Undergoing surgery later in life is not associated with an increased risk of long-term cognitive decline, according to the results of a large twin study involving more than 8,500 middle-aged and elderly Danish twins.
The study, which was published this week in the journal Anesthesiology, is the latest in a growing body of research that appears to debunk the idea that the use of general anesthesia during surgery leads to what has been referred to as persistent postoperative cognitive decline (POCD).
This latest study may therefore offer some reassurance to older people who are reluctant to undergo surgery that might enhance their quality of life because they are worried it will permanently affect their cognitive abilities, including memory.
Although POCD is a well-recognized short-term complication of surgery, these findings suggest that surgery and anesthesia have little effect on memory or other thinking skills beyond the initial period after the procedure.
For the study, researchers at the University of Southern Denmark analyzed data collected from 4,299 middle-aged twins (aged 69 or under) and 4,204 elderly twins (aged 70 or older). All the twins were participants in long-term aging studies that had periodically assessed their cognitive abilities.
About 65 percent of the twins had undergone major, minor, hip or knee replacement, or other surgery 18 to 24 years before taking the cognitive tests used for the current study’s analysis.
When the cognitive test scores of the twins who had undergone surgery were compared with those who had been surgery-free, the analysis revealed that the twins who had undergone major surgery (such as heart surgery) had slightly lower scores. That difference, however, was not statistically significant.
And, indeed, further intrapair analysis — comparing the cognitive test results of a twin who had undergone surgery with his or her twin who hadn’t — revealed no cognitive differences.
Interestingly, twins who had undergone hip and knee replacement surgery scored slightly higher on the cognitive tests, although, again, the finding was not statistically significant.
The study’s authors offer a couple of possible reasons for this particular finding, however. It may be due, they write “to the fact that the best functioning individuals are offered hip and knee replacements. Furthermore, it could also be explained by reduction in pain and increased mobility after successful joint replacements and subsequent improvement in level of functioning.”
The researchers also examined data for twins who had undergone surgery within a shorter time frame — three months to two years before taking the cognitive tests. They found no association between surgery and cognitive function.
Together, these results suggest, say the researchers, “that preoperative cognitive functioning and underlying diseases were more important for cognitive functioning in mid- and late life than surgery and anesthesia.”
Limitations and implications
The study has several limitations. Most notably, it involved a highly homogenous group of Northern Europeans. The results might not be replicable in other populations.
Still, the study is not alone in its findings. As an editorial that accompanies the study points out, another twin study — one that followed World War II veterans between 1990 and 2002 — found that heart surgery had no effect on long-term cognition.
“It is interesting to consider why the perceptions of persistent POCD and dementia attributable to surgery endure despite the refutation studies,” write the editorial’s authors, Michael Avidan and Dr. Alex Evers, both professors of anesthesiology at Washington University School of Medicine in St. Louis. “It is likely that persistent POCD is a powerful example of a post hoc ergo propter hoc (after this, therefore because of this) misattribution fallacy. Anecdotes can be very compelling and one often hears about people whose cognitive abilities were permanently diminished after their surgery. It might, therefore, be assumed that the surgery or the anesthesia caused the cognitive change.”
The idea that surgery, specifically heart surgery, could lead to long-term cognitive decline can be traced back to a 1955 paper published in the Lancet — a paper that other research has since disproved, say Avidan and Evers.
“It is difficult to change a firmly entrenched belief among many researchers, clinicians, and the general public,” they add. “For all these reasons, when elderly people become demented or experience persistent cognitive decline after a surgical procedure, we suggest that the surgery is usually a coincidence masquerading as the cause.”