As many as 80 percent of all amputees experience pain in their stumps or a continued perception of a phantom limb long after the surgical wounds have healed. The pain that amputees feel in their stumps comes from a different source than the pain that some experience from phantom limbs, researchers suggest.
Dr. Cathy Stannard and researchers from the Johns Hopkins University in Baltimore have carried out tests that show stump and phantom pain are quite two distinct types of pain with different sources. The result of their study provides doctors clues as to what type of pain relief medication is best for amputee patients.
Presently, stump and phantom pain treatments are inadequate because little is known about the underlying mechanisms.
"Overall, the treatment of phantom and stump pain has been disappointing, in part due to the uncertain nature of the mechanisms behind the pain and the lack of well-controlled clinical studies," said Srinivasa N. Raja, M.D., lead author of the study and professor of anesthesiology and critical care medicine at Hopkins.
The researchers found that the powerful pain relief drug morphine significantly relieved both stump and phantom pain, whereas the local anesthetic lidocaine relieved only the stump pain.
Stump pain is believed to arise from nerve injuries at the site of the amputation. Nerve injuries result in the formation of neuromas, non-cancerous tumors that grow out of the injured nerve fibers. These tumors may trigger pain impulses in a random fashion or may give off pain signals when trapped by other tissue, such as muscle.
Phantom pain is thought to reside in the brain. Other areas of the brain fill in the part of the brain that used to control the limb before the limb was amputated.
The Hopkins data supports that theory. Since lidocaine is a drug that predominantly works on the peripheral nervous system, it did not relieve phantom pain. Morphine works well on both types of pain because it acts on the peripheral and central nervous systems.
The researchers studied 32 patients with an average age of 54, who had amputated limbs: twelve patients had pain in the region of the stump, nine had pain in the missing part of the limb and eleven patients experienced both types of pain.
On three consecutive days, each patient was given an intravenous injection of any of the following: morphine, lidocaine or a placebo. The study was double-blind, meaning neither the patients nor the researchers knew which injection was being given.
Then pain measures and patient satisfaction scores were recorded every five minutes from 30 minutes before the injection until 30 minutes after.
Patient satisfaction scores were similar and significantly higher for both lidocaine and morphine compared to placebo, which did not significantly reduce either type of pain.
Dr Cathy Stannard, a consultant in pain medicine at the Frenchay Hospital, Bristol, told BBC News Online that the conclusions of the study were similar to the general consensus among experts.
Dr Stannard said, "Stump and phantom pain are quite possibly two very distinct things, but there is quite a strong relationship between the two, people who have stump pain are more likely to also experience phantom pain. It may be that although stump pain has a local pathology, it feds into the general process that generates the phantom pain."
Also, Stannard said some patients were reluctant to admit to phantom pain, possibly because of embarrassment.
She also said that the strong emphasis on rehabilitation meant that some patients were not given sufficient information about the types of pain that they might experience.
The charity Action Research is currently funding research to find out if the use of the drug ketamine prior to amputation operations could be effective in preventing phantom pain.
Lead researcher Dr Alistair Nimmo of the Royal Infirmary of Edinburgh agreed that stump and phantom limb pain could be very different. But he stressed that the latter is particularly difficult to treat.
He said,"There is still no effective remedy for phantom limb pain, which can bother amputees for many years after their operation."
"Treatment is difficult and standard pain killers are often ineffective. It is hoped that our Action Research funded project will lead to treatment to be directed at preventing this disabling pain from developing," Nimmo continued.
The research was presented on Oct. 16 in New Orleans at the annual meeting of the American Society of Anesthesiologists.
"Our results suggest that different therapeutic sensitivities of stump and phantom pain to these drugs exist, and that the mechanisms of these two types of pain may differ," said Srinivasa N. Raja, M.D.
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