These changes are similar to those that occur in intracranial hypertension, a potentially serious condition in which pressure builds within the skull.
A retrospective analysis of the MRI data appears online today in the journal Radiology.
A team of researchers performed MRIs on the eyes and brains and analyzed the data on the 27 astronauts, each of whom were exposed to microgravity, or zero gravity, for an average of 108 days while on the International Space Station (ISS), a habitable research facility that has been orbiting the earth since 1998 and/or the Space Shuttle. Eight of the 27 astronauts underwent a second MRI exam after a second space mission that lasted an average of 39 days.
“The MRI findings revealed various combinations of abnormalities following both short- and long-term cumulative exposure to microgravity that we believe may be related to intracranial hypertension,” said Larry A. Kramer, M.D., professor of diagnostic and interventional imaging at The University of Texas Medical School at Houston, part of UTHealth. “The physiologic changes that occur during exposure to microgravity may help scientists to better understand the mechanisms responsible for intracranial hypertension in non-space traveling patients.”
Among astronauts with more than 30 days of cumulative lifetime exposure to microgravity, findings included optic nerve sheath distension (an expansion of the cells surrounding the optic nerve, which transmits visual information from the retina to the brain) in nine of the 27 (33 percent) astronauts; posterior globe flattening (a flattening of the rear of the eyeball) in six (22 percent); optic nerve protrusion (or bulging) in four (15 percent); and moderate concavity of the pituitary dome with posterior stalk deviation (a change in the pituitary gland and its connection to the brain) in three (11 percent) of the astronauts.
The same type of abnormalities are observed in idiopathic intracranial hypertension, a condition in which no cause can be found for increased pressure around the brain, resulting in a swelling of the cranial nerves and the optic disc, the juncture between the optic nerve and the eyeball.
Bone mineral loss and muscle atrophy are some of the known effects of zero gravity on astronauts. With the onset of longer excursions in space afforded by the ISS, visual changes have also been observed and are now being studied.
“Microgravity-induced intracranial hypertension represents a hypothetical risk factor and a potential limitation to long-duration space travel,” Kramer said.
William J. Tarver, M.D., M.P.H., chief of flight medicine clinic at NASA/Johnson Space Center, said the agency has noted transient changes in vision in some ISS astronauts, the etiology of which is not yet fully understood. No astronauts have been considered ineligible for space flight duties as a result of the findings, which he said are suspicious but not conclusive of intracranial hypertension.
“NASA has placed this problem high on its list of human risks, has initiated a comprehensive program to study its mechanisms and implications, and will continue to closely monitor the situation,” Tarver said.
The study is titled “Orbital and Intracranial Effects of Microgravity: Findings at 3-T MR Imaging.” UTHealth co-authors include Khader M. Hasan, Ph.D., associate professor of radiology.
Deborah Mann Lake
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