Brain scans of astronauts who have spent more than a month in space have shown serious damage in their eyeballs and brain tissue that could jeopardise the future of long-term space missions.
Researchers used MRI scans to examine the eyes and brains of 27 astronauts, who had flown long-duration Nasa missions, and found a pattern of deformities in their eyeballs, optic nerves and pituitary glands that remains unexplained.
This ranged from a flattening and bulging of the eyeball to damage to the connections between the brain and the pituitary gland -- one of the key glands governing bodily functions, the researchers said.
The astronauts examined had been exposed to zero or micro gravity for an average of 108 days, either on space shuttle missions or on-board the International Space Station, Daily Mail reported.
The problems found in the astronauts are similar to those caused by intracranial hypertension - a rare and dangerous condition where pressure inside the brain rises and presses against the skull and eye sockets.
Prof Larry Kramer, who led the tests at the University of Texas Medical School at Houston, said: "Microgravity-induced intracranial hypertension represents a hypothetical risk factor and a potential limitation to long-duration space travel.
Medical crews at Nasa and four other major space agencies in Europe, Russia, Japan and Canada are now investigating the issue.