When the Pressure Moves, Ask What Happened to the Eye

Lower body negative pressure can change a pressure measurement during spaceflight. That finding matters. It still leaves the clinical question at the center of spaceflight-associated neuro-ocular syndrome: did the eye change as well? NASA can obtain more value from the next countermeasure study by assigning those two observations different evidentiary roles from the outset. Pressure and fluid measures show physiological action. Retinal, choroidal, optic nerve, globe, and visual measures carry the ocular claim.

The distinction is practical for crews travelling farther from clinical support. A plausible mechanism helps explain an intervention, but the endpoint tied to crew health determines what mission planners can conclude. Keeping both measurement classes in one protocol preserves the mechanism without asking it to substitute for a clinical result.

One inflight experiment produced two answers

Greenwald and colleagues studied 14 International Space Station crewmembers in a prospective, within-subject crossover. Intraocular pressure rose during flight. With 25 millimeters of mercury of lower body negative pressure, it returned to 14.2 millimeters of mercury, statistically equivalent to the preflight seated value. Subfoveal choroidal thickness had risen from 339 to 374 micrometers during flight and measured 381 micrometers during the intervention (P = 0.99) (Greenwald et al. 2021).

The pressure response showed that the intervention acted on one part of the physiology. The choroidal measurement recorded a different ocular result. Neither needs to be diminished to make the other meaningful.

Related inflight studies reinforce that separation. Pardon and colleagues followed 14 crewmembers. By flight day 150, four optical coherence tomography parameters had changed, including a 33.8-micrometer increase in minimum rim width and a 0.038-cubic-millimeter decrease in optic cup volume. Brief lower body negative pressure did not change those parameters (Pardon et al. 2022). Jasien and colleagues examined 13 crewmembers and found no change in four noninvasive indicators of intracranial pressure during the same acute intervention (Jasien et al. 2022).

These studies need not be compressed into one efficacy verdict. One measure moved, others remained stable, and the designs recorded both. The next investigation can test whether exposure schedule, endpoint selection, or another condition accounts for the pattern.

The literature has measured mechanism more often than structure

Gastronaut’s bounded ocular-health synthesis assessed 785 records and retained 388 analysable studies from 3,375 unique records. Its human evidence is substantial relative to the other portfolio domains: 100 studies concern human spaceflight and 163 concern human ground analogs. Full-text retrieval did not run. The counts describe the indexed corpus and do not establish absence elsewhere in the literature.

Within that corpus, fifty-six analysable studies used lower body negative pressure or venoconstrictive thigh cuffs. Five measured an ocular structural endpoint. The remaining 51 measured jugular vein area, intracranial pressure, intraocular pressure, central venous pressure, cerebral blood velocity, heart rate, or tolerance. Each proxy can reveal physiological action. Optic disc edema, choroidal folds, globe flattening, and retinal change answer the clinical question about the eye.

This distribution supports high confidence that the reviewed mechanical-countermeasure literature gives more attention to fluid and pressure proxies than to ocular structure. Confidence in a general efficacy conclusion remains low because five of the 56 relevant records measured ocular structure, and the studies differ in setting, dose, duration, and outcome. The evidence boundary is useful: it shows where the record is developed and where a new study can contribute.

Duration offers one line of inquiry. Hearon and colleagues applied lower body negative pressure for eight hours each night in a three-day bed-rest crossover involving 10 participants. Choroid area and volume rose during bed rest. The intervention attenuated the increase in area by 74 percent and in volume by 53 percent (Hearon et al. 2022).

That study used a longer daily exposure in a ground analog and measured a related, though different, ocular outcome. It provides a basis for investigating dose and timing. Transfer to flight and prevention of SANS remain prospective questions, to be answered by designs created for those purposes.

Design the claim before collecting the image

A decision-ready protocol can retain the proxies while reserving a SANS mitigation claim for a prespecified ocular endpoint, comparator, imaging schedule, and uncertainty statement. A change in a declared fluid or pressure measure would establish physiological action. The ocular record would establish what happened to the eye.

Optical coherence tomography could be paired with a declared segmentation method and observations of optic disc edema, choroidal folds, globe shape, and visual function. Intraocular pressure and noninvasive indicators of intracranial pressure would stay in the protocol as mechanistic measures. Exposure dose, duration, adherence, tolerance, recovery, preflight anatomy, biochemical status, cabin conditions, medication use, and collection time for every sample or image would provide the context needed for interpretation.

Imaging specifications deserve the same care. The synthesis identified 82 records that used optical coherence tomography, while 11 abstracts named the device or segmentation software. Instruments and software can change retinal-layer values. Recording both permits reviewers to compare results produced years apart while leaving investigators free to pursue different scientific questions.

One bounded question can govern the work: under a specified exposure schedule, does the intervention change a prespecified ocular structure in the expected direction, and does the proposed physiological mechanism accompany that change? The result may show a mechanism, an ocular response, both, or neither. Because the claims were assigned before the observations arrived, each outcome remains interpretable.

Gastronaut proposes a measurement-design partnership that connects nutritional exposure records, environmental conditions, and cultivation provenance with NASA’s ocular and biochemical observations. ORCA is a ground-stage cultivation system at approximately TRL 3 to 4. It has no flight or lunar operating history, and its use would not establish ocular efficacy. Its role would be to supply a controlled, traceable food and nutrient exposure whose intake and environmental record can share a time base with biological measurement.

NASA would retain the clinical question and the health inference. Gastronaut would make the research exposure reproducible and auditable. That division yields an endpoint-governed study and a record that later teams can understand without having stood beside the original investigators.

The pressure reading tells us whether the mechanism moved. The ocular measurement tells us what that movement meant for the eye. Giving each result its proper place is a measured act of care for the crew members whose sight gives this research its purpose.

References

Gastronaut. Ocular Health and Spaceflight-Associated Neuro-ocular Syndrome: Evidence Synthesis. Evidence version 22 Aug. 2026. Research synthesis.

Greenwald, Scott H., et al. “Intraocular Pressure and Choroidal Thickness Respond Differently to Lower Body Negative Pressure During Spaceflight.” Journal of Applied Physiology, 2021. https://doi.org/10.1152/japplphysiol.01040.2020.

Hearon, Christopher M., et al. “Effect of Nightly Lower Body Negative Pressure on Choroid Engorgement in a Model of Spaceflight-Associated Neuro-ocular Syndrome: A Randomized Crossover Trial.” JAMA Ophthalmology, 2022. https://doi.org/10.1001/jamaophthalmol.2021.5200.

Jasien, Jessica V., et al. “Noninvasive Indicators of Intracranial Pressure Before, During, and After Long-Duration Spaceflight.” Journal of Applied Physiology, 2022. https://doi.org/10.1152/japplphysiol.00625.2021.

Pardon, Laura P., et al. “Changes in Optic Nerve Head and Retinal Morphology During Spaceflight and Acute Fluid Shift Reversal.” JAMA Ophthalmology, 2022. https://doi.org/10.1001/jamaophthalmol.2022.1946.