Nerve injury


What are nerves?
Nerves are the “telephone wiring” system that carries messages from the brain to the rest of the body. A nerve is like a telephone cable wrapped in insulation. An outer layer of tissue forms a cover to protect the nerve, just like the insulation surrounding a telephone cable (see Figure 1). A nerve contains millions of individual fibers grouped in bundles within the “insulated cable.”

Nerves serve as the “wires” of the body that carry information to and from the brain. Motor nerves carry messages from the brain to muscles to make the body move. Sensory nerves carry messages to the brain from different parts of the body to signal pain, pressure, and temperature. While the individual axon (nerve fiber) carries only one type of message, either motor or sensory, most nerves in the body are made up of both.

What happens when a nerve is injured?
Nerves are fragile and can be damaged by pressure, stretching, or cutting. Pressure or stretching injuries can cause the fibers carrying the information to break and stop the nerve from working, without disrupting the insulating cover. When a nerve is cut, both the nerve and the insulation are broken. Injury to a nerve can stop the transmission of signals to and from the brain, preventing muscles from working and causing loss of feeling in the area supplied by that nerve.

When nerve fibers are broken, the end of the fiber farthest from the brain dies, while the insulation stays intact, leaving empty tubes which used to carry the nerve fibers. The end that is closest to the brain does not die, and after some time may begin to heal. If the insulation was not cut, the nerve fibers may grow down the empty tubes until reaching a muscle or sensory receptor. If both the nerve and insulation have been cut and the nerve is not fixed, the growing nerve fibers may grow into a ball at the end of the cut, forming a nerve scar called a ‘neuroma’. A neuroma can be painful and cause an electrical feeling when touched.

Types

Neurapraxia

This is the least severe form of nerve injury, with complete recovery. In this case, the actual structure of the nerve remains intact, but there is an interruption in conduction of the impulse down the nerve fiber. Most commonly, this involves compression of the nerve or disruption to the blood supply (ischemia). There is a temporary loss of function which is reversible within hours to months of the injury (the average is 6–9 weeks). Wallerian degeneration does not occur, so recovery does not involve actual regeneration. There is frequently greater involvement of motor than sensory function with autonomic function being retained. In electrodiagnostic testing with nerve conduction studies, there is a normal compound motor action potential amplitude distal to the lesion at day 10, and this indicates a diagnosis of mild neuropraxia instead of axonotmesis or neurotmesis.[2]

Axonotmesis

This is a more severe nerve injury with disruption of the neuronal axon, but with maintenance of the myelin sheath.[clarification needed] This type of nerve damage may cause paralysis of the motor, sensory, and autonomic. Mainly seen in crush injury.

If the force creating the nerve damage is removed in a timely fashion, the axon may regenerate, leading to recovery. Electrically, the nerve shows rapid and complete degeneration, with loss of voluntary motor units. Regeneration of the motor end plates will occur, as long as the endoneural tubules are intact.

Axonotmesis involves loss of the relative continuity of the axon and its covering of myelin,[clarification needed] but preservation of the connective tissue framework of the nerve ( the encapsulating tissue, the epineurium and perineurium, are preserved ). Because axonal continuity is lost, Wallerian degeneration occurs. Electromyography ( EMG ) performed 2 to 4 weeks later shows fibrillations and denervation potentials in musculature distal to the injury site. Loss in both motor and sensory spines is more complete with axonotmesis than with neurapraxia, and recovery occurs only through regenerations of the axons, a process requiring time.

Axonotmesis is usually the result of a more severe crush or contusion than neurapraxia, but can also occur when the nerve is stretched (without damage to the epineurium). There is usually an element of retrograde proximal degeneration of the axon, and for regeneration to occur, this loss must first be overcome. The regeneration fibers must cross the injury site and regeneration through the proximal or retrograde area of degeneration may require several weeks. Then the neuritis tip progresses down the distal site, such as the wrist or hand. Proximal lesion may grow distally as fast as 2 to 3 mm per day and distal lesion as slowly as 1.5 mm per day. Regeneration occurs over weeks to years.

Neurotmesis

Neurotmesis is the most severe lesion with potential of recovering. It occurs on severe contusion, stretch, laceration, or Local Anesthetic Toxicity. Not only the axon, but the encapsulating connective tissue lose their continuity. The last (extreme) degree of neurotmesis is transsection, but most neurotmetic injuries do not produce gross loss of continuity of the nerve but rather internal disruption of the architecture of the nerve sufficient to involve perineurium and endoneuruim as well as axons and their covering. Denervation changes recorded by EMG are the same as those seen with axonotmetic injury. There is a complete loss of motor, sensory and autonomic function. If the nerve has been completely divided, axonal regeneration causes a neuroma to form in the proximal stump. For neurotmesis, it is better to use a new more complete classification called the Sunderland System.
[edit]Regeneration
Main article: Neuroregeneration

Physiological mechanisms or neuroregeneration may include remyelination, generation of new neurons, glia, axons, myelin orsynapses. Neuroregeneration differs between the Peripheral Nervous System (PNS) and the Central Nervous System (CNS) by the functional mechanisms and especially, the extent and speed.

Surgery can be done in case a nerve has become cut or otherwise divided. Recovery of a nerve after surgical repair depends mainly on the age of the patient. Young children can recover close-to-normal nerve function.[3] In contrast, a patient over 60 years old with a cut nerve in the hand would expect to recover only protective sensation, that is, the ability to distinguish hot/cold or sharp/dull.[3] Many other factors also affect nerve recovery.[3]

In contrast, repair after damage to the central nervous system is limited.

How is it treated?

To fix a cut nerve, the insulation around both ends of the nerve is sewn together. A nerve in a finger is only as thick as a piece of thin spaghetti, so the stitches have to be very tiny and thin. The repair may need to be protected with a splint for the first 3 weeks to protect it from stretching apart since it is so delicate. The goal in fixing the nerve is to repair the outer cover so that nerve fibers can grow down the empty tubes to the muscles and sensory receptors and work again (seeFigure 2). The surgeon tries to line up the ends of the nerve repair so that the fibers and empty tubes match up with each other as best as possible, but with millions of fibers in the nerve, not all of the original connections are likely to be re-established. If a wound is dirty or crushed, your physician may wait to fix the nerve until the skin has healed. If there is a gap between the ends of the nerve, the doctor may need to take a piece of nerve (nerve graft) from another part of the body to fix the injured nerve. This may cause permanent loss of feeling in the area where the nerve graft was taken. Smaller gaps can sometimes be bridged with “conduits” made from a vein or special cylinder.

Once the nerve cover is fixed, the nerve fibers generally begin to start growing across the repair site after three or four weeks. The nerve fibers then usually grow down the empty nerve tubes up to one inch every month, depending on the patient’s age and other factors. This means that with an injury to a nerve in the arm 11 or 12 inches above the fingertips, it may take as long as a year before feeling returns to the fingertips. The feeling of pins and needles in the fingertips is common during the recovery process. While this can be uncomfortable, it usually passes and is a sign of recovery.
Treatment


Surgical Treatment


Nerve repair with realignment of bundles.The insulation around both ends of the injured nerve is sewn together. The goal in fixing the nerve is to save the insulating cover so that new fibers can grow and the nerve can work again.

If a wound is dirty or crushed, surgery may be delayed until the skin has healed.

If there has been some loss, leaving a space between the ends of the nerve, it may be necessary to take a piece of nerve (nerve graft) from a donor part of the body to fix the injured nerve. This may cause permanent loss of feeling in the area where the donor nerve graft was taken.

Once the insulating covering of the nerve is repaired, the nerve generally begins to heal three or four weeks after the injury. Nerves usually grow one inch every month, depending on the patient's age and other factors. With an injury to a nerve in the arm above the fingertips, it may take up to a year before feeling returns to the fingertips. The feeling of pins and needles in the fingertips is common during the recovery process. While this can be uncomfortable, it usually passes and is a sign of recovery.

Therapy

Several things can be done to keep up muscle activity and feeling while waiting for the nerve to heal.

Physical therapy will keep joints flexible. If the joints become stiff, they will not work, even after muscles begin to work again.

If a sensory nerve has been injured, care must be taken not to burn or cut fingers because there is no feeling in the affected area.

With a nerve injury, the brain may need to be "re-educated." After the nerve has recovered, sensory re-education may be needed to improve feeling to the hand or finger. This involves physician therapy and the appropriate therapy based on the nature of the injury will be recommended by the physician.

Factors that may affect results after nerve repair include age, the type of wound and nerve, and location of the injury. Although nerve injuries may create lasting problems for the patient, care by a physician and proper therapy help two out of three patients return to more normal use.

What is my role in recovery and what kind of results can I expect?

The patient should be aware of several things while waiting for the nerve to heal. Your doctor may recommend therapy to keep joints flexible. If the joints become stiff, they will not work even after muscles begin to work again. When a sensory nerve has been injured, the patient must be extra careful not to burn or cut their fingers since there is no feeling in the affected area. After the nerve has recovered, the brain gets “lazy,” and a procedure called sensory re-education may be needed to improve feeling to the hand or finger. Your doctor will recommend the appropriate therapy based on the nature of your injury.

Factors that may affect results after nerve repair include age, the type of wound and nerve, and location of the injury. While nerve injuries may create lasting problems for the patient, care by a physician and proper therapy help return to more normal use.

Diagram of nerves with bundles of individual nerve fibers

Figure 1: Nerve with bundles of individual nerve fibers and surrounding outer sheath (“insulation”)

Nerve repair with realignment of bundles

Figure 2: Nerve repair with realignment of bundles

NerveInjury.pdf


Building nerve bridges
How nerve regeneration works 
How nerve regeneration works
"The nerve is made up of many thousands of fibres like you can see in a telephone cable.

"These need to be reconnected for the telephone to ring."

Dr Gary Coulton, of St George's Hospital Medical School in London is one of the leading experts treating nerve damage, suffered by thousands of people each year.

The causes can range from household accidents to stabbing, or casualties in war - and are notoriously difficult to treat and slow to heal.

Now Dr Coulton and his colleagues have discovered key proteins which are essential for nerve regeneration.

BBC News Online investigates how scientists plan to "bridge the gap".


Nerve damage can cause long-term problems.

The exact nature of the damage depends on where the affected nerves are, but injuries tend to heal very poorly and cause extensive disability, often wrecking patients' lives.

And the longer patients wait for their injury to be treated, the smaller the possibility of successful treatment.

At the moment, nerve damage is repaired either by sewing the nerve ends back together or by a graft of a nerve from the lower leg, which causes more damage, prolonging the recovery time.


This could be useful for any patient who is undergoing nerve repair

Dr Giorgio Terenghi,
Royal Free and University College Medical School
But scientists at the Royal Free and St George's Hospitals in London are hoping key scientific discoveries will improve patients' chances.

Special proteins have been identified which encourage growth, speeding up the repair process.

Two have been identified - NGF and NT-3.

NGF increases the number of nerve fibres reaching the skin, which improves the sensitivity of skin.

NT-3 seems to act on the nerve cells responsible for fast contracting muscles.

The researchers say this means there must be another growth factor responsible for slow contracting, weight-bearing muscles, which are responsible for posture.

Therapy

Action Research says more information about how specific nerve cells work will help surgeons repairing nerves.

A member of the research team, Dr Giorgio Terenghi, head of the Nerve Regeneration Group at the Blond McIndoe Centre, Royal Free and University College Medical School, added: "It could be useful for any patient who is undergoing nerve repair.

"Although this new therapy will be applicable only to recent nerve injury, we cannot exclude that development in this field might eventually lead to the treatment of patients who have suffered nerve injury in the past."

Scientists had already developed "bridges" - very small tubes that can link healthy and damaged nerves - which can be filled with the growth factor proteins.

The team have also carried out further work into the use of special genetically modified cells called Schwann cells.

These wrap around nerve fibres, helping one cell communicate with another via electrical impulses.

Schwann cells can be injected into the "bridges", producing extra amounts of growth factors.

Because they can be labelled and identified, they can be tracked once they are transplanted.

Dr Coulton added: "Identification of all these different factors contributing to nerve regeneration will be a crucial step forward."


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