Patients 40 years old and above who underwent anesthesia prior to surgery showed no evidence of any subsequent associated mild cognitive impairment in their lives, a study has found.

Mild cognitive impairment is considered a condition somewhere between normal cognitive declines that come with aging and true dementia, explain Mayo Clinic researchers studying whether surgical anesthesia is associated with any prolonged cognitive impairment.

While some elderly patients may develop delirium following anesthesia and a surgical procedure, the condition normally resolves after days, or at most, weeks, they say.

For the study published in Mayo Clinic Proceedings, the researchers followed more than 1,700 Minnesotans aged 70 to 89 with normal mental function at the study's beginning in 2004.

Around 85 percent of the study participants had undergone at least one surgical procedure under general anesthesia after age 40.

The mental functioning of the participants was evaluated every 15 months during the study period.

"The bottom line of our study is that we did not find an association between exposure to anesthesia for surgery and the development of mild cognitive [mental] impairment in these patients," says study senior author and anesthesiologist Dr. David Warner.

While 31 percent of the participants were diagnosed with mild cognitive and memory issues during the research period, the problems were not associated with their exposure to anesthesia, the researchers noted.

The work follows earlier Mayo Clinic research that found elderly patients were no likelier than any other age group to show signs of developing dementia after surgery.

While the study found no association between anesthesia and mild cognitive impairment as a result of surgery done after age 40, the researchers said they couldn't rule out such a link in surgeries after age 60, especially in cases of vascular surgery.

"That may not be surprising, because there is increasing evidence that some of the problems that we see with cognition in the elderly may be caused by vascular problems that cause stroke and other sorts of problems like that," says Warner, suggesting more research in that area is required.

At the other end of age range in surgery patients — children — there is some evidence of associations between exposure to anesthesia for surgery and issues with memory and learning later on in life, Warner points out, although he emphasized it is "by no means established yet."

"Right now it's just associations, and we and many other people are doing a lot of work to try to see if this really is a problem in children or not," he notes.