Head-Down Tilt Bed Rest Studies as a Terrestrial Analog for Spaceflight Associated Neuro-Ocular Syndrome.

Heather E Moss,Joshua Ong,Andrew G Lee

doi:10.3389/fneur.2021.648958

Abstract

Astronauts who undergo prolonged periods of spaceflight may develop a unique constellation of neuro-ocular findings termed Spaceflight Associated Neuro-Ocular Syndrome (SANS). SANS is a disorder that is unique to spaceflight and has no terrestrial equivalent. The prevalence of SANS increases with increasing spaceflight duration and although there have been residual, structural, ocular changes noted, no irreversible or permanent visual loss has occurred after SANS, with the longest spaceflight to date being 14 months. These microgravity-induced findings are being actively investigated by the United States' National Aeronautics Space Administration (NASA) and SANS is a potential obstacle to future longer duration, manned, deep space flight missions. The pathophysiology of SANS remains incompletely understood but continues to be a subject of intense study by NASA and others. The study of SANS is of course partially limited by the small sample size of humans undergoing spaceflight. Therefore, identifying a terrestrial experimental model of SANS is imperative to facilitate its study and for testing of preventative measures and treatments. Head-down tilt bed rest (HDTBR) on Earth has emerged as one promising possibility. In this paper, we review the HDTBR as an analog for SANS pathogenesis; the clinical and imaging overlap between SANS and HDTBR studies; and potential SANS countermeasures that have been or could be tested with HDTBR.

Highlights

During long-duration spaceflight (LDSF) missions, astronauts undergo a number of microgravity-induced physiological changes such as skeletal muscle atrophy, decreased bone mass, and height change [1,2,3]
Understanding the pathogenesis; identifying those at increased risk; and mitigating the effects of Spaceflight Associated Neuro-Ocular Syndrome (SANS) are of critical importance so that astronauts can complete LDSF missions safely
Terrestrial analogs, that is, experimental models of SANS that can be completed on Earth, are an important tool with which to achieve this and Head-Down Tilt Bed Rest (HDTBR) is a promising one as, with the right parameters, it is associated with many of the ophthalmic findings seen in SANS

Summary

INTRODUCTION

During long-duration spaceflight (LDSF) missions, astronauts undergo a number of microgravity-induced physiological changes such as skeletal muscle atrophy, decreased bone mass, and height change [1,2,3]. Supporting this hypothesis is a cohort study of twenty-two astronauts with post-flight MRI scans demonstrating a 0.80 ± 0.74 mm (average ± SD) increase in optic nerve length from globe to chiasm compared to pre-flight scans This was associated with forward displacement of the optic nerve head, which was related to duration of spaceflight and clinical signs of SANS [42]. Choroid thickness showed the opposite pattern; there was a larger increase in astronauts compared to the strict HDTBR subjects with the average difference being 27 μm Another study furthered this investigation of the mild hypercapnic environment with HDBTR and observed that individuals who developed SANS features from HDTBR demonstrated elevated reliance on visuals cues when tested on cognitive performance compared to non-SANS HDTBR individuals [54]. A postural study on ambulatory neurosurgical patients with continuous ICP monitoring observed an increase in ICP during postural change from standing or supine to 10◦ & 20◦ head-down tilt [4, 59]. 6◦, 60-min head-down

LIMITATIONS

Findings

DISCUSSION AND FUTURE