General anesthesia during surgery may increase a patient's pain after they regain consciousness, U.S. researchers said.
They said "noxious" or chemically irritating anesthesia drugs, which include most of those used in general anesthesia, sensitize nerves that sense pain and cause inflammation.
"The choice of the anesthesia may be a contributing factor to post-surgical pain and inflammation," said Gerard Ahern of Georgetown University Medical Center in Washington, whose study appears in the Proceedings of the National Academy of Sciences.
Ahern said doctors have known for some time that anesthesia drugs can cause pain at the injection site or in the lungs, and anesthesiologists often administer drugs first to dull that pain.
"That was thought to be a temporary thing," Ahern said in a telephone interview.
Ahern and colleagues suspected that the noxious chemicals in most general anesthetics were acting on two specific sites on nerve cells known as TRPV1 and TRPA1. Both are involved in sensing pain from irritants in plants, like wasabi.
In lab experiments, they found that TRPA1 — more commonly known as the mustard oil receptor — becomes activated when exposed to noxious anesthetics.
"It's a major pain receptor on peripheral nerves," Ahern said. "When they are activated they cause burning pain."
To see if TRPA1 was involved, they tested the drugs on mice genetically engineered to lack the TRPA1 gene and found they showed no pain when the drugs were used, unlike normal animals.
Next, they simulated surgery by putting normal mice under general anesthesia and applying a bit of mustard oil to their ears. They used both noxious anesthetics and an anesthesia drug called sevoflurane that does not have this quality.
The mice that were treated using noxious anesthesia drugs had much more inflammation, which lasted for several hours.
While the findings would need to be verified in human studies, Ahern said they do help explain why these drugs cause pain and suggest the need for more study on ways to prevent or avoid it.
"I think it's a factor that should be considered," he said.