You might expect to get temporarily knocked out by general anesthesia during surgery, but new research has found that it may have lasting impacts on memory and cognition.

Researchers have already found evidence that anesthesia can increase risk of cognitive decline in the elderly, but a new study published today in the medical journal Anaesthesia found evidence it may also affect people in middle age.

Anesthesia remains a mystery for doctors in many ways, despite its widespread use for over a century. Doctors still don’t understand anesthesia’s mechanism and how it works to help render patients unconscious.

In this study, researchers examined 964 participants with a mean age of 54 from the Wisconsin Registry for Alzheimer’s Prevention (WRAP).

The participants underwent two cognitive assessment tests over four years to see if one group was more likely to experience a cognitive decline or impaired memory.

Of the 670 participants with normal memory at the start of the study, those who had surgery during the study period were nearly twice as likely to show signs of “abnormal memory” than those who did not have surgery.

In total, 21 of the 114 people who underwent surgery developed abnormal memory by the end of the study.

This percentage was significantly higher than the 56 of 556 participants who developed abnormal memory and did not have surgery.

Overall, the team found that participants who had surgery were more likely to have more abnormal memory and issues with executive function, although the memory changes were fairly small.

“These data suggest that patients having surgery and anesthesia are more likely to experience impaired performance on neuropsychological tests of memory and executive function,” the study authors wrote.

They also found that having surgery was associated with a decline of immediate memory and verbal learning that was double the rate of decline among participants who did not have surgery.

‘Small but significant’

Dr. Kirk Hogan, a senior study author from the University of Wisconsin-Madison School of Medicine and Public Health, explained that the changes to memory may be small but are still significant.

“The cognitive changes we report are highly statistically significant in view of the internal normative standards we employ,” Hogan said in a statement. “But the cognitive changes after surgery are small — most probably asymptomatic and beneath a person’s awareness.”

Hogan and his co-authors said in the study that they still “observed small but significant declines in tests of memory and executive function.”

They also found the rate of deterioration was accelerated for people who already had lower cognitive performance at the beginning of the study.

Hogan did say more research was needed to better understand the long-term effects of anesthesia in other populations.

“The results await confirmation both in follow-up investigations in our own population sample after more surgeries in aging participants, and by other investigators with other population samples,” he said.

Local vs. general anesthesia

Dr. Gayatri Devi, a neurologist at Lenox Hill Hospital in New York City, said the study revealed how much more needs to be done to understand all of the effects of anesthesia, especially if a patient is already at risk for memory disorders.

“I have always been leery of general anesthesia in my memory disorders practice because I have found that it worsens cognition in my patients,” Devi, also the author of The Spectrum of Hope: An Optimistic and New Approach to Alzheimer's Disease and other Dementias, told Healthline.

Devi said that she recommends local anesthesia if possible to help diminish the associated risk of general anesthesia.

“There are, of course, unavoidable surgeries without any option but general anesthesia,” she said. “But whenever possible, I advocate for local or regional anesthesia.”

“This study underlines that anesthesia, even for uncomplicated procedures, in younger patients in their fifties, may transiently worsen cognition, although the long-term effects are not known,” Devi explained. “More studies need to be done on the effects of anesthesia on long-term cognition.”