HOUSTON — NASA this spring plans to introduce its first systematic study of the vision problems that surfaced two years ago as an unanticipated ailment among some astronauts assigned to long-duration missions aboard the International Space Station.

The Ocular Health investigation, which will use tonometry to gather regular measurements of intraocular pressures from astronauts, ultrasound scans of their eyes and intracranial physiology, is scheduled to begin with ISS Expedition 35 in late March. It will last two years.

An estimated 20% of astronauts assigned to ISS missions, which can span four to seven months, report blurred vision linked to what space medical experts now term microgravity-induced visual impairment and intracranial pressure. The blurring typically disappears during the post-mission physical readaptation phase.

“This was a process that was not predicted by what we know about human health on Earth,” said NASA ISS program scientist Julie Robinson, who summarized the research plan during a Jan. 17 briefing on upcoming space station activities. “We will be taking systematic measurements to really try and understand this process for the first time.”

Researchers expect the long-running study to pay terrestrial dividends as well as address potential obstacles to the human deep-space missions that NASA is planning. Those include possible links between the blurred vision symptoms, cardiovascular disease and  high blood pressure, Robinson said.

NASA astronaut Chris Cassady, who is training to launch to the ISS with two Russian cosmonauts on March 27, will be among the first to carry out the investigation and serve as a subject.

“This is very important data we will be collecting not only for our own health, but also as it translates to the whole planet for ocular health,” said Cassady, who stressed that the need for careful data gathering is likely to require some trial and error to perfect. “We will be prepared to iterate the process until we deliver the right data,” he said.

The National Space Biomedical Research Institute, a NASA-funded consortium of U.S. medical schools and health research organizations, launched eight new ground-based investigations into the vision issue in 2012.