Changes in the Optic Nerve Head and Choroid Over 1 Year of Spaceflight

Neuro-ophthalmological changes in healthy females exposed to a 5-day dry immersion: a pilot study

After exposure to microgravity, astronauts undergo microgravity-induced thoraco-cephalic fluid shift, which may lead to ocular changes called "spaceflight associated neuro-ocular syndrome" (SANS). The onset of SANS may be multifactorial, including a potential elevation in intracranial pressure. Moreover, little is known about the impact of spaceflight on SANS in women due to the fact that fewer female astronauts have spent time in long-term missions. The objective is to determine whether similar ophthalmological changes occur in healthy women after short-term exposure to microgravity. The auto-refractometer was used to determine objective refraction. The best corrected distance visual acuity was assessed with a Monoyer chart. The ocular axial length was assessed using optical biometry. The applanation tonometry was used to determine intraocular pressure. Peripapillary retinal nerve fibre layer thickness (pRNFLT), macular total retinal thickness, and ganglion cell complex (GCC) were measured using optical coherence tomography. Ocular axial length is reduced after DI. pRNFL is thickest after DI specifically in the temporal, temporal-inferior, and nasal-inferior quadrants. Macular total retinal at the inferior quadrant of the 6-mm ring is thickest after DI. Global GCC is thinnest after DI. In this study, 5 days of DI induces slight but significant ophthalmological changes in women. However, these subtle changes do not correspond to criteria defined in SANS.

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.

Incidence and Progression of Chorioretinal Folds During Long-Duration Spaceflight

Importance

The primary contributing factor for development of chorioretinal folds during spaceflight is unknown. Characterizing fold types that develop and tracking their progression may provide insight into the pathophysiology of spaceflight-associated neuro-ocular syndrome and elucidate the risk of fold progression for future exploration-class missions exceeding 12 months in duration.

Objective

To determine the incidence and presentation of chorioretinal folds in long-duration International Space Station crew members and objectively quantify the progression of choroidal folds during spaceflight.

Design, Setting, and Participants

In this retrospective cohort study, optical coherence tomography scans of the optic nerve head and macula of crew members completing long-duration spaceflight missions were obtained on Earth prior to spaceflight and during flight. A panel of experts examined the scans for the qualitative presence of chorioretinal folds. Peripapillary total retinal thickness was calculated to identify eyes with optic disc edema, and choroidal folds were quantified based on surface roughness within macular and peripapillary regions of interest.

Interventions or Exposures

Spaceflight missions ranging 6 to 12 months.

Main Outcomes and Measures

Incidence of peripapillary wrinkles, retinal folds, and choroidal folds; peripapillary total retinal thickness; and Bruch membrane surface roughness.

Results

A total of 36 crew members were analyzed (mean [SD] age, 46 [6] years; 7 [19%] female). Chorioretinal folds were observed in 12 of 72 eyes (17%; 6 crew members). In eyes with early signs of disc edema, 10 of 42 (24%) had choroidal folds, 4 of 42 (10%) had inner retinal folds, and 2 of 42 (5%) had peripapillary wrinkles. Choroidal folds were observed in all eyes with retinal folds and peripapillary wrinkles. Macular choroidal folds developed in 7 of 12 eyes (4 of 6 crew members) with folds and progressed with mission duration; these folds extended into the fovea in 6 eyes. Circumpapillary choroidal folds developed predominantly superior, nasal, and inferior to the optic nerve head and increased in prevalence and severity with mission duration.

Conclusions and Relevance

Choroidal folds were the most common fold type to develop during spaceflight; this differs from reports in idiopathic intracranial hypertension, suggesting differences in the mechanisms underlying fold formation. Quantitative measures demonstrate the development and progression of choroidal folds during weightlessness, and these metrics may help to assess the efficacy of spaceflight-associated neuro-ocular syndrome countermeasures.

Implantable Collamer Lens Use in a Spaceflight Participant During Short Duration Spaceflight

BACKGROUND: The purpose of this report is to document the first use of a single piece, posterior chamber phakic implantable collamer lens (ICL) with a central port in the right eye (OD) of a spaceflight participant (SFP) during a 12-d Soyuz mission to the International Space Station (ISS). We also briefly document the stability of a pre-existing pachychoroid pigment epitheliopathy (PPE) in the macula of his left eye (OS) during this mission.CASE REPORT: Ocular examination, including refraction, slit lamp examination, macular examination by optical coherence tomography (OCT), and tonometry were performed before and after his mission and he was questioned regarding visual changes during each portion of his flight.DISCUSSION: We documented no change in ICL position during his spaceflight. He reported stable vision during liftoff, entry into microgravity, 12 d on the ISS, descent, and landing. Our results suggest that the modern ICL with a central port is stable, effective, and well tolerated during short duration spaceflight. His PPE also remained stable during this mission as documented by OCT.Gibson CR, Mader TH, Lipsky W, Brown DM, Jennings R, Law J, Sargsyan A, Brunstetter T, Danilichev SN, Maezawa Y. Implantable collamer lens use in a spaceflight participant during short duration spaceflight. Aerosp Med Hum Perform. 2023; 94(1):48-50.

Identification of Factors Associated With the Development of Optic Disc Edema During Spaceflight

Importance

Approximately 70% of crew members who complete long-duration missions to the International Space Station develop signs of optic disc edema, a hallmark finding of spaceflight-associated neuro-ocular syndrome. The onset and magnitude of edema differ across individuals, and the reason for this variability remains unknown. Identifying risk factors for spaceflight-induced disc edema is important because this condition may become more severe during extended-duration missions to the moon and Mars and could be associated with irreversible vision loss.

Objective

To assess whether preflight indicators of crowded optic nerve head morphology, other ocular measures (such as choroid thickness and axial length), body weight, body mass index, sex, age, and previous flight experience are associated with optic disc edema development.

Design, Setting, and Participants

This cohort study analyzed ocular, body weight, and demographic data collected from 31 US and international crew members before, during, and after spaceflight at the NASA Johnson Space Center and International Space Station. Ocular factors assessed included preflight and in-flight peripapillary total retinal thickness, minimum rim width, optic cup volume, mean cup depth, mean cup width, cup-disc ratio, Bruch membrane opening area, retinal nerve fiber layer thickness, choroid thickness, axial length, and refractive error. In addition, body weight, body mass index, sex, age, and previous spaceflight experience were assessed for associations with optic disc edema development. The data were analyzed from August 2021 to June 2022.

Exposure

Approximately 6 to 12 months of spaceflight.

Main Outcomes and Measures

In-flight increases in peripapillary total retinal thickness. Linear mixed models were used to assess for associations between a wide range of risk factors and in-flight increases in peripapillary total retinal thickness, which is a sensitive objective measure for detecting optic disc edema.

Results

This study included 31 International Space Station crew members with a mean (SD) age of 46.9 (6.0) years (25 men [80.6%]). During spaceflight, mean (SE) peripapillary total retinal thickness increased from 392.0 (5.8) μm to 430.2 (9.6) μm (P < .001), and greater individual changes were associated with smaller preflight cup volume (slope [SE], -62.8 [18.9]; P = .002), shallower preflight cup depth (slope [SE], -0.11 [0.03]; P < .001), and narrower preflight cup width (slope [SE], -0.03 [0.01]; P = .03). No associations were observed between changes in peripapillary total retinal thickness and any other variable evaluated.

Conclusions and Relevance

Findings of this cohort study suggest that smaller optic cup morphology may be associated with optic disc edema development during spaceflight. Crew members with this cup profile may benefit from enhanced ophthalmic monitoring during spaceflight and use of countermeasures against spaceflight-associated neuro-ocular syndrome.

Composites Additive Manufacturing for Space Applications: A Review

The assembly of 3D printed composites has a wide range of applications for ground preparation of space systems, in-orbit manufacturing, or even in-situ resource utilisation on planetary surfaces. The recent developments in composites additive manufacturing (AM) technologies include indoor experimentation on the International Space Station, and technological demonstrations will follow using satellite platforms on the Low Earth Orbits (LEOs) in the next few years. This review paper surveys AM technologies for varied off-Earth purposes where components or tools made of composite materials become necessary: mechanical, electrical, electrochemical and medical applications. Recommendations are also made on how to utilize AM technologies developed for ground applications, both commercial-off-the-shelf (COTS) and laboratory-based, to reduce development costs and promote sustainability.

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.

Optic Disc Edema in Astronauts from a Choroidal Point of View

INTRODUCTION: Optic disc edema has been well documented in astronauts both during and after long-duration spaceflight and is hypothesized to largely result from increased pressure within the orbital subarachnoid space brought about by a generalized rise in intracranial pressure or from sequestration of cerebrospinal fluid within the orbital subarachnoid space with locally elevated optic nerve sheath pressure. In addition, a recent prospective study documented substantial spaceflight-associated peripapillary choroidal thickening, which may be a contributing factor in spaceflight-associated neuro-ocular syndrome. In the present article, based on the above, we offer a new perspective on the pathogenesis of microgravity-induced optic disc edema from a choroidal point of view. We propose that prolonged microgravity exposure may result in the transudation of fluid from the choroidal vasculature, which, in turn, may reach the optic nerve head, and ultimately may lead to fluid stasis within the prelaminar region secondary to impaired ocular glymphatic outflow. If confirmed, this viewpoint would shed new light on the development of optic disc edema in astronauts.Wostyn P, Gibson CR, Mader TH. Optic disc edema in astronauts from a choroidal point of view. Aerosp Med Hum Perform. 2022; 93(4):396-398.

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

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.

Evaluating Ocular Response in the Retina and Optic Nerve Head after Single and Fractionated High-Energy Protons

There are serious concerns about possible late radiation damage to ocular tissue from prolonged space radiation exposure, and occupational and medical procedures. This study aimed to investigate the effects of whole-body high-energy proton exposure at a single dose on apoptosis, oxidative stress, and blood-retina barrier (BRB) integrity in the retina and optic nerve head (ONH) region and to compare these radiation-induced effects with those produced by fractionated dose. Six-month-old C57BL/6 male mice were either sham irradiated or received whole-body high energy proton irradiation at an acute single dose of 0.5 Gy or 12 equal dose fractions for a total dose of 0.5 Gy over twenty-five days. At four months following irradiation, mice were euthanized and ocular tissues were collected for histochemical analysis. Significant increases in the number of apoptotic cells were documented in the mouse retinas and ONHs that received proton radiation with a single or fractionated dose (p < 0.05). Immunochemical analysis revealed enhanced immunoreactivity for oxidative biomarker, 4-hydroxynonenal (4-HNE) in the retina and ONH following single or fractionated protons with more pronounced changes observed with a single dose of 0.5 Gy. BRB integrity was also evaluated with biomarkers of aquaporin-4 (AQP-4), a water channel protein, a tight junction (TJ) protein, Zonula occludens-1 (ZO-1), and an adhesion molecule, the platelet endothelial cell adhesion molecule-1 (PECAM-1). A significantly increased expression of AQP-4 was observed in the retina following a single dose exposure compared to controls. There was also a significant increase in the expression of PECAM-1 and a decrease in the expression of ZO-1 in the retina. These changes give a strong indication of disturbance to BRB integrity in the retina. Interestingly, there was very limited immunoreactivity of AQP-4 and ZO-1 seen in the ONH region, pointing to possible lack of BRB properties as previously reported. Our data demonstrated that exposure to proton radiation of 0.5 Gy induced oxidative stress-associated apoptosis in the retina and ONH, and changes in BRB integrity in the retina. Our study also revealed the differences in BRB biomarker distribution between these two regions. In response to radiation insults, the cellular response in the retina and ONH may be differentially regulated in acute or hyperfractionated dose schedules.

Effects of Venoconstrictive Thigh Cuffs on Dry Immersion-Induced Ophthalmological Changes

Neuro-ophthalmological changes named spaceflight associated neuro-ocular syndrome (SANS) reported after spaceflights are important medical issues. Dry immersion (DI), an analog to microgravity, rapidly induces a centralization of body fluids, immobilization, and hypokinesia similar to that observed during spaceflight. The main objectives of the present study were 2-fold: (1) to assess the neuro-ophthalmological impact during 5 days of DI and (2) to determine the effects of venoconstrictive thigh cuffs (VTC), used as a countermeasure to limit headward fluid shift, on DI-induced ophthalmological adaptations. Eighteen healthy male subjects underwent 5 days of DI with or without VTC countermeasures. The subjects were randomly assigned into two groups of 9: a control and cuffs group. Retinal and optic nerve thickness were assessed with spectral-domain optical coherence tomography (OCT). Optic nerve sheath diameter (ONSD) was measured by ocular ultrasonography and used to assess indirect changes in intracranial pressure (ICP). Intraocular pressure (IOP) was assessed by applanation tonometry. A higher thickness of the retinal nerve fiber layer (RNFL) in the temporal quadrant was observed after DI. ONSD increased significantly during DI and remained higher during the recovery phase. IOP did not significantly change during and after DI. VTC tended to limit the ONSD enlargement but not the higher thickness of an RNFL induced by DI. These findings suggest that 5 days of DI induced significant ophthalmological changes. VTC were found to dampen the ONSD enlargement induced by DI.