The central nervous system includes the brain and spinal cord. The peripheral nervous system is comprised of all the nerve branches that leave the skull or spine to feed the individual muscles and areas of sensation. The most dramatic difference between the two systems is that, unlike the peripheral nerves, once the human brain or spinal cord is injured, they are essentially incapable of significant regeneration. Damage for the most part is permanent, and this limitation is consistent for all mammals, not just for humans. Scientists continue to study ways to change this natural boundary hoping to someday stimulate the spinal cord or brain to repair itself, but the inroads have been small.
The peripheral nerves, however, have been given the miraculous capability to regenerate and repair themselves. The process of how a nerve dies back after injury, and then begins the process of repair, has been studied in detail since the first experiments by Augustus Waller in 1951. Waller was a British physician who made his discoveries through detailed studies on frogs, identifying the first steps that occur when a nerve is injured. The process remains well known today among scientists as Wallerian degeneration.
Once a peripheral nerve injury occurs, and the nerve begins to die back, there is a rapid process, whereby, specific cells in the blood stream flow into the injured area and begin to digest and clean up the damaged tissue. This process prevents unhealthy scarring. As the tissue debris is cleared, the stump of the nerve develops finger-like sprouts and begins to regrow looking for its partner end. At the partner end, without nutrient support, the nerve begins to die back, or experience Wallerian degeneration. In doing so, it smartly repairs and protects the channel within the nerve trunk so the sprouts can find their way, unobstructed, by scar tissue. There are many chemical growth factors that pour into the area to stimulate the regrowth which can occur maximally at 1-2 mm per day. This growth rate seems large on the small scale, but in life it can mean 6-9 months of growth for a nerve to regenerate from a torn branch in the neck to the end of the arm.
Surgery can be done to facilitate regrowth of a nerve that has become completely cut in two, rather than just crushed. The mutilated ends of the nerve are trimmed to a fresh healthy surface and then the two ends are carefully sutured back together. Without such direct contact, it would be very hard for the nerve sprouts to find their way. Without a proper pathway, the regrowth will end in a scarred ball, with all the tender fibers exposed. This is called a neuroma, which locally, can be very painful. For large gaps between the two stumps, where the ends are no longer long enough, a graft can be sewn between the ends. The healthy nerve graft is obtained by sacrificing a less important nerve from somewhere else in the body. Grafting is rarely as successful as a direct end to end repair because the nerve, harvested from elsewhere, likely differs in thickness and fiber organization. Also, in this scenario, the nerve has to heal in two places, rather than just one.
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Injuries to the peripheral nerves can begin early in life, even at birth during delivery. The most common injury here is when the baby gets stuck in the birth canal. The baby’s head may be released, but the broader shoulders may get hung up, resulting in stretch to either shoulder, that in turn stretches the group of nerve roots, called the brachial plexus. The trauma can be significant due to the forces involved, sometimes resulting in a broken collar bone as well. This injury is also known as Erb’s Palsy, named after Wilhelm Heinrich Erb in 1874, a German neurologist who first described the precise anatomic location of the injury. Fortunately, since these nerves are part of the “miraculous peripheral nervous system,” most of these stretch injuries can repair themselves within 6-12 months. Treatment involves intense physical therapy until the nerves have a chance to regenerate. If the stretching is too great, the nerves can be torn in two, and a complex surgery is required to rejoin the nerves.
Young adults most often injure their peripheral nerves through trauma. I remember a patient during my training years who was thrown from his motor boat by a sharp wave. He was the only person on board and after being thrown, the boat was left in a sharp turn, without a driver, circling back over him in the water. As the boat was about to run over him, he dove under water to escape. Each time he came up for air, the boat was there again, with its hard bow and spinning propeller. After several successful escapes, but during his third or fourth dive, the back of his leg was cut in a spiral pattern by the whirling propeller which also severed his sciatic nerve. The good news was he survived this massive injury to his leg, and the cut to the nerve was relatively clean and sharp, so the nerve could be surgically reattached, end to end, without special grafting.
Adults tend to injure their nerves from more repetitive minor trauma related to daily activities of work. A very common injury to the median nerve in the wrist is called carpal tunnel syndrome. Repetitive bending of the wrist can lead to nerve inflammation. The patient then experiences pain in the hand, sometimes radiating up the nerve pathway to the shoulder, as well as numbness and weakness. These symptoms can often be more prevalent during the night, when one is trying to rest. If changes in activity, splinting for rest, and anti-inflammatory medications do not work, a relatively simple surgery can be performed at the wrist to take pressure off the nerve, called a carpal tunnel release.
Another spot that frequently gets irritated is the ulnar nerve, or the “funny bone nerve,” as it passes around the back of the elbow through a bony groove. Most of us have bumped our funny bone, which is actually the nerve itself, being temporarily squashed up against the elbow bone. This sends pain and tingling down our arm to the fourth and fifth fingers, which it supplies. With frequent bumping or more significant trauma to the nerve, which might even result in a broken elbow, the nerve can become scarred and squeezed permanently within the bony groove. In these cases, a surgeon can rescue the nerve by dissecting it free from scar tissue, and rerouting it away from the bony elbow and into the soft elbow crease, where traumatic bumps rarely occur. This procedure is called an ulnar transposition.
If you have an injured nerve in your arm or leg, and want to seek further information on surgical treatments, contact the Rocky Mountain Neurosurgery Center, Marcus Daly Memorial Hospital, in Hamilton, MT 406-375-4714, or visit mdmh.org.