Comparison of Dural Venous Sinus Volumes Before and After Flight in Astronauts With and Without Spaceflight-Associated Neuro-Ocular Syndrome

Prevention of Cerebrovascular Emergencies in Spaceflight: A Review and a Proposal for Enhanced Medical Screening Guidelines

Purpose of Review

A growing number of opportunities for paying customers to travel to space are becoming available. Preflight medical screening of these potential private astronauts will likely be performed by local physicians, with referral to specialists in aerospace medicine as required for more in-depth evaluation before flight qualification. Neurologic concerns contribute a portion of the potential medical risks for these private astronauts, especially with the participation of more diversified crews than traditional governmental astronauts. The objective of this article was to review the current knowledge base concerning cerebrovascular adaptation to spaceflight to inform risk factor assessment for flight-associated cerebrovascular emergencies by the neurologic community when performing initial medical screening of potential private astronauts.

Recent Findings

A review of published human spaceflight studies and medical guidelines regarding cerebrovascular risks for spaceflight was conducted. Most of the available literature describes cohorts of a small number of astronauts undergoing spaceflight missions of various flight profiles. While there are gaps in the literature, cerebrovascular adaptation to spaceflight occurs, which may alter the medical risk profile in susceptible individuals. The occurrence of an inflight cerebrovascular emergency could have devastating consequences; therefore, additional screening tests may be required, based on risk level and mission profile, in assessing the more diverse commercial spaceflight population expected over the next decade.

Summary

With increasing interest in commercial space tourism among diverse participant populations, the stroke risk in microgravity/reduced gravity environments is unknown. Furthermore, stresses of rocket ascent/descent, abnormal fluid dynamics in microgravity, altered atmospheric conditions, and other unknown occupational hazards add additional complexity. Because inflight emergency management protocols have yet to be developed, the most effective tool to ensure spaceflight participant safety is comprehensive preflight preventative screenings. Determining neurologic risk factors is critical for developing evidence-based guidelines for preventative measures and treatment protocols in the future.

Spaceflight Associated Neuro-ocular Syndrome (SANS) and its countermeasures

Astronauts can develop a distinct collection of neuro-ophthalmic findings during long duration spaceflight, collectively known as Spaceflight Associated Neuro-ocular Syndrome (SANS). These clinical characteristics include optic disc edema, hyperopic refractive shifts, globe flattening, and chorioretinal folds, which may pose a health risk for future space exploration. Obtaining knowledge of SANS and countermeasures for its prevention is crucial for upcoming crewed space missions and warrants a multidisciplinary approach. This review examines the potential causes and countermeasures of SANS, including space anticipation glasses, lower body negative pressure, venoconstrictive thigh cuffs, impedance threshold devices, translaminar pressure gradient modulation, centrifugation, artificial gravity, pharmaceuticals, and precision nutritional supplementation. This paper highlights future research directions for understanding the genetic, anthropometric, behavioral, and environmental susceptibilities to SANS as well as how to use terrestrial analogs for testing future mitigation strategies.

The optic nerve in spaceflight: novel concepts in the pathogenesis of optic disc edema in microgravity

PURPOSE OF REVIEW

Spaceflight-associated neuro-ocular syndrome (SANS) encompasses a unique constellation of neuro-ocular findings in astronauts, including optic disc edema (ODE), globe flattening, chorioretinal folds, and hyperopic refractive shift. Although there are numerous neuro-ocular findings in SANS, the purpose of this review is to describe the novel, emerging concepts of the pathogenesis for the ODE specifically in SANS.

RECENT FINDINGS

While the initial hypotheses on the pathogenesis of ODE in SANS focused on possible elevated intracranial pressures (i.e., papilledema), the most prominent current hypothesis is microgravity-induced cephalad fluid shift. More recent studies however suggest that the pathogenesis of the ODE in SANS is likely multifactorial including possible underlying metabolic and genetic components.

SUMMARY

We review the literature on ODE in SANS including recent work integrating the complex physiologic interactions of microgravity-induced disruption in intracerebral and intraocular fluid dynamics, vascular congestion, cellular stress responses, and genetic predisposition. We believe that the development of ODE in SANS is likely multifactorial in origin, and further understanding of the mechanical, cellular, metabolic, and genetic components is of utmost importance to develop future countermeasures in preparation for possible future crewed missions to the moon, the asteroid belt, and Mars.

Hypothesis on the outflow of optic nerve cerebrospinal fluid in spaceflight associated neuro ocular syndrome

Spaceflight-associated neuro-ocular syndrome (SANS) has been well documented in astronauts. However, its pathogenesis is not fully understood. New findings indicate the impaired outflow of the optic nerve cerebrospinal fluid may participate or contribute to some changes in SANS. In this perspective, we generated a hypothesis that the outflow of cerebrospinal fluid through the optic nerve sheath may be impaired under micro-gravity and then may potentially lead to SANS-related alterations.

Ocular Biomechanical Responses to Long-Duration Spaceflight

Objective: To assess the impact of microgravity exposure on ocular rigidity (OR), intraocular pressure (IOP), and ocular pulse amplitude (OPA) following long-term space missions. OR was evaluated using optical coherence tomography (OCT) and deep learning-based choroid segmentation. IOP and OPA were measured with the PASCAL Dynamic Contour Tonometer (DCT). Results: The study included 26 eyes from 13 crew members who spent 157 to 186 days on the International Space Station. Post-mission results showed a 25% decrease in OPA (p < 0.005), an 11% decrease in IOP from 16.0 mmHg to 14.2 mmHg (p = 0.04), and a 33% reduction in OR (p = 0.04). No significant differences were observed between novice and experienced astronauts. Conclusions: These findings reveal previously unknown effects of microgravity on the eye's mechanical properties, contributing to a deeper understanding of Spaceflight-Associated Neuro-ocular Syndrome (SANS). Long-term space missions significantly alter ocular biomechanics and have the potential to become biomarkers of disease progression.

Effects of spaceflight on the brain

The number of long duration human spaceflights has increased substantially over the past 15 years, leading to the discovery of numerous effects on the CNS. Microgravity results in headward fluid shifts, ventricular expansion, an upward shift of the brain within the skull, and remodelling of grey and white matter. The fluid changes are correlated with changes to perivascular space and spaceflight associated neuro-ocular syndrome. Microgravity alters the vestibular processing of head tilt and results in reduced tactile and proprioceptive inputs during spaceflight. Sensory adaptation is reflected in postflight effects, evident as transient sensorimotor impairment. Another major concern is that galactic cosmic radiation, which spacefarers will be exposed to when going beyond the magnetosphere around Earth, might have a negative effect on CNS function. Research with rodents points to the potential disruptive effects of space radiation on blood-brain barrier integrity and brain structures. More work is needed to understand and mitigate these effects on the CNS before humans travel to Mars, as the flight durations will be longer than anyone has previously experienced.

A perspective on the evidence for glymphatic obstruction in spaceflight associated neuro-ocular syndrome and fatigue

Spaceflight associated neuro-ocular syndrome (SANS) alters the vision of astronauts during long-duration spaceflights. Previously, the current authors have discussed the similarities and differences between SANS and idiopathic intracranial hypertension to try to elucidate a possible pathophysiology. Recently, a theory has been advanced that SANS may occur secondary to failure of the glymphatic system caused by venous dilatation within the brain and optic nerves. There is recent evidence to suggest glymphatic obstruction occurs in childhood hydrocephalus, multiple sclerosis and syringomyelia due to venous outflow dilatation similar to that proposed in SANS. The purpose of the current paper is to discuss the similarities and differences between the known CSF and venous pathophysiology in SANS with these other terrestrial diseases, to see if they can shed any further light on the underlying cause of this microgravity-induced disease.

Microglial activation in spaceflight and microgravity: potential risk of cognitive dysfunction and poor neural health

Due to the increased crewed spaceflights in recent years, it is vital to understand how the space environment affects human health. A lack of gravitational force is known to risk multiple physiological functions of astronauts, particularly damage to the central nervous system (CNS). As innate immune cells of the CNS, microglia can transition from a quiescent state to a pathological state, releasing pro-inflammatory cytokines that contribute to neuroinflammation. There are reports indicating that microglia can be activated by simulating microgravity or exposure to galactic cosmic rays (GCR). Consequently, microglia may play a role in the development of neuroinflammation during spaceflight. Prolonged spaceflight sessions raise concerns about the chronic activation of microglia, which could give rise to various neurological disorders, posing concealed risks to the neural health of astronauts. This review summarizes the risks associated with neural health owing to microglial activation and explores the stressors that trigger microglial activation in the space environment. These stressors include GCR, microgravity, and exposure to isolation and stress. Of particular focus is the activation of microglia under microgravity conditions, along with the proposal of a potential mechanism.

Modelling large scale artery haemodynamics from the heart to the eye in response to simulated microgravity

We investigated variations in haemodynamics in response to simulated microgravity across a semi-subject-specific three-dimensional (3D) continuous arterial network connecting the heart to the eye using computational fluid dynamics (CFD) simulations. Using this model we simulated pulsatile blood flow in an upright Earth gravity case and a simulated microgravity case. Under simulated microgravity, regional time-averaged wall shear stress (TAWSS) increased and oscillatory shear index (OSI) decreased in upper body arteries, whilst the opposite was observed in the lower body. Between cases, uniform changes in TAWSS and OSI were found in the retina across diameters. This work demonstrates that 3D CFD simulations can be performed across continuously connected networks of small and large arteries. Simulated results exhibited similarities to low dimensional spaceflight simulations and measured data-specifically that blood flow and shear stress decrease towards the lower limbs and increase towards the cerebrovasculature and eyes in response to simulated microgravity, relative to an upright position in Earth gravity.

Insights into spaceflight-associated neuro-ocular syndrome with review of intraocular and orbital findings

PURPOSE OF REVIEW

Spaceflight-associated neuro-ocular syndrome (SANS) remains a phenomenological term, and advances in ophthalmic imaging as well as new insights from ground-based experiments have given support to new theories of how SANS develops and what may be done to counter it.

RECENT FINDINGS

SANS has been postulated to arise from elevated intracranial pressure (ICP) during long-duration spaceflight (LDSF). However, recent work has shown that acute microgravity exposure does not increase ICP, and the effect of cephalad fluid shifts on ICP in microgravity remain unknown. In addition, structural imaging of the retina and optic nerve show changes after LDSF that are distinct from findings in terrestrial patients with elevated ICP. Since astronauts have not reported symptoms that would be expected with chronic ICP elevation, new theories that orbital and/or intracranial venous pressure may be the primary contributors to the development of SANS.

SUMMARY

Research has been filling knowledge gaps that exist regarding the cause(s) of SANS, and these advances are crucial steps in the effort to design countermeasures that will be required before human deep space exploration missions can be undertaken.

Longitudinal Changes in Cerebral Perfusion, Perivascular Space Volume, and Ventricular Volume in a Healthy Cohort Undergoing a Spaceflight Analog

BACKGROUND AND PURPOSE

A global decrease in brain perfusion has recently been reported during exposure to a ground-based spaceflight analog. Considering that CSF and glymphatic flow are hypothesized to be propelled by arterial pulsations, it is unknown whether a change in perfusion would impact these CSF compartments. The aim of the current study was to evaluate the relationship among changes in cerebral perfusion, ventricular volume, and perivascular space volume before, during, and after a spaceflight analog.

MATERIALS AND METHODS

Eleven healthy participants underwent 30 days of bed rest at 6° head-down tilt with 0.5% atmospheric CO2 as a spaceflight analog. For each participant, 6 MR imaging brain scans, including perfusion and anatomic-weighted T1 sequences, were obtained before, during, and after the analog period. Global perfusion, ventricular volume, and perivascular space volume time courses were constructed and evaluated with repeated measures ANOVAs.

RESULTS

Global perfusion followed a divergent time trajectory from ventricular and perivascular space volume, with perfusion decreasing during the analog, whereas ventricular and perivascular space volume increased (P < .001). These patterns subsequently reversed during the 2-week recovery period.

CONCLUSIONS

The patterns of change in brain physiology observed in healthy participants suggest a relationship between cerebral perfusion and CSF homeostasis. Further study is warranted to determine whether a causal relationship exists and whether similar neurophysiologic responses occur during spaceflight.

External to internal cranial perfusion shifts during simulated weightlessness: Results from a randomized cross-over trial

The exact pathophysiology of the spaceflight-associated neuro-ocular syndrome (SANS) has so far not been completely elucidated. In this study we assessed the effect of acute head-down tilt position on the mean flow of the intra- and extracranial vessels. Our results suggest a shift from the external to the internal system that might play an important role in the pathomechanism of SANS.

Identification of a novel path for cerebrospinal fluid (CSF) drainage of the human brain

How cerebrospinal fluid (CSF) drains from the human brain is of paramount importance to cerebral health and physiology. Obstructed CSF drainage results in increased intra-cranial pressure and a predictable cascade of events including dilated cerebral ventricles and ultimately cell death. The current and accepted model of CSF drainage in humans suggests CSF drains from the subarachnoid space into the sagittal sinus vein. Here we identify a new structure in the sagittal sinus of the human brain by anatomic cadaver dissection. The CSF canalicular system is a series of channels on either side of the sagittal sinus vein that communicate with subarachnoid cerebrospinal fluid via Virchow-Robin spaces. Fluorescent injection confirms that these channels are patent and that flow occurs independent of the venous system. Fluoroscopy identified flow from the sagittal sinus to the cranial base. We verify our previous identification of CSF channels in the neck that travel from the cranial base to the subclavian vein. Together, this information suggests a novel path for CSF drainage of the human brain that may represent the primary route for CSF recirculation. These findings have implications for basic anatomy, surgery, and neuroscience, and highlight the continued importance of gross anatomy to medical research and discovery.

Future research directions to identify risks and mitigation strategies for neurostructural, ocular, and behavioral changes induced by human spaceflight: A NASA-ESA expert group consensus report

A team of experts on the effects of the spaceflight environment on the brain and eye (SANS: Spaceflight-Associated Neuro-ocular Syndrome) was convened by NASA and ESA to (1) review spaceflight-associated structural and functional changes of the human brain and eye, and any interactions between the two; and (2) identify critical future research directions in this area to help characterize the risk and identify possible countermeasures and strategies to mitigate the spaceflight-induced brain and eye alterations. The experts identified 14 critical future research directions that would substantially advance our knowledge of the effects of spending prolonged periods of time in the spaceflight environment on SANS, as well as brain structure and function. They used a paired comparison approach to rank the relative importance of these 14 recommendations, which are discussed in detail in the main report and are summarized briefly below.

A perspective on spaceflight associated neuro-ocular syndrome causation secondary to elevated venous sinus pressure

Spaceflight associated neuro-ocular syndrome (SANS) alters the vision of astronauts during long-duration spaceflights. There is controversy regarding SANS being similar to patients with idiopathic intracranial hypertension (IIH). IIH has been shown to be due to an elevation in venous sinus pressure. The literature suggests an increase in jugular vein pressure secondary to a headward shift of fluid occurs in SANS but this may not be enough to significantly alter the intracranial pressure (ICP). The literature regarding cardiac output and cerebral blood flow (CBF) in long-duration spaceflight is contradictory, however, more recent data suggests increased flow. Recent modelling has shown that an increase in CBF can significantly increase sinus pressure. The purpose of the present paper is to review the SANS vascular dynamics literature and through mathematical modelling suggest the possible underlying cause of SANS as an elevation in venous sinus pressure, secondary to the redistribution of fluids towards the head, together with a significant increase in pressure drop across the venous system related to the CBF.