Abstract
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.
Highlights
As a result of extended spaceflight, some astronauts experience ophthalmic changes collectively referred to as spaceflightassociated neuro-ocular syndrome (SANS)[1,2]
Semi-automated magnetic resonance imaging (MRI)-based quantification of intraorbital anatomy revealed that spaceflight did not result in changes in optic nerve sheath (ONS) anatomy for the majority of individuals after long-duration spaceflight in our astronaut cohort
One subject with significant retinal thickening, who was diagnosed with Frisen grade 1 optic disc edema, displayed the largest bilateral increases in ONS area in our cohort, this increase is relatively small compared to clinical populations with increased ICP19
Summary
As a result of extended spaceflight, some astronauts experience ophthalmic changes collectively referred to as spaceflightassociated neuro-ocular syndrome (SANS)[1,2]. The single subject diagnosed with grade 1 optic disc edema in our study had an ON cross-sectional area reduction of −0.85 mm[2] at R + 1 in that eye, a value within the 95% CI for that measurement.
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