Despite ONSD measurement’s potential benefits in detecting elevated ICP terrestrially and monitoring SANS manifestation during space flight, ONSD utilization is limited by variability in measurement technique and positive test cutoff values 6, 12. Magnetic resonance imaging (MRI) of the brain and orbits is also a part of the routine terrestrial pre- and post-space flight surveillance in astronauts, and while not portable, it remains a valuable tool to further our understanding of SANS 11. ONSD ultrasonography (US) is portable and non-invasive, making it ideal for use in evaluating unstable patients, in low resource settings 9, and during space flight where a spectrum of ocular US measurements has been performed for years 10. In multiple meta-analyses and systemic reviews, increased ONSD has been found to be a highly sensitive (ranging from 90% to 93%) and moderately specific (ranging from 74% to 85%) metric for detecting elevated ICP 4, 5, with an overall mean ONSD difference of 1.3 mm (95% CI 0.6–1.9 mm) between elevated and normal ICP 7. Optic nerve sheath diameter (ONSD) has been proven to reliably detect elevated ICP in terrestrial clinical studies 4, 5, 6, 7, 8. A portable, reliable, non-invasive measurement tool for evaluating the magnitude of optic nerve sheath expansion in SANS remains an area of need. Therefore, SANS is a crucial NASA research priority to further understand and prevent visual compromise during LDSF 3. Normal visual function is vital for mission performance and safety. Although the specific etiology of SANS is unclear, possible etiologies include a cephalad fluid shift in microgravity that may cause chronically elevated intracranial pressure (ICP), cerebral venous congestion, lymphatic dysfunction, compartmentalization of cerebrospinal fluid (CSF) within the optic nerve sheath, disturbances in the one-carbon metabolic pathway, increased CO 2 exposure, or a combination of these mechanisms 2. Together, these findings in astronauts have been termed “spaceflight associated neuro-ocular syndrome” (SANS). These findings include optic disc edema, globe flattening, choroidal folds, hyperopic shifts in refraction, and optic nerve sheath distension.

Up to 60% of astronauts exposed to microgravity during long-duration space flight (LDSF) develop some degree of neuro-ocular changes 1.

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