Ocular Changes: Spaceflight Versus Bed Rest
By Lynda Seminara
Selected and Reviewed By: Neil M. Bressler, MD, and Deputy Editors
Journal Highlights
JAMA Ophthalmology, February 2020
Download PDF
Optic disc edema is a risk for all astronauts and is most common in lengthy spaceflights. Laurie et al. looked at whether the ocular changes in individuals exposed to a ground-based analogue of weightlessness would be similar to those observed in astronauts. Since intracranial pressure (ICP) is higher when the head is tilted downward than when it is in a supine position, the authors hypothesized that the magnitude of optic disc edema and choroidal thickening may be greater during bed rest than in spaceflight. They found that peripapillary total retinal thickness became greater during bed rest than in spaceflight—and that choroidal thickening developed only during spaceflight.
For this cohort study, the researchers enrolled 11 healthy civilians and 20 astronauts. The civilian participants were examined before, during, and after 30 days of strict bed rest with the head tilted 6 degrees downward, while the astronauts were examined before and during approximately 30 days of spaceflight. Main outcome measures were peripapillary total retinal thickness and peripapillary choroid thickness, determined from optical coherence tomography images.
Participants on bed rest had a greater increase in peripapillary total retinal thickness than did the astronauts (mean difference of 37 μm; p = .005). Conversely, choroidal thickness did not increase with bed rest but did in spaceflight (mean difference of 27 μm; p < .001). The authors posit that the mild long-term elevation in ICP that occurs with bed rest may explain the differences in optic disc edema between the study groups. Gravitational gradients that remain present during bed rest may prevent choroidal thickening; during spaceflight, there are no such gravitational gradients.
Despite the possibility that different mechanisms may underlie optic disc edema development in different conditions, use of the terrestrial model of spaceflight-associated neuro-ocular syndrome will help in developing effective countermeasures for protecting astronauts’ eyes during space missions, said the authors. They emphasized that replication of their findings is needed and that identifying the factors underlying these differences should provide new insight into the mechanisms of optic disc edema in astronauts. Although both groups have long-term mild elevation of ICP, the authors surmise that small differences in ICP between the two settings may dictate the severity of optic disc edema. (Also see related commentary by Ari Shinojima, MD, PhD, in the same issue.)
The original article can be found here.