MRI-based quantification of posterior ocular globe flattening during 60 days of strict 6° head-down tilt bed rest with and without daily centrifugation

Abstract

Spaceflight Associated Neuro-ocular Syndrome (SANS) is associated with acquired optic disc edema, hyperopia, and posterior globe flattening in some astronauts during long-duration spaceflight possibly due to the headward fluid redistribution in microgravity. The goal of this study was to assess whether strict head-down tilt (HDT) bed rest as a spaceflight analog would produce globe flattening and whether centrifugation could prevent these changes. Twenty-four healthy subjects separated into 3 groups underwent 60 days of strict 6° HDT bed rest: one control group with no countermeasure (n=8) and two countermeasure groups exposed to 30 min daily of short-arm centrifugation as a means of artificial gravity (AG), either intermittent (iAG, n=8) or continuous (cAG, n=8). Magnetic resonance images (MRI) were collected at baseline, HDT-day 14, HDT-day 52, and 3 days after bed rest. An automated method was applied to quantify posterior globe volume displacement compared to baseline scans. On average, subjects showed an increasing degree of globe volume displacement with bed rest duration (mean ± SE: 1.41 ± 1.01 mm3 on HDT14 and 4.04 ± 1.19 mm3 on HDT52) that persisted post-bed rest (5.51 ± 1.26 mm3)). Application of 30 min daily AG did not have a significant impact on globe volume displacement (p = 0.42 for cAG and p = 0.93 for iAG compared to control). These results indicate that strict 6° HDT bed rest produced displacement of the posterior globe with a trend of increasing displacement with longer duration which was not prevented by daily 30 min exposure to AG.

Anemia and Spaceflight Associated Neuro-Ocular Syndrome (SANS)

“…12 Increased posterior globe flattening has been a SANS finding that has been replicated in head-down tilt bed-rest studies. 13,14…”

“…12 Increased posterior globe flattening has been a SANS finding that has been replicated in head-down tilt bedrest studies. 13,14 In recent research, Ferguson, et al compared results in patients with increased ICP from IIH with observations in astronauts following long-duration spaceflight to determine the frequency and location of choroidal and retinal folds. The scientists concluded that there are parallels between the choroidal and retinal fold patterns in SANS and IIH, indicating that elevated ICP might play a role in the onset of SANS.…”

Spaceflight-Associated Neuro-Ocular Syndrome and Increased Intracranial Pressure—Are We Closer to Understanding the Relationship?

“…While short-term/intermittent short-radius centrifugation (artificial gravity induction) of participants exposed to HDT reduces muscular deconditioning and orthostatic intolerance, it does not appear to change SANS-like findings. 5,6 We still do not know if ICP is elevated in microgravity, although there are pending efforts that may answer this question. The work presented here 1 gives further evidence that neither ICP nor venous outflow changes are sufficient to explain key findings in SANS such as choroidal folds beyond the optic disc area, and further studies are needed to identify the forces acting on the globe and orbit in microgravity.…”

“…However, these folds were peripapillary and resembled those seen in individuals with terrestrial IIH and not LDSF. A recently published study using magnetic resonance imaging to assess globe deformation in participants exposed to HDT showed that posterior globe flattening was a frequent occurrence and could persist or even progress in the days following cessation of HDT . Again, this globe flattening was identified in the peripapillary region and did not extend to the macula, making it difficult to implicate presumed peripapillary forces in the development of the choroidal folds noted by Ferguson and colleagues .…”

“…A better understanding of the forces that cause SANS is critical to direct the development of SANS countermeasures. While short-term/intermittent short-radius centrifugation (artificial gravity induction) of participants exposed to HDT reduces muscular deconditioning and orthostatic intolerance, it does not appear to change SANS-like findings . We still do not know if ICP is elevated in microgravity, although there are pending efforts that may answer this question.…”