Lower body negative pressure reduces optic nerve sheath diameter during head-down tilt

Optic Nerve Sheath Diameter Point-of-Care Ultrasonography Quality Criteria Checklist: An International Consensus Statement on Optic Nerve Sheath Diameter Imaging and Measurement

OBJECTIVES

To standardize optic nerve sheath diameter (ONSD) point-of-care ultrasonography (POCUS) and improve its research and clinical utility by developing the ONSD POCUS Quality Criteria Checklist (ONSD POCUS QCC).

DESIGN

Three rounds of modified Delphi consensus process and three rounds of asynchronous discussions.

SETTING

Online surveys and anonymous asynchronous discussion.

SUBJECTS

Expert panelists were identified according to their expertise in ONSD research, publication records, education, and clinical use. A total of 52 panelists participated in the Delphi process.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Three Delphi rounds and three asynchronous discussion rounds generated consensus on quality criteria (QC). This started with 29 QC in addition to other QC proposed by expert panelists. The QC items were categorized into probe selection, safety, body position, imaging, measurement, and research considerations. At the conclusion of the study, 28 QC reached consensus to include in the final ONSD POCUS QCC. These QC were then reorganized, edited, and consolidated into 23 QC that were reviewed and approved by the panelists.

CONCLUSIONS

ONSD POCUS QCC standardizes ONSD ultrasound imaging and measurement based on international consensus. This can establish ONSD ultrasound in clinical research and improve its utility in clinical practice.

Lower body negative pressure as a research tool and countermeasure for the physiological effects of spaceflight: A comprehensive review

Lower Body Negative Pressure (LBNP) redistributes blood from the upper body to the lower body. LBNP may prove to be a countermeasure for the multifaceted physiological changes endured by astronauts during spaceflight related to cephalad fluid shift. Over more than five decades, beginning with the era of Skylab, advancements in LBNP technology have expanded our understanding of neurological, ophthalmological, cardiovascular, and musculoskeletal adaptations in space, with particular emphasis on mitigating issues such as bone loss. To date however, no comprehensive review has been conducted that chronicles the evolution of this technology or elucidates the broad-spectrum potential of LBNP in managing the diverse physiological challenges encountered in the microgravity environment. Our study takes a chronological perspective, systematically reviewing the historical development and application of LBNP technology in relation to the various pathophysiological impacts of spaceflight. The primary objective is to illustrate how this technology, as it has evolved, offers an increasingly sophisticated lens through which to interpret the systemic effects of space travel on human physiology. We contend that the insights gained from LBNP studies can significantly aid in formulating targeted and effective countermeasures to ensure the health and safety of astronauts. Ultimately, this paper aspires to promote a more cohesive understanding of the broad applicability of LBNP as a countermeasure against multiple bodily effects of space travel, thereby contributing to a safer and more scientifically informed approach to human space exploration.

Ocular perfusion pressure is not reduced in response to lower body negative pressure

Lower body negative pressure (LBNP) has been proposed as a countermeasure to mitigate the cephalad fluid shift occurring during spaceflight, which may be associated with the development of Spaceflight Associated Neuro-ocular Syndrome (SANS). This study quantifies the effect of LBNP on intraocular pressure (IOP), mean arterial pressure at eye level (MAPeye), and ocular perfusion pressure (OPP). Twenty-four subjects (12 male, 12 female) were subjected to graded LBNP in 0° supine and 15° head-down tilt (HDT) postures from 0 mmHg to -50 mmHg in 10 mmHg increments. IOP decreased significantly with LBNP pressure in 0° supine (by 0.7 ± 0.09 mmHg per 10 mmHg LBNP pressure, p < 0.001) and in 15° HDT (by 1.0 ± 0.095 mmHg per 10 mmHg of LBNP pressure, p < 0.001). MAPeye significantly decreased by 0.9 ± 0.4 mmHg per 10 mmHg of LBNP pressure in 0° supine (p = 0.016) but did not significantly change with LBNP in 15° HDT (p = 0.895). OPP did not significantly change with LBNP in 0° supine (p = 0.539) but it significantly increased in 15° HDT at 1.0 ± 0.3 mmHg per 10 mmHg of LBNP pressure (p = 0.010). Sex did not have a significant effect on OPP, MAPeye, or IOP in any condition. In 15° HDT, the reduction in IOP during increasing negative pressure, combined with the relatively constant MAPeye, led to the increase in OPP. Furthermore, results suggest that LBNP, while effective in reducing IOP, is not effective in reducing OPP across all postures investigated.

Quantitative ultrasound image assessment of the optic nerve subarachnoid space during 90-day head-down tilt bed rest

The elevation in the optic nerve sheath (ONS) pressure (ONSP) due to microgravity-induced headward fluid shift is the primary hypothesized contributor to SANS. This longitudinal study aims to quantify the axial plane of the optic nerve subarachnoid space area (ONSSA), which is filled with cerebrospinal fluid (CSF) and expands with elevated ONSP during and after head-down tilt (HDT) bed rest (BR). 36 healthy male volunteers (72 eyes) underwent a 90-day strict 6° HDT BR. Without obtaining the pre-HDT data, measurements were performed on days 30, 60, and 90 during HDT and at 6 recovery time points extended to 180-days (R + 180) in a supine position. Portable B-scan ultrasound was performed using the 12 MHz linear array probe binocularly. The measurements of the ONS and the calculation of the ONSSA were performed with ImageJ 1.51 analysis software by two experienced observers in a masked manner. Compared to R + 180, the ONSSA on HDT30, HDT60, and HDT90 exhibited a consistently significant distention of 0.44 mm2 (95% CI: 0.13 to 0.76 mm2, P = 0.001), 0.45 mm2 (95% CI: 0.15 to 0.75 mm2, P = 0.001), and 0.46 mm2 (95% CI: 0.15 to 0.76 mm2, P < 0.001), respectively, and recovered immediately after HDT on R + 2. Such small changes in the ONSSA were below the lateral resolution limit of ultrasound (0.4 mm) and may not be clinically relevant, possibly due to ONS hysteresis causing persistent ONS distension. Future research can explore advanced quantitative portable ultrasound-based techniques and establish comparisons containing the pre-HDT measurements to deepen our understanding of SANS.

Comprehensive assessment of physiological responses in women during the ESA dry immersion VIVALDI microgravity simulation

Astronauts in microgravity experience multi-system deconditioning, impacting their inflight efficiency and inducing dysfunctions upon return to Earth gravity. To fill the sex gap of knowledge in the health impact of spaceflights, we simulate microgravity with a 5-day dry immersion in 18 healthy women (ClinicalTrials.gov Identifier: NCT05043974). Here we show that dry immersion rapidly induces a sedentarily-like metabolism shift mimicking the beginning of a metabolic syndrome with a drop in glucose tolerance, an increase in the atherogenic index of plasma, and an impaired lipid profile. Bone remodeling markers suggest a decreased bone formation coupled with an increased bone resorption. Fluid shifts and muscular unloading participate to a marked cardiovascular and sensorimotor deconditioning with decreased orthostatic tolerance, aerobic capacity, and postural balance. Collected datasets provide a comprehensive multi-systemic assessment of dry immersion effects in women and pave the way for future sex-based evaluations of countermeasures.

Spaceflight associated neuro-ocular syndrome: proposed pathogenesis, terrestrial analogues, and emerging countermeasures

Spaceflight associated neuro-ocular syndrome (SANS) refers to a distinct constellation of ocular, neurological and neuroimaging findings observed in astronauts during and following long duration spaceflight. These ocular findings, to include optic disc oedema, posterior globe flattening, chorioretinal folds and hyperopic shifts, were first described by NASA in 2011. SANS is a potential risk to astronaut health and will likely require mitigation prior to planetary travel with prolonged exposures to microgravity. While the exact pathogenesis of SANS is not completely understood, several hypotheses have been proposed to explain this neuro-ocular phenomenon. In this paper, we briefly discuss the current hypotheses and contributing factors underlying SANS pathophysiology as well as analogues used to study SANS on Earth. We also review emerging potential countermeasures for SANS including lower body negative pressure, nutritional supplementation and translaminar pressure gradient modulation. Ongoing investigation within these fields will likely be instrumental in preparing and protecting astronaut vision for future spaceflight missions including deep space exploration.

Wearable Sensing System for NonInvasive Monitoring of Intracranial BioFluid Shifts in Aerospace Applications

The alteration of the hydrostatic pressure gradient in the human body has been associated with changes in human physiology, including abnormal blood flow, syncope, and visual impairment. The focus of this study was to evaluate changes in the resonant frequency of a wearable electromagnetic resonant skin patch sensor during simulated physiological changes observed in aerospace applications. Simulated microgravity was induced in eight healthy human participants (n = 8), and the implementation of lower body negative pressure (LBNP) countermeasures was induced in four healthy human participants (n = 4). The average shift in resonant frequency was -13.76 ± 6.49 MHz for simulated microgravity with a shift in intracranial pressure (ICP) of 9.53 ± 1.32 mmHg, and a shift of 8.80 ± 5.2097 MHz for LBNP with a shift in ICP of approximately -5.83 ± 2.76 mmHg. The constructed regression model to explain the variance in shifts in ICP using the shifts in resonant frequency (R² = 0.97) resulted in a root mean square error of 1.24. This work demonstrates a strong correlation between sensor signal response and shifts in ICP. Furthermore, this study establishes a foundation for future work integrating wearable sensors with alert systems and countermeasure recommendations for pilots and astronauts.

Optic nerve sheath diameter and spaceflight: defining shortcomings and future directions

Neuro-ocular changes during long-duration space flight are known as spaceflight-associated neuro-ocular syndrome (SANS). The ability to detect, monitor, and prevent SANS is a priority of current space medicine research efforts. Optic nerve sheath diameter (ONSD) measurement has been used both terrestrially and in microgravity as a proxy for measurements of elevated intracranial pressure. ONSD shows promise as a potential method of identifying and quantitating neuro-ocular changes during space flight. This review examines 13 studies measuring ONSD and its relationship to microgravity exposure or ground-based analogs, including head-down tilt, dry immersion, or animal models. The goal of this correspondence is to describe heterogeneity in the use of ONSD in the current SANS literature and make recommendations to reduce heterogeneity in future studies through standardization of imaging modalities, measurement techniques, and other aspects of study design.

Acute effects of postural changes and lower body positive and negative pressure on the eye

Introduction: Entry into weightlessness results in a fluid shift and a loss of hydrostatic gradients. These factors are believed to affect the eye and contribute to the ocular changes that occur in space. We measured eye parameters during fluid shifts produced by lower body negative pressure (LBNP) and lower body positive pressure (LBPP) and changes in hydrostatic gradient direction (supine-prone) in normal subjects to assess the relative effects of fluid shifts and hydrostatic gradient changes on the eye.

Methods: Ocular parameters (intraocular pressure (IOP), ocular geometry, and optical coherence tomography measures) were measured in the seated, supine, and prone positions. To create a fluid shift in the supine and prone positions, the lower body chamber pressure ranged from -40 mmHg to +40 mmHg. Subjects maintained each posture and LBNP/LBPP combination for 15 min prior to data collection. A linear mixed-effects model was used to determine the effects of fluid shifts (as reflected by LBNP/LBPP) and hydrostatic gradient changes (as reflected by the change from seated to supine and from seated to prone) on eye parameters.

Results: Chamber pressure was positively correlated with both increased choroidal thickness (β = 0.11 ,p = 0.01) and IOP (β = 0.06 p < 0.001). The change in posture increased IOP compared to seated IOP (supine β = 2.1, p = 0.01, prone β = 9.5, p < 0.001 prone) but not choroidal thickness. IOP changes correlated with axial length (R = 0.72, p < 0.001).

Discussion: The effects of hydrostatic gradients and fluids shifts on the eye were investigated by inducing a fluid shift in both the supine and prone postures. Both hydrostatic gradients (posture) and fluid shifts (chamber pressure) affected IOP, but only hydrostatic gradients affected axial length and aqueous depth. Changes in choroidal thickness were only significant for the fluid shifts. Changes in hydrostatic gradients can produce significant changes in both IOP and axial length. Fluid shifts are often cited as important factors in the pathophysiology of SANS, but the local loss of hydrostatic gradients in the head may also play an important role in these ocular findings.

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.

Gravitational Dose‐Response Curves for Acute Cardiovascular Hemodynamics and Autonomic Responses in a Tilt Paradigm

Background

The cardiovascular system is strongly dependent on the gravitational environment. Gravitational changes cause mechanical fluid shifts and, in turn, autonomic effectors influence systemic circulation and cardiac control. We implemented a tilt paradigm to (1) investigate the acute hemodynamic response across a range of directions of the gravitational vector, and (2) to generate specific dose‐response relationships of this gravitational dependency.

Methods and Results

Twelve male subjects were tilted from 45° head‐up tilt to 45° head‐down tilt in 15° increments, in both supine and prone postures. We measured the steady‐state hemodynamic response in a range of variables including heart rate, stroke volume, cardiac output, oxygen consumption, total peripheral resistance, blood pressure, and autonomic indices derived from heart rate variability analysis. There is a strong gravitational dependence in almost all variables considered, with the exception of oxygen consumption, whereas systolic blood pressure remained controlled to within ≈3% across the tilt range. Hemodynamic responses are primarily driven by differential loading on the baroreflex receptors, combined with differences in venous return to the heart. Thorax compression in the prone position leads to reduced venous return and increased sympathetic nervous activity, raising heart rate, and systemic vascular resistance while lowering cardiac output and stroke volume.

Conclusions

Gravitational dose‐response curves generated from these data provide a comprehensive baseline from which to assess the efficacy of potential spaceflight countermeasures. Results also assist clinical management of terrestrial surgery in prone posture or head‐down tilt positions.

Changes in Optic Nerve Head and Retinal Morphology During Spaceflight and Acute Fluid Shift Reversal

Importance

Countermeasures that reverse the headward fluid shift experienced in weightlessness have the potential to mitigate spaceflight-associated neuro-ocular syndrome. This study investigated whether use of the countermeasure lower-body negative pressure during spaceflight was associated with changes in ocular structure.

Objective

To determine whether changes to the optic nerve head and retina during spaceflight can be mitigated by brief in-flight application of 25-mm Hg lower-body negative pressure.

Design, Setting, and Participants

In the National Aeronautics and Space Administration's "Fluid Shifts Study," a prospective cohort study, optical coherence tomography scans of the optic nerve head and macula were obtained from US and international crew members before flight, in-flight, and up to 180 days after return to Earth. In-flight scans were obtained both under normal weightless conditions and 10 to 20 minutes into lower-body negative pressure exposure. Preflight and postflight data were collected in the seated, supine, and head-down tilt postures. Crew members completed 6- to 12-month missions that took place on the International Space Station. Data were analyzed from 2016 to 2021.

Interventions or Exposures

Spaceflight and lower-body negative pressure.

Main Outcomes and Measures

Changes in minimum rim width, optic cup volume, Bruch membrane opening height, peripapillary total retinal thickness, and macular thickness.

Results

Mean (SD) flight duration for the 14 crew members (mean [SD] age, 45 [6] years; 11 male crew members [79%]) was 214 (72) days. Ocular changes on flight day 150, as compared with preflight seated, included an increase in minimum rim width (33.8 μm; 95% CI, 27.9-39.7 μm; P < .001), decrease in cup volume (0.038 mm3; 95% CI, 0.030-0.046 mm3; P < .001), posterior displacement of Bruch membrane opening (-9.0 μm; 95% CI, -15.7 to -2.2 μm; P = .009), and decrease in macular thickness (fovea to 500 μm, 5.1 μm; 95% CI, 3.5-6.8 μm; P < .001). Brief exposure to lower-body negative pressure did not affect these parameters.

Conclusions and Relevance

Results of this cohort study suggest that peripapillary tissue thickening, decreased cup volume, and mild central macular thinning were associated with long-duration spaceflight. Acute exposure to 25-mm Hg lower-body negative pressure did not alter optic nerve head or retinal morphology, suggesting that longer durations of a fluid shift reversal may be needed to mitigate spaceflight-induced changes and/or other factors are involved.

Lower body negative pressure reduces jugular and portal vein volumes and counteracts the elevation of middle cerebral vein velocity during long-duration spaceflight

Cephalad fluid shifts in space have been hypothesized to cause the spaceflight-associated neuro-ocular syndrome (SANS) by increasing the intracranial-ocular translaminal pressure gradient. Lower body negative pressure (LBNP) can be used to shift upper-body blood and other fluids toward the legs during spaceflight. We hypothesized that microgravity would increase jugular vein volume (JVvol), portal vein cross-sectional area (PV), and intracranial venous blood velocity (MCV) and that LBNP application would return these variables toward preflight levels. Data were collected from 14 subjects (11 males) before and during long-duration International Space Station (ISS) spaceflights. Ultrasound measures of JVvol, PV, and MCV were acquired while seated and supine before flight and early during spaceflight at day 45 (FD45) and late at day 150 (FD150) with and without LBNP. JVvol increased from preflight supine and seated postures (46 ± 48% and 646 ± 595% on FD45 and 43 ± 43% and 702 ± 631% on FD150, P < 0.05), MCV increased from preflight supine (44 ± 31% on FD45 and 115 ± 116% on FD150, P < 0.05), and PV increased from preflight supine and seated (51 ± 56% on FD45 and 100 ± 74% on FD150, P < 0.05). Inflight LBNP of -25 mmHg restored JVvol and MCV to preflight supine level and PV to preflight seated level. Elevated JVvol confirms the sustained neck-head blood engorgement inflight, whereas increased PV area supports the fluid shift at the splanchnic level. Also, MCV increased potentially due to reduced lumen diameter. LBNP, returning variables to preflight levels, may be an effective countermeasure.NEW & NOTEWORTHY Microgravity-induced fluid shifts markedly enlarge jugular and portal veins and increase cerebral vein velocity. These findings demonstrate a marked flow engorgement at neck and splanchnic levels and may suggest compression of the cerebral veins by the brain tissue in space. LBNP (-25 mmHg for 30 min) returns these changes to preflight levels and, thus, reduces the associated flow and tissue disturbances.

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

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.

Reviving lower body negative pressure as a countermeasure to prevent pathological vascular and ocular changes in microgravity

Mitigation of spaceflight-related pathologies such as spaceflight-associated neuro-ocular syndrome (SANS) and the recently discovered risk of venous thrombosis must happen before deep space exploration can occur. Lower body negative pressure (LBNP) can simulate gravitational stress during spaceflight that is likely to counteract SANS and venous thrombosis, but the ideal dose and method of delivery have yet to be determined. We undertook a review of current LBNP literature and conducted a gap analysis to determine the steps needed to adapt LBNP for in-flight use. We found that to use LBNP in flight, it must be adapted to long time duration/low pressure use that should be compatible with crew activities. A lack of understanding of the etiology of the pathologies that LBNP can counteract hinders the application of LBNP as a countermeasure during spaceflight. Future research should aim at filling the knowledge gaps outlined in this review.

Ex-vivo quantification of ovine pia arachnoid complex biomechanical properties under uniaxial tension

Background

The pia arachnoid complex (PAC) is a cerebrospinal fluid-filled tissue conglomerate that surrounds the brain and spinal cord. Pia mater adheres directly to the surface of the brain while the arachnoid mater adheres to the deep surface of the dura mater. Collagen fibers, known as subarachnoid trabeculae (SAT) fibers, and microvascular structure lie intermediately to the pia and arachnoid meninges. Due to its structural role, alterations to the biomechanical properties of the PAC may change surface stress loading in traumatic brain injury (TBI) caused by sub-concussive hits. The aim of this study was to quantify the mechanical and morphological properties of ovine PAC.

Methods

Ovine brain samples (n = 10) were removed from the skull and tissue was harvested within 30 min post-mortem. To access the PAC, ovine skulls were split medially from the occipital region down the nasal bone on the superior and inferior aspects of the skull. A template was used to remove arachnoid samples from the left and right sides of the frontal and occipital regions of the brain. 10 ex-vivo samples were tested with uniaxial tension at 2 mm s⁻¹, average strain rate of 0.59 s⁻¹, until failure at < 5 h post extraction. The force and displacement data were acquired at 100 Hz. PAC tissue collagen fiber microstructure was characterized using second-harmonic generation (SHG) imaging on a subset of n = 4 stained tissue samples. To differentiate transverse blood vessels from SAT by visualization of cell nuclei and endothelial cells, samples were stained with DAPI and anti-von Willebrand Factor, respectively. The Mooney-Rivlin model for average stress–strain curve fit was used to model PAC material properties.

Results

The elastic modulus, ultimate stress, and ultimate strain were found to be 7.7 ± 3.0, 2.7 ± 0.76 MPa, and 0.60 ± 0.13, respectively. No statistical significance was found across brain dissection locations in terms of biomechanical properties. SHG images were post-processed to obtain average SAT fiber intersection density, concentration, porosity, tortuosity, segment length, orientation, radial counts, and diameter as 0.23, 26.14, 73.86%, 1.07 ± 0.28, 17.33 ± 15.25 µm, 84.66 ± 49.18°, 8.15%, 3.46 ± 1.62 µm, respectively.

Conclusion

For the sizes, strain, and strain rates tested, our results suggest that ovine PAC mechanical behavior is isotropic, and that the Mooney-Rivlin model is an appropriate curve-fitting constitutive equation for obtaining material parameters of PAC tissues.

Quantitative magnetic resonance image assessment of the optic nerve and surrounding sheath after spaceflight

A subset of long-duration spaceflight astronauts have experienced ophthalmic abnormalities, collectively termed spaceflight-associated neuro-ocular syndrome (SANS). Little is understood about the pathophysiology of SANS; however, microgravity-induced alterations in intracranial pressure (ICP) due to headward fluid shifts is the primary hypothesized contributor. In particular, potential changes in optic nerve (ON) tortuosity and ON sheath (ONS) distension may indicate altered cerebrospinal fluid dynamics during weightlessness. The present longitudinal study aims to provide a quantitative analysis of ON and ONS cross-sectional areas, and ON deviation, an indication of tortuosity, before and after spaceflight. Ten astronauts undergoing ~6-month missions on the International Space Station (ISS) underwent high-resolution magnetic resonance imaging (MRI) preflight and at five recovery time points extending to 1 year after return from the ISS. The mean changes in ON deviation, ON cross-sectional area, and ONS cross-sectional area immediately post flight were −0.14 mm (95% CI: −0.36 to 0.08, Bonferroni-adjusted P = 1.00), 0.13 mm² (95% CI −0.66 to 0.91, Bonferroni-adjusted P = 1.00), and −0.22 mm² (95% CI: −1.78 to 1.34, Bonferroni-adjusted P = 1.00), respectively, and remained consistent during the recovery period. Terrestrially, ONS distension is associated with increased ICP; therefore, these results suggest that, on average, ICP was not pathologically elevated immediately after spaceflight. However, a subject diagnosed with optic disc edema (Frisen Grade 1, right eye) displayed increased ONS area post flight, although this increase is relatively small compared to clinical populations with increased ICP. Advanced quantitative MRI-based assessment of the ON and ONS could help our understanding of SANS and the role of ICP.

Daily generation of a footward fluid shift attenuates ocular changes associated with head-down tilt bed rest

Astronauts have presented with a constellation of visual changes referred to as spaceflight-associated neuro-ocular syndrome (SANS). However, neither have early markers of microgravity-induced optic remodeling been fully identified nor have countermeasures been developed. To identify early markers of SANS, we studied 10 subjects with optical coherence tomography and ultrasonography when upright and supine and again after 24 h of 6° head-down tilt (HDT) bed rest. Upon acute transition from the upright to the supine position, choroid area (2.24 ± 0.53 to 2.28 ± 0.52 mm², P = 0.001) and volume (9.51 ± 2.08 to 9.73 ± 2.08 mm³, P = 0.002) increased. After 24 h of HDT bed rest, subfoveal choroidal thickness (372 ± 93 to 381 ± 95 µm, P = 0.02), choroid area (2.25 ± 0.52 to 2.33 ± 0.54 mm², P = 0.08), and volume (9.64 ± 2.03 to 9.82 ± 2.08 mm³, P = 0.08) increased relative to the supine position. Subsequently, seven subjects spent 3 days in -6°HDT bed rest to assess whether low-level lower body negative pressure (LBNP) could prevent the observed choroidal engorgement during bed rest. Maintaining the -6° HDT position for 3 days caused choroid area (Δ0.11 mm², P = 0.05) and volume (Δ0.45 mm³, P = 0.003) to increase. When participants also spent 8 h daily under -20 mmHg LBNP, choroid volume still increased, but substantially (40%) less than in the control trial (Δ0.27 mm³, P = 0.05). Moreover, the increase in choroid area was diminished (Δ0.03 mm², P = 0.13), indicating that low-level LBNP attenuates the choroid expansion associated with 3 days of -6° HDT bed rest. These data suggest that low-level LBNP may be an effective countermeasure for SANS.NEW & NOTEWORTHY Choroid measurements appear to be sensitive to changes in gravitational gradients, as well as periods of head-down tilt (HDT) bed rest, suggesting that they are potential indicators of early ocular remodeling and could serve to evaluate the efficacy of countermeasures for SANS. Eight hours of lower body negative pressure (LBNP) daily attenuates the choroid expansion associated with 3 days of strict -6° HDT bed rest, indicating that LBNP may be an effective countermeasure for SANS.

[From human terrestrial models to new preventive measures for ocular changes in astronauts : Results of the German Aerospace Center studies]

BACKGROUND

Ocular changes in astronauts, particularly the spaceflight associated neuro-ocular syndrome (SANS), pose a medical challenge for which no suitable preventive measures exist. During long-duration spaceflight missions, e.g. to the Moon and Mars, SANS and radiation-induced cataract could affect the health and performance of crews and jeopardize the success of missions. Mechanistic studies and development of preventive measures require suitable terrestrial models.

OBJECTIVE

Overview on the most recent research and future plans in space medicine.

MATERIAL AND METHODS

Search for relevant publications using PubMed.

RESULTS

Bed rest studies at the German Aerospace Center (DLR) demonstrated that strict bed rest in a -6° head down tilt position reproduces changes just like SANS on Earth. This model including creation of optic disc edema is applied in human studies testing influences of artificial gravity through short arm centrifugation as a preventive method. The unique research facility :envihab provides the opportunity to also simulate the ambient conditions of the International Space Station during bed rest studies.

CONCLUSION

Future head down tilt bed rest studies will serve to systematically test preventive measures for SANS. Similar investigations would be difficult to realize under real space conditions. Through close collaboration between space medicine and terrestrial ophthalmology, this research can benefit patients on Earth.

Visual changes after space flight: is it really caused by increased intracranial tension? A systematic review

INTRODUCTION

Spaceflight-Associated Neuro-ocular Syndrome (SANS) was linked to increased intracranial pressure (ICP) attributable to the combined effects of microgravity and environmental conditions encountered during spaceflight. Microgravity countermeasures as lower body negative pressure (LBNP) are potential interventions for SANS. Our aim is to provide a comprehensive qualitative analysis of literature contrasting simulation and spaceflight studies, focusing on the pathophysiology of SANS, and highlighting gaps in current knowledge.

EVIDENCE ACQUISITION

We systematically searched PubMed electronic database for English primary research published until February 2019 discussing intracranial changes in spaceflight or simulated microgravity, excluding animal and experimental studies. Two authors screened all the abstracts with a third author resolving disagreements. The full-text manuscripts were analyzed in pilot-tested tables.

EVIDENCE SYNTHESIS

Nineteen studies were reviewed; 13 simulation, and two out of six spaceflight studies were prospective. ICP changes were investigated in 11 simulation studies, where eight demonstrated a significant increase in ICP after variable periods of head-down tilt. three showed a significant increase in intraocular pressure (IOP) in conjunction with ICP elevation. With increasing ambient CO<inf>2</inf>: one showed an increase in IOP without further increase in ICP, while another showed a slight further decrease in ICP. LBNP demonstrated no significant effect on ICP in one and a decrease thereof in another study. After spaceflight, increased ICP on lumbar puncture was demonstrated in five studies.

CONCLUSIONS

Exposure to microgravity increases ICP possibly precipitating ocular changes. Whether other factors come into play is the subject of investigation. Further randomized studies and methods of direct ICP measurement during spaceflight are needed.

Spaceflight associated neuro-ocular syndrome (SANS) and the neuro-ophthalmologic effects of microgravity: a review and an update

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). We 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. In this manuscript, we update and review 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). We also discuss the knowledge gaps for in-flight and terrestrial human research including potential countermeasures for future study. We recommend that NASA and its research partners continue to study SANS in preparation for future longer duration manned space missions.

Haemodilution and head-down tilting induce functional injury in the rat optic nerve: A model for peri-operative ischemic optic neuropathy

BACKGROUND

Mechanisms of peri-operative ischaemic optic neuropathy remain poorly understood. Both specific pre-operative and intra-operative factors have been examined by retrospective studies, but no animal model currently exists.

OBJECTIVES

To develop a rodent model of peri-operative ischaemic optic neuropathy. In rats, we performed head-down tilt and/or haemodilution, theorising that the combination damages the optic nerve.

DESIGN

Animal study.

SETTING

Laboratory.

ANIMALS

A total of 36 rats, in four groups, completed the functional examination of retina and optic nerve after the interventions.

INTERVENTIONS

Anaesthetised groups (n>8) were supine (SUP) for 5 h, head-down tilted 70° for 5 h, head-down tilted/haemodiluted for 5 h or SUP/haemodiluted for 5 h. We measured blood pressure, heart rate, intra-ocular pressure and maintained constant temperature.

MAIN OUTCOME MEASUREMENTS

Retinal function (electroretinography), scotopic threshold response (STR) (for retinal ganglion cells) and visual evoked potentials (VEP) (for transmission through the optic nerve). We imaged the optic nerve in vivo and evaluated retinal histology, apoptotic cells and glial activation in the optic nerve. Retinal and optic nerve function were followed to 14 and 28 days after experiments.

RESULTS

At 28 days in head down tilted/haemodiluted rats, negative STR decreased (about 50% amplitude reduction, P = 0.006), VEP wave N2-P3 decreased (70% amplitude reduction, P = 0.01) and P2 latency increased (35%, P = 0.003), optic discs were swollen and glial activation was present in the optic nerve. SUP/haemodiluted rats had decreases in negative STR and increased VEP latency, but no glial activation.

CONCLUSION

An injury partly resembling human ischaemic optic neuropathy can be produced in rats by combining haemodilution and head-down tilt. Significant functional changes were also present with haemodilution alone. Future studies with this partial optic nerve injury may enable understanding of mechanisms of peri-operative ischaemic optic neuropathy and could help discover preventive or treatment strategies.

Lower Body Negative Pressure: Physiological Effects, Applications, and Implementation

This review presents lower body negative pressure (LBNP) as a unique tool to investigate the physiology of integrated systemic compensatory responses to altered hemodynamic patterns during conditions of central hypovolemia in humans. An early review published in Physiological Reviews over 40 yr ago (Wolthuis et al. Physiol Rev 54: 566-595, 1974) focused on the use of LBNP as a tool to study effects of central hypovolemia, while more than a decade ago a review appeared that focused on LBNP as a model of hemorrhagic shock (Cooke et al. J Appl Physiol (1985) 96: 1249-1261, 2004). Since then there has been a great deal of new research that has applied LBNP to investigate complex physiological responses to a variety of challenges including orthostasis, hemorrhage, and other important stressors seen in humans such as microgravity encountered during spaceflight. The LBNP stimulus has provided novel insights into the physiology underlying areas such as intolerance to reduced central blood volume, sex differences concerning blood pressure regulation, autonomic dysfunctions, adaptations to exercise training, and effects of space flight. Furthermore, approaching cardiovascular assessment using prediction models for orthostatic capacity in healthy populations, derived from LBNP tolerance protocols, has provided important insights into the mechanisms of orthostatic hypotension and central hypovolemia, especially in some patient populations as well as in healthy subjects. This review also presents a concise discussion of mathematical modeling regarding compensatory responses induced by LBNP. Given the diverse applications of LBNP, it is to be expected that new and innovative applications of LBNP will be developed to explore the complex physiological mechanisms that underline health and disease.