NERVE RELATED INJURIES
Peripheral nerves are formed after a complex brachial plexus merging of nerve roots from the spinal cord. The peripheral nerves of the upper limb are the median, ulnar and radial nerves.
All three of these nerves are exposed during trauma – whether blunt or sharp. A loss of the nerve function causes a change in sensation (sensory part) as well as a loss of function of the muscles it innervates. The normal nerve is a fascinating structure resembling an electric cable, so to speak, with millions of electric wires connecting the outer world, like your fingertips, to the brain. These axons are insulated electrically by Schwann cells which form a myelin layer around the “electric cables”. Millions of these axons are wrapped up by an outer layer called the epineural sheath. This epineural sheath forms the basis of accurate nerve repairs.
The nerve physiology is very sensitive and even pressure can cause failure of function (neuropathy). In the chronic setting, pressure on the median nerve in the carpal tunnel causes carpal tunnel syndrome. The radial nerve crosses the humerus in the upper arm. Pressure in the inner arm is felt, similar to when hanging the arm over the backrest of a seat for a prolonged time, and can cause a radial nerve palsy (so called Saturday night palsy) with complete loss of radial nerve function.
The physiology behind nerve injuries and the recovery thereof is fascinating: The axon has a cell body with multiple dendrites (hair like structures) that communicate with other cells. The cell body has a long axon or electrical cable that can stretch all the way to the skin. This cable is covered by a myelin sheath formed by a Schwann cell. When a peripheral nerve is cut, electric cable and Schwann cells distal to the injury degenerate in a process called Wallerian degeneration, leaving. The proximal part of the nerve senses that there is an injury and stimulates the axon or electric cable to grow.
One of two things can happen: Either it finds the tube and grows down the tube. If it can’t find the tube, the axon grows out wildly and sprouts out to form nerve tissue outside the nerve tube. This is called a neuroma which causes excruciating painful electric symptoms when touch stimulates this electric active nerve tissue around the original injury.
Repairing a nerve endeavours to realign a nerve to ensure that these axons find the empty tubes to reach its end receptors in the skin (sensory) or motor end plates in muscles. The recovery can be in full, but can take 12 - 18 months. Once scarring is established in these tubes, recovery is less likely.
As a hand surgeon, I repair the injured nerve microsurgically in such a way that all the blood vessels are lined up in the epineurium to ensure that the new fibres can grow down their original tubes. We can do this for the small nerves of the fingers or the peripheral nerves in the arm. The further away from the hand it is, the longer the recovery.
The radial nerves main function is to open or clear the palm before something can be picked up. When you want to pick up a glass, your wrist has to extend and your thumb and fingers have to extend to clear the palm in order to close it or wrap the fingers around the object. The loss of radial function causes a dropped wrist and inability to extend the fingers.
The median nerve is the eye of the hand. It is the thickest of the peripheral nerves because it supplies most of the sensory supply within the hand. If you lose the function of the median nerve, you basically have an insensate hand that can’t feel. Losing sensation of most of the hand is very debilitating.
The ulnar nerve runs on the ulnar boundary of the forearm (little finger side). It supplies sensation to the little finger and half of the ring finger. It is called the “powerhouse of the hand” as it supplies all the small muscles of the hand that give you a strong grip strength. If you lose the intrinsic muscle function, your hand assumes a claw position as a result of the imbalance of the hand muscles.
Finally, if you have sustained a nerve injury repair, a reconstruction can be done. Time frames are important and the sooner the better. Accurate microsurgical repair is critical. Rather wait longer though to see a hand surgeon who specialises in this form of surgery. There are multiple options to reconstruct lost motor function in the form of tendon transfers.