Nerves transmit impulses from the central nervous system (brain and spinal cord) to muscles (motor fibres) and from sensory endings in the skin and other tissues back to the central nervous system (sensory fibres). A separate system (autonomic) provides control and feedback to and from structures that do not come under conscious control, such as sweat glands and blood vessels. (See reflex dystrophy, under Complex Regional Pain syndrome).
Nerve fibres have a cell body in or close to the spinal cord, and each fibre is an extension of that cell or neuron out to the periphery. Division of the thousands of such fibres in an injured nerve profoundly damages the whole cell from which each fibre arises. A small proportion die altogether, and all the remaining fibres have to degenerate and then slowly regrow. They can only do this if the growing fibres at the cut end of the nerve can grow into the nerve beyond the cut and continue down to the target structure, either a muscle fibre or a sensory ending. Although careful repair of the nerve can facilitate this, it remains a complex biological process, and our ability to facilitate good recovery of nerve function is frustratingly limited. Various factors are favourable, including youth, early repair without unnecessary extra damage, rapid uncomplicated wound healing and healthy surrounding body tissues. Conversely, old age, delay, traumatic conditions of the wound or surgery carried out, and infection are adverse factors. Tension in the nerve repair is also unhelpful, and this is usually a feature of late repair, because the rounding off structure that forms (neuroma) has to be removed, causing a gap. If this gap is large enough a nerve graft may be needed, but this means sacrificing another small nerve, and is therefore only done when essential. There has been some success in the development of artificial conduits to bridge nerve gaps, but grafts are not yet superseded.
Nerve surgery is generally done with magnification to ensure accurate delicate handling, and an operating microscope has the advantage of a good light source in the line of the surgeon’s view.
Apart from the loss of muscle and sensory function, the greatest problem resulting from nerve injury is pain. This is very variable, and even damage to small nerves in susceptible individuals can result in severe pain. The pain resulting from major nerve injury, such as for example injury to the brachial plexus in the neck, can in some people be unbearable; this is usually referred to as causalgia. It is interesting that children are not susceptible to this type of pain.
The neuroma that forms on a cut nerve may be the source of pain, and is always tender and uncomfortable. The best way of improving matters is to repair the nerve, but when this is impossible, for example after amputation, it may help to move the neuroma into a position where it is less vulnerable, inside bone or deep in a muscle compartment.
When nerve repair fails to restore movement, it is often possible to move another tendon (or muscle) to provide the movement, since many movements have more than one muscle contributing to them. This type of surgery is called a tendon transfer. Lost sensation is less easy to replace; occasionally an area of skin that still has sensation can be moved from a less important area to one in which it is critical (neurovascular island flap).