03/13/2012 12:09 EDT | Updated 05/13/2012 05:12 EDT

Space flight linked to eye, brain problems

Astronauts who have spent prolonged periods in the zero gravity of space tend to show eye abnormalities linked to pressure around the brain, another study has confirmed.

The new study, which involved magnetic resonance imaging of the eyes and brains of 27 astronauts – a larger sample than previous studies — also found abnormalities in the pituitary gland and its connection to the brain in three cases. The gland, found at the base of the brain, secretes and stores a number of important hormones that regulate growth, metabolism and reproduction.

The findings, published online in the journal Radiology on Tuesday, may point to a "hypothetical risk factor and potential limitation to long-duration space travel," said study co-author Dr. Larry Kramer, a professor of diagnostic and interventional imaging at the University of Texas Medical School at Houston, in a statement. That means they could pose a problem on future missions to places such as Mars.

William J. Tarver, chief of the flight medicine clinic at NASA's Johnson Space Center, said in a statement that the U.S. space agency 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."

Astronauts have complained for decades about vision problems such as blurriness following trips into space. A recent NASA survey of 300 astronauts found correctible near and distance vision problems in 48 per cent of astronauts who had been on extended missions and 23 per cent of those who had been on brief missions. In some cases, they lasted for years after the astronauts returned to Earth.

Fluid shifting toward head causes problems

In the new study, the astronauts had spent an average of 108 days in space. Their eye abnormalities were similar to those seen in patients on Earth with idiopathic intercranial hypertension. Patients with the condition have increased pressure around their brains for no apparent reason.

Among the astronauts in the study:

- 33 per cent had expansion of the space filled with cerebral spinal fluid that surrounds the optic nerve, which connects the eye to the brain.

- 22 per cent had flattening of the rear of the eyeball.

- 15 per cent had bulging of the optic nerve.

- 11 per cent had changes in the pituitary gland and its connection to the brain.

An earlier NASA-sponsored study of seven astronauts, published last November in the journal Ophthalmology, found similar abnormalities and also noted that they were similar to those experienced by patients on Earth suffering from pressure in the head. But it noted that astronauts did not experience symptoms usually associated with that problem on Earth, such as chronic headache, double vision or ringing in the ears.

The earlier study suggested that the problems might be caused by fluid shifting toward the head during extended periods of time in microgravity. This could result in abnormal flow of spinal fluid around the optic nerve, changes in blood flow in the vessels at the back of the eye, or chronic low pressure within the eye, the researchers said.