Topical Ketamine Promising in Relieving Allodynia from Complex Regional Pain Syndrome

Complex regional pain syndrome is a little understood but very painful syndrome that can begin after a seemingly minor injury. What makes this syndrome even more puzzling is the allodynia, severe pain results from a relatively harmless action, such as brushing lightly against the affected limb. Because it is neuropathic, or nerve pain, it is particularly hard to treat. Many of the medications that are tried have side effects that may make them unbearable for the patient, regardless of the ability to relieve pain. For this reason, researchers are constantly working on finding an alternative for people living with this disorder.

Ketamine is a powerful anesthetic that is used mostly by veterinarians, but may be used for humans. It causes the person to feel as if he or she isn't part of the pain, it's often called a dissociative anesthetic. Using smaller doses, not enough to induce unconsciousness, may help relieve some types of pain, most often after surgery, but also chronic pain. But it is still not a commonly used medication.

The authors of this study wanted to determine if topical ketamine, ketamine that is placed on top of the skin, rather than ingested or injected, could help patients with complex regional pain syndrome. To do this, researchers recruited 20 patients (14 female) who had been diagnosed with the syndrome. Twelve had developed it in the arm and eight in the leg. Five patients developed it after a fracture, six after a sprain or soft tissue injury, three patients after surgery or receiving a injection, and four after an infection, clotting, electric shock, or anaphylaxis (allergic reaction).

All patients were tested for their sensation, their ability to feel a light touch, their pain-pressure threshold, sharpness stimulation, light brushing sensation, and cold and heat thresholds. The patients were evaluated at the time the cream was applied to both their affected limb and their non-affected limb - and then again 30 minutes later, in two sessions, one week apart. What the researchers found was that although the ketamine cream didn't decrease the actual pain from the complex regional pain syndrome, it reduced the allodynia and this could be an important finding. Allodynia is reported as one of the most frustrating parts of the disorder. The authors of this article hope that these findings will lead to more research into using ketamine for complex regional pain syndrome.

Reference: 

Philip M. FInch, Lone Knudsen, and Peter D. Drummond. Reduction of allodynia in patients with complex regional pain syndrome: A double-blinded placebo-controlled trial of topical ketamine. In PAIN. Vol. 146. Pp. 18 to 25.

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