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Comment on: ‘A multifactorial, evidence-based analysis of pathophysiology in Spaceflight Associated Neuro-Ocular Syndrome (SANS)’

Eye volume 39, page 1431 (2025)Cite this article

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The Galdemez et al. review of the spaceflight-associated neuro-ocular syndrome contains an important physiological misconception [1]. The authors use the phrase “venous congestion” to describe the venous circulation in the neck and brain in space. In clinical settings, the term “venous congestion” is often used when venous pressures increase, such as when right heart failure leads to venous congestion in the liver. Venous pressures are not abnormally increased in spaceflight.

The authors note correctly that entering weightlessness produces a head-ward fluid shift compared to the upright position on Earth. They also suggest that head-down tilt (HDT) can simulate the fluid shift in microgravity. On Earth, moving from the upright to the HDT position initiates a head-ward fluid shift, and central venous pressure, jugular pressure, intraocular pressure, and intracranial pressure all increase. If the fluid shift in weightlessness were similar to this Earth-based HDT shift, all these pressures would be expected to increase above supine values because leg volume measurements suggest the head-ward shift in space exceeds what occurs when moving from upright to supine on Earth. But multiple studies show peripheral venous pressure [2], central venous pressure [3], intraocular pressure [4], and intracranial pressure [5] are all usually below supine levels in weightlessness. The data on elevated jugular vein pressure the authors cite is based on a flawed, non-invasive technique [6].

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References

  1. Galdamez LA, Mader TH, Ong J, Kadipasaoglu CM, Lee AG. A multifactorial, evidence-based analysis of pathophysiology in Spaceflight Associated Neuro-Ocular Syndrome (SANS). Eye 2025;1–0.

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Authors and Affiliations

  1. Geisel School of Medicine at Dartmouth, One Medical Center Drive, Lebanon, NH, USA

    Jay C. Buckey

  2. Thayer School of Engineering at Dartmouth, One Medical Center Drive, Lebanon, NH, USA

    Mimi Lan

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  1. Jay C. Buckey
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  2. Mimi Lan
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Jay Buckey and Mimi Lan jointly contributed to the conception, drafting, and revision of this Letter to the Editor. Both authors critically reviewed the content, approved the final version for submission, and agreed to be accountable for all aspects of the work.

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Correspondence to Jay C. Buckey.

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Buckey, J.C., Lan, M. Comment on: ‘A multifactorial, evidence-based analysis of pathophysiology in Spaceflight Associated Neuro-Ocular Syndrome (SANS)’. Eye 39, 1431 (2025). https://doi.org/10.1038/s41433-025-03727-z

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