Spaceflight Associated Neuro-Ocular Syndrome

By Andrew G. Lee

Prolonged microgravity exposure during long-duration spaceflight (LDSF) produces unusual physiologic and pathologic neuro-ophthalmic findings in astronauts. These microgravity-associated findings collectively define the Spaceflight Associated Neuro-ocular Syndrome (SANS). In this book, the editors compare and contrast prior published work on SANS by the National Aeronautics and Space Administration’s (NASA) Space Medicine Operations Division with retrospective and prospective studies from other research groups. The book describes the possible mechanisms and potential etiologies for SANS, and provides an update and review on the clinical manifestations of SANS including: unilateral and bilateral optic disc edema, globe flattening, choroidal and retinal folds, hyperopic refractive error shifts, and focal areas of ischemic retina (i.e., cotton wool spots). The ocular imaging findings (e.g., retinal nerve fiber layer, optic disc, and choroidal changes on optical coherence tomography) of SANS is also described, including the intraorbital and intracranial findings on orbital ultrasound and magnetic resonance imaging. The knowledge gaps for in-flight and terrestrial human research including potential countermeasures for future stud is also explored, including reports on the in-flight and terrestrial human and animal research being investigated by NASA and its partners to study SANS both prospectively and longitudinally and in preparation for future long duration manned missions to space including the moon, the asteroid belt, or Mars. We think this is a unique topic and hope that NASA and its research partners continue to study SANS in preparation for future longer duration manned space missions.

• Written in an easy-to-read manner, the book adopts a translational approach and explores the science and the clinical manifestations of Space flight associated neuro-ocular syndrome. It is also multi-disciplinary and suitable for both clinicians and researchers in ophthalmology, neurology, and aerospace medicine interested in SANS
• SANS is a unique space flight disorder that has no terrestrial equivalent. The book involves contributions from international experts across multiple disciplines to tackle the problem of SANS
• Summarizes and reviews the current findings of SANS, including possible mechanisms and potential etiologies, clinical manifestations, current reports on the in-flight and terrestrial human and animal research, and ocular imaging findings

• Language: English
• Publisher: Academic Press
• Release date: Jul 16, 2022
• ISBN: 9780323915250

Book preview

Chapter 1: An introduction to space medicine and the physiological effects of spaceflight on the human body

Joshua Ong ¹ , and Andrew G. Lee ² , ³ , ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹       ¹ University of Pittsburgh School of Medicine, Pittsburgh, PA, United States      ² Center for Space Medicine, Baylor College of Medicine, Houston, TX, United States      ³ Department of Ophthalmology, Blanton Eye Institute, Houston Methodist Hospital, Houston, TX, United States      ⁴ The Houston Methodist Research Institute, Houston Methodist Hospital, Houston, TX, United States      ⁵ Departments of Ophthalmology, Neurology, and Neurosurgery, Weill Cornell Medicine, New York, NY, United States      ⁶ Department of Ophthalmology, University of Texas Medical Branch, Galveston, TX, United States      ⁷ University of Texas MD Anderson Cancer Center, Houston, TX, United States      ⁸ Texas A and M College of Medicine, Bryan, TX, United States      ⁹ Department of Ophthalmology, The University of Iowa Hospitals and Clinics, Iowa City, IA, United States

Abstract

Long-duration spaceflight introduces the human body to various risks and physiological changes, including increased radiation exposure, bone density loss, skeletal muscle atrophy, and vision changes. The field of space medicine seeks to further understand and potentially mitigate these risks to protect astronaut health during spaceflight. Protection and mitigation of these consequences on the human body may also help to uphold mission performance for prolonged space missions. In this chapter, we discuss some of the various hazards that have been identified in the spaceflight environment, as well as several countermeasure strategies that have been tested. This chapter serves as an introduction to the field of space medicine and a primer for the following textbook chapters on Spaceflight Associated Neuro-Ocular Syndrome.

Keywords

Microgravity; Physiology; Space medicine; Spaceflight; Spaceflight Associated Neuro-Ocular Syndrome

Introduction

In 1961, the first human traveled into space, literally and figuratively opening the doors to the field of space medicine. Our understanding of the human body's response to short- and long-duration spaceflight (LDSF) has evolved over time, and human beings face unique challenges and hazards from LDSF. Longitudinal studies of astronaut health have shown that the microgravity environment impacts nearly every organ system including the eyes, muscles, bones, kidneys, and brain (Gundel et al., 1997; Lee et al., 2020; Patel et al., 2020; Pavlakou et al., 2018; Stein, 2013).

In addition to the effects of microgravity, astronauts are also exposed to elevated levels of radiation and carbon dioxide, leading to a multitude of potential side effects (Patel et al., 2020; Chancellor et al., 2014). Many of these exposures in the space exposome may produce severe consequences on the human body for future manned interplanetary missions (Fig. 1.1). The National Aeronautics and Space Administration (NASA) in the United States has established the Human Research Program (HRP) to comprehensively identify, assess, and mitigate these potential hazards during LDSF (Patel et al., 2020). In this chapter, we introduce the physiological changes observed during LDSF; discuss the serious potential implications on human beings; and describe a primer for the following chapters in this textbook on Spaceflight Associated Neuro-Ocular Syndrome (SANS).

Health hazards during spaceflight

The NASA HRP has assigned human health risks with a rating scale termed likelihood and consequence (LxC). LxCs ratings are assigned based on the impact the human health risk has on the quality of life and impact on mission performance (Patel et al., 2020). Several risks may have lower LxC ratings due to established countermeasures or carry low occurrence/consequences during spaceflight. Human health risks that have elevated LxC ratings are prioritized and require implantation of countermeasure strategies, particularly for long-duration missions. For example, SANS carries an elevated LxC rating; thus, mitigation strategies for this neuroophthalmic phenomenon are required for future planetary missions.

Figure 1.1  Illustration overviewing the documented effects of spaceflight on the human body including vision impairment, behavioral changes, muscle loss, and increased exposure to radiation.

Radiation

One of the most consequential systemic health considerations during LDSF is increased exposure to radiation. Beyond low earth orbit, astronauts are exposed to multiple types of ionizing radiation that can cause numerous harmful effects. The most common radiation that astronauts will experience beyond low earth orbit will be galactic cosmic radiation (GCR). Astronauts are also exposed to solar particle events (SPEs), which are unpredictable events of high-energy protons and ions from the sun (Chancellor et al., 2014). This potentially toxic combination of radiation from GCR and SPEs can produce various and complex effects to the human body (e.g., genetic mutations, tissue degeneration, skin burns, expedited cataract formation, immune system dysregulation, and cardiovascular disorders) (Chancellor et al., 2014; Chylack et al., 2009; Patel, 2020; Patel et al., 2020). However, the average amount of time an astronaut spends in space (e.g. onboard the International Space Station (ISS)) is anticipated to be significantly less than planetary travel. Therefore, terrestrial studies have been conducted to further understand the longitudinal effects and potential countermeasures against GCR and SPEs. The National Council on Radiation Protection and Measurements has recommended the use of specific settings and ions during terrestrial radiobiology experiments to mimic the radiotoxic environment during spaceflight (Chancellor et al., 2014). Several countermeasures have been suggested, ranging from identifying composite materials for spacecraft shielding to targeting biomolecular oxidative pathways (Kennedy, 2014; Naito et al., 2020; Patel, 2020). While radiation remains a high risk for human spaceflight, extensive and diverse research is being conducted to attenuate the severe physiologic complications that may arise from prolonged GCR exposure (Cucinotta et al., 2013; Kennedy, 2014; Montesinos et al., 2021). Whether some of the findings in SANS (e.g., cotton wool patches) are related to radiation exposure rather than microgravity remains ill