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Sometimes, when people are injured on one side of the body they feel pain in the exact same spot on the other side, creating somewhat of a medical mystery. As this ScienCentral News video explains, researchers are now showing that the phenomenon is real.
Phantom Pain
As far back as the American Civil War doctors have documented cases where patients seemed to experience phantom pains in an uninjured arm or leg after suffering an injury to their opposite limb.
While working to understand the biological causes of chronic pain, neurologist Anne Louise Oaklander became curious about these types of phantom pain, having heard surprising complaints from some of her patients. "I had a number of patients who mentioned to me that they had symptoms of injury in the opposite limb, as well as in the injured limb," explains Oaklander, MD, director of the Nerve Injury Unit at Massachusetts General Hospital in Boston.
Anne Louise Oaklander, Massachusetts General Hospital
Oaklander's research group has shown that the so-called "mirror-image pain" can't be simply explained away by overuse of the uninjured limb, or as psychological. "These seemingly bizarre complaints have contributed to the impression that some chronic pain patients are crazy," she says. But, studying mirror-image pain in patients suffering from shingles – a condition caused by the virus that produces chicken pox in children, which inflicts adults with a painful rash or blisters on one side of the body – she showed that the answer is even more of a mystery – nerve damage on one side can actually lead to "crossover" nerve damage in the exact same spot on the opposite side of the body.
"Many people interpreted our shingles study as due to just the virus getting into the spinal chord and traveling over to the opposite side of the body, and that certainly is a reasonable interpretation, and the most obvious one," Oaklander says. But she had seen this crossover effect in patients with all kinds of injures, such as cut fingers, sprained ankles and broken legs. So she went on to study the mirror-pain related to direct injuries where no viruses were involved.
As was reported in Discover magazine, Oaklander's research group carefully made a tiny cut in one of the nerves in the hind paws of rats to see what would happen. This killed all nerve endings in the area of the cut. Then, counting the number of nerve endings per square millimeter in the skin of both the injured and uninjured legs, within a few weeks they saw that more than half the nerves in the spot on the uninjured paw had also died. "In the opposite paw, the opposite leg, which as far as we knew was totally healthy, fifty percent of the nerve endings had been lost," Oaklander explains.
The mirror nerve injuries are so precise that Oaklander concludes they must result from a specific nerve signal. "We've been trying to figure out how this signal spreads, because it's really remarkable the changes that we detect, in the limbs of either humans or animals, somehow are traveling up several feet to the spinal chord and then down the other limb," she says. "The real question of interest is what is the path that they take?"
She believes it may be a side effect of something our body does normally, perhaps allowing us to coordinate information coming from both sides of our body. That would mean that matching areas on two sides of the body communicate with each other in ways that no one was aware of before. There are no known pathways connecting matching sensory nerves on opposite sides of the body – if such pathways are found through further investigation, changes will have to be made to the textbooks.
While her research was done in lab rats, Oaklander says it's already benefiting pain patients simply by proving their pain is not just in their heads. "We really showed hardcore anatomical data, with facts and numbers, to prove this is a real phenomenon," she says. She hopes that with further study her work will also add to general understanding of the underlying biological changes that occur with pain, and will get chronic pain sufferers the treatment they need. "Not only validating their complaints, but also providing the tools necessary to develop new treatments and hopefully a cure," says Oaklander.
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