Bridging the Gap: three questions about nerve grafts
On February 14, a team of UC San Diego Health System surgeons performed a complicated cancer surgery on Hall of Fame outfielder, Tony Gwynn. The process involved utilizing a nerve graft from Gwynn’s neck to reconstruct the facial nerve.
We asked Justin Brown, MD, director of the Neurosurgery Peripheral Nerve Program and co-director of the Center for Neurophysiology and Restorative Neurology three questions about this type of nerve grafting.
Question: Where are nerves for graft typically harvested? Can nerves be donated?
Answer: Usually a nerve is harvested from a part of the patient that is not critical. We often use a sensory nerve from the side of the foot or inside of the arm, often a region that is close to the nerve being reconstructed as in this case. The removal of this nerve from its normal location causes an area of numbness, but usually it is small and not a problem. It does not cause any weakness to take these nerves because they are purely sensory nerve. The nerve is a graft – that is, a bridge to replace the piece of nerve that is missing.
An actual nerve transplant from another person is possible, but requires taking immunosuppression drugs. In the case of a nerve transplant, however, a patient wouldn’t have to take these drugs forever as they would with an organ transplant. Nerve transplants are therefore only used in severe nerve injuries when the patient’s own nerve grafts do not appear to be enough.
Q: How long does it take for nerves, once grafted, to work?
A: In this diagram, the part of the nerve that still is in contact with the spinal cord (A) has the “wires” or axons that need to grow through the graft (C) to the paralyzed muscle in order to recover it. Those wires grow through the graft and into the distal nerve (B) and finally to the muscle. They typically grow at 1 mm per day-a little less than the thickness of a dime. So measure the distance from the proximal stump (A) to the muscle (not pictured) and you have a rough idea of how long it will take. Typically it is three months for a short distance and even more than a year for long distances.
Q: Do nerves have memory? How does a transplanted nerve know how to behave once it has been moved?
A: It’s all about the “wires” or axons within the nerve. With a nerve graft we are trying to help the original wires re-grow to the original target so often there’s nothing to learn. In complex nerves sometimes the correct wires do not go to the correct target. When this happens in the face it can cause the whole side of the face to contract instead of just the mouth when a patient tries to smile. This is called “synkinesis.”
When the proximal nerve stump (A) is not available for a graft reconstruction, sometimes we use a nerve that originally went to a different target muscle to bring axons to the distal nerve stump (B). This is called a nerve transfer. For the face, part of the nerve to the chewing muscle (masseter) can be used to recover a smile. When this is done the smile usually occurs whenever the patient bites down (the movement that made this nerve work when it was involved in chewing). With time the brain can learn which axons are going to the face and which are still involved in chewing and the patient may learn to smile more naturally, without biting down.