In Spaceflight, Lengthier Lower-Body Negative Pressure is Needed for Edema

Astronauts’ missions are progressively longer, with the impacts of weightlessness on the human body of greater concern, such as spaceflight-associated neuro-ocular syndrome (SANS) and subsequent risk for chronic optic disc edema. Thickness of tissues at the optic nerve head have exhibited changes during extended assignments, with SANS thought to develop from headward fluid shift. In a study, researchers evaluated in-flight sessions of a possible preventive measure — 25 mm Hg lower-body negative pressure (LBNP) — and its impact on optic nerve head and retinal morphology, with data reported in JAMA Ophthalmology.

Six of the 14 astronauts and cosmonauts had previous spaceflight experience, and mean mission time to the International Space Station was 214±72 days. The National Aeronautics and Space Administration’s (NASA’s) Fluid Shifts Study included crew members with a mean age of 45±6 years, 79% were men, 21% were women. They underwent optical coherence tomography (OCT) imaging before, during, and after missions. In-flight imaging took place in typical weightless conditions and after 10 to 20 minutes of LBNP, during sessions scheduled on or near flight days 50, 150; and for 4 individuals, day 250. Left eye data was analyzed between 2016 and 2021.

Meaningful differences arose by flight days 50 and 150. Bruch’s membrane opening (BMO) height trended lower and posteriorly shifted by flight day 50 — significantly lower by day 150 (P =.009). In fact, a number of parameters meaningfully changed between flight days 50 and 150: MRW, cup volume, peripapillary total retinal thickness (TRT) 250 μm and 500 μm from BMO (all P <.001), as well as TRT 1000 μm (P =.04), and macular thickness (MT) at within 500 μm foveal radius (P =.046). Thus, optic disc edema and macular thinning were time-dependent.

MRW and cup volume changes continued until 30 days after missions, but returned to baseline by post-flight day 180. Notably, distance from the optic nerve head was linked to TRT changes; TRT 1000 increase resolved by 10 days post-flight, compared with TRT 250 and 500 that returned to pre-launch level after 180 days. MT 500 decrease resolved 180 days after return, although MT 1500 μm recovered by 30 days post-flight.

Since jugular vein pressure and IOP decreased during LBNP sessions, 25 mm Hg proved adequate to reverse headward fluid shift. Yet, this countermeasure failed to have a significant impact on optic nerve head or retinal morphology, so longer LBNP treatment may be needed, according to the analysis.

Previous research has also shown increases in MRW and TRT by flight day 150. In the present investigation, macular thinning was consistent across time but recovered after return to earth, suggesting thinning reflected mechanical force rather than loss of ganglion cells. In spaceflight, prolonged, slightly raised IOP and choroidal thickening may compress the macula, the researchers speculate.

The astronauts’ various schedules limited testing availability to the first half of the day instead of an exact time. No control group was employed, and cause and effect was not established. The study concludes that enlarged peripapillary TRT and reduced optic cup size indicates mild optic disc edema. “However, in-flight posterior ONH displacement and thinning of the central macula suggest that ICP alone cannot explain the ocular findings associated with long-duration spaceflight,” the researchers add.

Reference

Pardon LP, Macias BR, Ferguson CR, et al. Changes in optic nerve head and retinal morphology during spaceflight and acute fluid shift reversal. JAMA Ophthalmol. Published online June 16, 2022. doi:10.1001/jamaophthalmol.2022.1946