NERVE DAMAGE IN THE ARM & HAND

(INJURY TO ULNAR, MEDIAN, OR RADIAL NERVE)

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What is Nerve Damage to the Arm & Hand?

Nerves give sensation to our hands and strength to our muscles.

The Radial nerve provides the sensation to the back of our hand, and also allows our brain to control the muscles that extend (straighten) our fingers and wrist.

The palm of our hand is given sensation by two nerves. The Median nerve, gives sensation to the thumb, index and middle finger, and gives muscle function to flex (bend) our thumb and other fingers. The Ulnar Nerve gives sensation to the ring and pinky finger, and gives muscle function to the smaller muscles within the hand that allows fine dexterity (like writing and gripping things). Each of these nerves starts within your neck as a branch off the spinal cord, and these nerves travel down your arm along different paths.

If one of these nerves is injured along their pathway, either by being crushed or by being cut, the nerve cannot pass information to and from your brain. This prevents you from being able to flex muscles or feel sensation along the injured nerve's path.

Lets look at each of these three nerves and how an injury will affect our hand function.

What is a Median Nerve Injury?

The Median Nerve travels down the front of your arm and enters into the palm of your hand.

The classic Median Nerve injury is carpal tunnel syndrome (see talk here), where pressure slowly develops within the carpal tunnel and pinches this nerve causing numbness in the hand and weakness in the thumb. Our "Opposible Thumb" is critical for hand dexterity, and injury to the Median Nerve may cause weakness in the Abductor Pollicus Brevis muscle (the main muscle for thumb opposition).

Carpal Tunnel syndrome, or a laceration to the Median Nerve at the wrist, is called a low Median Nerve injury because it occurs in the lower part of the arm (near the wrist). Most of the muscles that are given function by this nerve are not affected by a "low nerve injury".

However, a "High Median Nerve Injury" (where the nerve is cut above your elbow) is a lot more complicated because it paralyzes all of the muscles that flex (bend) your wrist and hand . Only the Flexor Carpi Ulnaris (a muscle that flexes your wrist) and the Flexor Digitorum Profundus (a muscle that bends your ring and pinky finger) are spared. This type of injury can occur during a car accident, or if someone is stabbed in the arm during a street fight.

You are probably wondering why is this a big deal, why not just find the cut nerve and tie it back together. Simply tying the nerve back together is an option lower in the arm (near the hand), but it doesnt work as you get closer to your armpit. Here is why: When a nerve is cut, the end of the nerve dies no matter what. The whole reason for tying the cut ends back together is like plugging a fuse into the wall: it allows an electric current to enter this dead segment and bring it back to life. Slowly the electricity will stimulate a new nerve ending to grow along the same path. But it takes a very long time for the new nerve to grow (it grows about 1 mm per day, or 1 inch a month). Meanwhile, as this nerve is growing back, the muscle that is waiting for a signal from the nerve is starting to shrivel up and die (its true that if you dont use it, you lose it). Basically its a very slow race against time: your muscle will forever lose function if it doesnt make contact with the nerve in about 1 year (maybe up to 16 months). That means your nerve cannot be cut much more than 12 inches, or else it will never reach the muscle in time. Therefore, in "High Median Nerve Injuries" your doctor can tie the nerve back together, and sometimes you will regain sensation in the hand, but you will rarely regain muscle strength because its too far for the nerve to travel to save the muscles in time.

If you cannot save the muscles, what can you do for the hand so that it has some function, and isnt just flopping around? There is a solution and its based on Two Principles: 1) Some muscles receive signals from the Median Nerve, but others receive signals from the Ulnar or Radial Nerve; and 2) Our arms have some muscles that do the same thing (overlapping function) and therefore, you can take one muscle and move it somewhere else to do a new job. Moving a muscle from one place to another is called a Tendon Transfer. Tendon Transfers can be used to give function to an area thats paralyzed by a nerve injury. A doctor can borrow the Brachioradialis muscle (which get its signal from the Radial Nerve) and attach it to the thumb so it can bend, and also borrow the ECRL muscle and attach it to the Index finger.

What is a Radial Nerve Injury?

The Radial Nerve travels down the back of your arm, innervates your Triceps, and then just past your elbow it divides into a Muscle Branch (called the PIN) and a Sensation Branch (called the Superficial Nerve). The Radial Nerve activates the muscles that straighten (extend) our fingers and wrist. Without being able to extend our thumb and fingers, its near impossible to grasp objects. And without being able to extend our wrist, we lose a lot of strength when holding objets.

A High Radial Nerve injury (which occurs above the elbow) is a topic of great debate among orthopedic surgeons. Injury to this nerve commonly occurs with a broken arm (about 10% of the time, for more on humeral shaft fracture, see talk) because the nerve is traveling right alongside the bone. If the arm breaks, the bone fragment can poke, pull or crush the nerve. There is a lot of controversy regarding treatment of this nerve injury, and currently there is no great solution. Generally, if the nerve is injured at the same time that the bone breaks, there is a good chance full nerve function will return. If 2 months go by, and the nerve is still not showing any signs of life, meaning no increased sensation (seen with Tinel's sign), no wrist extension (the brachioradialis and ECRL muscles are the first to start working again), and no signs on EMG test (test muscle stimulation with needles), then many doctors will perform surgery to look at the nerve and see if its completely cut, or squished by the bone fragment. They would fix any problem they see at that time. Many other doctors will wait about 3 months before performing surgery because many people's nerves dont show signs of life until after 8 weeks. Remember that the nerve regrows about 1 inch every month (with a 1 month delay at the beginning), so doctors need to time their surgery so that if they need to repair the nerve, it still has enough time to regrow and make contact with the muscles.

The results of a radial nerve repair are often very good. The ends are tied back together, or if a large segment of nerve is completely destroyed, a small tube can connect the two ends, and a new nerve end will grow down that tube. However, if the segment of destroyed nerve is more than 5 cm (about 2 inches) then theres a very low chance the nerve will ever recover.

If the nerve fails to recover in time, and the muscles lose their function, then Tendon Transfers are possible. Additionally, most nerve injuries to the PIN (branch of the radial nerve) cannot be repaired because the PIN fans out like a spider web into many tiny branches. Surgery to repair one branch would likely cause an injury to another, and therefore a tendon transfer is often the best treatment. In fact, Tendon Transfers for a Radial Nerve Injury are some of the most successful of any upper extremity transfer. The most common transfers are the Pronator Teres to the ECRB to enable wrist extension, and thus return a person's power grip. The FCR tendon can be tied to the Extensor digitorum Communis tendon to restore finger extension, and the Palmaris Longus to the Extensor Policis Longus to return thumb extension and abductio.

What is an Ulnar Nerve Injury?

The Ulnar Nerve can also be cut "High" in the arm (above the elbow), or "Low" in the arm (near the wrist).

A Low Ulnar Nerve injury affects sensation on the side of the hand and also makes the little muscles within the hand very weak. This causes the ring and pinky finger to bend into the palm (called a "claw hand"), and also causes overall weakness and clumsiness in the hand. A High Ulnar Nerve Injury will also affect function of the wrist.

Remember that an injured nerve will grow back about 1 cm every month, so lacerations near the hand have a good chance of returning some (not perfect) function to the hand if repaired quickly. In High nerve injuries, where the nerve will not regrow fast enough to prevent the muscles from shriveling up, another option is to take a branch of the Median Nerve (called the AIN) and plug it into the Ulnar Nerve, creating a new electrical connection. This surgery has also been successful at returning some (but not totally normal) function to the hand.

Sometimes the nerve doesnt grow back, and hand weakness persists. This can be a problem for the Ulnar Nerve because it gives function to so many small muscles in your hand, there arent enough tendon transfers available to restore near normal function. Therefore, tendon transfers for the Ulnar Nerve focus on restoring the most important functions, but a person will not regain all of the fine dexterity. The FDS tendon can be used to correct the "Claw Hand" deformity, and sometimes the Thumb Pinch strength can be improved with a ECRB tendon transfer (although the adductor muscle, which gives pinch strength, usually gets innervation from both the Ulnar and Median nerve, so it remains strong enough for daily activity even when the Ulnar Nerve is damaged).

References

1. Lubahn JD, Cermak MB. Uncommon nerve compression syndromes of the upper extremity. JAAOS 1998. 6:378-386. see article. review.