher classes reserved for acute functional impact or potential long-term structural harm.
NASA’s current monitoring pathway includes standardized eye examinations and MRI before and after missions, plus in-flight assessments around flight days 30 and 90 and again 30 days before return on six-month missions. Astronauts now train to perform OCT, fundus imaging, and orbital ultrasound in orbit, and visual field testing became available on the International Space Station in late 2025 for clinically concerning cases.
Treatment remains the biggest gap. Apart from refractive correction and return to Earth, no established therapy exists. Potential countermeasures include lower body negative pressure, venoconstrictive thigh cuffs, resisted breathing devices, and periocular pressure devices designed to alter the translaminar pressure gradient. Drugs used in idiopathic intracranial hypertension, such as acetazolamide and topiramate, carry side effects that may be unacceptable during spaceflight (e.g., topiramate can cause significant cognitive impairment when given at therapeutic doses). GLP-1 receptor agonists, which can reduce intracranial pressure in terrestrial IIH studies, are proposed as a possible future avenue.
The consensus is necessarily provisional: astronaut cohorts are small, evidence is limited, and mechanisms still remain uncertain. However, the authors predict that as extended duration spaceflight continues to advance, so too will the prevalence and severity of SANS. As such, it represents an emerging neuro-ophthalmic condition requiring standardized diagnosis, careful monitoring, and credible countermeasures before longer missions make mild ocular changes harder to ignore.
