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Structural changes in the brain and retina after long-duration spaceflight were largely uncoupled from each other, an observational study suggested.

While weightlessness-induced fluid redistribution during spaceflight may be common stressors to both the brain and the eye, development of optic disc edema appeared to be isolated from changes in the intracranial compartment, reported Karina Marshall-Goebel, PhD, of KBR, a Houston-based NASA contractor, and co-authors.

Among 19 International Space Station crew members in space for an average of 191 days, changes in total retinal thickness (an objective measure of optic disc edema) were not associated with brain white matter volume or with intracranial volume changes, the researchers wrote in . A positive association between spaceflight-induced changes in retinal thickness and lateral ventricle volume was suggested but was not definitive.

"We found a very small relationship between the changes in lateral ventricle volume -- which is the main change we're seeing in the brain -- and total retinal thickness after spaceflight," Marshall-Goebel said in a JAMA Ophthalmology . "The statistical model we ran predicted that for every 1-mL increase in lateral ventricle volume, you would see a total retinal thickness increase by 5 microns, which is quite small."

Optic disc edema, globe flattening, and chorioretinal folds are features of (SANS) and have the potential to cause irreversible damage and vision loss.

Eye problems after long spaceflights are not uncommon: earlier research reported that many International Space Station crew members experienced at least minimal subjective , with some vision changes persisting for years.

"Some of these problems really weren't identified until we got more into long-duration spaceflight," noted co-author Larry Kramer, MD, of the University of Texas Health Science Center at Houston, in an interview with MedPage Today. "Some changes weren't really apparent during the shuttle era, when spaceflight was only a few weeks."

"Almost everything going on in the space program now is long-duration spaceflight, as they try to ramp up to go to Mars and to the moon," he added. "What we're seeing are prospective studies that took 5 or 6 years to get enough data," including both eye and brain assessments. Last year, Kramer and co-authors published research demonstrating that long spaceflights significantly increased brain white matter volume from pre- to post-flight.

Of the 19 crew members Marshall-Goebel and colleagues evaluated before and immediately after long-duration spaceflight, five were women and 14 were men. Mean age was 45. Forty-one healthy non-crew members were included in the analysis to compare lateral ventricle volumes.

When adjusted for preflight total retinal thickness, a model showed that for each 1-mL increase in lateral ventricle volume after spaceflight, postflight total retinal thickness increased 4.7 μm (95% CI -1.5 to 10.8 μm, P=0.13). Adjusting for spaceflight mission duration improved prediction of postflight total retinal thickness to 5.1 μm (95% CI -0.4 to 10.5 μm, P=0.07) per 1 mL of lateral ventricle volume increase.

There was no association between postflight total retinal thickness and change in white matter volume (0.02 μm, 95% CI -0.5 to 0.5 μm, P=0.94) or intracranial volume change (brain tissue plus cerebrospinal fluid volume: 0.02 μm, 95% CI -0.6 to 0.6 μm, P=0.95). Absolute ventricular volume after spaceflight remained within normal population sizes.

"A weak association between spaceflight-induced changes in lateral ventricle volume and total retinal thickness in this study suggests that cerebrospinal fluid redistribution that likely underlies brain structural changes does not play a substantial role in the development of optic disc edema; ocular changes are likely complex and multifactorial in nature," Marshall-Goebel and co-authors wrote.

"Future research might focus on identifying factors contributing to the inter-individual variability in brain and ocular structural changes to better understand the cause of these spaceflight-induced neuro-ocular findings," they added.

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