Some effects persist one year later

Structural brain changes were seen in an MRI study of astronauts before and after long-duration spaceflight.

“Long-duration spaceflight was associated with increased pituitary deformation, augmented aqueductal cerebrospinal fluid (CSF) hydrodynamics, and expansion of summated brain and CSF volumes,” wrote Larry Kramer, MD, of the University of Texas Health Science Center at Houston, and coauthors in Radiology.

“Summated brain and CSF volumetric expansion persisted up to 1 year into recovery, suggesting permanent alteration,” they added.

The effects of microgravity on the nervous system and brain are in the early stages of study. Earlier studies have suggested changes in CSF flows and pulsatility, and have demonstrated narrowing of the central sulcus, upward shift of the brain, and narrowing of CSF spaces at the vertex after long-duration flights. Changes in aqueductal peak CSF velocity have also been reported.

Space-flight-related changes have been found to affect cognitive and motor function, and up to 60% of returned astronauts experience subjective change in visual acuity that may persist for years, a phenomenon known as spaceflight-associated neuro-ocular syndrome. MRIs have shown variable optic disc edema, posterior globe flattening, optic nerve protrusion, pituitary dome concavity, and posterior stalk deviation. Along with optic nerve and sheath changes, the findings were thought to resemble idiopathic intracranial hypertension.

In their study, Kramer and colleagues analyzed data from 11 astronauts, 5 of whom had previously experienced space flight. The average age of the group was 45 and 10 of the 11 astronauts were men. All were evaluated with 3 Tesla MRI before planned long-duration spaceflight (mean length 171 days), then at days 1, 30, 90, 180, and 360 postflight. Intracranial volumetry and aqueductal CSF hydrodynamics were tracked, along with pre- to postflight (day 1) pituitary changes.

At the day 1 post-flight evaluation, mean volume was increased for whole brain (28 mL; P<0.001), white matter (26 mL; P<0.001), mean lateral ventricles (2.2 mL; P<0.001), and mean summed brain and CSF (33 mL; P<0.001).

By 360 days, summated mean brain and CSF volume remained increased (28 mL; P<0.001), which was “predominantly composed of expanded white matter (26 mL; P<0.001) and, to a lesser degree, lateral ventricular (2.2 mL; P<0.001) volumes,” the authors noted. Brain, lateral ventricular, total ventricular, and white matter volumes individually also remained significantly elevated at 1 year.

“The results of our study agree with previous work demonstrating enlargement of the ventricles after spaceflight and confirm that the changes are small in magnitude and remain within the range for healthy adults of similar age,” Kramer and co-authors wrote. “Normative data showing that expected ventricular growth of 0.65 mL per year was exceeded within our pre- to postflight time range suggest that ventricular expansion is not exclusively age related.”

“This distinction is important because increased summated brain and CSF volumes can potentially augment intracranial pressure (ICP) according to the cranial pressure-volume relationship,” they added.

Day 1 increases seen in mean aqueductal stroke volume (14.6 mL; P=0.045) and mean CSF peak-to-peak velocity magnitude (2.2 cm/sec; P=0.01) returned to baseline by 1-year follow-up.

Noting that normal pressure hydrocephalus similarly shows ventricular enlargement and increased aqueductal stroke volume associated with impaired intracranial compliance and augmented ICP pulsatility, Kramer and colleagues observed that “glymphatic pathway dysfunction has been implicated in normal pressure hydrocephalus and, if applicable in microgravity, it could help explain increased white matter free water in postflight astronauts.”

Average midline pituitary height decreased from 5.9 mm at baseline to 5.3 mm at day 1 (P<0.001). Six of the 11 astronauts had pituitary dome depression compared with baseline.

“Our combined observations could support an ICP model of spaceflight-associated neuro-ocular syndrome pathogenesis,” Kramer and colleagues wrote. “The development of pituitary gland deformation in the majority of astronauts indicates that summated expansion of brain and CSF volumes is incompletely offset by a decrease in blood volume, resulting in elevated ICP.”

The constellation of findings about pituitary depression, hyperdynamic CSF flow, and persistent white matter and ventricular intracranial volume increase, “is presumed to result in increased intracranial pressure and pulsatility, which may help to explain (and develop strategies to avoid) potential long-term effects of microgravity such as space flight associated neuro-ocular syndrome,” observed Michael Lev, MD, of Massachusetts General Hospital in Boston, in an accompanying editorial.

Study limitations include time delays between preflight baseline imaging and launch, reliance on literature-based normative data to help distinguish the effects of space flight from those of normal aging, and inaccuracy in the segmentation of compressed dural venous structures affecting white matter, ventricular, and cerebral blood volume measurements.

There also was no direct preflight, inflight, or postflight measurement of ICP or compliance, Kramer and colleagues noted. They suggested caution interpreting these findings as inflight altered ICP or compliance as opposed to a readaptation response to postflight gravity.

  1. Long-duration spaceflight (average 171 days) was associated with increased pituitary deformation, augmented aqueductal cerebrospinal fluid (CSF) hydrodynamics, and expansion of summated brain and CSF volumes.

  2. Summated brain and CSF volumetric expansion persisted up to 1 year after astronauts returned to earth.

Paul Smyth, MD, Contributing Writer, BreakingMED™

Kramer disclosed money to his institution from NASA Human Research Program and from Kellogg Brown and Root.

Lev disclosed money received for consultancy from GE Healthcare and Takeda.

Cat ID: 130

Topic ID: 82,130,730,130,192,925,96

References

Kramer LA, et al “Intracranial effects of microgravity: A prospective longitudinal MRI study” Radiology 2020; DOI: 10.1148/radiol.2020191413.

Lev MH “The long-term effects of spaceflight on human brain physiology” Radiology 2020; DOI: 10.1148/radiol.2020201164.