Two years ago, Dr Daniel Cole's 85-year-old father had heart bypass surgery. He has not been quite the same since.
"He forgets things and will ask you the same thing several times," said Dr Cole, a professor of clinical anaesthesiology at UCLA and a past president of the American Society of Anaesthesiologists.
"He never got back to his cognitive baseline," Dr Cole continued, noting that his father was sharp as a tack before the operation.
His father probably has post-operative cognitive dysfunction (POCD) - a little-known condition that affects a substantial number of older adults after surgery, he said.
Some patients with POCD experience memory problems; others have difficulty multitasking, learning new things, following multistep procedures or setting priorities.
"There is no single presentation for POCD. Different patients are affected in different ways," said Dr Miles Berger, an assistant professor of anaesthesiology at Duke University School of Medicine.
Unlike delirium - an acute, sudden-onset disorder that affects consciousness and attention - POCD can involve subtle, difficult-to-recognise symptoms that develop days to weeks after surgery.
LIKELY TO BE AFFECTED
People who are older, with some unrecognised 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.
DR CHARLES HUGH BROWN, assistant professor of anaesthesiology and critical care medicine, on post-operative cognitive dysfunction in older patients.
Most of the time, POCD is transient and patients get better in several months. But sometimes - how often is unclear - this condition lasts up to a year or longer.
There are many unanswered questions about POCD. How should it be measured? Is it a stand-alone condition or part of a continuum of brain disorders after surgery? Can it be prevented or treated? Can it be distinguished in the long term from the deterioration in cognitive function that can accompany illness and advanced ageing?
Here's what scientists know about POCD.
It was first studied systematically about 20 years ago. But reports of patients appearing cognitively compromised after surgery date back about 100 years, said Dr Roderic Eckenhoff, vice-chair for research and a professor of anaesthesiology at the Perelman School of Medicine at the University of Pennsylvania.
An influential 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."
To recognise the condition, typically, a series of neuropsychological tests are administered before and after surgery - a time-consuming process. Often, tests are given one week and again three months after surgery.
But the tests and time frames differ in various studies. Studies also define POCD differently, using varying criteria to assess the kind and extent of cognitive impairment that patients experience.
How common is it? The first international study of older adults with POCD in 1999 suggested that 25.8 per cent of patients had this condition one week after a major non-cardiac surgery, such as a hip replacement, while 9.9 per cent had it three months after surgery.
Two years later, a study by researchers at Duke University Medical Centre found that 53 per cent of adults who had heart-bypass surgery showed significant evidence of cognitive decline when they were discharged from the hospital; 36 per cent were affected at six weeks; 24 per cent, at six months; and 42 per cent, five years after their operations.
Another Duke study of older adults who had knee and hip replacements found that 59 per cent had cognitive dysfunction immediately after surgery; 34 per cent, at three months; and 42 per cent, at two years.
The risk of experiencing POCD after surgery is enhanced in those who are older, have low levels of education or have cognitive concerns that predate surgery.
Adults age 60 and older are twice as likely as younger adults to develop POCD.
"People who are older, with some unrecognised 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 assistant professor of anaesthesiology and critical care medicine Charles Hugh Brown at Johns Hopkins Medicine.
Currently, the evidence implicates the stress of surgery rather than the anaesthesia.
"Most surgery causes peripheral inflammation," Dr Eckenhoff said. "In young people, the brain remains largely isolated from that inflammation, but with older people, our blood-brain barrier becomes leaky.
"That contributes to neuroinflammation, which activates a whole cascade of events in the brain that can accelerate the ageing process."
Many patients are not told of the risk of POCD before they have surgery. That should change, said experts. "Beyond question, patients should be informed that the 'safety step' of not undergoing surgery is theirs to choose," wrote professor of anaesthesiology Kirk Hogan at the University of Wisconsin at Madison School of Medicine and Public Health, in an article published this year.
"Each patient must determine if the proposed benefits of a procedure outweigh the foreseeable and material risks of cognitive decline after surgery."