Two years ago, Daniel Cole’s 85-year-old father had heart bypass surgery. He hasn’t been quite the same since.

“He forgets things and will ask you the same thing several times,” said Cole, a professor of clinical anesthesiology at UCLA and a past president of the American Society of Anesthesiologists. “He never got back to his cognitive baseline. He’s more like 80 percent.”

His father probably has postoperative cognitive dysfunction (POCD) — a little-known condition that affects a substantial number of older adults after surgery, Cole said.

Some patients with POCD experience memory problems; others have difficulty multi-tasking, learning new things, following multistep procedures or setting priorities.

“Different patients are affected in different ways,” said Miles Berger, an assistant professor of anesthesiology at Duke University School of Medicine.

Unlike delirium — an acute, sudden-onset disorder that affects consciousness and attention — POCD can involve subtle, difficult-to-recognize symptoms that develop days to weeks after surgery. Most of the time, POCD is transient and patients get better in several months. But sometimes it lasts a year or longer.

POCD was first studied systematically about 20 years ago. But reports of patients who appeared cognitively compromised after having surgery date back about 100 years, said Roderic Eckenhoff, vice chair for research and a professor of anesthesiology at the Perelman School of Medicine at the University of Pennsylvania.

A 1955 report in the Lancet noted common complaints by family or friends after someone dear to them had surgery: “He’s become so forgetful. … She’s lost all interest in the family. … He can’t concentrate on anything. He’s just not the same person since.”

There is no short, simple test for POCD. Typically, a series of neuropsychological tests are administered before and after surgery.

The risk is enhanced in those who are older, have low levels of education or have cognitive concerns that predate surgery. Adults 60 and older are twice as likely as are younger adults to develop POCD.

“People who are older, with some unrecognized brain pathology, or people who have some trajectory of cognitive decline at baseline, those are the patients who you’re going to see some change in one, two or three years out,” said Charles Hugh Brown IV, assistant professor of anesthesiology and critical-care medicine at Johns Hopkins Medicine.

Researchers have examined whether the type of anesthetic used during surgery or the depth of anesthesia affects the risk of developing POCD. So far, results have been inconclusive.