SANS: Clinical Monitoring and Management

What to do

Seeing a returning long-duration flyer (eye-focused checklist)

When is lumbar puncture considered?

NASA’s clinical practice (summarized in the SANS brief) considers postflight LP when there is Frisén grade ≥ 1 disc edema on fundoscopy and/or an average peripapillary total retinal thickness increase ≥ 55 µm on OCT in either eye. CSF opening pressure above 18 cm H2O is “high normal,” and ≥ 25 cm H2O prompts treatment consideration (particularly with new disc edema or scotoma) and Aerospace Medicine Board review for continued flight [1]. This is reported as operational practice for education, not an SMF recommendation.

Overview

SANS was first described in detail in a 2011 series of seven long-duration ISS astronauts with disc edema (5), globe flattening (5), choroidal folds (5), cotton-wool spots (3), nerve fiber layer thickening on OCT (6) and reduced near vision (6), alongside questionnaire data from about 300 astronauts reporting in-flight vision changes [3]. The name changed from VIIP (visual impairment and intracranial pressure) to SANS to reflect uncertainty about mechanism [4].

Clinical considerations

• Likely mechanism. Loss of gravity-driven fluid gradients shifts fluid headward; the absence of normal daily (upright) reductions in intracranial and venous pressure may create a small but persistent pressure gradient at the back of the eye [1][7]. In flight, internal jugular vein cross-sectional area and pressure rise markedly and stagnant or reverse flow occurs in some crew [8].
• Contributors under study: cabin CO2, B-vitamin/one-carbon pathway variants, impaired sleep/glymphatic clearance, small or crowded optic nerve heads, repeat flights and longer missions [1].
• Management in flight is mostly monitoring and optical correction; all crew fly with at least three pairs of glasses of varying power (“space anticipation glasses”) [1]. Acetazolamide is carried on ISS but side effects limit its use; no medication is proven to prevent SANS [1].

Mission-type differences

Terrestrial parallels

• Idiopathic intracranial hypertension (IIH): shared disc edema and globe flattening, but NASA notes the two “share more differences than similarities”; mildly elevated intracranial pressure is hypothesized in SANS, but it has not been measured in flight [1]. In practice, a returning astronaut with disc edema still needs the same exclusion of other causes as any patient. See IIH.
• Head-down tilt bed rest: 30 days of strict 6° head-down tilt bed rest produced optic disc edema in volunteers [6] — the closest analog.
• Choroidal folds from orbital mass: globe flattening on Earth usually means something pushing from behind; in astronauts the force is presumed hydrostatic [1].

Gaps

• No validated countermeasure; LBNP (lower-body negative pressure), thigh cuffs and B-vitamin supplementation are under investigation [1]. In a bed-rest crossover study, 8 hours of nightly LBNP (−20 mmHg) attenuated choroid engorgement, an early marker [7].
• In-flight intracranial pressure has never been directly measured [1].
• Long-term consequences of persistent globe flattening are unknown [1].

Commercial spaceflight notes

Short orbital trips are unlikely to cause clinically significant SANS, but commercial stations with multi-month stays will. Operators should obtain a preflight OCT/fundus baseline for any long-stay participant so postflight change can be interpreted.

Clinical pearls

• Always compare with preflight images — a “normal-looking” disc can be swollen relative to baseline.
• A returning astronaut needing stronger reading glasses is common; disc edema with headache and diplopia is not — work it up.