Effect of Nightly Lower Body Negative Pressure on Choroid Engorgement in a Model of Spaceflight-Associated Neuro-ocular Syndrome: A Randomized Crossover Trial

Key Points

Question  Does nightly (8 hours) lower body negative pressure (LBNP) attenuate microgravity-induced choroid engorgement, an early marker of ocular remodeling related to space-associated neuro-ocular syndrome?

Findings  In this randomized crossover study of 10 individuals, overnight LBNP reintroduced hydrostatic gradients during sleep without compromising hemodynamic stability and significantly attenuated the choroid engorgement observed during 3 days of simulated microgravity.

Meaning  These results suggest nightly LBNP may be an effective countermeasure against early markers of ocular remodeling associated with space-associated neuro-ocular syndrome.

Importance  Astronauts returning from long-duration spaceflight experience ocular remodeling related to cephalad fluid shifts induced by microgravity. It is hypothesized that the absence of diurnal reductions in intracranial pressure in microgravity creates a low but persistent pressure gradient at the posterior aspect of the eye, which results in ocular remodeling and space-associated neuro-ocular syndrome (SANS) over many months.

Objective  To determine whether partial reintroduction of footward fluid shifts during simulated microgravity via lower body negative pressure (LBNP) during sleep attenuates choroid engorgement, an early marker of ocular remodeling related to SANS.

Design, Setting, and Participants  Between May 2019 and February 2020, participants with no major cardiovascular, kidney, or ophthalmic disease completed 3 days of supine (0°) bed rest with and 3 days without 8 hours of nightly LBNP in a randomized, crossover design. This single-center investigation took place at the UT Southwestern Medical Center. All analyses were conducted blinded to condition and time point.

Interventions  Eight hours of nightly LBNP (−20 mm Hg) vs no LBNP.

Main Outcomes and Measures  The primary outcome was the change in choroid area and volume after 3 days of bed rest measured by optical coherence tomography.

Results  Of 10 participants, 5 were female, the mean (SD) age was 29 (9) years, and the age range was 18 to 55 years. Central venous pressure increased from the seated to supine position (mean [SD], seated: −2.3 [2.0] vs supine: 6.9 [2.0] mm Hg; P < .001), leading to choroid engorgement over 3 days of bed rest (Δ area: +0.09 mm² [95% CI, 0.04-0.13]; P = .001; Δ volume: +0.37 mm³ [95% CI, 0.19-0.55]; P = .001). Nightly LBNP caused a sustained reduction in supine central venous pressure (mean [SD], 5.7 [2.2] mm Hg to 1.2 [1.4 mm Hg]; P < .001) and attenuated the increase in choroid area (74%) (Δ: 0.02 mm² [95% −0.02 to 0.06]; P = .01) and volume (53%) (Δ: 0.17 mm³ [95% CI, 0.01-0.34]; P = .05) compared with control.

Conclusions and Relevance  Nightly LBNP reinstated a footward fluid shift and mitigated the increase in choroid area and volume. LBNP during sleep may be an effective countermeasure for ocular remodeling and SANS during long-duration space missions.

Quiz Ref IDAstronauts develop spaceflight-associated neuro-ocular syndrome (SANS) during long-duration missions that is characterized by optic disc edema, chorioretinal folds, globe flattening, choroid thickening, and hyperopic refractive error shift.^1,2 While 15% to 20% of crew members develop SANS, the prevalence of ocular remodeling is much higher with 70% of crew members having documented optic disc edema.^3,4 In some cases, structural abnormalities such as choroid thickening and globe flattening may persist for years after returning to Earth.³ Because future spaceflight missions are anticipated to exceed 6 months in duration, it is important to develop safe, effective, and practical countermeasures to prevent the ocular remodeling responsible for SANS.⁴

Loss of hydrostatic gradients in microgravity and a headward shift in vascular and cerebrospinal fluid are hypothesized to be a primary physiological stress responsible for ocular remodeling. While microgravity lowers intracranial pressure (ICP) compared with the supine position on Earth,⁵ the prevailing ICP in microgravity remains mildly elevated compared with the upright position. The inability to lower ICP in microgravity results in a mild but persistent elevation in daily ICP and chronic reduction in the translaminar pressure gradient that contribute to optic remodeling.^5-8 Reinstating footward fluid shifts using lower body negative pressure (LBNP) lowers ICP acutely^9,10 and attenuates choroid engorgement when administered daily for 8 hours during simulated microgravity (3-day head down tilt [HDT] bed rest).¹⁰ Therefore, LBNP may be a potential countermeasure for SANS if it can be used safely without interfering with crew activities.^11,12

Administration of LBNP nightly while crew members sleep is one strategy to reinstate footward fluid shifts while not interfering with daily activities. The efficacy of such an approach to elicit sustained decreases in central pressure overnight and prevent neuro-ocular remodeling is unknown. The purpose of this study was to test the hemodynamic stability and efficacy of a specialized sleeping sack capable of delivering nightly LBNP. In a randomized crossover design, participants completed 3 days of supine bed rest¹³ with or without nightly LBNP. We hypothesized that nightly LBNP would induce a footward fluid shift (measured by central venous pressure [CVP]) that would attenuate the increase in choroid area and volume observed during 3 days of simulated microgravity without compromising hemodynamic stability.

Trials were conducted at UT Southwestern Medical Center, Dallas, Texas, from May 2019 to February 2020. Participants provided written informed consent and were compensated for their time. The investigation was approved by the institutional review board and followed guidelines set forth in the Declaration of Helsinki.¹⁴ This study was not required to be registered by the funding agency but was registered retrospectively (NCT05016414) in compliance with International Committee of Medical Journal Editors recommendations. This study follows the Consolidated Standards of Reporting Trials (CONSORT) reporting guideline for crossover randomized trials.

Quiz Ref IDThe trial protocol and statistical analysis plan are available in Supplement 1. Volunteers, aged 18 to 55 years, free from cardiovascular, kidney, or ophthalmological disease completed 2 three-day bouts of strict supine (0°) bed rest, separated by at least a 10-day washout. Supine bed rest was used, as opposed to 6° HDT because the supine posture more closely simulates the increase in ICP experienced during spaceflight and is sufficient to induce choroid remodeling after 3 days.^5,13,15 Participants were randomized (1:1, simple randomization) to receive 8 hours of LBNP (−20 mm Hg), a dose previously shown to acutely reduce intracranial pressure without compromising cerebral perfusion pressure.⁹ LBNP was applied overnight (10 pm to 6 am) by a custom designed sleeping sack (eFigure 1 in Supplement 2) or control conditions. Primary outcome measures were the change in choroid area and volume from the morning of day 1 to the morning of day 3, assessed by optical coherence tomography in the supine position. For the primary outcome, optical coherence tomography (OCT) was assessed with LBNP off (>2 hours) to avoid acute effects of LBNP (eFigure 2 in Supplement 2). Data on race and ethnicity were collected by self-report.

Participants arrived to the laboratory on the first morning of the study for placement of a peripherally inserted central catheter for the measurement of CVP and supine blood sampling. Participants were then transported to the hospital and admitted for the bed rest protocol.

After 20 minutes of seated rest, steady state hemodynamics (heart rate, blood pressure, CVP, cerebral blood flow, end-tidal CO₂) and optic parameters (OCT) were measured in the 90° upright seated posture. Participants were then rested supine for 20 minutes prior to baseline measures. Measurements were repeated each day at 8 am and 8 pm for 3 days and nights in the supine position.

Participants spent 8 hours per night inside a prototype sleeping bag sealed at the level of the iliac crest. The chamber was designed to rotate 360° while maintaining a seal to facilitate natural sleeping positions. Control sessions were subject to an identical protocol without LBNP. Beat-by-beat blood pressure by finger photoplethysmography¹⁶ and LBNP were measured continuously overnight. LBNP levels and vital signs were assessed by a nurse every 2 hours to document hemodynamic stability (hypotension defined as blood pressure <90/60 mm Hg) during LBNP and standardize sleep disruptions between treatments. Participants were allowed to lower LBNP briefly when turning over and once each night for a bathroom break lasting no more than 20 minutes if needed. The use of pillows was prohibited to ensure the head remained flat on the bed (even when eating), and participants were monitored to ensure strict adherence to LBNP and bed rest.⁵

Upright and supine ocular measurements were performed in quadruplicate on the right eye (Spectralis OCT; Heidelberg Engineering) as described previously.^10,17 Acquisition parameters are outlined in the eMethods in Supplement 2. Choroid and retinal boundaries, subfoveal layer thickness, and macular retinal thickness were quantified by automated software segmentation (Heidelberg Spectralis OCT software HEYEX version 6.4.8.116) with manual adjustments as needed by an experienced observer blinded to all experimental conditions. The coefficient of variation for choroid area and volume was less than 1.1%. The typical error of measurement for area was 0.028 mm² and for volume was 0.105 mm³. Choroid volumes were restricted to a 6-mm diameter circle centered on the fovea. Single subfoveal line scans were taken in each position to measure the choroid area. The HEYEX software computed the choroid area after the choroid was manually demarcated with a nasal boundary set as the optic nerve head and a temporal boundary set at 3000 mm from the fovea. Retinal nerve fiber layer thickness and Bruch membrane opening–minimum rim width (BMO-MRW) were obtained through the optic nerve head scan and quantified by automated software segmentation.

A fluid-filled 4F catheter was placed into a brachial vein.¹⁸ The catheter was advanced to the level of right atrium, and the correct position was confirmed with fluoroscopy and through the identification of CVP waveforms. Position of the right atrium was marked on the body in the anteroposterior and lateral planes, and the pressure transducer (Transpac IV; ICU Medical) was leveled at the right atrium and zeroed to atmospheric pressure in all conditions.

Heart rate was monitored from a 3-lead electrocardiogram. Arterial blood pressure was measured in duplicate at each time point by electrosphygmomanometry (SunTechMedical Instruments). Beat-by-beat blood pressure was monitored by finger photophlethysmography (Portapres; Finapres Medical Systems). Expired CO₂ was sampled by capnography (Novametrix; Criticare Systems) from a nasal cannula.

Cerebral blood flow velocity of the middle cerebral artery was recorded using transcranial Doppler ultrasound (Multigon Industries) as described previously.¹⁰

Based on a previous investigation in our laboratory using LBNP and OCT,¹⁰ a minimum sample size of 8 was needed to detect a similar effect of LBNP on choroid area and volume with an unadjusted α = .05 and (1 - β) of .80. To determine the acute effects of posture on choroid area and volume, OCT and hemodynamics were measured on day 1 (day 1 am upright vs day 1 am supine) and compared using a paired t test. To determine the acute effects of LBNP on choroid area and volume, measurements were made on the last morning of bed rest before and 2 hours after turning LBNP off (day 4 am LBNP on vs day 4 am LBNP off) and compared using a paired t test. The chronic effect of LBNP on choroid area and volume was assessed by comparing day 1 am and day 4 am with LBNP off using 2-way repeated measures (condition × time point) analysis of variance with post hoc Sidak tests adjusted. The magnitude of increase in choroid area and volume (Δ = day 4 am – day 1 am) for each condition were compared using paired t test. All values are expressed as means (SDs) or means (95% CIs) (GraphPad version 9.0), and significance was unadjusted and set at a priori P < .05.

Eleven participants were randomized and 10 participants (5 men; mean [SD] age, 29 [9] years; mean [SD] height, 171.7 [11.5] cm; mean [SD] weight, 68.4 [10.4] kg; mean [SD] body mass index, 23.1 [1.5], calculated as weight in kilograms divided by height in meters squared) completed the intervention (Figure 1). One participant randomized to first receive the control intervention withdrew prior to completing the first full day of bed rest owing to gastrointestinal discomfort. No adverse events related to hemodynamic stability were reported.

Compared with seated, the supine position induced a headward fluid shift and subsequent increase in CVP (Δ: +9.6 mm Hg [95% CI, 8.8-10.5]; P < .001). CVP remained elevated in each condition throughout the 3 days of bed rest when LBNP was not in use (Δ day 4 am: control: +6.4 mm Hg [95% CI, 4.46-8.2]; LBNP: +7.0 mm Hg [95% CI, 5.1-8.9]; P = .83) (Figure 2). The acute increase in CVP from upright to supine was associated with an increase in choroid area (Δ: +0.10 mm² [95% CI, 0.05-0.13]; P < .001) and volume (Δ: +0.24 mm³ [95% CI, 0.15-0.32]; P < .001) but no acute increase in global BMO-MRW or retinal nerve fiber layer (Table 1).

Quiz Ref IDLBNP each evening lowered CVP acutely (Δ: −4.4 mm Hg [95% CI, −5.6 to −3.1]; P < .001) and maintained lower CVP over the course of 8 hours (CVP: LBNP on pm: mean [SD], 1.3 [1.5] mm Hg; LBNP on am: mean [SD], 0.68 [1.4] mm Hg; P = .98). Hemodynamics were stable throughout the 8-hour exposure with no difference in overnight mean pressure or heart rate between conditions (Table 2). Acute LBNP lowered cerebral blood velocity (middle cerebral artery velocity: day 4 LBNP on: mean [SD], 55 [12]; day 4 LBNP off: mean [SD], 60 [13] cm/s; P = .02), with no hypotensive events requiring termination of LBNP. On cessation of LBNP each morning, CVP increased immediately (Δ: +4.7 mm Hg [95% CI, 3.4-5.9) back to supine levels (Figure 2).

Quiz Ref IDCVP decreased similarly in both conditions over 3 days (day 1 am: mean [SD], 7.4 [1.6]; day 4 am: mean [SD], 4.4 [1.7]; P = .001) but remained higher than the upright posture (upright: mean [SD], −2.3 [2.0] mm Hg; day 4 am: mean [SD], 4.4 [1.7]; P < .001). There was a compensatory increase in heart rate (Δ: 3.2 bpm [95% CI, 0-6]; P = .04), a maintenance of mean arterial pressure (Δ: −0.2 mm Hg [95% CI, −3.0 to −2.6]; P = .31), and a decline in middle cerebral artery velocity (Δ: −9.3 cm/s [95% CI, −12.68 to −5.82]; P < .001) and end-tidal CO₂ (Δ: 1.2 mm Hg [95% CI, 0.2-2.1]; P = .02). Hemodynamics were similar in both conditions when LBNP was not in use with no group × time interaction. Bed rest without LBNP increased choroid area (Δ: 0.09 mm² [95% CI, 0.05-0.13]; P < .001) and volume (Δ: 0.37 mm³ [95% CI, 0.21-0.53]; P < .001). Nightly LBNP attenuated the magnitude of increase in choroid area by 74% (Δ: 0.02 mm² [95% CI, −0.02 to 0.06]; P = .02 control vs LBNP) and volume by 53% (Δ: 0.17 mm³ [95% CI, 0.01-0.34]; P = .004 control vs LBNP) (Figure 3).

Three days of supine (0°) bed rest (an established model of spaceflight-associated fluid shifts)¹³ increased choroid area and volume, which are early markers of ocular remodeling related to the development of SANS. Partial reintroduction of footward fluid shifts with LBNP (−20 mm Hg) for 8 hours overnight had no untoward effect on hemodynamic stability during sleep and attenuated ocular remodeling. These results are the first to demonstrate the hemodynamic stability of prolonged LBNP during sleep and support the feasibility of LBNP delivered nightly as a practical and effective countermeasure against SANS.

On Earth, two-thirds of each day are spent with low ICP due to gravitational hydrostatic gradients associated with the upright posture.^9,10,19 In microgravity, headward fluid shifts cause a mild elevation in ICP compared with upright that is likely persistent for the duration of microgravity exposure and may contribute to pathological ocular remodeling.^1,5,7 Nearly all astronauts display some degree of ocular remodeling during long-duration spaceflight including optic disc edema, choroid thickening, and axial length shortening.^1,3,4,13 These structural changes may be early markers related to the development of SANS.

Ground-based spaceflight analogs such as bed rest are used to replicate the headward fluid shift observed in microgravity.²⁰ Historically, 6° HDT bed rest was commonly used to generate a mild foot to head Gz hydrostatic gradient and marked increases in ICP.⁵ However, previous investigations found that 30 days of HDT bed rest caused more extensive optic disc edema than what is observed in individuals who completed a comparable duration of spaceflight.^13,15 Further, the marked choroid thickening observed during spaceflight was not consistently present in HDT bed rest.¹³ The discrepancy in the magnitude and pattern of ocular remodeling may be due to the exaggerated ICP during HDT bed rest compared with the supine position and microgravity.⁵ The current investigation used strict supine (0°) bed rest without any head elevation to more closely simulate the increase in ICP experienced during spaceflight.

As expected, transition from upright to supine posture increased CVP (Δ 9 mm Hg) and acutely increased choroid area, volume, and subfoveal choroid thickness indicative of choroidal vascular engorgement.^6,10,19 In contrast RNFL, global BMO-MRW, and macular retinal thickness were unaffected by the acute change in posture consistent with retinal autoregulation of blood flow during short-duration hemodynamic stress.^10,21-24 Importantly, the magnitude of acute choroidal engorgement in the supine posture was similar to that observed on acute exposure to microgravity.⁶ After 3 days, there was a further increase in choroid area, volume, and subfoveal choroid thickness beyond the acute adjustment to the supine posture. These data confirm findings from bed rest^10,25 and spaceflight³ that the choroid is susceptible to remodeling when exposed to chronic vascular congestion. There was an increase in global BMO-MRW, a potential marker of early optic disc edema, that was smaller in magnitude compared with 6° HDT.¹⁰ These findings indicate that supine (0°) bed rest causes continued optic remodeling beyond the acute effects of supine posture, potentially mimicking the progressive development of ocular remodeling observed in astronauts.⁴

LBNP is a promising countermeasure to counteract the headward fluid shift experienced in microgravity and has been investigated extensively as a countermeasure to orthostatic intolerance and other the hemodynamic maladaptations to spaceflight.^12,26-28 Mild LBNP (−20 mm Hg) can lower ICP without eliciting significant hemodynamic adjustments or compromising cerebral perfusion pressure,^9,19,29 and the reduction in ICP can be maintained when LBNP is applied for long durations (8 hours).¹⁰ A recent investigation found that LBNP applied for 8 hours during the day can attenuate early markers of ocular remodeling during 6° HDT bed rest.¹⁰ However, administering LBNP while crew members are awake is a substantial logistical burden. Therefore, it is important to establish the hemodynamic stability and efficacy of LBNP administered for long durations at night when it is superimposed on a naturally occurring nocturnal dip in blood pressure and plasma volume.

Our results indicate that LBNP administered at −20 mm Hg for 8 hours during sleep did not compromise blood pressure regulation or cerebral blood flow. Hemodynamic stability was observed across all 3 nights despite a gradual reduction in CVP seen from night 1 to night 3 (Figure 2A). Acute administration of LBNP reinstated approximately 50% of the pressure gradient observed during upright seated measures (Figure 2A). Unloading the choroid nightly for 3 days attenuated the remodeling of choroid area, volume (Figure 3), and to a lesser extent, subfoveal choroid thickness (Table 2). Importantly, the changes in choroid area and volume were observed with LBNP off indicating that influence of LBNP extends beyond its acute effect and can attenuate the chronic remodeling observed during simulated microgravity. In contrast, the increase in global BMO-MRW observed during 3 days of bed rest was not consistently attenuated by LBNP (Table 2). Similarly, a previous study of 3-day 6° HDT with daily LBNP showed inconsistent effects on global BMO-MRW,¹⁰ although both studies report a numerical decrease. Because optic disc edema may develop more gradually over time, further studies are needed to determine if LBNP can attenuate changes in global BMO-MRW during longer-duration bed rest.

Future investigations are needed to determine whether LBNP can prevent development of SANS during long-duration bed rest/spaceflight. Because bed rest does not eliminate the tissue compressive forces of the Gx gravitational gradient on Earth, LBNP may not be sufficient to fully address all aspects of ocular remodeling related to the complete loss of gravitational gradients. Finally, the best use of nightly LBNP to prevent SANS needs to be determined. Considerations, including application in all crew members vs just those deemed at highest risk, application prophylactically or only in response to evolving SANS, and use of nightly LBNP at a lower frequency (eg, every other night or only a few times per week),³⁰ would all be reasonable approaches to study for long-duration missions.

Supine bed rest for 3 days increased choroid area, volume, and global BMO-MRW, potential markers of early ocular remodeling associated with SANS. Low-level LBNP (−20 mm Hg) decreased CVP and an acutely unloaded the choroid to approximately 50% of normal gravitational unloading and was sustained overnight for 8 hours. Administration of LBNP nightly attenuated the increase in choroid area, volume, and to a lesser degree BMO-MRW with no hemodynamic instability during sleep. These findings suggest that nightly LBNP is a feasible countermeasure for early ocular remodeling. Future investigations are needed to determine if LBNP can be used to prevent the clinical manifestation of SANS during longer-duration bed rest and spaceflight.

Corresponding Author: Benjamin D. Levine, MD, Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital Dallas, 7232 Greenville Ave, Ste 435, Dallas, TX 75231 (benjaminlevine@texashealth.org).

Accepted for Publication: October 16, 2021.

Published Online: December 9, 2021. doi:10.1001/jamaophthalmol.2021.5200

Author Contributions: Dr Levine had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.

Concept and design: Hearon, Dias, Babu, Marshall, Levine.

Acquisition, analysis, or interpretation of data: All authors.

Drafting of the manuscript: Hearon, Dias, Campain.

Critical revision of the manuscript for important intellectual content: Hearon, Dias, Babu, Marshall, Peters, Leidner, Ivey, MacNamara, Levine.

Statistical analysis: Hearon, Dias, Campain.

Obtained funding: Dias, Levine.

Administrative, technical, or material support: Hearon, Dias, Babu, Marshall, Leidner, Levine.

Supervision: Hearon, MacNamara, Levine.

Conflict of Interest Disclosures: None reported.

Funding/Support: This study was supported by NASA (grants 80NSSC19K0300 and 80NSSC19K0300-P00001).

Role of the Funder/Sponsor: NASA had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.

Data Sharing Statement: See Supplement 3.

Additional Contributions: We would like to thank all volunteers for dedicating their time and effort towards these challenging experiments. We would also like to thank the supporting staff at the Institute for Exercise and Environmental Medicine including Dean Palmer, MS, Mitchel Samels, MS, Teverick Boyd, BS, Cyrus Oufi, BS, and Ramanathan Murugappan, PhD, for their technical support and Sheryl Livingston, RN, and Margot Morris, RN, for nursing support. No compensation outside of standard salary was received.