HOUSTON — Astronauts aboard the International Space Station this week kicked off a four-year study of vision problems that surfaced among crew members several years ago and now rank among the top health concerns facing fliers selected for future deep-space missions.

Nineteen ISS astronauts have developed symptoms of impaired vision since the ailment was first recognized in 2005, according to Dr. Christian Otto, a Universities Space Research Association remote medicine specialist who serves as the principal investigator for the NASA-sponsored Prospective Observational Study of Ocular Health.

The study, ultimately involving a dozen closely followed international astronauts, will search for a link between the blurred vision and the long observed shift of fluid from the lower torso to the chests and heads of fliers as they adjust to weightlessness. The fluid shift now appears to affect the eyes as well as the cardiovascular and central nervous systems.

“We are very excited to see this data as it comes down in the various parameters and get a better understanding of what is happening in spaceflight to the cardiovascular, central nervous systems and the eyes so we can develop countermeasures,” said Otto in a June 5 NASA video status report.

“This is probably a dose response. The longer you are in flight, it’s likely the worse this problem gets.”

The vision issue joins much better-characterized bone and muscle losses and radiation exposures as top heath concerns for long-duration space flight.

“This is our first formal take at doing a study to document systematically eye health,” Otto said. “Our hypothesis is that the intracranial pressure actually elevates in flight due to the fluid shift in zero gravity.”

NASA astronaut Karen Nyberg and European Space Agency astronaut Luca Parmitano, who began a 5-6 month stay aboard the ISS on May 28, became the first of the subjects expected to undergo closer vision scrutiny before, during and after their space flights with regular funduscopy and tonometry exams. The two techniques are used to observe the retina and back of the eyeball and measure intraocular pressures.

In addition, they are undergoing on-board ocular ultrasound exams to observe changes to the eye and optic nerve, which appears to undergo a flattening as intracranial pressure builds.

The subjects will be tracked for a year post-flight with similar exams to assess whether the symptoms abate.

“We are finding they are not returning to normal as quickly as we would like, and in some cases the abnormalities are persisting for years,” Otto said.

Currently, veteran U.S. and Russian astronauts Scott Kelly and Mikhail Kornienko are training for a one-year ISS stay in 2015. They too will be the focus of an ocular research strategy that has been two years in the planning.