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The question: I recently had knee-replacement surgery, and I just don’t feel like myself. I’m a retired school principal and I used to have an excellent memory. Now I struggle to remember the simplest things – even my husband’s name. I am slowly getting better, but it has been very frustrating and frightening. My doctor said I could be suffering a side effect from the anesthetic or the operation. Is this a common problem after surgery?

The answer: Doctors have long assumed that patients return to normal once an anesthetic wears off. But in recent years, a growing body of research suggests that some people experience lingering cognitive impairment after being wheeled out of the operating room.

“We now know that between 10 and 20 per cent of patients will have problems at the time they are discharged from hospital,” says Dr. Beverley Orser, an anesthesiologist at Sunnybrook Health Sciences Centre in Toronto.

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“And one in 10 of those affected will still have problems three to six months later. What happens at the one-year mark is still debated,” she says, adding that some may never fully recover.

The cognitive impairment seems to affect memory, word-finding and attention as well as certain “executive functions” of the brain, such as decision-making and problem-solving.

In some patients, the effects can be very subtle. They may not feel mentally impaired, although family members may notice a difference in their memory or behaviour.

Experts believe the anesthetic drugs used to induce an unconscious state – so patients don’t feel pain or remember the surgery – may be partly responsible for the cognitive changes.

But the biggest contributor to the mental deficits, known as postoperative cognitive dysfunction (POCD), might be the operation itself.

“Surgery is not a natural thing, and the body goes into a high state of alert,” says Dr. Roderic Eckenhoff, vice-chair of research in the department of anesthesiology and critical care at the University of Pennsylvania in Philadelphia.

In particular, he adds, the immune system unleashes a cascade of cells that produce an inflammatory response, which continues to ramp up in intensity several days after the operation.

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The brains of older patients and those with complex medical conditions appear to be especially vulnerable to this inflammatory assault.

“For people with existing mild cognitive impairment, the surgery may be tipping them over the edge,” says Orser, who is also chair of the department of anesthesia at the University of Toronto.

Researchers are looking for ways to reduce the risks of POCD. For instance, they are studying whether certain drugs that are given to surgical patients may have fewer negative effects than others. As well, in some cases, a local nerve block can be used to ease pain without the need for a full anesthetic.

“With a nerve block, you recover faster from the anesthetic because you get less of the drug,” says Dr. Stephen Choi, a Sunnybrook anesthesiologist.

Even so, it’s not clear that the use of a nerve block actually protects the brain from cognitive impairment. The powerful effects of postsurgical inflammation may simply overwhelm the brains of some patients.

Researchers agree that there are gaps in their understanding of POCD, and they can’t identify with certainty which patients will run into problems. Although seniors tend to be at an elevated risk of post-surgery memory lapses, some do just fine.

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So what are patients and health-care providers supposed to do while researchers probe for solutions?

Good postoperative nursing care is probably the best prevention strategy for all patients, says Eckenhoff. That means making sure patients resume a normal routine as soon as possible. “If they use glasses or hearing aids, they should get them back quickly so they can reorient themselves,” says Eckenhoff.

Returning to a natural sleep cycle is also critically important, he adds. “Poor sleep hygiene is thought to translate into a greater degree of inflammation of the brain.”

Choi says patients should at least be aware they might feel off-kilter after an operation, but he stresses that the risk of possible cognitive impairment is not a good enough reason to forgo an operation.

After all, the goal of most surgeries is to improve a patient’s quality of life or even extend life. Those benefits will likely outweigh any possible memory deficits – whether short or long term.

Paul Taylor is a Patient Navigation Advisor at Sunnybrook Health Sciences Centre. He is aformer Health Editor of The Globe and Mail. You can find him on Twitter and online at Sunnybrook’s Your Health Matters.

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