ELEVATED INTRACRANIAL PRESSURE DOES NOT EXPLAIN SPACEFLIGHT-INDUCED OPTIC DISC EDEMA

Abstract

Purpose : Spaceflight-associated neuro-ocular syndrome (SANS) is reported to affect ~40% of astronauts on long-duration spaceflights as assessed by one or more findings: optic disc edema, hyperopic shifts, globe flattening, cotton-wool spots, or choroidal folds. The purpose of this study was to determine if noninvasive measures of intracranial pressure (ICP) were elevated during long-duration spaceflight relative to the supine posture on Earth, and if peripapillary retinal thickening provided a quantification of optic disc edema.

Methods : Ten astronauts were tested before and ~150 days (FD150) into spaceflight. Noninvasive ICP (nICP) was assessed using ultrasound measures of optic nerve sheath diameter (ONSD) and the cerebral and cochlear fluid pressure (CCFP) device. Optic nerve head structure was assessed using fundoscopy and radial pattern optical coherence tomography (OCT) images (Spectralis, Heidelberg Engineering). Retinal layers were segmented by two independent observers and total retinal thickness (TRT) was calculated from internal limiting membrane to Bruch’s membrane over a region from BMO outward 250 µm. ONSD was measured 3 mm posterior to the optic nerve head. The values from each observer varied by <10% and the mean value was used for analysis. Data are presented as 95% confidence intervals. One-way ANOVA with Dunnett’s multiple comparisons was used for statistical testing.

Results : Peripapillary TRT increased significantly on FD150 (-1 to +27 µm, P<.05), yet measures of nICP from the CCFP (P=.103) and ONSD (P=.729) were not greater than those in supine posture. One of the 10 subjects demonstrated Frisèn grade 1 edema during flight, as determined by fundoscopy, and TRT increased by 105 µm. However, this subject’s nICP measures were similar to preflight values.

Conclusions : These data suggest ICP during spaceflight is not pathologically elevated, yet optic nerve head changes consistent with edema developed in over half of this astronaut cohort. Thus the headward fluid shift associated with weightlessness induces optic disc edema independent of pathologically elevated ICP. Quantitative OCT-based metrics of optic nerve head morphology during long-duration spaceflight may provide early objective measures of subclinical edema.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.