Using an innovative surgical technique, researchers have rerouted major nerves to give amputees greater control of prosthetic arms — and unwittingly restored the sense of touch and temperature in their "phantom" limbs.
The surgery connects residual arm nerves to a nerve below the pectoral muscle in the chest. The new connection allows nerve growth into the skin of the chest.
When certain spots on the chest are touched, the amputee "feels" the sensation in their missing hand.
"It appears that the body has a really surprising ability to heal itself, and the idea that these nerves were able to regenerate through the skin over such large distances did come as quite a surprise to us," said co-author Paul Marasco, a sensory neurophysiologist and post-doctoral fellow at the Rehabilitation Institute of Chicago.
In fact, the researchers did not set out to restore a sense of feeling in an amputee's phantom hand. They were trying to improve the person's ability to better use their prosthetic arm, which is controlled by electrical signals.
"The idea was that this would help these individuals control limbs much more effectively because they're using the signals," Marasco said Monday from Chicago. "Essentially, what they're doing is thinking in a normal way to move their hand in particular ways and the computerized prosthetic limb picks up that signal and then
transfers it to the appropriate motion in the arm."
In their study, published in Tuesday's Proceedings of the National Academy of Sciences, the researchers report on two patients: a 54-year-old man who lost both arms after being electrocuted while working on high-tension lines and a 24-year-old woman who lost one arm after a motorcycle crash.
After the man's surgery, the technician was preparing to attach the prosthetic arm when the man said, "You're touching me. I feel that in my hand," said Marasco.
"And everybody kind of stopped and looked at him and pushed on (his chest) a few times and realized that he was feeling like he was actually being touched in his hand," he said. "It was all a surprise."
The surgery also seems to affect an amputee's sense of hot and cold.
Marasco said the woman complained before her surgery that her phantom limb always felt cold.
"But what happened after her sensation developed, she just on her own took a hot water bottle and put it on her chest and actually got relief for the cold," he said of the woman after her surgery. "So it doesn't bother her any more. Her limb doesn't feel cold any more."
Surgery fairly easy: researchers
Although powerful in its effects, the surgery isn't complex or extremely invasive, Marasco said. Surgeons take the ends of several large nerves that remain after amputation of the arm — nerves which previously controlled limb and hand movement — and hook them up to the nerve under the pectoral muscle in the chest.
"We take each of those individual big nerves and then we plug right onto the little nerve that we cut on the underside of the muscle," he said. "And once that's done, then the motor parts, or the motor nerves, grow into muscle and re-establish connections with the muscle in the chest instead of the muscles they once served in the arm."
"For all intents and purposes, as far as we can tell right now, those sensory parts of those nerves … appear to push their way out through the muscle into the skin, then reinnervate the skin."
Dr. Milos Popovic, a senior scientist in spinal cord rehabilitation at Toronto Rehab, agreed the work is "a very exciting development … that you can potentially get ways to communicate sensation back to the individual who has lost the arm."
"And any way we can provide this feedback would be very beneficial."
But he said reinnervation could also have a downside, increasing the amount of phantom limb pain that amputees experience. That pain, which is often excruciating, seems to arise from the neurological map of the limb retained by the brain and may emanate from severed nerves at the amputation site.
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