The Effect of Zero Gravity on Eyelid and Brow Position.

Abstract

To describe the effect of spaceflight on upper eyelid and brow position. Images of astronauts on earth and in space were collected from a National Aeronautics and Space Administration image library. For each astronaut, front facing portraits captured within a 5-year period were obtained. At least 6 photos were collected per astronaut, 3 taken on earth, and 3 in zero gravity. Measurements from the center of the pupil to the upper eyelid margin (MRD1) and center of the pupil to the inferior eyebrow margin [pupil-to-brow (PTB)] were performed digitally for each image and averaged for each condition and each individual. Differences in mean MRD1 and PTB were assessed to illustrate the effects of zero gravity on eyelid and eyebrow position respectively. Linear-mixed effects modeling, analysis of variance and post hoc power analysis were performed. One hundred and twelve photographs (55 earth, 57 space) from 18 astronauts (4 women, 14 men; mean age 45.9 years) were retrieved. Zero gravity resulted in a mean PTB increase of 3.7 mm (p < 0.01) but did not result in significant change in MRD1 (p = 0.21). Post hoc power analysis of the measured MRD1 difference was performed by simulation. At an MRD1 difference of 0.5 mm, the probability of a type II error is 2.6%, equivalent to a beta power of 97.4%. Thus, we can be 97.4% confident that a difference in MRD of 0.5 mm or greater would be detected. While spaceflight led to significant elevation of PTB, it did not consistently alter MRD1. As such, the brow and eyelid appear to be independently regulated with respect to gravitational changes.