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FORT LAUDERDALE, Fla. -- In fibromyalgia patients undergoing knee or hip replacement surgery, low-dose, intraoperative ketamine appeared to reduce the need for post-surgical opioid therapy, researchers said here.

Based on a retrospective review, the dose requirement for oral morphine to control pain in the 48 hours after surgery was 135 mg for patients who received intraoperative ketamine versus 250 mg for those who did not (P=0.054), reported Jonathan Day, MD, a major in the U.S. Air Force, who led the study while a fellow in anesthesiology at the Mayo Clinic in Jacksonville, Fla.

"Despite the small sample size – 15 patients who were not given ketamine versus 10 patients getting the sub-anesthetic dose of ketamine -- this was marginally statistically significant and consistent with anecdotal reports," Day said in a presentation at the American Academy of Pain Medicine meeting.

Ketamine is a dissociative anesthetic and has been used in human and veterinary medicine since 1963.

After hearing anecdotal reports from fibromyalgia patients of fibromyalgia symptom improvement after receiving ketamine, Day's group decided to analyze patient records at their clinic to confirm these reports.

They performed a chart review of 25 consecutive patients with fibromyalgia. Ten of the patients were undergoing total hip arthroplasty and 15 were having total knee arthroplasty. The same regional anesthetic blockade was used in all cases. Five patients in each surgical group were also given low-dose ketamine infusion.

"Our focus was on 48 hour postoperative oral morphine equivalent use," the authors explained.

The researchers reviewed outcomes involving 25 patients who underwent 26 procedures. Two of the patients were men. The median age of the patients was 60 (range 28 to 82). They were treated between November 2009 and July 2011.

"Fibromyalgia is a complicated disease process which despite modern treatment often leaves these patients in significant pain," Day said. "The subsequent high use of opioids further complicates their care. Low-dose ketamine, with its proven opioid sparing effects, may provide a novel avenue of treatment for patients with fibromyalgia."

His group called for more in-depth, prospective studies.

The study results suggest that "in a sub-anesthetic dose, [ketamine] can be synergistic with other analgesics and can reduce the amount of opioids, and that is a good thing," commented Lynn Webster, MD, president of the American Academy of Pain Medicine and medical director of CRILifetree in Salt Lake City.

Sub-anesthetic doses of ketamine can range from 0.1 mg/kg to 0.5 mg/kg, but dosing of the drug is highly individualized, he cautioned.

Paul Sloan, MD, from the University of Kentucky in Louisville, also warned that "chemically, ketamine is a cousin of the hallucinogen LSD [lysergic acid diethylamide]. Some of my patients have experienced hallucinogenic episodes with the drug," he said.

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