Brain scans of NASA astronauts who were in space for more than a month revealed potentially serious abnormalities that could set back plans for longer deep space missions, according to a US study published Tuesday.

Researchers from The University of Texas Medical School in Houston scanned the eyes and brains of 27 astronauts who had spent an average of 108 days in space, either on space shuttle missions or aboard the International Space Station.

They found that those who spent more than a month in space were more likely to suffer from intracranial hypertension -- a potentially serious condition that occurs when pressure builds within the skull.

"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."

- Dr. William Tarver, chief of the flight medicine clinic at NASA's Johnson Space Center

The symptoms included excess cerebral-spinal fluid around the optic nerve in 33 percent of the astronauts studied, while a fifth showed a flattening of the back of the eyeball, which affects the ability to focus, research published in the journal Radiology showed.

The scans also showed that 15 percent of the astronauts had a bulging optic nerve and 11 percent experienced changes to the pituitary gland, which is located between the optic nerves and secretes and stores hormones that regulate a variety of important body functions.

Professor Larry Kramer, lead author, said, "The MRI findings revealed various combinations of abnormalities following both short and long-term cumulative exposure to microgravity also seen with idiopathic intracranial hypertension."

Kramer said the impact of space travel on astronauts' brains and eyes represented a "potential limitation to long-duration space travel."

Bone mineral loss and temporary muscular aches have been known to affect astronauts in the past and NASA was now focusing on concerns over the eye health of its space flight corps.

Dr. William Tarver, chief of the flight medicine clinic at NASA's Johnson Space Center, said, "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."

He described the findings as "suspicious" but not conclusive of intracranial hypertension, and added that no astronauts have been considered ineligible for space flight duties as a result of the findings.