Abstract
BACKGROUND:
“Spaceflight associated neuro-ocular syndrome” (SANS) represents a challenging health condition in modern space medicine. Forty-eight percent of astronauts are diagnosed with SANS after long-term space missions. The pathophysiological mechanism seems to be multifactorial, and yet remains unknown. In this proof-of-concept study we plan to investigate retinal microcirculatory changes in weightlessness and aim to identify their role in the development of SANS.
METHODS AND DESIGN:
Healthy individuals will take part in a parabolic flight campaign, which recreates fractioned total weightlessness periods. The airplane is specifically equipped, and designed for the execution of parabolic flight maneuvers and scientific research in microgravity. Retinal microcirculation will be assessed with a modified fundus camera, which allows dynamic vessel analysis. We will additionally measure intra-ocular pressure and hemodynamic changes during each phase of the flight. Blood samples will be analyzed at baseline, one hour and 24 hours after exposure to weightlessness.
CONCLUSIONS:
This pilot study aims to investigate the feasibility of retinal microcirculation assessment during varying gravity. Results of this study may generate insights whether venous stasis in the eye, surrogated by the dilatation of retinal vessels and increase in intraocular pressure as signs of venous insufficiency, may potentially contribute to the development of SANS.
Keywords
Abbreviations
Dynamic Vessel Analyzer Head Down tilt Bed Rest National Aeronautics and Space Administration Spaceflight Associated Neuro-ocular Syndrome
Introduction
Space environmental conditions, such as microgravity, extreme temperatures, and radiation exposure present a significant challenge to the human body. The National Aeronautics and Space Administration (NASA) Artemis aims to send humans to the moon and farther away in space in 2024. Hence, future space missions will include a significantly longer stay in space with very limited access to medical care. Next to governmental programs, commercial spaceflights evolve rapidly, potentially exposing older adults with increased cardiovascular risk to microgravity conditions. Weightlessness causes many adaptions and alterations in the human body including muscular atrophy or impairment of the immune system [1–3]. Moreover, effects of microgravity on the circulatory system have been described with reduced blood pressure and increased cardiac output [4]. Dynamic changes of heart failure biomarkers and circulating micro-RNA have been measured after parabolic flight maneuvers which simulate fractioned total weightlessness periods [5, 6]. In zero-gravity, body fluids shift to the upper body and the head. The cephalad fluid overload leads to congestion in the neck veins, higher intracranial pressure and venous statis, and potential alterations in the microcirculatory system. Additionally, weightlessness induces changes in endothelial function, blood vessel drainage alterations [7], compartmentalization, and blood rheology which may be associated with an increased risk for thrombus formation. Recently, blood rheology and hypercoagulability got into the focus of space research as an obstructive left internal jugular venous thrombosis was found in an astronaut during a spaceflight misson [8, 9], comparabe to clinical settings, such as found in COVID-19 or essential thrombocytosis [10, 11]. Amongst others (i.e., enzymatic deficiencies in the cyanocobalamin- and folate-dependent-1-carbon pathways), the altered venous pressure to volume ratio is suspected to mediate spaceflight associated neuro-ocular syndrome (SANS) [7, 12–14]. SANS, previously known as “visual impairment and intracranial pressure syndrome”, constitutes a series of symptoms and findings, that have been detected in astronauts, especially after long lasting spaceflight missions and expeditions [15, 16]. About 23% of astronauts were diagnosed with SANS after short term flight missions; 48% experienced SANS after long-lasting flight missions. Lee et al. proposed to define SANS [7] by the following criteria: a) unilateral and bilateral optic disc edema, b) globe flattening, c) choroidal and retinal folds, d) hyperopic refractive error shifts (>0,75 diopters), and e) focal areas of ischemic retina which have not been present before the space mission. Clinical symptoms range from impaired near-visual acuity, and scotoma to headaches [16].
Studies investigating hemodynamic changes in the microcirculatory system in microgravity are rare: we successfully analyzed sublingual microcirculation via intravital microscopy in a previous parabolic flight campaign [4], and new technological developments now allow the examination of the retinal fundus.
Hence, the here presented study outline describes the rational and design of the REMOTE trial which will investigate retinal microcirculation in weightlessness.
Methods
Study outline, primary research question, and statistical analysis
This exploratory, pilot study will examine the retinal microcirculation, the intraocular pressure, and retinal vessel diameters of nine certified airworthiness individuals during a parabolic flight campaign, which recreates weightlessness conditions. This investigation is designed to generate first insights whether venous stasis in the eye, surrogated by the dilatation of retinal vessels and intraocular pressure, rises as a sign of venous pooling, potentially contributing to the development of SANS.
All participants will provide written informed consent prior to study inclusion. The study will be conducted in accordance with the Declaration of Helsinki and the protocol was approved by the Ethics Committee of the Medical Faculty of the University Hospital Duesseldorf, Germany (approval date: 29th of July 2020; project identification code: 2020-929).
Inclusion and exclusion criteria
Parabolic flight maneuvers
The microcirculation experiments will take place onboard of an Airbus A310 aircraft, owned by the French company Novespace (Bordeaux, France). The A310 Zero G is specifically equipped, and designed for the execution of parabolic flight maneuvers and scientific research in microgravity.
To perform these parabolic flight maneuvers, three jet pilots will simultaneously operate in the aircraft cockpit. Each pilot is assigned to a specific task and coordinates one of the three flight dimensions (speed, vertical movement, and horizontal movement). To perform a parabolic flight maneuver, the aircraft will leave its horizontal flight path and climbs up steeply (“pull-up” phase) to follow a parabolic trajectory thereafter, as the pilots throttle back the propulsive force of the engines. This will result in a free fall, during which the passengers experience nearly zero gravity (0 g) onboard, lasting for approximately 22 seconds. The phase of microgravity will be followed by a “pull-out” phase, during which the pilots steer the aircraft back to its horizontal flight path. The phases of “pull-up” and “pull-out” comprise states of hypergravity (up to 1.5 g–1.8 g). As the duration of zero-gravity and hypergravity phases differ slightly between each parabola, a protocol of the exact durations of the phases as well as the exact G-forces will be provided by the organizers of the parabolic flight campaign. An example of a standardized parabola with descriptions of altitude, speed, gravity conditions, and gravity duration can be found in Fig. 1.
A parabolic flight day typically lasts for approximately three hours and includes 31 parabolic flight maneuvers. The first parabola on each flight day will be a test run for all individuals onboard. Overall, the parabolic flight campaign will cover up to four flight days within a period of two weeks. We plan to participate in all available time slots. For each flight day, test subjects will be assigned to ten parabolic flight maneuvers. During each flight maneuvers, investigations will be performed at all gravity phases.
Example of a standardized parabola with descriptions of altitude, speed, gravity conditions, and gravity duration.
Retinal microcirculation will be measured using a dynamic vessel analyzer (DVA) manufactured by Imedos® (iMEDOS Health GmbH, Jena, Germany). The DVA is a modified fundus camera, which analyzes the retinal vessel network and its diameters over time after adequate vessels have been identified and markey by two independent investigators (Fig. 2). The DVA will measure the diameters of both marked vessels each 0.04 seconds simultaneously. The retinal vessel analysis is a unique, radiation-free and non-invasive method for the evaluation of endothelial dysfunction and microcirculation. It can measure flicker light-induced vasodilatation of the examined vessels, likely driven by nitric oxide signaling pathways.[17, 18] Hence, the DVA retinal scanner enables the in vivo visualization of the retinal capillary network, which can provide indirect information for potential microcirculatory alterations in the eye during the state of microgravity.
Example of a retinal fundus examination and a regular result of retinal vessel assessment over time for a venule (marked by V2; upper panel) and an artery (marked by A1; lower panel) using the DVA.
The DVA will be attached to a computer unit and a monitor for direct visualization of test results. In order to meet all required safety guidelines, the DVA is going to be installed within a rack, which is equipped and securely attached to the airplane with various screws, hinges and cushions (Figs. 3 4). As we expect flight-related vibrations to complicate the DVA measurements and, moreover, to maximize safety, an individual cushion will be designed and produced for each test subject. The DVA is certified and corresponds with the European guidelines for medical devices and is approved for measurements in humans.
Bird’s eye view of the complete rack, the DVA, and the participant’s location as planned. The study participant is marked by a red dot.
First, baseline measurements will be conducted in the morning hours (approximately 8 a.m. Central European Time). Those include the heart frequency, blood pressure levels, oxygen saturation, intraocular pressure, blood samplings (amongst others for rheological markers) and baseline DVA measurements with flicker light induction. On the flight day, scopolamine will be injected by a flight physician to avoid motion sickness in a weight-adapted dose. Afterwards, hemodynamic parameters will be collected once again to rule out any systemic hemodynamic effect of scopolamine. Thereafter, 3–5 drops (i.e. 0,6–1,0mg) of the mydriatic agent tropicamide will be applied to each eye, which will be evaluated for adequate mydriasis. This offers sufficient access to the retinal fundus and enables reliable DVA measurements.
Picture of the corpus of the rack, the DVA, and the monitor for visualization aboard the airplane.
During the parabolic flights, retinal microcirculation will be assessed in all phases of the previously described flight maneuver without flicker light induction: in the steady-state (approximately 1 g), states of hypergravity (“pull-up” and “pull-out”; approximately 1.5 g–1.8 g), and in microgravity (approximately 0 g). To gain adequate images and to offer stable image conditions and a representative retinal area, the DVA will record and store all collected data and video material continuously. Every data set of each participant will additionally include hemodynamic parameters like blood pressure, oxygen saturation, heart rate and intraocular pressure during all phases of the flight maneuvers. Each parameter will be documented once in each flight phase of every single parabola. To maximize safety and to avoid any possible damage to the eyes of the participants, intraocular pressure will be measured with a tonometer that is going to be attached to a modified helmet (Fig. 5). The examined eye will be protected by an ultra-thin contact lens (0,07 mm; ACUVUE 1-Day MOIST). To rule out circadian interferences, blood samples will be collected again one hour and 24 hours post landing of the aircraft. Moreover, DVA measurements with flicker light induction will be performed after landing.
Picture of the prototype of a modified helmet for securing the tonometer during the flight maneuvers.
The here introduced proof-of-concept study aims to evaluate potential retinal microcirculatory alterations during weightlessness. We will use a widely modified fundus camera (i.e. dynamic vessel analyzer), which allows us to detect retinal vessel adaptions in vivo, and in real-time. Next to intravital microscopy, we will evaluate intraocular pressure, perform hemodynamic measurements of the whole circulatory system, and test blood rheology markers. It is already known, that intraocular pressure is increased in microgravity conditions [19, 20]. Although the tonometry assessment of intraocular pressure appears not as reliable marker for the development of SANS [21], we decided to evaluate the intraocular pressure additionally to the retinal vessel analysis. The combination of both investigations may offer additional information on hemodynamic adaption processes and acute retino-orbital changes.
In a first step, we want to investigate the feasibility of our research design and set-up under varying gravity conditions during parabolic flight maneuvers. In a second step, we hope to gather knowledge about the hemodynamic adaption processes during microgravity, specifically focusing on possible microcirculatory alterations in the eye.
As described previously, SANS is a frequent and challenging health condition in modern space medicine [15, 16], and fluid shifts to the upper part of the body and venous congestion may play a critical role in mediating SANS [7]. However, the function and autoregulation mechanism of the eyes microcirculatory system in weightlessness has not been investigated yet. It remains unknown whether venous stasis in the eye itself could play a key role in the development of SANS. Our planned in vivo, real-time investigation of retinal small vessels diameters, and their changes over time in different phases of gravity may contribute significantly to the understanding of SANS.
If we find conclusive results supporting our hypothesis, a comprehensive evaluation of retinal microcirculation with the DVA could serve as an additional diagnostic tool to detect SANS before, and early during flight missions. About 23–48% of all astronauts are diagnosed with SANS after returning from spaceflight missions, highlighting the relevance of this health issue [12]. Currently, magnetic resonance imaging, optical coherence tomography, and orbital ultrasound are used to assess the astronauts’ risk for developing SANS prior to space missions [16]. So far, the diagnostic capability on a space mission itself is extremely limited by equipment portability and primarily based on findings using orbital ultrasound and optical coherence tomography [7, 16]. The retinal DVA is relatively small, lightweight and an easy-to-use device, which could serve as an additional diagnostic tool during future space missions (e.g., onboard the International Space Station). It may offer the opportunity to start specific treatment for SANS early (i.e., application of Acetazolamide 500mg and 250mg for six and two weeks which has not been tested in space yet), next to general management during the mission such as proper nutritional supplementation and targeted exercise regimes mimicking earth-like gravitational environment in space.
Limitations
This proof-of-concept study will only have an exploratory character with a limited number of participants. During parabolic flights, only short, fractioned total weightlessness periods can be simulated which may influence the results. The time in microgravity could be too short to induce relevant retinal changes. We will also face technical limitations of the DVA: the assessment of endothelial function and microcirculation of retinal vessels is mainly based on the evaluation of vessel dilatation after flicker light induction [17, 22–24]. As only approximately 20 seconds of microgravity will be available during each parabola, we will not be able to use flicker light during the flight and, therefore, focus on the change in vessel diameters over time.
An alternative approach could be the evaluation of retinal microcirculation in a head down tilt bed rest (HDBR) study. Although the microgravity environment in space is unique, and can only partly be mimicked by parabolic flights or head down tilt studies, HDBR studies revealed similar optical coherence tomography findings as observed after spaceflights [25].
Footnotes
Acknowledgments
None.
Funding
Institutional support has been received by the German Aerospace centre (DLR) and the German Federal Ministry for Economic Affairs and Energy (#50WB1714 and #50WB1914).