Spaceflight consistently elevates the lower eyelid, according to a new study. Analyzing 115 NASA photographs (52 Earth, 63 space) from 13 astronauts, researchers found that MRD2—the distance from the corneal center to the lower eyelid margin—decreased by a mean of 1.0 mm in microgravity (from 4.9 ± 0.7 mm on Earth to 3.9 ± 0.7 mm in space, p < 0.001). All astronauts exhibited this “reverse ptosis,” with 62% showing a change of ≥1 mm—a threshold considered clinically significant. The effect was highly consistent between eyes (r = 0.870, p < 0.001) and comparable in magnitude to changes observed during short-term parabolic flights, suggesting the phenomenon occurs rapidly and persists through long-duration missions.

The authors propose two main mechanisms. First, cephalad fluid shift—about 2 liters of fluid moves toward the head in microgravity, with approximately 50 mL redistributing into head and neck tissues—causes pretarsal swelling that mechanically pushes the lower eyelid upward. Second, elastic recoil of facial tissues plays a role: on Earth, gravity pulls tissues downward, but in microgravity, elastic forces reposition them to minimize tension, contributing to the upward displacement. Notably, the changes were similar to those seen in parabolic flights, aligning with reports of persistent periorbital edema throughout spaceflight.

While reverse ptosis can alter corneal topography, reduce visual field, and affect visual acuity, this study was not designed to establish a direct link to the dry eye symptoms reported by approximately 30% of astronauts. Limitations include the retrospective use of publicity images (often with smiling or angled faces), reliance on average corneal diameters for calibration rather than individual measurements, and absence of symptom data. The authors call for future studies with standardized imaging protocols to clarify mechanisms and health implications for extended lunar and Martian missions.

The work titled “The Effect of Microgravity on Lower Eyelid Position in Astronauts”, was published on Eye & ENT Research (published on June 17, 2026).
DOI：10.1002/eer3.70043