The Effect of Microgravity on Lower Eyelid Position in Astronauts

Abstract

Background: Specific facial changes are known to occur in microgravity. A higher lower eyelid position has been observed during short-term microgravity but has not been analyzed in astronauts during spaceflight.

Purpose: To describe the effect of spaceflight on lower eyelid position.

Method: Images of astronauts captured on Earth and in space were collected from the National Aeronautics and Space Administration online image library. For each astronaut, at least 6 front-facing portraits (at least 3 captured on Earth and at least 3 in space) were selected. Measurements from the center of the pupil to the lower eyelid margin (margin reflex distance 2 [MRD2]) were determined digitally for each image and compared between Earth and space images using a Wilcoxon signed-rank test. Additionally, per-astronaut pooled mean values were descriptively compared with reference values from parabolic flight.

Results: 115 photographs (52 Earth, 63 space) from 13 astronauts (12 men, 1 woman; mean age 46 ± 7 years, range 35–60) were retrieved from the database. Microgravity induced a mean MRD2 decrease of 1.0 mm (p < 0.001).

Conclusion: Spaceflight led to a significant decrease in MRD2. These findings are comparable to those obtained from short-term microgravity experiments on parabolic flights. Further studies implementing standardized imaging techniques are needed to uncover the mechanism and health implications.

Graphical abstract

Keywords

astronaut / eyelid / microgravity / ptosis

1 Introduction

Microgravity affects the position of several periocular landmarks in both short-term microgravity during parabolic flight and long-term microgravity during spaceflight. Facial changes are a possible contributor to eye-related issues experienced by astronauts such as dry eye symptoms and spaceflight associated neuro-ocular syndrome [1–3]. Microgravity-induced changes in upper eyelid and eyebrow position have been described, whereas changes in lower eyelid position in astronauts have not been considered [4].

In clinical practice, the vertical position of the eyelids is measured in relation to the corneal light reflex with margin reflex distance 1 (MRD1) to the upper eyelid margin and margin reflex distance 2 (MRD2) to the lower eyelid margin. Normal MRD1 is typically around 3.5 mm and MRD2 is approximately 5 mm [5]. These measurements are used to diagnose various types of abnormalities of the ocular adnexa, and if such changes occur as a matter of course in microgravity, it is likely that they will cause physiological changes to the eyelids and their function [6].

Our studies have shown that almost identical changes in MRD1 and eyebrow height measured in astronauts occurred in the short periods of microgravity during parabolic flight [4,7]. In addition, we observed a previously unnoticed decrease in MRD2 (a consistent elevation of the lower eyelid). We therefore hypothesize that a decrease in MRD2 will also be found in astronauts.

2 Materials and Methods

2.1 Study Design

This was an observational study using publicly available images which were of National Aeronautics and Space Administration (NASA) astronauts, Roscosmos State Corporation for Space Activities cosmonauts, Japan Aerospace Exploration Agency (JAXA) astronauts, and European Space Agency (ESA) astronauts. Data were accessed via the NASA Image and Video Library (Available at: images.nasa.gov/. Accessed August 5, 2025). The images being in the public domain meant that they were not subject to copyright, and permission for use was not required (Ethics exemption: EX-H17195, Western Sydney University Human Research Ethics Committee, Sydney, Australia).

For subjects to be included in the study, at least 3 high-resolution photographs captured on Earth and at least 3 captured during spaceflight needed to be available. Further conditions for inclusion were: subjects must be looking directly at the camera in primary position with no more than an estimated 10° of chin inclination and must show a neutral facial expression, that is, no obvious frontalis contraction, smiling nor squinting of the eyes. Smiling and squinting both lead to a reduction in MRD2 through orbicularis muscle contraction [8]. In some cases, the capture of spaceflight photos preceded the capture of Earth portraits, and in other cases, this order was reversed.

2.2 MRD2 Measurements

White-to-white corneal diameters of the astronauts were not available. Instead, images were calibrated to the average corneal diameter, 11.60 mm for men and 11.71 mm for women [9]. Image analysis was performed using Adobe Photoshop (Adobe Inc., San Jose, Cal, USA) starting with the corneal diameter calibration. The geometric corneal center was then used because the corneal center (pupillary) light reflex was not present in every picture analyzed. Corneal centers were aligned using the spirit level tool. Measurements of MRD2, that is, the distance between the corneal center and lower eyelid margin was measured at a 90° angle to the spirit level ruler originating in the corneal center. This method has been described in detail previously [7].

2.3 Statistics

MRD2 values were first averaged per astronaut and condition, yielding one Earth and one space value per astronaut. Since the data were not normally distributed (Shapiro–Wilk test, p < 0.05), a Wilcoxon signed-rank test was applied to assess differences between terrestrial and spaceflight images. This analysis was done separately for the left and right eye as well for the mean of both eyes. The results were Bonferroni–Holm corrected to compensate for the multiple analysis.

Descriptive statistics were calculated, including mean, median, and standard deviation. Concordance in changes between the eyes was assessed with a linear regression analysis. Further, the results of this study were descriptively compared with the results from short-term microgravity experiments during parabolic flight. To mirror the approach from these experiments, the mean data for both eyes was pooled per astronaut for each condition, resulting in one discrete datapoint for Earth and inflight data per astronaut [7].

3 Results

Of several thousand images viewed, only a few image sets satisfied the criteria. In total 52 Earth, and 63 space images from 12 men and 1 woman (mean age 46 ± 7 years, range 35–60) were analyzed (Figure 1, representative images). Mean MRD2 on Earth was 4.9 ± 0.7 mm and 3.9 ± 0.7 mm in space (Figure 2). Thus, microgravity induced a mean MRD2 decrease of 1.0 mm (p < 0.001). Eight subjects (62%) had a mean change of MRD2 of ≥ 1 mm. In all cases, there was a decrease in MRD2 and high concordance between eyes (r = 0.870, p < 0.001).

4 Discussion

This analysis shows that the lower eyelid sits higher on the eye (i.e., the lower eyelid margin rests closer to the corneal center) during spaceflights than it does on Earth. Such elevation of the lower eyelid is called upside-down ptosis or reverse ptosis. Reverse ptosis may result from neurogenic causes, including sympathetic or oculomotor pathway lesions, or from mechanical factors such as lower eyelid retractor weakness, globe malposition, or trauma. Measurement of MRD2 in such cases is used for diagnosis and a reduction of > 1 mm is considered clinically significant [10]. The mean change in lower eyelid height in the current study was 1.0 mm with 62% of subjects having a mean change of 1 mm or more. Deleterious consequences to vision associated with reverse ptosis have been described and may have implications for long-haul missions [11,12]. In brief, alternations of corneal topography, a reduction of visual field and visual acuity might occur [7].

The results presented here were similar in extent to the changes in lower eyelid height observed during short-term microgravity brought about by parabolic flight (Figure 3) and suggest that this microgravity-induced reverse ptosis occurs instantly (parabolic flight data) and appears to persist through many months of spaceflight as indicated by the astronaut images. Moreover, persistent periorbital edema has been reported in a study over 7 spaceflight days and frequently during long-duration spaceflight in general [13,14].

4.1 Possible Mechanisms for Reverse Ptosis in Microgravity

Cephalad fluid shift (CFS) occurs upon entering microgravity [15]. Of the about 2 L of fluid that shifts toward the head, around 50 mL was calculated to redistribute into the superficial tissues of the head and neck region causing the face to become swollen (“puffy face”) [16]. In the current study, periocular swelling was particularly evident in the pretarsal area of the lower lid, leading to a prominent plump pretarsal roll. Such pretarsal rolls can be created surgically for cosmetic reasons by fat transposition or by adding volume to the lower eyelid tissues [17,18]. Therefore, it stands to reason that the accentuated pretarsal roll observed in the current study has its origins in added volume. This pushes the lower eyelid mechanically upwards through mass effect, thereby reducing MRD2 [19]. Added fluid volume is also consistent with the decreased MRD2 not resolving because the tissues of the head have fewer regulatory mechanisms for their microcirculation to handle fluid storage [20]. By contrast, the feet do have such a mechanism which counters filtration and storage of fluid in the interstitial tissue matrix [21]. It is also possible that redistribution of motile interstitial free water, already located in the face, adds to the changes observed [22]. Passive elastic forces exerted by the fasciae and tendons around the eye are similarly likely contributors to MRD2 reduction [23]. On Earth, gravity tends to pull the facial tissues down toward the chin which is resisted by internal tension within the skin, fascia, and muscles and tendons of the face [22]. In microgravity, elastic recoil would reposition facial tissue so that tension is minimal [24–26].

Studies of facial changes in healthy subjects placed in supine position on Earth support a contributing effect of elastic recoil to a decrease in MRD2 [27]. Although the decrease (about 0.5 mm) was less than that observed in microgravity, this can be attributed to still having a gravity vector pulling the tissues toward the back of the head [22,27]. This would also explain the finding of a significant reduction of MRD1 (i.e., the upper eyelid position) whereas there is no change of MRD1 in microgravity [4,7,27]. Elastic recoil has not been considered in a previous study as a mechanism for the MRD changes [27]. Instead, it was speculated that the change in lid position was due to an autonomic response because sympathetic activity decreases in the supine position [28–30]. Mueller's muscle in the upper eyelid and the inferior tarsal muscle in the lower eyelid retract the eyelid under sympathetic innervation [31]. It was thus argued that reduced sympathetic activity due to assuming a supine position would lead to a decrease in both MRD1 and MRD2 [27]. It is unlikely that this sympathetic effect would influence MRD2 in space, because it has been shown that the autonomic challenge of spaceflight is small and represents an orthostatic stress less than that of upright posture on Earth [32].

Possible countermeasure options have been discussed in the previous publication [7].

4.2 Limitations

This study has several limitations. Firstly, the number of suitable images was low because they were not taken with the explicit purpose of being analyzed with regard to periocular facial changes. Public images available were most often taken for publicity purposes and therefore unsuitable because the astronauts were often smiling and the face was angled > 10° to the camera. Secondly, there was no individual standard available for measurements. Instead, an estimate of the white-to-white diameter was used, and so values were less precise. If the white-to-white diameter were an underestimate, it might explain the differences to parabolic flight values where there appears to be a systematic error with the MRD2 being similarly different under the different gravitational conditions but not identical in absolute value (Figure 3). An alternative explanation for the apparent systematic error could be that the parabolic flight participants were on average 9 years younger than the astronauts in the current study [7].

While the findings of this study might have implications for eye health in general and ocular surface health of astronauts in particular, this study was not designed to establish a connection between lower lid position and eye symptoms since data on these symptoms (e.g., through questionnaires like the Ocular Surface Disease Index) were not available. It is, however, known that around 30% of astronauts reported symptoms of dry eye disease [2,33]. Furthermore, a thorough understanding of how the face changes in microgravity is relevant when performing surgery in that region [34].

5 Conclusion and Outlook

A decrease in MRD2 is a consistent and persistent feature in microgravity. This finding provides a basis upon which to investigate the mechanisms and effects of such a decrease in MRD2. However, future studies should implement strict image standardization. The plump pretarsal roll could be examined for volume change using 3D facial analysis. An effect could be on tear film dynamics, and so these could be explored for changes in microgravity as well.

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