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Thursday, July 9, 2015
The peripheral nervous system links brain and body, relaying electrical signals that control the movement or function of every muscle, tissue and organ. When peripheral nerves are injured, communication is blocked. Damage to motor nerves leads to loss of muscle strength and coordination; sensory nerve disruption causes numbness and pain.
Damaged peripheral nerves have a native ability to heal themselves, but only to a point. When they cannot, neurosurgeons from the Peripheral Nerve Surgery Service at Mass General Hospital step in to help repair and restore the function of these important conduits.
Directed by Ziv M. Williams, MD, the service is one of the only centers in the northeastern US specializing in surgical treatments of peripheral nervous system disorders. Through an active clinical and research program, Williams and colleagues are constantly exploring new techniques for reconstruction of peripheral nerves.
Many patients come to the nerve surgery unit after motor vehicle accidents, a major cause of traumatic peripheral nerve injury. Nerves might be severed, or pulled out from their roots in the spinal cord. Other patients have nerve damage due to surgery or other medical procedures. The good news is that in many cases the nerves are still intact above and below the injury, says Williams, allowing surgeons to aim for physical reconnection.
When nerves are cleanly cut, end-to-end rejoining can sometimes do the trick, Williams says. Attaching abutted nerve ends with a few hair-thin sutures allows for natural regeneration and reconnection. “Recovery can take several months but without surgically reconnecting the nerve there’s basically no chance for natural recovery in cases where there is a complete nerve injury,” says Williams.
For cases where the gap between ends is too wide to span with only sutures, Williams and colleagues use the latest tissue engineering tools, employing synthetic collagen conduits to guide regrowth and rejoining of nerves. Some conduits come impregnated with protein factors to promote tissue survival and regrowth. Generally, these scaffolds provide the capability to bridge short gaps up to several millimeters in length.
For more difficult mends, where nerve ends are seriously damaged or separated by long distances, the team employs sophisticated nerve transfer techniques, where they rewire a damaged nerve using a nearby healthy nerve that would normally serve a different part of the body. In one technique, the surgeons take a healthy, noncritical motor nerve serving the shoulder muscle, cut it and reconnect it to an injured nerve that controls the bicep. The shoulder can do with one fewer nerve, and, thanks to neuroplasticity, the brain is able to reorganize itself to conform to the new wiring. After several months, the nerves and muscles function fairly normally.
“We do all kinds of things—we’ll connect nerves that normally go to the lungs to nerves that normally go the arm, for example. For very severe cases, we’ll even connect multiple nerves,” Williams says.
Currently, nerve transfer is most often used to treat injuries to the brachial plexus, a nerve network that controls the shoulder, arm and hand. But Williams and coworkers are leading efforts to extend the use of the technique to nerves in the legs as well. In 2011, the Mass General group published the first example of a nerve transfer to correct damage to the obturator nerve that controls muscles in the inner thigh. The surgeons made a femoral-to-obturator nerve connection in a woman who suffered loss of muscle strength and difficulty walking from a nerve injury incurred during pelvic surgery and were able to restore her normal thigh strength and gait.
Ultimately, Williams would like to develop computer-brain interfaces to bridge the most severe brain-body disconnections, like spinal cord injury. These devices promise to bypass nerve injuries by detecting and decoding brain activity responsible for movements, and then directly supplying the appropriate electrical stimulation to peripheral nerves and muscles to generate willful action.
Williams, who is also an Associate Professor of Neurosurgery, Harvard Medical School, and a 2009 winner of the prestigious Presidential Early Career Award for Scientists and Engineers from the White House Office of Science and Technology, mixes neuroscience and engineering in a research program aimed at developing such computer-driven neural prosthetics. Last year, in a widely publicized pre-clinical research study, Williams and colleagues showed how such a system allowed one animal to use its thoughts to control the arm movements of a second “avatar” animal. While the technology is still far from human application, in the future neural prosthetic devices may allow a person with spinal cord or peripheral nerve injury to regain control of his or her own intact limbs.
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