One of the studies I saw was from Sweden and it looked specifically at twins where one had been injured. This is probably about as close to an RCT as you can g

One of the studies I saw was from Sweden and it looked specifically at twins where one had been injured. This is probably about as close to an RCT as you can get. That said, it doesn't separate the anesthesia from damage due to the injury itself, or the trauma of the surgery. Still, getting anesthesia for something like a colonoscopy or a root canal strikes me as a very disproportionate risk given the evidence.